Relationship of Water Uptake Depth with Tree Height and Resource-Use Strategies in a Warm-Temperate Secondary Forest in Central Japan

Understanding water uptake depth and its relationship with functional traits offers valuable insights into resource-use partitioning among coexisting tree species as well as forest responses to drought. However, knowledge about water uptake patterns in vertical soil layers, especially among increasingly widespread secondary forest tree species, remains limited. In this study, we investigated interspecific and seasonal variations in water uptake depth among seven coexisting tree species over a 2-year period in a warm-temperate secondary forest in central Japan. We also analyzed the relationships of water uptake depth with tree height and functional traits, including specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen (N) content, and wood density (WD), to discern resource-use and -acquisition strategies. Results revealed that taller trees, especially when soil water is scarce, tend to access deeper soil water sources, indicating that water source partitioning is correlated with tree height. This interspecific and temporal variation in water sources likely stratifies trees to facilitate coexistence within the forest. Water uptake depth was primarily associated with WD and LDMC: trees absorbing more water from shallow soils during dry conditions exhibited lower WD and LDMC, indicating a proactive resource-use strategy. Conversely, SLA and leaf N content were orthogonal to water uptake depth, suggesting that strategies for acquiring belowground and aboveground resources may differ. Considering the alternation of tree species composition during secondary forest succession, our study highlights the importance of further data collection regarding root water uptake depth along successional stages to understand dynamic shifts in water uptake sources.