Mortality and utilisation of Sclerocarya birrea subsp. Caffra between 2001 and 2008 in the Kruger National Park, South Africa

A resurvey of a sample of marula trees (n = 474) in the Kruger National Park (KNP) originally documented in 2001, was conducted in 2008 to determine the response of this population segment to the conditions prevalent during that time. These included an increasing elephant population and changes to the KNP fire policy. The overall mortality rate was estimated to be 2.6% per annum, with the mortality rate for individuals > 5 m in height being 3.7% per annum. The highest level of mortality was found in the 5-8 m height class (7.1% per annum), while the highest number of dead individuals occurred in the 11-15 m height class (25). In addition minimal recruitment into the seedling size class was observed (n = 6). No growth of saplings less than 2 m in height to heights greater than 2 m was observed. Almost half the individuals that were heavily damaged in 2001 had died by 2008. Heavy elephant utilisation and tree mortality was concentrated in the Sabie thorn thickets ecozone on granites where marula density was highest. Utilisation levels on the surviving individuals throughout the sampled area had almost tripled between 2001 and 2008, with individuals between 5 and 8 m in height accumulating the most additional elephant utilisation. It was concluded that the sampled marula population was not sustaining itself under the conditions prevalent between 2001 and 2008. Since sampled trees > 5 m in height were strongly biased towards the Sabie thorn thickets, any projections for the likely fate of marulas in terms of adult mortality would mainly apply to this ecozone. The spatial variability described in this study suggests that further sampling is needed to adequately represent trends elsewhere.