Impact of Drought and Legacy Effects on Tree Crown Biodiversity: Elucidating the Interplay Among Phytochemistry, the Leaf Microbiome and Insect Herbivores

• Applicants: Matthew Agler, Ph.D.; Dr. Andrea Ghirardo; Professor Dr. Thorsten Grams
• Subject Area: Organismic Interactions, Chemical Ecology and Microbiomes of Plant Systems ; Microbial Ecology and Applied Microbiology ; Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 568946422

Forests are increasingly impacted by climate change. The growing frequency of summer droughts and associated canopy mortality pose a serious threat to forest biodiversity, as tree crowns host species-rich communities essential for ecosystem resilience and function. This challenge is further compounded by the difficulty of fully quantifying forest biodiversity and understanding how extreme droughts influence the links between biodiversity facets and their ecological consequences. Tree canopies harbor high biodiversity due to vertical stratification, which creates diverse microclimatic gradients and habitat structures that support unique species communities. Additionally, within-canopy variation in light and temperature conditions generates a spatial phytochemical mosaic. This distinct phytochemical diversity likely plays a crucial role in shaping higher trophic levels, including leaf-associated bacteria, herbivore communities, and related ecological processes such as feeding behavior and leaf damage patterns. However, despite the ecological importance of tree crown biodiversity, the mechanisms shaping it remain poorly understood. Moreover, knowledge of how drought affects canopy communities is still emerging, limiting our ability to protect forest biodiversity. This project aims to investigate how drought severity and legacy effects influence the interactions among phytochemical, bacterial, and herbivore diversity, as well as herbivore feeding behavior and leaf damage patterns, in Norway spruce and European beech, two dominant tree species in Central Europe. We will combine long-term drought manipulation in the field with controlled laboratory experiments to examine tree-bacteria-insect interactions. Using the established throughfall exclusion experiment in Kranzberg Forest (Germany), we will assess how drought severity and legacy effects alter leaf chemodiversity and, in turn, affect the relationships between leaf bacterial and herbivore diversity as well as herbivory. The project will also determine whether these effects vary across crown strata or influence the entire tree and how they impact generalist versus specialist herbivores. Additionally, we will explore the feeding preferences of sucking and chewing herbivores under drought conditions and identify the key factors driving these preferences. Finally, we aim to provide insights into how drought-induced phytochemical changes influence bacterial recruitment and whether these bacteria, in turn, affect host phytochemical diversity and herbivore preferences. The project will advance our understanding of the factors shaping biodiversity and tree-bacteria-herbivore interactions in two dominant temperate forest species increasingly affected by extreme droughts. Given the ongoing biodiversity loss and rising frequency of drought events, this research will provide an important reference for assessing the consequences of repeated extreme drought summers on forest ecosystems in Europe.

• DFG Programme: Research Grants
• International Connection: Switzerland
• Cooperation Partner: Michael Eisenring, Ph.D.