Assessment of Physiological Stress and Bark Beetle-Induced Mortality in Fir Trees, Zao Mountains, Japan

Abstract. In the Northern Hemisphere, bark beetles are responsible for high tree mortality rates in forest ecosystems. In recent decades, forest pest outbreaks have increased in frequency and scale related to climate change. Although, many studies have focused on the effect of pest outbreak on forests, there are few studies focusing on the early physiological stress on trees preceding the infestations. In the treeline of Zao Mountains in northeastern Japan, a double pest infestation of totrix moth (Epitonia piceae) and bark beetles (Polygraphus Proximus) that caused severe tree mortality in a natural fir forest (Abies mariesii), which is the first reported case worldwide of treeline retreat caused by bark beetle infestation. In order to understand forest dynamics prior to the outbreak, tree rings samples were collected from 20 trees in the treeline (dead trees) and 40 trees from living trees (healthy and damaged) at lower altitudes. In these samples a dendrochronological and carbon stable isotope analysis (Δ¹³C) was performed. Results indicate a declining trend in tree-ring indices (TRI) for dead trees, while living trees showed a strong an annual fluctuation, but did not show any declining trend. Healthy and damaged trees maintained relatively stable Δ¹³C values (14.9 ‰–18.5 ‰), reflecting stable physiological activity even in the partially defoliated damaged trees. During the years the infestation lasted, there was no response from tree rings Δ¹³C (16.2 ‰ and 16.4 ‰) to its surrounding environment in trees prior to their death. The decreasing trend of Δ¹³C values in tree rings prior to pest infestation in dead trees indicate a continuous decline in tree physiological activity caused by a tendency to close the stomata due to environmental stress. In Zao Mountains evidence shows that extreme events in winter lead to severe physical damage in trees, including fallen trees, caused by a combination of heavy snow, strong winds and recently observed high snow density. We speculate that this event gradually weakened trees in the treeline. Another factor that is probably related to this trend is the earlier snowmelt observed in the last two decades, which leads to decreases in soil moisture during spring, when precipitation is the lowest. These findings suggest that Δ¹³C values in tree rings can serve as early warning indicators of stress preceding severe natural disturbance and can contribute to scientifical based informing forest management strategies.