Ecosystem Modeling and Flows of Energy and Matter in Grasslands

Although the first interest in ecosystem modeling peaked in the 1970s with the International Biological Program (IBP), it is now having a rebirth with the recent interest in predicting ecosystem effects of global change. In the 1960s and 1970s 'big biology' projects were initiated by G. M. Van Dyne to study ecosystems including grasslands. One of the main problems of this big biology project was the attempt to model everything without a clear and focused research question. At the end, the resulting models were nearly as complex as nature itself and they could not be properly analyzed and thus understood. The objective of the IBP grassland simulation model was to simulate biomass dynamics in a variety of grassland types and the response of the system to irrigation, fertilization, and cattle grazing.

The model comprises several submodels, that is, abiotic, producers, mammals, grasshoppers, decomposers, nitrogen, and phosphorus. Some of the submodels originally designed to be incorporated into the ELM model never reached this objective but developed a life of their own.

Plant growth and production

A number of models, reviewed in detail by J. D. Hanson and colleagues in the mid-1980s, simulated/predicted plant growth and production of grassland ecosystems. For example, the AFRICA model included processes like shoot growth, tillering, root growth, photosynthesis, and nitrogen uptake for single plants. The aim of AFRICA was to model primary production of perennial graminoids and it unites physiological processes and morphometric traits. In a study in the late 1990s, M. B. Coughenour and D.- X. Chen linked models of photosynthesis, plant growth, and biophysical processes with models that simulate water, nutrient, and carbon flows through plant-soil ecosystems. The linked ecosystem model was applied to examine ecosystem-level responses to CO2, temperature, precipitation, and global-warming scenarios in grasslands of Colorado and Kansas (USA) and Kenya. Using similar ecosystem model approaches, several models have been developed for semiarid perennial grasslands that allow multiyear simulations of plant growth patterns by accounting for carbohydrate storage in root systems and further translocation to aboveground regrowth.

Biochemistry models

Other models analyzed the soil organic matter dynamics in response to changes in management and climate. These models described the flow of energy and matter (Conceptual Diagrams and Flow Diagrams) in form of balance equations. Following the approach of H. T. Odum, the focus of these models was not on biotic interactions between species but rather on the flows of energy and nutrients, treating plants basically as composers and decomposers.

The most prominent model of this type is the 'CENTURY model' (Figure 4) developed by W. J. Parton in the 1980s. CENTURY is a model of terrestrial biochemistry of grasslands based on the relationships between climate, human management (fire, grazing), soil properties, plant productivity, and decomposition. Studies performed with this model include efforts to link models describing plant and soil responses to the large-scale modeling of global change effects. The model is intended as a generic model whose basic balances of the different flows in grasslands can be calibrated to specific systems. In the 1990s, Parton and colleagues parametrized CENTURY for the world's major grassland types to predict the biomass and soil organic matter dynamics of the grassland biome worldwide.

In the late 1990s, the performance of CENTURY was assessed by T. G. Gilmanov and colleagues using long-term

Flowchart Grassland Ecosystem

Figure 4 Flowchart of the CENTURY model. Supplement to Metherell et al. 1993. Century Manual. CENTURY Soil organic matter model environment. Technical documentation. Agroecosystem version 4.0. Great Plains System Research Unit Technical Report No.4. USDA-ARS, Fort Collins, Colorado, USA. Freely available at http://www.nrel.colostate.edu/projects/century/Century_Slides.ppt

Figure 4 Flowchart of the CENTURY model. Supplement to Metherell et al. 1993. Century Manual. CENTURY Soil organic matter model environment. Technical documentation. Agroecosystem version 4.0. Great Plains System Research Unit Technical Report No.4. USDA-ARS, Fort Collins, Colorado, USA. Freely available at http://www.nrel.colostate.edu/projects/century/Century_Slides.ppt

data collected under IBP and at research stations within the former USSR. They found that CENTURY reproduced the seasonal, mid-term, and, in some cases, long-term dynamics in aboveground biomass in a wide range of grassland ecosystems. Model discrepancies were attributed to changes in species composition and short-term responses to intermittent rainfall that are missed by the monthly timestep of the model. In another application, K. A. Hibbard and colleagues assessed in the 2000s the magnitude of changes in plant and soil carbon and nitrogen pools in a subtropical landscape undergoing succession from grassland to thorn woodland in southern Texas (USA). They linked CENTURY to a transition matrix model and parametrized grass and tree production submodels of CENTURY with field data. The Markov transition matrix model simulated the displacement of grassland communities under land-use practices (heavy livestock grazing, no fire) and climate events.

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Responses

  • omar
    What is the flow matter/ energy in grasslands?
    6 years ago

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