Neighbourhood and stand structure affect stemflow generation in a heterogeneous deciduous temperate forest

Abstract. Although stemflow oftentimes represents only a small portion of net precipitation in forests, it creates hot spots of water input that can affect subsurface stormflow dynamics. The distribution of stemflow over different trees is assumed to be temporally stable, yet often unknown. Therefore, it is essential to know the systematic factors driving stemflow patterns. Several drivers have been identified in the past, mainly related to tree traits. Less attention has yet been paid to tree neighbourhood interactions impacting stemflow generation and creating stand patches with enhanced or reduced stemflow. We recorded stemflow in 26 precipitation events on 65 trees, growing in 11 subplots (100 m² each), in a temperate mixed beech forest in the Hainich National Park, Germany. We used linear mixed effects models to investigate how traits of individual trees (tree size, tree species, number of neighbouring trees, their basal area, and their relative height) affect stemflow and how stemflow is affected by stand properties (stand, biomass and diversity metrics). As expected, stemflow increased with event and tree size. Stemflow was highly variable at both tree and subplot scale. Especially in large rainfall events (> 10 mm), tree/subplot ranking was almost identical between events, probably due to fully developed flow paths bringing out the full stemflow potential for each tree. Neighbourhood and stand structure were increasingly important with event size (15 % of fixed effects on the tree scale, ca. 65 % on the subplot scale for large events). Subplot scale stemflow was especially enhanced by a higher proportion of woody surface, expressed by a high number of trees, low leaf area and a large maximum tree size. Simpson’s diversity index contributed positively to stemflow yield in large events, probably by allowing more efficient space occupation. Also, our models suggest that neighbourhood impacts individual tree morphology, which may additionally increase stemflow in dense, species diverse neighbourhoods. Unexpectedly, rain shading within the canopy had little impact on stemflow spatial variation. Overall, we find a strong cross-scale temporal stability. Tree size and tree density were the main drivers, independently increasing stemflow, creating forest patches with strongly enhanced or reduced stemflow. Our results show that, besides tree metrics, also forest structure and potentially diversity affect stemflow patterns and associated potentially biogeochemical hotspots.