Where are the young baobabs? Factors affecting regeneration of Adansonia digitata L. in a communally managed region of southern Africa

摘要

Large baobabs are prominent in many African savannas, but the apparent lack of young trees suggests that recruitment is limited and possibly episodic. This study aims to determine if recruitment was seed or establishment (microsite) limited. From five annual seed crops, baobab seed consistently exhibits high viability (>89%) and a field trial shows they form persistent soil seed banks. Seed production is substantial (5500 ± 2334 seed/ha) and thus recruitment does not appear to be seed limited. In closed plots (excluding livestock) 6.33% of seeds emerged after 328 ± 28 (mean ± SE) days and of these 94.4% died within 21 ± 5 days of moisture stress and insect browsing. In open plots (exposed to livestock) 2% of seeds emerged after 377 ± 5 days and all died within 12 ± 5 days from goat browsing. Planted sapling survival in closed plots (65%) was much higher than in open plots (10%), with an average time to death of 10.45 ± 0.97 and 4.33 ± 0.45 months respectively. Hence seedling establishment is poor in response to infrequent rainfall and survival of seedlings and saplings, that do manage to persist, is further hampered by high livestock numbers. Recruitment is thus microsite limited in communal lands. Planting and protection from livestock is required to overcome the recruitment bottleneck.