Arid and semiarid regions represent a large fraction of global land, but most of the existing Dynamic Global Vegetation Models (DGVMs) (e.g., as used in global climate system models) do not include shrubs or do not effectively distinguish shrubs from grasses, and hence cannot realistically reproduce the ecosystem formation and variability there. A shrub submodel is developed here for the Community Land Model-DGVM (CLM-DGVM), and the major revisions include: 1) explicit consideration of shrubs' drought tolerance in the photosynthesis computation; 2) use of appropriate phenology type and morphology parameters for shrubs; 3) consistent treatment of fractional vegetation coverage; 4) development of tree/grass/shrub hierarchy for light competition; and 5) improvement of the allocation scheme to avoid unrealistic behaviors.

Preliminary global offline CLM-DGVM simulations for 400 years show that, with the shrub submodel, the simulated global distribution of temperate shrubs agrees with MODIS land cover type and fractional vegetation cover (FVC) data. The simulated shrub coverage reaches its peak around annual precipitation (Pann) of 300 mm, the grass coverage reaches its peak over a broad range of Pann (from 400-1100 mm), and the tree coverage reaches its peak for Pann=1500 mm or higher, all in good agreement with MODIS data. Furthermore, it is found that, compared with the (correct) combined land cover and FVC data, using MODIS land cover type data alone for DGVM evaluation would yield incorrect results.

With the new shrub submodel, global competition among trees, grasses, and shrubs become possible in the CLM-DGVM which can then be used for climate system modeling studies of energy, water, and carbon cycles as well as dynamic vegetation.

Additional details are provided in our recent paper: Zeng, X.D., X. Zeng, and M. Barlage, 2008: Growing temperate shrubs over arid and semiarid regions in the NCAR Dynamic Global Vegetation Model (CLM-DGVM). Global Biogeochemical Cycles, 22, GB3003, doi: 10.1029/2007GB003014.