355
(2001) evaluated leaf ontogeny and found that spring leaf expansion was usually
correlated with high anthocyanin pigments in the mesophyll tissue. Along with phenolic compounds, anthocyanins provide protection of the pigment molecules during
senescence (Matile 2000) and defensive functions against herbivory (Hamilton and
Brown 2001) and during leaf senescence. Some molecules absorb in the UV and
blue wavelengths, ranging from the simplest phenol to complex polyphenols like
tannic acid. Kokaly and Skidmore (2015) recently demonstrated detection of a phenolic absorption in plants at 1660 nm. Phenolic compounds often provide regulatory
and defensive functions. Because of the great diversity of non-photosynthetic pigments and photosynthetic accessory pigments (carotenoids), pigments provide a
basis for discriminating among plant species in optical data. Suites of pigments
often occur in specific families or clades in agreement with molecular phylogeny
(Lee and Collins 2001).
In the NIR, there are no strongly absorbing compounds, so a high proportion of
light is reflected or transmitted. Light is often scattered multiple times, increasing
the probability of absorption before being reflected or transmitted out of the leaf.
For example, Wooley (1971) reported 4% NIR absorptance in soybean (Glycine
max), and Everitt et al. (1985) found only 5% NIR absorptance in buffalo grass
(Bouteloua dactyloides) leaves. Scattering within the leaf is related to the internal
cellular structure, especially at cell membrane and air interfaces, where light can be
reflected and refracted. Allen et al. (1970) showed the volume of intercellular air
spaces was highly correlated with NIR reflectance. Multiple scattering of photons
causes the NIR reflectance to be much higher than reflectance of visible or SWIR
wavelengths, where absorptions by pigments and water result in single scattering
processes (light is absorbed or scattered on its first interaction).
Secondary water absorption features in leaves are found around 980 nm and
1240 nm (Carter 1991). The diversity of plant adaptations to different water regimes
results in a wide range of leaf water contents among species. The percent water
content is generally positively correlated with increasing leaf thickness, but the
opposite may occur. Thus, a sclerophyllous leaf species like Adenostoma fasciculatum (chamise) can have higher leaf mass area (LMA = 1 divided by specific leaf
area) and low water content, but a succulent species may have high water content
and a high leaf mass per area (Ackerly et al. 2002; Vendramaini et al. 2002).
The SWIR part of the leaf spectrum is dominated by water absorption when the
leaves are living and by leaf chemical constituents when dry. Many plant compounds have absorptions in the SWIR, including cellulose, lignin, nitrogen (N),
sugars, starch, and waxes. Interpretation of these absorption features in dry leaves is
complicated because many molecules have absorptions at overlapping wavelengths
and we lack the specific absorption coefficients to identify them in the data. The cell
wall C compounds comprise the largest fraction of the dry biomass of a leaf, and the
absorption feature around 1750 nm is generally attributed to these materials (Kokaly
et al. 2009). Kokaly (2001) also identified two absorptions at 2054 nm and 2172 nm
that cause broadening of the 2100 nm absorption feature due to N compounds. The
complexity of relationships among species adaptations for water, structural carbohydrates, and nutrients provides a strong basis for detecting species diversity in RS
imagery, at least in local to regional studies (Asner and Martin 2016).
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…
(2001) evaluated leaf ontogeny and found that spring leaf expansion was usually
correlated with high anthocyanin pigments in the mesophyll tissue. Along with phenolic compounds, anthocyanins provide protection of the pigment molecules during
senescence (Matile 2000) and defensive functions against herbivory (Hamilton and
Brown 2001) and during leaf senescence. Some molecules absorb in the UV and
blue wavelengths, ranging from the simplest phenol to complex polyphenols like
tannic acid. Kokaly and Skidmore (2015) recently demonstrated detection of a phenolic absorption in plants at 1660 nm. Phenolic compounds often provide regulatory
and defensive functions. Because of the great diversity of non-photosynthetic pigments and photosynthetic accessory pigments (carotenoids), pigments provide a
basis for discriminating among plant species in optical data. Suites of pigments
often occur in specific families or clades in agreement with molecular phylogeny
(Lee and Collins 2001).
In the NIR, there are no strongly absorbing compounds, so a high proportion of
light is reflected or transmitted. Light is often scattered multiple times, increasing
the probability of absorption before being reflected or transmitted out of the leaf.
For example, Wooley (1971) reported 4% NIR absorptance in soybean (Glycine
max), and Everitt et al. (1985) found only 5% NIR absorptance in buffalo grass
(Bouteloua dactyloides) leaves. Scattering within the leaf is related to the internal
cellular structure, especially at cell membrane and air interfaces, where light can be
reflected and refracted. Allen et al. (1970) showed the volume of intercellular air
spaces was highly correlated with NIR reflectance. Multiple scattering of photons
causes the NIR reflectance to be much higher than reflectance of visible or SWIR
wavelengths, where absorptions by pigments and water result in single scattering
processes (light is absorbed or scattered on its first interaction).
Secondary water absorption features in leaves are found around 980 nm and
1240 nm (Carter 1991). The diversity of plant adaptations to different water regimes
results in a wide range of leaf water contents among species. The percent water
content is generally positively correlated with increasing leaf thickness, but the
opposite may occur. Thus, a sclerophyllous leaf species like Adenostoma fasciculatum (chamise) can have higher leaf mass area (LMA = 1 divided by specific leaf
area) and low water content, but a succulent species may have high water content
and a high leaf mass per area (Ackerly et al. 2002; Vendramaini et al. 2002).
The SWIR part of the leaf spectrum is dominated by water absorption when the
leaves are living and by leaf chemical constituents when dry. Many plant compounds have absorptions in the SWIR, including cellulose, lignin, nitrogen (N),
sugars, starch, and waxes. Interpretation of these absorption features in dry leaves is
complicated because many molecules have absorptions at overlapping wavelengths
and we lack the specific absorption coefficients to identify them in the data. The cell
wall C compounds comprise the largest fraction of the dry biomass of a leaf, and the
absorption feature around 1750 nm is generally attributed to these materials (Kokaly
et al. 2009). Kokaly (2001) also identified two absorptions at 2054 nm and 2172 nm
that cause broadening of the 2100 nm absorption feature due to N compounds. The
complexity of relationships among species adaptations for water, structural carbohydrates, and nutrients provides a strong basis for detecting species diversity in RS
imagery, at least in local to regional studies (Asner and Martin 2016).
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…