Spatial-temporal differentiations in water use of coexisting trees from a subtropical evergreen broadleaved forest in Southwest China

Southern China houses the largest subtropical broadleaf forest in the world, with abundant rainfall brought by the Pacific and Indian monsoons. As the intensity and frequency of drought are increasing, understanding the water use patterns of co-occurring trees is a key issue for predicting the response of these subtropical forests. The water uptake depths and water use-related leaf traits of three dominant canopy tree species at different ages (DBH) were investigated in a subtropical forest (2018–2019). We found that the δ2H and δ18O of various water pools, such as fog, soil, stream, lake, and groundwater, plotted almost entirely on the local meteoric water lines (2018: y = 8.2x +11.5; 2019: y = 8.1x +10.4). This result suggested that water bodies within the forest experienced minor evaporation. An interspecific partitioning of water acquisition was detected among tree species of similar ages in 2018 (a regular year), but they tapped soil water from similar depths after experiencing drought event in 2019. Most large (DBH > 50 cm) and middle-sized (15 cm < DBH < 30 cm) trees switched their major water sources from shallow (2018) to deep (2019) soil layers, whereas the seedlings (DBH < 2 cm) relied on shallow soil water during the study periods. The water use patterns of different trees were related to the leaf-level intrinsic water use efficiency (WUEi) and specific leaf area (SLA). The spatial-temporal differentiation of water use in regular years can facilitate plant co-existence among the canopy trees, but the interspecific competition for similar water sources in drought years may increase the risk of tree dieback in the subtropical forests. These fingdings provide a mechanistic explanation for the high diversity of subtropical forests in China from the perspective of differentiation in plant water use.