Stomatal openings are the pathways through which both water and carbon dioxide are exchanged between vegetation and the atmosphere. Consequently, net leaf photosynthesis and stomatal conductance to water vapour are linked. The fluxes of carbon and water vapour are proportional to the gradient of water vapour and carbon dioxide respectively.
Leaf photosynthesis is dependent on a number of environmental variables as well as the internal CO2 concentration. The closure suggested by Jacobs (1994) is used in JULES (Cox et al, 1998, Cox et al, 1999). The leaf photosynthesis models are based on the work of Collatz et al (1991) and Collatz et al (1992) for C3 and C4 plants respectively. However, an additional direct soil moisture dependence is introduced as suggested by Cox et al (1998).
The approach of Sellers et al (1992) is used for scaling up from the stomatal conductance to the canopy conductance, in which the primary determinants of photosynthesis, mean incident photosynthetically active radiation (PAR) and the maximum rate of carboxylation of Rubisco are assumed to be proportional throughout the plant canopy. The photosynthesis limiting factors are assumed to be the same at every depth in the canopy. As a consequence it is straightforward to integrate the leaf conductance and photosynthesis over the canopy leaf area index to yield canopy conductance and net canopy photosynthesis. The net leaf photosynthesis is calculated by subtracting the rate of dark respiration from the gross photosynthetic rate.
An alternative scaling up procedure is available in JULES. In this case the leaf-to-canopy scaling makes explicit calculations at different levels in the canopy, and with the emphasis on a more sophisticated representation of light-interception. Light interception is simulated using the two stream approximation approach from Sellers (1985) which describes absorption and scattering losses of incident radiation for both direct and diffuse radiation separately in the visible and near infrared wavebands. The two stream approach provides a set of equations for variation of direct and diffuse, upward and downward beams through the canopy, calculating PAR as a function of L. The calculated values of PAR(L) also depend on solar zenith angle, direct and diffuse radiation incident at the top of the canopy, leaf angle distribution and leaf radiation properties for each waveband. Using the calculated absorbed light at each layer of the canopy, leaf photosynthesis, leaf respiration and stomatal conductance are calculated at each layer and summed to provide a canopy level value. Based on this additive calculation through the canopy, a canopy level stomatal conductance is defined to allow calculation of overall surface energy partitioning. See HCTN 63.
Plant respiration is split into maintenance and growth respiration. Growth respiration is assumed to be a fixed fraction of the net primary productivity. Leaf maintenance respiration is equivalent to the moisture-modified canopy dark respiration while root and stem respiration is assumed to be independent of soil moisture, but to have the same dependences on nitrogen content and temperature. Thus total maintenance respiration is dependent on the mean leaf nitrogen concentration.
The nitrogen concentrations of root and stem are assumed to be fixed (functional type dependent) multiples of the mean leaf nitrogen concentration. The respiring stemwood is calculated using a “pipemodel" approach in which live stemwood is proportional to leaf area and canopy height. The constant of proportionality is approximated from Friend et al (1993).
The dynamic global vegetation model, “TRIFFID (Top-down Representation of Interactive Foliage and Flora Including Dynamics)", updates the plant distribution depending on the CO2 fluxes at the land-atmosphere interface. The surface CO2 fluxes associated with photosynthesis and plant respiration are calculated at every model timestep (normally 30 minutes), for each of 5 plant functional types while the area covered by a plant type is updated every 10 days. The area covered by each plant type is based on the net carbon available to it and on the competition with other plant types, which is modelled using a Lotka-Volterra approach to intraspecies and interspecies competition (see for example Silvertown, 1987). Competition between natural PFTs is based on a tree-shrub-grass dominance heirachy, with dominant types limiting the expansion of subdominant types. However, the tree types (broadleaf and needleleaf) and grass types (C3 and C4) co-compete with competition coefficients dependent on their relative heights. Some allowance is made for agricultural regions, from which the woody types (i.e. trees and grasses) are excluded, and C3 and C4 grasses are reinterpreted as “crops".
Vegetation carbon content and LAI
JULES includes parameters describing the maximum and minimum leaf area index values for the given plant functional type, and a “balanced" LAI which would be reached if the plant was in “full leaf" can be derived. The actual LAI depends on the balanced LAI and the phenological status of the vegetation type, which is updated as a function of temperature.
Changes in vegetation carbon density are related allometrically to changes in the balanced LAI. First, it is broken down into leaf, root, and total stem carbon. Then each of these components are related to the balanced LAI. Root carbon is set equal to leaf carbon, which is itself linear in LAI, and total stem carbon is related to the balanced LAI by a power law (Enquist et al, 1998). The local litterfall rate consists of contributions from leaf, root and stem carbon. The leaf turnover rate is calculated to be consistent with the Leaf Phenology model.
The root turnover rate is set equal to the minimum leaf turnover rate for all PFTs, but the total stem turnover is PFT-dependent to react to the different fractions of woody biomass. There is an additional litter contribution arising from large-scale disturbance which results in loss of vegetated area at the prescribed rate.
Leaf mortality rates for the tree-types are assumed to be a function of temperature, increasing as the leaf temperature drops below a threshold value. The leaf turnover rate increases by a factor of 10 when the temperature drops 1 C below the threshold value. The phenological status is updated on a daily basis assuming leaves are dropped at a constant absolute rate when the daily mean value of leaf turnover exceeds twice its minimum value. Budburst occurs at the same rate when leaf mortality drops back below this threshold, and “full leaf" is approached assymptotically thereafter.
This process amounts to a “chilling-days" parametrization of leaf phenology. A similar approach may be taken for drought-deciduous phenology and for the cold-deciduous phenology of the other (non-tree) PFTs, but neither is included in this version of JULES.
Collatz, G. J., J. T. Ball, C. Grivet, and J. A. Berry, 1991: Physiological and environmental regulation of stomatal conductance, photosynthesis and transpiration: A model that includes a laminar boundary layer. Agric. and Forest Meteorol., 54, 107-136
Collatz, G. J., M. Ribas-Carbo, and J. A. Berry, 1992: A coupled photosynthesis- stomatal conductance model for leaves of C4 plants. Aus. J. Plant Physiol., 19, 519-538
Cox, P. M., C. Huntingford, and R. J. Harding, 1998: A canopy conductance and photo-synthesis model for use in a GCM land surface scheme. J. Hydrology, 212- 213, 79-94
Cox, P. M., R. A. Betts, C. B. Bunton, R. L. H. Essery, P. R. Rowntree, and J. Smith, 1999: The impact of new land surface physics on the GCM simulation of climate and climate sensitivity. Clim. Dyn., 15, 183-203
Enquist, B., J. Brown, and G. West, 1998: Allometric scaling of plant energetics and population density. Nature, 395, 163-166
Friend, A. D., H. H. Shugart, and S. W. Running, 1993: A physiology-based model of forest dynamics. Ecology, 74, 797-797
Jacobs, C., 1994: Direct impact of atmospheric CO2 enrichment on regional transpiration. PhD thesis, Wageningen Agricultural University
Sellers, P. J., 1985: Canopy reflectance, photosynthesis and transpiration. Int. J. Remote Sens., 6(8), 1335-1372
Silvertown, J., 1987: Introduction to Plant Population Ecology. Longman Scientific and Technical, second edition