The relationship of forest structural and spectral diversity to biotic resistance to invasion

R. Chelsea Nagy, Jennifer K. Balch, Nayani Ilangakoon, Adam L. Mahood, Joseph M. McGlinchy and Victoria M. Scholl, Earth Lab, University of Colorado Boulder, Boulder, CO, R. Chelsea Nagy, Jennifer K. Balch, Nayani Ilangakoon, Adam L. Mahood, Joseph M. McGlinchy and Victoria M. Scholl, CIRES, University of Colorado Boulder, Boulder, CO, Jennifer K. Balch, Adam L. Mahood and Victoria M. Scholl, Department of Geography, University of Colorado Boulder, Boulder, CO

Background/Question/Methods
Controls on plant species invasion and invasibility are related to regional forces (e.g., climate and climate change) and site level factors (e.g., soil properties, topography, local disturbance, existing native species). The biotic resistance hypothesis (or the diversity-invasibility relationship) suggests that more diverse ecosystems are more resistant to invasion as native species occupy more of the available niche space. This hypothesis has been tested across scales with different metrics of species diversity (e.g., species richness). But less is known about how forest structural and spectral diversity relate to resistance to invasion. Therefore, we asked: do greater forest structural and spectral diversity lead to greater resistance to invasion? We selected 15 forested sites across the U.S. from the National Ecological Observatory Network (NEON). Using NEON point cloud lidar data, we calculated five existing metrics of forest structural diversity for each site: canopy external heterogeneity, canopy internal heterogeneity, mean canopy height, maximum canopy height, and canopy gap fraction. We used NEON hyperspectral data to calculate spectral diversity at each site as the coefficient of variation of reflectance. Lastly, we used NEON plot level plant cover data to determine exotic and native species richness and percent cover.
Results/Conclusions
Linear regression indicated that exotic plant species richness was not significantly related to the structural diversity metrics (canopy external heterogeneity, canopy internal heterogeneity, mean canopy height, maximum canopy height, and canopy gap fraction). Similarly, the mean plot level percent cover of exotic species was not significantly related to the forest structural diversity metrics, except for maximum canopy height (p = 0.07). Exotic species richness and cover were not significantly related to spectral diversity (both p > 0.1). Using multiple regression, no significant relationships were found between exotic species richness or cover and the structural diversity metrics, spectral diversity, or native species richness (all p > 0.1). These preliminary results indicate that unlike species diversity, structural and spectral diversity may not be closely related to biotic resistance to invasion across forest types. However, further analysis is needed to determine the relationships within forest types (e.g., deciduous, evergreen, mixed) and how these relationships change with scale from the plot level to the site level. Additional metrics of forest spectral diversity and canopy chemistry should also be considered.