Effects of Acacia melanoxylon, an alien tree species to South Africa, on Afrotemperate forest tree sapling composition

Abstract

The alien tree Acacia melanoxylon (R.Br.) occurs extensively in the Southern Cape Afrotemperate Forest (South Africa). Concerns exist  about its potential negative effects on indigenous tree sapling communities, ultimately with implications for forest composition and  structure. We investigated whether Acacia melanoxylon affects Afrotemperate forest tree sapling composition in the Garden Route  National Park. Using a paired plot design, we compared sapling species richness, diversity and density underneath 30 overstorey Acacia  melanoxylon trees and 30 indigenous counterparts of the species Podocarpus latifolius (Thunb.) R.Br. ex Mirb, Afrocarpus falcatus (Thunb.) C.N.Page, or Olea capensis macrocarpa (C.H.Wright) I.Verd. We recorded 2 503 tree saplings from 27 indigenous species and  only three Acacia melanoxylon saplings in the 60 plots. There were no significant differences in the richness, diversity or density of  saplings underneath Acacia melanoxylon trees versus their indigenous counterparts. Non-metric multidimensional scaling and cluster  analysis accordingly showed only minor differences in sapling species composition and abundance between Acacia melanoxylon and  indigenous overstorey trees. We concluded that indigenous tree sapling composition is not significantly altered by the presence of Acacia  melanoxylon trees at the densities (< 3 trees ha⁻¹) at which this species commonly occurs in Afrotemperate forest in the Garden Route  National Park. However, populations of Acacia melanoxylon in the forest interior act as a source of propagules for invasion in forest  margins, riparian areas and neighbouring fynbos shrubland, while at high density the species may alter forest gap-forming dynamics.  These invasive attributes need to be considered in the management of the species in the region.