Disentangling controls on drought stress in urban trees in a Central European city

Highly urbanised areas are tough environments for trees, often compared to arid lands. Drought stress detection and prediction in urban trees is of utmost importance for the sustainable management of the urban forest. The high heterogeneity of the urban fabric presents a major challenge for identifying the hierarchy of controls on tree drought stress. In this contribution, we combine field observations of soil moisture and sap flow measurements with mechanistic plant hydraulic modelling to disentangle this hierarchy in the city of Braunschweig, Germany. We use a modified version of the plant hydraulic model SurEau-Ecos for inverse hydraulic trait estimation of different tree species at sites spanning a gradient of surface sealing. The investigated species are cypress oak (Quercus robur 'Fastigiata'), Turkish hazel (Corylus colurna), and littleleaf linden (Tilia cordata). Anthropogenic structures such as drainage pipes and other elements of the urban karst significantly affect both soil and plant water dynamics, driving high intra-species trait variation across sites. This suggests co-adaptation of tree hydraulic traits and micro-environmental conditions at the patch scale. Our results indicate that intra-species hydraulic trait plasticity and soil moisture availability are the main factors controlling drought stress in urban trees.