Tree water-use strategies and growth performance along a hillslope transect in a diverse Central European Forest

Consecutive dry periods (e.g., 2014–2016, 2018–2019, 2022) led to persistent long-term impairments in maintaining tree functions such as growth and canopy structure thereby exacerbating drought stress and mortality in temperate forests. Despite growing attention to compound drought impacts on forest ecosystems, the role of deep-water sources at varying positions on hillslopes remains unclear.

In this study, we investigated how hillslope position influences growth dynamics and water-use strategies of co-occurring tree species in an unmanaged, structurally diverse forest stand in Lower Saxony, Germany. The stand is composed of the broadleaf deciduous tree species Fagus sylvatica (L.), Carpinus betulus (L.), Fraxinus excelsior (L.), and Quercus robur (L.) which differ in their root structure, stomatal regulation and growth strategies. Over the three years (2023-2025) we employed continuous point-dendrometer, sap flow and soil moisture measurements to monitor growth, soil and stand water use and water potential. Further destructive samples for verifying water potential and stable carbon isotopes of phloem sap were measured.

We found that growth patterns were strongly species-specific and closely aligned with contrasting tree water-use strategies. Despite similar climatic conditions in 2023 and 2024, pronounced interannual differences in growth were observed. These differences suggest a delayed recovery from previous long-term drought events (2018-2022), particularly for the shallow-rooted species F. sylvatica, C. betulus and F. excelsior, compared to deep-rooted Q. robur, highlighting long-term effects of compound droughts on productivity. It was notable that the species were able to adapt their strategies according to their position. Additionally, we observed in F. excelsior and Q. robur that high growth rates can be supported by using water storage (e.g., via deep roots and access to deep water sources or via stem water use) or by maintaining high transpiration rates during drought at the risk of cavitation. In conclusion, we postulate that drought mitigation strategies not only depend on species traits, but also on tree positioning and climatic conditions.