State-and-Transition Models, Thresholds, and Rangeland Health: A Synthesis of Ecological Concepts and Perspectives
Abstract
This article synthesizes the ecological concepts and perspectives underpinning the development and application of state-and-transition models, thresholds, and rangeland health. Introduction of the multiple stable state concept paved the way for thedevelopment of these alternative evaluation procedures by hypothesizing that multiple stable plant communities can potentiallyoccupy individual ecological sites. Vegetation evaluation procedures must be able to assess continuous and reversible as well asdiscontinuous and nonreversible vegetation dynamics because both patterns occur and neither pattern alone provides a completeassessment of vegetation dynamics on all rangelands. Continuous and reversible vegetation dynamics prevail within stablevegetation states, whereas discontinuous and nonreversible dynamics occur when thresholds are surpassed and one stable statereplaces another. State-and-transition models can accommodate both categories of vegetation dynamics because they representvegetation change along several axes, including fire regimes, weather variability, and management prescriptions, in addition tothe succession-grazing axis associated with the traditional range model. Ecological thresholds have become a focal point ofstate-and-transition models because threshold identification is necessary for recognition of the various stable plant communitiesthan can potentially occupy an ecological site. Thresholds are difficult to define and quantify because they represent a complexseries of interacting components, rather than discrete boundaries in time and space. Threshold components can be categorizedbroadly as structural and functional based on compositional and spatial vegetation attributes, and on modification of ecosystemprocesses, respectively. State-and-transition models and rangeland health procedures have developed in parallel, rather than ascomponents of an integrated framework, because the two procedures primarily rely on structural and functional thresholds,respectively. It may be prudent for rangeland professionals to consider the introduction of these alternative evaluationprocedures as the beginning of a long-term developmental process, rather than as an end point marked by the adoption of analternative set of standardized evaluation procedures.