Spatio‐temporal variation in deep soil water use patterns of overstorey and understorey layers in subtropical plantations predicts community assembly.

Deep soil water utilization allows plants to cope with drought stress. However, little is known about the roles of the understorey layers in driving spatio‐temporal variations of deep soil water in forests and how the patterns of deep soil water use among life‐forms contribute to community assembly processes.We assessed the spatio‐temporal patterns and determinants of deep water utilization of tree, shrub and herb layers in subtropical coniferous plantations and investigated associations between deep water use parameters and dominance and richness of understorey vegetation.We found that the understorey layer had a higher reliance on deep soil water in the dry season, a larger seasonal plasticity of deep soil water uptake, but lower spatial variability in deep soil water utilization than the tree layer. We showed that greater reliance of the tree layer on deep soil water was associated with decreased shrub layer diversity, whereas greater reliance of the shrub layer on deep water was associated with increased herb layer diversity.Synthesis. Our results highlight the roles of understorey layers in driving the temporal dynamics of deep soil water in forests and improve our understanding of how deep soil water use patterns among life‐forms shape community assembly in forests. [ABSTRACT FROM AUTHOR]