Growth of seedlings of the invasives,Acacia salignaandAcacia cyclopsin relation to soil phosphorus

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Abstract
Acacia salignaandAcacia cyclopsare the dominant invasive alien plants of phosphorus‐poor, sand‐plain, lowland fynbos and the relatively phosphorus‐rich strandveld vegetation of the southwestern Cape of South Africa, respectively, but their ranges overlap. Seedlings of the two species were grown in pots, in isolation and mixed, in up to seven treatments supplying a broad gradient in phosphorus (P) availability.Acacia salignaseedlings grew taller and had greater dry mass than those ofA. cyclopsat each level of P, but both peaked in response to the same relatively high soil P level and then tended to decline. Root : shoot ratios did not differ in response to P, but were greater inA. saligna. In mixture,A. salignahad a higher dry mass thanA. cyclopsat each level of P, but the relative differences between species were no greater than in isolation. Depth penetration of the soil by the tap root ofA. salignaseedlings was over three times as rapid as that ofA. cyclopsover a 30 day period. The higher absolute growth rates ofA. salignawere not related to seed size or seed nitrogen and P contents, as these were greater inA. cyclops. The contrasting distributions of the acacias do not appear to be a response to P availabilityper se, but possibly to the interaction of P with other factors such as moisture availability.

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