Abstract: Although water use strategies of tree species are critical for maintaining tropical rainforest ecosystem function, the water use pattern of dominant tree species in tropical secondary forests remain poorly understood. In this study, with stable isotopes, we analyzed the plant water use sources and intrinsic water use efficiency (WUE_i) of five coexisting dominant tree species in tropical secondary rainforests on Hainan Island during the wet season. The results showed that the shallow soil (0–40 cm) had higher water content than that in deep soil (40–100 cm), the five dominant tree species in tropical secondary forests mainly utilized shallow soil water, with an absorption contribution of 57% compared to deep soil water (43%). Besides, there was no correlation between soil water contribution rate and soil water content in the shallow soil layer, but a significant positive relationship was observed in the deep soil layer (P < 0.01), indicating that deep soil water content has a driving effect on the deep water contribution rate. Meanwhile, the proportional similarity of water uptake between five species exceeded 0.9, suggesting that the water use source of the coexisting dominant species is highly similar. Moreover, the WUE_i of five dominant tree species differed significantly between species (P < 0.05). Both the plant water use source and WUE_i were affected by plant functional traits. Our study demonstrated that there was no soil water partitioning among five dominant tree species in tropical secondary forests, which may intensify water competition amid projected seasonal drought intensification.

Key words: stable isotopes, water use source, water use efficiency, tropical secondary forests, coexisting tree species