'Vitellaria paradoxa' in Uganda : population structures and reproductive characteristics
An understanding of population structures and reproductive characteristics is fundamental to effective plant conservation and management, yet often such information is lacking even for important tree taxa such as Vitellaria paradoxa subsp. nilotica. Vitellaria paradoxa subsp. nilotica is a vital source of vegetable oil for communities in the parklands of northern Uganda. This study investigates population dynamiCS, leafing, flowering and fruiting phenologies, and reproductive processes and success in eight populations of Vitellaria paradoxa subsp. nilotica in Otuke County, Lira District, Uganda. Currently, in the populations of this tree there are many disproportionately high numbers (6.5-7.5 ha-1) of individuals >30 cm dbh but fewer (0.66-2.5 ha-1) young individuals < 30 cm dbh. Although seedlings regeneration is abundant (185 ha-1), few survive to pole stage «3 ha-1) because of cultivation and fire. Reproductive maturity and activity begins when individuals reach approximately 10 cm dbh. Full fruit production levels (mean: 500-1,500) are reached in trees >30 cm dbh. Flowering begins in early November and continues into late April, peaking from January to February. Fruiting peaks in Marchi April but only a low proportion (25%) of flowers develops into fruit. A high proportion (75%) of all the individuals:2: 10 cm dbh in the study populations flowered, the proportion rising with increasing dbh. However, monitoring showed that only 67% of the 800 trees flowered in two successive years of study. Environmental cues seem likely to synchronise an individual's flowering with conspecifics. Flowering is apparently affected by land use and home-compound trees enjoyed more favourable conditions for reproduction-presumably from better indigenous management, protection and less competition. Flowers are self-compatible but out-crossing is favoured by the floral structure and the sequence of events in anthesis. A vector is necessary for pollen movement from anther to stigma. Most trees have synchronous flowering and fruiting with fruiting being highly synchronised with relative humidity and precipitation Experimental investigation showed that fruit set in open pollination conditions (26%) was lower than fruit set with hand cross-pollination (32%). Fruit ripening during rainy season could be an adaptation for maintenance of the species in this harsh savanna environment prone to strong fires. This study indicates an old resource, giving very variable fruit production in Vitellaria individuals in time and space- further study of this variation is needed to allow more informed proper decision-making on stand management and better yield predictions.