Demographic rate directional change and its determinants reveal how plant species win in a patchy landscape with frequent droughts

Relative growth rate (RGR) has been a core demographic performance trait in community ecology because its species-specific divergent responses to light often determine community structure and dynamics. Nevertheless, how divergent responses of plant species in RGR to water stress govern community assembly, alongside the mechanistic basis of inter- and intra-specific variations in functional traits, remain elusive. We propose a theoretical framework for describing how the directional change of RGR across dry and wet patches drives community assembly in a landscape with frequent droughts. The framework was applied to a subtropical understory shrub community where rock fragment content, an important proxy for water stress, shows strong spatial variability. Our empirical evidence demonstrated that RGR directional change (RGR_{dir change}) was more robust than RGR in wet patches, RGR in dry patches and the mean RGR in predicting species' dominance hierarchy. Inter-specific variation in functional traits contributed equally to inter-specific variation in mediating shifts in species' RGR_{dir change}. Species with increasing RGR as substrate water availability decreased were dominant (with higher relative abundance and importance). Dominant species generally had acquisitive roots and hydraulically safe stems. In response to decreasing substrate water availability, dominant species further increased their leaf drought tolerance and optimized root specific length, which not only contributed to their faster radial stem growth but also likely contributed to their release from light competition. Our results highlight the roles of RGR directional change across contrasting water stress situations in driving community assembly in patchy landscapes with frequent droughts and may improve our understanding of how global climate change-related droughts shape plant community assembly.