Abstract
Increased tree mortality has become a widespread phenomenon and is largely attributed to climate change. Little field research has addressed the complex interactions between trees, herbivores, and their natural enemies as affected by temperature. We recorded the densities of bark insects and their natural enemies emerging from felled trees in Scots pine forests at 17 study sites along 6 elevation gradients encompassing different temperature ranges in 3 regions in Switzerland and Italy. We additionally measured tree resin defense at different elevations. The density of aggressive bark beetles decreased with increasing temperatures while that of non-aggressive species did not respond to temperature. Contrasting patterns were also found for natural enemies, with the densities of most predatory taxa decreasing with increasing temperature whereas densities of parasitoids increased. Consequently, bark beetle mortality by predators decreased and that by parasitoids increased with temperature. Exudation of resin increased with temperature. As the number of resin ducts did not change with temperature, this is assumed a physical effect of reduced viscosity. Despite lower densities of aggressive bark beetles and improved tree resin flow under higher temperatures, the currently experienced drought-induced reduction in tree vigor is likely to increase tree mortality under the ongoing climate warming.
Highlights
In the past few decades, increased mortality of trees has been observed worldwide, becoming manifest on all continents and in various climates [1,2,3]
Bark beetle-mediated tree mortality is the result of complex interactions between trophic levels: (i) number of available trees and their susceptibility, (ii) population size, colonization density, and reproduction of bark beetles, and (iii) the regulation of these pests by natural enemies (e.g., [4,13])
Elevation gradient studies in general showed, that non-bark-beetle predators can exhibit quite heterogeneous distributions [76,77,78]. We suggest this is due to the fact that predators, in contrast to parasitoids, have very diverse lifestyles and can switch between numerous prey species
Summary
In the past few decades, increased mortality of trees has been observed worldwide, becoming manifest on all continents and in various climates [1,2,3]. This is mainly attributed to an increased frequency, duration, and severity of drought and heat stress brought about by climate change. (ii) population size, colonization density, and reproduction of bark beetles, and (iii) the regulation of these pests by natural enemies (e.g., [4,13]) These multifaceted, trophic interactions between host tree, phloem feeders, and their natural enemies are intricate per se and difficult to analyze. Such multitrophic interactions have been identified as important gaps in process-based ecosystem models [6,14,15]
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