A PHYSIOLOGY-BASED GAP MODEL OF FOREST DYNAMICS

A computer model of forest growth and ecosystem processes is presented. The model, HYBRID, is derived from a forest gap model, an ecosystem process model, and a photosynthesis model. In HYBRID individual trees fix and respire carbon, and lose water daily; carbon partitioning occurs at the end of each year. HYBRID obviates many of the limitations of both gap models and ecosystem process models. The growth equations of gap models are replaced with functionally realistic equations and processes for carbon fixation and partitioning, resulting in a dynamic model in which competition and physiology play important roles. The model is used to predict ecosystem processes and dynamics in oak forests in Knoxville, Tennessee (USA), and pine forests in Missoula, Montana (USA) between the years 1910 and 1986. The simulated growth of individual trees and the overall ecosystem-level processes are very similar to observations. A sensitivity analysis performed for these sites showed that predictions of net primary productivity by HYBRID are most sensitive to the ratio of CO2 partial pressure between inside the leaf and the air, relative humidity, ambient CO2 partial pressure, precipitation, air temperature, tree allometry, respiration parameters, site soil water capacity, and a carbon storage parameter.