MAPSS (Mapped Atmosphere-Plant-Soil System) is a static biogeography model that projects potential vegetation distribution and hydrological flows on a grid. It simulates type of vegetation and density for all upland vegetation from deserts to wet forests. It uses long term, average monthly climate data (mean monthly temperature, vapor pressure, wind speed, and precipitation) as well as soils information (texture, depth). MAPSS has been used widely for various climate change assessments including the 2000 National Assessment Synthesis Team's report.
MAPSS assumes that vegetation distribution is constrained either by the availability of water or of energy for growth. In temperate latitudes, water is the primary constraint while at high latitudes energy is the primary constraint (exceptions occur particularly in areas that are nutrient limited). The energy constraints on vegetation type and leaf area index (LAI) are simulated by calculating growing degree days as a surrogate for net radiation.
The model simulates infiltration, saturated, and unsaturated percolation. Water holding capacities at saturation, field potential, and wilting point are calculated from soil texture, as are soil water retention curves. Transpiration is driven by potential evapotranspiration (PET) while actual transpiration is being constrained by soil water, leaf area and stomatal conductance. Stomatal conductance varies as a function of PET (a surrogate for vapor pressure deficit) and soil water content. Canopy conductance (i.e., actual transpiration) is an exponential function of LAI, modulated by stomatal conductance.
The model calculates the leaf area index (LAIs) for both woody (either trees or shrubs) and grass lifeforms competing for 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 lifeforms in relation to their relative LAIs and stomatal conductance, i.e., canopy conductance, while woody vegetation alone has access to deeper soil water.
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 interpreted as a reduction in stomatal conductance. Since MAPSS simulates carbon/biomass indirectly (through LAI), a WUE effect can be imparted directly as a change in stomatal conductance, which results in increased LAI and usually a decrease in transpiration per unit land area.
Based on a set of climatic thresholds, the model defines the following plant functional types:
- Evergreen needleleaf or broadleaf trees/shrubs
- Deciduous broadleaf or needleleaf trees/shrubs
- C3 and C4 grasses
The model uses thresholds of LAI and climatic zone thresholds to identify potential vegetation types. The model has been used at various spatial scales (10x10 km over the continental U.S. and 50x50km globally) determined by the spatial grain of the available climate inputs. It was partially validated within the U.S. for vegetation distribution, LAI, and runoff.
Several important findings emerged from running the MAPSS model with future climate scenarios:
- Global patterns show a shift to the north of vegetation zones, some dieback of boreal forests, particularly along edges of interior grasslands, and in the USA large changes in eastern forest types are projected with increased fire vulnerability.
- Early stages of global warming could see increases in productivity and density of forests worldwide -"green up phase"- as increased carbon dioxide acts as a fertilizer. Continued elevated temperatures, however could strain water resources, in time producing drought-induced stress and broad-scale dieback - "brown down"- , as wildfire increases.
- Increased carbon sequestration from more productive vegetation growth under warmer conditions could be offset by pulses of carbon release from increased wildfire.
- Shifting distribution and altered productivity of forests would affect regional as well as global forest markets.
- Long-term forest management plans are based on assumptions of a stable climate and fixed tree growth rates. Expectations from these plans need a significant overhaul to accommodate the range of possibilities (increased fire danger, pest outbreaks, drought stress) that come with the projections of future climate impacts.
