Growth rate of selected woody species in northern Maputaland, KwaZulu–Natal, South Africa

Abstract

Annual stem circumference growth rates of 23 woody species of the Sand Forest and woodland in Maputaland are presented for the first time. The rare Sand Forest, has been identified as an endemic, diverse vegetation type that is under threat from land transformation and human utilisation outside conservation areas. The growth rate of the selected woody species was evaluated over a 71-month survey that encompassed climatic extremes, oscillating from heavy rainfall to drought in a short period of time. The mean annual stem circumference growth rate among the investigated species varied thirteen fold, from a low of 2.04 mm/yr for Brachylaena huillensis to 26.28 mm/yr for Garcinia livingstonei. When the data of all species were considered, there was a significant positive relationship between stem circumference and growth rate, but no significant relationship between wood density and growth rate. The present results suggest that there is no significant difference in terms of mean annual growth rate between the Sand Forest and woodland vegetation types. In general, woodland species showed larger fluctuations in growth rate and the temporal pattern of Sand Forest species seemed to lag behind that of the woodland species. These trends can in part be ascribed to the woodland species all being deciduous, whereas the Sand Forest suite included both deciduous and evergreen species. Annual growth rates measured from dry season to dry season were not related to the seasonal rainfall pattern, but appeared to be highly dependent on the water availability at the time of the enumeration. The larger size classes reacted sooner to decreased water availability and a reduction in growth of larger trees was accompanied by an increase in growth in the smaller size classes.