Spectral characterization of Australian arid zone plants

Abstract

Visible – near-infrared reflectance spectra were measured for a selection of common trees, shrubs, ephemerals, and grasses in the rangelands of the southeastern Australian arid zone. The plants varied in leaf structure, leaf coatings, woodiness, succulence, chlorophyll and other pigment content, and physiological status, and the spectra showed considerable variation within and between species. Factors contributing to their spectral variation, identified by principal components analysis, were albedo over the entire spectral range, followed by contrast between near-infrared and visible reflectance, and blue and red contrast. Visible – near-infrared regions most important in variation within the spectral collection were the red edge, visible green, near infrared beyond 760 nm, blue, and red. Between 1000 and 1800 nm variation could be explained by albedo and the water absorption region near 1420–1450 nm. Functional and taxonomic groups of plants were classified using discriminant analysis of the narrow-band visible – near-infrared spectra. Spectral regions most influential in the discrimination were the chlorophyll absorption near 680 nm, the infrared beyond 720 nm, and the blue–green and other visible wavelengths. Fair discrimination (k = 0.48) was achieved with 70 wavebands spanning the visible – near infrared but was slightly reduced with 16 selected narrow bands (k = 0.44). Better discrimination was achieved with five rather than eight plant groups (k = 0.53). Both principal components analysis and discriminant analysis pointed to broad spectral regions that were important in the arid plant spectral variation, rather than specific narrow bands.