components. Precipitation and temperature primarily govern the absorption of photosynthetically active radiation and its conversion into dry matter—that is, the net primary productivity (NPP) of the biosphere. Nitrogen and changes in its availability, as well as changes in other nutrient cycles, are the key biogeochemical controls on productivity.

At present, several rather complex models are being developed to account for the ecophysiological and biophysical processes that determine the spatial and temporal features of NPP.21 Their goal is to provide a prognostic capability. The major modeled processes are photosynthesis, growth and maintenance respiration, evapotranspiration, uptake and release of nitrogen, allocation of photosynthates to the various parts of the plant, litter production and decomposition, and phenological development. Some models focus on detailed mechanistic relationships for some processes (e.g., water and CO2 fluxes and the nitrogen cycle), while others rely on simple empirical relationships or satellite observations to derive or constrain important features (e.g., canopy characteristics and phenology).

The challenge is not simply to calculate NPP but rather to develop coherent explanations for past changes in the total carbon fluxes and/or storage, to test hypotheses about the underlying causes of these changes, and to establish the capability for estimating future changes. It is now becoming evident that models of the terrestrial carbon cycle and of terrestrial ecosystem processes in general will play an overriding role in addressing many of the issues posed by global environmental change. The question of climate change is a case in point. Describing, characterizing, and eventually understanding and predicting the spatial patterns of changes in terrestrial carbon storage and associated fluxes are essential to the assessments undertaken by the Intergovernmental Panel on Climate Change (IPCC22).23 These patterns and allied issues lie at the heart of analyzing any atmospheric CO2 stabilization policy.24 Moreover, these issues must be far better resolved if there is to be an adequate verification scheme to confirm national performance in meeting targets for CO2 emissions. From a broader perspective, the prognostic models of terrestrial carbon cycle and terrestrial ecosystem processes are central for any consideration of the effects of environmental change and analysis of mitigation strategies; moreover, these demands will become even more significant if countries begin to adopt carbon emission targets.25 Finally, while progress will be made (and is needed) on modeling terrestrial processes, more integrative studies also are needed wherein terrestrial systems are coupled to models of the physical atmosphere and eventually to the chemical atmosphere as well.26 Tying in the human component is clearly important.27

Soil Moisture

Modeling studies of extreme (theoretical) deforestation in the Amazon region have indicated a severe weakening of the water cycle attributable solely to



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