How Rangelands Change Through Time

Written by Brad Schultz, Extension Educator, Winnemucca, Nevada

The vegetation on rangeland is always changing toward one of several or more plant communities. Understanding why vegetation changes and how to manipulate that change is critical for rangeland managers to ensure that rangelands continue to provide the goods and services needed by society.

Early research about changes in plant communities across time, or plant succession, on rangelands resulted in the belief that vegetation change was linear. That is, community A changed into community B, which ultimately changed into community C, and so forth. After many decades, rangeland scientists and managers realized that vegetation change is not always linear. Changes in management did not always result in the return to the previous, more desired plant community, and normal disturbances often resulted in new plant communities that were stable and responded very little to management actions. In essence, the response of the plant communities to management and disturbance was non-linear.

Land managers need to understand which plant communities can ultimately occupy a site, given the current plant composition, the inherent potential of the soil on the site to produce specific plant communities, the probable climatic patterns and environmental conditions or constraints that will occur, and the suite of management actions available within the aforementioned constraints. To describe the non-linear vegetation changes that occur on most rangelands, rangeland scientists developed a conceptual approach called state and transition models. State and transition models use box-and-arrow diagrams to describe and understand non-linear vegetation change, or plant succession. That is, changes in community composition, vegetation structure, and ecological function are illustrated that do not follow the one-dimensional pathway, forward or backward, described in the previous paragraph.

The specific plant communities that ultimately develops depends upon which management actions are applied; the intensity, frequency, and duration of those actions; and how those management actions interact with environmental conditions and their variation across time. To help you understand the concepts behind and advantages of state and transitions models we will briefly discuss plant succession on rangelands, why the previous models used to describe plant succession on rangelands do not work and can result in poor management decisions, the components of a state and transition model, and the structure of a complete state and transition model. Data and information from a Wyoming sagebrush ecological site will be used to illustrate the problems with previous plant succession models and the components of state and transition models.

Forces that Shape Rangelands. Rangelands are a dynamic landscape, composed of many resources, that produce many products. The rangeland landscape and its resources are constantly being modified by a suite of non-human forces, including:

  • Grazing

  • Fire

  • Climate or Weather

Humans also modify rangelands directly through development (e.g., energy, mining, and transporation and communications infrastructure) and recreation. People also affect the other forces of change by introducing invasive species, controlling or igniting fires, managing grazing and potentially impacting the climate and weather patterns through human caused changes in atmospheric chemistry .

Managers need a way to predict how management practices or natural disturbance will impact the vegetation on rangelands, so they developed State and Transition Models. State and transition models are box-and-arrow diagrams used to describe vegetation change, or plant succession, from a specific disturbance based on the current vegetation community, the soils and climate of a site.

Relevant Research

Allen-Diaz, B., and J. W. Bartolome. 1998. Sagebrush–grass vegetation dynamics: comparing classical and state-transition models. Ecological Applications 8:795–804.

Bestelmeyer, B. T. 2006. Threshold concepts and their use in rangeland management and restoration: the good, the bad and the insidious. Restoration Ecology 14:325-329.

Bestelmeyer, B.T., J.E. Herrick, J.E. Brown, D.A. Trujillo, K.M. Havstad. 2004. Land management in the American Southwest.: a state and transition approach to ecosystem complexity. Environmental Management 34:38-51.

Briske, D. D., B. T. Bestlemeyer, T.K. Stringham and P.L Shaver. 2008. Recommendations for development of resilience-based state and transition models. Rangeland Ecology and Management 61:359-367.

Briske, D.D., S.D. Fuhlendorf and F.E. Smeins. 2003. Vegetation dynamics on rangelands: a critique of the current paradigms. Journal of Applied Ecology 40:601-614.

Briske, D.D., S.D. Fuhlendorf and F.E. Smeins. 2005. State and transition models, thresholds and rangeland health: a synthesis of ecological concepts and perspectives. Rangeland Ecology and Management 58:1-10.

Chartier, M. P. and C.M. Rostagno. 2006. Soil erosion thresholds and alternative states in northeastern Patagonian rangelands.

Dyksterhuis, E. J. 1949. Condition and management of rangeland based quantitative ecology. Journal of Range Management 2:104–115.

Illius, A. W., and T. G. O’Connor. 1999. On the relevance of non-equilibrium concepts to arid and semiarid grazing systems. Ecological Applications 9:798–813.

Laycock, W. A. 1991. Stable states and thresholds of range condition on North American rangelands: a viewpoint. Journal of Range Management 44:427–433.

Stringham, T. K., W. C. Krueger, and P. L. Shaver. 2001. States, transitions, and thresholds: further refinement for rangeland applications. Special Report 1024, Agricultural Experiment Station, Oregon State University, Corvallis, Oregon, USA.

Stringham, T. K., W. C. Krueger, and P. L. Shaver. 2003. State and transition modeling: an ecological process approach. Journal of Range Management 56:106-113.

Walker, B.H. 1993. Rangeland ecology: understanding and managing change. Ambio 22:80-87.

Westoby, M., B. H. Walker, and I. Noy-Meir. 1989. Opportunistic management for rangelands not at equilibrium. Journal of Range Management 42:266–274.

Westoby, M., B. Walker, and I. Noy-Meir. Range management on the basis of a model which does not seek to establish equilibrium. Journal of Arid Environments 17:235-239.

Wiens, J. A. 1984. On understanding a non-equilibrium world: myth and reality in community patterns and processes. Pages 439–457 in D. R. Strong, D. Simberloff, K. G. Abele, and A. B. Thistle, editors. Ecological communities: conceptual issues and the evidence. Princeton University Press, Princeton, New Jersey, USA.