Mesoscale assessment of changes in tropical tree species richness across a bioclimatic gradient in Panama using airborne imaging spectroscopy

We used imaging spectroscopy to perform a top-down mesoscale analysis of tropical tree species richness across a bioclimatic gradient in Panama. The expressed precipitation gradient from the wet Caribbean side to the dry Pacific side makes Panama an excellent study area for performing a mesoscale assessment of climate effects on tropical tree species richness. Spatial patterns in local spectral variability (expressed as the coefficient of variation) and spectral similarity (expressed as the spectral similarity index) were used as proxies for species area curves and species distance decay curves. Our analysis revealed significant spectral changes along the precipitation gradient. Highest spectral diversity was observed for moist forest sites while lowest diversity was observed for the driest forest sites. Most of the spectral variation came from changes in the visible (VIS) and shortwave-infrared (SWIR) reflectance. Variation in the VIS was significantly higher for the dry compared to the moist and wet forests, while the opposite was true for the NIR and SWIR reflectance. Our spectral mesoscale analysis extends previous results suggesting that niche differentiation with respect to soil water availability is a direct determinant of both local- and regional-scale distributions of tropical trees. A next step would be to test the accuracy and scalability of our results with lower spatial resolution spectrometer data, simulating the observing conditions that will be achieved with future satellite missions such as the European Union's EnMap and NASA's HyspIRI missions.