3D canopy structure and biotic resistance to invasion in North American forests

Structurally complex forest canopies with large volume are hypothesized to host greater species diversity by increasing niche dimensionality. Canopy structure may therefore contribute to the biotic resistance of recipient forest systems. Here, we tested whether canopy height and complexity were related to native and exotic plant species richness at continental and local scales. Canopy height and structural complexity were measured from aerial LiDAR data at 19 forested NEON sites across North America. We used canopy height as a proxy for canopy volume. Complexity was measured as horizontal and vertical variation in vegetation height and top of canopy rugosity. Species richness of native and exotic forest communities was calculated from NEON plant survey data at each of the 19 sites. At the continental scale, we found that canopy height and complexity were positively correlated with native, but not exotic species richness. At the local scale, canopy height and structural complexity were more often positively correlated with native species richness and more often negatively correlated with exotic species richness. Ongoing analyses will examine whether these patterns are also sensitive to phylogenetic diversity and growth form of the vegetation community. Canopy complexity and volume are associated with native diversity at both scales. However, our study suggests that canopy volume and complexity are associated with biotic resistance at a local scale only, which is consistent with previous studies showing a cross scale interaction such that biotic resistance occurs at small and not large spatial extents. Future research should focus on the local scale to understand mechanisms underlying the role of canopy structural complexity in biotic resistance to forest invasion.