The Hidden Strength of Urban Trees: Growth Patterns, Drought Resilience and Climate Change Adaptation

Urban trees provide positive ecosystem services (ESS), including mitigating urban heat islands. However, climate change and the increasing frequency of heatwaves and droughts in Central Europe have intensified stress on urban trees, threatening their vitality. While European-scale research has extensively focused on common urban tree species, the drought resilience of less common species remains underexplored. This study evaluates the growth rate and drought resilience of rare urban tree species to assess their suitability for future dry climates. It investigates the growth performance and drought resilience of 11 urban tree species, comprising five rare species (Corylus colurna, Gleditsia triacanthos, Populus nigra 'italica', Styphnolobium japonicum, Sorbus intermedia) and six common species (Acer platanoides, Fagus sylvatica, Platanus acerifolia, Quercus robur, Robinia pseudoacacia, Tilia cordata), to evaluate their adaptability to increasingly arid future urban climates. A total of 450 trees were sampled and cored across two German cities with contrasting climates: Munich (wet) and Würzburg (dry). The initial results of the rare tree species showed lower tree vitality for all studied species in Würzburg compared to Munich. Dendrochronological analyses revealed that P. nigra 'italica' exhibited the highest annual growth, while S. intermedia had the lowest. Linear mixed models demonstrated significant growth declines across all species after 2000 compared to earlier decades. Superposed Epoch Analysis (SEA) indicated growth reductions during Germany’s major drought years (1976, 2003, 2015), particularly for C. colurna, P. nigra 'italica', S. japonicum, and G. triacanthos. Notably, S. intermedia displayed a delayed growth response, suggesting a potential legacy effect.

These findings highlight the importance of further research on less commonly planted tree species to deepen our understanding of their long-term resilience and support the design and planning of cities better adapted to climate change.