The Lund-Potsdam-Jena DGVM (LPJ-DGVM) (Sitch et al., in prep) was constructed in a modular framework. Individual modules describe key ecosystem processes, including vegetation establishment, resource competition, growth, fire and mortality. Vegetation is grouped into ten plant functional types (PFTs) which are distinguished according to their plant physiological (C3, C4 photosynthesis), phenological (deciduous, evergreen) and physiognomic (tree, grass) attributes. The model is run on a grid cell basis with input of soil texture, monthly fields of temperature, precipitation and percentage sunshine hours. Each grid cell is divided into fractions covered by the PFTs and bare ground. The presence and fractional coverage of an individual PFT depends on PFT specific environmental limits, and on the outcome of resource competition with the other PFTs. A daily photosynthesis calculation is made for each PFT based on its fractional coverage current phenology and water availability in the rooting zone. Photosynthesis is modelled according to a simplified Farquhar approach (Haxeltine and Prentice, 1996a,b), including a dynamic feedback between photosynthesis and soil hydrology through modulation of canopy conductance. A two layer soil water balance model is based on the approach of Haxeltine and Prentice, 1996 b, c. The moisture status in each layer, expressed as a fraction of water holding capacity, is updated daily. Percolation from the upper to the lower layer, and absolute water holding capacity is soil texture dependent. The moisture status in the upper layer is taken as a surrogate for litter moisture status, important in the fire disturbance calculation. The PFT fractional coverage is derived from the number of average individuals in the grid cell. An individual is described in terms of its leaf, sapwood, heartwood and fine root biomass. Each tissue is assigned a C:N ratio and a tissue turnover time. Maintenance and growth respiration is calculated daily for each PFT based on tissue biomass, C:N ratio, and environmental conditions. Processes affecting vegetation structure are simulated annually. Annual net primary production (NPP) is allocated to the leaf, sapwood and fine root pools satisfying prescribed structural constraints. Establishment and mortality are modelled on an annual basis. Plant establishment, in terms of additional PFT individuals, depends on the fraction of bare ground available for seedlings to successfully establish. The fraction of the grid cell affected by fire is calculated based on the length of the dry season and the amount fuel. The effect of disturbance on vegetation structure is PFT specific, accounting for the different levels of plant adaptations in ecosystems. Natural mortality is taken as a function of PFT vigour, and corresponds to an annual reduction in the number of PFT individuals. A decrease in PFT vigour will lead to an increased rate of mortality. Light competition favours woody over grass PFTs. Any dead biomass enters the litter pool, and the soils pools.
References:
Haxeltine, A. and Prentice, I.C., 1996a. A general model for the light-use efficiency of primary production. Functional Ecology, 10:551-561.
Haxeltine, A. and Prentice, I.C., 1996b. BIOME3: An equilibrium terrestrial biosphere model based on ecophysiological constraints, resource availability, and competition among plant functional types. Global Biogeochemical cycles Vol10.,NO4., pp693-709,1996.
Haxeltine, A., Prentice, I.C., Creswell, I.D., 1996c. A coupled carbon and water flux model to predict vegetation structure. Journal of vegetation Science, 7(5):651-666.
Sitch, S., Prentice I.C., Smith B., Kaplan J. LPJ- a coupled model of vegetation dynamics and the terrestrial carbon cycle. In prep.