Germination strategy of the East African savanna tree Acacia tortilis

Abstract

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.

Seed viability and germination success of Acacia tortilis along land‐use and aridity gradients in the Eastern Sahara

“…Others report a negative effect (Miller ; Oconnor ; Loth et al. ), possibly related to dung hardness, fibrousity, and speed of drying (Coe and Coe ; Wilson and Witkowski ). Camel and ovicaprids have relatively small, dry, and fibrous pellets with expected good moisture holding capacity; however, we observe no effect probably because of regular watering in our experiment.…”

Seed production, infestation, and viability in Acacia tortilis (synonym: Vachellia tortilis) and Acacia robusta (synonym: Vachellia robusta) across the Serengeti rainfall gradient

“…The former, also known as the "umbrella acacia", is a medium to large-sized tree producing pods that are variable in size and are indehiscent (pods do not open to expose their seeds while on the tree). In East Africa, A. tortilis seed production peaks between October and December (Loth et al 2005). This species has a broad geographic range, being widespread throughout Africa and the Middle East and locally abundant in Serengeti.…”

Grass competition overwhelms effects of herbivores and precipitation on early tree establishment in Serengeti

“…For instance, seed predation by small mammals, long ignored as a factor in the demography of savanna trees (Midgley & Bond, 2001;Shaw, Keesing, & Ostfeld, 2002), appears to limit seed germination in a number of ecosystems (Goheen, Keesing, Allan, Ogada, & Ostfeld, 2004;Vaz Ferreira, Bruna, & Vasconcelos, 2010;Walters, Milton, Somers, & Midgley, 2005). At the seed germination stage, a complex suite of factors-chief among them soil moisture availability-controls rates of germination in savannas (Barnes, 2001;Kraaij & Ward, 2006;Loth, Boer, Heitkonig, & Prins, 2005;Wilson & Witkowski, 1998). At the postgermination stage, targeted experiments have found that soil moisture availability, grass biomass, herbivory, and fire affect seedling establishment rates across a range of savanna types (Dickie, Schnitzer, Reich, & Hobbie, 2007;Goheen et al, 2010;Hoffmann, 1996;Inouye, Allison, & Johnson, 2004;Tyler, Kuhn, & Davis, 2006).…”