1. General Model Information

Name: SORTIE - Model of Forest Dynamics

Acronym: SORTIE

Main medium: terrestrial
Main subject: forestry, populationdynamics
Organization level: organism, population
Type of model: individual-based, not specified (2D)
Main application:
Keywords: forest, species distribution, spatially explicit, long term effects, management, stochastic


Dr. Charles Canham
65 Sharon Turnpike; P.O. Box AB
Millbrook NY 12545-0129,

Phone: (845) 677-7600 Ext. 139 (Direct)
Fax: (845) 677-5976
email: ccanham@ecostudies.org
Homepage: http://www.ecostudies.org/people_sci_canham.html


Steve Pacala, modified by Douglas Deutschman, Simon Levin, Linda Buttel, Catherine Devine


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 (4200’N, 7315’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.)

II. Technical Information

II.1 Executables:

Operating System(s): UNIX

II.2 Source-code:

Programming Language(s): C

II.3 Manuals:

Manual at Princeton University

II.4 Data:

Example Files at Princeton University

III. Mathematical Information

III.1 Mathematics

Model Parameters

III.2 Quantities

SORTIE documentation at SCIENCE ONLINE

III.2.1 Input

SORTIE documentation at SCIENCE ONLINE

III.2.2 Output

IV. References

Bolker, B.M., Pacala, S.W. 1999. Spatial moment equations for plant competition: understanding spatial strategies and the advantages of short dispersal. The American Naturalist 153(6): 575-602.
Moorcroft, P.R., Hurtt, G.C., and Pacala, S.W. 1999. Scaling rules for vegetation gap dynamics: a new terrestrial biosphere model for regional and global change studies. Global Biogeochemical Cycles. In review.
Deutschman, D.H., S.A. Levin and S.W. Pacala. 1999. Error propagation in a forest succession model: The role of fine-scale heterogeneity in light. Ecology 80:1927-1943.
Pacala, S.W. and Rees, M. 1998. Models suggesting field experiments to test two hypotheses explaining successional diversity. The American Naturalist 152: 729-737.
Pacala, S.W., and Levin, S.A. 1997. Biologically generated spatial pattern and the coexistence of competing species. In Spatial Ecology, eds. D. Tilman and P. Kareiva, in press.
Pacala, S.W. 1997. Dynamics of plant communities. In Plant Ecology, ed. M. Crawley. Blackwell Scientific.
Pacala, S., Canham, C., Saponara, J., Silander, J., Kobe, R., and Ribbens, E. 1996. Forest models defined by field measurements: II. Estimation, error analysis and dynamics, Ecological Monographs 66: 1-44.
Deutschman, D. H., S. A. Levin, C. Devine and L. A. Buttel 1997. Scaling from Trees to Forests: Analysis of a Complex Simulation Model. Published in Science Online: A new electronic journal published by AAAS.
Deutschman, D. H. 1996. Scaling from trees to forests: The problem of relevant detail. Ph.D. dissertation, Department of Ecology and Systematics, Cornell University.
Pacala, S. W. and D. H. Deutschman. 1995. Details that matter: The spatial distribution of individual trees maintains forest ecosystem function. Oikos 74(3):357-365.

more references

V. Further information in the World-Wide-Web

  • Home-Page of SORTIE-ND
  • SORTIE documentation at SCIENCE ONLINE
  • Press release related to visualization of SORTIE output

    VI. Additional remarks

    Last review of this document by: T. Gabele : Jan 21 1998
    Status of the document:
    last modified by Joachim Benz Tue May 9 21:36:29 CEST 2006

    Go back to Register of Ecological Models (R E M)