350
may have different concentrations of biochemicals that together produce a distinct
pattern. Minor biochemicals, such as specific defensive compounds, may reveal more
about biodiversity by their presence or absence than by their concentrations.
In their paper on the worldwide leaf economics spectrum, Wright et al. (2004)
showed that plant investments in leaf traits represent long-term adaptations to climate
characteristics such as length of the growing season, air temperature, and precipitation. Global research on the concept of relating leaf traits to ecosystem functionality
(e.g., Wright et al. 2005; Ordoñez et al. 2009; Kattge et al. 2011) and the use of these
properties to better understand functional adaptations has rapidly expanded.
In recent years, there has been strong interest in using optical properties to elucidate biodiversity patterns, and identification of functional traits with phylogenetic
associations has become a key new objective of spectroscopy and remote sensing
(RS) (Martin, Chap. 5; Meireles et al. Chap. 7. With rapid losses of biodiversity,
there is a need to improve understanding, identify hot spots, and predict how patterns of biodiversity may change in the future. This has led to renewed interest in
whether the optical properties of plants can be understood in a phylogenetic context
as well as their functional processes.
14.2 On the Optical Spectrum of Seed Plants
There are many definitions of what constitutes the optical spectrum. Most narrowly,
it is visible light—the part of the electromagnetic spectrum that can be seen by the
human eye (wavelengths 400–700 nm). The full solar spectrum includes all the
wavelengths of electromagnetic energy from the sun that reach the Earth’s surface.
These wavelengths start in the zone of ultraviolet (UV) A (generally longer than
370 nm) and include visible light, near-infrared (NIR, 700–1000 nm), and shortwaveinfrared (SWIR, 1000–3000 nm, also termed middle-infrared in some disciplines).
Solar energy interacts with a leaf across the full range of wavelengths to produce the
leaf’s optical properties, which are determined by its biochemical and biophysical
characteristics. In recent years, with improved detector technology, it has become
possible to measure reflected sunlight with satellite and airborne imagers that have
sufficient spectral resolution to access the absorption patterns of an increasing number of chemical compounds. It is the variation in the full suite of chemistry and
scattering properties that allows identification of plant species from their leaf spectra— the patterns of absorption and reflection across all wavelengths that can be
measured in the solar spectrum.
Seed plants have three basic types of leaves. Monocot and dicot leaves of angiosperms typically have a wide blade, and conifers have needle-shaped leaves.
Figure 14.1 shows examples of typical spectra of evergreen (Quercus wislizeni) and
deciduous (Quercus douglasii) dicot leaves, deciduous leaves of a monocot (Zea
mays), and evergreen conifer needles (Pinus ponderosa). The overall shape of the
leaf spectra is similar, with low reflectance across visible wavelengths due to absorption by photosynthetic pigments (Gates et al. 1965). High reflectance is
S. L. Ustin and S. Jacquemoud
may have different concentrations of biochemicals that together produce a distinct
pattern. Minor biochemicals, such as specific defensive compounds, may reveal more
about biodiversity by their presence or absence than by their concentrations.
In their paper on the worldwide leaf economics spectrum, Wright et al. (2004)
showed that plant investments in leaf traits represent long-term adaptations to climate
characteristics such as length of the growing season, air temperature, and precipitation. Global research on the concept of relating leaf traits to ecosystem functionality
(e.g., Wright et al. 2005; Ordoñez et al. 2009; Kattge et al. 2011) and the use of these
properties to better understand functional adaptations has rapidly expanded.
In recent years, there has been strong interest in using optical properties to elucidate biodiversity patterns, and identification of functional traits with phylogenetic
associations has become a key new objective of spectroscopy and remote sensing
(RS) (Martin, Chap. 5; Meireles et al. Chap. 7. With rapid losses of biodiversity,
there is a need to improve understanding, identify hot spots, and predict how patterns of biodiversity may change in the future. This has led to renewed interest in
whether the optical properties of plants can be understood in a phylogenetic context
as well as their functional processes.
14.2 On the Optical Spectrum of Seed Plants
There are many definitions of what constitutes the optical spectrum. Most narrowly,
it is visible light—the part of the electromagnetic spectrum that can be seen by the
human eye (wavelengths 400–700 nm). The full solar spectrum includes all the
wavelengths of electromagnetic energy from the sun that reach the Earth’s surface.
These wavelengths start in the zone of ultraviolet (UV) A (generally longer than
370 nm) and include visible light, near-infrared (NIR, 700–1000 nm), and shortwaveinfrared (SWIR, 1000–3000 nm, also termed middle-infrared in some disciplines).
Solar energy interacts with a leaf across the full range of wavelengths to produce the
leaf’s optical properties, which are determined by its biochemical and biophysical
characteristics. In recent years, with improved detector technology, it has become
possible to measure reflected sunlight with satellite and airborne imagers that have
sufficient spectral resolution to access the absorption patterns of an increasing number of chemical compounds. It is the variation in the full suite of chemistry and
scattering properties that allows identification of plant species from their leaf spectra— the patterns of absorption and reflection across all wavelengths that can be
measured in the solar spectrum.
Seed plants have three basic types of leaves. Monocot and dicot leaves of angiosperms typically have a wide blade, and conifers have needle-shaped leaves.
Figure 14.1 shows examples of typical spectra of evergreen (Quercus wislizeni) and
deciduous (Quercus douglasii) dicot leaves, deciduous leaves of a monocot (Zea
mays), and evergreen conifer needles (Pinus ponderosa). The overall shape of the
leaf spectra is similar, with low reflectance across visible wavelengths due to absorption by photosynthetic pigments (Gates et al. 1965). High reflectance is
S. L. Ustin and S. Jacquemoud