EFFECTS OF INVASIVE ALIEN ACACIAS ON NUTRIENT CYCLING IN THE COASTAL LOWLANDS OF THE CAPE FYNBOS

(1) Production, nitrogen and phosphorus return, and decomposition of leaf litter of the invasive alien, Acacia saligna, was compared with that of the indigenous sclerophyllous shrub, Leucospermum parile, in sand-plain lowland fynbos with acid soils low in P during the early stages of alien invasion. The same was done for A. cyclops and Pterocelastrus tricuspidatus in strandveld with alkaline soils high in P. (2) The Acacia spp. had twice the leaf N concentrations of the indigenous species, whereas P concentrations were highest in the strandveld species. (3) 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 acacias and indigenous species in either vegetation. (4) 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 acacias varied little. Phosphorus was immobilized in the fynbos species compared with a release of about 50% from P. tricuspidatus after 2 years. (5) Soil N concentrations and litter-layer N contents were elevated under acacia canopies. (6) The N status of the fynbos and strandveld ecosystems is elevated by the invasion of alien acacias. The results for P cycling are equivocal and P availability does not appear to limit plant growth in the strandveld.