This chapter describes how JULES and CTESSEL represent the main aspects of the land surface. Particular emphasis has been placed on the parameterization of vegetation pro- cesses. JULES photosynthesis model is based on Farquhar et al. (1980), and calculates the leaf net photosynthetic rate as a co-limited function of the three limiting regimes (carbon, light and export). CHTESSEL/CTESSEL photosynthesis model is based on Goudriaan et al. (1985) as modified by Jacobs (1994); Jacobs et al. (1996). This formulation does not calculate separate limiting regimes but does account for carbon and light limitation. It does not explicitly account for the export or phosphate limitation, although it could be argued that a parameter Am,max could play the role of a physiological limitation by
imposing a maximum carboxylation rate attainable by each plant species. JULES and CTESSEL calculate the stomatal conductance based on the A-gs relationship that corre-
lates the photosynthetic rate with the stomatal aperture. CHTESSEL however uses the Jarvis approach to determine the stomatal conductance. In the rest of the thesis, the cou- pled version CTESSEL, is used for comparison with JULES. Furthermore, both models calculate the Ci/Ca ratio as a function of air humidity based on Jacobs (1994) closure
equation, although JULES neglects the term associated to transport through the cuticula when stomata are closed.
The treatment of the soil moisture stress is quite different in both models. While JULES uses a simple linear scaling of the photosynthetic rate based on volumetric water content, CTESSEL uses a complex parameterization based on Calvet (2000) and Calvet et al. (2004), that affects gm and f0 or Dmax. It provides the choice of offensive or defensive
chapters, CTESSEL uses the offensive strategy.
JULES provides a variety of methods to upscale the leaf level photosynthesis to the canopy level. The most advanced version divides the canopy into several layers. CTESSEL uses the simple big-leaf approach but introduces a radiative transfer through the canopy with different attenuation coefficients for direct and diffuse radiation.
One potential shortcoming of both formulations is the treatment of the leaf temperature. The leaf temperature for the whole canopy is the tile’s surface temperature. A dedicated canopy/leaf energy balance could provide a more accurate temperature to regulate the photosynthesis reaction.
In the next chapter the photosynthesis schemes of both models are isolated, in order to be tested against leaf level measurements and to compare their response to environmental variables.
Leaf level photosynthesis
3.1
Introduction
In this chapter the focus is on the photosynthesis schemes at the leaf level. Photosynthesis is a key process in the modelling of carbon cycle, and one that is prone to variations in a changing climate due to its direct dependence on environmental factors such as tem- perature, radiation, humidity, soil moisture and ambient CO2 (Nemani et al., 2003). The
trade off between carbon assimilation and transpiration is also affected by climatic changes (Keenan et al., 2013; De Kauwe et al., 2013). Model intercomparison studies have revealed uncertainties in the response of modelled carbon absorption to climate change (Cramer et al., 2001; Friedlingstein et al., 2006). Vegetation models are becoming increasingly com- plex, with parameters describing traits such as leaf nitrogen concentration or leaf lifespan, and upscaling methods with multiple canopy layers and different treatments for diffuse radiation. However, the core of the photosynthesis activity and its dependence on the environment variables remain dictated by the the leaf level processes. Consequently, the equations at the leaf level are of primary importance in determining how the projected increase in temperature and CO2 will affect vegetation. These equations are based on
biochemical photosynthesis models (Farquhar et al., 1980; Goudriaan et al., 1985) and the empirical A-gsrelationship (Ball et al., 1987; Leuning, 1995) and were described in Chapter
2. In this chapter, the sensitivity of leaf level photosynthesis (and stomatal conductance) to environmental factors as modelled in JULES and CTESSEL is assessed. In particular,
the fertilisation effect on photosynthesis due to enhanced atmospheric CO2 as reproduced
by each model is compared as well as the effects that temperature and radiation exert on it. Each model’s photosynthesis scheme at the leaf level has been isolated from the rest of the land surface model in order to reproduce the direct response of photosynthesis to ambient factors at the leaf environment. This analysis leads to a better understanding of how differences in photosynthesis parameterization translate in terms of vegetation’s modelled climate response. The variation across the different plant functional types that are represented by land surface models has also been explored. The different species are represented by the same functional relationships and characterised by specific values of the model parameters, except C4 grasses which possess differentiated formulation for some
processes.
The three limiting situations for photosynthesis described in Chapter 2 play an important role in determining the photosynthesis sensitivity to environmental factors. The influence of the limiting regimes on CO2fertilisation has been analysed in JULES. The main limiting
factors for plant growth are CO2, even with the increasing levels of this atmospheric
molecule, and RuBP regeneration, related to light absorption and patent at low radiation intensities. JULES also incorporates an export limiting regime. The limiting factor for photosynthesis reaction varies throughout the day and geographically and is determined by the combined levels of incoming radiation and temperature, as well as CO2 concentration.
An increase in atmospheric CO2affects the three potential photosynthetic rates to different
degrees or has no effect for export limiting regime. The rise in atmospheric CO2 enhances
plant productivity, and the enhancement is more pronounced if carbon is the limiting factor. However, as the CO2 supply increases, photosynthesis will reach other limitations.
Due to these heterogeneous responses, the actual effect of CO2 on carbon assimilation is
not easy to infer directly. Since stomatal conductance is linked to photosynthetic activity, variations in gas exchange due to enhanced CO2 also depend on the limiting factor on
photosynthesis. Using JULES photosynthesis model, the conditions that determine each limiting regime have been analysed and the modifications that a changing climate might bring.
Section 3.2 describes the stand alone versions of the photosynthesis schemes. In Section 3.3, the leaf photosynthesis schemes from both models are validated for present day cli-
mate by comparing with field measurements of photosynthesis and stomatal conductance from vineyards (Jacobs, 1994). These data provide insight into the diurnal dependence of photosynthesis on leaf level incident radiation, temperature and humidity for a particular field site. The occurrence of the limiting regimes in this set of observations has been iden- tified (as modelled by JULES). The remainder of the Chapter considers the photosynthesis response to the whole environment including atmospheric CO2 concentration. In Section
3.4 the photosynthesis CO2 fertilisation effect at the leaf level as reproduced by JULES
and CTESSEL is analysed, as well as photosynthesis responses to changes temperature and radiation. These results can be related to the climate projections derived from models induced by vegetation’s response to enhanced CO2, increasing temperatures or changes
in radiation levels. The underlying assumptions in plant modelling described in Chapter 2 are linked in this chapter to the CO2 assimilation response to the main drivers. Using
the JULES photosynthesis model, the conditions of radiation and temperature that foster each limiting regime have been identified as well as the changes that enhanced CO2 inflict
on these conditions. In Section 3.5 the responses of stomatal conductance are analysed. Finally, in Section 3.6 a global sensitivity analysis using the Extended Fourier Amplitude Sensitivity Test (FAST) is performed on the leaf level photosynthesis schemes. This anal- ysis determines the relative importance of each input variable and the models parameter on the photosynthesis model output.