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INTRODUCTION
Biodiversity is the “variety of life on Earth”—including both species and functional diversity—and this diversity is declining at an alarming rate (United Nations Convention on Biological Diversity, 2020). Reduced diversity results in ecosystems that are less productive and less resistant to perturbations resulting from changing climate, pests, or disease, which in turn can have significant effects on human health (Millennium Ecosystem Assessment, 2005). Imaging spectroscopy has been used to great effect to investigate plant diversity in terrestrial ecosystems. The spectral variability hypothesis (SVH) states that species richness will be positively related to variation of spectra in remotely sensed imagery (Palmer et al., 2000), and testing found that spectral variation (in terms of mean distance from spectral centroid) is especially well correlated with the number of rare species (Palmer et al., 2002). Studies on airborne and spaceborne spectral imaging have found empirical relationships between spectral diversity and ecosystem-level in situ plant diversity metrics (Draper et al., 2019; Féret & Asner, 2014; Schmidtlein et al., 2007), and many studies have estimated specific plant structural and functional traits from spectra (e.g., Féret & Asner, 2014; Gholizadeh et al., 2020; Leitão et al., 2019; Schneider et al., 2017; Singh et al., 2015; Wang & Gamon, 2019). In moving from the fine (1–10 m) pixel resolution of airborne sensors—such as the NASA Airborne Visible/Infrared Imaging Spectrometer (AVIRIS)—to the coarser resolution of spaceborne sensors (30–100 m)—such as the upcoming NASA mission to map Earth's Surface Biology and Geology (SBG)—analysis must account for the scale transition from individual plant observations to mixtures of multiple species within a single pixel.
Biodiversity can include taxonomic, phylogenetic, functional, and spectral diversity for vegetation (Cavender-Bares et al., 2020). Some studies consider terrestrial vascular plant species quantified in terms of richness and evenness of spectral “species” (alpha diversity), as in Féret and Asner (2014). Other studies have considered structural diversity using canopy height and foliar density variation (e.g., Schneider et al., 2017), or functional diversity using foliar chemical and morphological traits (e.g., Singh et al., 2015). Yet others consider the turnover of composition across space (beta diversity; Draper et al., 2019; Leitão et al., 2019), or regional diversity (gamma diversity, which is the diversity across an entire ecosystem or region). Some definitions of biodiversity are centered around insect...