Reductions in plant species richness under stands of alien trees and shrubs in the fynbos biome.
One of the problems associated with the presence of dense stands of exotic trees and shrubs in the fynbos biome of the Cape Province, South Africa, is the reduction of indigenous plant species diversity. Shrubs, often forming a major part of fynbos vegetation, have been shown to be particularly vulnerable to invasive exotic species. A review was made of published data on plant species richness in fynbos (in 4 to 256 m2 quadrats); in addition, a survey was made of 4 m2 quadrats randomly distributed at 13 sites representative of various alien plant densities or control histories. The invasive species included Pinus pinaster, P. radiata, Hakea sericea, Acacia saligna, A. melanoxylon and A. cyclops. Linear regressions of species richness on the log of quadrat size were significant for both uninvaded fynbos and fynbos under dense stands of alien trees and shrubs. The slopes of the regression equations did not differ significantly between invaded and uninvaded sites, but elevations [y-intercepts] were significantly different, indicating a marked reduction in richness of indigenous plant species in invaded areas. The linear regression of species richness on quadrat size for cleared areas was not significant, but quadrats at most cleared sites showed species richness values intermediate to those of uninvaded fynbos and dense stands of aliens. Reductions in species richness at the scale of the sample quadrats evaluated in this study occur once the canopy cover of aliens exceeds about 50%, and there is evidence of reduced species richness as suppression continues. It is recommended that alien stands should be cleared before canopy closure.