Mapped Atmosphere-Plant-Soil System Study

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About Us

The MAPSS Model

• MAPSS is a model that was developed to simulate the potential natural vegetation that could exist on any upland site in the world under present, past, or future climate.

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• MAPSS operates on the fundamental principle that ecosystems will tend to maximize the leaf area that can be supported at a site by available soil moisture or energy. The model also has been adapted to the landscape scale, providing a single ecosystem simulation paradigm applicable to a full range of scales from landscape (watershed) to global.

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• MAPSS is fully integrated with a continentally calibrated hydrologic model, linking hydrologic and vegetation processes, and has been validated over the globe.

Conceptual Framework

The conceptual framework for the MAPSS model is that vegetation distributions are, in general, constrained either by the availability of water in relation to transpirational demands, or the availability of energy for growth (Neilson and Wullstein 1983, Neilson et al. 1989, Stephenson 1990, Woodward, 1987). In temperate latitudes, water is the primary constraint, whereas at high latitudes, energy is the primary constraint (exceptions occur, of course, particularly in some areas that may be nutrient limited.) The energy constraints on vegetation type and leaf area index (LAI) are currently modeled in MAPSS by using a growing-degree-day algorithm as a surrogate for net radiation (e.g., Botkin et al. 1972, Shugart 1984).

The model calculates the leaf area index of both woody and grass life forms (trees or shrubs, but not both) in competition for both light and water, while maintaining a site water balance consistent with observed runoff (Neilson 1995). Water in the surface soil layer is apportioned to the two life forms in relation to their relative LAIs and stomatal conductance, i.e., canopy conductance, while woody vegetation alone has access to deeper soil water.

Biomes are not explicitly simulated in MAPSS; rather, the model simulates the distribution of vegetation life forms (trees, shrubs, grass), the dominant leaf form (broadleaf, needleleaf), leaf phenology (evergreen, deciduous), thermal tolerances and vegetation density (LAI). These characteristics are then combined into a vegetation classification consistent with the biome level (Neilson 1995).

Model Workings

The principal features of the MAPSS model include algorithms for:

1. Formation and melt of snow
2. Interception and evaporation of rainfall
3. Infiltration and percolation of rainfall and snowmelt through three soil layers
4. Runoff
5. Transpiration based on LAI and stomatal conductance
6. Biophysical "rules" for leaf form and phenology
7. Iterative calculation of LAI
8. Assembly rules for vegetation classification

Infiltration, and saturated and unsaturated percolation, are represented by an analog of Darcy's Law specifically calibrated to a monthly time step. Water-holding capacities at saturation, field potential, and wilting point are calculated from soil texture, as are soil water-retention curves (Saxton et al. 1986). Transpiration is driven by potential evapotranspiration (PET) as calculated by an aerodynamic turbulent transfer model based upon Brutsaert's (1982) ABL model (Marks 1990; Marks and Dozier, 1992), with actual transpiration being constrained by soil area, and stomatal conductance. Stomatal conductance is modulated as a function of PET (a surrogate for vapor pressure deficit) and soil water content (Denmead and Shaw 1962). Canopy conductance (i.e., actual transpiration) is an exponential function of LAI, modulated by stomatal conductance.

Elevated CO₂ can affect vegetation responses to climate change through changes in carbon fixation and water use efficiency (WUE, carbon atoms fixed per water molecule transpired). The WUE effect is often noted as a reduction in stomatal conductance (Eamus 1991). Because MAPSS simulates carbon indirectly (through LAI), a WUE effect can be imparted directly as a change in stomatal conductance, which results in increased LAI (carbon stocks) and usually a small decrease in transpiration per unit land area.

MAPSS has been implemented at a 10-km resolution over the continental United States and at a 0.5^o resolution globally (Neilson 1993, 1995; Neilson and Marks 1994). The model has been partially validated within the United States and globally with respect to simulated vegetation distribution, LAI, and runoff (Neilson 1993, 1995; Neilson and Marks 1994). MAPSS has also been implemented at the watershed scale (MAPSS-W, 200-m resolution) via a partial hybridization with a distributed catchment hydrology model (Daly 1994, Wigmosta et al. 1994).

MAPSS findings

Model Output

References

US Forest Service - Pacific Northwest Research Station, Mapped Atmosphere-Plant-Soil System Study
Last Modified: Monday, 17 March 2008 at 13:07:01 EDT