Germination strategy of the East African savanna tree Acacia tortilis

Germination of Acacia tortilis seeds strongly depends on micro-site conditions. In Lake Manyara National Park, Tanzania, Acacia tortilis occurs abundantly in recently abandoned arable fields and in elephant-mediated gaps in acacia woodland, but does not regenerate in grass swards or beneath canopies. We examined the germination of Acacia tortilis using field and laboratory experiments. Seeds placed on top of the soil rarely germinated, while seeds covered with elephant dung or buried under the soil surface had a germination success between 23–43%. On bare soil 39% of both the dung-covered and buried seeds germinated, in perennial grass swards 24–43%, and under tree canopies 10–24% respectively. In laboratory experiments, seed water absorption correlated positively with temperature up to 41 °C, while subsequent germination was optimal at lower (21–23 °C) temperatures. Seeds that had absorbed water lost their viability when kept above 35.5 °C. The absence of light did not significantly influence germination success. Acacia tortilis does not actively disperse its seeds, but regeneration outside tree canopies was substantial. The regeneration potential thus strongly depends on the physiognomy of the vegetation.