A 10-year demographic study of rabbitbrush (Chrysothamnus nauseosus): growth, survival and water limitation.

A population of 126 adult rabbitbrush (Chrysothamnus nauseosus var. consimilis) was followed for 10 consecutive years from 1983 through 1992 in a sanddune ecosytem on the north shore of Mono Lake, California. The study area received an average (1981-1992) of 160 mm total annual precipitation of which 55 mm fell from April through October, making this site drier than other known sites in the Great Basin desert where C. nauseosus occurs. Cover of all shrubs was 18.0%, and that of C. nauseosus 7.5%, in the 3077 m study plot. Water was limiting for this population of C. nauseosus, exerting large effects on new canopy growth and reproductive effort in a given year, on survival, and on establishment of juveniles into the adult population. Growth and survivorship of the adult shrubs were significantly correlated with the depth to ground water, which varied from 3.6 to 6.0 m below the surface during the study, and not with any measure of precipitation in a given year or adjacent years. Shrubs in this population exhibited a net decrease in size between 1983 and 1992. The region experienced a significant drought from 1987 onwards, with some increase in precipitation after 1990. Mortality of shrubs began in 1987 and reached 22% by 1992. Individuals that died were smaller on average initially than individuals surviving throughout the study: 50% or more of the individuals having 4000 cm of canopy or less at the start of the study died by 1992. Elevation (m msl) of each shrub in the study area, presumably correlated with depth to ground water, had no effect on growth but a weak effect on survival during the study period. Specific microsite had significant effects, with shrubs on steep slopes being smaller, having more negative "growth" slopes, and being more likely to die during the drought compared to individuals occurring on flatter gradients. Recruiment of juveniles into the population did not occur after 1985. Competition between shrubs could not be definitively demonstrated during the study, but only indirect tests for competition were used. Adult C. nauseosus and adults of all species combined (Sarcobatus vermiculatus with 9.6% cover, Tetradymia tetrameres with 0.8% cover and Chrysothamnus viscidiflorus with 0.02% cover) were randomly distributed, failing to support the hypothesis of competition if competition for water results in a uniform distribution of shrubs. Moreover, comparison of the spatial dispersion of C. nauseosus individuals before (1985) and after (1992) mortality did not reveal competitive thinning toward uniformity. On the other hand, larger individuals were farther from their nearest neighbor than were smaller individuals, an effect postulated to result from competitive thinning.