Assessing the plasticity of drought tolerance in urban trees growing in different tree pit surface cover conditions in Munich, Germany

With climate change on the rise, increasing the frequency and intensity of drought stress, we aim to define drought tolerance by assessing the plasticity of several physiological parameters in urban trees. Urban trees face unique sets of challenges compared to trees in the forest, leading them to be more exposed to extreme conditions, as they are restricted to tree pits surrounded by impervious structures (e.g., size, depth, morphology, surface cover and connectedness to other trees). Understanding the impacts of tree pit surface cover can help gain better insights for planting a more resilient urban forest.

To advance our understanding of urban European tree species, we investigate the plasticity of the turgor loss point (TLP), xylem potential when 50% conductivity is lost (P50), specific leaf area (SLA) and Huber Values (HV) for four widespread urban tree species, Acer platanoides (L., Sapindaceae), Ginkgo biloba (L.), Platanus x hispanica (Münichh.) and Tilia cordata (Mill., Malvaceae), growing in different tree pit surface cover conditions (e.g., concrete, exposed soil, grass or vegetation) in the city of Munich, Germany. We used different growing periods of the 2025 growing season for 70 individuals; TLP was measured in July and September, while P50, SLA and HV were measured in August.

Our results indicate that the TLP did not change between early and late season sampling for any of the species or tree pit surface cover types. Moreover, TLP varied between species, while tree pit surface cover influenced TLP only within species. P50, however, was strongly related to species identity and was also affected by tree pit surface cover within species. In particular, A.platanoides P50 was less negative in the concrete tree pit surface cover type, while the other tree pit surface covers have more negative P50’s, suggesting that A.platanoides growing in a concrete tree pit surface cover is less drought tolerant, than when A.platanoides is growing in a tree pit with exposed soil, grass or vegetation surface cover.

When assessing drought tolerance of urban trees, the TLP and P50 provides insights on how different tree species respond to different tree pit surface cover growing conditions. To strengthen urban forests’ resilience to drought stress, our work suggests that tree pit surface covers should be taken into consideration when designing urban forests for specific tree species.