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

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 evaluated the impact of nutrient supplementation and a drought event on insect herbivory in cool-temperate Nothofagus pumilio forests located in southern South America. We examine how these factors affect herbivory by various feeding guilds and the rate of parasitism in leaf miners. Previous research in this ecosystem has demonstrated that habitat type (dry versus wet) and soil nutrient levels play a role in the regulation of insect folivory by insectivorous birds. However, there is limited understanding of the influence of nutrients and drought, as well as their potential interaction with insect folivory and parasitism in controlling leaf damage in temperate Patagonian forests. This focus is relevant when considering that Patagonian forests, along with other forest ecosystems around the world, are being affected by episodes of extreme climatic variability, as well as the advancement of anthropic activities (e.g., livestock) which modify soil nutrient dynamic. Collectively, these factors may modify tri-trophic interactions in forest ecosystems. Specifically, we asked the following questions: (i) what is the effect of nitrogen addition and a drought event on the leaf quality, damaged leaf area, and frequency of damage to N. pumilio saplings caused by different insect guilds? (ii) What is the effect of nitrogen addition and a drought event on parasitism rates of different morphotypes of leaf miners?
Our results showed that nitrogen 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 of the average year. The positive effect of N 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.