Same damage, different story: Vertical decoupling of herbivore abundance and beech herbivory across forest layers - Data and Code

Invertebrate herbivory is a key ecological process shaping ecosystem multifunctionality, especially in forest habitats. The vertical stratification of forests is characterized by unique structural niches, microclimatic conditions, the availability of resources for organisms, and biotic assemblages. Nevertheless, a comprehensive understanding of the processes driving plant-herbivore interactions across forests’ complex three-dimensional systems remains poorly understood. We selected forests stands along physiological and structural axes and used high-resolution LiDAR data on stand structure, together with microclimate and biodiversity data, to investigate patterns of invertebrate herbivory on European beech (Fagus sylvatica) leaves and herbivore abundance across different spatial scales, in the canopy and understorey of a beech-dominated forest. Herbivore abundance in the canopy was 67% higher than in the understorey, yet relative herbivory did not differ between layers. This decoupling between herbivore abundance and herbivory suggests a resource dilution effect: greater foliage availability in canopies leads to herbivore aggregation but proportionally dilutes relative leaf damage. We found layer-specific drivers, with herbivore abundance responding negatively to vertical structure and positively to microclimate in both layers, while woody plant diversity had contrasting effects across layers. Relative herbivory was only influenced positively by woody plant diversity in the understorey. Stand-specific factors explained substantial variance in both metrics, suggesting that important drivers operate simultaneously across stands and vertical layers. This study shows that the relationship between herbivore abundance and herbivory can be decoupled across vertical forest layers through resource dilution, challenging the assumption that more herbivores result in more per-leaf damage. The effects of woody plant diversity and structure depend on whether a stratum functions as a resource-rich or resource-limited environment for beech leaf herbivores. These findings highlight that understanding the mechanisms of resource availability as drivers of herbivory in forests requires vertically resolved, multi-scale approaches.