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.
Gourlay, I.D. Growth ring characteristics of some African Acacia species. Journal of Tropical Ecology (1995) 11 (1) 121-140.