Effects of invasive alien acacias on nutrient cycling in the coastal lowlands of the Cape fynbos.
Production, N and P return, and decomposition of leaf litter of the invasive alien Acacia saligna were compared with that of the indigenous sclerophyllous shrub, Leucospermum parile, during the early stages of invasion in sand-plain lowland fynbos with acid soil low in P. A similar comparison was made for the invasive alien A. cyclops and Pterocelastrus tricuspidatus in strandveld with alkaline soils high in P. The 2 sites were in the SW Cape, South Africa. Measurements were made from 1986 to 1988. The Acacia spp. had twice the leaf N concentrations of the indigenous species. Leaf P concentrations were greatest in the strandveld species. The Acacia spp. tended to produce more litter, with three times the N content of that of the indigenous species. No significant differences in P return were found between the Acacia spp. and indigenous species in either vegetation. Decomposition turnover times were longer in the fynbos species than those of the strandveld. Nitrogen was immobilized in the leaf litter of the indigenous species, while the N contents of the Acacia spp. varied little. Phosphorus was immobilized in the fynbos species compared with a release of about 50% from P. tricuspidatus after 2 years. Soil N concentrations and litter-layer N contents were elevated under Acacia spp. canopies. The N status of the fynbos and strandveld ecosystems is elevated by the invasion of alien Acacia spp. The results for P cycling were equivocal and P availability does not appear to limit plant growth in the strandveld.