353
the NIR and SWIR (Fig. 14.1). Phenological changes over the growing season are
expressed in leaf reflectance by declining pigment and water contents and increases
in the mass of secondary cell wall components as the growing season nears its end.
Similar patterns are also observed when comparing leaves from mesic to arid
habitats.
In late spring, when the leaves shown in Fig. 14.1 were measured, phenological
differences are generally minimized because leaves are near their growth peak. The
leaves in Fig. 14.1 show they have high water content as the water absorption features in the NIR, around 970 nm and 1240 nm, are relatively deep for non-succulent
leaves. Water causes absorption at all wavelengths longer than 1400 nm, decreasing
reflectance across the spectrum. The small absorption feature observed near
1800 nm is from cellulose and other related structural C (C) compounds. In addition
to these few large absorption features, many small absorptions exist, only some of
which are identified with a specific biochemical. The wide variety of secondary
biochemicals that exist in plant leaves and their possible range of concentrations
provides a spectral palette that can be used to identify individual species in optical data.
Clearly, the environmental conditions that a plant is exposed to, including soil
properties, weather, and its phenological age, alter the leaf’s optical properties. The
genetic heritage modulates the types of responses of a species to environmental
conditions. The reflection of light from the leaf and transmission through the leaf
are determined by what wavelengths of light are absorbed by the various biochemical compounds in leaves (chlorophylls, carotenoids, water, cellulose and lignin, proteins, etc.) and the relative strength of the absorptions.
The scattering of light at the leaf surface depends on the structure of the epidermis, the waxes, cutin, and protrusions such as leaf hairs (Ehleringer et al. 1976) and
on the orientation of the leaf to the beam of light (Comstock and Mahall 1985;
James and Bell 2000). The variety of leaf properties are expressions of different
adaptive strategies among species and are related to their functional traits (Serbin
and Townsend, Chap. 3). For example, differences in epidermal structure cause
leaves of one species to have a bluish powder coating, those of a different species to
appear white, and those of another to have a shiny smooth green surface.
Within the leaf, scattering occurs between cells and between organelles within
cells (Vogelmann 1993). Vogelmann et al. (1996a) used fiber optics to study scattering processes within cells and tissues to show how leaf anatomy modifies the internal light environment to optimize photosynthetic performance under different
habitat conditions.
14.4 Leaf Transmittance Patterns
Transmittance is the fraction of light that enters a leaf and is eventually scattered out
the opposite surface. The transmittance spectrum approximates the reflectance
spectrum, but they are not exact copies of each other. Transmittance can be greater
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…
the NIR and SWIR (Fig. 14.1). Phenological changes over the growing season are
expressed in leaf reflectance by declining pigment and water contents and increases
in the mass of secondary cell wall components as the growing season nears its end.
Similar patterns are also observed when comparing leaves from mesic to arid
habitats.
In late spring, when the leaves shown in Fig. 14.1 were measured, phenological
differences are generally minimized because leaves are near their growth peak. The
leaves in Fig. 14.1 show they have high water content as the water absorption features in the NIR, around 970 nm and 1240 nm, are relatively deep for non-succulent
leaves. Water causes absorption at all wavelengths longer than 1400 nm, decreasing
reflectance across the spectrum. The small absorption feature observed near
1800 nm is from cellulose and other related structural C (C) compounds. In addition
to these few large absorption features, many small absorptions exist, only some of
which are identified with a specific biochemical. The wide variety of secondary
biochemicals that exist in plant leaves and their possible range of concentrations
provides a spectral palette that can be used to identify individual species in optical data.
Clearly, the environmental conditions that a plant is exposed to, including soil
properties, weather, and its phenological age, alter the leaf’s optical properties. The
genetic heritage modulates the types of responses of a species to environmental
conditions. The reflection of light from the leaf and transmission through the leaf
are determined by what wavelengths of light are absorbed by the various biochemical compounds in leaves (chlorophylls, carotenoids, water, cellulose and lignin, proteins, etc.) and the relative strength of the absorptions.
The scattering of light at the leaf surface depends on the structure of the epidermis, the waxes, cutin, and protrusions such as leaf hairs (Ehleringer et al. 1976) and
on the orientation of the leaf to the beam of light (Comstock and Mahall 1985;
James and Bell 2000). The variety of leaf properties are expressions of different
adaptive strategies among species and are related to their functional traits (Serbin
and Townsend, Chap. 3). For example, differences in epidermal structure cause
leaves of one species to have a bluish powder coating, those of a different species to
appear white, and those of another to have a shiny smooth green surface.
Within the leaf, scattering occurs between cells and between organelles within
cells (Vogelmann 1993). Vogelmann et al. (1996a) used fiber optics to study scattering processes within cells and tissues to show how leaf anatomy modifies the internal light environment to optimize photosynthetic performance under different
habitat conditions.
14.4 Leaf Transmittance Patterns
Transmittance is the fraction of light that enters a leaf and is eventually scattered out
the opposite surface. The transmittance spectrum approximates the reflectance
spectrum, but they are not exact copies of each other. Transmittance can be greater
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…