Seed banks, bark thickness and change in age and size structure (1978-1999) of the African savanna tree, Burkea africana

Abstract

The population dynamics of the tree, Burkea africana Hook. was assessed within six sites in a nutrient-poor broad-leaf savanna, and seed banks were assessed within three of these sites. Population size and age structure in 1978 (representing the effects of 60 years cattle farming) differed little to those in 1999 (representing 20 years of conservation management). Greatest differences were in the smaller size classes, possibly in response to increased fire intensities over the last 20 years, although both periods displayed inverse J-shaped curves, typical of strongly recruiting populations. Growth rings were used to age trees, but estimates of age are actually the age since the last resprouting event, thus underestimating total age. Bark thickness increased almost linearly with plant size up to 400 mm trunk circumference, whereafter the relationship was poor for both currently fruiting and non-fruiting plants. This appears to represent a size at which trees are sufficiently protected by their bark to become fire resistant. It is also the typical minimum size for sexual maturity (flower and pod production), suggesting that the attainment of fire resistance immediately precedes the development of sexual maturity. Even though scarring of B. africana trees by porcupines was rare, the significant relationship between scarring and fire damage and consequent tree death resulted in the creation of gaps for regeneration. Densities of small plants (< 1 m in height) were 363 ± 341 plants ha−1 for those that had resprouted (94% of total), with only 7 ± 37 (1.7%) current and 13 ± 43 (4.3%) previous year's seedlings ha−1. Similar proportions occurred subcanopy (48%) and in the open (52%). Most seeds in the soil seed bank (69%) were recently dispersed and occurred in the litter (94%), while non-current seeds (31%) occurred mainly in the soil (55%). Buried seeds had greater viability than those at the surface. Seeds were effectively dispersed to all microsites but with highest densities below parent trees. At the landscape level, seed densities were about 750 000 ha−1. Presently, B. africana appears to be resilient to the change in management of the reserve.