Prolonged experimental drought reduces plant hydraulic conductance and transpiration and increases mortality in a piñon–juniper woodland

Robert E Pangle,Patrick J Hudson,Jennifer A Plaut,William T Pockman,Jean‐Marc Limousin,Nathan Gehres,Nate G Mcdowell,Enrico A Yepez,Amanda L Boutz

doi:10.1002/ece3.1422

Abstract

Plant hydraulic conductance (ks) 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 piñon pine (Pinus edulis) and juniper (Juniperus monosperma) woodland. We examined the relationship between ks 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 (EC) and conductance (GC). For both species, we observed significant reductions in plant transpiration (E) and ks under experimentally imposed drought. Conversely, supplemental water additions increased E and ks in both species. Interestingly, both species exhibited similar declines in ks under the imposed drought conditions, despite their differing stomatal responses and mortality patterns during drought. Reduced whole-plant ks also reduced carbon assimilation in both species, as leaf-level stomatal conductance (gs) and net photosynthesis (An) declined strongly with decreasing ks. Finally, we observed that chronically low whole-plant ks was associated with greater canopy dieback and mortality for both piñon and juniper and that subsequent reductions in woody canopy biomass due to mortality had a significant impact on both daily and annual canopy EC and GC. Our data indicate that significant reductions in ks 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 piñon–juniper woodlands can be expected due to reduced plant hydraulic conductance and increased mortality of both piñon pine and juniper under anticipated future conditions of more frequent and persistent regional drought in the southwestern United States.

Highlights

Global analyses suggest an increased frequency of droughtinduced forest and woodland tree mortality (Allen et al 2010; Carnicer et al 2011; Peng et al 2011; Das et al 2013)
We examined the relationship between ks 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 (EC) and conductance (GC)
We observed that chronically low whole-plant ks was associated with greater canopy dieback and mortality for both pin~on and juniper and that subsequent reductions in woody canopy biomass due to mortality had a significant impact on both daily and annual canopy EC and GC

Summary

Introduction

Global analyses suggest an increased frequency of droughtinduced forest and woodland tree mortality (Allen et al 2010; Carnicer et al 2011; Peng et al 2011; Das et al 2013). Xylem Conductance Decline and Tree Mortality tion patterns will impact vegetation has motivated efforts to more fully understand the physiological mechanisms that contribute to and cause tree mortality, and utilizing these crucial insights, refine mortality routines in models such as dynamic global vegetation models (DGVM) to better predict terrestrial vegetation responses to anticipated climate change, both at the local and global scale (Sitch et al 2008; Fisher et al 2010; McDowell et al 2011, 2013; Jiang et al 2013; Xu et al 2013; Zeppel et al 2013). Recent modeling efforts utilizing longterm tree ring records and estimates of future drought severity (Seager et al 2007; Dominguez et al 2010) suggest significant, or even complete, conifer mortality in some parts of the SWUS by 2050 (Jiang et al 2013; Williams et al 2013). Pin~on–juniper is an excellent system to study long-term physiological responses to imposed drought because these two species exhibit differing plant hydraulic response strategies to water stress

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Conclusion