Growing temperate shrubs over arid and semiarid regions in the Community Land Model-Dynamic Global Vegetation Model

Arid and semiarid regions represent a large fraction of global land, but most of the existing dynamic global vegetation models (DGVMs) 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 Moderate Resolution Imaging Spectroradiometer (MODIS) data. The simulated shrub coverage reaches its peak around annual precipitation (P_ann) of 300 mm, the grass coverage reaches its peak over a broad range of P_ann(from 400 to 1100 mm), and the tree coverage reaches its peak for P_ann = 1500 mm or higher, all in good agreement with MODIS data.