Plant Ecology 150: 7–26, 2000. 7 © 2000 Kluwer Academic Publishers. Printed in the Netherlands. Long-term growth patterns of Welwitschia mirabilis, a long-lived plant of the Namib Desert (including a bibliography) Joh R. Henschel & Mary K. Seely Desert Ecological Research Unit, Desert Research Foundation of Namibia, Gobabeb Training and Research Centre, P.O. Box 953, Walvis Bay, Namibia (E-mail:
[email protected]) Key words: Episodic events, Long-term ecological research, Namibia, Population dynamics, Seasonality, Sex ratio Abstract Over the past 14 years, long-term ecological research (LTER) was conducted on the desert perennial, Welwitschia mirabilis (Gnetales: Welwitschiaceae), located in the Welwitschia Wash near Gobabeb in the Central Namib Desert. We measured leaf growth of 21 plants on a monthly basis and compared this with climatic data. The population structure as well as its spatial distribution was determined for 110 individuals. Growth rate was 0.37 mm day−1, but varied 22-fold within individuals, fluctuating seasonally and varying between years. Seasonal patterns were correlated with air humidity, while annual differences were affected by rainfall. During three years, growth rate quadrupled following episodic rainfall events >11 mm during mid-summer. One natural recruitment event followed a 13-mm rainfall at the end of summer. Fog did not appear to influence growth patterns and germination. Plant loca- tion affected growth rate; plants growing on the low banks, or ledges, of the main drainage channel grew at a higher rate, responded better and longer to rainfall and had relatively larger leaves than plants in the main channel or its tributaries.