LoTEC Carbon Model

LoTEC

Authors

Anthony W. King and W. Mac Post III, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831

Links

http://www.ccs.ornl.gov/eagle/Carbon.html

http://stonesoup.esd.ornl.gov/apps/gtec/

Description

LoTEC is a mechanistic soil-plant-atmosphere model of ecosystem carbon storage and CO₂ and H₂O flux. Canopy photosynthesis is described by a "Bigleaf" implementation of either a C3 or C4 biochemical model of photosynthesis combined with a sub-model of stomatal conductance. Maintenance respiration for four plant compartments is a function of tissue nitrogen concentration and temperature, while growth respiration is proportional to the change in compartment size.

Litter produced in the turnover of vegetation compartments is passed to the decomposable and resistant plant material compartments of the soil module. Decomposition of litter and soil organic carbon and the associated release of CO₂ in heterotrophic respiration are modeled with a daily time step implementation of the Rothamsted soil carbon turnover model. Incoming organic matter passes through the decomposable and resistant plant material compartments once. Both decomposable and resistant materials decompose to CO₂ (which is lost to the atmosphere), microbial biomass, and humified organic matter. When microbial biomass and humus decomposes, CO₂ is released and microbial biomass and humus are formed. The microbial biomass and humus are subject to further decomposition. The soil is also assumed to contain a small amount of inert organic matter. The characteristic decay rates of each soil compartment are modified by environmental factors of temperature and soil moisture deficit. The difference between net primary production (NPP) and the CO₂-carbon released during decomposition is net ecosystem production (NEP) and represents any change in total ecosystem carbon storage and net carbon exchange with the atmosphere.

Model scale and resolution

Canopy photosynthesis and maintenance respiration are calculated hourly. Carbon allocation, growth, and growth respiration are calculated daily. Litter and soil carbon dynamics are simulated with a monthy time step. The spatial scale of the model is a half-degree grid cell. The model uses a scale-translation factor to scale from the local photosynthetic response to elevated CO₂ to the response of ecosystem NPP at the spatial scale of half-degree grid cells (King et al. 1997).

A complete run of these models generally requires three phases of simulation:

1. A spin-up phase to obtain initial conditions (> 2,000 simulation years), which is run once for every production run that uses the same initial and boundary conditions.
2. Historical time-dependent (100 simulation years, 1895 to 1993)
3. Future scenarios (30 - 100 simulation years, 1994 onward, provided by atmospheric general circulation model (AGCM) output)

This model differs from others in that it uses the empirical Miami model, including a factor that represents the response to changing CO₂, as a basis for estimating steady-state NPP instead of the Farquahar model or other process-based models. Because rubisco-limited photosynthesis is not simulated, use of LoTEC is best justified when light is the limiting factor (King et al. 1997).

Precursors

GTEC - global model contains 21,600 1-degree terrestrial cells. The carbon dynamics of each vegetated

land cell (1-degree latitude x 1-degree longitude resolution) is described by a mechanistic soil-plant-atmosphere model (LoTEC) of ecosystem carbon storage and CO2 and H2O flux. Each grid cell is assigned to one of 15 ecosystem types and one of 105 soil types.

vGTEC - continental U.S. model contains 3160 terrestrial 0.5 degree cells. The carbon dynamics of each vegetated land cell is described by a mechanistic soil-plant-atmosphere model (LoTEC) of ecosystem carbon storage and CO₂ and H₂O flux. Vegetation, soil and climate data are provided by the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP) to facilitate comparison between terrestrial carbon cycle model results.

Inputs

Climate variables

1. Solar radiation
2. Air temperature
3. Precipitation

Site variables

1. Changes in leaf area index derived from AVHRR/NDVI data determine canopy dynamics and phenology.
2. Area of each of 31 ecosystem types (million sq. km)
3. The partitioning coefficient for leaves in each ecosystem
4. The partitioning coefficient for branches in each ecosystem
5. The partitioning coefficient for stems in each ecosystem
6. The partitioning coefficient for roots in each ecosystem
7. Average life span for leaves in each ecosystem (y)
8. Average life span for branches in each ecosystem (y)
9. Average life span for stems in each ecosystem (y)
10. Average life span for roots in each ecosystem (y)
11. Rooting depth in each ecosystem (m)
12. Classification into one of 105 soil types

Testing and validation

References

King, A.W., W.M. Post, S.D. Wullschleger. 1997. The potential response of terrestrial carbon storage to changes in climate and atmospheric CO2. Climate Change 35: 199-227.

Post, W. M., A. W. King, and S. D. Wullschleger 1997. Historical variations in terrestrial biospheric carbon storage. Global Biogeochemical Cycles 11:99--109.

Post, W. M., King, A. W., and S. D. Wullschleger 1996. Soil organic matter models and global estimates of soil organic carbon. pp. 201-222. IN (P. Smith, J. Smith and D. Powlson, eds.) Evaluation of Soil Organic Matter Models Using Existing Long-Term Datasets. Springer-Verlag, Berlin.

Post, W. M., A. W. King, S. D. Wullschleger, and F. Hoffman 1997. Historical Variations in Terrestrial Biospheric Carbon Storage. DOE Research Summary, No. 34, June 1997. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory. Oak Ridge, Tennessee.