Trees play an essential role in the city: in improving air quality. as promoting the well-being of city dwellers. as refreshing the surrounding air by perspiration fresh water from the ground, from the roots to the leaves. This function of natural conditioner becomes essential in the current and future context of increasing frequency of summer heat waves.
Our UN forecast a growth in the world's human population, which is expected to reach 8,5 billion people in 2030. Knowing that more than half of the world's population lives in cities (4,2 billion people), the cooling power of urban forests constitutes a major challenge to counteract the harmful effects of urban heat islands.
But what about the capacity of urban forests to resist in an increasingly hot and dry climatic context? Our team of scientists, bringing together French and Australians, examined the impact of future changes in temperature and precipitation for a total of 3129 species of trees and shrubs present in 164 cities across 78 countries.
If no mitigation measures are taken by 2050, two thirds of tree and shrub species could be at risk in cities around the world with disastrous consequences for the quality of life. and the habitability of our cities. Despite this worrying observation, there are solutions to help the trees of our cities to ensure the sustainability of their role as natural air conditioners.
The multifunctionality of trees
There is no doubt that trees have many advantages. By refreshing the surrounding air by transpiration of water from the roots to the leaves, they buffer the climatic extremes and make it possible to reduce the electricity bill linked to the operation of air conditioners, while absorbing the carbon dioxide emitted by vehicle traffic. This natural air purification is also accompanied by a reduction in ambient noise.
Trees also form the basis of green spaces in the city, essential to the well-being of city dwellers lacking greenery and ensuring a certain comfort of life. City dwellers saw this clearly during periods of confinement linked to the coronavirus pandemic. Covid-19.
The World Tree Yggdrasil (here an ash tree), and some of its inhabitants. Engraving by Friedrich Wilhelm Heine (1845-1921), taken from Asgard and the gods ("Asgard and the Gods") by Wilhelm Wägner.
Wikimedia, CC BY-NC-SA
Finally, in all civilizations, the tree occupies an important place: sometimes symbolic, like the willow tree signifying immortality, sometimes associated with tales and legends, or sometimes even sacred or divinatory; this is the case of Yggdrasil, the world-tree often represented by an ash tree in Norse mythology.
A very concrete expression of this strong bond that unites trees and people is reflected in our desire to name and label the most remarkable of them by giving them the status of "exceptional natural monument".
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For all these reasons, ensuring the sustainability of trees and shrubs in the city, collectively referred to as urban forests, is a major challenge in terms of strategies for mitigating and adapting to the harmful effects of climate change.
There is nevertheless very little information on the ability of trees and shrubs in our cities to withstand the stress of urban environments in the context of global warming temperatures and increased frequency of heat waves and droughts.
Threat to urban forests
The results of our study, published this Monday, September 19, 2022 in the journal Nature Climate Change, reveal that species as common as ashes, oaks, maples, poplars, elms, limes, chestnut trees, pines and many other species, are among the more than a thousand species of trees and shrubs identified as being at risk in the face of climate change. current and future climates, where they are present in most cities of the world.
Currently, between 56 and 65% of these species are already at risk, either because the temperatures in some cities already exceed the upper limit tolerable for the species or because the level of current rainfall in some cities is already below the lower limit tolerable by the species.
More worrying: by 2050, between 68 and 76% of these species will be at risk, with a greater danger for cities closer to the equator, in the intertropical zone, where the species safety margin is the narrowest.
When maximum temperatures exceed the tolerable limits for a species or when the cumulative annual precipitation falls below the vital minimum required, this not only leads to a weakening and a drop in growth at the level of the individual, but also a reduction of the natural air conditioning effect.
Without water in the system, or in the event of excessive heat, the tree stops its photosynthetic activity, no longer sweats and sometimes even, for hardwoods such as oak, gets rid of its leaves to limit exchanges with the atmosphere. This means that just when we need it mostcooling effect of trees, the air conditioning breaks down!
The case of French cities
As part of our study, we had data for five French cities (Paris, Bordeaux, Montpellier, Grenoble, Lyon) and 506 different species of trees and shrubs, for a total of 1254 observations.
By 2050, 71% of tree and shrub species in these five French cities will be at risk from the increase in average annual temperatures, 69% of species will be at risk from face of the decrease in the cumulative annual precipitation and 49% of species will be at risk for both phenomena at the same time.
In a city like Montpellier, these percentages reach 83%, 66% and 55%, respectively, and species such as common ash, small-leaved linden, large-leaved linden, Norway maple, silver birch, aspen poplar, white alder or Scots pine will then be among the species most at risk.
This means that the individuals of these species will undergo increasingly stressful climatic conditions, whether in terms of extreme temperatures or lack of water, which will require the provision by town halls of increasingly financial means. costly to maintain favorable water supply conditions in the event of a water deficit and thus limit health risks of mortality.
What are the possible mitigation measures?
Our results are important overall, because they provide managers of green spaces, across 164 cities around the world, with crucial information on the species of trees and shrubs most at risk and for which health monitoring will be necessary for future as well as the species most resilient to change, thus allowing better planning of the selection of future species to be planted in the city.
Impregnated soils for better management of rainwater (Actu-environnement, October 5, 2021).
There are a few simple solutions to help urban forests endure in this transition to a warmer, drier climate in most cities around the world:
- ensure privileged access of tree roots to rainwater by reducing impermeable surfaces and by redirecting water towards the ground rather than into the gutters;
- plant more trees and shrubs in the city by choosing species that are resistant to water stress and if possible indigenous or from neighboring biogeographical zones;
- Above all, do not sacrifice what already exists in favor of more asphalted surfaces intended for parking spaces, for example.
Jonathan Lenoir, Senior Researcher in Ecology & Biostatistics (CNRS), University of Picardy Jules Verne (UPJV); Jaana Dielenberg, University Fellow, Charles Darwin University; Manuel Esperon Rodriguez, Lecturer and Research Fellow, Hawkesbury Institute for the Environment, Western Sydney University; Mark G Tjoelker, Professor and Associate Director, Hawkesbury Institute for the Environment, Western Sydney University et Rachel Gallagher, Associate Professor, Hawkesbury Institute for the Environment, Western Sydney University