Changes in the woody component of an East African savanna during 25 years

Abstract

Changes in density, structure and species composition of the woody component of the two predominant savanna types in Tarangire National Park, northern Tanzania, during a period of 25 y were investigated. The park is known for its large, increasing elephant numbers and high frequency of fires. In 1996 a study on woody species density, composition and age structure, which was first performed in 1971, was repeated, using the same transects and method. Access to the original data of 1971 allowed for a full comparison of the changes and an investigation whether these could be related to changes in elephant numbers and fire frequency. The total tree density declined during the 25 y, but the decline was not evenly distributed over the different height classes. Although the density of trees taller than 5 m declined significantly, the greatest decline occurred in the density of trees shorter than 1 m. The density of trees in the intermediate height class of 1-5 m did not decline. Although damage to trees by elephants increased during the 25-y period, c. 25% showed no browse damage and, except for some severely damaged trees, elephant damage was not found to reduce tree vigour. Elephants affected the size distribution of the savanna woody component much more than the density, while the data suggest no significant effect of fire on changes in tree density. The large decline in density of small trees was attributed to a severe drought in 1993. Based on large numbers of elephants during the past decades and on relatively low elephant impact on the total tree density, the present study suggests that the current elephant number of 2300 can be sustained in the park without causing detrimental effects, provided that their current range is maintained.