Effect of temperature on fertilisation

In this section, the combined effect of temperature and CO2 is explored. Only one plant

(a) JULES (b) CTESSEL

Figure 3.12: CO2 response curve (Ca) with varying temperatures for broadleaf trees,

PPFD = 1600 µmol photon m−2 s−1

synthetic rates of this species, providing a better comparison. In Figures 3.12a and 3.12b the photosynthesis rate is shown (color and z-axis) as a function of both atmospheric CO2

and leaf temperature. The lines across y-axis represent the effects of temperature on the kinetic activity behind photosynthesis. Initially there is a steady increase in photosynthe- sis rate with temperature, reaching a maximum at the optimum temperature and a steep drop thereafter, when high temperatures hamper plant activity. The fertilisation effect can be seen along the other axis (x-axis), from left to right. The slope of these lines is modulated by temperature dependence (y-axis), being virtually flat at low temperatures, and increasing slope and therefore fertilisation effect with temperature up to the optimum, after which the slope is reduced again. A more pronounced fertilisation effect is found for CTESSEL for the middle range of temperatures.

In Figure 3.13, the A-Cacurves introduced in Figure 3.12 are shown for three temperatures:

6◦C, 18◦C and 30◦C, all below the optimum temperature, and at low radiation. It is evident how the fertilisation effect is greater at higher temperatures, as long as temperature remains below the optimum temperature (once past the optimum the slopes would be reduced again as seen in Fig. 3.12). Figure 3.14 also shows the fertilisation effect for the same temperatures but in this case for high radiation intensity PPFD = 1600 µmol photon m−2 s−1. Again the higher temperatures foster the CO2 fertilisation effect and CTESSEL

presents the strongest CO2 effect, with an almost linear increase with CO2.

(a) JULES (b) CTESSEL

Figure 3.13: CO2 response curve (Ca) with varying temperatures, for broadleaf trees,

PPFD = 200 µmol photon m−2 s−1.

factor on photosynthesis, as shown for JULES in Figures 3.14a and 3.13a, where the main limitation on photosynthesis has been indicated with coloured circles. To exemplify how the three photosynthetic rates (Wc in Equation 2.44, Wj in Equation 2.45 and We in

Equation 2.46) respond to increased CO2 and how they are combined together to yield

the leaf net photosynthesis via a co-limitation quadratic equation (Eq. 2.52), their values are shown in Figure 3.15. Specifically, these values correspond to Figure 3.13a for T = 18◦C. The carbon limiting regime has the strongest dependence with CO2. The light

limiting regime has weaker dependence and the export limiting regime is independent of CO2. For this case (as was shown by the dashed line in Figure 3.13a) carbon limits the

reaction when the Cais up to 400 ppm, then there would be light limitation for Ca = 450

ppm and finally it is the export limitation that dominates, reducing the fertilisation effect as Ca increases. Due to the co-limitation there is still a subtle slope in An at the higher

Ca, although We is flat.

At low irradiance, as can be seen in Figure 3.13a (PPFD = 200 µmol photon m−2 s−1), export is the sole limitation for temperatures below 6◦C and there is virtually no fertili- sation effect. For temperatures between 6-36◦C the limitation comes from either CO2 or

light (or RuBP regeneration); there is therefore CO2 fertilisation effect which is eventually

reduced at higher Ca when export dominates. At high irradiance (Figure 3.14a) the light

limitation plays no direct role. In this case, again export is the dominating regime at low temperatures (up to 6◦C) hampering the fertilisation. For moderate temperatures (18-

(a) JULES (b) CTESSEL

Figure 3.14: CO2 response curve with varying temperatures for broadleaf trees, PPFD =

1600 µmol photon m−2 s−1. In (a) the regime limiting photosynthesis is shown by the coloured circles.

Figure 3.15: Dependence with ambient carbon of the three limiting photosynthetic regimes in JULES and the resulting leaf net photosynthesis. Broadleaf trees, PPFD = 200 µmol photon m−2 s−1 and T = 18◦C.

26◦C) there is a combination of carbon at low Ca and export at high Ca, and for higher

temperatures only CO2 dominates.

At low temperatures, under export limiting regime, there is hardly any increase in pho- tosynthetic rate with enhanced ambient carbon. This represents the lack of demand of carbon compounds from other organs of the plant, as the low temperatures reduce all physiological activity. Therefore enhanced CO2 does not translate into increased photo-

synthetic activity. Export limiting regime in JULES is only dependent on temperature (via Vcmax, see Equation 2.45 in Chapter 2), and it is independent of carbon dioxide and

radiation. As temperatures increase, some carbon limitation appears at the lower ambient carbon dioxide levels, combined with export for higher Ca. When carbon limits the pho-

tosynthesis reaction, the curve starts to show some steepness, to then flatten as it reaches the export limitation. The Ca value at which the shift from carbon to export limiting

regime occurs increases rapidly with temperature. On the other hand, for temperatures above 28◦C the limitation is entirely by carbon or light.

CTESSEL shows a similar behaviour at low radiation (Figure 3.13b), except for the coldest case, with a slightly steeper curve at 6◦C. The fertilisation effect in CTESSEL diminishes for low temperature, but is not suppressed as in JULES, as this model has no export limitation. At low radiation both models exhibit similar predictions, as in CTESSEL the light limits photosynthesis at high Ca. At high radiation however, with no limitation from

light in CTESSEL, there is a steady increase in photosynthesis in response to a carbon dioxide rise, with the fertilisation rate maintained up to the double CO2 levels. This

linear fertilisation is a consequence of the lack of export limiting regime. Although the highlighted limitation for JULES at high Cain Figure 3.14a is carbon, the effect of export

is manifested as a saturation through the co-limitation.

3.4.2.1 Effect of CO2 on optimum temperatures for photosynthesis

The temperature at which the photosynthetic rate is maximum is determined by the combined temperature dependencies of the regimes in JULES, or temperature dependent parameters in CTESSEL. In JULES each limiting regime has a distinct temperature de- pendency. The export limiting regime presents a temperature optimum corresponding to

the Vcmaxtemperature optimum, whereas the carbon limiting regime presents an optimum

for temperatures a few degrees lower, due to the effects of the other temperature dependent parameters. The light limited regime, although not directly affected by changes in tem- perature, can experience indirect variation via changes in humidity. If Ds is reduced due

to an decrease in temperature, Ci increases following the Jacobs (1994) closure, increasing

the apparent quantum efficiency (Eq. 2.45 and Eq. 2.56). The temperature at which each PFT is able to assimilate the maximum amount of carbon is the result of the co- limitation between the three situations. The effect of temperature is ultimately regulated by the Tupp and Tdown parameters that regulate all temperature dependent parameters in

JULES. These parameters are PFT dependent. In CTESSEL there are two pairs of pa- rameters T1 and T2, one pair for mesophyll conductance and one pair for Am,max. However

these parameters are the same for all C3 species and only vary for C4 species (see Table

2.2).

The leaf net photosynthesis as a function of temperature for each plant type in both models is shown in Figure 3.16. The effect that CO2increase has on the response of photosynthesis

to temperature is shown by the different colour lines, indicating the atmospheric CO2

from 200 ppm to 800 ppm. As well as an increase in the photosynthetic rate due to fertilisation, the optimum temperature for photosynthesis sees an increase in a few degrees

◦_{C towards higher temperatures. The shift is particularly evident in JULES, although}

also found in a more reduced amount for some low vegetation plant species in CTESSEL (shrubs and C3 grasses). Table 3.6 shows the values for the optimum temperatures for

photosynthesis for each species and model for present day CO2 (400 ppm) and doubled

CO2 (800 ppm). JULES shows an increment of 2◦C on average by a doubling of present

day atmospheric CO2 concentrations. There is an exception: the optimum temperature

for C4 photosynthesis is not altered by CO2 increase in neither model. Because of the

PFT dependence of the Tupp and Tdown parameters in JULES, the optimum temperatures

for different species are more varied, from 20◦C for needle leaf trees to 41◦C for C4 species.

In CTESSEL, the optimum temperatures are more similar for all plant types.

The reason for the shift in the optimum temperature for photosynthesis is a consequence of the uneven response of the different limiting regimes to CO2 increase in JULES. The

CO2 induced photosynthesis increase is more pronounced if the process is carbon limited.

(a) JULES, Broadleaf trees (b) CTESSEL, Broadleaf trees

(c) JULES, needle leaf trees (d) CTESSEL, needle leaf trees

(e) JULES, C3 grasses (f) CTESSEL, C3 grasses

Figure 3.16: Leaf level photosynthesis as a function of leaf temperature for each PFT, with varying ambient CO2 indicated by colour. PPFD = 1000 µmol photon m−2 s−1.

(g) JULES, C4 (h) CTESSEL, C4

(i) JULES, shrubs (j) CTESSEL, shrubs

Figure 3.16: (Cont.) Leaf level photosynthesis as a function of leaf temperature for each PFT, with varying ambient CO2 indicated by colour. PPFD = 1000 µmol photon m−2

s−1.

Table 3.6: Optimum temperature (◦C) for photosynthesis, at Ca= 400 ppm and 800 ppm

PPFD = 1000 µmol photon m−2 s−1

PFT JULES CTESSEL

400 ppm 800 ppm 400 ppm 800 ppm

Broadleaf trees 27 29 31 31

Needle leaf trees 20 21 31 31

C3 grasses 27 30 28 29

C4 grasses 41 41 32 32

effect (from the co-limitation).

In JULES at relatively high radiation (1000µmol photon m−2 s−1) photosynthesis is in- creasingly less carbon and export limited as CO2levels rise; since the export limited regime

has its optimum at a higher temperature, the actual photosynthesis peaks at temperatures closer to the export limit optimum. At low light, the effect of optimum temperature shift is less severe. The A-T curve is more blunt around the maximum, due to the light lim- itation, which only has an indirect dependence on temperature through possible changes in humidity. Although CTESSEL’s parameterization presents differences in the way the limitations of photosynthesis are brought together, there is also a slight increase of the optimum temperature. In this case the shift is more subtle, and not appreciated in the plots.

There are some differences in how this effect appears in the different PFTs in JULES. C3,

shrubs and broadleaf trees show a similar response to enhanced CO2, a substantial incre-

ment in photosynthesis and 2-3◦C increment in the optimum temperature for a doubling in the atmospheric CO2. Needle leaf trees are less responsive to the CO2 increase and

show a 1◦C shift. On the other hand, C4 plants are virtually insensitive to the carbon

increase (as seen in Figure 3.11a) and show no shift in the optimum temperature for pho- tosynthesis. In CTESSEL there is less variation in the behaviour of the different PFTs. C4 plants photosynthesis also see a photosynthesis increase with enhanced CO2 but no

variation on the optimum temperature. The effective increase in the optimum tempera- ture for photosynthesis can be interpreted as an acclimation of the plants. However, the driver of the acclimation is not the temperature itself but the enhanced CO2. This is an

interesting result that emerges in a implicit way from the construction of photosynthesis models, and is in agreement with observations/predictions of acclimation (Yamori et al., 2005).