GROWTH RING CHARACTERISTICS OF SOME AFRICAN ACACIA SPECIES

The age of most temperate tree species can be estimated accurately by counting the annual rings revealed by concentric changes in their wood anatomy. In the tropics and sub-tropics, this growth periodicity is seldom clearly and unambiguously defined. This research investigates whether a wood anatomical feature is present that delimits annual periods in one of the most widely distributed genera in the semi-arid and arid areas of Africa, Acacia. Most research on this topic has been based on sample trees from natural stands, often the largest and putatively oldest trees. In contrast, the trees sampled for this study were actively sought from material of known age, particularly those for which the history of management was known. Several African Acacia species were examined for growth rings. These were apparent in most species as narrow bands of marginal parenchyma filled with long crystal chains. The crystals were subsequently identified as calcium oxalate through the use of a scanning proton microprobe. The number of bands formed annually corresponded to the number of peaks in the annual rainfall distribution. Ring widths were highly correlated with total annual rainfall.