Canopy spectral variability is a weak proxy for plant alpha diversity

Canopy spectral variability is widely used as a proxy for plant alpha diversity, yet this relationship often varies among plant communities and spatial scales. This instability may partly arise because canopy spectral variability responds not only to species richness and evenness but also to community composition, including differences in species identity and dominance structure. We examined the influence of community composition on canopy spectral variability using two herbaceous datasets: airborne hyperspectral imagery from 706 National Ecological Observatory Network (NEON) grassland plots across the United States and UAV multispectral imagery from 330 quadrats in the Northeast China marshes (NECM). For each dataset, we quantified canopy spectral variability using the coefficient of variation of reflectance (Refl CV ), and calculated alpha diversity indices and Bray–Curtis‐based community‐composition gradients. We then assessed how canopy spectral variability varied along community‐composition gradients, compared the variance contributions of community‐composition gradients and alpha diversity, and evaluated how relationships between Refl CV and alpha diversity changed after accounting for community composition. Canopy spectral variability changed significantly along community‐composition gradients, and community composition accounted for more variance in Refl CV than alpha diversity in both datasets. Accounting for community composition also altered the estimated relationships between Refl CV and alpha diversity, but in contrasting directions: these relationships generally weakened in NECM but strengthened in NEON. These contrasting responses indicate that community composition can either enhance or obscure apparent associations between canopy spectral variability and alpha diversity, depending on the underlying compositional context. Synthesis . Canopy spectral variability is a context‐dependent and incomplete proxy for plant alpha diversity in herbaceous communities. Explicit consideration of community composition is therefore important when canopy spectral variability is used to infer plant alpha diversity across plant communities.