VEGETATION CANOPY PAR ABSORPTANCE AND NDVI - AN ASSESSMENT FOR 10 TREE SPECIES WITH THE SAIL MODEL

The relation between the normalized difference vegetation index (NDVI) and the fraction of absorbed photo-synthetically active radiation (f( APAR)) was examined for ten different forest types by using the scatte ring-from-arbitrarily-inclined-leaves (SAIL) radiative transfer model. Leaf reflectance and transmittance, twig reflectance, and background reflectance data were collected as part of field experiments whose sit es contain species whose ranges cover a significant part of western an d northern North America. This provides a sense of variations that occ ur at continental scales. Actual backgrounds of forests include litter and mosses; these materials did not fall along a soil line in red-nea r infrared reflectance space. The simulations indicated that, at low v alues of the leaf area index (LAI), the background reflectance had a s ignificant effect on the canopy reflectance, although little effect on photosynthetically active radiation (PAR) absorption. At higher value s of LAI, leaf optical properties were the factors that dominated cano py reflectance and NDVI. Variations in canopy reflectance due to leaf optical properties were large, but most species had similar reflectanc e patterns. Green leaf optical properties, among the species studied, had little effect on f(APAR). The presence of twigs in the canopy had a noticeable effect on canopy reflectance and absorption of PAR, but t hese effects were secondary to the effects of background and leaf opti cal properties. (C) Elsevier Science Inc.