Reductions in Plant Species Richness under Stands of Alien Trees and Shrubs in the Fynbos Biome

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Abstract
The reduction of species richness of indigenous plants is one of the major problems associated with the presence of dense stands of invasive alien trees and shrubs in the Fynbos Biome of the Cape Province, South Africa. A synthesis was made of published and unpublished data on plant species richness in fynbos with different levels of invasion and different histories of control. 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 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 used in this study (4–256 m2) occur once the canopy cover of aliens exceeds about 50% and there is evidence of reduced species richness with increased time of suppression. For this reason, stands should be cleared before canopy closure is achieved.