Experimental effects of water stress on semi-arid Karoo seedlings: implications for field seedling survivorship

Very little is known about seedling establishment in the semi-arid Karoo of southern Africa. In a greenhouse study, we investigated the responses of seedlings of three shrub species to water stress by withholding water at different stages of seedling development. Mortality of the two Asteraceous species, Pteronia pallens and Osteospermum sinuatum was more rapid where seedlings were provided with water for longer periods before being subjected to water stress. Hardened seedlings, exposed to water stress at an earlier stage, survived the longest. These seedlings were the smallest with respect to root and shoot mass. Seedlings of the leaf succulent Ruschia spinosa were extremely hardy and after 400 days without water, approximately 80% of the seedlings were still alive. The last P. pallens seedling died 160 days after water had been withheld. Osteospermum sinuatum seedlings were the least tolerant to water stress and 100% mortality occurred within 120 days. These results are discussed in terms of microhabitat preferences of the seedlings in field situations.