An overview of three main types of simulation approach (explanatory, abstraction, and estimation) is presented, along with a discussion of their capabilities limitations, and the steps required for their validation. A process model being developed through the Forest Response Program is used as an example. The lower hierarchical level of processes, such as photosynthesis and nutrient allocation, is emphasized in simulation of the activities of the foliar organ. A branch growth module with which the foliar module will be coupled in the system is also a process model, but is under development using the abstraction approach. A stand model has been developed empirically, using the estimation approach. The development of modular simulators of sub-processes, utilizing various of the three developmental approaches, then coupling them is a robust approach to meeting information needs. The resultant aggregate models provide tools with the capabilities of incorporating current scientific knowledge to provide predictions which possess the characteristics of realism, generality and precision.

"EPA/600/D-89/164." Includes bibliographical references. "Presented at the Proceedings of the 2nd US/USSR Symposium Air Pollution Effects on Vegetation, September, 1988, Raleigh, NC." Microfiche.