Morphological and heat-tolerance traits are associated with progression and impact of, but not vulnerability to, tree decline

Abstract

Warming and drying climate trends have been linked to tree-dieback phenomena worldwide with broad-reaching impacts on ecosystem services. Studying tree decline is unavoidably a retrospective exercise in which a challenge lies in determining whether trait values contribute to, or are consequences of, decline. Here we used sub-alpine snow gum (Eucalyptus pauciflora ssp. niphophila) to test whether plant traits explain vulnerability of individual trees to decline, assess how progression of dieback symptoms affect traits and physiological tolerance, and ask whether those responses could exacerbate decline. Snow gum woodlands are in widespread decline reflecting the compounding effects of climate warming and drought, and infestation by the wood-boring cerambycid, Phoracantha mastersi. While the impact of drought on tree mortality is well documented, we considered the potential role of heat stress, given exposure of high-elevation forests to increasing temperature. We measured changes in leaf and stem morphology, and stomatal anatomy across orthogonal dieback severity and elevation gradients. Trees showing severe dieback exhibited trait values indicating water stress, while less severely- and un-affected trees did not differ. This suggests observed differences are responses to stress caused by the impacts of wood-borer infestation and provide no evidence of underlying differences in vulnerability. We also modelled the viability of photosynthetic machinery in leaves under current and future climate scenarios; models indicated that leaves on severely-affected trees were likely to accumulate lethal damage to photosystems within a growing season. Even under the current thermal regime, dieback affected trees have lower capacity to tolerate compounded extreme events, contributing to a feedback cycle of decline.