PNNL

Abstract

Increasing sea level and storm surges associated with climate change are threatening the survival of coastal forests, yet the mechanism by which seawater exposure causes tree death remains poorly understood. Despite the potentially crucial role of nonstructural carbohydrate (NSC) reserves in tree survival, their dynamics in the process of death under seawater exposure are unknown. Here we monitored progressive tree mortality and associated NSC storage in Sitka-spruce trees dying under ecosystem-scale increase in seawater exposure in western Washington, USA. All trees exposed to seawater experienced declining crown foliage during the sampling period, and individuals with lower percentage of live foliated crown (PLFC) died faster. Tree PLFC was strongly correlated with subsurface salinity and needle ion contents. Total NSC concentrations in trees declined remarkably with crown decline, and reached extremely low levels at death (2.4%, 1.6% in leaves and branches respectively, and 0.4% in stems and roots). Starch in all tissues was almost completely consumed while sugars stayed at a homeostatic level in foliage. The decreasing NSC with closer proximity to death, and near zero starch at death, are evidence that carbon starvation occurred during Sitka-spruce mortality during seawater exposure. Our results highlight the importance of carbon storage as an indicator of tree mortality risks under seawater exposure.