Disentangling the effect of drought on stand mortality and productivity in northern temperate and boreal forests.
Abstract
It has recently been reported that changing precipitation patterns increase tree mortality and reduce biomass accumulation in northern temperate and boreal forests. Functional diversity can mitigate the impacts of climate extremes in different types of ecosystems, but few studies have focused on tree functional diversity in forest ecosystems. It is critical to identify functional traits that could help anticipate the impact of drought on tree mortality, and how these affect above-ground tree biomass productivity at the ecosystem level. We tested how three functional traits (ratio of dry leaf mass per unit area [LMA], xylem pressure at which 50% of stem xylem conductivity is lost through cavitation [ψ50] and leaf area to sapwood ratio) influence severe drought impacts on stand mortality and productivity. We used structural equation modelling to compare the effect of a latent variable composed of these three traits between plots that had suffered drought and plots that had not, on the mortality and productivity of northern temperate and boreal forests of Québec, Canada. A latent variable composed of LMA and ψ50 significantly explained drought-induced tree mortality, but did not explain stand productivity response to severe drought. Therefore, even if these traits relate to the ability of species to survive drought (drought tolerance) at the tree level, they did not affect the maintenance of plant productivity during drought events (drought resistance) at the stand scale. We hypothesize that the effect of tree mortality on productivity was likely compensated by the formation of canopy openings that stimulate the growth of surviving trees. Synthesis and applications. Our results show that mitigation of water loss and xylem resistance to cavitation contribute similarly to severe drought tolerance in trees within northern temperate and boreal forests. Therefore, including drought-resistant trees in fine-scale mixtures might mitigate the impacts of drought on forest productivity due to the survivors' response to released resources.