Canopy reflectance, photosynthesis, and transpiration. III. A reanalysis using improved leaf models and a new canopy integration scheme

The theoretical analyses of Sellers (1985; 1987), which linked canopy spectral reflectance properties to (unstressed) photosynthetic rates and conductances, are critically reviewed and significant shortcomings are identified. These are addressed in this article principally through the incorporation of a more sophisticated and realistic treatment of leaf physiological processes within a new canopy integration scheme. It is assumed, based on ecophysiological observations and arguments, that leaf physiological properties vary throughout the plant canopy in response to the radiation-weighted time-mean profile of photosynthetically active radiation (PAR). These modifications yield a simpler and more robust theoretical relationship between canopy biophysical rates (photosynthesis, conductance) and spectral vegetation indices (SVI). The results indicate that area-averaged SVI, as obtained from coarse resolution satellite sensors, may give good estimates of the area-integrals of photosynthesis and conductance even for spatially heterogenous (though physiologically uniform) vegetation covers.