Plant trees in the city, yes, but which ones? A million urban trees scrutinized
The numerous revegetation projects envisaged by all the metropolises, communities and towns throughout the territory are faced with various very engaging issues, including this one: what tree species should be planted today to resist the climate? of tomorrow ? A drier climate, warmer during periods extending from spring to autumn. The summer of 2022 gives us a taste of this situation, which could well happen again this year. Let’s turn it into an opportunity, by studying the species in place in different territories with regard to their resistance to water stress, and hope that the results will help guide our choices. To do this, we studied the behavior of a million urban trees, corresponding to more than 800 different species, during the last three summers. Here is a first feedback on this study.
The framework of the study: resistance to water stress like all living organisms, trees have a vital need for water. Without sufficient water resources, their growth will be limited, their ecosystem contributions will be restricted and their survival will be at stake. See how this soap maker sees its cooling capacity regain its vigor the day after watering, whereas the day before, it had ceased all evapotranspiration.
Various resources are available to help communities and all those involved in greening to choose suitable species, such as SESAME from CEREMA or Canographia in Lyon. For our part, we were interested in the resistance of species to water stress.
This study started last year, with the analysis of tree water stress in different urban parks. By way of illustration, here is a comparison of water stress (characterized from the NDWI2 tree index on the tree heritage of Bordeaux between July 2021 and July 2022.
Two paths have been pursued: The first consisted in monitoring the water stress of trees dynamically, as was done in Bordeaux Métropole, with monitoring of the water stress of the tree heritage from May to September, using satellite data. The second consisted in applying the same method to the documented tree heritage (having the location of the trees and at the very least the genus and species of the trees) over three summers with different characteristics:
– 2020 warm without excessive drought (in general)
– 2021 rather cool, with rare periods of heat and high humidity
– 2022 very hot and very dry
The drought we are currently experiencing raises fears of serious consequences for spring and summer. Climatic conditions that are likely to become the standard regime for years to come. It is therefore essential to identify the species that will resist the best, which will suffer the least and which will therefore have the greatest chance of developing and lasting.
This work is completed by systematic monitoring of the daily behavior of the trees, which will not be detailed here.
The data used: We have worked on documented tree heritage, available in opendata on various community or state sites. In total, our study covers just over a million urban trees, representing more than 800 species. The map below shows the territories concerned over a vast area stretching from Brussels to Montpellier and from Bayonne to Strasbourg. This first sample makes it possible to cover various climates and typologies of spaces: oceanic, Mediterranean, continental, plain, mountain…
Not all assets (in their version accessible online) have the same level of documentation, but this can be completed later. The parameters accessible in the study concern:
– Obviously the species
– The place of planting: streets, parks, gardens, etc.
– The nature of the soil
– age
– Crown size
The resulting multiparametric analysis is therefore very broad and rich in information. We present here only a few examples of use.
Some results and available tools: All the data is accessible via a statistical tool allowing you to choose the territory, the species and the period of analysis, and the parameters studied. It is associated with a GIS cartography, of which here are two views of Strasbourg and Lyon.
For what it is worth, Garnett’s carefully selective allusions to the local cannot be explained merely in terms of a simple modernist tension between the universal and the provincial; or in terms of the tourism industry’s preference for the exotic. The architect’s vague allusions to local history and identity were also strongly linked to the reality of Cyprus’s war and the intense desire to avoid them. These situations, contrary to what he had planned, would not provide clear, legible design inspiration for travelers seeking an escape from their own reality.
This study led to the creation of an already quite substantial database that we wish to continue to enrich with new heritage, in territories varied. Any suggestion in this direction will be welcome. The analysis of the behavior of a particular species according to the territories is also interesting, as well as its evolution over the years.
The use of this information, cross-referenced with other data available on trees (Sesame, etc.) on the one hand, and with the local monitoring of species on the other hand, constitutes support for revegetation projects in order to propose the palettes best suited to each site.
