The single largest immediate threat to sagebrush steppe ecosystems is wildland fire. Although sagebrush losses from fire are quite dramatic because of their “acute” nature, “chronic” declines in sagebrush abundance are equally as concerning. Long-term data from the Idaho National Laboratory Site indicate that between 1975 and 2006 average big sagebrush (primarily Artemisia tridentata ssp. wyominensis) cover in unburned plant communities declined from more than 20% to less than 10% absolute cover. The cause of this trend is unknown, but previous investigators suggested that it began as part of a widespread big sagebrush die-off in the mid- 1970s and has persisted because of a general lack of vigor. To better understand losses in Wyoming big sagebrush abundance and declines in stand condition, we conducted a study on population structure. We sampled fourteen 10,000 m2plots, representing a range of stand condition, for shrub cover by species, sagebrush density, sagebrush condition rank, and we collected stem cross sections for 50 live and 20 dead individuals. Big sagebrush stem sections were prepared and we counted annual rings in the laboratory. Because this study is exploratory in nature, we used bar charts and summary statistics to characterize stand age distribution and compare age structure among stands.
A total of 636 big sagebrush stems were collected from live shrubs and ring counts from those stem cross sections indicate mean age of all live individuals pooled across all sample plots is 14.9 years. The pooled mean age of sagebrush from this study was much younger than we had anticipated. All 14 stands exhibited an uneven age structure and we found no evidence of strong dominance by one age cohort in any stand. While there does appear to be some correspondence between gross precipitation patterns and the number of individuals recruited into the big sagebrush population annually, not all recruitment patterns are directly attributable to precipitation trends. Variability among age distributions by plot suggests that local conditions may contribute to population structure at least as much as general precipitation patterns. Recruitment occurred in most stands during most years; however, declines in cover may potentially be attributed to a decrease in the total number of individuals establishing most years. Our results suggest that declines in recruitment coupled with the shorter than expected turnover rate for Wyoming big sagebrush may be contributing to declining cover trends over the past three decades.