Phenology, flowering and fruit-set patterns of baobabs, Adansonia digitata, in southern Africa

摘要

Baobabs (Adansonia digitata) are iconic and highly valued trees that characterise many semi-arid environments across Africa. The aim of this study was to describe leaf, flowering and fruit phenology, flower production and fruit-set patterns of southern African baobabs. This was done on a sample of 106 trees across five land-use types at monthly intervals over two-years. Rainfall in the first year (2006/7, Year 1) was only 275 mm, but doubled in the second year (516 mm; 2007/8, Year 2), being below and above the long term mean of 461 mm, respectively. Leaf flush preceded the onset of rains (October) in 88% of trees in Year 1, but after the onset of rains (August) in all trees in Year 2. Leaves flushed in November and were retained until April and in October and retained until March, respectively. Leaf fall occurred one month later in Year 1 (May) than in Year 2 (April). Flowering followed a steady-state pattern, lasting for 1-5 months with peak flowering in November in both years. For adult trees, flower number/tree (Year 1: 711 +/- 72 (S.E.) and Year 2: 287 +/- 33), but not fruit-set (mean of 20 +/- 4%) varied significantly between years. Flower number showed a logarithmic relationship with tree size (stem diameter) (R-2 = 0.3830, P 0.0001), while fruit-set was unrelated to tree size (R-2 = 0.0045, P = 0.5081). Flower number and fruit-set did not vary between five land-use types, but length of flowering did, with village trees flowering for the longest period. Baobabs are hermaphrodite plants with both male and female reproductive structures in the same flower. Yet, across Africa many people refer to individual trees as being 'male' (fruiting is absent or minimal) or 'female' (substantial fruiting). Producer 'female' and poor-producer 'male' trees, did not differ in flowering phenology (number, timing and length of flowering), but fruit-set over two sequential years differed greatly between producer (33.5 +/- 5.2%) and poor producer (0.2 +/- 0.1%) trees. Leaf flush was responsive to early rains and hence baobabs appear to be facultative early greeners. However flowering and fruit-set patterns were not significantly different between these two years, despite the large rainfall difference. Although flower production was not different between producer and poor-producer trees in either year, fruit set was three orders of magnitude higher in producer than poor-producer trees. These quantitative results suggest that baobabs may be functionally dioecious and thus a complete characterization of the reproductive biology is required. Mechanisms underlying this pattern are discussed in terms of tree age, environment, pollination, genetics and evolutionary biology.