Model Description

Many direct and indirect interactions among interacting ecosystem processes result in responses to changing environmental conditions at many different scales of time and space. Large-scale responses over several years pose the greatest challenges to elucidate and quantify. We used the global terrestrial ecosystem carbon (GTEC) model to analyze terrestrial carbon storage and exchange with the atmosphere over the 1930-2100 period. In this model, the carbon dynamics of each 1° latitude by 1° longitude vegetated land cell are described by a mechanistic soil-plant-atmosphere model of ecosystem carbon cycling and exchange.

The GTEC model has implemented a big-leaf version of the so-called Farquhar model for C3 plants (Farquhar et al., 1980) and a similar model for C4 plants (Collatz et al., 1992). Photosynthesis is coupled to a description of the dependence of stomatal conductance on assimilation rate, temperature, and available soil moisture to form a leaf productivity model. Autotrophic maintenance respiration is a function of tissue nitrogen concentration and temperature, while growth respiration is proportional to the change in biomass.

Soil moisture is calculated using a multilayer soil with simple piston flow dynamics. Canopy photosynthesis and maintenance respiration are calculated hourly, while carbon allocation, growth, and growth respiration and soil water balance are calculated daily. Carbon in dead organic matter is partitioned as in the Rothamsted (RothC) model (Jenkinson, 1990) with litter inputs assigned to decomposable and resistant plant material compartments. The model is thus capable of responding to interactions among climate, rising atmospheric CO2 concentration, soil moisture, and solar radiation. This detailed physiological model is considerably more sensitive to rising atmospheric CO2 concentration than most bio-geochemical terrestrial ecosystem models.

The model requires many inputs and parameters for a simulation. Some inputs remain constant during the simulations and also between different simulations. These include vegetation type for each grid cell (Table 7.1), and associated photosynthesis parameters, respiration coefficients, partitioning and turnover parameters for plant tissue allocation and longevity, and litter quality characteristics. Soil type and associated hydrological parameters are also required.

Initial carbon pools for plant and soil C compartments must also be supplied. We supplied this information by spinning the model up using climate data for the past 70 years. The model was run from very small pool sizes using yearly climate data chosen at random from the historical record until all the model compartments remained more or less constant without a strong time-dependent trend. Using the

Table 7.1 Ecosystem Types in Global Terrestrial Ecosystem Carbon Model

Global Area

Terrestrial Ecosystem Type (106 ha)

Broadleaf evergreen forest 1342

Broadleaf deciduous forest and woodland 330

Mixed coniferous and broadleaf deciduous forest and 660 woodland

Coniferous forest and woodland 1298

High-latitude deciduous forest and woodland 575

C4 Wooded grassland 1710

C4 Grassland 893

Shrubs and bare ground 1100

Tundra 710

Desert, bare ground 1687

Cultivation 1328

C3 Wooded grassland 460

C3 Grassland 1152

Notes: Each grid square is assigned to an ecosystem type. The ecosystem types are used to assign photosynthesis, respiration, allocation, and tissue turnover parameters. Ecosystem type and associated areas may be used to sum geographically extensive fluxes and pools by report to ecosystem type as is done in Figures 7.1, 7.2, and 7.3.

simulated initial conditions, the model was then run using inputs that varied during the time-dependent simulations that included atmospheric CO2 concentration and the climate variables temperature, precipitation, solar radiation, and relative humidity.

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