Prolonged experimental drought reduces plant hydraulic conductance and transpiration and increases mortality in a pinon-juniper woodland

Plant hydraulic conductance (k(s)) is a critical control on whole-plant water use and carbon uptake and, during drought, influences whether plants survive or die. To assess long-term physiological and hydraulic responses of mature trees to water availability, we manipulated ecosystem-scale water availability from 2007 to 2013 in a pinon pine (Pinus edulis) and juniper (Juniperus monosperma) woodland. We examined the relationship between k(s) and subsequent mortality using more than 5 years of physiological observations, and the subsequent impact of reduced hydraulic function and mortality on total woody canopy transpiration (E-C) and conductance (G(C)). For both species, we observed significant reductions in plant transpiration (E) and k(s) under experimentally imposed drought. Conversely, supplemental water additions increased E and k(s) in both species. Interestingly, both species exhibited similar declines in k(s) under the imposed drought conditions, despite their differing stomatal responses and mortality patterns during drought. Reduced whole-plant k(s) also reduced carbon assimilation in both species, as leaf-level stomatal conductance (g(s)) and net photosynthesis (A(n)) declined strongly with decreasing k(s). Finally, we observed that chronically low whole-plant k(s) was associated with greater canopy dieback and mortality for both pinon and juniper and that subsequent reductions in woody canopy biomass due to mortality had a significant impact on both daily and annual canopy E-C and G(C). Our data indicate that significant reductions in k(s) precede drought-related tree mortality events in this system, and the consequence is a significant reduction in canopy gas exchange and carbon fixation. Our results suggest that reductions in productivity and woody plant cover in pinon-juniper woodlands can be expected due to reduced plant hydraulic conductance and increased mortality of both pinon pine and juniper under anticipated future conditions of more frequent and persistent regional drought in the southwestern United States.