Long-term growth patterns of Welwitschia mirabilis, a long-lived plant of the Namib Desert (including a bibliography)

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 w ith climatic data. The population structure as well as its spatial distribu tion was determined for 110 individuals. Growth rate was 0.37 mm day(-1), b ut varied 22-fold within individuals, fluctuating seasonally and varying be tween years. Seasonal patterns were correlated with air humidity, while ann ual differences were affected by rainfall. During three years, growth rate quadrupled following episodic rainfall events > 11 mm during mid-summer. On e natural recruitment event followed a 13-mm rainfall at the end of summer. Fog did not appear to influence growth patterns and germination. Plant loc ation affected growth rate; plants growing on the low banks, or ledges, of the main drainage channel grew at a higher rate, responded better and longe r to rainfall and had relatively larger leaves than plants in the main chan nel or its tributaries. This could be due to better water and nutrient cond itions on the ledges than elsewhere. The population appears to be growing o utwards, with the smallest (youngest?) plants highest. Sex ratio was male-b iased and males grew larger than females. Our study, in conjunction with th e extensive literature base on Welwitschia, published here in a bibliograph y comprising 297 papers, indicates the knowledge gaps and needs for further ecological studies, including the continuation of our LTER programme. This should elucidate the reproductive output, seed dispersal, recruitment, wat er availability, age structure, and ecological differences between the sexe s, and long-term life history strategies. Such knowledge would contribute t o desert ecology and improve the management strategies of this unique Namib Desert perennial.