干旱区生态环境知识资源中心

Reflectance and transmission properties of savanna trees were measured using a pole-mounted radiometer for four Sahelian and two Sudanian species in West Africa. The measurements showed that canopy spectral components, that is, shadowed and sunlit tree crown and background, have distinct reflectance characteristics in red and infrared wavebands as modelled by Li and Strahler (1985). Sunlit canopy is the greenest component (greatest infrared to red contrast), and sunlit background (consisting mostly of bare soil), the brightest (greatest summed red and infrared reflectance). Shadowed crown, the darkest component, is greener than shadowed background. The field radiometer measurements were used to calculate the normalized difference vegetation index (NDVI), and the spatial integral of the NDVI over the canopy was related to crown volume yielding per species r2 of 0.75 to 0.81. Measurements of canopy transmission indicated that only 12 to 47 per cent of incoming radiation was absorbed in these wavebands.