MC1 Dynamic Vegetation Model

MC1 Dynamic Vegetation Model


Dominique Bachelet, Ph.D.
Senior Climate Change Scientist, Team Lead- Global Change
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MC1 is a  widely used dynamic global vegetation model (DGVM) that has been used to simulate potential vegetation shifts in California and Alaska, all of North America, and over the entire globe under various climate change scenarios. MC1 was originally designed by a team of OSU scientist  (Jim Lenihan, Chris Daly, and Dominique Bachelet) in collaboration with the CENTURY group in the Natural Resources Ecology Laboratory at Colorado State, with USFS-PNW financial support within Ron Neilson's MAPPS team. Several programmers (e.g. Jesse Chaney, David Conklin) contributed to the various version of MC1 used in the published reports. It is currently maintained and documented by a group of Conservation Biology Institute scientists in close collaboration with a newly established group of MC1 developed communicating improvement to the code to the overall MC1 users community, and making available the latest version of the code on the OSU subversion repository.

MC1 is a model that simulates vegetation types, ecosystem fluxes of carbon, nitrogen, and water, as well as wildfire occurrence and impacts. MC1 is routinely implemented on spatial data grids of varying resolution (i.e., grid cell sizes ranging from 900 m2 to 2500 km2). The model reads climate data at a monthly time step and calls interacting modules that simulate:

  • biogeography
  • biogeochemistry
  • fire disturbance

For a complete list of MC1 variables, here is an excel spreadsheet.

The biogeography module simulates the potential life-form mixture of evergreen needleleaf, evergreen broadleaf, and deciduous broadleaf trees, as well as C3 and C4 grasses. The tree lifeform mixture is determined at each annual time-step as a function of annual minimum temperature and growing season precipitation. The C3/C4 grass mixture is determined by reference to their relative potential productivity during the three warmest consecutive months. The tree and grass lifeform mixtures together with growing degree-day sums and biomass simulated by the biogeochemistry module are used to determine which possible potential vegetation types (~20) occurs at each grid cell each year.

The biogeochemistry module is a modified version of the CENTURY model, which simulates plant productivity, organic matter decomposition, and water and nutrient cycling once the vegetation type has been determined by the biogeography module. Plant growth is determined by empirical functions of temperature, moisture, and nutrient availability which decrement set values of maximum potential productivity. Plant growth is generally assumed not to be limited by nutrient availability. The direct effect of an increase in atmospheric carbon dioxide (CO2) is simulated using a beta factor that increases maximum potential productivity and reduces the moisture constraint on productivity. Grasses compete with woody plants for soil moisture and nutrients in the upper soil layers where both are rooted, while the deeper-rooted woody plants have sole access to resources in deeper layers. The growth of grasses may be limited by reduced light levels in the shade cast by woody plants. The values of model parameters that control woody plant and grass growth are adjusted with shifts in the lifeform mixture determined annually by the biogeography module. Plant parts simulated include leaves, fine branches, coarse branches and boles, fine and coarse roots. Litter pools include surface litter and standing dead, belowground litter and 3 soil carbon organic matter pools with increasing degrees of resistance to decomposition based on their chemical composition.

The fire module simulates the occurrence, behavior, and effects of fire. It simulates the behavior of a fire event in terms of the potential rate of fire spread, fireline intensity, and the transition from surface to crown fire. Several measurements of the fuel bed are required for simulating fire behavior, and they are estimated by the fire module using information provided by the other two MC1 modules. The current lifeform mixture is used by the fire module to select factors that allocate live and dead biomass into different classes of live and dead fuels. The moisture content of the two live fuel classes (grasses and leaves/twigs of woody plants) are estimated from moisture at different depths in the soil provided by the biogeochemical module. Dead fuel moisture content is estimated from climatic inputs to MC1 using different functions for each of four dead fuel size-classes. Fire events are triggered in the model when the moisture content of coarse woody fuels, and the flammability of fine fuels all meet set thresholds. Sources of ignition (e.g., lightning or anthropogenic) are assumed to be always available. Area burned is not simulated explicitly as fire spread within a given cell. Instead, the fraction of a cell burned by a fire event is estimated as a function of set minimum and maximum fire return intervals for the dynamically-simulated vegetation type and the number of years since a simulated fire event. Simulated fire effects include consumption and mortality of dead and live vegetation carbon, which is removed from (or transferred to) the appropriate carbon pools in the biogeochemistry module. Live carbon mortality and consumption are simulated as a function of fireline intensity and the tree canopy structure, and dead biomass consumption is simulated using functions of fuel moisture that are fuel-class specific.

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