Simulation of Disturbances and Recovery in Shortgrass Steppe Plant Communities

Simulation modeling is a complementary tool to field observation and experimentation in understanding ecological systems (Lauenroth et a l., 1998). The overall objective of our plant community modeling is to allow us to evaluate the importance of gap dynamics concepts of succession for understanding shortgrass plant community recovery after disturbances. A gap dynamics approach focuses on individual plants, and the interactions between disturbance characteristics and plant life history traits in explaining successional patterns (Watt, 1947). Simulation models have been used extensively to evaluate the importance of gap dynamics processes to short- and long-term vegetation dynamics in temperate and tropical forests (e.g., Botkin et al., 1972; Shugart, 1984). We developed a gap dynamics model for shortgrass steppe plant communities (STEPPE [Coffin and Lauenroth, 1990]) based upon the conceptual and modeling framework provided by forest models, modifying it to represent Great Plains grasslands (Coffin and Lauenroth, 1996; Coffin and Urban, 1993). We used STEPPE in several capacities: (1) to synthesize and integrate existing knowledge to improve our understanding of recovery processes after disturbance, (2) to identify key processes limiting recovery, and (3) to predict long-term recovery dynamics for different climate and disturbance characteristics—in particular, soil texture and disturbance size. Our approach to modeling shortgrass community dynamics was to incorporate only the most important processes needed to address specific research questions. We added processes through time either because the model did not sufficiently represent ecosystem dynamics or because we posed more complicated research questions. STEPPE simulates the recruitment, growth, and mortality of individual plants on a small plot through time at an annual time step (Fig. 7.1) (Coffin and Lauenroth, 1990). Recruitment and mortality both have stochastic elements. Growth is deterministic and is based upon competition for resources among plants. A key difference between STEPPE and the forest models from which it was derived is that belowground resources are the most frequently limiting resources in semiarid grasslands compared with aboveground resources (light) in forests (Lauenroth and Coffin, 1992).