Massive windfalls boost an ongoing spruce bark beetle outbreak in the Southern Alps

Abstract

European coniferous forests are currently threatened by bark beetles (e.g. Ips typographus) because of an increasing incidence of triggering factors, such as drought and windstorms. Furthermore, such natural disturbances are expected to increase in terms of magnitude and frequency due to climate change, and thus interacting with each other. Here, we present a particular case study in the Southern Italian Alps (Gares, Canale d’Agordo, Belluno), in which wind disturbance interacts with an ongoing outbreak of I. typographus, probably associated with an extended drought in the previous three years. By combining remote sensing and field surveys, we spatially reconstructed the bark beetle attack in the period 2015-2021, which includes the Vaia windstorm in October 2018. Although the windstorm occurred in an expanding phase of the bark beetle outbreak, attacks on standing trees did not occur during the first year after the windstorm but were observed two years later. Our findings suggest that an overlap of a large availability of wind felled trees with an incipient outbreak of I. typographus resulted in an immediate decrease of standing trees mortality in the year following the storm. However, the fallen trees worked as a hidden sink for the beetle population, which in the following years massively attacked the standing trees that survived the storm.