Toft C A
Division of Biological Sciences, Section of Evolution and Ecology and Center for Population Biology, University of California, 05616, Davis, CA, USA.
Oecologia. 1995 Jan;101(1):1-12. doi: 10.1007/BF00328893.
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.
1983年至1992年的连续10年间，在加利福尼亚州莫诺湖北岸的一个沙丘生态系统中，对126株成年兔耳草（Chrysothamnus nauseosus var. consimilis）进行了跟踪研究。研究区域（1981 - 1992年）的年总降水量平均为160毫米，其中4月至10月的降水量为55毫米，这使得该地点比大盆地沙漠中已知的其他有 nauseosus生长的地点更为干旱。在3077平方米的研究样地中，所有灌木的覆盖度为18.0%，兔耳草的覆盖度为7.5%。水分对该兔耳草种群来说是有限的，在特定年份对新的冠层生长和繁殖努力、存活率以及幼株进入成年种群都有很大影响。成年灌木的生长和存活与地下水位深度显著相关，在研究期间地下水位深度在地表以下3.6至6.0米之间变化，与特定年份或相邻年份的任何降水量指标均无关联。该种群中的灌木在1983年至1992年间大小出现净减少。该地区从1987年起经历了严重干旱，1990年后降水量有所增加。灌木的死亡率始于1987年，到1992年达到22%。死亡的个体最初平均比整个研究期间存活的个体小：在研究开始时冠层面积为4000平方厘米或更小的个体中，有50%或更多在1992年死亡。研究区域内每株灌木的海拔（平均海平面以上米数），大概与地下水位深度相关，对生长没有影响，但在研究期间对存活有微弱影响。特定的微生境有显著影响，与生长在较平缓坡度上的个体相比，生长在陡坡上的灌木较小，“生长”斜率更负，并且在干旱期间更有可能死亡。1985年后没有幼株进入该种群。在研究期间无法明确证明灌木之间存在竞争，但仅使用了竞争的间接测试方法。成年兔耳草与所有物种的成年个体总和（覆盖度为9.6%的 vermiculatus、覆盖度为0.8%的 Tetradymia tetrameres 和覆盖度为0.02%的 viscidiflorus）呈随机分布，如果对水的竞争导致灌木均匀分布，这就不支持竞争假说。此外，对兔耳草个体在死亡前（1985年）和死亡后（1992年）的空间分布进行比较，并未发现向均匀性的竞争性疏伐。另一方面，较大的个体比较小的个体距离其最近邻更远，这一效应被推测是由竞争性疏伐导致的。
