264
The Light Environment of Plant Canopies
15.10 Remote Sensing of Canopy Cover and
IPAR
Remote sensing is a name associated with inferring characteristics of
surfaces from measurements of radiance. In environmental biophysics,
remote sensing usually refers to the interpretation of radiometric measurements made above soil-vegetation systems from towers, aircraft, or
satellites. A more general term is indirect measurement, which refers to
any measurement made without directly contacting an object. Technically, our eyes indirectly sense the environment around us so an absurd
interpretation might infer that all information obtained with our eyes (e.g.,
reading a ruler) could be considered remote sensing; however, this is not
what we mean. In environmental biophysics, some examples of remote
sensing include the following.
1. Infrared thermometer measurements of soil surface temperature.
2. Measuring soil or canopy roughness using the backscattered radiation
from a laser (these systems are called LIDAR).
3. Estimating the water content of the top 5 cm layer of soil using passive
microwave measurements of surface temperature and emissivity.
4. Estimating total forest-canopy water content to infer vegetation
biomass using RADAR.
5. Inferring canopy cover, leaf area index, or intercepted photosynthetically active radiation (IPAR) from measurements of visible (VIS) and
near-infrared (NIR) reflected radiance.
Another indirect measurement that is common in environmental biophysics, but not generally referred to as remote sensing, is the indirect
measurement of canopy architecture. This is discussed briefly in a later
section of this chapter.
Some of the fundamental characteristics of remote sensing data can be
understood using knowledge of canopy architecture by considering the relation between canopy cover, IPAR, and reflected VIS and NIR radiation.
In previous sections we discussed the penetration of radiation through
canopies, the reflection of radiation from canopies and the distribution of
radiation over the surface of leaves. Although all this is relevant to remote
sensing, a second consideration also is required; that is, the portion and
characteristics of the canopy and soil that occupy the field-of-view (FOV)
of the sensor. As mentioned in Ch. 10, bidirectional reflectance factors
(BRF) involve two directions; the direction of the source (usually the sun)
and the direction of the receiver (a sensor). To simplify the analysis that
follows, we do not consider finite solid angles of view, but only consider
particular directions as though the radiation were composed of parallel
rays all from that direction. Essentially this amounts to using data from
a narrow FOV sensor that is calibrated to read out the flux density emhating from the target surface by making the output proportional to the
radiance times the FOV of the sensor.
The Light Environment of Plant Canopies
15.10 Remote Sensing of Canopy Cover and
IPAR
Remote sensing is a name associated with inferring characteristics of
surfaces from measurements of radiance. In environmental biophysics,
remote sensing usually refers to the interpretation of radiometric measurements made above soil-vegetation systems from towers, aircraft, or
satellites. A more general term is indirect measurement, which refers to
any measurement made without directly contacting an object. Technically, our eyes indirectly sense the environment around us so an absurd
interpretation might infer that all information obtained with our eyes (e.g.,
reading a ruler) could be considered remote sensing; however, this is not
what we mean. In environmental biophysics, some examples of remote
sensing include the following.
1. Infrared thermometer measurements of soil surface temperature.
2. Measuring soil or canopy roughness using the backscattered radiation
from a laser (these systems are called LIDAR).
3. Estimating the water content of the top 5 cm layer of soil using passive
microwave measurements of surface temperature and emissivity.
4. Estimating total forest-canopy water content to infer vegetation
biomass using RADAR.
5. Inferring canopy cover, leaf area index, or intercepted photosynthetically active radiation (IPAR) from measurements of visible (VIS) and
near-infrared (NIR) reflected radiance.
Another indirect measurement that is common in environmental biophysics, but not generally referred to as remote sensing, is the indirect
measurement of canopy architecture. This is discussed briefly in a later
section of this chapter.
Some of the fundamental characteristics of remote sensing data can be
understood using knowledge of canopy architecture by considering the relation between canopy cover, IPAR, and reflected VIS and NIR radiation.
In previous sections we discussed the penetration of radiation through
canopies, the reflection of radiation from canopies and the distribution of
radiation over the surface of leaves. Although all this is relevant to remote
sensing, a second consideration also is required; that is, the portion and
characteristics of the canopy and soil that occupy the field-of-view (FOV)
of the sensor. As mentioned in Ch. 10, bidirectional reflectance factors
(BRF) involve two directions; the direction of the source (usually the sun)
and the direction of the receiver (a sensor). To simplify the analysis that
follows, we do not consider finite solid angles of view, but only consider
particular directions as though the radiation were composed of parallel
rays all from that direction. Essentially this amounts to using data from
a narrow FOV sensor that is calibrated to read out the flux density emhating from the target surface by making the output proportional to the
radiance times the FOV of the sensor.