Effects of forest management on herbivorous insects in temperate Europe

Herbivorous insects play an important role in forest ecosystems, but their response to forest management is not well known. We tested how various aspects of forest management affect the abundance and species richness as well as the community composition of herbivorous insects in temperate European forests, applying a large scale approach that included 126 forests in three regions, two forest strata (understory and canopy), and three herbivorous guilds (xylophages, leaf chewers, plant suckers). Forest management was characterized by a recently developed silvicultural management index, stand developmental stage, and dominating tree species. We sampled more than 44,710 individuals of 698 species of herbivorous Coleoptera and Hemiptera. Overall, forest management caused shifts in herbivore community composition. Responses of abundances and species richness varied between feeding guilds and forest strata. Forest management intensity had opposing effects on herbivores in the two forest strata, with abundances tending to be negatively affected in the canopy layer but positively in the understory. Effects on species richness were weak overall and only significant in plant suckers, with slightly higher species richness at high management intensities. Stand developmental stage of beech forests mainly impacted plant suckers, with abundance increasing with stand age. However, a general positive effect of stand age on forest herbivores can be suggested, as abundances of all feeding guilds responded positively to stand biomass. Oak forests were found to harbor a significantly higher species richness of leaf chewers and the highest abundance of plant suckers compared to all other stands. And while canopies of pine stands exhibited low abundances of leaf chewers, they were found to have positive effects on plant sucker species richness compared to pure beech stands. Our results show that management intensity significantly alters the species composition and abundances of forest herbivores, albeit effects vary depending on forest stratum. This strongly suggests that the forest canopy needs to be considered for a reliable estimation of the consequences of forest management on herbivorous insects. Decisions on tree species composition should be made diligently, as they have significant impact on the abundance and species richness of some forest herbivores and, consequently, also on the ecosystem processes they shape.