CTCD vegetation models

        Sam Evans, Sebastien L afont, Mark Lomas, Jennifer Pellenq , Ghislain Picard, Mat Williams, Ian Woodward

          1. SDGVM (F I Woodward, M R Lomas, G Picard)

          2. ForestETP (S P Evans, S Lafont, J Pellenq)

          3. SPA (M Williams)

          SDGVM (Sheffield Dynamic Global Vegetation Model)

          DVMs were originally designed to model the response of terrestrial ecosystems to long-term atmospheric changes in temperature, precipitation and gas concentrations (CO2 and N). The typical structure of a DVM is shown in Figure 2. A core set of coupled modules represents the interactions of ecosystem carbon and water exchanges with vegetation dynamics, under given soil and atmospheric conditions. The biochemical processes of photosynthesis and the dependence of gas exchange on stomatal conductance are explicitly modelled; these depend on temperature and soil moisture. Canopy conductance controls soil water loss by transpiration. The assignment of nitrogen uptake to leaf layers is proportional to irradiance and respiration, and maximum assimilation rates depend on nitrogen uptake and temperature. Total nitrogen uptake is derived from soil carbon and nitrogen and depends on temperature. The intrinsic timescales of the processes are indicated in Figure 2. In practice, averaging and interpolation are used to deal with the most rapid effects (which are embodied in the vegetation physiology and biophysics module), thereby reducing the computational load.

          The great strength of such models is their generality and predictive capability: by accurately representing the biophysical processes involved, they allow credible calculations of the long-term behaviour of vegetation systems under changing climate. However, in order to achieve this, they necessarily use generalised descriptions of the system

          The SDGVM is a point model, with no horizontal fluxes. The model is driven by climate; site parameters are defined at run time or read in from existing maps. The overall structure of the SDGVM is shown in Figure 3. There are six main components: soil carbon and nitrogen dynamics, hydrology, calculation of NPP, phenology, vegetation growth and vegetation dynamics.


          The ForestETP model is an ecological model designed to predict water movement through the soil-plant-atmosphere continuum and carbon exchanges in UK forests. Actually, three versions are under development. The ForestETP-1D model is a point scale, daily timestep soil-vegetation- atmosphere transfer (SVAT) model. It simulates relevant terrestrial hydrological processes (interception, vertical and lateral soil water movement, runoff, soil and canopy evaporation, and N-sensitive photosynthesis-coupled transpiration) for a known tree species growing in a locally-defined soil and climate. ForestETP is coupled with a weather generator that allows the downscaling of summary meteorological data and the generation of climate change time series. The ForestETP-3D runs at the catchment scale and includes the lateral hydrological fluxes induces by topography, the soil and vegetation heterogeneities and the climate variability. In the ForestGrowth extention, vegetation growth has been included and the forest management is taken into account. Figure 4 shows the structure of the ForestETP model and its variants.

          SPA (Soil Plant Atmosphere Model)

          The Soil-Plant-Atmosphere model (SPA, Williams et. al 1996) is a process- based model that simulates ecosystem photosynthesis and water balance at fine temporal and spatial scales (30 minute time-step, multiple canopy and soil layers). The scale of parametrisation (leaf-level) and prediction (canopy level) have been designed to allow the model to diagnose eddy flux data, and to provide a tool for scaling up leaf level processes to canopy and landscape scales (Williams et al. 2001c). The model has been used in temperate (Williams et al. 1996, 2001) tropical (Williams et al. 1998), and arctic ecosystems (Williams et al. 2000). The model is written in FORTRAN 90. The code is divided among several files, each associated with a particular component of the overall model. This means that the core files can be shared among users, who should only have to alter the input/output file and the main program file in order to customise the model for their own uses. Figure 5 shows a flow diagram of the operation of the SPA model.

          The model is freely available to interested researchers. For more information, see http://www.ierm.ed.ac.uk/mw/mwhome.htm.

          Development of Vegetation Carbon Models
          Evaluation against independent data sources
          Future developments (community model)