pends on the intensity and length of the agricultural activity and the amount of soil organic matter on the site at the time of abandonment. To simulate the biogeochemistry of secondary vegetation, models must capture patterns of plant growth during secondary succession. These patterns depend substantially on the status of nutrient pools inherited from the previous stage. The changes in hydrology also need to be considered, since plants that experience water stress will alter the allocation of carbon (e.g., to allocate more carbon to roots). Processes such as reproduction, establishment, and light competition have been added to such models and interact with the carbon, nitrogen, and water cycles. Disturbance regimes such as fire are also incorporated into the models, and these disturbances (and potential changes in their frequency) are essential to include in order to successfully treat competitive dynamics and hence future patterns of ecosystem distribution. It should also be noted that these forcing terms themselves may be altered by the changes that result from changes in the terrestrial system. Finally, the issues of successional dynamics, which result from extending the temporal scale, also force more careful consideration of spatial scaling.

Research Priorities

Immediate challenges that confront models of the terrestrial-atmosphere system include exchanges of carbon and water between the atmosphere and land and the terrestrial sources and sinks of trace gases. An overarching grand challenge is to provide insight into the dynamics of a biosphere subjected to multiple stresses, which after all is the actual case that we confront (see Chapter 2). Hence, the development of dynamic vegetation models is, as stated, of central importance.

Carbon

In the past two decades the significant influence of the terrestrial biosphere on the global carbon balance and hence on the problem of timing and magnitude of possible climate change has been recognized. 20 Much of the remaining uncertainty in our understanding of the carbon cycle centers on the role of terrestrial ecosystems, in which at least two factors govern the level of carbon storage. First and most obvious is the anthropogenic alteration of the Earth's surface—for example, through the conversion of forest to agriculture—which can result in a net release of CO2 to the atmosphere. Second, and more subtle, are the possible changes in net ecosystem production (and hence carbon storage) resulting from changes in atmospheric CO2, other global biogeochemical cycles (particularly nitrogen), and/or the physical climate system.

The productivity of the terrestrial biosphere is primarily controlled by the radiation reaching terrestrial ecosystems, the availability of nutrients, and the climatic conditions in which they live, that is, by the conditions under which plants carry out photosynthesis and allocate photosynthates to various structural



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