Nutrients and drought alter tri‐trophic interactions in cool‐temperate forests

Abstract

Abstract Herbivorous insects are the main consumers in forest ecosystems, and the complexity of trophic interactions varies with resource availability, heterogeneity among feeding guilds and the presence of natural enemies. In this study, we examined tri‐trophic interactions in Nothofagus pumilio (Fagales: Nothofagaceae) Poepp. & Endl. Krasser forests in southern South America. Specifically, we focused on the effect of nitrogen addition and a drought event on levels of insect herbivory by different feeding guilds and the rate of parasitism on leaf miners throughout the growing season (middle and late). Nitrogen (N) addition increased the foliar nitrogen content but had no effect on total damage by arboreal insects. However, the different feeding guilds responded differently to nitrogen addition, and these responses varied between average or dry year and within the growing season. Particularly, miner activity responded positively to nitrogen addition, increasing their incidence towards the late season of the average year. The positive effect of nitrogen addition on miner activity did not cascade up to the trophic level of parasitoids, which varied with the climatic context and the growing season. These results suggest that tri‐trophic interactions were mainly controlled by the climatic context, since consumers and their natural enemy responses were more dependent on the climatic context and growing seasons than on plant quality. Overall, we highlight the role of fluctuating abiotic resources and the heterogeneity within each trophic level in structuring tri‐trophic interactions in forest ecosystems.