WATER REQUIREMENTS FOR GERMINATION AND EARLY SEEDLING ESTABLISHMENT IN 4 AFRICAN SAVANNA WOODY PLANT-SPECIES

The aim of this study was to determine the water requirements for germ ination and early seedling establishment of four African savanna tree species, namely Acacia karroo, A. nilotica, A. tortilis and Mundulea s ericea. The acacias are characteristic of nutrient-rich, and M. serice a 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 relat ive 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. k arroo and M. sericea kept at 40 degrees C dried to below their origina l moisture contents within 2 h. Seeds of A. nilotica and A. tortilis d ried at 40 degrees C lost viability relative to (undried) control seed s, while there was no significant loss of viability for A. Karroo and M. sericea. For fully imbibed seeds to germinate in sandy savanna soil s, all four species required at least the equivalent of 3 mm rainfall every 2 days under mild greenhouse conditions. However, most ungermina ted seeds were still viable despite drying-out from a fully imbibed st ate. Maintenance of soil at 50% field capacity (FC), or watering to FC every 9th day is the maintenance requirements for 2-week-old seedling s of A. nilotica and M. sericea to continue growth for a further 5 wee ks under greenhouse conditions, although A. nilotica mostly survived 2 5% 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 app ears to be essential for germination and seedling survival over the fi rst 7 weeks. (C) 1998 Academic Press Limited.