Comment on bg-2023-5

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Urban vegetation plays an important role in offsetting urban CO₂ emissions and mitigating heat through tree transpiration and shading. With frequent heatwave events and the accompanying drought, the functioning of urban trees is severely affected in terms of photosynthesis and transpiration rate. The detailed response is however still unknown despite tree functioning having crucial effects on the ecosystem services they provide. We conducted sap flux density (<em>J_s</em>) and leaf gas exchange measurements of trees (<em>Tilia cordata</em>, <em>Tilia</em> &times; <em>europaea</em>, <em>Betula pendula</em>, <em>Malus spp</em>.) located at four types of urban green areas (Park, Street, Forest, Orchard) in Helsinki, Finland, over two contrasting summers 2020 and 2021. Summer 2021 had a strong heatwave and drought, whereas summer 2020 was more typical for Helsinki. In this study, our aim was to understand the responses of urban tree transpiration and leaf gas exchange to heatwave and drought and examine the main environmental drivers controlling the transpiration rate during these periods in urban green areas. We observed varying responses of tree water use during the heatwave period at the four urban sites. <em>J_s</em> was found to be 35&ndash;67 % higher during the heatwave as compared to the non-heatwave period at the Park, Forest, and Orchard sites but no significant difference was found at the Street site. Our results showed that <em>J_s</em> was higher (31&ndash;63 %) at all sites during drought as compared to non-dry periods. The higher <em>J_s</em> during the heatwave and dry periods were mainly driven by the high atmospheric demand for evapotranspiration represented by the vapor pressure deficit (VPD), suggesting that the trees were not experiencing severe enough heat or drought stress that stomatal control would have decreased transpiration. Accordingly, maximum assimilation (<em>A_max</em>), stomatal conductance (<em>g_s</em>), and transpiration (<em>E</em>) at the leaf level did not change at the four sites during heatwave and drought periods. However, <em>g_s</em> was substantially reduced during the drought period at the Park site. VPD explained 55&ndash;69 % variations in the daily mean <em>J_s</em> during heatwave and drought periods at all sites except at the Forest site where the saturation of <em>J_s</em> at high VPD was evident due to low soil water availability. The heat and drought conditions were untypically harsh for the region but not excessive enough to restrict stomatal control and the increased transpiration indicating that ecosystem services such as cooling was not at risk.