Figure 3.1 provides a general framework for modeling carbon flow through ecosystems and is the basis for organizing the material presented in this chapter. All of the environmental variables associated with modeling the water cycle (Chapter 2) are closely linked with the carbon cycle. Atmospheric carbon dioxide concentrations and the availability of soil nitrogen (N) must also be considered when modeling photosynthesis, carbon allocation, and respiration. Confidence in the reliability of models has greatly increased with the development of an eddy correlation technique that uses fast-response sensors to record the net exchange of CO2 and water vapor from forests and other types of terrestrial
ecosystems.