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.
Similar content being viewed by others
References
Acocks J.P.H. 1953. Veld types of Southern Africa. Mem. Bot. Surv. S. Afr. 28.
Baker H.G. 1989. Some aspects of the natural history of seed banks. In: Leck M.A., Parker V.T. and Simpson R.L. (eds), Ecology of Soil Seed Banks. Academic Press, London, pp. 9–24.
Bullock J.M., Silverton J. and ClearHill B. 1996. Plant demographic responses to environmental variation: distinguishing between effects on age structure and effects on age-specific vital rates. Journal of Ecology 84: 733–743.
Christensen N.L. 1993. Fire regimes and ecosystem dynamics. In: Crutzen P.J. and Goldammer J.G. (eds), Fire in the Environ-ment – the Ecological, Atmospheric and Climatic Importance of Vegetation Fires. John Wiley & Sons, Chichester, England, pp. 233–244
Clark J.S. 1991a. Disturbance and population structure on the shifting mosaic landscape. Ecology 72: 1119–1137.
Clark J.S. 1991b. Disturbance and tree population life history on the shifting mosaic landscape. Ecology 72: 1102–1118.
Cole M.M. 1986. The Savannas: Biogeography and Geobotany. Academic Press, London.
Crawley M.J. 1990. The population dynamics of plants. Phil. Trans. R. Soc. Lond. B. 330: 125–140.
Frost P.G.H. 1987. The regional landscape: Nylsvley in perspective. South African National Scientific Programmes Report 133. CSIR, Pretoria, 30. pp.
Garner R.D. and Witkowski E.T.F. 1997. Variations in seed size and shape in relation to depth of burial in the soil and predispersal predation in Acacia nilotica, A. tortilis and Dichrostachys cinerea. South African Journal of Botany 63: 371–377.
Grossman D., Grunow J.O. and Theron G.K. 1980. Biomass cycles, accumulation rates and nutritional characteristics of grass layer plants in canopied and uncanopied subhabitats of Burkea savanna. Proc. Grassland Society of southern Africa 15: 157–161.
Grubb P.J. 1977. The maintenance of species-richness in plant communities: The importance of the regeneration niche. Biological Review 52: 107–145.
Grunow J.D. 1974. Savanna Ecosystem Research Project: General information on Nylsvleyfarm. Report to National Programme for Environmental Sciences. CSIR, 6pp.
Hare R.C. 1965. Contribution of bark to fire resistance of southern trees. Journal of Forestry 63: 248–251.
Henderson L. 1979. The age structure and dynamics of the Burkea Africana community at Nylsvley.
Huntley B.J. and Walker B.H. (eds) 1982. Ecology of tropical savannas, Ecological Studies 42. Springer-Verlag, Berlin.
Jackson G. 1974. Cryptogeal germination and other seedling adaptations to the burning of vegetation in savanna regions: the origin of the Prophytic habit. New Phtologist 73: 771–780.
Kalitz S. 1991. Experimental determination of seed bank age structure in the winter annual Collinsia verna. Ecology 72: 575–585.
Kalitz S. and McPeek M.A. 1992. Demography of an age-structured annual: resampling projection matrices, elasticity analyses, and seed bank effects. Ecology 73: 1082–1093.
Kalitz S. and McPeek M.A. 1993. Extinction dynamics, population growth and seed banks. Oecologia 95: 314–320.
Knoop W.T. and Walker B.H. 1985. Interactions of woody and herbaceous vegetation in a southern African savanna. Journal of Ecology 73: 235–253.
Lamont B.B., Groom P.K., Richards M.B. and Witkowski E.T.F. 1999. Recovery of Banksia and Hakea communities after fire in Mediterranean Australia-the role of species identity and functional attributes. Diversity and Distributions 5: 15–26.
Laws R.M. 1970. Elephants as agents of habitat and landscape change in east Africa. Oikos 21: 1–15.
Mbalo B.A. and Witkowski E.T.F. 1997. Tolerance to soil temperatures experienced during and after the passage of fire in seeds of Acacia karroo, A. tortilis and Chromolaena odorata: a laboratory study. South African Journal of Botany 63: 421–425.
Moore R.P. 1985. Handbook on Tetrazolium Testing. The International Seed Testing Association, Zurich, Switzerland.
Nicolia V. 1989. Thermal properties and fauna on the bark of trees in two different African ecosystems. Oecologia 80: 421–430.
Pate J.S., Froend R.H., Bowen B.J., Hansen A. and Kuo J. 1990. Seedling growth and storage characteristics of seeder and resprouter species of Mediterranean-type ecosystems of S.W. Australia. Annals of Botany 65: 585–601.
Pinard M.A. and Huffman A. 1997. Fire resistance and bark properties of trees in a seasonally dry forest in eastern Bolivia. Journal of Tropical Ecology 13: 727–740.
Silvertown J. 1991. Modularity, reproductive thresholds and plant population dynamics. Functional Ecology 5: 577–580.
Strang R.M. 1969. Soil moisture relations under grassland and under woodland in the Rhodesian highveld. Commonwealth Forestry Review 48: 26–40.
Thompson K., Band S.R. and Hodgson J.G. 1993. Seed size and shape predict persistence in soil. Functional Ecology 7: 236–241.
Trabaud L. 1987. Fire and survival traits of plants. In: Trabaud L. (ed.), The Role of Fire in Ecological Systems. SPB Academic Publishing, The Hague, The Netherlands, pp. 65–90.
Trollope W.S.W. 1982. Fire in savanna. In: Huntley B.J. and Walker B.H. (eds), Ecological Effects of Fire in South African Ecosystems, Ecological Studies 42. Springer-Verlag, Berlin.
Walker B.H. 1979. Management of Semi-arid Ecosystems. Elsevier Scientific Publishing Company, Amsterdam.
Walker B.H., Stone L., Henderson L. and Vernede M. 1986. Size structure analysis of the dominant trees in a South African savanna.South African Journal of Botany52: 397–402.
Wilson T.B. and Witkowski E.T.F. 1998. Water requirements for germination and early seedling establishment in four African savanna woody plant species. Journal of Arid Environments 38: 541–550.
Witkowski E.T.F. and Garner R.D. 2000. Spatial distribution of soil seed banks of three African savanna woody species at two contrasting sites. Plant Ecology 149: 91–106.
Witkowski E.T.F. and Lamont B.B. 1996. Disproportionate allocation of mineral nutrients and carbon between vegetative and reproductive structures in Banksia hookeriana. Oecologia105: 38–42.
Witkowski E.T.F., Lamont B.B. and Obbens F.J. 1994. Commercial picking of Banksia hookeriana in the wild reduces subsequent shoot, flower and seed production. Journal of Applied Ecology 31: 508–520.
Witkowski E.T.F., Weiersbye-Witkowski I.M., Przybylowicz W.J. and Mesjasz-Przybylowicz J. 1997. Nuclear microprobe studies of elemental distributions in dormant seeds of Burkea africana. Nuclear Instruments and Methods in Physics Research B 130: 381–387.
Yeaton R.I. 1988. Porcupines, fires and the dynamics of the tree layer of the Burkea africana savanna. Journal of Ecology 76: 1017–1029.
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Wilson, B., Witkowski, E. Seed banks, bark thickness and change in age and size structure (1978–1999) of the African savanna tree, Burkea africana . Plant Ecology 167, 151–162 (2003). https://doi.org/10.1023/A:1023999806577
Issue Date:
DOI: https://doi.org/10.1023/A:1023999806577