CE22 - Sociétés urbaines, territoires, constructions et mobilité

Modeling of street trees for urban micro-climatology – TIR4sTREEt

TIR4sTREEt (Thermal InfraRed for Street Trees)

The two major assets of trees, particularly appreciated during hot spells, are the shade they provide for passers-by and facades, and their ability to regulate the temperature of their foliage and the air around them, via the transpiration mechanism.

Quantifying the effect of street trees, of 3 species, on urban climate

Transpiration is a physiological phenomenon whereby a tree emits water into the air in gaseous form, via the stomata of its leaves, to ensure photosynthesis and, at the same time, regulate its temperature. Transpiration also activates the circulation of sap from the roots to all the plant's organs. However, the effect of the tree's environment on leaf transpiration, for example, remains poorly understood, especially in urban environments. While the cooling effect of trees has clearly been proven, it has yet to be quantified, given that it varies over the course of a day and according to atmospheric conditions. Studying the interactions between trees and their environment should enable us to identify planting methods to improve living conditions and comfort for city dwellers during hot spells. It will also help to answer a question that is particularly relevant in the context of global warming. The trees that line our streets may not be able to withstand future weather conditions. Which tree species should be planted, in what quantities and in what configurations in relation to buildings and roadways?<br /><br />In this project, which brings together more than 15 scientists from a variety of backgrounds, we aim to reproduce the physical impact of buildings and street trees on the urban microclimate, using measurements and 3D site modeling. The main objective is to understand, and then better characterize, the physical impact of buildings and street trees, of different species, on urban micro-climatology. Strasbourg's tree population is made up of numerous species, some of which are more common than others, such as lime trees and plane trees, for example. The hackberry, more widespread in Mediterranean regions, is interesting for its ability to withstand water stress and high temperatures. This is why the project focuses on 3 different species of avenue trees: lime, hackberry and plane, which line 3 adjacent streets in Strasbourg. To monitor and understand them, we are equipping these trees with a large number of devices and carrying out thermal, meteorological, eco-physiological and geometric measurements for at least two years. These measurement campaigns are the first part of the project.<br /><br />The 2nd objective of TIR4sTREEt is to develop a methodology for merging the geometry of an urban scene with thermal infrared measurements. <br /><br />The result of coupling all the data will be used, in a 3rd objective of TIR4sTREEt, to validate estimates produced by microclimatic simulation tools, currently being developed by the consortium. At the end of the project, this dataset, both measured and simulated, will be made available to the scientific community.

The numerous measurements collected will first be analyzed to deduce behaviors by tree species and over different time periods. Cross-checking these data will give us a fairly complete picture of the soil-plant-atmosphere continuum.
The resulting database will be used not only to support our analyses, but also to refine our simulation models. These simulation models, operating at tree scales and district scale, can be used to estimate the benefits of potential street greening on urban climate. In the medium term, this project will help guide decision-makers in choosing the most appropriate tree species and planting scenarios.

The expected results can be listed as follows:
- Creation of a comprehensive database characterizing trees and their interaction with their urban environment.
- Development of a mobile thermal mapping system for buildings.
- Creation of a 4D thermal model of urban objects with facade thermography.
- Development of an urban microclimate simulation tool with planting modalities.
- Distribution of a decision-making tool for towns and cities.

The project's prospects can be broken down into various types of spin-offs:
1) Scientific spin-offs:
- Estimation of interactions between trees and building facades in a real urban context, and effects on street microclimate.
- Sharing of measured data, via a database, and dissemination of developments (at the end of the project).
2) Economic benefits:
- Help select the species to be favored, to mitigate future urban heat island episodes.
- Provide simulation software as a decision-making tool.
- Develop a mobile thermal mapping system for buildings.
3) Social and cultural impact :
- Help understand and quantify the benefits of trees in the city
- Evaluate greening strategies in future urban projects.

- Development of a mobile building thermal mapping system (a dynamic terrestrial system and an aerial system)
- Publication of articles via HAL-ANR (open archive)
- Development of a decision-making tool

In the continuity of the approach implemented in previous works (ANR COOLTREES 2018-2021), the aim of TIR4sTREEt is firstly to carry out a new measurement campaign to acquire thermal infrared (TIR), meteorological, eco-physiological as well as geometric data, at different seasons, around street trees. The acquisition will be enriched by measurements from an innovative aerial robotic system, never deployed for this kind of topic. The second objective is to develop a methodology for merging the 3D geometry of an urban scene with the measured surface temperatures. The fusion of 3D and thermal infrared data at the scale of a street will then be used to validate estimations produced by microclimatic models. Finally, no study has yet investigated the impact of interspecific variability of trees and their environment on the urban microclimate. This project aims to fill this gap by carrying out measurements at the scale of three streets and on three tree species in the city of Strasbourg.

Project coordination

Tania LANDES (Laboratoire des sciences de l'Ingénieur, de l'Informatique et de l'Imagerie (UMR 7357))

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

SILVA Institut national de recherche pour l'agriculture, l'alimentation et l'environnement Nancy
EMS Service espace verts, Géomatique et connaissance du territoire et Eco Ter, Eurométropole Strasbourg
INSA Strasbourg - ICube Laboratoire des sciences de l'Ingénieur, de l'Informatique et de l'Imagerie (UMR 7357)
PIAF Physique et Physiologie Intégratives de l'Arbre en environnement fluctuant
Unistra - ICube Laboratoire des sciences de l'Ingénieur, de l'Informatique et de l'Imagerie (UMR 7357)

Help of the ANR 514,324 euros
Beginning and duration of the scientific project: - 48 Months

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