Abstract

SummaryDuring winter, timberline trees are exposed to drought and frost, factors known to induce embolism. Studies indicated that conifers cope with winter embolism by xylem refilling. We analysed the loss of hydraulic conductivity (LC) in Picea abies branch xylem over 10 years, and correlated winter embolism to climate parameters. LC was investigated by direct X‐ray micro‐computer tomography (micro‐CT) observations and potential cavitation fatigue by Cavitron measurements. Trees showed up to 100% winter embolism, whereby LC was highest, when climate variables indicated frost drought and likely freeze–thaw stress further increased LC. During summer, LC never exceeded 16%, due to hydraulic recovery. Micro‐CT revealed homogenous embolism during winter and that recovery was based on xylem refilling. Summer samples exhibited lower LC in outermost compared to older tree rings, although no cavitation fatigue was detected. Long‐term data and micro‐CT observations demonstrate that timberline trees can survive annual cycles of pronounced winter‐embolism followed by xylem refilling. Only a small portion of the xylem conductivity cannot be restored during the first year, while remaining conduits are refilled without fatigue during consecutive years. We identify important research topics to better understand the complex induction and repair of embolism at the timberline and its relevance to general plant hydraulics.

Highlights

  • Xylem embolism is a major risk for plants as it impairs the water supply of distal tissues (Tyree & Zimmermann, 2002; McDowell et al, 2008)

  • The long-term data of mean and maximum loss of hydraulic conductivity (LC) collected between winter 2003/2004 and summer 2013 at the alpine timberline (Fig. 1) demonstrate that winter LC occurs frequently and can be dramatically high, as illustrated by Norway spruce that suffers from high embolism during most winters

  • Intra- and inter-rings water status analyses show that refilling, besides embolism avoidance, has to be taken into account as a key strategy of timberline trees to overcome hydraulic limitations caused by the combination of frost drought and freeze–thaw stress during winter

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Summary

Introduction

Xylem embolism is a major risk for plants as it impairs the water supply of distal tissues (Tyree & Zimmermann, 2002; McDowell et al, 2008). Trees suffer from frost drought, because the ice in soil, roots and frozen trunk parts block any uptake of soil water for months, while cuticular transpiration causes relevant water losses (Mayr et al, 2003b; Duursma et al, 2019). This is especially true in evergreen conifers as needles can reach substantially higher temperatures than the air during sunny winter days (Mayr et al, 2006b). Experimental as well as field data indicated the combination of frost drought and freeze–thaw stress to be responsible for pronounced winter embolism in conifers at higher elevation (Sparks & Black, 2000; Mayr et al, 2003b, 2006a, 2014)

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