The idea of the forest carries deep cultural significance, from the sacred forest groves of pagan tribes and Gothic romantics, to the verdant transcendental “cathedral groves” of John Muir, Yosemite Valley, and the American frontier [1]. Within the urban landscape, this ecologically complex, spatially layered, dynamic system is today also understood to perform a wide range of essential ecosystem services, from increasing property values to mitigating climate change. Reforesting cities is one of the defining trends of twenty first century urbanism, but there is little agreement about how our urban forests are to be designed, planned and managed.

How we define urban forests, what and where they exist, varies between disciplines and individuals. Conceptually, we may think of the urban forest either as a forest within the city or a forest upon which a city relies.  To some, urban forests are the aggregation of woody parts, a collection of isolated trees within the urban landscape.  Alternatively, others may view urban forests as an ecosystem, including not just trees, but their dynamic relationships and interactions with factors biotic and abiotic. Extending this view even further, urban forests may be seen as a critical piece of the larger urban infrastructure.  Ultimately, how we define the urban forest reflects our training, our experience, and our perception of the opportunities for acting on and within this landscape.

Scenario 4: Building the Urban Forest seeks to reconsider this spatial, biological, and metaphorical construct of the “urban forest” from several disciplinary directions: as living machine, as novel ecosystem, as a provider of ecosystem services, and as a spatially and culturally rich landscape, among others. How does the urban forest, and urban forestry, act as operative terms for a set of related disciplines? The issue brings together submissions from ecologists, architects, artists, and landscape architects, all using the term “urban forest” in diverging and overlapping ways.

Image by Nicholas Pevzner

Trends

Among designers, the forest has served as inspiration, as model, and as tool. Architects are incorporating increasing amounts of vegetation into buildings and tapping into the urban forest vocabulary as a means of exploring adaptive and reactive living systems. Malaysian architect Ken Yeang’s projects set an early and enthusiastic precedent, energetically incorporating trees into his buildings for the last three decades, with recent built projects seeking to continuously up the bar. Boeri Studio’s Bosco Verticale, currently under construction, is slated to incorporate 900 trees on the facades of two residential towers in Milan. We appreciate the gusto, but as Tim DeChant elaborates in his essay, “Can we please stop drawing trees on top of skyscrapers?,” the outcome of architecture attempting to foster more complex biological systems takes considerable expense and effort, is difficult for the trees, and is certainly a long way from creating a truly self-regulating forest environment.

Recent projects from landscape architects tend to focus less on trees as a design objects or as a protective [2] or decorative skin on a building, but instead explore forest elements and ecological structure for their cultural value and their productive function. Embracing the process-based routines of forest management, speculative projects such as MVVA and Peter Rose + Partners’ Low2No Master Plan, PORT A+U’s Re-cultivating the Forest City, Atlas Lab’s Sacramento Capitol Canopy, and the winning but ultimately unsuccessful Tree City proposal for Downsview Park by OMA and Bruce Mau, have all proposed long-term afforestation projects that improve streets, build soil, fight vacancy, and build up the spatial qualities of place in cities by tapping into cultural narratives of former regional forest landscapes. This trend also taps into a shared project of expanding the scope of landscape architectural practice to a systemic and infrastructural role essential to the functioning of urban environments and entire cities.

Many design projects that engage the vocabulary of the urban forest, however, remain firmly rooted in the territory of metaphor, borrowing either the spatial qualities, the performative attributes, or the terminology of forests without attempting to actually create a functioning forest ecosystem. From “The Forest,” Peter Walker’s grove of oak trees at the National 9/11 memorial at Ground Zero complete with “Memorial Glades,” to the impressively engineered “Gardens by the Bay” project in Singapore, to Stoss LU’s “Energy Forest” plaza in Pittsburgh, we see designers deploying the rhetorical power of the grove and the forest to lend a deeper reading of engineered infrastructural or formal constructions.

While designers have grappled with the possibilities of largely theoretical urban forests, forest managers and cities have been wrestling with the real challenges of maintaining existing tree canopy and forested land while setting ambitious goals for combatting a range of ecological, social and economic challenges– from global warming to poverty and obesity — through the expansion and emergence of new forestry projects.  In New York, Los Angeles, London, and other cities, Million Trees initiatives have emphasized planting goals that most often target street tree sites, but have also been extended to include afforestation efforts on vacant land.  In other cities, such as Denver, planting goals have been reduced in favor of managing and sustaining the existing canopy in the face of future pressures from urban development, climate change, and emerging pests.  Finally, in cities such as Seattle and Austin, among other exemplary urban forest programs, we find comprehensive, long-term management plans, grounded in ecological and social assessments, that employ a range of urban forestry management strategies and prescriptions. Supporting all of these efforts, is a rapidly growing body of applied ecological research that seeks to better understand the dynamics of urban forest systems, their function and performance, and means in which we may manage, plant, and direct future forest growth so as to maximize the benefits experienced and achieved.

Image by JB 

The presence and performance of urban forests is one of the most pressing topics in contemporary urbanism. As an increasing number of projects and programs concern themselves with shaping, managing and advocating for a new generation of urban forests, we see an outpouring of ideas and techniques emerging, creating a new ground for innovation. Scenario 4: Building the Urban Forest brings together a collection of pieces that explores the potential of the urban forest from a number of scales and disciplinary perspectives. These visions of forests, forestry and foresters examine the spatial, imaginative, active and political dimensions of the topic.

To ground our discussion of contemporary practice and new possibilities, three pieces look to the origins, history, and evolution of urban forestry.  In Deep Roots, Roxi Thoren tells the story of Frederick Law Olmsted and Gifford Pinchot’s work at the Biltmore Estate — a foundational project and a moment in history where the young fields of landscape architecture and forestry converged. Max Piana and Blake Troxel’s essay, Beyond Planting, presents the ecological foundations of urban forests and forestry management, exploring the history of these practices and looking to future trends.  And in Forests as Landscape Artifacts, Brian Davis and Jamie Vanucchi discuss the management promise of the USFS Multiple Use Mandate and how looking to forests rather than parks can enable urban landscapes to be more productive, active, and responsive elements of urban life.

Urban forestry draws from the deep traditions of classical forestry management and environmental science, but it is constantly inventing new techniques for unique urban situations. Constructed and spontaneous urban forests represent novel ecosystems and offer new possibilities for urban design and urban ecology. Putting these concepts into practice, designers have begun to explore the opportunities provided by introducing forests into urban environments. West 8’s piece on Airport Landscape: Schiphol presents a strategy by which nearly a million trees were planted on an unloved piece of urban infrastructure. Jill Desimini’s To Multiply or Subdivide critiques plans to bulldoze an emergent forest fragment that has grown on the abandoned but highly politicized site of the former Pruitt Igoe towers, while Sarah Moos’ 50,000 Trees explores the possibility of using another abandoned and overlooked urban space — the ubiquitous freeway right-of-way — as a site for afforestation and carbon sequestration.

Other projects in the issue propose using the forest to shape urban process and form, and argue for forestry as a legitimate type of urban program. Treating afforestation as an urban design tool, Karen Lutsky’s Big Old Tree, New Big Easy imagines care and stewardship for the next generation of New Orleans’ beloved and character-defining trees as a way of building and servicing a community. Chieh Huang’s project, Urban Regeneration: Foresting Vacancy in Philadelphia imagines a novel land bank strategy that uses urban forestry to redefine the meaning and function of vacant land.

In addition to improving the economic and ecological function of urban environments, urban forests have the capacity to capture the imagination and create curious and mysterious places. This issue features three graphically-rich photo essays that present forested spaces and objects both real and imagined that express a mixture of the wonder and banality of the urban landscape. Invasive Species, Dillon Marsh’s photographs of cellphone towers camouflaged as trees reveals the continuing construction of nature, while The Goodweather Collective’s “retroprojective” photographic project, Roundabout Vancouver, imagines what Vancouver could have looked like had it preserved and incorporated traces of the old-growth forest that predated it, rather than displacing them completely. Keith Chung and Anna Misharina’s Paradoxes of Archetypes: The Urban and the Forest suggests that the conceptual power of the urban forest lies not in the merging of these two traditionally opposed conditions, but in their inherent tension.

Evaluating the success of current urban forestry programs and techniques is challenged by the dearth of available, long-term data. Lara Roman’s piece, How Many Trees Are Enough? calls attention to the need to better understand tree population dynamics in order to set more informed canopy and planting goals, while in Constructing Native Urban Forests as Experiments to Evaluate Resilience, Alex Felson calls out the opportunities for researchers and designers to work together, designing experiments that collect actionable data to improve management techniques while creating new urban spaces to serve and engage communities.

In Building the Global Forest, Richard Weller and Tatum Hands question the scale of our ambitions and interventions, and discuss the measures necessary to achieve global biodiversity and conservation goals which cannot be fully addressed by individual cities or isolated projects. In the process, they call attention to a global condition that calls upon the skills of many disciplines into a unified, strategic, global project.

The pieces represented in this collection have all grappled with what it means to describe, design and represent the urban forest. We hope that you enjoy this issue of Scenario Journal and that you will continue this conversation in your own work and in your communities.

Stephanie Carlisle is a designer and environmental researcher whose work focuses on the relationship between the built and natural environment. She works in the research group at KieranTimberlake Architects. She is a co-editor of this issue. 

Nicholas Pevzner’s research focuses on the public and civic potential of infrastructure, and explores the role that infrastructural landscape moves can play in structuring and sustaining healthy cities. He teaches in the Department of Landscape Architecture and Regional Planning at the University of Pennsylvania School of Design. He is a co-editor of this issue. 

Max is an urban ecologist and environmental researcher at KieranTimberlake.  His work at KT employs a range of quantitative and qualitative assessment methods to inform high performance building and landscape design, emphasizing an understanding of the ecology of site. Max received a Master in Environmental Management from the Yale School of Forestry and Environmental Studies.

Notes

[1] Simon Schama, Landscape and Memory (New York: Vintage Books, 1995)

[2] Landscape elements do of course provide many opportunities for decreasing the thermal loads on a building facade through shading and other micro-climate strategies, and there is considerable exploration of the use of green walls, green screens and even woody elements being used as shading elements to control temperature and moisture.

What would a metropolis in the Pacific Northwest look like if urban planners at the turn of the twentieth century had recognized and exploited the spatial potential of old-growth trees rather than their resource potential? Employing techniques of photomontage and urban mapping, we have proposed an anachronistic detour that decouples empirical fact from historical memory.

While in the present city of Vancouver, the center space of traffic roundabouts is given over to various sanctioned treatments—community gardens, a monumental rock, and so on—in this “retroprojective” proposal an alternative vision of the not-so-distant past is offered, one in which forward-thinking city planners leave an old-growth tree in the middle of each future roundabout.

With this simple gesture, we can envisage an entirely different city, one in which massive trees are no longer a rarity but instead fundamentally define and shape our movement through the urban fabric of Vancouver. While the singular presence of each tree is in itself remarkable, their collective existence is a legacy comparable in size and density to that of Stanley Park, Vancouver’s beloved urban green space. With this action on the civic imagination, the city becomes a forest, and the forest a city.

Roundabout Vancouver: Wires, ca. 1914

Roundabout Vancouver: Hastings St, ca. 1920s

Roundabout Vancouver: Mt Pleasant, ca. 1930s

Roundabout Vancouver: East Village, ca. 1970s

Roundabout Vancouver: Bidwell/Nelson, ca. 2010

  Roundabout Vancouver: Britannia, ca. 2010

Roundabout Vancouver: a retroprojective proposal

This piece was originally produced for the exhibition WE: Vancouver – 12 Manifestos for the City at the Vancouver Art Gallery, February-May 2011. It has appeared in Cabinet Magazine, Issue 43: Forensics and on the CCA Recommends list of the Canadian Center for Architecture.

Project Credits: Michael Lis, Daniel Irvine, Chad Manley

Goodweather is an interdisciplinary design collective based in Vancouver, Canada.

 Formally established in 2012, Goodweather is both an operating platform and alias, functioning as a ‘nom-de-plûme’ that permits work to be undertaken in a diverse field of creative engagements; from self-initiated works to collaborative projects, from private commissions to commercial work and design destined for industry or the public sphere. Drawing primarily on a background in architecture, but recognizing that contemporary design demands an interlacing of associated practices, Goodweather operates within and among a number of allied fields including architecture, design and fabrication.

An airport’s irregular distribution of open space and built-up areas; the variety in building sizes, shapes and architecture; the contrast in size of craft, equipment and vehicles and the multitude of activities and logistics of airport operations, require careful visual management to prevent the appearance of visual chaos.

The Schiphol Airport project is not about theory but about images. This absence of sound theoretical knowledge is perhaps indicative of a moment in landscape architecture history in the late 80’s/early 90’s. When West 8 was asked to take on the project, the question emerged of whether it was actually possible to ‘landscape’ an airport. In an environment that is so dynamic, there seemed to be nothing to design; no durable plan drawing to be delivered. The West 8 designers felt impotent. There were no precedents or examples; it had not been done before. This forced the team to start thinking in terms of a ‘menu’ rather than about a final design. A list was compiled of landscape elements which could be applied in certain locations under certain conditions. The objective was not to develop a lasting landscape rather, to address short term landscape aesthetics. The effectiveness of that menu became very much an eye opener and maybe it can now be considered theory in its own right.

Traveling is unpleasant and after 9/11 the experience became even more painful. For the privilege of air travel, we agree to spend long periods of time in lousy, dirty, smelly places, stranded without our family or friends in anonymous landscapes that hate people and constantly dehumanize the individual. For elderly people, these environments are particularly aggressive. This is just not OK. We are born as Homo sapiens, and it seems that it is only our natural talent of living in and navigating through the wilderness, which allows us to us survive and get through this ‘hell’.

The Growth of Schiphol Airport

This is the context in which West 8 commenced their work. Responding to an ever-increasing global demand for air travel, Schiphol Airport continued to grow. In two decades, Schiphol has since become the fourth busiest airport in Europe.

Considering that Holland is a rather small country, it is remarkable that it has such a large airport and exceptional connectivity, and West 8 is particularly fortunate to have been given the opportunity, as local landscape architects, to assist with the development of such a significant piece of public realm.

By the early 90s, Schiphol had transitioned surprisingly quickly from a small airport in an agricultural landscape setting – into a large airport, with terminals, piers, parking lots and business parks, and everything else. The climax of development activity occurred in the late 80’s, at the same moment that in Holland an environmental awareness had begun to flourish and complaints about air and noise pollution for nearby neighborhoods were front page news on a daily basis. This was exacerbated by a disaster which happened early one evening when a fuel laden freight plane on take-off crashed into a suburban apartment building, killing its crew and scores of local residents.

Schiphol Airport, circa 1960

Opposition against airport development was growing especially amongst the free spirited residents of Amsterdam who saw Schiphol as an ever increasing polluting and damaging cancer. Political campaigns became a national concern, as everyone also knew that if Schiphol failed to grow, the Dutch economy would falter and fall into ruins. The Dutch are principally traders and they need the means to do so. Schiphol management and its directors were increasingly forced to concentrate on public relations campaigns, promoting Schiphol as a quality place, creating feel-good imagery, and highlighting the airport’s responsibilities addressing environmental concerns.

As luck would have it, West 8 was offered an opportunity to work on a part of the Airport which was then under construction. In light of the airport’s budgetary constraints, it was initially intended as a small commission. As a first step, for about 2 or 3 days of work, we set out to analyze this 200,000ha study area – a vast, windy, concrete and tarmac-clad environment with parking lots and buildings of glass and steel, just the type of environment people hate.

It’s not that Schiphol was so bad. Such environments are common at airports, but it is bad that people are forced to deal with them. Airports are very complex places and due to ongoing economic changes and growth they are effectively a constantly changing building site. They are never finished; they are open ended with open edges, full of dirt and junk spaces. For the untrained eye it is difficult to even grasp what you see and visually they are often so complex that their appearance falls outside design ‘theory’.

 The visual complexity of Schiphol 

The overall airport territory was completely fragmented — an aggregation of plots and lots and medians and road verges. All these little bits together constitute a gigantic surface area which was essentially untreated. And then there were the construction sites. The airport seemed to be in a constant flux with building activity never completed, constantly changing to accommodate growth, new machines, new equipment, new techniques, and everything new. So to make sense of this chaos and as a means of organizing this mosaic of untreated bits of land, West 8 introduced a strategy, a menu, which was very clear, very simple and could be summarized on one poster.

West 8 pitched to Schiphol management, that they should get rid of all the patches of dirt, all the unnecessary tarmac, all the vacant gravel lots, the unused sidewalks, the maintenance intensive garden beds, half empty storage compounds, etc. Everything was to be taken away and all those vacant bits of land should be planted with trees. Young small trees, planted in masses, which were cheap, would be spread across the airport area like a veil. This approach had nothing to do with design, but focused on obscuring the airport’s infrastructural complexity.

 Scenario for a green airport 

Rather than starting with an image, West 8 began by reconsidering the process by in which an airport landscape is designed and built. West 8 abandoned the notion that that there should be a project landscape architect which attends weekly meetings with the traffic authority, and the green department, and the commercial property developers, and the utility providers, to negotiate ideal streetscape designs. Instead the approach was that as soon as a site was cleaned up, became vacant, or was left abandoned, a bombardment of trees took place which required very little maintenance.

Through this simple provocation, the landscape area was vastly expanded, the landscape budget was more than halved, and there was no need for another coordination meeting. The bureaucratic site development approach of which landscape architecture had been integral part had suddenly become obsolete. The rationalization and the cost savings impressed the client greatly.

The airport strategy had four simple layers:

• Runway verges: For those arriving at Schiphol, Holland’s green and tidy image needs to be confirmed. The green grassed verges are well maintained at all times.

• Green route: Various airport services, facilities and centers, are positioned along a loop road. A uniform landscape treatment links and characterizes these auxiliary areas.

• Infill planting: In amongst the airport buildings, facilities and services are many areas of open space and vacant land. All those areas, without an identifiable purpose, are planted with trees.

• Visual access: The most impressive visual quality of an airport is the landing and take-off of planes. Coinciding with air-safety, visual corridors are kept open for people to enjoy.

The Right Tree

Special attention had to be paid to the selection of the tree species for the project. Species selection was conducted in collaboration with the Agricultural University Research Centre (Dorskamp). Schiphol Airport as a territory has numerous soil types ranging from coarse sand to heavy clay and peat. Due to extensive previous earthworks, soil storage, and infrastructure development soil types have become heavily disturbed and unevenly mixed across the site. There is the further complication that the airport is situated in a polder (the Haarlemmermeerpolder), which is up to five meters below sea level. The water tables – albeit diligently monitored and maintained – vary significantly across the site.

Additionally, any airport landscape design brief underscores that whatever landscape development is proposed, it is not acceptable to harbor birds. Schiphol is not different and demanded certification that proposed tree species would not attract or support any bird populations. Further, the site, especially along the exposed western edges, has high wind levels and any trees planted need to be able to withstand these harsh airport conditions.

As West 8 intended to plant many trees across the site, the species needed to be cheap and easy to propagate. Also, if the trees ended up in a wrong location and were obstructing new development or utilities, these trees should not impede work in any way. They should be easily removed and be cheaply replaced. And finally, the species selected needed to be clean, low maintenance and make minimal mess when dropping their foliage in autumn.

After considerable investigation, it turned out that Betula pubescens, the ‘Soft Birch’ or ‘European White Birch’ was the tree that ticked all the boxes and that is how the strategy unfolded and hundreds of thousands of these trees were planted all across the airport area. As a natural pioneer species the birches were hardy and adaptable to the numerous growing conditions that occur at Schiphol. As unlikely as it may sound there are no birds perching in these trees, ever. The exception of course confirms the rule: in the past twenty years there has been only a single pair of European magpies who, in 2006/07 had a nest two years running, but they have since moved on.

West 8 in the early 90’s also introduced the idea of promoting natural soil mitigation through the extensive seeding of clover, a commonly used agricultural practice in those days, in order to bring nutrients into the poor roadside soils. In the early years, as the clover colonized all the territories under the birch trees, it proved very effective at improving soils, showing remarkable results in just 2 to 3 years. By mowing just the road edges, the landscape always had a soft clean edge but under the trees the vegetation was kept more natural.

Additionally, it was also decided to place beehives along the roadsides to make an ecological statement. The grotesque, ever-expanding Schiphol Airport, flying more and more planes each year, could now also play the card that they were busy with ecology, on both a large and small scale.

Construction every day

Whenever building or construction or excavation work was being carried out and completed – something something which happens at Schiphol every year, every week, every day – new trees were planted with a bang. With hundreds, sometimes thousands at the same time, it was a strategy that worked everywhere.

Normally in an urban situation, landscape cannot be treated this way. In a number of meetings it would be determined how an existing tree needs to be treated, or protected, or repotted, or possibly even, how the utility needs to be diverted around the tree. Not so at Schiphol Airport. Here the utilities are always number one. The airport is infrastructure. Landscape architects need not worry. After planting has been implemented, everything needs to be let go with an “everything is okay mentality”. Just as long as any damaged or removed planting is replanted in winter!

In the context of Dutch polder land, water management is a very precise exercise and with the growth of Schiphol, and inherent increase of runways, aprons and hard stand areas, storage of the peak run-off needed to be accommodated in the landscape. Several lakes and ponds were therefore incorporated in the terminal district. In that same area, with the introduction of more advanced communication techniques and new fiber-optic cabling, utilities were restructured and concentrated in one corridor. Because of these works, the birch planting and re-planting was carried out three times. The planting here is now in its third life.

Traffic and parking became so dominant that a multi-pronged approach to deal with the visual impact was needed. Across the airport, views towards large parking lots are screened with the trees. At a more local level, mounds were shaped around the parking areas with recycled soil to hide the cars. Where there was not enough room for such mounds, hedges were planted.

For new car park development the idea was introduced of concentration and dedication, using optimally utilitarian parking – by means of ‘off the shelf’ concrete parking garage solutions – while still achieving a high degree of daylight penetration by the incorporation of courtyard areas. Of course, birch trees were also placed within the courtyards and surrounding the parking structures. So even looking out from the parking building you see the trees that provide vistas, express the seasons and filter the daylight. There is only one repertoire – that of birch trees.

In time the result became more and more interesting, as long as the tree planting practice was kept up. In first 6 years hundreds of thousands of trees were installed. The strategy works both in central district environments with offices and business parks, as well as in the large territories at the airport periphery. In central areas the emphasis was more on pushing back the ‘static’, the visual complexity which is so typical for airports. On the larger parcels at the Airport fringes the trees stretches out for miles and because of its simplicity became an extraordinarily beautiful landscape. Birch trees are quite delicate in appearance and in their masses their fine and transparent texture appear as a veil over the landscape enveloping the business parks and cargo districts. The trees bring harmony and identity and their seasonality is wonderful.

So the diligent planting of trees and treating every spare, unused bit of airport surface with grass is now a 20 years strategy. West 8 is still commissioned on an ongoing basis to advise on landscape matters but essentially the project and the ever-changing landscape look after themselves.

Enrichment

To avoid a notion of over-simplification, a number of other interventions were applied. The idea was introduced that visitors arriving at Schiphol should have an intense first impression, a clear sense of arrival in Holland, the country of flowers.

At every front door of all the terminal and key airport buildings flowers of the best (Dutch) quality were placed. Commercial signage was pushed to one side of the main plaza to make one large display of a multitude of billboards and a space was cleared where people could escape the busy terminal, sit down and relax. In this space, the primary seating is the edges of large bowls with flowers. Here you literally sit in the flowers, you can touch the flowers. For these flowers Schiphol pays annually 1,000 euros per bowl. They are planted with 5 or 6 different species throughout the seasons. The quality is superb. The concept plays shamelessly with identity, with what is quintessentially Holland, but people love it. It emphasizes the notion of being outside the terminal, it provides a very attractive social ambiance and the flowers can be photographed the whole year through.

Finally, in the busy central terminal district, as the airport terminal building was renovated, a sedum roof was introduced, which in those days, was a new phenomena. The approach fitted Schiphol. Again there was no undue reliance on ‘landscape’ design. The green roof brought visual relief and was attractive because of its simplicity.

Entrance in the mid 80’s (expansion under construction) [left]
Arrival Hall with the large sedum roof [right]

For about ten years West 8 had only occasional involvement at Schiphol Airport. The planting strategy, without West 8’s attention, became extraordinarily successful and was proving to be self-sustaining. Unfortunately in the following decade we saw a trend of intensification of commercial use both inside and outside the terminal buildings. The plaza which was once cleared of all commercial signage to provide relief and relaxation is increasingly abused by commerce. The airport receives generous payments for such ‘pop-ups’, displays and marketing activities but this is not the intent of West 8’s approach.

In recent times, West 8 has again become more actively involved in the landscape and public realm development of the airport. As time had passed, work practices were changing and some people ‘in the know’ had moved on. The shared understanding of the strategy had faded somewhat and West 8 was asked to evaluate the actual landscape as it has established and to reassess and reinvigorate the strategy. One of the successful and much appreciated actions in this respect was a photographic record that was taken of all key areas. On the photographs was marked-up how items and issues could be addressed and improved.

 Photographic record

By these simple means West 8 could for example easily explain that by adding a few trees the visual aggression of infrastructure could be taken away. A comprehensive set of images was produced which provided clear guidance. Unfortunately works at the airport are increasingly carried out by sub-contractors and such images were not adequate to quantify and assess contractual performance. An engineering firm, hired by Schiphol, converted the images into working documents, which goes against our idea of not having documents. New people in Schiphol management cannot cope with design intent alone and have started to document ‘things’ again.

As the airport has come of age, the urban structure of office precincts has become established and decisions can now be made about permanency including selection of trees and ‘hard landscape’ elements that will now be considered permanent. The landscape and public realm evaluation at Schiphol also brought to light that coordination of work across the airport had grown difficult and over time the clarity of the project had devolved. The photo record, presented to Schiphol management took many by surprise. In response, West 8 was asked to produce a design manual that provided comprehensive, fully coordinated guidance in respect to the recommended material selection throughout the entire Schiphol territory – pulling the project back into a cohesive whole.

Another, more striking occurrence that pointed out the danger of design without coordination, threatened not only the image of the airport landscape, but also cost the airport a great number of trees in exchange for advertising.

Infrastructural development without meetings: The danger

Slowly but surely, over the last 20 years, the number of large pillars with commercial signage at Schiphol Airport has gradually increased. There is a special division at Schiphol who leases out advertising space and the associated contracts are worth significant amounts of money. And of course, in order for the signs to be most effective, they must be seen.

Between the commercial and legal department it was agreed that the advertising contracts would provide for 100% visibility and without consultation, a landscape contractor was called in to chop off all birch trees within the billboards’ sightlines. Not just a few trees, but tens of thousands of mature birches, comprising several kilometers of planting screen were removed – actively reversing twenty years of growth and landscape development.

March 2013, trees chopped off at knee height [left]
Stumps were inter-planted with new stock [right]

Analysis of positioning & visibility from A4 freeway [left]
Trees soften advertising without obstructing visibility [right]

Airport management, au fait with the original landscape design intent, was rather displeased. This action was in stark contrast with Schiphol’s original approach as a contributor to ecological development. It was ordered that the clear felled areas were replanted immediately. Considering the value of the advertising revenue, there were no significant budgetary constraints but availability of so many Betula pubescens at short notice was a problem. So it was therefore recommended to leave the tree stumps, hoping that they may sprout again, even though these remnants of the devastation were rather confrontational.

 Trees chopped off at eye-level sprouted again

Airport – Forest

The most recent development in the long progression of the Schiphol landscape occurred just 2 years ago, when West 8 was speaking about airport management with individuals from the Amsterdam Municipal Authority, who are responsible for the management and development of ‘Het Amsterdamse Bos’, a very successful park from the 1930’s. We asked why the park had not ‘jumped’ over the canal, which surrounds the Haarlemmermeerpolder, the polder in which Schiphol is situated. Would it not be possible to link Schiphol Airport’s green territories with Het Amsterdamse Bos? They too found this to be an interesting provocation. So a drawing was produced, in which the Amsterdamse Zuid-as, with the financial district to the right and Schiphol Airport at the left, become linked. By simply constructing a few bridges and laying out a bicycle loop road connecting people who live and work in these neighborhoods, we could produce a seamless recreational zone without segregation, between the airport landscape, the traditional polder landscape, canals and the Amsterdam forest. At last Schiphol’s landscape strategy can be seen as more than a way to fill dysfunctional space. Through an extension of Het Amsterdamse Bos, the airport landscape is recognized for its role in providing a critical mass of trees and vegetation, and even some ecological potential. Of course there is no money for the bridges yet but that will come. One day.

Het Amsterdamse Bos

Adriaan Geuze founded West 8 urban design & landscape architecture, a leading urban design practice in Rotterdam in 1987. Geuze received a Masters degree in Landscape Architecture from the Agricultural University of Wageningen. After winning the prestigious Prix-de-Rome award in 1990, Geuze, with his office West 8, established an international reputation with his unique approach to planning and design of the public environment. By founding the SLA Foundation (Surrealistic Landscape Architecture) in 1992, Geuze increased public awareness of his profession. Adriaan Geuze and Edzo Bindels brought West 8 to the frontline of international urban design and landscape architecture. West 8 developed a technique of relating contemporary culture, urban identity, architecture, public space and engineering within one design, while always taking the context into account. Geuze frequently lectures and teaches at universities worldwide. With West 8, Adriaan Geuze has been honoured with the success of winning various international design competitions such as Governors Island in New York, Playa de Palma in Mallorca, Toronto’s New Central Waterfront design in Canada, Yongsan Park in Seoul and most recently the Ontario Place Park and Trail project.

Maarten Buijs has been with West 8 since 2007 as Senior Project Manager. After completing his Master in Landscape Architecture at the Agricultural University of Wageningen in 1988, Maarten gained a large part of his professional experience internationally, especially in Australia, where he worked in Melbourne (1988) and Cairns (1993). His expertise encompasses a particularly broad range of landscape architecture, planning and urban design skills. His specific fields of expertise include, amongst many others, landscape master planning, visual analysis, concept design and construction detailing. Maarten’s work with West 8 has focused on projects in the UK. His role as project director for the concept design stages for Stratford Town Centre and London Olympic Village, the design and implementation stages of Jubilee Gardens, and the design for Elizabeth House (Waterloo Station) and the former Commonwealth Institute/British Design Museum site. Maarten has also been involved in projects in North America, Europe, the Middle East and Asia.

The urban forest is a public source of incredible environmental services [1] and deep cultural meaning — it is also a resource that we are rapidly losing. In the urban areas of the United States the forest cover is estimated to be disappearing at 7900 ha/year, or 4.0 million trees/year [2]. There is an urgent need to more effectively and sustainably manage urban forests.

Over the past three decades, a diverse community of environmental managers, researchers, municipalities, and communities have taken on this issue through engaged, on-the-ground actions — representing an expansion in both the science and management of the urban forest [3].  Along the way, our perspective on urban forests has evolved and deepened, reminding us that there is a great deal that we need to learn about the functions and principles of these urban environments.

Urban forests are complex ecosystems, and increasingly we have come to understand that urban forests both reside within the dynamic landscape of the city and are integral to the city’s function and experience.  As such, we are beginning to understand that to achieve ambitious management goals, we must not limit the practice of urban forestry to only foresters, arborists, and horticulturalists. Urban forests and urban forestry represent a site, system, and practice full of opportunity for expanded research, innovation, and cross-disciplinary collaboration.  But as with all interdisciplinary mixing, entry into the foreign vocabulary, theoretical frameworks, and trade of another field is often difficult to initiate and navigate. In this piece we broadly introduce and examine some of the key concepts, techniques, and challenges of urban forestry, with the hopes of providing a resource and a point of departure for a non-forester seeking to explore and engage with this complex and rapidly evolving field.

Images by Steve Harris [Left] and Anthony Fine [Right]

Defining urban forests & urban forestry

At a fundamental level, a forest may be defined as “a three-dimensional ecological system dominated by trees and other woody vegetation that exists in dynamic interaction with the air-earth matrix of the landscape” [4].  An urban forest, however, is less clearly delineated. In practice, the scope of what is considered an urban forest can differ based on three defining components: which structural elements are considered (from a single tree to connected canopy or greenspace), what scale is counted, and what benefits or services are prioritized for management [5,6].  Are trees isolated in curb cuts a piece of the urban forest? Are saplings and emergent vegetation in a vacant lot? Or is the urban forest only found in large tracts of connected canopy on the urban edge?

Urban forestry, as a practice and a field of study, is also a challenge to define and is both historically and culturally contextual [7].  Historically, in the United States, the term “urban forestry” first appeared in the late 1890s [8], and was associated with arboriculture, which focuses on the management of individual trees or collections of highly managed trees in parks and gardens. The practice of professional forestry in America, which also emerged in the late 1800s, did not focus on urban forests, which were at the time viewed as systems too fragmented and broken to be evaluated or managed using traditional methods.  As a result, the ecological study and management of urban forests lagged behind their more rural counterparts. Conversely, in Europe, urban forestry historically concerned itself with the management of peri-urban forests and urban woodlots [9], and as a result remained more closely connected to traditional forestry practice, but located itself at the periphery of the city, and therefore was more spatially and typologically constrained.  Since the 1960s and 70s however, we have seen a rapid expansion of the practice of urban forestry in the United States and a broadening of the definition of the urban forest that has become increasingly comprehensive [10].

Presently, within the United States urban forests are defined, by the U.S. Forest Service and many practitioners and municipalities, as “all publicly and privately owned trees within an urban area — including individual trees along streets and in backyards, as well as stands of remnant forest” [11]. When considering the U.S. Forest Services definition of “urban” [12], this classifies nearly 25% of the U.S. land area as urban forest [13]. As a practice, the American Society for Foresters defines urban forestry as “the art, science, and technology of managing trees and forest resources in and around urban community ecosystems for the physiological, sociological, economic, and aesthetic benefits trees provide society” [14]. In both of these definitions we see the forest described as more than a collection of trees, but rather a complex system defined by multiple and layered functional parts.

The recognition of an urban forest as a system is perhaps best exemplified by the increasingly common description of urban forests as elements of green infrastructure [15]. Such definitions challenge us to extend our consideration of the urban forest to include the entire urban forest ecosystem as a functional whole. This conceptual lens requires the integration of expertise from outside the realm of ecological science, to include engineers, architects, and landscape architects — those practiced in building the urban landscape. As we begin to embrace such broad definitions of urban forestry in our study, practice, and discourse, this new integrated community of urban foresters will be tasked to develop and refine the operational definitions that direct future action.

Image by Remy Rolf Lambert

What makes urban forests unique?

Foresters rely on silvics — the knowledge of how trees grow, reproduce, and respond to environmental changes — to help define and describe forests, understand growth, and guide management over time [16]. Fundamentally, forests are classified by their composition and structure:  trees’ distribution in space (vertically and horizontally), as well as the size, age, and condition of species [17,18,19].  Characteristics such as climate, temperature, water availability, and soil nutrient capacity are determinants of a site – “the sum of environmental factors surrounding and available to the plant at a specific geographic place” [20] — and describe the natural potential for given forest type at a given location. Forest managers use their knowledge of site and silvics to alter forest structure and composition in order to meet a desired set of economic, social, and ecological goals [21,22]. Therefore, the observed composition, structure, and function of a forest are a reflection of both biophysical site factors and deliberate management regimes [23].

While urban forests are characterized under similar principles, the complexity of urban site conditions results in fragmented forest structure and composition, as well as accelerated disturbance regimes and changes in resource availability, all of which alters the growth and silvics of individual trees. Research has found inhibited tree growth rates to be correlated with many urban site factors: constrained growing space [24], low soil moisture [25], excessive soil moisture [26], increased evaporative demand [27], limited nutrient availability [28], disease and pathogens [29], pests [30], competition with understory vegetation [31], and competition with neighboring trees [32].

Urban environments are not entirely hostile to tree growth. Warmer and more stable temperatures, high CO2 concentration, and altered rates of nutrient deposition in cities have been shown to cause accelerated growth rates [33].  Contributing still more complexity, human dynamics can also be correlated to forest demographics, both positively and negatively, adding a social dimension to urban forest systems [34]. Over time, these physiological stress responses result in larger changes in forest structure and composition. This high degree of complexity and variability is a significant challenge to developing appropriate policy, analyzing management scenarios, planning for spacing and infrastructure constraints, and determining best management practices for the selection, siting, planting, and maintenance of urban forests [35,36,37,38]. Urban ecology is beginning to map the variable site conditions that define urban areas, but our understanding of the physiological response of trees and forests to urban stressors is still relatively nascent when compared to traditional forest ecology.

The value of urban forests

The description of urban forests as elements of green infrastructure is a relatively recent trend, driven by the recognition, and valuation, of ecosystem services — the measureable benefits that people can derive, directly or indirectly, from a suite of environmental functions [40,41]. The most basic of such benefits include, but are not limited to: thermal regulation, improved air quality, stormwater management, and provision of habitat.  At the level of the individual tree, these functions are often correlated with tree and crown size, two measures most often used as indicators in tree surveys and management plans. Recent studies illustrate a direct relationship between the benefits of trees and their leaf-atmosphere interactions, suggesting that these services are a function of tree canopy volume and leaf area [42,43,44]. When scaled up, the components of forest structure — the distribution of species, size, age, and condition — determine the quality and quantity of ecosystem services that the urban forest is able to provide [45].

There are also many social and cultural benefits for individuals and communities offered by urban forests, although these are highly variable and more difficult to measure. The aesthetic values of urban forests contribute to recreational enjoyment, quantifiable improvements in mental and physical health, significant emotional, spiritual and personal well-being, reductions in stress and anxiety, and perceptions of privacy, safety and community cohesion [46].  Similarly, community forestry and citizen-based tree planting programs have been correlated with benefits for participants and the associated neighborhoods [47].

Urban Resources Initiative in New Haven, CT. Image by Christopher Schaefer.

At a fundamental level, people like trees and value the benefits that they provide. However, within urban forestry and ecosystem service literature, there is a noticeable gap in defining and quantifying these social and cultural values — a reflection of the need and opportunity for urban forestry to integrate with other urban practitioners, most notably designers, who routinely describe the social and cultural benefits of their interventions.

This increasing recognition of ecosystem service value is changing the perception of nature in cities and is a result of the recent surge in the systematic assessment and the quantification of urban forests benefits [48,49]. Additionally, software that assesses and models ecosystem benefits, such as iTree and inVest, has made the quantification and comparison of ecosystem services both commonplace and cost-effective. Any discussion of urban forest benefits — aesthetic and socio-cultural values, energy and climate management, air and water quality, storm water and soil retention, habitat and biodiversity enhancement, carbon sequestration, or economic benefit — will now assuredly include an estimated dollar amount.

Foxboro, Massachusetts [Left] and Berlin Park System Plan (1909) [Right].  Image by NASA Goddard Space Flight Center [Left]; Penn State University Libraries [Right]

Planning, planting, & growing the urban forest

Recognizing both the many benefits of urban forests and the continued decline in urban canopy cover, cities and municipalities across the country have developed urban forestry programs, management plans, and urban forest goals driven by public policy and municipal mandates.  Within the United States, the majority of municipal afforestation goals and urban forestry plans are tied to a single metric — urban tree canopy, or canopy cover [50]. A recent study determined that 38.9% of 329 U.S. cities with populations of 50,000 or greater have adopted a canopy cover goal [51], most of which reference targets established by American Forests, who recommends an canopy cover of 40%, with more specific recommendations based on land use and geographic location [52].

At the same time, canopy cover assessment, while relatively straightforward and intuitive, has been critiqued by many urban foresters and researchers as “essentially a two-dimensional measurement of the horizontal surface area of the forest” [53], which alone is an incomplete means from which to establish a truly effective urban forestry plan. Recognizing these limitations, researchers and urban foresters have suggested more comprehensive assessment strategies, similar to the study of site and silvics applied to traditional forests, which examine urban forest composition, structure, demographics [54] and growth [55,56] across, and in relation to, the entire urban landscape [57,58,59,60,61].

To reach these canopy goals, many cities emphasize tree-planting programs. The most publicized of these afforestation efforts are the “Million Trees” programs — famously connected to NYC, but having been adopted as well by other cities such as Los Angeles, Denver, and Salt Lake City. The success and efficacy of these programs has been questioned — both Million Trees L.A. and Denver’s Mile High Million have abandoned their planting initiatives and are reorienting their programs towards maintenance, now simply aspiring to sustain existing canopy [62]. Of course even sustaining existing canopy is a challenge. More than half of the existing canopy in U.S. cities resides on private land, requiring different mechanisms for incentivizing and supporting afforestation programs [63]. In combination with ordinances targeting tree management and replacement on private lands, zoning and land use may be used to strategically direct urban forest composition and structure at a citywide scale [64,65]. Another emerging approach is the release of products such as iTree and inVest that begin to quantify and communicate the value of these forests to landowners, which may incentivize private land management and conservation when coupled with economic policy tools such as cap-and-trade or portfolio standards.

Images by Andrew J Faulkner [Left]; Bill Benzen [Right]

Alternatively, recognizing the challenge of increasingly ambitious canopy goals, the continued loss of canopy despite planting efforts, and shrinking municipal budgets, some urban foresters have begun to ask, must the urban forest be planted, or can natural tree regeneration be incorporated into urban forest management? Examining several cities’ entire tree populations, David Nowak of the U.S. Forest Service found that only one in three trees within a city are likely to have been planted [66] — findings that suggest that at least some species of trees are effectively regenerating on their own, and that yes, canopy may “grow” without planting. But what species or populations of trees are successfully colonizing and regenerating — are they natives or planted trees?

Ultimately, this and similar studies raise further questions about how natural forest regeneration in the city may affect urban forest composition, structure, ecosystem health and function. How well will native species recruit, colonize, and regenerate in urban forests or adjacent land? What is the threshold for coexistence and integration between native and non-native populations? Do non-native species significantly decrease urban forest performance? What is the value of Ailanthus? Recent research is beginning to answer some of these questions [67,68] but ultimately, more research is needed to develop robust management strategies that leverage and direct natural regeneration, succession, recruitment, and colonization in combination with strategic plantings to meet our lofty goals for these forests. Planting isn’t the only option that exists — cities may look beyond traditional forestry and adopt creative methods of urban forest regeneration to help them effectively and efficiently sustain or grow their urban canopy.

Old City, Philadelphia.  Image by Dominic Lacivita

Urban silviculture: A hybrid management system for the future?

While the complexity of urban forests presents challenges, the principles of ecological management and silviculture offer a framework for effectively managing this public resource to both maximize and sustain the environmental and social benefits they provide. Silviculture, as a method of ecologically based forestry, has been defined as the “art of producing and tending a forest: the application of knowledge of silvics in the treatment of a forest; and the theory and practice of controlling forest establishment, composition, structure, and growth” [69]. In essence, silviculture is the applied ecological practice of growing trees and vegetation.

The application of these traditional forestry methods is already present in contemporary urban forest research, planning, and practice. Recently, American Forests published a series of case studies examining exemplary urban forestry programs in the United States [70], and while varying in strengths and weaknesses, all these programs demonstrate assessment and management strategies relating to the principles of silviculture and applied ecology. Ultimately, urban forestry is moving towards a perspective that looks beyond the individual tree, and rather to the whole urban forest system, both constructed and natural.

Through the theoretical framework of silviculture, professions outside of forestry may find potential points of connection and opportunity to engage. In a classic silvicultural text, David M. Smith et al. state,

“Silviculture is a kind of process engineering or forest architecture aimed at creating structure or developmental sequences that will serve the intended purposes, be in harmony with the environment, and withstand the loads imposed by environmental influences.” [71].

Additionally these authors suggest that silviculture can be seen as a “biological technology that carries ecosystem management into action”. As we have begun to consider urban forests as elements of urban green infrastructure, and manage this resource with intention and precision, collaboration and coordination with other disciplines such as engineering, architecture, and landscape architecture has become essential. This is probably best evidenced and observed through the collaboration of ecology, forestry, and engineering to deliver urban environmental services, especially those related to water and stormwater management.

Water management, however, represents only one point of intersection for “outside” disciplines to enter the practice of urban forestry. While forests are a driver of ecological services, they can be managed for a variety of different outcomes related to both ecosystem services as well as social and cultural value. Ironically, it is in shaping urban forests for their social and cultural potential that urban forestry arguably struggles the most. The framework for an urban silviculture may provide great opportunity for designers to collaborate with foresters in creatively managing and choreographing urban forest composition and structure, through both space and time, for play, learning, engagement, imagination — as well as ecological health and function.

We can’t simply plant ourselves out of canopy decline, but by continuing to research this complex urban system, integrate allied disciplines, and apply novel management strategies, urban forestry is rapidly evolving and advancing the tools and techniques required to both build and grow the future urban forest. There is both great opportunity and need for non-foresters to engage with this dynamic public resource of our cities.

Max Piana is an urban ecologist and environmental researcher at KieranTimberlake. His work at KT employs a range of quantitative and qualitative assessment methods to inform high performance building and landscape design, emphasizing an understanding of the ecology of site. Max received a Master in Environmental Management from the Yale School of Forestry and Environmental Studies. He is a guest-editor of this issue.

Blake Troxel is a natural resource manager with a particular interest in the connection between forests and people. His efforts aim to refine our understanding of urban forest structure and function; encourage the innovative application and practice of urban forestry; and explore the use of market tools as an incentive for forest management and conservation. He currently works as a consultant for the Vanuatu Department of Forests, and as a consulting forester and urban forester in California. 

Notes

[1] Christiane Weber, “Ecosystem Services Provided by Urban Vegetation: A Literature Review,” Urban Environment (2013):119-131.

[2] David J. Nowak and Eric J. Greenfield, “Tree and impervious cover change in US cities,” Urban Forestry & Urban Greening 11(1) (2012): 21-30.

[3] Georgia Silvera Seamans, “Mainstreaming the environmental benefits of street trees,” Urban Forestry & Urban Greening 12(1) (2013): 2-11.

[4] Burton Verne Barnes, et al., Forest ecology (John Wiley and Sons, 1997).

[5] T.B. Randrup, et al., “The concept of urban forestry in Europe,” Urban forests and Trees, eds. Konijnendijk, C.C., Nilsson, K., Randrup, T.B.,Schipperijn, J. (Berlin, Springer, 2005), 9-21.

[6] Rowan A. Rowntree, “Toward Ecosystem Management: Shifts in the Core and the Context of Urban Forest Ecology.” Urban Ecology, pp. 661-675. Springer US, 2008.

[7] Cecil C. Konijnendijk, Robert M. Ricard, Andy Kenney, and Thomas B. Randrup, “Defining urban forestry–A comparative perspective of North America and Europe,” Urban Forestry & Urban Greening 4(3) (2006): 93-103.

[8] Cecil C. Konijnendijk, et al., “Defining urban forestry–A comparative perspective of North America and Europe,” Urban Forestry & Urban Greening 4(3) (2006): 93-103.

[9] Due in part to the relative scarcity of remote, natural forests and wilderness.

[10] Cecil C. Konijnendijk, et al., “Defining urban forestry–A comparative perspective of North America and Europe,” Urban Forestry & Urban Greening 4(3) (2006): 93-103.

[11] David J. Nowak, Mary H. Noble, Susan M. Sisinni, and John F. Dwyer, “People and trees: assessing the US urban forest resource,” Journal of Forestry 99(3) (2001): 37-42.

[12] Defining what is “urban” is perhaps as contentious as defining “urban forest”.  Often where the boundaries are drawn are dependent upon one’s goals and utility, be it project or research site.  In this instance, we draw from the U.S. Forests Service and U.S. Census Bureaus definition related to specific population density (urban areas and urban clusters), which as of Nowak and Greenfield’s 2012 study, designates 3.1% US Land, 79% of population resides there definition of urban.  See: David J. Nowak, and Eric J. Greenfield, “Tree and impervious cover change in US cities.” Urban Forestry & Urban Greening 11, no. 1 (2012): 21-30.

[13] David J. Nowak, et al., “People and trees: assessing the US urban forest resource.” Journal of Forestry 99, no. 3 (2001): 37-42.

[14] J.A. Helms (Ed.), The Dictionary of Forestry. (Wallingford, UK, CAB International, 1998),193.

[15] Mark A. Benedict, Edward T. McMahon, “Green infrastructure: smart conservation for the 21st century.” Renewable Resources Journal. 20(3) (2002): 12-17.

[16] David M. Smith, et. al., The practice of silviculture: Applied forest ecology 9th edition, (Canada, John Wiley & Sons,1997).

[17] Rowan A. Rowntree, “Toward Ecosystem Management: Shifts in the Core and the Context of Urban Forest Ecology.” Urban Ecology, pp. 661-675. Springer US, 2008.

[18] David J. Nowak, “Understanding the structure,” Journal of forestry 92(10) (1994): 42-46.

[19] Ralph A. Sanders, “Urban vegetation impacts on the hydrology of Dayton, Ohio.” Urban Ecology 9 (3–4) (1986):361–376.

[20] Burton Verne Barnes, et al., Forest Ecology (John Wiley and Sons, 1997).

[21] David J. Nowak, “Understanding the structure,” Journal of forestry 92, (10) (1994): 42-46.

[22] David J. Nowak and John F. Dwyer, “Understanding the benefits and costs of urban forest ecosystems,” Urban and community forestry in the Northeast (2007)25–46.

[23] From an ecological perspective, site is, “the sum total of environmental factors surrounding and available to the plant at a specific geographic place,” including atmospheric factors such as climate, sun temperature and water, as well as physiographic characteristics, soil, and biotic factors that impact plants, including but not limited to, competing vegetation, people, animals, pests, and pathogens. See (Barnes et al.)  Burton Verne Barnes, Donald R. Zak, Shirley R. Denton, and Stephen H. Spurr. Forest ecology. No. Ed. 4. John Wiley and Sons, 1997.

[24] Richard W. Rhoades, and R. Jay Stipes, “Growth of trees on the Virginia Tech campus in response to various factors.” Journal of Arboriculture 25 (1999): 211-217.

[25] Nina Bassuk, and Thomas Whitlow, “Environmental stress in street trees.” The Scientific Management of Vegetation in the Urban Environment 195 (1985): 49-58.

[26] P. Berrang, D.F. Karnosky, and B.J. Stanton, “Environmental factors affecting tree health in New York City.” Journal of Arboriculture 11, (1985):185–1.

[27] Roger Kjelgren, and James R. Clark, “Microclimates and tree growth in three urban spaces.” Journal of Environmental Horticulture 10, no. 3 (1992): 139.  See also:  R. Close, P. Nguyen, and J. Kielbaso, 1996. Urban vs. natural sugar maple growth: I. Stress symptoms and phenology in relation to site characteristics. Journal of Arboriculture 22, 144–150

[28] G.A. Ruark, D.L. Mader, P.L.M. Veneman, and T.A. Tattar,  “Soil factors related to urban sugar maple decline [Acer saccharum].” Journal of Arboriculture (1983).

[29] K. I. Mallett, and Winston Jan Anthony Volney, “The effect of Armillaria root disease on lodgepole pine tree growth.” Canadian Journal of Forest Research 29, no. 2 (1999): 252-259.

[30] Richard W. Rhoades, and R. Jay Stipes, “Growth of trees on the Virginia Tech campus in response to various factors.” Journal of Arboriculture 25 (1999): 211-217.

[31] R. Close, P. Nguyen, J. Kielbaso, “Urban vs. natural sugar maple growth: I. Stress symptoms and phenology in relation to site characteristics.” Journal of Arboriculture 22 (1996): 144-150.

[32] David J. Nowak, Joe R. McBride, and Russell A. Beatty, “Newly planted street tree growth and mortality.” Journal of Arboriculture 16, no. 5 (1990): 124-129.

[33] Jillian W. Gregg, Clive G. Jones, and Todd E. Dawson, “Urbanization effects on tree growth in the vicinity of New York City.” Nature 424, no. 6945 (2003): 183-187.

[34] Emily Jack-Scott, Max Piana, Blake Troxel, Colleen Murphy-Dunning, and Mark S. Ashton, “Stewardship Success: How Community Group Dynamics Affect Urban Street Tree Survival and Growth.” Arboriculture & Urban Forestry 39, no. 4 (2013): 189-196.

[35] Flemming Kjølstad Larsen, and Palle Kristoffersen, “Tilia’s physical dimensions over time.” Journal of Arboriculture 28, no. 5 (2002): 209-214.

[36] G.H. Stoffberg, M. W. Van Rooyen, M. J. Van der Linde, and H. T. Groeneveld, “Predicting the growth in tree height and crown size of three street tree species in the City of Tshwane, South Africa.” Urban Forestry & Urban Greening 7, no. 4 (2008): 259-264.

[37] E. Gregory McPherson, Tree guidelines for coastal southern California communities. Local Government Commission, 2000.

[38] P.J. Peper, E.G. McPherson, and S.M. Mori, “Equations for predicting diameter, height, crown width, and leaf area of San Joaquin Valley street trees.” Journal of Arboriculture 27, no. 6 (2001): 306-317.

[39] S. T. A. Pickett, M.L. Cadenasso, J. Morgan Grove, Christopher G. Boone, Peter M. Groffman, Elena Irwin, Sujay S. Kaushal et al., “Urban ecological systems: Scientific foundations and a decade of progress.” Journal of Environmental Management 92, no. 3 (2011): 331-362.

[40] Robert Costanza, and Carl Folke, “Valuing ecosystem services with efficiency, fairness and sustainability as goals.” Nature’s services: societal dependence on natural ecosystems. Island Press, Washington, DC (1997): 49-70.

[41] Puspam Kumar, ed., The Economics of Ecosystems and Biodiversity: ecological and economic foundations. UNEP/Earthprint, 2010.

[42] Xiao, Qingfu, E. Gregory McPherson, Susan L. Ustin, and Mark E. Grismer, “A new approach to modeling tree rainfall interception.” Journal of Geophysical Research: Atmospheres (1984–2012) 105, no. D23 (2000): 29173-29188.

[43] Xiao, Qingfu, E. Gregory McPherson, James R. Simpson, and Susan L. Ustin, “Rainfall interception by Sacramento’s urban forest.” Journal of Arboriculture 24 (1998): 235-244.

[44] G. H. Stoffberg, M. W. Van Rooyen, M. J. Van der Linde, and H. T. Groeneveld, “Carbon sequestration estimates of indigenous street trees in the City of Tshwane, South Africa.” Urban forestry & urban greening 9, no. 1 (2010): 9-14.

[45] David J. Nowak, “Understanding the structure.” Journal of forestry 92, no. 10 (1994): 42-46.

[46] David J. Nowak, John F. Dwyer, J.F., Understanding the benefits and costs of urban forest ecosystems. Urban and community forestry in the Northeast, (2007): 25–46; See also Wendy Y. Chen, and C. Y. Jim. “Assessment and valuation of the ecosystem services provided by urban forests.” In Ecology, Planning, and Management of Urban Forests, pp. 53-83. Springer New York, 2008.  Note: The number of studies reflecting the cultural benefits of urban forests are numerous.  Some examples of these research findings include, the correlation between the view of trees and improved psychological health, productivity, personal well-being (Kaplan 1993), as well as increased recovery time for patients (Ulrich 1984).  One study in Chicago found the presence of trees to increase community ties, the utilization of outdoor spaces, “healthier patterns of children’s play”, and fewer property and violent crimes Kuo, 2003; Kuo et al., 1998; Kuo and Sullivan, 2001; Sullivan and Kuo, 1996).

[47] Active involvement in tree-planting programs has been shown to enhance a community’s sense of social identity, self-esteem, and territoriality; it teaches residents that they can work together to choose and control the condition of their environment. Planting programs also can project a visible sign of change and provide the impetus for other community renewal and action programs (Feldman and Westphal, 1999; Westphal, 1999, 2003)

[48] Christine Weber, “Ecosystem Services Provided by Urban Vegetation: A Literature Review.” Urban Environment, pp. 119-131. Springer Netherlands, 2013.

[49] Georgia Silvera Seamans, “Mainstreaming the environmental benefits of street trees.” Urban Forestry & Urban Greening 12.1 (2013): 2-11.

[50] Canopy cover is defined by Sarah Mincey et al in “Zoning, land use, and urban tree canopy cover: the importance of scale.” as the “areal extent of the canopy across a city” and is calculated through various methodologies that utilize aerial imagery. Ultimately urban tree canopy assessments establish a potential canopy for a given area, which can be translated into planting or canopy goals. (Mincey 2013)

[51] Marie Rachel Krause, “An assessment of the greenhouse gas reducing activities being implemented in US cities.” Local Environment 16, no. 2 (2011): 193-211.

[52] “Setting urban tree canopy goals,” American Forests website, American Forests, Washington, DC.  Accessed on 8/1/09 http://www.americanforests.org/resources/urbanforests/treedeficit.php

[53] Andy W. Kenney, Philip JE van Wassenaer, and Alexander L. Satel, “Criteria and indicators for strategic urban forest planning and management.” Arboriculture & Urban Forestry 37, no. 3 (2011): 108-117.

[54] Lara A. Roman, and Frederick N. Scatena, “Street tree survival rates: Meta-analysis of previous studies and application to a field survey in Philadelphia, PA, USA.” Urban Forestry & Urban Greening 10.4 (2011): 269-274.

[55] Gregory E. McPherson, “Selecting reference cities for i-Tree Streets.” Arboriculture & urban forestry 36, no. 5 (2010): 230-240.

[56] Blake Troxel, Max Piana, Mark S. Ashton, and Colleen Murphy-Dunning, “Relationships between bole and crown size for young urban trees in the northeastern USA.” Urban Forestry & Urban Greening 12, no. 2 (2013): 144-153.

[57] Andy W. Kenney, Philip JE van Wassenaer, and Alexander L. Satel, “Criteria and indicators for strategic urban forest planning and management.” Arboriculture & Urban Forestry 37, no. 3 (2011): 108-117.

[58] James R. Clark, Nelda P. Matheny, Genni Cross, and Victoria Wake, “A model of urban forest sustainability.” Journal of Arboriculture 23 (1997): 17-30.

[59] Sarah K. Mincey, Mikaela Schmitt-Harsh, and Richard Thurau, “Zoning, land use, and urban tree canopy cover: the importance of scale.” Urban Forestry & Urban Greening 12, no. 2 (2013): 191-199.

[60] David J. Nowak, Rowan A. Rowntree, E. Gregory McPherson, Susan M. Sisinni, Esther R. Kerkmann, and Jack C. Stevens, “Measuring and analyzing urban tree cover.” Landscape and Urban Planning 36, no. 1 (1996): 49-57.

[61] Elizabeth Hill, Jeffrey H. Dorfman, and Elizabeth Kramer, “Evaluating the impact of government land use policies on tree canopy coverage.” Land Use Policy 27, no. 2 (2010): 407-414.

[62] Mark Hough, “Can Urban Forests Save the Planet?,” last modified 9/19/13. http://www.planetizen.com/node/65217

[63] James R. Clark, Nelda P. Matheny, Genni Cross, and Victoria Wake, “A model of urban forest sustainability.” Journal of Arboriculture 23 (1997): 17-30.

[64] In a recent study, Mincey et al. suggest that fine-scale variation in land use policies and related canopy cover policies may be instrumental in sustaining and influencing urban forest canopy cover, structure and composition. (Mincey et al. 2013)

[65] Sarah K. Mincey, Mikaela Schmitt-Harsh, and Richard Thurau, “Zoning, land use, and urban tree canopy cover: the importance of scale.” Urban Forestry & Urban Greening 12, no. 2 (2013): 191-199.

[66] David J. Nowak,”Contrasting natural regeneration and tree planting in fourteen North American cities.” Urban Forestry & Urban Greening 11, no. 4 (2012): 374-382.

[67] Emily E. Oldfield, Robert J. Warren, Alexander J. Felson, and Mark A. Bradford, “FORUM: Challenges and future directions in urban afforestation.” Journal of Applied Ecology 50, no. 5 (2013): 1169-1177.

[68] Sarah L. Robinson, and Jeremy T. Lundholm, “Ecosystem services provided by urban spontaneous vegetation.” Urban Ecosystems 15.3 (2012): 545-557.

[69] David M. Smith, B. C. Larson, M. J. Kelty, and P. M. S. Ashton, The practice of silviculture: Applied forest ecology 9th edition. (1997).

[70] “Urban Forest Case Studies,” American Forests, accessed on 3/1/2014, http://www.americanforests.org/our-programs/urbanforests/urban-forests-case-studies/

[71] David M. Smith, B. C. Larson, M. J. Kelty, and P. M. S. Ashton, The practice of silviculture: Applied forest ecology 9th edition. (1997).

The New York City Afforestation Project (NY-CAP) constitutes a new approach to urban ecology that combines scientific research with landscape architecture to advance understanding of constructed urban ecosystems. This approach bridges disciplines through creative partnerships between designers, ecologists, and city agencies to combine hypothesis driven research with public park design. This model produces an urban research platform facilitating experimentation to guide urban forestry, urban design, and land management.

Figure 1. Cities provide opportunities for the invasion of exotics leading to declines in native populations. Within this context, planting a forest composed of natives presents both opportunities and problems. (Image Citation: YALE UEDLAB)

Background

Novel urban ecosystems are increasingly becoming a part of urban sustainable landscapes. As cities go green, a new generation of green and grey infrastructure, as well as other constructed parkland and public spaces are being built to aid in storm water mitigation, pollution removal, carbon sequestration, and biodiversity enhancement. In the creation and restoration of urban landscapes, planting natives is seen as a way to promote native regional biodiversity of plants and animals [1,2]. Other novel ecosystems, such as successional and invasive plant communities, may be less planned but also contribute vital ecosystem services. Continued investment in these projects begs the question: Do they carry out their intended functions? [3] Landscape infrastructure project have the potential to transform the urban landscape, but such projects can also fail to supply the ecosystem services they were designed to provide and may have unintended consequences [4]. The New York City Afforestation Project (NY-CAP) uses an emerging approach in urban ecology, ‘Designed Experiments,’ to answer this question in situ with a research experiment designed as public parkland [5].

Figure 2. NYC and Five Boroughs: New York City is limited in its capacity to increase canopy. (Image Citation: YALE UNIVERSITY UEDLAB)

The New York City Afforestation Project (NY-CAP) an ecological research component of MillionTreesNYC, an initiative launched as part of New York City’s sustainability-driven agenda PLANYC2030 to increase native tree canopy and to promote human and environmental health. The research opportunity for the NY-CAP arose from Mayor Bloomberg’s allocation of US$400 million over 10 years to the New York City Department of Parks and Recreation (NYCDPR) to plant 600,000 trees, including ~800 ha of forest restoration.

This funding enabled a research oriented afforestation project in Kissena Corridor Park in Queens, NY, in which the selected project team proposed to incorporate basic and applied research into the design, going beyond the usual post-implementation monitoring of tree mortality and survival. During project design and implementation, ecologists in the group worked with park managers to develop experiments that would generate new knowledge on the performance and resilience of urban forests. Designers, in turn, incorporated these experiments into an urban aesthetic and functional park design. For the NY-CAP, the designed experiments approach provided a framework for the collaboration of designers and ecologists that allowed each to employ their respective expertise, and at the same time inform and complement each other’s role [6].

The NY-CAP experiments are designed to explore the impact of differences in species richness, vegetative structure and soil amendment on plant performance and survival. The designed experiments approach offers (A) a new platform for ecologists to design and situate hypothesis-driven research in urban areas on sites that have been historically inaccessible to ecological experiments; (B) a new method for generating replicated scientific data about the ecological processes of constructed urban ecosystems; (C) a new framework for connecting ecologists with stakeholders, including city agencies, contractors and park users; and (D) a new means for designers and park maintenance staff to update their underlying assumptions informing urban land management and expand their ecological knowledge.

The construction of this research platform facilitates urban forest research to further understand mechanisms by which the performance and viability of a constructed urban forest are affected by land management practices, initial conditions of the site, as well as human interventions and socio-economic behaviors. The approach embeds applied ecological science into ongoing city planning through a robust and unprecedented study of urban forestry.

Figure 3. The NY-CAP provides multiple values for stakeholders including contributions to evidence-based management, advance sin ecological understanding of constructed ecosystems, the facilitation of novel relationships between ecologists and designers, and opportunities to engage community members and students in the experiment. (Diagram by Yale University UEDLAB)

Challenges to urban afforestation projects

Cities have recently been highlighted as areas of opportunity for the enhancement and conservation of biodiversity [7, 8]. However, cities also provide opportunities for the invasion of exotic species that lead to declines in native populations [9,10,11].  Species invasion is recognized as posing a significant threat to global biodiversity; exotics threaten to homogenize the world’s flora and fauna, displace native species, modify habitats, and alter ecological function [12,13,14].  Understanding these threats, land managers across the world have enacted programs intended to eradicate and mitigate the presence and impacts of exotic species [15]. Notably, urban areas have been highlighted as locations especially susceptible to invasion [16,17,18]. Higher influxes of exotics through transportation, trade, and horticulture have increased species richness in cities but have also displaced native vegetation [19]. Within this context, the decision to plant a forest composed wholly of native species presents both opportunities and problems: Will planted natives survive and outcompete incoming vegetation? Will the original composition persist through successful recruitment of native vegetation? Our research, the New York City Afforestation Project (NY-CAP), seeks answers to these questions, ultimately assessing the sustainability and resilience of a planted native forest [20].

Figure 4. Urbanization alters resources and physical environments, creating rich niche opportunities for exotic species to capitalize upon and putting natives at competitive advantage. The NYC-CAP explores the resiliency of native forests in the face of urban stressors. (Yale University UEDLAB)

As urbanization continues, and human action continues to modify land, novel urban ecosystems are likely to proliferate [21]. Urban areas have recently been highlighted as opportunities for conservation of native species; however, it is important to note that presence is not necessarily a proxy for persistence or sustainability. There has been little research done assessing the resilience and sustainability of native plant species in these novel urban ecosystems [22]. The NY-CAP provides an empirical testing ground to fill this gap in knowledge, enabling the exploration of urban forest resiliency and sustainability. The work is potentially transformative in that it will establish a robust empirical and conceptual framework for asking whether constructed, native, urban ecosystems can persist, by sustaining the planted vegetation through recruitment and resistance to invasive plant species.

 Testing Resilience

In order to develop the experiments on forest sustainability in Kissena Corridor Park, we used funds directed towards landscape design and implementation were used both for recreation, green infrastructure, carbon storage and habitat and as part of a well-replicated, large-scale, urban forestry experiment integrated as a component of urban public park design within a densely inhabited city.

The experiments test three central questions: 1) How do urban environmental stressors (i.e. pollution, vandalism, drought) affect the health and survival of planted trees? 2) How does tree health and survival respond to management and planting practices, such as plant diversity and organic amendments? 3) Will planted trees recruit to form the urban forest of the future or will this constructed forest instead be overwhelmed by invasive species? These questions derive from the fact that little data exists on how planted, native vegetation survives and functions in urban settings [23,24,25]. Situating ecological research as a component of green infrastructure presents an opportunity to fill these knowledge gaps.

(Yale University UEDLAB)

The 56 research plots are embedded within public parkland. They are separated into 8 different treatments, consisting of a factorial arrangement of tree species richness (6 species versus 2 species), stand complexity (with shrubs and herbs versus without), and soil amendment (with compost versus without). The research design allows us to determine factors impacting tree survival and seedling recruitment across treatments. To balance the interests of stakeholders (e.g. land managers, park-users, the general public), the NY-CAP was, from the outset, designed to generate useful results for guiding decision making about land management issues.

Figure 5. In the summer of 2011, researchers tracked survival, growth, pest damage, photosynthetic capacity, and drought stress across 1,300 of the planted trees. In summer 2012, researchers tracked seedling recruitment of woody vegetation to determine how incoming vegetation competes with natives. (Yale University UEDLAB)

Measurements include assessing the survival, health, and performance of planted vegetation through a series of metrics including root collar diameter and height, tree vigor, pest incidence, drought stress, and vandalism, specific leaf area, and chlorophyll fluorescence. The research team will track seedling recruitment from natural regeneration through estimates of abundance and survival within the research plots.

Figure 6. This boxplot shows a wide degree of variability between tree species, suggesting that some natives are performing better than others. This information will help inform land managers and other policy makers when implementing large scale plantings in temperate urban areas.  (Yale University BRADFORD AND FELSON LABS)

In an environment characterized by pollution, vandalism, drought, pests and disease, it is difficult to say whether or not planted vegetation will survive, persist and be able to recruit its next cohort. The research will test the resiliency and sustainability of this constructed urban forest, applying ecological theory in a constrained urban setting to analyze the survival, growth and recruitment of native species.

Conclusion

“Designed Experiments,” advocate a strong partnership between ecologists and designers through the use of urban design projects as ecological experiments in cities. Expanding the researcher’s role in the design process has the potential to integrate current research into the design and management of the urban built environment and establish a new paradigm for both studying and actively shaping urban ecosystems. There are concerns for limited application of research results to management issues. In confronting these concerns, urban ecological researchers will need to proactively assert the value of ecological sciences, research, and experimentation in urban design projects. To this end, urban ecologists will need to address the broader objective their research will accomplish; namely while basic ecological research seeks to study a system, and design seeks to shape the system, the urban ecology researcher will now have to study a system in order to better shape that system.

The role of designed experiments is not simply to generate statistically viable data but also a multipurpose urban mechanism for revitalization, education and environmental improvement. The design, establishment, and on-going success of the NY-CAP relies on thorough input from a number of stakeholders including, scientists, designers, the New York City Department of Parks and Recreation, as well as local community members. In this way, designed experiments foster collaboration from a number of sources not traditionally involved in ecological research or urban design. Coordinated research and knowledge exchange, advanced data and information sharing, and new research technologies, as well as education and public outreach strategies, are all practices that will support the growth of urban ecology and the integration of ecological principles into the design process, ultimately providing the baseline data and proof-of-concept to implement tested urban sustainability strategies on sites world-wide.

Acknowledgements

The authors would like to acknowledge Robert J. Warren II, Assistant Professor at SUNY Buffalo State and Richard A. Hallett from USDA Forest Service for comments and ideas that contributed to this piece. Thanks to many students from the Urban Ecology and Design Lab, from Mark Bradford’s lab, and from Yale School of Forestry and Yale School of Architecture.  Thanks also for funding from Yale’s Hixon Center and Global Institute of Sustainable Forestry. Thanks to the New York City Department of Parks and Recreation, AECOM and the USDA Forest Service for contributions to the project.

Dr. Alexander Felson is an Assistant Professor at Yale University, where he is jointly appointed between the School of Architecture and the School of Forestry and Environmental Studies. He also directs the joint degree program between these two schools and is the founder of Urban Ecology and Design Lab (UEDLAB). In partnership with The Nature Conservancy, the UEDLAB is developed the Coastal Resilience Plan for Guilford, Connecticut. Dr. Felson is a Co-PI on the NY-CAP project and served as Project Director for AECOM’s work with MillionTreesNYC. Dr. Felson has built bioretention gardens in Bridgeport and is leading Yale’s efforts in a federal HUD and Hurricane Sandy Task Force initiative, Rebuild by Design.  His research focuses on linking urban and landscape design with applied ecological research to study, adapt, and reshape human settlements in response to pressing sustainability challenges.

Instead of trying to fill up the “black hole” of vacancy, Urban Regeneration: Foresting Vacancy in Philadelphia envisions a different future for the city of Philadelphia — one that not only embraces, but expands vacancy [1]. With the assistance of a proposed Philadelphia Land Bank, the system accumulates and transforms vacant property as well as under-used infrastructure into an adaptive and productive urban forest, not only increasing the biodiversity of the urban environment, but in the long run, also lowering maintenance costs and balancing the real estate market for the city.

Contemporary American cities such as Detroit, Cleveland, and Philadelphia have been struggling with population decline and consequently with the problem of vacancy. Property values in the vacant areas of these cities continue to plunge, while crime rates sky-rocket. Tax delinquent properties turn into city’s burdens instead of revenue sources, and infrastructures become inefficient and redundant. Altogether these issues start snowballing into an irreversible vicious cycle.

Upon acknowledging that there is “more vacant property than can be filled,” [2] the question is no longer “how to fill up the void?” but rather “how to utilize the void?” Under the complex settings, this project proposes a landscape planning and land management strategy that adapts both spatially and economically to a shrinking population.

Land Bank Management Strategy: The Philadelphia Land Bank collects vacant land by taking back tax delinquent property and reassembles lots with various ownerships into larger units for urban forestry.

The project is inspired by the remnants of post-industrial train tracks that formerly stretched into the urban grid. By thickening the Northeast Corridor with robust forests, wetland and habitat patches, a river-to-river connection is established. The urban fabric is also diversified with successional forests, while stormwater is managed locally in acupunctural wetlands, reducing CSO overflows into the Schuylkill River and Delaware River [3]. The system then expands, with spurs growing out from the thickened spine into the city. Along these former under-used streets, active programs like community farms, rain gardens, and linear parks will become the new urban edge that redefines its adjacent neighborhoods. Superimposed on top of these, a bike trail network interconnects the city with its two riverfronts.

In addition to the spatial intervention, a new land bank system is proposed as a tool of land management for the project. Unlike a conventional land bank, the Philadelphia Land Bank is set up not only to acquire and manage vacant property but to take them permanently off the market for urban forestry, allowing the population to concentrate in livable areas. By cutting down the housing supply to match the low demand in the area, a balance in the real estate market will be achieved. Once the physical conditions of the areas have been improved and the market structure been restored, the remaining lands in North Philadelphia can begin to recover in market value.

Urban Forestry System Framework: By taking vacant property off the market, and under-used streets off the grid, an urban forest is grown, the city is resized and reshaped, and a new way of living is introduced to its inhabitants.

Unlike the over-sized Fairmount Park with its large portions of green space hidden deep inside the park, the systematic, growing network of linear urban forests creates a maximum number of green interfaces directly to the urban fabric, stretching nature into the heart of the city. Along these interfaces, new urban lifestyles will emerge: kids biking through the green linear parks to get to school; the community gathering in the urban farm to grow their own fresh produce; couples taking a romantic stroll in the woods after dinner; recycled materials from abandoned factories and rail constructions being used to build low-budget scaffolding bridges, providing easy and scenic access across the railway and the forests.

By strategically accumulating and turning vacancy into an urban forest, the project reorganizes the spatial disruption of the existing fabric. By reshaping the city and its neighborhoods into an economically efficient and manageable size, and reducing service costs by replacing under-used infrastructure with a sustainable landscape, this project aims to redefine the meaning and function of vacancy in the city.

Forest Succession: To grow an urban forest, a two-stage process is proposed. During primary succession, shallow-rooted trees such as pines are planted to break apart existing built surfaces and loosen the soil underneath, before being culled. In the secondary succession, a hardwood forest along with shrubs as sub-canopy begins to establish a more robust and diverse environment.

Operation Typology: According to the different spatial conditions of each vacant lot, a diverse menu of landscape operations is prepared. Collectively, they to transform and integrate the site into a part of a larger, robust growing structure.

Process of Making: Through a series of interwoven operations, inputs of energy, material, and planting, vacant lots and underused streets yield in a host of beneficial products and services, ultimately resulting in a remediated, replanted, and expanded interactive and productive urban forest for the city.

A thickened forest band, separating the rail line from the city, is nurtured both as a buffer zone and as a habitat corridor to the city.

Vehicular and pedestrian access to enclosed areas is established; cross-connections are created utilizing adjacent post-industrial structures and materials, linking both sides of the corridor.

Green spurs are extended into the neighborhoods. Linear parks, urban farms, and bike trails work as the active interface between forested areas and the urban fabric.

Year 1: An under-used street with vacant land and buildings. The city still pays to provide services.

Year 5: The vacant property and the empty street are taken offline and start the process of primary succession with fast-growing, shallow-rooted pine trees.

Year 15: Towards the end of the primary succession, the pine trees have broken apart the road structure and prepared the soil for the secondary succession. Adjacent lots are turned to productive programs such as nurseries or community farming.

Year 50: As the secondary succession matures, the interface transitions from forest to clearing/farm to park to urban fabric. The trail network crosses through these adaptive landscapes and links them to the rest of the city.

Chieh Huang is a Master of Landscape Architecture candidate at the University of Pennsylvania School of Design. Before joining PennDesign, he finished his Bachelor Degree in Urban Planning at National Cheng Kung University in Taiwan. His research is focused on how landscape interventions serve not only as public spaces and ecological habitats in the city, but also as a new type of urban infrastructure and land management. Unlike conventional urban planning predicated on expansion, his work provides an alternative vision for the future of blighted cities, embracing nature into the heart of the city.

[3] “Green City, Clean Waters,” Philadelphia Water Department, accessed 3/6/14, http://www.phillywatersheds.org/what_were_doing/documents_and_data/cso_long_term_control_plan

[4] “Land Bank Now,” Philly Land Bank Alliance, accessed 3/6/14, http://www.phillylandbank.org/land-bank-now

“The conservation idea covers a wider range than the field of natural resources alone. Conservation means the greatest good to the greatest number for the longest time…It proclaims the right and duty of the people to act for the benefit of the people. Conservation demands the application of common-sense to the common problems for the common good.”

– Gifford Pinchot, The Fight for Conservation, 1910

“The Third Landscape can be considered as the genetic reservoir of the planet, the space of the future.”

– Gilles Clément, The Third Landscape, 2003 [1]

In America, the forests of our past stretched across the continent, in wide bands, with distinct species marking the different longitudes and latitudes of the vast territory. Surveyors had to devise new systems of measurement to address the wooded landscape—the European precedent only worked for a denuded terrain with vast open views. The American forest was dense and occluded, unmatched in its richness and diversity. The famed naturalist John Muir described them as the best that God ever planted [2]. The transect was impressive from the eastern spruce-fir and the beech-maple-birch forests of the northeast to the loblolly pinelands of the southeast through oak forests and aspen-birch tracts, to the dry ponderosa pines and the deciduous conifer larches onward to the immense western hardwood forests and the coastal lands of Douglas fir, Sitka spruce and redwoods.

 Diagram of United States Forest Types

These trees, as decried by Muir, gave way to urban progress with its agricultural fields, cityscapes, infrastructural projects and industry. But as the cycles dictate, the forests are re-emerging, as spontaneous woodlands on abandoned farms, foreclosed subdivisions, vacant parcels, un-mown right-of-ways and former factory sites. As landscape architect Gilles Clément indicates, these are the forests of the future, the sites to be targeted, re-invented and claimed as the next commons.

Aerial View of Pruitt Igoe Woodlands taken from the City Museum

One such tract—a 33-acre parcel—sits less than one mile from downtown Saint Louis. The site is stunning, both from a distance and on the ground. To penetrate the overgrown perimeter (the fence is a key element in the ecological toolbox, collecting seeds, protecting seedlings and fostering incredible spontaneous plant growth), is to discover a magical interior. Trees loom over forgotten streets with occasional street lamps and manhole covers signaling former development. Rolling topography, formed from the dumping of construction waste, supports varied vegetal communities—a testament to the idea that complex, urban land use patterns yield high biodiversity [3]. Perfect sumac domes have found their ideal home and the site has attracted a suite of species capable of both thriving in the harsh environment and contributing ecosystem and aesthetic benefit. 

Rich Spontaneous Vegetation Covers the Site

The place is remarkable both for its present constitution and past legacy. Once the home of the infamous modernist public housing project—the Captain Wendell O. Pruitt Homes and the William L. Igoe Apartments—the contested site has remained in limbo since their demolition, long enough to grow into a substantial wild urban woodland [4]. Yet, after forty years, the tower demolition imagery remains the most well-known and proliferated symbol of the site [5], while the burgeoning forest is unrecognized, overlooked as a resource worth mentioning, much less celebrating or conserving.

Saint Louis, the Gateway to the West, was founded in 1764. Underlaid by Karst geology, the village site was chosen for its prominent trade location rather than its sound bedrock. With sink holes and surface water filled, expansion began. The city grew throughout the 1800s and early 1900s. The Desoto-Carr neighborhood, located 2 miles north of downtown and just west of the industrial waterfront, developed to accommodate the growing working population. By the 1930s, the neighborhood had aged and was home to some of the city’s poorest residents. Living conditions were substandard and the City of Saint Louis was looking for a housing solution. The Federal Housing Acts of 1937 and 1949 provide an answer. Funds were available for cities to acquire land, clear it and construct new modern housing. Saint Louis led the nation in the envisioning, planning and construction of these towers—deemed a welcome sanitary alternative to the previous condition.

The Saint Louis Housing Authority committed to building over 5,000 units, more than half located in the 33- 11 story towers of Pruitt-Igoe. The racially-segregated buildings opened to residents in 1956, and many loved their new apartments. But with changing federal policies, design-flaws, high crime rates and a maintenance budget dependent on occupancy, the buildings quickly fell into disrepair. Facing economic and political pressure, the Housing Authority elected to demolish the buildings in 1972, only 18 years after it opened.

Post-demolition, the site sat unused, with high levels of debt and social stigma discouraging development but welcoming floral and faunal invaders. It was used as a construction dumping ground, taking much of the material from the stadiums and the convention center, but otherwise sitting fallow as a scar in the city. In the early 1990s, 57 acres were given to the construction of the Gateway school complex. The site was capped, with a large berm burying the toxins below. The remaining 33 acres continue to be richly colonized by a robust successional forest forming a “green hole” in the city’s conscience.

Diagrams Showing Major Site Transformations from Karst Geology through Development Cycles to Wild Urban Woodland 

The unique circumstances surrounding the Pruitt-Igoe site have protected it from development, especially the often seen transition from modernist public housing towers to HOPE VI style suburban enclave. This transformation can be seen on the adjacent site. However, after forty years, the pressure seems more imminent. A local suburban developer, Paul McKee, known for his corporate campuses on greenfield sites, has targeted the neighborhood for his next project, Northside Regeneration. He has purchased over 2,220 parcels in the surrounding neighborhoods, including an option on the Pruitt-Igoe site [6]. The project has garnered political support but the vision remains nebulous. There is no question that the area is in need, as one of the hardest hit areas of population and economic loss, and McKee’s emphasis has been on job creation. Yet, he is treating the land as vacant, as a brownfield tabula rasa akin to his greenfield sites, disregarding past use, and present occupation. It is true that many have left the neighborhood, but those who have remained have chosen to do so. Any future development should respect the resilient, both the human and the floral and faunal.

In Detroit, by contrast, the urban forest is becoming an alternative development model for the extensive, yet perforated, open land. Local planners champion a 140-acre project advertised as a reforestation initiative [7]. John Hantz, a local financier, has obtained a series of non-contiguous parcels on the east side of the city. His proposal, dubbed the Hantz Woodlands, is for the planting of 15,000 trees. The project is unabashedly an economic one, designed for profit through tree harvesting. Much like the Northside Regeneration project in Saint Louis, the details have yet to surface, but Hantz vows for the sites to be open to the public, without fences, allowing free passage among the trees. The implication is a hybrid, civic-economic forest, an alternative urban infill scheme. The idea of a forest running through a city is not a new one—but the idea of a managed, productive urban forest is intriguing and untested. The Hantz project, however, despite self-proclamation, is not an urban forest. Instead, it is more akin to a large scale nursery demonstration. The ornamental planting seems out of place amid the extensive urban wilds. The plots of volunteer vegetation found across the city which is slowly moving into an early successional forest (as evidenced by the Pruitt-Igoe site) have greater potential to seed the next urban forests. The Hantz proposal actively intervenes, by planting woodlands as if they were gardens, when embracing and managing the emergent woodlands is likely a better alternative in a resource-strapped environment.

A small stand of trees is being planted in Detroit while another is being eradicated in Saint Louis. The Northside Regeneration project in Saint Louis seeks to remove the successional forest on the Pruitt-Igoe site, a landscape forty years in the making [8]. Perhaps the argument is that Saint Louis already has a Forest Park – a much-loved and well-frequented park that belies its name. There are countless Forest Parks in the United States, many developed during periods of industrialization as ways to escape the evils of urban life. Some of these parks, like the 5,100 acre woodland in Portland, Oregon remain true to their name whereas others, and Saint Louis’s fits in this category, are forest parks largely in name only. In Saint Louis, a corner of the 1,371 acre park remains treed, while the rest is heavily programmed with recreational activities. Overall, the site has an average of only thirteen trees per acre. While it hosts a primary forest, a secondary successional forest, and several fragile ecosystems, the park is best known for its cultural amenities including a free zoo and a museum housed in Cass Gilbert’s World’s Fair Exposition Palace of Fine Arts building. So while the name is taken, there is still room for a second forest park in Saint Louis. This second generation Forest Park has the ability to capitalize on the urban successional woodlands as a resource capable of providing ecosystem value—carbon sequestration, heat island reduction, stormwater mitigation, biodiversity—while anchoring neighborhood transformation and development.

Ecologists have recognized the potential of the wild urban woodlands. Cities have rich ecosystems, often demonstrating higher biodiversity than areas often considered “near-natural,” such as agricultural areas [9]. This is due to the complex and compact land use patterns of urban areas that allow for many habitats in a relatively small area [10]. The volunteer species that establish in these swaths of woodland are productive ecosystems, “‘hotspots’ of botanic and animal diversity.” [11] These spaces of newfound environmental significance—dubbed novel ecosystems [12] —are the contribution of the anthropocene. The forests of our future are hybrid spaces, rich and layered biomes shaped directly and indirectly by human activity. Their design and management must address ecological, as well as cultural and economic requirements. It is time for urban designers to join their ecological cohort in championing the successional urban forest as an important element in urban restructuring. The insertion builds on past landscape infrastructures, large tracts of land like the Forest Parks or the 1,100 acre Emerald Necklace in Boston, set aside as civic amenities. It recognizes both the need to adopt former sites of human occupation as the next fodder for conservation and the impetus to restore balance to the urban environment. The successional urban forest recycles land and provides breathing room in formerly dense conurbations with species well-adapted to the given conditions.

Key Wild Urban Woodland Species Found on Site

The tools of design are twofold: one at the regional or city-wide scale and one at the site level. The existing and potential forested tracts must be identified, drawn and configured to form the backbone of future development. They must be set aside, acquired much in the same way early conservation pushed the creation of America’s National Parks. The decision as to what to un-build is as fundamental as what to build. Conserved forests may become one element in a larger citywide development strategy that addresses the full complexity of issues facing cities losing population. Then, at the site scale, the forests require greater interpretation. They cannot be simply conserved as ecological preserves or held to an idealized pre-human nature, which National Parks at times foolishly attempt. The latter is counter to the very definition of the novel ecosystem while the former would ignore Gifford Pinchot’s key notion of conservation. To conserve, or preserve, Pruitt-Igoe and analogous sites simply as a wild urban woodlands would be to ignore “the right of the present generation to the fullest necessary use” of the sites themselves [13]. Instead, the woodland conservation demands careful management—allowing for on-going succession intermingled with cultural programming. 

The Site as a National Park

Speculative Program Distribution

Designed experimental landscapes afford the chance to vary the extent of intervention, with some areas left to continue unabated growth, others arrested in states of incomplete succession and still others design for active human use [14]. The strategies evoke those used in early American parks, with sheep grazing and visitors ambling, but the presentation is direct, with a contemporary view of the ecological and cultural landscape as slowly evolving, dynamic space meant to reconcile past occupation with present demands. Resources are limited and they must be understood, respected, conserved, and adapted rather than wiped clean for the next wave of short-sighted subdivisions. The nascent forests emerge from the overgrown vines and scrubby trees. Inside there is enchantment awaiting discovery. 

Jill Desimini is an Assistant Professor of Landscape Architecture at the Harvard University Graduate School of Design. Prior to joining the full-time faculty, she was a Senior Associate at Stoss Landscape Urbanism in Boston. She holds master of landscape architecture and master of architecture degrees from the University of Pennsylvania and a bachelor of arts in urban studies from Brown University. Her research focuses on reproductive strategies for abandoned urban lands.

In 1996 a palm tree appeared almost overnight in a suburb of Cape Town, South Africa. This was supposedly one of the world’s first disguised cell phone tower. Since then these trees have spread across the city, the country and the rest of the world. Invasive Species explores the relationship between the environment and the disguised towers of Cape Town and its surrounds.

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Dillon Marsh is an artist from Cape Town, South Africa. He has a Bachelor’s Degree in Fine Art from the University of Stellenbosch.  During his studies he was drawn to photography and has remained passionate about it ever since. He focuses on natural and manmade features within landscapes that reveal curious details about ourselves and our relationship with the environment.  He has participated in two solo and several group exhibitions, and will be taking part in the Saatchi Gallery’s Pangaea exhibition from April to August this year.

urban [ˈɜːbən]    adj

1. (Social Science / Geography) of, relating to, or constituting a city or town.
2. (Social Science / Geography) living in a city or town.

forest [ˈfɒrɪst]    n

1. (Forestry) a large wooded area having a thick growth of trees and plants.
2. (Law) An area of woodland, especially one owned by the sovereign and set apart as a hunting ground with its own laws and officers.

In the last decade, contemporary urbanism has undergone a radical change in its conceptualization and description of the urban landscape and its relationship to its surroundings.  The emergence of the mega-urban landscape has led even the most rational voices in urbanism to become tethered to a discourse of sustainability. Within this paradigm, the urban forest has become both a troupe and a programmatic stalwart – acting as a largely symbolic gesture serving to legitimize the production of cities through the notion of a “green” urbanism.

On its own, the “urban forest” seems self-explanatory and banal, characterized largely by spatial proximity and contrasting territories and functions. However, in addition to its literal definition – a forest within or adjacent to the city – the urban forest can also be understood in the context  of hybrid progressions, wherein it serves as a rhetorical device that challenges the idea of the classical urban core and questions the possibility of a new form and pattern of urban expansion. In this sense, the urban forest highlights the potential for forested land to serve as a meaningful urban typology capable of promoting a topographic and ecological evolution in a mega-urban landscape.

Romanticized binaries: the urban person in the forest is the same as wild animal in the city  

As the title of this issue (Scenario 4: Building the Urban Forest) suggests, there is a possibility of constructing a new paradigm of landscape urbanism today, one which does away with the paradoxical and unhelpful binaries of urban versus forest, of nature versus nurture, but instead promotes a synthetic project by exploring the productive possibilities of fusing together urban and forest. It’s a proposition to recognize the significance and instrumentality of forests as ecologically complex, spatially layered and dynamic systems that contribute to contemporary urban production. It suggests that the forest can, and does grow within human settlements; forming novel ecosystems that fuse wilderness and metropolis; a sophisticated natural system that could be managed and sustained within the city. Perhaps, as forests are considered through the lens of urban ecology, we might recognize the ways in which they benefit citizens and add value to cities – by mediating water, clean air, sunlight, shade, shelter to animals, and local climate. Ultimately, the urban forest is a place for citizens to escape the city and it’ll be an inexorable rebellion…

The urban forest is a mixture of two archetypes brought together to create an entirely new condition. However, creating such a synthesis simply through a superficial combination of heterogeneous archetypes and features can only result in a problematic and sloppy synthesis. Therefore, the following collages question the expedient definition of urban forest as a total environmental and urban system by stressing the fundamental differences between these two ideas. The drawings examine a paradoxical inversion of these archetypes, in which the city and the forest are each articulated through their contextual nature, and when merged together, maintain traces of their problematic encounter while also denouncing their incapability to function as a synthetic whole.

In order for the urban forest to have social meaning, it must be legible as a form and as a system – revealing the workings of each entity through their juxtaposition. The justification of these archetypes does not rely solely on their conjunct symbolism, meaning and quality, but builds off each idea to stand individually and create something new through a productive tension.

 Tender junction: The city and the forest can never be brought together. They either oppose to each other or take over one another. It is in-between… It cannot serve as none of them. It is a dead city and a ruined forest, it is post-apocalyptical place where neither a human nor an animal can exist as it used to before, it is forced to learn to survive again.

City Decoration: The forest in the city can be considered as green facade: plants are constrained to grow on the vertical surface being victims of the fashion for the nature. They are just a decoration without its initial features and abilities. Same is the forest which by the whim becomes a decoration for the city.

Cliché: It was nature through the lens of the city. It was as the city itself created by the society for society and the forest in the city can never be the real forest as it will be created by the very same principle.

Masquerade of the City: The forest caged in the city is swallowed by it, digested. It can never remain the same. It has to live by the rules set in the city, it has to survive and adapt. It has to wear masks that are for the city embedded faces. It has to hide, it has to suffer. It is colonized.

Anna Misharina completed a Bachelor degree in architecture from the Moscow Architectural Institute and graduated with a Master degree in architecture from the Umea School of Architecture in Umea, Sweden.

Keith Chung completed a Bachelor degree in architecture from the Boston Architectural College and graduated with a Post-Master degree in architecture from the Berlage in Delft, Netherlands.

Keith and Anna are the co-founders of studio CRIT, a research studio that stresses the capacity of architecture as a multi-disciplinary practice with writing, research and design intervention. The studio focuses on understanding spatial conceptualization and production in relation to the changing context of both increasing globalization and the dispersal of local cultures.