BIOME2 uses a coupled carbon and water flux simulation model to capture the broadscale environmental controls on the distribution of vegetation structural and functional types (Haxeltine et al. 1996, Haxeltine and Prentice 1996a, Haxeltine and Prentice 1996b). Model input consists of latitude, soil texture, and mean monthly climate data (temperature, precipitation, and sunshine hours).
A rule-based biogeography module based largely on the biome model of Prentice et al. (1992) is first used to select which plant types may potentially be present at a particular site. This rule-base captures the effects of minimum temperature tolerances and chilling requirements on determining the distributions of different plant types. Starting from the set of plant types that may potentially be present at a certain site the model then finds the combination of plant types which maximizes the whole ecosystem NPP. Gross primary production (GPP) is calculated as a linear function of absorbed photosynthetically active radiation based on a optimized version of the Farquhar photosynthesis equation (Haxeltine and Prentice 1996a). GPP is reduced by drought stress and low temperatures. Respiration costs are currently estimateed simply as being 50% of the non-water-limited GPP. Through the effects of drought stress on NPP, the model correctly reproduces changes in FPC along moisture gradients. A simple two-layer hydrology model allows a realistic simulation of the competitive balance between grass and woody vegetation, including the effects of soil texture. The prescribed CO2 concentration has a direct effect on GPP through the photosynthesis algorithm and greatly effects the competitive balance between C3 and C4 plants. The water balance calculation is based upon equilibrium evapotranspiration theory (Jarvis and McNaughton 1986) which suggests that the large-scale potential evapotranspiration rate is determined by the energy supply for evaporation. Stomatal conductance is not explicitly included in the water balance calculation and there is no direct effect of CO2 on the water balance in the model.
Model output consists of net primary production (NPP) and leaf area (as foliar projective cover, FPC) for the combination of major plant types (e.g., evergreen and cold deciduous woody plants and C3 and C4 grasses) that maximizes whole ecosystem NPP. A rule-base is then used to translate the model output into vegetation structural categories which can be directly compared with those of the VEMAP vegetation data set.
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