Secondary succession in Acacia nilotica (L.) savanna in the Hluhluwe Game Reserve, South Africa

Analysis of aerial photographs indicates that woody plant biomass has rapid ly increased in Hluhluwe Game Reserve over the last 40 years. Open Acacia n ilotica savanna is being replaced by broadleaf species, especially Euclea s pp. We were interested in whether this secondary successional shift was due to high numbers of seedlings establishing and growing to maturity under ac acias (facilitation) or due to the release of already established, but supp ressed individuals ('gullivers') of the resprouting broadleaf species. We e xamined the recruitment patterns and size-class distributions (height, basa l diameter) of important species in this savanna. Densities of euclea seedlings (< 0.6 cm basal diameter) under Acacia niloti ca were low (median of 0 and mean of 0.06 m(2)) below adult canopies and ef fectively zero in adjacent interspaces. No differences in numbers of other broadleaf species were found between open sites and under Acacia nilotica s ites. Few large eucleas or other broadleaf species occurred under Acacia ni lotica. Few Acacia nilotica recruits were found either under adults or in t he open whereas Acacia karroo recruits were more common. The overall size class distribution for eucleas was dominated by individual s in the intermediate size class, suggesting that recruitment is not the do minant demographic process. We propose that the escape of intermediate size d eucleas from the fire trap has caused the increase in woody plants. Analysis of a time sequence of aerial photographs shows that invasion occur red rapidly between 1954 and 1975. A common feature for sites where woody p lant invasion has taken place, was the presence of barriers to fire (especi ally roads). We suggest that the recent and rapid increase in woody vegetat ion is due to a decrease in the frequency of intense fires, rather than the recent absence of megaherbivores which allowed Acacia nilotica establishme nt. Few intense fires allow suppressed tree or shrub individuals, ('gullive rs'), to escape the grass/fire layer and thereby become tall and fire-resis tant. This release may explain the rapid rate of invasion by inherently slo w growing broad-leaf species.