Range trend refers to the change in the status of resources at a site detected by monitoring and is usually expressed as improving, declining, or stable. It originally pertained to any goal defined by management such as changing vegetation cover by adjusting stocking rates or grazing practices. The general association of range trend with data describing any vegetation attribute in a monitoring program is still theoretically valid, but today the term carries a more specific interpretation relating to the comparison of consecutive assessments of range condition in a monitoring program.
Therefore, weaknesses in methods to assess range condition are manifested in the evaluation of range trend. Most important, range trend is an ecological assessment relating current species composition to that perceived as the climax vegetation at a site, and without connotations to the goals of management. For example, improving range trends usually reflect more desirable conditions for livestock production and watershed stability, but could have undesirable consequences to the habitat of wildlife species that require a high proportion of forbs in their diet, (e.g., pronghorn). These interpretations are better indicated from other concepts such as the desired plant community or resource value ratings.
Because land resources must be monitored for an extended period to assess range trend, apparent trend is sometimes appraised from data describing site conditions collected at a single point in time. Apparent trend is determined by ranking soil and water criteria believed to reflect the probable changes in resource status, including the presence of unpalatable species, plant vigor, and soil surface conditions. Although a subjective appraisal relying on professional judgement, apparent trend provides land managers with useful information to guide short-term tactical decisions.
Site responses to climate and management must be distinguished to credibly interpret range trends and apparent trends. Repeated records based on quantitative comparisons over a long period provide the best indication of overall trends due to management, but such intensive monitoring strategies can be impractical in many situations. Even then, range trend assessments can be insensitive to detect early phases of improvement or deterioration, when masked by the yearly fluctuations in species composition caused by variable precipitation patterns. Although evaluation of range trends should be restricted to data collected at the same site to avoid the possible confounding by site variability, exclosures can act as valuable comparison areas to provide supplementary evidence describing the effect of current weather conditions.
References and Further Reading
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Barker, S., and K.M. Egen. 1993. Range trend monitoring in southern Arizona. Rangelands 15:166-167.
Bonham, C.D. 1989. Measurements for Terrestrial Vegetation. John Wiley Sons, New York, NY. pp. 273-275.
Holechek, J.L., Pieper, R.D., and C.H. Herbel. 1995. Range Management Principles and Practices. Prentice Hall, Englewood Cliffs, NJ. 2nd ed. p. 166.
National Research Council. 1994. Rangeland Health: New Methods to Classify, Inventory and Monitor Rangelands. National Academy Press. pp. 81-82.
Ratliff, R.R. 1993. Viewpoint: trend assessment by similarity index. Journal of Range Management 46:139-141. (pdf)
Tueller, PT., and W.H. Blackburn. 1974. Condition and trend of the big sagebush/needle-and-thread habitat type in Nevada. Journal of Range Management 27:36-40. (pdf)
West, N.E., McDaniel, K., Smith, E.L., and S. Leonard. 1994. Monitoring and Interpreting Ecological Integrity on Arid and Semi-Arid Lands of the Western United States. Range Improvement Task Force, Las Cruces, NM. p. 11.
Wikeem, B.M., and M.D. Pitt. 1991. Grazing effects and range trend assessment on California bighorn sheep range. Journal of Range Management 44:466-470. (pdf)