SORTIE Model Structure

SORTIE is a mechanistic, spatially explicit, stochastic model of forests in the northeastern United States that describes local competition among nine species of trees in terms of empirically derived responses of individuals. The nine species modeled are all dominant or subdominant species found in mid- and late-successional stands: American beech (Fagus grandifolia; Be), eastern hemlock (Tsuga canadensis; Hm), sugar maple (Acer saccharum; SM), red maple (Acer rubrum; RM), yellow birch (Betula alleghaniensis; YB), white pine (Pinus strobus; WP), red oak (Quercus rubra; RO), black cherry (Prunus serotina; BC), and white ash (Fraxinus americana; WA). (For more detailed species descriptions, see Pacala et al. 1993). Data for the model were collected in northwestern Connecticut (42°00’N, 73°15’W) at elevations between 350 and 550 m.

SORTIE is conceptually a simple model consisting of two subunits: (i) a routine that measures the local availability of the critical resource, light, and (ii) the life history responses for all nine species, including patterns of growth, reproduction, and mortality as direct or indirect functions of light. It is important to note that although field data were collected on water and nitrogen relations in this forest, they were not strongly related to tree performance and therefore were not incorporated in the model (Pacala et al. 1993; Pacala et al. 1996).

Four-panel diagram of SORTIE structure

Four components of SORTIE for two hypothetical species measuring local light availability, growth, mortality risk, and reproduction. The darker species has a higher rate of growth and a higher mortality risk, but a shorter dispersal.

LIGHT AVAILABILITY: Local light availability is determined for each tree by means of a modification of the general light index (GLI) (Canham 1988; Canham et al. 1994; Chazdon and Field 1987). A focal tree’s GLI is determined by finding all neighboring trees that shade the focal tree. Information on the spatial relations among these neighboring tree crowns is combined with the movement of the sun throughout the growing season in order to determine the total, seasonally averaged light, expressed as a percentage of full sun. This process is repeated for every tree on the landscape in each time step.

SPECIES LIFE HISTORY: The response of each tree to its local light environment is based on empirically estimated life-history information. Light values are used to predict relative radial growth rates based on species-specific Michaelis-Menten functions. Radial growth is related to height growth, canopy width, and canopy depth in accordance with estimated allometric relations. Fecundity is estimated as an increasing power function of tree size, and seeds are dispersed stochastically according to a relation whereby the probability of dispersal declines with distance. Mortality risk is also stochastic and has two elements: random mortality and mortality associated with suppressed growth (Pacala et al. 1993; Pacala et al. 1996).

Visualization of local dispersal and shading