Tree diversity reduces risk of bark beetle infestation for preferred conifer species, but increases risk for less preferred hosts

1. In recent decades, European temperate forests have repeatedly suffered from severe droughts. Drought-weakened forests have often become more susceptible to pest outbreaks such as bark-beetle infestations. Tree-species diversity is expected to increase resistance to drought and pests, but evidence for a positive tree diversity effect on insect pest reduction is largely circumstantial.
2. Here we tested the effects of tree diversity and biogeographic origin of tree species on bark-beetle infestation in a large, young tree-diversity experiment, with six broadleaved and six conifer species from Europe and North America. Lower infestation risk was expected for the exotic tree species in each congeneric pair (spruce, larch and pine) and for mixtures with higher species richness and higher broadleaf proportion. Following a severe drought in summer 2018, the conifer trees were attacked by the six-toothed spruce bark beetle (Pityogenes chalcographus). Bark-beetle boreholes were recorded in wi...