Competitive resistance of a native shrubland to invasion by the alien invasive tree species, Acacia cyclops

Abstract

Biotic resistance is infrequently considered when investigating the potential spread of invasive woody species. In this study, we determined whether native vegetation of Mediterranean-climate shrublands in the Cape Floristic Region of South Africa constrains woody invasive seedling establishment and performance through above- and below-ground competition for resources. Seedlings of the invasive tree, Acacia cyclops, were transplanted into native shrubland vegetation in a three-way factorial design where light availability, below-ground root competition and soil nutrient availability were manipulated. Survival, growth, biomass allocation, water use efficiency, foliar nutrients and δ15N were assessed after ca. 7 months. Control seedlings had a ca. 20 % survival, which increased to a maximum of ca. 80 % depending on treatments. Root exclusion tubes increased seedling survival and heights by ca. 1.6-fold and total biomass by ca. 4.8-fold. In contrast, above-ground availability of light did not influence seedling survival or height, although seedlings grown under the lowest light availability (0–20 %) had biomass ca. threefold lower than seedlings grown under the highest light availability (80–100 %). Competition from native vegetation, particularly below-ground competition, impedes survival, growth and biomass accumulation of these woody invasives, presumably constraining invasion spread. We conclude that the maintenance of intact, native ecosystems is important in limiting woody invasion spread and also suggest that biotic components should be included in invasion distribution and spread models in order to aid in optimization of scarce management resources.