Tuesday, November 6, 2007 - 12:30 PM
173-4
Canopy Reflectance Principles and Their Potential Use in Grassland Management.
Numerous physical and biochemical factors affect the reflectance properties of a crop canopy, even when the crop is grown in monoculture. The spectral reflectance signatures of grassland canopies in the field are far more complex and often very different from that of a single green leaf isolated within a well-illuminated chamber. The objective of this work is to outline the primary factors affecting canopy reflectance and identify additional challenges in grassland systems. On an elementary level, canopy reflectance (R) is a product of the quantity of light that is intercepted by the crop canopy (Qi) and the proportional reflectance of that light by the canopy (ρc). Assuming that changes in Qi can be accounted for by measuring insolation or by providing a light source that maintains a given Qi, the fraction of intercepted light at a given wavelength that will be reflected by a crop canopy is a function of three primary parts: the limiting (i.e., asymptotic maximum/minimum) coefficient of reflection for the canopy (ρc*), the coefficient of reflection by the soil (ρs), the leaf area index (LAI), and a canopy attenuation coefficient (A) (which is analogous to Beer’s extinction coefficient, ε). Therefore, the wavelength-specific fraction of light reflected from a canopy is a function of the LAI and the biochemical and morphological constraints of canopy, mesophyll cell, and cell wall microfibril architecture, but is limited by ρc* and ρs. These biochemical and morphological factors will likely differ between grassland species and management schemes. Thus, use of canopy reflectance in grassland systems is further complicated by the diversity of species present in polycultural grasslands and the frequency and selectivity of defoliation. Despite these difficulties, measurements of large swings in canopy reflectance have been shown to be of significant research value and have the potential to be useful to astute grassland managers in the field.
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