Bird occurrence and trophic interactions vary across gradients of tree diversity and microclimate in a planted forest

Abstract

Deforestation reshuffles communities across landscapes with myriad consequences for ecosystem function. Following deforestation, rapid exposure to novel microclimates can act as a strong environmental filter, favoring warm-adapted species and decoupling trophic interactions. Forest restoration may partly reverse this process through increased habitat structure, food resources, and buffering of microclimates - each potentially modified by tree diversity. Despite growing evidence that tree diversity and cool microclimates help maintain animal diversity in natural forests, less is known about how these factors shape species assemblages or multitrophic dynamics in restoration areas. Here, using surveys and two field experiments within a long-term tree planting experiment, we assessed the relative effects of tree diversity, forest structure, and associated microclimate on fine-scale space use by birds and their top-down impacts on insects. Surveys showed that fine-scale occurrences of birds increased in cooler plots, which were associated with higher tree diversity and structural complexity. The strength of microclimate effects on bird occurrences was strongest for species that are forest specialists. To assess risk to insect herbivores from avian predation, we used a sentinel prey experiment and found that predation risk increased in warmer plots, counter to our expectations based on bird surveys. Last, we examined top-town effects of bird exclusion on leaf herbivory, finding that skeletonizing patterns of herbivory increased in exclosures and in cooler plots. Taken together, these results suggest that microclimate resulting from variation in forest structure shapes space use of birds at fine scales with complex outcomes for bird-herbivore-tree interactions in planted forests. Active restoration methods that enhance below-canopy cooling may improve biodiversity outcomes and help maintain species interactions that underlie many ecosystem functions.