The seasonal exchange of carbon dioxide between the atmosphere and the terrestrial biosphere : extrapolation from site-specific models to regional models

Ecological models of the seasonal exchange of carbon dioxide (CO₂) between the atmosphere and the terrestrial biosphere are needed in the study of changes in atmospheric CO₂ concentration. In response to this need, a set of site-specific models of seasonal terrestrial carbon dynamics was assembled from open-literature sources. The collection was chosen as a base for the development of biome-level models for each of the earth's principal terrestrial biomes or vegetation complexes. The primary disadvantage of this approach is the problem of extrapolating the site-specific models across large regions having considerable biotic, climatic, and edaphic heterogeneity. Two methods of extrapolation were tested.

The first approach was a simple extrapolation that assumed relative within-biome homogeneity, and generated CO₂ source functions that differed dramatically from published estimates of CO₂ exchange. The differences were so great that the simple extrapolation was rejected as a means of incorporating site-specific models in a global CO₂ source function.

The second extrapolation explicitly incorporated within-biome variability in the abiotic variables that drive seasonal biosphere-atmosphere CO₂ exchange. Simulated site-specific CO₂ dynamics were treated as a function of multiple random variables (i.e., the model driving variables). The predicted regional CO₂ exchange is the computed expected value of simulated site-specific exchanges for that region times the area of the region. The extrapolation was tested for the circumglobal latitude belt between 64°N and 90°N. The test involved the regional extrapolation of a tundra and a coniferous forest carbon exchange model. Comparisons between the CO₂ exchange estimated by extrapolation and published estimates of regional exchange for the latitude belt support the appropriateness of extrapolation by expected value. Extrapolation by mathematical expectation is a promising technique for extrapolating from site-specific models to regional and biome-level models.