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 winter 2018/19 on all conifer species. 3. Norway spruce (Picea abies) and European larch (Larix decidua) were the most infested species and thus considered main hosts of the bark beetle. For these two species, probability of infestation decreased with increasing tree diversity (although this was only significant for Larix). In contrast, Pinus, which were less infested overall, were more likely to be infested in plots with high tree diversity. Exotic trees tended to be less infested, with clearest support for enemy release found at the level of infestation intensity when considering pure conifer stands. Overall, effects of tree diversity and tree-species origin were not as strong as the effect of position within the experimental site, where higher rates of infestation were observed at the edge than in the centre. 4. Synthesis. Increasing tree diversity may reduce the risk of bark-beetle infestation for genera prone to high infestation rates (Picea and Larix), but risk for less preferred genera (Pinus, and to some extent the exotic tree species) may increase with tree diversity due to spill-over from preferred hosts. In mixed forests, risk of infestation, even by relatively specialized insect pests, may be re-distributed among tree species rather than reduced for all.

1. 近数十年来，欧洲温带森林反复遭受严重干旱侵袭。经干旱削弱的森林通常更易爆发树皮甲虫（bark-beetle）等虫害。树木物种多样性有望提升森林对干旱与虫害的抵抗力，但有关树木多样性可抑制虫害的积极效应的相关证据，在很大程度上仍属间接证据。
2. 本研究依托一项大型幼树多样性试验，探究了树木多样性以及树木物种的生物地理起源对树皮甲虫侵染情况的影响。试验涵盖源自欧洲与北美的6种阔叶树与6种针叶树物种。我们推测，各同属配对组（云杉属、落叶松属与松属）中的外来树种，以及物种丰富度更高、阔叶树占比更高的林分，其树皮甲虫侵染风险会更低。2018年夏季遭遇严重干旱后，针叶树受到了六齿小蠹（*Pityogenes chalcographus*）的侵染。2018/2019年冬季，研究人员对所有针叶树物种的树皮甲虫蛀孔进行了记录。
3. 挪威云杉（*Picea abies*）与欧洲落叶松（*Larix decidua*）是受侵染程度最高的物种，因此被视为该树皮甲虫的主要寄主。针对这两个物种，侵染概率随树木多样性提升而降低（不过该效应仅在落叶松中达到显著水平）。与之相反，整体受侵染程度较低的松属物种，在树木多样性较高的样地中反而更易受到侵染。外来树种的受侵染程度往往更低，当仅考虑纯针叶林林分时，“天敌释放（enemy release）”效应的佐证最为明确。总体而言，树木多样性与树木物种起源的效应，并未如试验样地内位置的效应那般显著：林分边缘的侵染率显著高于林分中心。
4. 综合分析结果显示：对于侵染率较高的类群（云杉属与落叶松属），提升树木多样性可降低其遭受树皮甲虫侵染的风险；但对于受偏好程度较低的类群（松属，以及一定程度上的外来树种），由于侵染会从偏好寄主发生溢出，其受侵染风险可能随树木多样性提升而升高。在混交林中，即便针对相对特化的植食性昆虫，虫害侵染风险也可能在不同树木物种间重新分配，而非全面降低。