Effects of repeated burning on woody vegetation structure and composition in a semi-arid southern African savanna

The objective of this study was to investigate the effects of repeated dry season annual hot fires on woody plants in a semi-arid southern African savanna in Zimbabwe. Parts of the National University of Science and Technology (NUST) research fields in Bulawayo, Zimbabwe have been burnt annually in the dry season between 1994 and 2003 in order to control bush encroachment. The present study was carried out in both the burnt and unburnt sites of the NUST research fields consisting of Acacia karroo-Colophospermum mopane vegetation. The study adopted a randomised block design and woody vegetation data were collected from a total of 10 plots. Variables measured and recorded included woody plant height, density, number of stems per plant, proportion of multi-stemmed plants, proportion of dead stems, basal area, fire damage and number of species per plot. The study results indicate that there were significant differences (P < 0.05) in woody plant heights, proportion of multistemmed plants, proportion of dead stems and basal areas between the burnt and unburnt sites. However, there were no significant differences (P > 0.05) in density, number of species per plot and number of stems per plant in woody plants between the burnt and unburnt sites. The study results suggest that repeated dry season annual hot fires leads to thinner and shortstemmed plants in semi-arid savanna ecosystems. Repeated burning also increased the proportion of multi-stemmed plants and proportion of dead stems in the burnt site. Despite burning sections of the study area annually, bush encroachment control has not been effectively achieved. The study findings points to the need of adaptive management strategies in the use of fires in managing vegetation in semi-arid savanna ecosystems.