Considering increasing environmental risk, the perspective of a +2 °C/3.6 °F temperature rise, and a 50% increase in mortality due to fine particulate matter in megalopolises by 2050,Jos Lelieveld et al., “The Contribution of Outdoor Air Pollution Sources to Premature Mortality on a Global Scale,” Nature, 525 (2015): 367–71, as quoted in the “Planting Healthy Air” study conducted by The Nature Conservancy and C40 Cities. cities will need to develop good habits in order to enjoy the benefits derived from nature. Procuring benefits from nature without considering its own fundamental needs would amount to an often-maligned utilitarian approach, however. Is there no reasonable course of action, somewhere between the two extremes of greenwashing and placing nature in inaccessible reserves, that could enable urban greenspace to flourish in harmony with an anthropized environment?
Caring for service-providing nature
Plants, and trees in particular, play a real role in preventing risks to public health and climatic hazards. First, by generating cooling, through the shade they provide as well as evapotranspiration,During the daytime, spaces with a canopy of mature trees benefited from a temperature lower by 2.7–3.3 °C compared with those devoid of trees in James R. Simpson and Edward G. McPherson, “Simulation of Tree Shade Impacts on Residential Energy Use for Space Conditioning in Sacramento,” Atmospheric Environment, 32, 1 (1998): 69–74. at a rate equivalent to five air conditioning units running twenty hours a day (in the case of an adult tree).Calculations drawn from a study by Johnston and Newton (2004) on mature trees transpiring 450 liters per day as mentioned in the Greater Lyon’s Charter for Trees. Then, by converting CO2 into O2 via photosynthesis (excluding the carbon stored in the soil) and by capturing airborne particulate matterBy investing $3.2 billion a year in planting the maximum possible number of trees in our cities, the mortality rate linked to fine particulate matter could be decreased by 2.7–8.7%. This would save 11,000–36,000 lives per year in the cities considered in the “Planting Healthy Air” study released at the American Public Health Association annual meeting on October 31, 2016. in their leaves and bark. Direct effects of the presence of nature in urban environments on asthma, hypertension, stress levels, patient recovery, aggressivity, and mood have been recorded.Workplace-related stress is lower among workers given access to a window overlooking a tree. Rachel Kaplan and Stephen Kaplan, “The Role of Nature in the Context of the Workplace,” Landscape and Urban Planning 26, no. 1–4 (1993): 193–201. Hospitalized patients given rooms with a garden view exhibited lower pain, fewer complications, and healed 10% faster. Roger S. Ulrich, “View Through a Window May Influence Recovery From Surgery,” Science 224, no. 4647 (1984): 420–1.
These benefits provided by nature are called “ecosystem services” and their economic value can be estimated in monetary terms. For example, the City of New York has estimated that each dollar invested in maintaining or increasing its tree heritage generated more than $5New York City, New York Municipal Forest Resource Analysis, Center for Urban Forest Research, USDA Forest Service, Pacific Southwest Research Station, 2007. by reducing costs incurred for healthcare, air conditioning, and stormwater management infrastructure.Depending on their characteristics, trees intercept up to 25% of rainfall with their leaves, thus limiting stormwater runoff. Furthermore, the combination of increased soil permeability and root development helps them achieve 25% rainwater retention the soil. Susan Downing Day and Sarah Beth Dickinson (Eds.), Managing Stormwater for Urban Sustainability Using Trees and Structural Soils (Blacksburg, VA: Virginia Polytechnic Institute and State University, 2008). This ability to lower flooding risks has led certain thinkers to refer to it as “green infrastructure,” though this is a divisive term among urban developers. Indeed, its drawback is that it objectifies/reifies nature by making it “utilitarian.” Nature thus remains viewed through a human-centric lens, given that the interest generated stems from the services it provides to humans and not what it represents in and of itself. Nevertheless, the terminology would have the advantage of developing closer ties between city departments in charge of public roads and green spaces, which would not only open up dialogue between these key protagonists and improve the understanding of the global challenges related to urban management, but also allow urban nature to tap into municipal infrastructure funds.
In any case, trees can only provide ecosystem services if they are in good health and these services are commensurate with their age and size. This implies going beyond planting a lot and planting well, at the right place, and, in order to do so, increasing our understanding of the subject.
Small deeds for great trees
Well aware of the benefits provided by urban revegetation, cities are taking concrete action. Paris is planning to plant 170,000 new trees by 2026 and to ensure a strong expansion in the creation of fast-growing “urban forests.” The Lyon metropolis aims to plant at least 300,000 trees, which would raise the surface of the urban canopy to 30% by 2030. Grenoble is aiming for 100,000 trees, and Montpellier for 50,000, while Milan is promising 3 million new trees by the same date. This is a pointless numbers race if the basic conditions of tree growth aren’t satisfied, however.
A plantation (whether urban or not) is by no means a forest, which results from spontaneous growth and highly diverse phytosociological communities. Trees happen to have root systems that are once to twice as big as their foliage. This implies that for trees to develop under optimal conditions, their roots mustn’t be covered with any impervious surface, nor enclosed in pits or planter boxes. Taking care of the tree heritage also implies not disrupting circadian cycles with night-time lighting, not conducting unnecessary pruning, and diversifying tree species in order to reduce pathogen-induced mortality. Such actions are only seldom implemented, as reflected in the low life expectancy of urban trees. Their short lifespan is probably due to a modern unfamiliarity with urban agroforestry, given that the trees that were planted in the days of Alphand—the two remarkable plane trees of Grand Palais being one such example—demonstrate that, when properly planted, trees can live upwards of 100 years in urban environments.
Unfortunately, many urban projects continue emphasizing design over plant biology, causing the underdevelopment of urban trees and lifespans as low as 20 years. Potted trees are still used to decorate city squares for instance, and, at the Angers railway station, the height of soilless culture is achieved, as the planters hang from the ceiling, while, in Copenhagen,Marshall Blecher and Studio Fokstrot, Copenhagen Islands. a widely acclaimed project proposes having them float on platforms. Specialists including Frédéric Ségur,Frédéric Ségur is the head of the Landscape and Urban Forestry Department at Lyon Metropolis. There, he is in charge of Plan Canopée, which puts trees at the service of the adaptation of the city to climate change. Forthcoming interview in Stream 05. Véronique Mure,Véronique Mure is a botanist and professor at the École Nationale Supérieur du Paysage de Marseille landscaping school. She collaborated with PCA-STREAM in the “Re-enchanting the Champs-Élysées“ study. Caroline Mollie-Stefulesco,Landscape planner and author of Des arbres dans la ville. L’urbanisme végétal [Trees in the City. Plant-based Urban Planning] (Paris: Actes Sud, 1993, 2009, 2020). and Francis HalléFrancis Hallé is a botanist specializing in the study of trees and tropical rainforests. He is the author of Du bon usage des arbres, Un plaidoyer à l'attention des élus et des énarques [The Proper Use of Trees. An Appeal to Politicians and our Administrative Elite] (Paris: Actes Sud, 2011). Forthcoming interview in Stream 05. nevertheless enable us to take a step back from such operations.
The time of trees is much longer than the time of humans, and planting a tree means looking several decades ahead. This is why project developers must agree to plant young trees, though the impression may seem disappointing at the project inauguration. Trees that are planted young adapt better to the constraints of their environment and therefore become more robust and resistant. It is also becoming crucial to adapt the selection of tree species to the estimated temperature increases in order to avoid condemning young trees to premature death as soon as they are planted. The availability of water and nutrients, and, as a result, soil type and depth are of key importance to the lifespan of urban tree heritage. This imperative sounds like a paradox given their largely impervious urban environment and the fact that they are seldom provided with open ground and their soil cluttered with underground networks.
Planting the city over the city
Curbing urban sprawl to preserve fertile lands implies densifying cities at the same time just as we are trying to make nature penetrate within them. Municipalities and metropolises are therefore facing a spatial contradiction that incites them to favor soilless ecosystems. Roof gardens, “vertical forests,” and slab gardens—the greatest success probably being Michel Péna’s Jardin Atlantique—flourish in urban environments. We urban developers wholeheartedly address this justified demand. It is nevertheless necessary to evaluate the balance between costs and benefits of creating these “natural artifacts”Marion Waller, Artefacts naturels : Nature, réparation, responsabilité [Natural Artifacts – Nature, Reparation, Responsibility] (Paris: Éditions de l’Éclat, 2016). in order to offer projects with genuine qualities.
One of the main constraints to which trees that are planted on rooftops or slabs is that they are subject to concerns related to the bearing capacity of infrastructure. Load capacity rarely allows for the required soil depths needed for tall trees to develop (ideally 1.50 m), thus we are condemning them to stunting and exposing them to water stress. As for the CO2 emitted by the required structural reinforcements, it will only be offset by the plants themselves after a few decades.Architecte Philippe Rahm calculated in a study on La Défense that the CO2 emissions due to the structural reinforcements needed to support the additional weight of plants on the slab would be offset by the growth of the trees only after 70 years. “Urbanisme : la poussée des villes-forêts divise les architectes paysagistes,” [Urban Planning: the Growing Trend of Cities as Forests divides landscape architects], Le Monde, 2020.
Another constraint results from the waterproofing of the roof garden and its necessary repair after a certain number of years (especially when the space beneath is inhabited). This results in the destruction of the slab garden at best 20 years after being installed, dooming its ecosystem to a premature disappearance. Moreover, the watertightness constraint stands in contradiction to the needs of the plants in water and its storage in the ground. In order to compensate for this, innovative approaches are being developed, such as alveolar surfaces doubling as water reservoirs, though their weight remains a limiting factor.
Finally, the maintenance of high-perched trees can prove expensive in certain cases, as with Milan’s Bosco Vertical, where pruning operations using ropes and cranes take place several times a year. Considerable experimental research had nonetheless been conducted on this project in order to adapt the 700 trees to the extreme conditions intended. The trees were first raised in a nursery and subjected to strong winds and root entanglements similar to those prevalent in their future soilless environment. But rather than focusing the research on the constraints to be applied to the plants in order to adapt them to the building’s architecture, wouldn’t it be more sensible to design an architecture that is conducive to the needs of the living? Until then, there is no doubt that the best way of combining vegetation and architecture remains using climbing plants, which exhibit fast growth, fragrant flowering, much lower water needs than trees, and leaf cover that is as extensive (when they grow on pergolas).
From natural artifacts to plant-based urbanism: cosigning projects with nature
There is no life in the absence of water, and no tree in the absence of earth. In addition to its weight, its origin also raises an important question. In natural settings, 30 cm of topsoil takes 3,000 years to generate,Frédérique Ségur, “Nature-Based Solutions,” Stream 05. New Intelligences. Forthcoming. which corresponds to the layer that is ripped off the ground to supply urban greening projects. Research teams from Greater Lyon and Seine-Saint-Denis’ Plan CanopéeTerres Fertiles 2.0 [Fertile Soils 2.0] project in Lyon’s Chemical Valley, and Lil’O (in L’Île-Saint-Denis). are now trying to recreate a living soil artificially by recycling inert soil, enriching the soil microbiome, and accelerating the process of organic soil production. This “nature-based solution” could address the material shortage that is impacting the revegetation of artificialized soils, whether streets, slabs, or roofs.
But the best way of taking into account how ecosystems operate remains to “cosign a project with nature.” Planting in the ground, making due with existing elements, relying on active natural processes, considering nature as a prerequisite to urbanization, naturalizing inner blocks, and densifying urban natural spaces, etc. All draw the outlines of plant-based urbanism. Architect Georges Descombes’s river rewilding project (with the Swiss river Aire), which lets the water trace its own course, or Michel Desvignes’ concept of “intermediary natures,” where the landscape is made a precedent for the construction of neighborhoods, are perfect examples of this. Other attitudes of care towards nature, its needs, and its processes are possible, for example, densifying plantations in the poorly named “urban voids,” which are often brimming with life. Planting around trees and on sidewalks, digging connected planting pits and reservoir pavements, redeveloping urban squares to make them permeable, reconnecting with landscape bodies such as the Seine River or the woodlands, creating continuities between existing parks in order to create a large central park in Paris, etc.PCA-STREAM – Pavillon de l’Arsenal, Champs-Élysées : Histoire et perspectives [Champs Élysées — History & Perspectives] (Paris, 2019). Exhibition catalog. These are the principles that contribute to writing a new urban narrative so that cities can stop being a symbol of pollution and suffocation and respect the conditions of the deployment of the living.
Researchers, entrepreneurs, and planners (among others) are increasingly building a positive vision of the future based on qualitative rather than merely quantitative data.Benoît Labbouz, Sarah Lumbroso, Isabelle Vial, and Denis Lacroix, “Biodiversity: Visions and strategies. Six visions for preserving biodiversity to 2050,” Futuribles 441, no. 2 (2021): 71–84. See also AXA’s 2021 Foresight Report, “2040: Exploring society's future challenges.” The outlook for trees is very promising as they might soon have their own legislationThe nonprofit A.R.B.R.E.S. is working on proposing new legislation recognizing the right to protection of trees based on the idea that trees are living beings in their own right. and governance.
Landscape architect, researcher and deputy editor-in-chief of Stream