Understanding of the hydric functioning of vegetation in an urban park

Abstract

MasterIn cities, airflows, radiative and water budgets are deeply modified by concrete and impervious surfaces, inducing an increase of temperature and a difficult water management. That can have negative impacts on human health especially during extreme events (heatwave, heavy rainfall or pollution peak). One solution is to re-green the cities with vegetation such as parks, trees in streets, green walls and roofs. Indeed, the vegetation transpiration and the shade produced contribute to cool the surrounding atmosphere. Nevertheless, cities are a constraining environment for vegetation especially because of higher temperatures and uncertain access to water. Our study registers in this context and focus more particularly on the trees hydric functioning in urban park. The studied site is an urban park (Jardin du palais Universitaire) located in the downtown of Strasbourg (France). The park is surrounded by buildings and composed of Tilia tomentosa Moench trees and lawn. We studied two years (2014 and 2015) of micrometeorological, vegetation and soil-related experimental data. By analyzing the climate of the two years, we identified two water-limited periods, one in the 2014 spring and another one in summer and fall 2015. We observed that in 2015, the climatic demand of the garden was higher due to a higher hydric deficit. Despite the dry conditions, the measured tree transpirations are nearly constant over the summer but we observe a higher transpiration in 2015 than in 2014, as the climatic demand increases. Regarding the lawn, we noticed a decrease of the evapotranspiration during the water-limited periods. We conclude from this experimental results analysis that the tree water stress is limited and we assume that they are able to draw in a water table which is at about 2m deep. On the contrary, the lawn which has shorter roots cannot reach this groundwater and suffers of water stress, which causes lawn death in summer 2015. To go further in our analysis, we use a numerical model that couples water and energy budget in the tree. The soil-plant-atmosphere continuum model was first design for a forest, so to take into account the fact that the park is located in a city and that we have a sparse trees canopy, we modified the radiation transfer model, the wind profile and the vegetation and soil characteristics. Besides, we modeled the groundwater with a saturated soil layer and the lawn death with a stomatal conductance decrease. Then, we compared the outputs of the model (especially the evapotranspiration) with the measurements. We observed that the inclusion of a ground water table allows a better simulation of the tree transpiration, the water table limits the drop of the leaf water potential and maintains tree transpiration even during water-limited periods. In 2015, the lawn transpiration decrease is well reproduced thanks to the reduction of the lawn stomatal conductance. Finally, we can say that the lawn is progressively dying because of the water stress, so its cooling effect is limited. On the contrary, the tree shade and transpiration are maintained improving human’s thermal comfort and reducing urban heat island