Climatic and stand drivers of forest resistance to recent bark beetle disturbance in European coniferous forests

Bark beetle infestation is a major driver of tree mortality that may be critical for forest persistence under climate change and the forecasted increase of extreme heat and drought episodes. Under this context, the environmental position of host tree populations within the species' climatic niche (central vs. marginal populations) is expected to be a determinant in the dynamics of insect-host systems. Here, we analyzed the recent patterns of bark beetle disturbance and forest resistance across European coniferous forests during the 2010-2018 period. We obtained bark beetle attack and tree mortality data from successive continental-scale forest condition surveys on 130 plots including five host trees and five bark beetle species, and characterized the climatic niche of each species. Then, we analyzed the overall forest resistance and species-specific responses, in terms of bark beetle attack and induced tree mortality, in relation to the distance to the niche optimum of both host tree and beetle species, previous drought events, and plot characteristics. Regional patterns of recent disturbance revealed that forests in central, north, and east of Europe could be at risk under the attack of multivoltine bark beetle species. We found that overall forest resistance to beetle attack was determined by several driving factors, which varied among species responses. Particularly, the environmental position of the affected forest within the host and beetle species' climatic niche and plot characteristics mediated the influence of drought on the resistance to beetle attack. In turn, forest resistance to induced tree mortality was determined exclusively by the maximum intensity and duration of drought events. Our findings highlight the importance of disturbance interactions and suggest that the joint influence of drought events and bark beetle disturbance will threaten the persistence of European coniferous forests, even in those tree populations close to their species' climatic optimum.