Effect of invasive Australian acacias on the regeneration, growth and nutrient chemistry of South African lowland fynbos.
Abstract
Regeneration, mortality, local extinction, growth and foliar elemental concentrations of indigenous (ericoid, restioid, proteoid) and alien (acacia) plant forms and concentrations of soil elements were compared in stands of the invasive Australian Acacia saligna and in surrounding sand plain lowland fynbos vegetation in South Africa after an autumn fire. All indigenous forms regenerated less successfully in acacia-infested than in natural habitats. Seedling:parent ratios of proteoids were 32%, restioids 42% and ericoids 65% of those in natural vegetation. Seedling mortalities of all plant forms were not different in the two habitats. Simulated probabilities of extinction, based on measured spatial variation in seedling:parent ratios of different forms in acacia stands and in natural vegetation, showed that risk of local extinction after fire was 3-4 times greater for ericoids, 5 times greater for proteoids and 7 times greater for restioids in acacia-infested than in natural habitats. Within a 10-m2 acacia stand area, probability of extinction of proteoids was high (99.7%) and moderate (56-57%) for ericoid and restioid forms. Soil N, Ca, Mg, K, Mn and B were higher and soil Fe lower in acacia-infested than in natural habitats; but the only significant increases recorded for all plant forms were in N and K of seedling leaves in acacia stands. Seedling shoot and root dry masses of all forms, except restioids which had the lowest foliar elemental concentrations, were unaffected by the higher soil nutrients beneath acacias. Increased shoot:root ratios observed among indigenous forms in acacia-infested habitats reflected responses to shading by burnt acacia parental remnants rather than soil mineral enrichment. Acacia shoot:root ratios were unaffected by different environmental conditions in acacia stands. It is concluded that mineral enrichment of lowland fynbos soils by acacias has no detrimental effect on post-fire seedling growth and survival of indigenous taxa. Depletion of indigenous taxa, particularly obligate reseeding forms, beneath acacia stands results mainly from their poor seed regeneration success and associated increased risk of local extinction from stochastic causes in this habitat after fire.