POPULATION-DYNAMICS OF AN EGYPTIAN DESERT SHRUB, THYMELAEA-HIRSUTA

Natural populations of the evergreen shrub, Thymelaea hirsuta (L.) End l., were studied over 6 years at five desert habitats, in terms of see dling recruitment and adult survival and as a function of plant size a nd gender class. Habitat and time significantly influenced mortality o f both reproductive and non-reproductive plants. Plant size also signi ficantly affected adult mortality. Seedling recruitment varied signifi cantly with habitat and year and approached zero some years. Significa nt among-year and among-population variation in population growth rate s were observed over the 6 years of study, and all populations decline d in size (ranging from -1.7% per year at the coastal dune site to -10 .9% per year at the inland plateau site). Spearman rank correlation an alysis between habitats ranked according to a north-south gradient and demographic variables indicates that this gradient is associated with a pattern of lower seedling emergence and survival and a lower popula tion growth rate and greater mortality for all size-classes of Thymela ea plants. In experimental botanic garden plots, germination of seed c ollected from five natural populations, and seedling survival in the f ollowing year were assessed under conditions of high, medium, and low seedling density. Seedling emergency differed significantly according to maternal habitat. With regular watering, seeding survival to one ye ar was 72% (averaged across habitats and densities). This compares wit h 64% for seedlings grown at the highest density, suggesting that the intense mortality observed under field conditions is more likely to be a result of water shortage than intraspecific competition.