Here are my suggestions. The first citation for the actual software, the second for the approach.
Apex Resource Management Solutions (2014). ST-Sim state-and-transition simulation model software. Retrieved from http://www.apexrms.com/stsm
Daniel, Colin J.; Frid, Leonardo. 2012. Predicting landscape vegetation dynamics using state-and-transition simulation models. In: Kerns, Becky K.; Shlisky, Ayn J.; Daniel, Colin J., tech. eds. Proceedings of the First Landscape State-and-Transition Simulation Modeling Conference, June 14–16, 2011, Portland, Oregon. Gen. Tech. Rep. PNW-GTR-869. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station: 5-22.