Seed bank dynamics and the role of mutualisms in the early-life ecology of emerging alien invasive plants: Acacia paradoxa in South Africa
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Seed banks provide a persistent source of propagules for recruitment and establishment and are a very important factor for the success in recruitment, persistence and spread of many invasive alien plants worldwide. Where eradication of an invasive plant species is deemed feasible, the depletion of its seed bank is a crucial goal. The study of seed bank dynamics (in particular seed density, viability, longevity, and germinability) is a vital component of the quantification of invasion risks and of guidelines to direct eradication programmes. In this study the seed bank dynamics of Acacia paradoxa , an invasive alien tree from Australia, targeted for eradication on Table Mountain, South Africa, was investigated. Seed densities in the soil were as high as 3,567 seeds /m2. Seed viability was very high (97%) and there was no evidence of a decline in viability of seeds with increasing age. Natural germinability was very low (7%), but this is greatly enhanced by germination triggers such as heat (98% at 100 0C, 35% at 60 0C) and smoke (82%). Seed bank depletion, in burnt areas after the 2009 large and intense fire, ranged from 64.3% to 100% while natural germination depleted seed banks with 3% to 12%. The fire stimulated dense seedling recruitment (236 seedlings/m 2), and the density of A. Paradoxa seedlings was significantly negatively correlated with the density and number of species of taxa plant taxa ( p = 0.036 and 0.032, respectively). Implications of these results for the eradication effort include the following: 1) the species has a very large seed bank, and seeds are stimulated to germinate by fynbos fires; 2) prescribed fires can be used to reduce the size of the seed bank; 3) all control operations need to be carefully co-ordinated with fires; and 4) all seedlings must be removed before they reach reproductive age. The role of mutualisms has been determined. DNA sequencing of the intergenic spacer (IGS) region of bacterial symbionts isolated from the plant identified the bacteria associated with the roots of A. paradoxa as taxa in the genus Bradyrhizobium and Rhizobium leguminosarum viciae, both of which are known symbionts of Australian acacias in their native ranges. Experiments in the greenhouse showed that the bacterial strains form effective nodules. Nodulation significantly increased plant biomass (P= 0.0003). These results show that A. paradoxa benefits from its association with these bacteria, but it remains to be tested how this relationship affects the competitiveness of the species in its new range.