The late Professor Michael Evenari was a leader and agreat scientist with a very wide view and varied interests. Throughout the 26 years that I studied desert plants with Professor Evenari in the deserts of Israel and the Sinai Peninsula, he liked to summarize the seasonal field observations of seedling emergence with the words, “this particular year is a very special year.”
What are the reasons for such species' diversity, and what are the survival strategies of desert annuals'? Some species are common and others emerge only once in several years under unpredictable seasonal precipitation and massive seed consumption by ants. Escape dispersal strategies after maturation of the tiny, long-living seeds, and partial “opportunistic” germination strategies after only 10 mm of rain, are found in some common annuals such as Schismus arabicus and Spergularia diandra. Day lengthing seed maturation, and light and temperatures during seed wetting and germination, also affect their germinability. In S. diandra, nine types of seeds have been found (3 genotypes and 3 color phenotypes), which differ in coat structure, color, and germinability, and in Mesembryanthemum nodiflorum a position effect was found (3 groups of seeds in a capsule). The more opportunities for a small portion of seeds from the seed bank to germinate after several rainfalls, the greater the chances to germinate at suitable rain distribution. This enables these plants to develop and produce large numbers of seeds, even after a number of small rainfalls.
Responses of Desert Annual Plants to Ozone and Water Stress in anin situExperiment