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Succession: Changing landscapes

How does a plant community change? Disturbances, such as fire, drought, livestock grazing, insect outbreaks and off-road vehicle use, can impact certain plants and not others. When a plant dies, it can leave an opening for another plant to occupy. Over time, there can be a shift in the species that make up the plant community. State-and-transition models are part of a new framework that scientists and land managers use to describe vegetation change.

In the illustration below, the large box labeled Natural Historic Cycle represents a ‘stable state’ for a sagebrush steppe community. Within this state, there are smaller boxes that represent different ‘phases’ within the sagebrush community. Following a fire, the ‘sagebrush dominates phase’ will shift to the ‘Grasses and forbs dominate phase’. After several decades, sagebrush will re-establish and gradually shift the community to the ‘Grasses, forbs and shrubs phase’, until another fire occurs. This natural historic cycle remains stable as long as there aren’t large, frequent disturbances.

New plant species, especially aggressive ones like invasive weeds, can also change the plant community and lead to new stable states. Such is the case with cheatgrass, which is represented above by the large box labeled Cheatgrass/Fire Cycle. Vast areas of the Great Basin are being changed from sagebrush and perennial grasses and forbs to annual grasslands. With cheatgrass, fires occur much more frequently and perennial plants are eventually lost. The new plant community is a stable state because the competitiveness of cheatgrass and frequent fire prevent a transition back to perennial grasses, forbs, and sagebrush. Secondary weed invasions, like with knapweeds or yellow starthistle, could create new stable plant communities that are even less desirable than cheatgrass.

Once a plant community transitions from one stable state to another, it requires high levels of inputs, including weed control and seeding with native perennial plant species, to return to the original stable state. There is an analogy, referred to as a “ball and trough”, that is often used to describe this. The “ball” is the plant community and the “troughs” are stable states. Transitioning from one stable state to another requires overcoming a threshold where the ball can then drop into a new trough, or stable state.

Depiction of restoration efforts to get the plant community back to a more natural state

Researchers and land managers are working on ways to restore these perennial plants to the Great Basin and return to a more natural plant community. This process may involve an intermediate step in which exotic plants like forage kochia and crested wheatgrass are planted to stabilize the plant community, reduce the fire return interval, and make returning to a more natural state easier.

The Animated Cycle in this website is a depiction of this state and transition process.

Text references:
-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. Vol. 58 No. 1. pp. 1-10.
-Landres, P.B., P. Morgan, and F.J. Swanson. 1999. Overview of the use of natural variability concepts in managing ecological systems. Ecological Applications, Vol. 9 No. 4, pp. 1179-1188.
-Young, T.P., J.M. Chase, and R.T. Huddleston. 2001. Community succession and assembly: Comparing, contrasing, and combining paradigms in the context of ecological restoration. Ecological Restoration. Vol. 19 No. 1 pp. 5-18.