Letterman needlegrass, the second most frequent species (Fig. 12), changed over time in a manner that, like Kentucky bluegrass, suggests the overriding influence of stocking rate adjustment. It began the study in 1980 as the most abundant species (Fig. 12) but then declined (during favorable years) to a low in 1988, followed by a modest increase (during dry years) to the end of the study. These changes are consistent with the behavior of a classic “increaser” species (Dyksterhuis 1949), i.e., one that is a component of the potential plant community and that increases in abundance under heavy grazing. Increaser species usually decrease in abundance once grazing pressure is relaxed, often in concert with increases in decreaser species (e.g., Kentucky bluegrass). American vetch was generally unremarkable in its successional trajectories in the six treatments. A modest drought-induced decline was followed by an equally modest recovery, and, on average, all of the treatments ended the study with approximately the same amounts of vetch with which they began.
Changes in Range Trend Range trend is defined as the “direction of change in range condition” (Society for Range Management 1989). Measurement of range trend is a monitoring technique that is widely practiced on rangelands of the West. It can be a useful tool for judging the impact of management programs and changing weather conditions on the range resource. It is especially useful to ranchers and professional range managers who have neither the time nor resources to monitor plant community change on an annual or biennial basis as we did in this project. It, in effect, provides a series of ecological “snapshots” over time by which to monitor and judge change. Trend data tell the manager whether the range is improving in ecological condition (upward trend), declining in condition (downward trend), or is remaining static. We adapted those procedures for use in the current study to provide yet another way of viewing and interpreting project results that would perhaps be more familiar and useful to the applied range manager. Specifically, increases over time in the frequencies of the following four key species would indicate an improving (upward) range trend: slender wheatgrass, mountain brome, Kentucky bluegrass, and American vetch. Likewise, decreases over time in the following two species would also be indicative of improving range trend: tarweed and Letterman needlegrass. Ideally, trend assessments are conducted at regular points in time, generally at intervals of 5-10 years. For our purposes, we chose the mid-point (1986) and final (1992) years of the study as our time points to assess the combined effects of grazing treatments and climate on range trend. Changes in the amounts (frequencies) of our 6 key species over those two 6-year periods are summarized in Figs. 13 and 14. Clearly, all of the treatment regimes improved in range condition, some more 29