Abstract
Acacia albida Del. is a widespread tree in the Sudan (Andrews 1952, Hassan 1978). Wickens (1969) and Werger & Ellenbroek (1982) indicated that the species exhibits an unusual phenological cycle, i.e. it sheds it leaves at the start of the rainy season and the new leaves flush at the start of the dry season. The theories which have been put forward to explain this foliage phenology of Acacia albida were summarized by Werger & Ellenbroek (1982) as follows: 'A. albida occurs in a habitat where ground water tables are high and often even above the surface during the rainy season. This leads to more or less anaerobic conditions in the root zone, which cause the tree to shed its leaves. During the dry season, aeration is much better and as the tree possesses deep roots, it also has enough water available and keeps growing.' Werger & Ellenbroek (1982), during field work in the Kafue area of Zambia (south of the Equator), observed the deviating crown shape of A. albida. They further added that trees of A. albida showed asymmetrical crowns on straight non-slanting boles, whereas the crowns of other trees in the same area were more or less spherical (symmetrical). They attributed the peculiar shape of A. albida to the phenological rhythm of the species. Werger & Ellenbroek's hypothesis is that species other than A. albida grow and photosynthesize in the rainy summer season when the sun moves overhead. In the dry winter season, the sun is continuously to the north and therefore, during the season when A. albida is actively growing, the light conditions are more favourable in that half of the crown which is exposed to the north which, consequently, grows faster. Werger & Ellenbroek (1982) concluded that the phenological behaviour,
Published Version
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