Divergence in strategies for coping with winter embolism among co‐occurring temperate tree species: the role of positive xylem pressure, wood type and tree stature

Abstract

Summary In temperate ecosystems, freeze‐thaw events are an important environmental stress that can induce severe xylem embolism (i.e. clogging of conduits by air bubbles) in overwintering organs of trees. However, no comparative studies of different adaptive strategies among sympatric tree species for coping with winter embolism have examined the potential role of the presence or absence of embolism refilling by positive xylem pressure. We evaluated the degree of winter embolism and hydraulic architecture traits in 22 deciduous angiosperm tree species typical of temperate forest sites in NE China. Co‐occurring trees growing in a local botanical garden were used to minimize variation caused by differences in proximal environmental conditions and to ensure that interspecific variation reflected genetic differences between species. Four functional groups with potentially different strategies for coping with winter embolism were compared: positive xylem pressure‐ generating species (PXP) that are all diffuse‐porous, except a semi‐ring‐porous species, large (LDP) and small (SDP) statured diffuse‐porous tree species that are unable to generate positive xylem pressure, and ring‐porous species (RP). The PXP group exhibited nearly full recovery from winter embolism in contrast to the other three groups, which showed persistent and relatively high degrees of hydraulic dysfunction during the subsequent growing season. The absence of a functional trade‐off between hydraulic efficiency and safety against freeze‐thaw‐induced embolism in the PXP group and the presence of a trade‐off in the other three groups, suggests that the ability to generate root or stem pressure for embolism refilling may partially free some temperate tree species from adaptive constraints imposed by winter embolism formation. Efficient winter embolism reversal by positive pressure in PXP species did not distinguish them from their non‐xylem pressure‐generating LDP counterparts in terms of various measures of xylem hydraulic efficiency during the growing season. Divergence in the ability to refill winter embolism through generation of positive xylem pressure implies a series of functional trade‐offs that may partially explain the co‐existence of these two types of temperate tree species. A lay summary is available for this article.