Calculating Canopy Assimilation from Leaf Assimilation
259
will vary depending on their orientation, but the mean flux density on the
sunlit leaves can be shown to be
where Qsc is the flux density of down-scattered radiation from the solar
beam. The flux density on shaded leaves is the diffuse flux plus the downscattered flux from the solar beam:
The down-scattered radiation is the difference between Qbt(@) and
Qb(@):
The next problem is to know what fraction of the leaf area at depth L is
sunlit. The probability of finding a sunlit leaf area index in thickness S L
at depth L in the canopy is the product of the probability that a ray will
penetrate to depth L and the probability that it will be intercepted in the
layer SL, divided by Kbe(@) (the ratio of projections of leaf area on a
horizontal surface to actual leaf area). If L* is used to represent the sunlit
leaf area index, then
In the limit as 6 L becomes small, 6 L* = SL exp(- Kbe(@) L). The
fraction fsl(@) of sunlit leaves at depth L is SL*/SL, so
The fraction of shaded leaves is fsh (@) = 1 - fsl (@). If the LA1 of the
entire canopy is Lt, then the sunlit LA1 of the whole canopy LT is
and the shaded LA1 is L, - LT.
15.9 Calculating Canopy Assimilation from Leaf
Assimilation
Several methods are available for calculating canopy photosynthetic rate
from leaf photosynthetic rate based on the distribution of light over leaves,
including methods that consider additional factors such as wind and
humidity. Norman (1992) compared various simple methods for estimating canopy assimilation from leaf assimilation. The most robust method
seems to divide the canopy into sunlit and shaded leaf classes, calculate
the assimilation rate for representative members of each class, and sum
the two contributions according to the fraction of leaf area in each class.
One reason this method works so well is that it accommodates the nonlinear response of leaf assimilation to light. Light assimilation responses of
259
will vary depending on their orientation, but the mean flux density on the
sunlit leaves can be shown to be
where Qsc is the flux density of down-scattered radiation from the solar
beam. The flux density on shaded leaves is the diffuse flux plus the downscattered flux from the solar beam:
The down-scattered radiation is the difference between Qbt(@) and
Qb(@):
The next problem is to know what fraction of the leaf area at depth L is
sunlit. The probability of finding a sunlit leaf area index in thickness S L
at depth L in the canopy is the product of the probability that a ray will
penetrate to depth L and the probability that it will be intercepted in the
layer SL, divided by Kbe(@) (the ratio of projections of leaf area on a
horizontal surface to actual leaf area). If L* is used to represent the sunlit
leaf area index, then
In the limit as 6 L becomes small, 6 L* = SL exp(- Kbe(@) L). The
fraction fsl(@) of sunlit leaves at depth L is SL*/SL, so
The fraction of shaded leaves is fsh (@) = 1 - fsl (@). If the LA1 of the
entire canopy is Lt, then the sunlit LA1 of the whole canopy LT is
and the shaded LA1 is L, - LT.
15.9 Calculating Canopy Assimilation from Leaf
Assimilation
Several methods are available for calculating canopy photosynthetic rate
from leaf photosynthetic rate based on the distribution of light over leaves,
including methods that consider additional factors such as wind and
humidity. Norman (1992) compared various simple methods for estimating canopy assimilation from leaf assimilation. The most robust method
seems to divide the canopy into sunlit and shaded leaf classes, calculate
the assimilation rate for representative members of each class, and sum
the two contributions according to the fraction of leaf area in each class.
One reason this method works so well is that it accommodates the nonlinear response of leaf assimilation to light. Light assimilation responses of