Short-Term Regeneration Dynamics of Wyoming Big Sagebrush at Two Sites in Northern Utah

Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) is a widespread shrub across the western United States, and there is great interest among scientists and land managers in its ecology and conservation, particularly with regard to maintaining structural heterogeneity of sagebrush stands for wildlife habitat and livestock forage. Yet little is known about its short-term regeneration dynamics and the implications of those dynamics for changes in stand structure. We examined changes among sagebrush size classes across 3 years, as well as emergence of sagebrush from seed bank and seed rain samples at 2 sagebrush shrub land sites in northern Utah: a lower-density site (1.4 plants/m2, SE 0.11) with no recent history of manipulation and a higher-density site (1.9 plants/m2, SE 0.21) that had recently been treated with herbicide to reduce sagebrush cover. On both sites, numbers of sagebrush plants in the largest size class decreased over the 3-year time period, while dead and medium-sized sagebrush plants increased. At the higher-density herbicide-treated site, this size class shift appeared to be driven by growth of small plants into the medium size class, likely associated with reductions in numbers of (and competition from) large plants. At the lower density site, it appears that densities of large plants declined because the plants shrank in size, possibly due to herbivory. Sagebrush seed rain did not differ between fall and spring assessments. Forbs had the greatest representation in the seed bank, followed by grasses and then sagebrush, though the number of sagebrush seeds may be sufficient for seedling recruitment. These results illustrate that shifts among sagebrush size classes, especially transitions of small shrubs into the medium size class, may be a primary and immediate pathway of stand recovery, in addition to recruitment from seed. These findings underscore the importance of sagebrush stand structure to plant community health and may aid in anticipating responses to disturbances such as drought or herbivory