Indirect Sensing of Canopy Architecture
275
ray of canopy envelopes of known dimensions and locations, the beam
transmittance tb(@, A Z ) can be estimated from
where p is the leaf area density (m 2 of hemi-surface area per m
3 canopy
volume) and S(@, A Z ) is the path length of light rays through the array
of canopy envelopes between a particular point in a horizontal (at some
depth in the canopy or at the soil surface) plane and the sun.
Models of BRF in heterogeneous canopies are quite complicated and
several approaches are described in detail in a book edited by Myneni
and Ross (1 99 1).
15.14 Indirect Sensing of Canopy Architecture
A description of canopy architecture includes the position and orientation distributions of leaves, branches, stems, flowers, and fruit. For most
canopies, leaves dominate the canopy space so leaf area index, leaf angle
distribution and some measure of clumping provide most of the information needed to describe canopy architecture. If we limit our discussion to
canopies that approximate random positioning (most full-cover deciduous
forests, grasslands and crops), then LA1 and x are the minimum essential bits of information. Direct measurements of LA1 and x , by cutting
plants and measuring leaf areas and angles are exceedingly laborious, so
alternative measurement methods are desirable. Measurements of canopy
gap fraction as a function of zenith angle can be used to obtain estimates
of L, and Kbe(@). The strategy for using gap-fraction measurements to
estimate canopy architecture is illustrated in Fig. 15.3. The gap fraction
corresponds to the ordinate labeled transmission and the curves show the
effect of leaf angle distribution ( x ) on transmission or gap fraction as a
function of zenith angle for L, = 1. Given a number of measurements
of gap fraction as a function of zenith angle, the curve that best fits the
data can be chosen from numerous families of curves such as shown in
Fig. 15.3 calculated for a range of LA1 values. The values of x and L,
that best fit the data are assigned to the canopy where the gap-fraction
measurements originated (Norman and Campbell, 1989). Although this
method appears to be simple, the inversion procedure can be error prone
and must be done carefully. Several commercial instruments that use this
approach are available and have been discussed by Welles (1990).
Heterogeneous (nonrandom) canopies require some additional information about the characteristics of the heterogeneity. If canopy
heterogeneity can be represented by the parameter S2 (@) in Eq. (15.35),
then additional methods must be available for estimating S2 (@) (Chen,
1996) beyond the measurements of gap fraction as a function of zenith
angle.
If heterogeneous canopies are composed of regular geometric shapes
that contain foliage with large gaps between them, then the path length
S(@, A Z ) may be determined for the particular geometry (horizontal
275
ray of canopy envelopes of known dimensions and locations, the beam
transmittance tb(@, A Z ) can be estimated from
where p is the leaf area density (m 2 of hemi-surface area per m
3 canopy
volume) and S(@, A Z ) is the path length of light rays through the array
of canopy envelopes between a particular point in a horizontal (at some
depth in the canopy or at the soil surface) plane and the sun.
Models of BRF in heterogeneous canopies are quite complicated and
several approaches are described in detail in a book edited by Myneni
and Ross (1 99 1).
15.14 Indirect Sensing of Canopy Architecture
A description of canopy architecture includes the position and orientation distributions of leaves, branches, stems, flowers, and fruit. For most
canopies, leaves dominate the canopy space so leaf area index, leaf angle
distribution and some measure of clumping provide most of the information needed to describe canopy architecture. If we limit our discussion to
canopies that approximate random positioning (most full-cover deciduous
forests, grasslands and crops), then LA1 and x are the minimum essential bits of information. Direct measurements of LA1 and x , by cutting
plants and measuring leaf areas and angles are exceedingly laborious, so
alternative measurement methods are desirable. Measurements of canopy
gap fraction as a function of zenith angle can be used to obtain estimates
of L, and Kbe(@). The strategy for using gap-fraction measurements to
estimate canopy architecture is illustrated in Fig. 15.3. The gap fraction
corresponds to the ordinate labeled transmission and the curves show the
effect of leaf angle distribution ( x ) on transmission or gap fraction as a
function of zenith angle for L, = 1. Given a number of measurements
of gap fraction as a function of zenith angle, the curve that best fits the
data can be chosen from numerous families of curves such as shown in
Fig. 15.3 calculated for a range of LA1 values. The values of x and L,
that best fit the data are assigned to the canopy where the gap-fraction
measurements originated (Norman and Campbell, 1989). Although this
method appears to be simple, the inversion procedure can be error prone
and must be done carefully. Several commercial instruments that use this
approach are available and have been discussed by Welles (1990).
Heterogeneous (nonrandom) canopies require some additional information about the characteristics of the heterogeneity. If canopy
heterogeneity can be represented by the parameter S2 (@) in Eq. (15.35),
then additional methods must be available for estimating S2 (@) (Chen,
1996) beyond the measurements of gap fraction as a function of zenith
angle.
If heterogeneous canopies are composed of regular geometric shapes
that contain foliage with large gaps between them, then the path length
S(@, A Z ) may be determined for the particular geometry (horizontal