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 stateand-transition models, thresholds, and rangeland health. Introduction of the multiple stable state concept paved the way for the development of these alternative evaluation procedures by hypothesizing that multiple stable plant communities can potentially occupy individual ecological sites. Vegetation evaluation procedures must be able to assess continuous and reversible as well as discontinuous and nonreversible vegetation dynamics because both patterns occur and neither pattern alone provides a complete assessment of vegetation dynamics on all rangelands. Continuous and reversible vegetation dynamics prevail within stable vegetation states, whereas discontinuous and nonreversible dynamics occur when thresholds are surpassed and one stable state replaces another. State-and-transition models can accommodate both categories of vegetation dynamics because they represent vegetation change along several axes, including fire regimes, weather variability, and management prescriptions, in addition to the succession-grazing axis associated with the traditional range model. Ecological thresholds have become a focal point of state-and-transition models because threshold identification is necessary for recognition of the various stable plant communities than can potentially occupy an ecological site. Thresholds are difficult to define and quantify because they represent a complex series of interacting components, rather than discrete boundaries in time and space. Threshold components can be categorized broadly as structural and functional based on compositional and spatial vegetation attributes, and on modification of ecosystem processes, respectively. State-and-transition models and rangeland health procedures have developed in parallel, rather than as components 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 evaluation procedures as the beginning of a long-term developmental process, rather than as an end point marked by the adoption of an alternative set of standardized evaluation procedures. The Texas and Oklahoma Agricultural Experiment Stations and USDA-NRI Ecosystem Program (92-37101-7463) funded field research that contributed to this synthesis and the stations provided partial support to the authors during manuscript preparation.

The effects of grassland degradation on plant diversity, primary productivity, and soil fertility in the alpine region of Asia’s headwaters

“…The degradation threshold can be defined as the point at which the ecological conditions degrade so severely that the ecosystem diverges significantly from its initial state and can no longer recover without increased management intervention (Hobbs and Norton 1996;Briske et al 2005;Bestelmeyer 2006;Sasaki et al 2008;Baker and King 2010). This concept has several implications for promoting preventive management and restoration intervention to sustain the ecosystem stability and services of alpine grasslands in the headwater areas of Asia.…”

Rangeland degradation and restoration management in China

“…With temperature increase has come more dry and windy periods and hence increased erosion events (Chen et al 2003). As such the arid rangelands are non-equilibrium systems and management based on this paradigm should be opportunistic (Westoby et al 1989;Ellis and Swift 1988;Oba et al 2000;Briske et al 2005). Traditional management has assumed an equilibrium system but this is no longer appropriate, especially since there is ongoing climate change (Li 2007).…”

National‐scale assessment of ecological content in the world's largest land management framework

“…NRCS has provided assistance and monetary support on approximately 188,000,000 ha of private land for conservation-related projects (Briske 2011) while the other agencies focus on public land. The result is a government-sponsored interagency collaboration that solidifies state-and-transition models (Westoby et al 1989, Bestelmeyer et al 2003, Briske et al 2005 in ESDs as one of the world's largest guiding frameworks for the management and restoration of terrestrial ecosystem services.…”

“…Prior to state-and-transition models, rangeland classification and assessment relied on methodologies that compared the composition and biomass of current rangeland vegetation to that of a historical benchmark (BLM and USFS used the ecological status model, NRCS used the range condition model; NRC 1994). These assessment models grew from Clements' (1916) theory of succession and vegetation climax and its refinement by Dyksterhuis (1949) to suggest that succession and retrogression of vegetation are well-defined, predictable changes along a single reversible trajectory (Briske et al 2003(Briske et al , 2005. Such succession-retrogression models led to a grazing-centric rangeland discipline and the widespread belief that adjustments in grazing pressure could maintain rangelands at equilibrium (Fig.…”

“…State-and-transition models still include aspects of the classical range model, but they feature multiple drivers in addition to grazing, multiple alternative stable states, and various trajectories of vegetation change (Briske et al 2003). More recently, state-and-transition models have incorporated concepts of thresholds, resilience, and heterogeneity into the models (Briske et al 2005, 2006, Bestelmeyer et al 2009.…”