Impact of freeze-thaw-induced pit aspiration on stem water transport in a subalpine conifer (Abies veitchii)

Abstract

During winter, subalpine conifers experience frequent freeze-thaw cycles in stem xylem, which may cause embolism and pit aspiration due to increased water volume during the sap to ice transition. This study examined the occurrence and ecological impacts of a combination of freeze-thaw-induced pit aspiration and embolism. In subalpine Abies veitchii trees, the fraction of closed pits and embolized tracheids as well as conductivity losses were measured to examine pit aspiration and its effects, triggered by natural and artificial stem freezing. When trees incurred mild drought stress in February and early March, 70% to 80% of stem conductivity was lost. Cryo-scanning electron microscopy indicated <20% embolized tracheids but [~]90% closed pits. Severe drought stress in late March caused 96 {+/-} 1.2% (mean {+/-} SE) loss of stem conductivity, while the fraction of embolized tracheids increased to 64 {+/-} 6.6%, and aspirated pit fraction decreased to 23 {+/-} 5.6%. Experimental freeze-thaw cycles also induced from 7.1 {+/-} 0.89% to 49 {+/-} 10% pit aspiration, and the fraction of closed pits was positively correlated to the percent loss of stem hydraulic conductivity. Results indicated that freezing-induced pit aspiration is an important factor for stem xylem dysfunction under mild drought. Upon severe drought in winter, stem water transport is predominantly inhibited by xylem embolism.