Age determination of miombo species Brachystegia spiciformis (Leguminosae – Caesalpinoideae) in Zimbabwe using growth rings

Abstract

The objective of this study was to determine whether Brachystegia spiciformis Benth. (Leguminosae – Caesalpinoideae), one of the dominants of dry miombo woodland in Zimbabwe, forms annual growth rings. In the past the production of effective cutting rotations and sound management plans for native miombo woodland in southern Africa has been hindered by the difficulty in determining the ages of the trees, and therefore their growth rates, because it was assumed that tropical trees did not form annual rings. This research indicates that B. spiciformis does form annual rings, made up of xylem cells terminated (or marked) by a narrow band of marginal axial parenchyma cells, even in dry years. The rings may sometimes be associated with a darkening of the xylem due to the production of flatter and thicker fibres, but the darker bands are not always produced at the end of the growing season as they would be in temperate species and may also be present at the beginning of the new season. Only in particularly wet or dry years do the changes in the width of the annual rings correspond closely with fluctuations in mean annual rainfall. These results provide a means to estimate the age and annual increment of this species, which will enable woodland managers to predict future yields more accurately and monitor of the effects of management on tree growth.

Keywords: miombo, annual growth rings, fuelwood harvesting rotations, sub-tropical dry savanna

Southern African Forestry Journal No. 206, 2006: 5-12