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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