The Light
Environment of
Plant Canopies 15
In Ch. 14 plant canopies are treated as big leaves. We did not wony
about their structure or the details of how the leaves make up the canopy,
we just assumed that we could find a canopy conductance for vapor and
boundary layer conductances for heat and vapor. Combining these with
the absorbed radiation and soil heat flux densities allowed us to compute
canopy temperatures and transpiration rates. We even estimated carbon
assimilation rates by knowing transpiration rate or light interception.
In this chapter we look in more detail at the light environment of
plant canopies. Without knowing how the light is distributed on leaves
within the canopy we could not use detailed photosynthesis models like
the last one presented in Ch. 14 to estimate canopy photosynthesis, but
a study of the light environment of plant stands is useful for many other
purposes as well. In this chapter we show how to compute the fraction
of radiation intercepted by a canopy and the fraction transmitted to the
soil. These are important for computing assimilation using simple models
like Eq. (14.13), as well as for partitioning potential evapotranspiration
between evaporation (from soil) and transpiration (from leaves). We also
show how to compute the change in spectral composition of light as it
is transmitted and reflected by the canopy. These spectral changes have
application in predicting responses of organs or organisms which are
triggered by a specific ratio of red to far-red radiation and in radiometric
remote sensing.
15.1 Leaf Area Index and Light Transmission
Through Canopies
We use the cumulative hemi-surface area index (HSAI) L to measure the
optical pathlength of radiation from the top of the canopy downward.
The hemi-surface area index is one-half the surface area of leaves per
unit ground area. For thin, flat leaves, the hemi-surface area index is the
same as the leaf area index (LAI), which is the silhouette (one-sided)
area of leaves per unit ground surface area. For more complicated shapes,
like conifer needles or branches, the hemi-surface area index is not equal
to the silhouette leaf area index. For example, conifer needles shaped
Environment of
Plant Canopies 15
In Ch. 14 plant canopies are treated as big leaves. We did not wony
about their structure or the details of how the leaves make up the canopy,
we just assumed that we could find a canopy conductance for vapor and
boundary layer conductances for heat and vapor. Combining these with
the absorbed radiation and soil heat flux densities allowed us to compute
canopy temperatures and transpiration rates. We even estimated carbon
assimilation rates by knowing transpiration rate or light interception.
In this chapter we look in more detail at the light environment of
plant canopies. Without knowing how the light is distributed on leaves
within the canopy we could not use detailed photosynthesis models like
the last one presented in Ch. 14 to estimate canopy photosynthesis, but
a study of the light environment of plant stands is useful for many other
purposes as well. In this chapter we show how to compute the fraction
of radiation intercepted by a canopy and the fraction transmitted to the
soil. These are important for computing assimilation using simple models
like Eq. (14.13), as well as for partitioning potential evapotranspiration
between evaporation (from soil) and transpiration (from leaves). We also
show how to compute the change in spectral composition of light as it
is transmitted and reflected by the canopy. These spectral changes have
application in predicting responses of organs or organisms which are
triggered by a specific ratio of red to far-red radiation and in radiometric
remote sensing.
15.1 Leaf Area Index and Light Transmission
Through Canopies
We use the cumulative hemi-surface area index (HSAI) L to measure the
optical pathlength of radiation from the top of the canopy downward.
The hemi-surface area index is one-half the surface area of leaves per
unit ground area. For thin, flat leaves, the hemi-surface area index is the
same as the leaf area index (LAI), which is the silhouette (one-sided)
area of leaves per unit ground surface area. For more complicated shapes,
like conifer needles or branches, the hemi-surface area index is not equal
to the silhouette leaf area index. For example, conifer needles shaped