The effects of invasion by Acacia saligna on the guild structure and regeneration capabilities of South African fynbos shrublands.
Abstract
The impact of dense stands of the alien invasive species Acacia saligna on the guild structure of indigenous fynbos vegetation was investigated at 3 sites on the Cape Peninsula, South Africa. Two Acacia stands of either recent (1-2 fire cycles) or longer (>2 fire cycles) origin were compared with neighbouring uninvaded vegetation. At one site fynbos recruitment was monitored following complete removal of vegetation in the 3 different stands in order to assess the restoration potential of invaded vegetation. Fynbos species richness, cover and frequency all declined through each invasion stage in the standing vegetation. Guild structure also changed: species with vertebrate-dispersed seeds were relatively more frequent in long-invaded stands and at 2 of the 3 sites long-invaded stands had relatively more tall shrubs and fewer shrubs with leptophyllous (ericoid) leaves than fynbos. Serotinous shrubs were virtually absent in long-invaded stands. Richness per plot of species recruited by seedlings was highest in fynbos and declined both with stage of invasion and time after clearing. However, the total stand species richness was nearly as high in recently invaded as in uninvaded fynbos. Ephemeral forbs constituted the majority of seedlings in all stands, but after 2 yr they remained the most important growth form in terms of density and cover only in the long-invaded stand. At 18 months after clearing, indigenous canopy cover was about 50% in all stands, but only in uninvaded fynbos did this continue to increase over the second summer. After 2 yr, resprouters formed about half the canopy cover in fynbos and recently invaded stands, but were insignificant in the long-invaded stand. After clearing, many species recruiting in the long-invaded stand were not present in the standing vegetation, indicating that persistent seed banks exist. As representatives of all the major fynbos growth forms were recruited into the long-invaded stand after clearing, albeit at a low density, such stands could revert to vegetation resembling fynbos in structure. However, in order to speed up the restoration process and to improve the probability of a fully functioning ecosystem being established, guilds which have been eliminated, such as serotinous Proteaceae, should be reintroduced.