A physiology-based gap model of forest dynamics

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. 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 Quercus alba forests in Knoxville, Tennessee, and pine Pinus contorta forests in Missoula, Montana between 1910-1986. Simulated growth of individual trees and the overall ecosystem-level processes are very similar to observations. 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. -from Authors