Description of the Model

BIOME3.5 is an equilibrium terrestrial biosphere model based largely on the BIOME3 model of Haxeltine and Prentice (1996). BIOME3.5's main differences from its predecessor include the addition of a module to calculate isotopic discrimination during photosynthesis (Aleaf), the reparameterization of the original plant functional types (PFTs), and the addition of several new PFTs to reflect poorly represented vegetation types in the arctic and arid subtropics. Like BIOME3, BIOME3.5 is a coupled carbon and water flux model that predicts global vegetation distribution, structure, and biogeochemistry. The model is driven by an arbitrary global ambient C02 concentration (in this case 360 ppmv), and a globally gridded climatology dataset (for Tmin see Bartlein, 1998, other data Leemans and Cramer, 1991). In addition, the model uses global information on soil texture and soil depth and recently available global surveys on rooting depth, frost resistance, and photosynthetic pathway (Woodward, 1987; FAO, 1995; Haxeltine and Prentice, 1996; Jackson et al, 1996; Kern and Bartlein, 1998; Ehleringer et al, 1997). The model is run globally at a 0.5° resolution.

Model operation is based on a suite of 13 PFTs representing broad, physiologically distinct classes of vegetation from arctic cushion forbs to tropical rainforest trees. Each PFT is assigned absolute bioclimatic limits (Table 1) that determine whether or not its net primary productivity (NPP) is calculated for a given grid-cell. The core of the model is a coupled carbon and water flux scheme that determines the leaf area index (LAI) that maximizes NPP. Given a certain soil-water balance, calculated on a pseudo-daily timestep, the model iteratively calculates the LAI that yields the maximum gross photosynthetic uptake and the corresponding

TABLE 1 Absolute Biodimatic Limits'


Tmm min

GDD; min

Sd min

Trees Tropical evergreen Tropical raingreen Temperate broadleaved evergreen Temperate summergreen Subtropical/temperate conifer Boreal evergreen Boreal deciduous Nontrees Temperate grass Tropical grass Desert woody shrub Tundra woody shrub Cold herbaceous Cushion forb/lichen/moss

1200 1200 900 250 250

400 400 400

Tc, mean temperature of the coldest month in °C; T...:„, absolute minimum temperature in °C; GDD5, growing degree-days on a 5°C base; Sd, the minimum survivable winter snowpack in cm.

canopy conductance. NPP is then calculated as the difference between gross photosynthetic uptake and maintenance respiration. Various environmental factors including variation in soil texture with depth and seasonal patterns of precipitation as well as the ambient concentration of atmospheric CO, have an effect on transpiration and carbon gain. PFT-specific parameters determine the sensitivity of each PFT to environmental changes (Table 2). Photosynthetic pathway is also PFT-specific, with a C(-type for woody plants and a C4-type representing tropical and subtropical grasslands and C4 desert shrubs (such as some Atriplex species). For computational reasons, the C,, subtypes, NADP-ME, NAD-ME, and PCK, are not separated; CAM photosynthesis is not considered.

Monthly mean NPP is summed on an annual basis for each PFT. The woody PFT with maximum NPP is considered the dominant PFT, except in special cases where grass or mixtures of grass and trees would be expected to dominate because of an inferred disturbance regime or soil moisture constraints. The dominant and subdominant PFTs are expanded into 22 classes of terrestrial vegetation biomes. All of the biogeochemical output from the model represents the dominant PFT for a grid-cell, as there is no explicit accommodation for mixed-PFT grid-cells. However, in the case of savannas and some mixed tree-grass temperate plant communities, the output variables (including Aleaf) are given an NPP-weighted average of the grass and tree types.

BIOME3.5 has the new feature of calculating isotopic discrimination against l3CO, during photosynthesis at the leaf level (Aleaf) and total ecosystem discrimination (Ac). The discrimination model for Aleaf is closely related to that of Lloyd and Far-quhar (1994). The main difference is that the BIOME3.5 model explicitly simulates the concentration of CO, in the chloroplast through optimization calculations balancing carbon gain with water loss. Only a maximum c-Jc, ratio is prescribed for each PFT

(optratio, Table 2). The actual c-JcR is subsequently modeled by the optimization calculation. Maximum c-Jc^ ratios were compiled from a literature survey on laboratory studies (Kaplan, in preparation) and from maximum <513C values measured for leaf material of all PFTs (Lloyd and Farquhar, 1994; Lloyd, personal communication).

Additionally, we developed a model for Ae that is based on the theories presented by Buchmann et al. (1998) and Flanagan and Ehleringer (1998). Monthly Ae values are estimated as the flux-weighted difference in discrimination against l3C from NPP and heterotrophic respiration (_Rh). Photosynthate, with a specific l3C content determined by the Aleaf value, is incorporated into the plant on a seasonally integrated flux-weighted basis. A simple model for Rh determines the monthly flux of respired C02 and , 3C02 to the atmosphere (Sitch et al., 1999; Foley, 1995; Lloyd and Taylor, 1994). The source of respired C02 is the aggregated annual NPP for the dominant vegetation type in a grid-cell. This carbon stock is arbitrarily divided into three pools according to the scheme of Foley (1995). Each pool is subjected to a degree of isotopic fractionation during respiration based on the assumed decay rate of the pool. Because the processes underlying carbon isotope fractionation during respiration are poorly understood, fractionation in each pool is assigned a constant value. The fractionation factor increases with pool age (Buchmann et al., 1997; Ciais et al, 1995; Ehleringer et al, 2000).

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