Transition and Gap Models of Forest Dynamics

We describe and apply a correspondence between two major modeling ap- proaches to forest dynamics: transition markovian models and gap models or JABOWA- FORET type simulators. A transition model can be derived from a gap model by defining states on the basis of species, functional roles, vertical structure, or other convenient cover types. A gap-size plot can be assigned to one state according to dominance of one of these cover types. A semi-Markov framework is used for the transition model by considering not only the transition probabilities among the states, but also the holding times in each transition. The holding times are considered to be a combination of distributed and fixed time delays. Spatial extensions are possible by considering collections of gap-size plots and the proportions of these plots occupied by each state. The advantages of this approach include: reducing simulation time, analytica-l guidance to the simulations, direct analytical exploration of hypothesis and the possibility of fast computation from closed-form solutions and formulae. These advantages can be useful in the simulation of landscape dynamics and of species-rich forests, as well as in designing management strategies. A preliminary ap- plication to the H. J. Andrews forest in the Oregon Cascades is presented for demonstration.