Within-crown leaf variation: a driver of heterogeneous leaf death during drought

Drought often causes patchy leaf mortality within tree canopies, yet the mechanisms driving this heterogeneity remain poorly understood. We quantified variation in key leaf hydraulic and structural traits within the crowns of three mature Eucalyptus viminalis Labill. trees and assessed how this variability influences the water potential at which runaway cavitation occurs, and the predicted timing of leaf desiccation. Substantial variation in traits was observed across hierarchical levels, with the largest proportion occurring amongst leaves within branches. While the water potential at 50% loss of xylem water transport capacity (P50) exhibited within-individual variation, variance partitioning showed that much of its total variability occurred between individuals. In contrast, traits such as leaf hydraulic conductance (Kleaf) and minimum stomatal conductance (gmin) varied more strongly at the leaf and branch levels. The sensitivity analysis indicated that P50 had the strongest influence on runaway cavitation when traits were varied independently. However, when incorporating observed within-crown variation (calculated within individual canopies only), multiple traits contributed to variation in runaway cavitation and predicted desiccation timing. Modelled time to leaf death varied widely within canopies, suggesting a staged mortality process during drought. These findings indicate that even moderate within-crown trait variation can strongly influence canopy responses to water deficit.