Where are urban trees most vulnerable to water stress? A spatial analysis integrating environmental and inventory tree data

Urban trees are key components of urban climate adaptation and resilience, yet they are increasingly exposed to ecosystem vulnerability drivers such as soil compaction, pollution, limited root space, and more frequent droughts induced by climate change. The severe damage to trees in Berlin during the 2018 drought exemplified these pressures, highlighting the urgent need for predictive spatial water stress assessment tools to support urban tree management. In this study, we developed a GIS-based exploratory model to assess the water stress experienced by urban street trees in Berlin, integrating open-source climate and site data with hydro-pedo-transfer-functions. We specifically examined how tree attributes, climate factors, and site conditions interact to drive water stress. To this end, we compared modelled outputs with reported damage data from inventory tree datasets. The results revealed the complex relationships between soil sealing, tree age, species composition, precipitation variability, sky view factor, available soil water, groundwater depth, in determining potential tree water stress vulnerability and overall damage status. Spatial hotspots of elevated water stress risk were predominantly concentrated in areas exhibiting high radiation, extensive soil sealing and limited groundwater accessibility. The model identified potential water-stress-prone areas that partially corresponded with hotspot areas of reported damage data from the tree inventory. Nevertheless, discrepancies between the modelled outputs and reported damage data suggested that the visible damage symptoms and defined tree damage level alone are insufficient proxies for tree water stress. To enhance the exploratory model, additional variables including soil compaction, root space restrictions, and damage history should be incorporated. Overall, the GIS-based modelling approach demonstrates considerable potential as an exploratory spatial screening tool for identifying potential water-stress-prone areas and informing targeted management strategies to enhance urban tree resilience under climate change.