Water requirements for germination and early seedling establishment in four African savanna woody plant species

The aim of this study was to determine the water requirements for germination and early seedling establishment of four African savanna tree species, namely Acacia karroo, A. nilotica, A. tortilis and Mundulea sericea. The acacias are characteristic of nutrient-rich, and M. sericea of nutrient-poor savannas. Imbibition times of scarified seeds of A. karroo (4 h), M. sericea (6 h) and A. tortilis (8 h) were rapid relative to A. nilotica (28 h) and were inversely correlated with seed size. Imbibed water is lost in about 2 h after drying at 25 degrees C for ail species except A. nilotica (only 70% moisture loss). Seeds of A. karroo and M. sericea kept at 40 degrees C dried to below their original moisture contents within 2 h. Seeds of A. nilotica and A. tortilis dried at 40 degrees C lost viability relative to (undried) control seeds, while there was no significant loss of viability for A. Karroo and M. sericea. For fully imbibed seeds to germinate in sandy savanna soils, all four species required at least the equivalent of 3 mm rainfall every 2 days under mild greenhouse conditions. However, most ungerminated seeds were still viable despite drying-out from a fully imbibed state. Maintenance of soil at 50% field capacity (FC), or watering to FC every 9th day is the maintenance requirements for 2-week-old seedlings of A. nilotica and M. sericea to continue growth for a further 5 weeks under greenhouse conditions, although A. nilotica mostly survived 25% FC with one seedling surviving 12.5% FC. Root penetration was rapid in sandy soils, with A. nilotica and M. sericea attaining a depth of 40 cm within 15 days. Frequent, but not necessarily high, rainfall appears to be essential for germination and seedling survival over the first 7 weeks. (C) 1998 Academic Press Limited.