The future of urban trees – assessing the potential impact of climate change on urban tree populations in two European cities

Abstract: Due to climate change, cities are increasingly facing the situation of being exposed to more frequent and prolonged extreme weather events in the future, such as heat and droughts, which are exacerbated by urban heat island (UHI) effects. The implementation of green infrastructure (GI) in the form of trees is seen as a possible adaptation measure to regulate the microclimate. However, there is an increased risk that both existing and future urban tree populations will suffer significantly from changing climate conditions. The aim of this study was to assess to what extent the current tree population in Berlin and Vienna will be threatened by heat and drought periods in the future and to what extent adaptation measures might become necessary. To this end, future climate-related heat and drought threats were investigated based on two emission scenarios (RCP 4.5, RCP 8.5) and selected climate indicators for both the near future (2021-2050) and the distant future (2071-2100). The heat and drought tolerance of trees was assessed based on a literature review. In addition, the exposure of categorised trees in relation to UHI effects was investigated to identify particularly vulnerable locations and recurring patterns. The risk of extreme events (heat and drought) was found to increase in the future for both climate scenarios studied for both cities, especially in the distant future and at the beginning and end of the growing season. Of the urban tree species studied, the majority were classified as moderately sensitive and moderately tolerant. The distribution of highly sensitive trees within the city showed similar distribution patterns in the two cities with single hotspots in parks but different average UHI intensity exposures. It can be concluded that timely, targeted planning of GI adaptation measures is already essential for both cities to be able to provide adequate GI in the near, but especially in the distant future.