The Light Environment of Plant Canopies
14.6 are in units of pmol CO2 m-2(leaf surface area) s-', and the units
needed are pmol C02 m-2(leaf hemi-surface area) s-'. Therefore
pmol CO2
AZ& = 11 m 2 (leaf surface area) s
2m 2 (leaf surface area)
X m 2 (leaf hemi-surface area)
A sun
pm01 C02
n.leaf = 22 m 2 (leaf hemi-surface area) ,s '
Similarily, at 16212 = 81 pmol photons m-2 (leaf surface area) s-'
pmol CO2
A: :
:
$
= 6 m 2 (leaf hemi-surface area) s '
Therefore the canopy assimilation is given by
pmol CO2
A, = 22 m 2 (leaf hemi-surface area) s
m
2 (leaf hemi-surface area)
x 1.32
m 2 (ground area)
pmol C02
+
m 2 (leaf hemi-surface area) s
m
2 (leaf hemi-surface area)
x 1.68
m 2 (ground area)
pmol CO2
= 29.0 + 10.1 = 39.1 m 2 (ground area) s '
The approach used in Example 15.2 to scale leaf assimilation to canopy
assimilation accommodates the nonlinearity in the 10°C light assimilation curve in Fig. 14.6. If we had ignored the fact that the 10°C light
assimilation curve is not a straight line and used an average absorbed
PAR for the entire canopy to scale up the leaf assimilation rate, how large
would the error be? The average absorbed PAR for the canopy is the
mean of sunlit and shaded absorbed PAR weighted by the leaf area of
each:
-
- Q,lLT + =(L, - L: ) 997 x 1.32 + 162 x 1.68
Q =
- -
Lt
3.0
p mol photons
= 529 m 2 (leaf hemi-surface area) s '
From Fig. 14.6, the leaf assimilation rate corresponding to the average
absorbed PAR is 20 pmol C02 m-2(leaf hemi-surface area) s-', so the