Survival of Winter Annuals in the Northern Mojave Desert.

Following early autumn germination in Mojave Desert winter annual populations (53 taxa) sampled on 13 plots (total sample size, 16.4 m ) in three drainage basins in southern Nevada, 1963-64, there was 38% survival to maturity (plot range 10-63%). Death occurred in early spring, at the time of shift from the slow vegetative growth of winter to the beginning of stem elongation. Despite no marked precipitation deficiencies during the 7- to 8-month growing season, mortality apparently resulted from inadequate soil moisture to meet the demands of seedlings at the point in the life cycle of a manyfold increase in plant volume. Mean percentage survival to maturity of seedlings (58 taxa), sampled on 62 plots (total sample size, 62 m ) in sevean drainage basins, following spring germination after rains of 3-5.5 inches in March-April 1965, was 60% (range by basin, 44-83%). Mortality in these populations, whose life cycles were completed in 6-10 weeks, could not be attributed to inadequate moisture levels. In most seasons, regardless of precipitation regimes, the majority of seedlings of desert annuals probably do not survive to maturity.