Parton gave an overview of the different types of vegetation models, presented results from recent model comparisons and other research activities, and discussed how land use/land cover data are being incorporated into ecosystem models.

The three major types of vegetation models include vegetation distribution models, plant physiology models and ecosystem models. Vegetation distribution models fall into two general types: climate-based models and optimum water use models. The climatic vegetation distribution models are based on empirical correlation of patterns in climatic variables (e.g., precipitation, minimum and maximum air temperature, potential evapotranspiration [PET] and the ratio of rainfall to PET) to observed vegetation distributions. Optimum water use models calculate the optimum leaf area index (LAI) as a function of the environmental conditions. These models assume that plant LAI will be optimized as a function of water stress with lower LAI values for arid environments. The optimum water use models also use climatic factors such as maximum and minimum air temperatures to classify vegetation distributions.

Detailed plant growth models have been developed for a large number of crops ( e.g., corn, wheat, alfalfa) and natural systems (grasslands, coniferous forests, deciduous forests, and savannas). These models are designed to simulate the mechanisms that control plant growth as a function of the environmental conditions. The models use mechanistic representations of plant processes such as photosynthesis, root and shoot respiration, carbon allocation, and growth of the different plant parts. The driving variables for the models are daily or hourly microclimatic variables such as soil and air temperature, solar radiation, relative humidity, and wind speed. These models are primarily used at selected sites where detailed soils and microclimatic data are available and are difficult to use at regional scales because of the lack of sufficient data to run the models. Many of the detailed plant growth models do not consider the limitations of nutrients on plant growth.

Ecosystem models have been developed for all of the major natural ecosystems in the world and many of the major cropping systems. Ecosystem models include plant growth, nutrient cycling, soil organic matter cycling and water and temperature submodels. They generally represent all of the major components of the systems that are being modeled and use more simplified representations as compared to the detailed mechanistic process models. Most ecosystem models are designed to run at different sites and use data available at the regional and global scales. Many of the ecosystem models are now being combined with vegetation distribution models. The combined models will be able to simulate response of ecosystems variables and vegetation distributions to environmental changes. There is substantial interest in combining ecosystem biogeochemistry models with atmospheric circulation models. Figure 13.1 shows the major links of ecosystem models to atmospheric circulation models. The major goal of the linked biogeochemistry and general circulation models is to simulate the interactive impact of climate and vegetation changes on trace gas production (CO₂, CH₄ and N2_O) and atmospheric circulation patterns.

Model Comparisons

Recently there has been substantial interest in the comparison of process and ecosystem models. Some of the reasons for model comparisons include: 1) improving scientific understanding, 2) providing error bars on climatic change predictions, 3) development of community models, and 4) selection of the specific models for environmental assessment activities. Parton presented results from process-oriented decomposition model comparisons and comparison of ecosystem models used in climatic change assessment. The major goal of process-oriented model comparisons is to improve scientific understanding about specific processes. A decomposition model comparison was set up to compare the processes that control decomposition of surface litter. The models used a common data set to test and parameterize the models and an independent data set was used to validate and compare model predictions. The model comparison showed that all of the models simulated observed carbon dynamics well. None of the models, however, simulated the nitrogen dynamics with good accuracy, and this illustrates the lack of understanding about the dynamics of N in surface litter.

Parton discussed the results from ecosystem model comparisons from two projects which had the objective of using model comparisons to provide error bars on environmental change predictions. The forest model comparison was designed to compare eight forest growth models for coniferous forest sites in Sweden and Australia. A common data set was used to tune the models for the specific sites and then the model predictions of climatic change impacts were compared. There was a formal comparison of the model results with the observed data sets and the climatic change predictions. The results showed that there were large uncertainties in the model responses to changes in temperature, atmospheric CO₂ levels and the interactions between plant and soil systems, and the long-term response to increased atmospheric CO₂ levels.

The VEMAP model comparison considered three biogeochemistry models and three biogeography models. The models were all compared under contemporary conditions for sensitivity to increased CO₂ and climatic change. The models simulated dynamics of all the ecosystems in the U. S. at .5 degree resolution and used the same data bases for current climate, soils, vegetation and climatic change scenarios. The comparison of the biogeography models under contemporary climate shows that the models agree fairly well for the geographic distribution of the major vegetation types. All of the models showed some response to increased atmospheric CO₂ and the combined impact of changing climate and increased CO₂ was to reduce the divergence between the models.

The contemporary climate comparisons of the biogeochemistry models for NPP, total actual evapotranspiration and total storage of C showed substantial agreement among the models. The CENTURY and TEM models showed that increasing temperature resulted in increased decomposition, N availability and higher NPP. For BIOME-BGC, increasing temperature resulted in increased drought stress, decreased NPP and decreased total C storage. The differences among the models are most pronounced for the combined impact of increasing CO₂ and climatic change. With the CENTURY and TEM models, nutrient availability is the primary control on plant production, while BIOME-BGC is less controlled by nutrient dynamics and more controlled by water stress. This reflects the different conceptual frameworks used to develop the models.

Incorporation of Land Use in Ecosystem Models

Most of the global ecosystem models have focused on simulating the dynamics of natural ecosystems. Human land use has substantially altered natural ecosystems in the both developed and less developed counties. Land use practices such as forest clear cutting, planting of crops, and animal grazing have altered many of the natural ecosystems in the world, along with the global carbon, nutrient and water budgets. The CENTURY model (see Figure 13.2) is one of the few global ecosystem models that have been set up to include the impact of land use practices on ecosystem dynamics at the site, regional and global scale. The model has been set up to simulate all of the major land use systems and includes management practices such as cultivation, fertilization, irrigation, and grazing.

Figure 13.3 shows the simulation of winter wheat crop yields from 1920 to 1990 for Weld County in eastern Colorado. The model included the change in cultivation practices, crop varieties, and fertilization levels in order to simulate the changes in crop yields during that time period. This model allows the tracking of long term changes in soil organic matter and nutrient cycling, and projections of the impact of new land use practices such as the Conservation Reserve Program on agroecosystem dynamics. Most of the current global ecosystem models are in the process of being modified to incorporate the effect of land use practices on natural and managed ecosystems.