1. 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
Photo: Sonoran Desert, from Wikipedia
(Presented by Koichi Sakaguchi)

2. 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

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

4. 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

5. 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 ]

6. 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

7. 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

8. Results
400-Yr Simulation of new DGVM with shrub submodel

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

10. 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

11. 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)

12. 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 mm
Tree coverage peak: for 1500 mm or higher

13. 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