Scots pine (Pinus sylvestris L.) mortality is explained by the climatic suitability of both host tree and bark beetle populations

Abstract

Higher temperatures and extreme drought events are promoting insect-driven tree mortality. However, there is great uncertainty about the impact of extreme climatic variations on the susceptibility to infestation, as this depends on the suitability of climatic conditions to both host trees and insects. For instance, the consequences of infestation could be more intense in tree populations living closer to the limits of tree species’ climatic tolerance, where resource allocation to defenses could be compromised by other functions essential to tree survival. In this article, we explored 22 Scots pine populations (Pinus sylvestris L.) in the northeast of the Iberian Peninsula that experienced a tree mortality episode involving bark beetle infestation and drought. We hypothesized that the infestation and eventual tree mortality varied according to the climatic suitability of a particular location for the host tree and bark beetles. Climatic suitability values were estimated by using species distribution models (MaxEnt). Then, we developed generalized linear mixed-effects models and partial least squares-generalized linear models to study tree mortality and stand bark beetle infestation (proportion of dead trees presenting signs of a successful attack) in relation to: host and insect suitability, intensity of bark beetle attack, tree characteristics, and stand structure. Intensity of bark beetle attack and tree size were the main factors determining the probability of tree mortality. Smaller trees in particular exhibited a higher probability of dying at lower intensities of beetle attack. At stand-level, bark beetle infestation was negatively associated with tree density and basal area of Scots pine, and positively with tree species richness. Taken together, the response of Scots pine populations in relation to the tree characteristics and stand structure suggested low-density attack by bark beetles (i.e., not in eruptive phase) which affected weakened smaller trees. Remarkably, our results showed that Scots pine populations established in higher climatic suitability areas were vulnerable to infestation, particularly when such locations were also suitable for the bark beetle species. Therefore, previous drought events and differential resource allocation for growth and defenses among central and peripheral tree populations seem to regulate infestation patterns, as mediated by the climatic suitability for both hosts and insects. Our study demonstrates the importance of assessing species’ climatic suitability to better understand and anticipate tree mortality episodes driven by bark beetles, particularly under climate scenarios with increasingly frequent extreme climatic events.