352
differences in the amount of energy absorbed and the anatomical and morphological
differences that enhance scattering of light at different wavelengths allow us to differentiate related taxa. While this chapter is focused on the leaf scale, complications
arise at larger scales; even when just scaling to the canopy, it can be difficult to
detect leaf optical properties due to the presence of additional materials adding to
the measured spectrum (e.g., live and dead leaves, flowers, fruit, bark, and understory, both vegetation and soil). However, in some cases, if the absorptivity of the
material is weak, the absorption at the canopy [pixel] scale can be enhanced. An
example of this is observed for foliar water content when measured on a single leaf
or a spectrum from multiple leaves (e.g., Roberts et al. 2004; Kokaly et al. 2009).
Our ability to measure leaf optical properties from airborne and satellite sensors
varies with spatial scales, as discussed in Chap. 16.
14.3 Leaf Reflectance Patterns
Reflectance is the fraction of light reflected from the leaf surface (R s ). It is composed of two parts: specular reflectance, which reflects directly off the surface in the
forward direction, and diffuse reflectance, which scatters light in all directions from
the surface. Light can be specularly scattered at some wavelengths and diffusely
scattered at others, depending on the scale of the roughness of the surface. Specular
reflectance happens when light intersects a surface that is smooth, i.e., one with
particles much smaller than the wavelengths contacting it. If the surface is rough,
that is, composed of particles about the size of the wavelengths of light or larger, it
will scatter light diffusely. Specular reflection is a leaf property that is determined
by the structure and chemical composition of the cuticle; thus, differences among
species are potentially related to biodiversity questions.
The fraction of light that is reflected from the interior of the leaf, R i , is the diffuse
or multiply scattered component. Some fraction of the beam of light that enters the
leaf’s interior will be absorbed, some transmitted through the leaf, and some will be
scattered back out of the upper surface of the leaf. Only the fraction of the incident
light that is reflected from the interior of the leaf carries information about the biochemical and structural properties of the leaf.
Reflectance patterns in the visible spectrum are primarily due to photosynthetic
pigments that absorb about 90% or more of the incoming light (Gates et al. 1965).
Water is the second strongest absorbing molecule in leaves; it absorbs strongly in
the SWIR region, with several smaller vibrational overtone absorptions in the NIR
(Carter 1991). Because there are no strongly absorbing molecules in the NIR, plants
reflect or transmit all but about 10% of the incoming radiation in this region
(Jacquemoud and Ustin 2008).
While the spectral shapes of leaves are generally consistent across all green
plants, they differ between species and plant functional types. Evergreen leaves usually have thicker cell walls and smaller cells and are more compact than deciduous
leaves. Consequently, their spectral signatures generally have lower reflectance in
S. L. Ustin and S. Jacquemoud
differences in the amount of energy absorbed and the anatomical and morphological
differences that enhance scattering of light at different wavelengths allow us to differentiate related taxa. While this chapter is focused on the leaf scale, complications
arise at larger scales; even when just scaling to the canopy, it can be difficult to
detect leaf optical properties due to the presence of additional materials adding to
the measured spectrum (e.g., live and dead leaves, flowers, fruit, bark, and understory, both vegetation and soil). However, in some cases, if the absorptivity of the
material is weak, the absorption at the canopy [pixel] scale can be enhanced. An
example of this is observed for foliar water content when measured on a single leaf
or a spectrum from multiple leaves (e.g., Roberts et al. 2004; Kokaly et al. 2009).
Our ability to measure leaf optical properties from airborne and satellite sensors
varies with spatial scales, as discussed in Chap. 16.
14.3 Leaf Reflectance Patterns
Reflectance is the fraction of light reflected from the leaf surface (R s ). It is composed of two parts: specular reflectance, which reflects directly off the surface in the
forward direction, and diffuse reflectance, which scatters light in all directions from
the surface. Light can be specularly scattered at some wavelengths and diffusely
scattered at others, depending on the scale of the roughness of the surface. Specular
reflectance happens when light intersects a surface that is smooth, i.e., one with
particles much smaller than the wavelengths contacting it. If the surface is rough,
that is, composed of particles about the size of the wavelengths of light or larger, it
will scatter light diffusely. Specular reflection is a leaf property that is determined
by the structure and chemical composition of the cuticle; thus, differences among
species are potentially related to biodiversity questions.
The fraction of light that is reflected from the interior of the leaf, R i , is the diffuse
or multiply scattered component. Some fraction of the beam of light that enters the
leaf’s interior will be absorbed, some transmitted through the leaf, and some will be
scattered back out of the upper surface of the leaf. Only the fraction of the incident
light that is reflected from the interior of the leaf carries information about the biochemical and structural properties of the leaf.
Reflectance patterns in the visible spectrum are primarily due to photosynthetic
pigments that absorb about 90% or more of the incoming light (Gates et al. 1965).
Water is the second strongest absorbing molecule in leaves; it absorbs strongly in
the SWIR region, with several smaller vibrational overtone absorptions in the NIR
(Carter 1991). Because there are no strongly absorbing molecules in the NIR, plants
reflect or transmit all but about 10% of the incoming radiation in this region
(Jacquemoud and Ustin 2008).
While the spectral shapes of leaves are generally consistent across all green
plants, they differ between species and plant functional types. Evergreen leaves usually have thicker cell walls and smaller cells and are more compact than deciduous
leaves. Consequently, their spectral signatures generally have lower reflectance in
S. L. Ustin and S. Jacquemoud