Remote Sensing of Terrestrial Photosynthesis
519
pigments provides the primary probe for photosynthesis, though estimating
total column pigment from measures indicating primarily surface conditions
presents comparable problems for marine and terrestrial environments
(Parslow and Harris 1990).
While chlorophyll estimates are not widely used for remote sensing-based
estimates of photosynthetic capacity in terrestrial ecosystems, they are
incorporated in several kinds of estimates of environmental stress. For
example, a blue shift in the position of the "red edge" can indicate a
decrease in leaf chlorophyll characteristic of some instances of forest decline
(Rock et al. 1988). A second example involves the ratio of carotenoids to
chlorophyll. Because carotenoids increase in plants under stress and persist
longer than chlorophyll in senescing leaves (Peiiuelas 1984), estimates of the
ratio of carotenoids to chlorophyll provide an additional window on plant
function. Peiiuelas et al. (1993) found that, for a number of aquatic angiosperms, another normalized difference (R680 - R430/R680 + R430) , was
highly correlated with the ratios of total carotenoids to chlorophyll a and
neoxanthin to chlorophyll a.
24.3.3 Other Compounds
In laboratory settings, near infrared reflectance spectroscopy is the basis for
accurate, highly repeatable assays of many biological materials, including
digestibility, nitrogen, energy content, moisture, ash, crude fats, total reducing sugars, alkaloids, and a number of other compounds and classes of
compounds in plant matter (Clark 1989). Remote sensing indices of some of
these, for example total nitrogen, protein, or Rubisco, could be extremely
valuable as probes for photosynthetic capacity. While large-scale remote
sensing indices for these compounds are not yet available, some of the
results to date are encouraging.
Wessman et al. (1988) used the IR reflectance measured with NASA's
Airborne Imaging Spectrometer (AIS), an instrument with moderately
narrow wavebands, to develop an index for canopy lignin in Wisconsin
forest sites. The index accurately predicted not only lignin but also annual
net nitrogen mineralization. In ecosystems where production is limited by
nitrogen availability, remote sensing of lignin, often a major control on
decomposition rate (Melillo et al. 1982), may provide an indirect approach
to estimating canopy nitrogen, as well as growth and production.
Successful application of this technology in the laboratory and in the field
suggests that the near IR (700-2400 nm) may provide a fertile band in the
search for useful remote sensing indices of ecosystem function. Most of the
promising techniques use a large number of narrow spectral bands, beyond
the capabilities of current satellites but within those of instruments scheduled
for launch in the next decade (Wickland 1991).
519
pigments provides the primary probe for photosynthesis, though estimating
total column pigment from measures indicating primarily surface conditions
presents comparable problems for marine and terrestrial environments
(Parslow and Harris 1990).
While chlorophyll estimates are not widely used for remote sensing-based
estimates of photosynthetic capacity in terrestrial ecosystems, they are
incorporated in several kinds of estimates of environmental stress. For
example, a blue shift in the position of the "red edge" can indicate a
decrease in leaf chlorophyll characteristic of some instances of forest decline
(Rock et al. 1988). A second example involves the ratio of carotenoids to
chlorophyll. Because carotenoids increase in plants under stress and persist
longer than chlorophyll in senescing leaves (Peiiuelas 1984), estimates of the
ratio of carotenoids to chlorophyll provide an additional window on plant
function. Peiiuelas et al. (1993) found that, for a number of aquatic angiosperms, another normalized difference (R680 - R430/R680 + R430) , was
highly correlated with the ratios of total carotenoids to chlorophyll a and
neoxanthin to chlorophyll a.
24.3.3 Other Compounds
In laboratory settings, near infrared reflectance spectroscopy is the basis for
accurate, highly repeatable assays of many biological materials, including
digestibility, nitrogen, energy content, moisture, ash, crude fats, total reducing sugars, alkaloids, and a number of other compounds and classes of
compounds in plant matter (Clark 1989). Remote sensing indices of some of
these, for example total nitrogen, protein, or Rubisco, could be extremely
valuable as probes for photosynthetic capacity. While large-scale remote
sensing indices for these compounds are not yet available, some of the
results to date are encouraging.
Wessman et al. (1988) used the IR reflectance measured with NASA's
Airborne Imaging Spectrometer (AIS), an instrument with moderately
narrow wavebands, to develop an index for canopy lignin in Wisconsin
forest sites. The index accurately predicted not only lignin but also annual
net nitrogen mineralization. In ecosystems where production is limited by
nitrogen availability, remote sensing of lignin, often a major control on
decomposition rate (Melillo et al. 1982), may provide an indirect approach
to estimating canopy nitrogen, as well as growth and production.
Successful application of this technology in the laboratory and in the field
suggests that the near IR (700-2400 nm) may provide a fertile band in the
search for useful remote sensing indices of ecosystem function. Most of the
promising techniques use a large number of narrow spectral bands, beyond
the capabilities of current satellites but within those of instruments scheduled
for launch in the next decade (Wickland 1991).