Does leaf shedding protect stems from cavitation during seasonal droughts? A test of the hydraulic fuse hypothesis

During droughts, leaves are predicted to act as 'hydraulic fuses' by shedding when plants reach critically low water potential (Ψ_plant ), thereby slowing water loss, stabilizing Ψ_plant and protecting against cavitation-induced loss of stem hydraulic conductivity (K_s ). We tested these predictions among trees in seasonally dry tropical forests, where leaf shedding is common, yet variable, among species. We tracked leaf phenology, Ψ_plant and K_s in saplings of six tree species distributed across two forests. Species differed in their timing and extent of leaf shedding, yet converged in shedding leaves as they approached the Ψ_plant value associated with a 50% loss of K_s and at which their model-estimated maximum sustainable transpiration rate approached zero. However, after shedding all leaves, the Ψ_plant value of one species, Genipa americana, continued to decline, indicating that water loss continued after leaf shedding. K_s was highly variable among saplings within species and seasons, suggesting a minimal influence of seasonal drought on K_s . Hydraulic limits appear to drive diverse patterns of leaf shedding among tropical trees, supporting the hydraulic fuse hypothesis. However, leaf shedding is not universally effective at stabilizing Ψ_plant , suggesting that the main function of drought deciduousness may vary among species.