In 1994, the National Research Council Committee on Rangeland Classification Systems published a report addressing current methods and possible improvements to assess the status of rangeland resources. The report recommended that assessments be based on rangeland health, which embodies the degree to which the integrity of soil and ecological processes within the ecosystem are sustained. Proper functioning of these processes is assumed to ensure that rangelands have the capacity to produce commodities and satisfy values on a sustainable basis.
The report acknowledged that the changing status of rangeland resources involves complex interactions that cannot be adequately determined by a single factor. Instead, rangeland health should be judged with several indicators that include:
- Degree of soil stability and watershed function - assessed from soil surface characteristics such as depth of A-horizon, pedestalling, rills and gullies, sheet erosion, and sedimentation.
- Nutrient cycling and energy flow - assessed by the distribution of plants, litter, roots, and age-classes.
- Presence and functioning of recovery mechanisms - assessed by plant vigor, and the presence of seedlings and germination microsites.
From these measured indicators, rangeland health could be evaluated according to three categories:
- Healthy - the capacity to satisfy values and produce commodities is being sustained,
- At Risk - rangeland resources are indicating a reversible loss in capacity, or
- Unhealthy - rangeland resources are indicating that a threshold has been crossed causing an irreversible loss in capacity, so that external inputs (eg., brush control, reseeding) are required to regain ecosystem integrity.
Equally important, the measured indicators of rangeland health might identify conditions where the ecosystem is particularly vulnerable to undesirable change, an early warning point, that marks the boundary between healthy and at risk rangelands, or a threshold that signals the beginning of irreversible changes associated with unhealthy rangelands. This approach corresponds to the widely accepted 'state-and-transition' model that is at the center of current ecological theory. It also has many similarities to threshold concepts included in the site conservation rating approach.
The National Research Council proposal is an innovative scheme to assess the ecological status of rangelands, but much of the detail required for implementation and interpretation remains undeveloped. Nonetheless, the report represents a major change in the philosophy of range assessment, by emphasizing soil surface characteristics as a fundamental component of inventory and monitoring, and by accepting that the vegetation on many rangeland areas fails to follow the successional patterns expected under Clementsian ecological theory.
References and Further Reading
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Friedel, M.H. 1991. Range condition assessment and the concept of thresholds: A viewpoint. Journal of Range Management 44:422-426. (pdf)
Laycock, W.A. 1991. Stable states and thresholds of range condition on North American rangelands: A viewpoint. Journal of Range Management 44:427-433. (pdf)
National Research Council. 1994. Rangeland health: New methods to classify, inventory and monitor rangelands. National Academy Press. pp. 180.
Westoby, M., B. Walker, and I. Noy-Meir. 1989. Opportunistic management for rangelands not at equilibrium. Journal of Range Management 42:266-274. (pdf)