The Forest Model: SORTIE

SORTIE is a mechanistic, spatially explicit, stochastic model of northeastern forests that describes local competition among nine species of trees in terms of empirically derived responses of individuals. The most powerful aspect of SORTIE is its ability to predict large-scale forest dynamics from a model parameterized exclusively from individual trees.

Conceptually, SORTIE is a simple model. Light (the limiting resource) is measured for each tree on the landscape by means of an approach based on fish-eye photography of the forest canopy. The light available to each tree is then used to calculate species-specific growth rate and risk of mortality. Surviving trees produce seedlings as an increasing function of tree size, and the seedlings are dispersed away from the parent tree in accordance with estimated dispersal functions. Forest dynamics emerge as the collective result of these localized interactions among trees.

Four-panel diagram of SORTIE structure

Four components of SORTIE for two hypothetical species: local light availability, growth as a function of light, mortality risk as a function of growth, and finally, the production and dispersion of seedlings. In this example, the darker species has a higher rate of growth and a higher mortality risk, but a shorter dispersal.






Copyright 1997 by the American Association for the Advancement of Science.