Remote Sensing of Vegetation

Remote sensing of vegetation is accomplished by using the strong coupling between reflected visible and NIR radiation with the physiological condition of leaves and their density. Photosynthesis generally occurs through CO2 gas exchange between hydrated chloroplast cells that are in direct contact with the intercellular air spaces within the leaves (figure 5.2; Gates et al., 1965).

Chlorophyll is a strong absorber of visible energy, and the interaction of near-infrared wavelengths with vegetation provide important clues as to the structures of plant leaves. A consequence of the internal structure of leaves is that visible and near-infrared solar radiation passing through the leaves are deflected and scattered due to the refractive index, differences between hydrated cells ( n ~ 1.3 Mm) and air spaces (n = 1.0 ^m), and the irregular pattern of cell facets in the mesophyll leaf. These scattering effects increase the effective path length of radiation as it passes through the leaves, resulting in increased absorption of visible light by plant pigments and liquid water. In contrast, there is negligible absorption at NIR wavelengths, which results in an enhanced spectral reflectance response (figure 5.3; Tucker and Sellers, 1986).

In the natural world, plant leaves are organized into canopies of varying structures and leaf orientations. The spectral reflectance and absorptance of the leaf elements are modified by the canopy structure and the amount

Figure 5.2 A scanning electron micrograph of transverse view of a mature broad-bean leaf magnified 420 x. The upper half of the leaf interior is more disorganized, and extensive intercellular air spaces expose the surfaces of mesophyll cells directly to air contact (from Troughton and Donaldson, 1972).

Figure 5.2 A scanning electron micrograph of transverse view of a mature broad-bean leaf magnified 420 x. The upper half of the leaf interior is more disorganized, and extensive intercellular air spaces expose the surfaces of mesophyll cells directly to air contact (from Troughton and Donaldson, 1972).

of green leaf density (figure 5.4). Thus, the determinants of the spectral reflectance of plant canopies include physiological condition of leaves, their structural arrangement and density within the plant canopy, and the underlying substrate, when the plant canopy is open (Tucker, 1980a). Sensitivity of satellite data to these characteristics of leaves becomes the basis for monitoring drought conditions using satellite data.

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