Making remote sense of biodiversity in herbaceous ecosystems: Biological characteristics moderate the strength of the relationship between taxonomic and spectral diversity

The last decade has brought increased scientific interest in the use of remote sensing to map and monitor diversity patterns. While theory predicts that spectral diversity is an indicator of biodiversity (the spectral variation hypothesis), there is mixed empirical support for this relationship in herbaceous ecosystems, impeding the operational use of remote sensing in monitoring programs. It remains unclear why the strength of the biodiversity-spectral diversity relationships varies so much among herbaceous ecosystems. Scale is one recognized influence on this relationship, but the spatial resolution of spectral campaigns is typically predetermined. Therefore, we investigated three biological characteristics that may also affect the strength of the relationship between taxonomic and spectral diversity: vegetation density, spatial species turnover (beta-diversity) and invasion by non-native species.

For nine herbaceous sites in the National Ecological Observatory Network, we calculated taxonomic diversity from field surveys of 20 m × 20 m plots, and derived spectral diversity for those same plots from airborne hyperspectral imagery with a spatial resolution of 1 m. We found a significant relationship between taxonomic and spectral diversity at some, but not all, sites. Spectral diversity was a better proxy for taxonomic diversity in sites where within-plot spatial species turnover is high and invasion is low. The strength of the taxonomic diversity-spectral diversity relationship was indifferent to variation in vegetation density.

In this study, we demonstrated that, even when the spatial resolution of pixels does not match the spatial scale of plant individuals, certain biological characteristics may enable a positive relationship between taxonomic and spectral diversity. With this, we provide insight into when and why spectral diversity may serve as an indicator of taxonomic diversity in herbaceous ecosystems and be useful for monitoring.