Reflectance Characteristics and Film Image Relations Among Important Plant Species on South Padre Island, Texas

Abstract

Radiometric ground reflectance measurements were made on eight important plant species and two associated soil surface conditions on South Padre Island, Texas on three dates (July, September, and November). Plant species studied included beach croton (Croton punctatus), beach morningglory (Ipomoea imperatii), camphorweed (Heterotheca subaxillaris), partridge pea (Chamaecrista fasciculata), railroad vine (Ipomoea pes-capre), sea purslane (Sesuvium portulacastrum), seacoast bluestem (Schizachyrium scoparium), and seaoats (Uniola paniculata). Reflectance measurements were made at the visible green (0.52-0.60 µm), visible red (0.63-0.69 µm), and near-infrared (0.76-0.90 µm) wavelengths. Reflectance values differed significantly (p = 0.05) among the plant species and soil surfaces at all three wavelengths and on all dates. Differences in reflectance among the plant species were attributed to variable foliage coloration, phenology, and vegetative density. Spectral differences between soil surfaces were attributed to their contrasting surface conditions (wet, dry). Color-infrared (CIR) aerial photographs obtained of study sites on three dates showed that many plant species could be differentiated. More plant species could be distinguished on the September photographs than on the other dates. Spectral measurements of several species were related to their image tonal responses on the CIR aerial photography. However, growth form, geometric shape, and textural image response of species were also important in their discrimination on aerial photos.