Geoscience Reference
In-Depth Information
Table 9.5 Parameter values for the A-g s model of Ronda et al. ( 2001 )
Parameter
Function of
Plant type
C 3 C 4
0 (mg J -1 ) 0.017 0.014
Γ (mg kg -1 ) T 68.5 4.3
g m (mm s -1 ) T 7.0 17.5
A n,max (mg m -2 s -1 ) T 2.2 1.7
a d (k Pa -1 ) 0.07 0.15
f max (-) 0.89 0.85
f min (-) g 0 , g m 0.23 0.18
D 0 (kPa) f max , f min , a d 9.4 4.5
The values for Γ , g m and A m,max are in principle temperature-dependent. Here the values at 298
K are given. Note that the value for a d for C 4 plants differs from the one in Ronda et al. ( 2001 ),
which was erroneous (Ronda, pers. comm., 2012).
eficiency, that is, the amount of CO 2 ixed at a given input of PAR, at low levels of
PAR. The initial light use eficiency determines the initial slope in Figure 6.13b . The
initial light use eficiency in turn depends on the maximum light use eficiency ( 0 )
as 
ce ce Γ Γ . Taking Eq. ( 9.30 ) to represent the maximum attainable
assimilation rate under the given CO 2 conditions, the actual gross assimilation can be
determined from the following interpolation:
=
(
q
) /(
q
+
2
)
0
I
AR
AAR
=+ − −
(
) 1
exp
PAR
(9.31)
g,cl
n,c
d
+
n,c
d
where the subscript 'cl' denotes that this is the CO 2 and light-limited gross assimila-
tion. R d is parameterized as a fraction of the net assimilation at maximum light avail-
ability: R d = 0.11 A n,c .
The aforementioned model for A g contains a number of parameters that are depen-
dent on temperature owing to the fact that the associated chemical processes are
temperature-dependent. In that way, the temperature response of the model is natu-
rally incorporated (for the values at reference temperature, see Table 9.5 ; temperature
dependences are given in Ronda et al., 2001 ). The parameters also differ between
plant species (in particular C 3 vs. C 4 plants).
The soil moisture response of the A-g s model is incorporated empirically, based on
a reduction of gross assimilation rate. Ronda et al. ( 2001 ) suggest:
2
AAf
=
2
()
θ
f
()
θ
(9.32)
g,clw
g,cl
θ
θ
 
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