Department of Atmospheric Sciences Development of the Shrub Submodel for the Community Land Model-Dynamic Global Vegetation Model (CLM-DGVM) Xubin Zeng Xiaodong Zeng Mike Barlage Department of Atmospheric Sciences University of Arizona Tucson, AZ 85721 xubin@atmo.arizona.edu Photo: Sonoran Desert, from Wikipedia http://en.wikipedia.org/wiki/Sonoran_Desert (Presented by Koichi Sakaguchi)

Motivation Exclusion of Dynamic Global Vegetation Model (DGVM) and associated carbon cycle is recognized as one of the main deficiencies of IPCC AR4 model simulations; Most of the existing DGVMs do not include shrubs or do not effectively distinguish shrubs from grasses; Shrub is a major plant type in dry regions, which represents a large fraction of the global land Shrubs are significantly different from trees and grasses in its physical characteristics and productivity

Shrub distribution Wide distribution of shrub (from MODIS land cover product) is not modeled by NCAR CLM-DGVM

Shrub productivity Default DGVM photosynthesis over SW U.S. Shrub photosynthesis is too small to establish shrub Plant Functional Type (PFT) MODIS-based photosynthesis (from Running et al.) Shrub photosynthesis is close to that of grass BDT: broadleaf deciduous tree NET: needleleaf evergreen tree

Shrub Submodel Drought-tolerance - use of different soil moisture stress function for shrub photosynthesis computation Phenology - raingreen for shrubs; no air temperature limitation for establishment Appropriate morphology parameters Tree/grass/shrub hierarchy for light competition Additional revisions: Consistent treatment of fractional vegetation coverage in CLM-DGVM: Morphology parameters:Lower leaf area (kla:sa) for a given sapwood size, lower max crown area (Camax), lower height and crown area for a given stem diameter (kallm1,2). [ FPC: foliar projective cover < PCA: plant crown area ]

Results Default v.s. shrub submodel in dry regions % plant cover soil column water (mm) Shrubs can exist when grasses or trees cannot establish in the default DGVM Shrubs occupy the bare area and slightly reduce grass area from the default DGVM Solid: New Dashed: Default Red: Grass Green: Shrub Blue: Tree Brown: Bare New Mexico 196 mm/year Arizona 355 mm/year

Results Shrubs occupy the bare ground fraction from the default DGVM Default v.s Shrub submodel in wetter regions Shrubs occupy the bare ground fraction from the default DGVM Shrubs do not establish in wet regions

Results 400-Yr Simulation of new DGVM with shrub submodel

Results Difference in Plant cover: New -- Control Panel (c): In the new model, shrubs cover part of the bare over arid regions

Evaluation General agreement on shrub coverage New DGVM shrub submodel v.s MODIS New DGVM shrub cover Quantitative comparisons by combining the MODIS land cover and Fractional Vegetation Cover (FVC) MODIS shrub cover (aggregated to model grids) General agreement on shrub coverage

Evaluation Dependence of PFT coverage on annual precipitation (60°N ~ 60°S) New shrub submodel Default DGVM With the MODIS land cover data only, shrub cover is estimated too high (shrub FVC usually < 100%) % cover With the new shrub submodel, Shrub cover peak agrees with MODIS land cover + FCV data set Bare soil cover agrees better with MODIS land cover + FCV data set MODIS Land cover + FVC MODIS Land cover only % cover Annual Precipitation (mm) Annual Precipitation (mm)

Summary Developed a shrub submodel for the DGVM for the global competition of trees, grass, and shrubs Crucial to use MODIS land cover product combined with FVC data set for the DGVM model evaluation (particularly for shrubs) Shrubs grow primarily by reducing the bare soil coverage, and to a lesser degree, by decreasing the grass coverage Shrub coverage peak: around annual precipitation of 300 mm, Grass coverage peak: over a broad range from 400-1100 mm Tree coverage peak: for 1500 mm or higher

Shrub submodel w/ DGVM-CLM3.0 Boreal shrub submodel Further improvement in global shrub cover MODIS shrub cover >20% Shrub submodel w/ DGVM-CLM3.0 DGVM-BGC w/CLM3.5 Global shrub cover DGVM-BGC w/CLM3.5: 20 years average vegetation cover (year 2181 to 2000) Boreal region plant coverage From S.Levis / NCAR