Vein hierarchy mediates the two-dimensional relationship between leaf size and drought tolerance across subtropical forest tree species.

Previous studies have observed a two-dimensional relationship (i.e., decoupled correlation) between leaf size and leaf economics, as well as a tight correlation between leaf economics and drought tolerance. However, the underlying mechanism maintaining the relationship between leaf size and drought tolerance remains largely unknown. Here, we measured leaf size, water potential at 50% loss of hydraulic conductance, hydraulic safety margin and different orders of vein traits across 28 tree species in a subtropical forest in Southern China. We found that leaf size and drought tolerance were in two independent dimensions (R2 = 0.00, P > 0.05). Primary and secondary vein traits (i.e., vein diameter and density) explained the variation of leaf size with R2 ranging from 0.37 to 0.70 (all P < 0.01), while minor vein traits accounted for the variation of leaf drought tolerance with R2 ranging from 0.30 to 0.43 (all P < 0.01). Our results provide insight into the two-dimensional relationship between leaf size and drought tolerance, and highlights the importance of vein hierarchy in plant leaf functioning.