Impact of drought on urban trees – a modelling study

Urban green areas can be used as means for sequestering carbon, but also to manage run-off water and to reduce the urban heat island effect. Urban green spaces are often managed, including irrigation, removal of litter and modification of soil. In addition, urban green areas are often subjected to stress factors such as heat, pollution and drought. Here we study various carbon cycle components of urban trees using the JSBACH ecosystem model, focusing on the impact of drought. The study area is Kumpula, a semi-urban area situated in Helsinki, Southern Finland. In 2020 and 2021, an intensive measurement campaign took place involving a park area with tilia trees (Tilia cordata) and an urban forest dominated by Silver birch (Betula pendula). The observations included soil texture, soil temperature and moisture, and soil respiration. We used remote sensing data (Sentinel-2) for the leaf area index. Photosynthesis was measured using leaf cuvettes, and sap flow was measured for birch and lime trees. In addition, we used net ecosystem exchange from eddy covariance measurements at the Kumpula SMEAR III urban measurement station, operated by the University of Helsinki. The meteorological forcing data, to drive the model, was derived from observations at the Kumpula weather observation station, operated by the Finnish Meteorological Institute, and the SMEAR III station.