Vein hierarchy mediates the 2D relationship between leaf size and drought tolerance across subtropical forest tree species

Previous studies have observed a 2D relationship (i.e. decoupled correlation) between leaf size (LS) and leaf economics as well as a tight correlation between leaf economics and drought tolerance. However, the underlying mechanism maintaining the relationship between LS and drought tolerance remains largely unknown. Here, we measured LS, 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 LS and drought tolerance were in two independent dimensions (R-2 = 0.00, P > 0.05). Primary and secondary vein traits (i.e. vein diameter and density) explained the variation of LS, with R-2 ranging from 0.37 to 0.70 (all Ps < 0.01), while minor vein traits accounted for the variation of leaf drought tolerance, with R-2 ranging from 0.30 to 0.43 (all Ps < 0.01). Our results provide insight into the 2D relationship between LS and drought tolerance and highlight the importance of vein hierarchy in plant leaf functioning.