Hydraulic dysfunction due to root-exposure-initiated water stress is responsible for the mortality of Salix gordejevii shrubs on the windward slopes of active sand dunes

Abstract

Sand-fixing shrub Salix gordejevii plays an important role in sand dune stabilization but often shows severe decline and mortality on the windward slopes of active dunes. The objective of this study was to explore the environmental drivers and physiological mechanisms behind its decline and mortality on the windward slopes of active sand dunes. The xylem hydraulics, sap flow, water relations and non-structural carbohydrate reserves were measured for S. gordejevii plants that respectively inhabited interdune, leeward and windward positions of active dunes with contrasting soil water regimes. Root exposure caused by wind erosion, rather than low soil moisture, predisposed windward S. gordejevii plants to stronger water stress. Correspondingly, the windward plants showed reduced transpirational water use and exhibited a suite of acclimative adjustments in leaf traits favoring water conservation. Despite these adjustments, windward S. gordejevii plants still suffered from higher risks of hydraulic failure and branch mortality. No evidence of carbon depletion in windward S. gordejevii plants was found although the soluble sugar to starch ratios changed significantly. Our findings suggest that wind-erosion-induced root exposure triggers water stress and eventually causes the mortality of windward S. gordejevii plants, during which increased risk of hydraulic dysfunction is pronounced.