FLUCTUATIONS AND STABILITY IN CLIMAX SHRUB AND WOODLAND VEGETATION OF THE MOJAVE, GREAT BASIN AND TRANSITION DESERTS OF SOUTHERN NEVADA

ABSTRACT

Woody vegetation was documented over a 12-year period, and rainfall through 10 years of the period, in 56 climax shrub and woodland communities in the Mojave, Great Basin and transition deserts of the Nevada Test Site in southern Nevada. Line interception data (335 m) were collected in each community in 1963, during a period of below-average rainfall, and again in 1975 along the same sampling lines, by the same method and observer. From summaries of the data for all sites and from data for individual sites, at the end of the 12 years which included periods of unusually high rainfall, there were consistent increases in numbers of shrubs and trees, mean height and percentage cover, and decreases in proportions of dead plants in the populations; but, neither the qualitative species composition nor the relative proportions of the species changed. From plant- by-plant comparisons along the lines of nine representative sites, it is evident that most of the shrubs and trees living in 1963 were still present as living plants in 1975 and an average 14% were dead or had disappeared from the lines by 1975; around 20% to 30% of the 1975 plants were new to the lines and of seedling or, in some species, shoot origin. From the data it is predictable that during the recurring dry/moist cycles in desert regions there is a high rate of turnover in the shrub component of the climax vegetation. Plant fluctuations were closely correlated with regional and site rainfall fluctuations of the period. The magnitude of the fluctuations was within the range necessary for continuing stability of all of the communities and populations of woody species in the three desert areas.