Secondary succession in Acacia nilotica (L.) savanna in the Hluhluwe Game Reserve, South Africa
Tóm tắt
Analysis of aerial photographs indicates that woody plant biomass has rapidly increased in Hluhluwe Game Reserve over the last 40 years. Open Acacia nilotica savanna is being replaced by broadleaf species, especially Euclea spp. We were interested in whether this secondary successional shift was due to high numbers of seedlings establishing and growing to maturity under acacias (facilitation) or due to the release of already established, but suppressed individuals (‘gullivers’) of the resprouting broadleaf species. We examined the recruitment patterns and size-class distributions (height, basal diameter) of important species in this savanna. Densities of euclea seedlings (<0.6 cm basal diameter) under Acacia nilotica were low (median of 0 and mean of 0.06 m2) below adult canopies and effectively zero in adjacent interspaces. No differences in numbers of other broadleaf species were found between open sites and under Acacia nilotica sites. Few large eucleas or other broadleaf species occurred under Acacia nilotica. Few Acacia nilotica recruits were found either under adults or in the open whereas Acacia karroo recruits were more common. The overall size class distribution for eucleas was dominated by individuals in the intermediate size class, suggesting that recruitment is not the dominant demographic process. We propose that the escape of intermediate sized eucleas from the fire trap has caused the increase in woody plants. Analysis of a time sequence of aerial photographs shows that invasion occurred rapidly between 1954 and 1975. A common feature for sites where woody plant invasion has taken place, was the presence of barriers to fire (especially roads). We suggest that the recent and rapid increase in woody vegetation is due to a decrease in the frequency of intense fires, rather than the recent absence of megaherbivores which allowed Acacia nilotica establishment. Few intense fires allow suppressed tree or shrub individuals, ('gullivers'), to escape the grass/fire layer and thereby become tall and fire-resistant. This release may explain the rapid rate of invasion by inherently slow growing broad-leaf species.
Tài liệu tham khảo
Archer, S., Scifres, C., Bassham, C. R. & Maggio, R. 1988. Autogenic succession in asubtropical savanna: Conversion of grassland to thorn woodland. Ecol. Monogr. 58: 111–127.
Archer, S. 1990. Development and stability of grass/woody mosaics in a subtropical savanna parkland, Texas, USA. J. Biog. 17: 453–462.
Archer, S., Schimel, D. S. & Holland, E. A. 1995. Mechanisms of shrubland expansion: Land use, Climate or CO2? Climate Change 29: 91–99.
Bond, W. J. & van Wilgen, B. W. 1996. Fire and Plants. Chapman & Hall. London.
Brooks, P. M. and Macdonald, I. W. A. 1983. The Hluhluwe-Umfolozi Reserve: An ecological case history. Pp. 51–77. In Owen-Smith, R. N. (ed.), Management of large mammals in African conservation areas. Haum, Pretoria..
Brown, J. R. & Archer, S. 1989. Woody plant invasion of grasslands: establishment ofhoney mesquite (Prosopis glandulosa var. glandulosa) on sites differing in herbacious biomass and grazing history. Oecologia 80: 19–26.
Franco-Pizana, J. G., Fulbright, T. E., Gardiner, D. T. & Tipton, A. R. 1996. Shrub emergence and seedling growth in microenvironments created by Prosopis glandulosa. J. Veg. Sci. 7: 257–64.
Hall, M. 1980. An iron-smelting site in the Hluhluwe Game Reserve, Zululand. Ann. Natal Mus. 24 (1): 165–175.
Gourlay, I. D. 1995. Growth characteristics of some African Acacia species. J. Trop. Ecol. 11: 121–140.
King, N. 1987. Bush Encroachment in Hluhluwe Game Reserve. Masters Thesis, University of Witwatersrand.
Noble, I. R. & Slatyer, R. O. 1980. The use of vital attributes to predict successional changes in plant communities subject to recurrent disturbances. Vegetatio 43: 5–21.
O'Connor, T. G. 1995. Acacia karoo invasion of grassland: environmental and biotic effects influencing seedling emergence and establishment. Oecologia 103: 214–223.
Trollope, W. S. W. 1984. Fire in Savanna, in Ecological Effects of Fire in South African Ecosystems. In: Booysen, P. de V. and Tainton, N. M. (eds), Springer-Verlag, Berlin, pp. 199–306.
Smith, T. M. & Goodman, P. S. 1986. The effect of competition on the structure anddynamics of Acacia savannah in southern Africa. J. Ecol. 74: 1031–1041.
Smith, T. M. & Goodman, P. S. 1987. Successional dynamics in an Acacia nilotica-Euclea divinorum savannah in southern Africa. J. Ecol. 75: 603–610.
Vetaas, O. R. 1992. Micro-site effects of trees and shrubs in dry savannas. J. Veg. Sci. 3: 337–344.
Watson, H. K. & Macdonald, I. A. W. 1983. Vegetation changes in the Hluhluwe-Umfolozi Game Reserve Complex from 1937 to 1975. Bothalia 14 (2): 265–269.
Whateley, A. & Porter, R. N. 1983. The woody vegetation communities of the Hluhluwe-Corridor-Umfolozi Game Reserve Complex. Bothalia 14 (3 & 4): 745–758.
Young T. P. & Lindsay W. K. 1988. Role of even-age populations structure in the disappearance of Acacia xanthophloea woodlands. African J. Ecol. 26: 69–72.