Impacts of the urban environment on well-watered tree architectural development and tree climate services

International audience

anglais. Trees in cities can help improve thermal comfort by shading and transpiration, two processes intrinsically linked with tree architecture and ecophysiology. Urban trees experience particular microclimatic conditions, including sudden and extreme changes in radiation and high temperatures. Yet, most of the knowledge on tree functioning was acquired in forests or orchards. To better assess the level of climate services that can be expected, it is thus important to understand how the urban environment affects tree development and ecophysiology. Our objective was to study how the built environment of a canyon street impacts the architectural development of trees, and to quantify climate services provided by these trees. The study was carried out in Angers, France, using an original outdoor facility with alignments of ornamental apple trees: a group of trees grew inside a 1/5th scale canyon street, and the other group (same age and variety) benefited from a rural-like atmospheric environment outside the street. All trees were well-watered and grown inside individual pits. Street trees were more vigorous than those outside street: their axes were longer with more leaves and larger leaf size and they developed a 55% higher leaf area in July, which resulted in higher light interception. In the street the trees were able to improve by up to 7.3°C the thermal comfort index score at midday, lowering the heat stress category from high to moderate. The proportion of the benefits due to shading and transpiration were analyzed. Results evidence the need to consider the effects of the urban microclimate on tree architecture to assess their potential services. In the next stage, the effects of water shortage, frequent in cities, and the interaction with the genotype will be studied, to help stakeholders choose the most suitable species.