How Often to Sample

Data need only be collected on a single occasion for an inventory program, to document rangeland resources and to evaluate their current status. In contrast, monitoring must be conducted on a recurrent basis to detect changes in vegetation attributes. Although it may appear simpler to identify and interpret the causes of vegetation change under an annual sampling regime, such a protocol is rarely undertaken except when monitoring utilization. For example, vegetation changes large enough to assume practical significance often take more than one year to become evident, particularly after allowing for variability arising from weather, and sampling error factors. Consequently, the optimum interval between sampling depends on the longevity of key species. For example, long-lived perennial shrubs such as mesquite (Prosopis spp.) need not be monitored as frequently as populations of shorter-lived annuals or biennials such as Rothrock grama (Bouteloua rothrockii). In addition, frequent sampling is usually limited by time and budgetary constraints.

Therefore, with the exception of biomass data to evaluate utilization patterns or residual biomass levels, it is not unusual to return to sample sites only once every 3 to 5 years in a long-term monitoring program. More frequent monitoring may be desirable for newly developed schedules, to confirm that the selection of attributes, time of sampling, sampling methods and site selection are suited to the objectives of the program. In other situations, it may be plausible to a) measure a different subset of sites each year so that all sites are sampled at the end of 3 to 5 years, most years, b) measure all sites when changes are noted at a subset of sites, c) when altering stocking rates, or d) during extraordinary seasonal conditions. Photographs may also be considered as a quick and easy alternative to provide additional information for sites in years when sampling of vegetation attributes is not undertaken.

References and Further Reading

McClaran, M.P., and D.N. Cole. 1993. Packstock in wilderness: Use, impacts, monitoring, and management. General Technical Report INT-301. Intermountain Research Station, United States Department of Agriculture, Forest Service. Ogden, UT. p 14.

Smith, E.L., and G.B. Ruyle. 1991. Considerations when monitoring rangeland vegetation. G.B. Ruyle. (ed). Some methods for monitoring rangelands and other natural area vegetation. University of Arizona, College of Agriculture, Extension Report 9043. p 3.