Demographic characteristics of 3 Artemisia tridentata Nutt. subspecies

Previous research suggested that woody plant recruitment may occur in pulse s in semi-arid areas, The overall objective of this study was to determine if this pulse phenomena was recorded in the demographic structures of big s agebrush (Artemisia tridentata Nutt.) stands in Wyoming. In 1997, approxima tely 75 stem cross sections were collected from 9 stands of each of 3 subsp ecies of big sagebrush in Wyoming along elevation and climatic gradients. A nnual growth-rings were used to identify year of establishment and demograp hic characteristics were analyzed from age-class frequencies, Mean stand ag es of the 3 subspecies were different (P = 0.002), and analyses revealed th at Wyoming (A. tridentata ssp. wyomingensis) and mountain big sagebrush (A. tridentata ssp. vaseyana) stand ages (32 +/- 9 and 26 +/- 9 years, respect ively) were significantly older than basin big sagebrush (A. tridentata ssp . tridentata) (17 +/- 3) stands (P < 0.05). Mean recruitment intervals (yea rs) were shorter for basin (1.6) than for Wyoming (2.3) and mountain (2.2) sagebrush (P = 0.01), The number of cohorts did not differ among the subspe cies (P = 0.11), but the percent of years with recruitment was significantl y higher for basin (59%) compared to Wyoming (37%) and mountain (39%) subsp ecies (P < 0.0001), Age-class frequency distributions of each stand and reg ional stand combination were assessed for dispersion across each associated period of record. Chi-square goodness-of-fit tests were performed for the negative binomial distribution. All stands (with one exception) and all 3 r egional stand combinations fit the negative binomial distribution. Age-clas s frequency patterns indicate that recruitment is clustered or aggregated a cross each period of record, Recruitment in big sagebrush stands occurs in pulses throughout Wyoming.