Decline can affect the structure, resources, and microclimates of the forest canopy, and potentially have cascading effects on canopy-dwelling species. Our survey shows that an oak decline can promote saproxylic beetles, especially xylophagous ones, and generalist phyllophagous weevils. However, it negatively affects specialist leaf-eating species and has no effect on seed-eating weevils. Decline in a context of climate change is expected to induce considerable changes in forest structure, potentially affecting habitat opportunities and trophic resources for numerous species. Nonetheless, the consequences of decline in forest biodiversity have rarely been studied. We aimed to characterize the impact of oak decline on different guilds of canopy-dwelling beetles. Beetles were sampled for three consecutive years in oak stands exhibiting different levels of decline. Several guilds were considered: (i) Buprestidae, (ii) other saproxylic beetles split into wood-boring species and non-wood-boring species, (iii) seed-eating weevils, and (iv) specialist and generalist leaf-eating weevils. Overall, decline had positive effects on the abundance and biomass of beetles, though contrasting variations were observed at the species or guild levels. Wood-boring species, especially the main oak-associated buprestids, and other saproxylic species benefitted from decline conditions. However, at odds with the insect performance hypothesis, decline had a positive effect on generalist leaf-eating species, a negative effect on specialist leaf-eating species, and a null effect on seed-eating species. The increase in species richness for saproxylic and leaf-eating beetle communities suggests that decline might promote forest biodiversity. Our results call for further studies to thoroughly assess the functional outcomes of forest decline and to suggest management strategies for conservation biologists.

中文翻译：

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森林衰退对树冠甲虫的营养行会产生不同的影响

衰退会影响森林冠层的结构、资源和小气候，并可能对冠层栖息物种产生连锁效应。我们的调查表明，橡树的衰退会促进腐生甲虫，尤其是食木虫和多面食叶象甲。然而，它会对专门的食叶物种产生负面影响，并且对食种子的象鼻虫没有影响。预计气候变化背景下的衰退将导致森林结构发生相当大的变化，从而可能影响众多物种的栖息地机会和营养资源。尽管如此，很少有人研究森林生物多样性下降的后果。我们旨在表征橡树衰退对不同种类的冠层甲虫的影响。甲虫连续三年在橡木林中取样，表现出不同程度的下降。考虑了几个行会：（i）Buprestidae，（ii）其他腐生甲虫分为蛀木物种和非蛀木物种，（iii）食种子象鼻虫，以及（iv）专家和通才食叶象鼻虫。总体而言，下降对甲虫的丰度和生物量产生了积极影响，尽管在物种或行会水平上观察到了截然不同的变化。蛀木物种，尤其是主要的橡木相关丁香和其他腐木物种受益于衰退条件。然而，与昆虫性能假设不一致，下降对一般食叶物种有积极影响，对专业食叶物种有负面影响，对种子食物种产生无效影响。腐生甲虫和食叶甲虫群落物种丰富度的增加表明，下降可能会促进森林生物多样性。我们的结果需要进一步的研究来彻底评估森林衰退的功能结果，并为保护生物学家提出管理策略。