VARIABILITY IN LIFE-HISTORY PARAMETERS OF 4 SEROTINOUS PLANTS IN THE NAMIB DESERT

Plants growing in extreme habitats, such as deserts, are expected to show high plasticity (variability) in life history parameters to compensate for the unpredictability of their environment. In this study variability in life history parameters and possible sources of variability were investigated in four serotinous (= seed retaining) plant species from the Namib Desert. Blepharis grossa, Geigeria alata, Geigeria ornativa and Petalidium setosum were studied in a mosaic of spatio-temporal environmental conditions represented by three sites and three rain seasons. The coefficient of variation and whether or not significant differences occurred between environmental conditions were used to determine the most variable species and parameters. The most variable species was the strictly annual B. grossa, which might be related to its life form strategy and poor competitive ability as a pioneer species. Life history parameters which responded most frequently to varying environmental conditions were source pool, flowering time and seed production. Seedling density was influenced by the source pool and intensity of rainfall, seedling establishment by timing and intensity of rain events, as well as by seed mass. Although it was expected that high within population variability in life history parameters would also result in a response to differing environmental conditions, there was no direct correlation. Variable responses in life history parameters in most species were caused by differences in site condition as well as by differences in seasons, especially with regard to timing and intensity of rain events.