Abstract:
State-and-transition models (STMs) have been successfully used to
describe ecological dynamics in woodlands, shrublands, grasslands, and several other ecosystems. Changes in vegetation and soil are measured to gauge and predict plant community dynamics within ecological states and transitions between alternative ecological states. Ecological states and their boundaries are defined by a range of variability in vegetation and soil parameters. The ecological site classification system used in the United States requires the development of a unique STM model for each defined ecological site. The foundation of the
ecological site concept is the estimation and quantification of a historical reference plant community. Historical information, land manager experience, and scientific data are used to quantify the historical reference plant community and gauge changes towards alternative stable states. In regards to winterfat plant
communities and the Silty 6-10 PZ ecological site, conceptual STMs have not been fully developed or tested. The objective of this study was to develop, test, and refine a process-based, data supported, STM for the Silty 6-10 PZ ecological site. The predominant factor of disturbance on this ecological site for the last 100 years has been grazing by livestock. To develop the process-based STM for the ecological site, we conducted a piosphere study centered on a livestock watering point and a five-year grazing exclosure study. With these two studies we were able to quantify the resilience of the ecological site when protected from disturbance (exclosure study) and also to quantify the trends of the ecological site when under a continued grazing disturbance (piosphere study). We used the information gathered from both of these studies to refine the STM. The four strongest factors that indicated differences between ecological states were amount of biological soil crust, soil surface aggregate stability, density of winterfat, and frequency of Indian ricegrass. Data from this study supported the initial proposed
STM for the Silty 6-10 PZ ecological site. States defined were a Reference Winterfat State (State 1), Sickle Saltbush State (State 2), and a Creeping Wildrye State (State 3). Each ecological state corresponds to a site characterized by high, moderate, and low ecological functioning, respectively.
