Model runs

There were a total of 673 leaf observations, of which 414 were sunlit and 259 were shaded. Both photosynthesis models were run using as input the measurements of temperature, humidity and PPFD particular to each leaf photosynthesis observation.

3.3.2.1 Model driving data

Measurements of leaf temperature, humidity and incident PAR were used as driving vari- ables of the photosynthesis schemes of JULES and CTESSEL. Since the plants are assumed not to suffer from soil water stress, the normalised soil moisture factor in the models has been set to 1 (volumetric soil water above the critical point), so photosynthesis and stom- atal conductance are not reduced by water stress. For the atmospheric CO2 (Ca) a value

of 325 ppm was used, which is close to the average of measured CO2 concentration of the

fixed value calculated using Equation 2.33 assuming an average windspeed of 2.5 m s−1 and leaf width of Wl = 0.075 m as used in Jacobs (1994). The derived value for the CO2

flow in the leaf boundary layer is gb = 0.0323 m s−1.

3.3.2.2 Model parameters

Initially, model parameters were set to the default model characterization for a grapevine, broadleaf tree model settings (PFT 1) in JULES and deciduous broadleaf tree (vegetation type 5) in CTESSEL. These settings underestimate leaf photosynthesis, highlighting the drawbacks of using a single plant functional type to characterise biomes inhabiting different climatic zones. The work by Jacobs (1994) derived specific parameters for the grapevine, which have also been tested. The photosynthesis model used in Jacobs (1994) is funda- mentally similar to CTESSEL photosynthesis scheme; they both derive from Goudriaan et al. (1985). Consequently, in CTESSEL the adjusted parameters could be directly in- troduced but some transformations were required to derive the corresponding parameters for JULES. Both sets of parameters are presented in Tables 3.3 and 3.4.

Although the model parameters in CTESSEL correspond to the adjusted parameters in Jacobs (1994), CTESSEL’s formulation introduces some relationships between parame- ters, which were described in Chapter 2. Specifically, in tree species parameters f0 and

gm are not independent: f0 is derived from the gm according to Equation 2.59 following

Calvet et al. (2004). This coupling was removed to be able to prescribe f0 and gm simul-

taneously; introducing one extra degree of freedom. The use of adjusted parameters yields an increase in the photosynthesis capacity in carbon limited situations (higher gm and

Am,max in Equation 2.54). CTESSEL’s default value for the parameter  (Equation 2.55)

related to the response to radiation in light limiting situations, does not vary; it is the same as derived by Jacobs (1994). The temperature limits in the temperature dependence (Equations 2.61 and 2.62) are shifted upwards, yielding a higher optimum temperature. The increase of the humidity parameter f0 enhances the sensitivity of Ci to changes in

specific humidity deficit (Eq. 2.43) and consequently stomatal conductance and photo- synthesis. The maximum specific humidity deficit vegetation can cope with (Dmax) is

reduced, as an adaptation to a dry environment. In the case of JULES, to be able to adjust the model parameters consistently, some relationships that link the main param-

Table 3.3: CTESSEL model settings

CTESSEL Deciduous broadleaf Grapevine

gm(25) (mm s−1) 1.4 2 gc (mm s−1) 0.25 0 Am,max (mg CO2 m−2 s−1) 1.83 2.2 Dmax (kg kg −1) 0.109 0.0582 f0 0.623† 0.916 Γ (25) (ppm) 42 45 [T1gm-T2gm] (◦C) [5-36] [0-42] [T1Am,max-T2Am,max] (◦C) [8-38] [15-42] 0 (mg CO2 J−1PAR) 0.017 0.017 † _{(Eq.2.59 in Chapter 2)}

Table 3.4: JULES model settings

JULES Broadleaf Grapevine

n0 (kg N (kg C)−1) 0.046 0.125

Dcrit (kg kg −1) 0.09 0.0582

f0 0.875 0.916

[Tlow-Tupp] (◦C) [0-36] [0-42]

eters from both models were explored. There is not an unequivocal conversion between parameters from different models. Moreover, there is not necessarily a direct match be- tween the same parameter used in two different models. This is because the other parts of the model may exert another type of control on photosynthesis. For example, the widely used maximum velocity of carboxylation, Vcmax, presents a lower value in models that

contain a diffusive mesophyll conductance than in those which do not. Bearing this con- sideration in mind, two approaches were followed to find ‘equivalent’ parameters to the grapevine settings suggested by Jacobs (1994) for the JULES model. Both relationships are described in the appendixes in Jacobs (1994). The first attempt is to relate mesophyll conductance to Vcmax by making use of the relationship obtained by deriving photosyn-

thetic rate with respect to intercellular carbon concentration, as described in Chapter 2 (Equation 2.63). With the default settings, broadleaf trees have Vcmax of 35 µmol CO2

m−2 s−1, derived from leaf nitrogen content (Equation 2.48, Chapter 2). The resulting Vcmax, using Equation 2.63 in Chapter 2 and persevering through some unit conversions,

is 45 µmol CO2 m−2 s−1. This value increased slightly JULES photosynthesis, but there

was still a significant underestimation. The second approach relates Vcmax to Am,max,

instead of gm:

Vcmax= 2Am,max (3.2)

This relation was used in a comparison between MOSES and a A-gs scheme (Steeneveld,

2002). The Vcmax derived from Am,max was much higher, Vcmax = 100 µmol CO2 m−2

s−1 which corresponds to a leaf nitrogen content (n0) of 0.125 kg N (kg C)−1. This

value has been chosen to be used in JULES for the grapevine settings since the leaf level photosynthesis values resulting from it, although above observations, were closer than when using Vcmax from the first method (Equation 2.63). It should be noted than

this Vcmax is more than double the values used in JULES for any PFT. However, other

modelling studies have also suggested higher values of nitrogen content in JULES (den Hoof et al., 2013).

In terms of temperature settings, the parameters derived by Jacobs (1994) for gmhave been

used for temperature dependent parameters in JULES (Tlow and Tupp). As in CTESSEL,

the modifications applied for grapevines tend to increase photosynthesis especially in the carbon limited regime ( Vcmaxin Equation 2.44 is almost tripled) and also less significantly

increase the sensitivity to ambient humidity (via increased f0 in Equation 2.43) and the

resilience to the dry (decreased Dmax) and hot environment (increased Tlow and Tupp in

Equation 2.47).