Re-establishment of Protea repens after clearing invasive Acacia saligna: Consequences of soil legacy effects and a native nitrophilic weedy species

Invasive Australian acacias can alter soil chemistry and microbial communities in areas they invade. After clearing invasive acacias, these changes can persist, and previously invaded areas can become dominated by nitrophilic weedy species. Restoration of viable native plant communities in cleared sites often fails due to a lack of native species re-establishment. Therefore, to improve restoration outcomes, it is important to understand the effects of soil chemical and biotic legacies, and of nitrophilic weedy species, on native species re-establishment. To investigate the effect of soil chemical legacies, we germinated and grew Protea repens seedlings (a native proteoid shrub) as an indicator species in soil taken from areas cleared of Acacia saligna in lowland fynbos, as well as from non-invaded areas under controlled conditions. To investigate the effect of soil biotic legacies, we sterilized half the soil from each cleared or non-invaded area. We grew Ehrharta calycina (a native nitrophilic weedy grass species) in half of each treatment and measured the effect of treatments on P. repens germination and growth. Germination percentage, root and shoot dry mass of P. repens did not significantly differ between altered and native soil chemistry. The germination percentage of P. repens was significantly greater (93%) in the presence of soil microbial communities than in their absence. The presence of E. calycina significantly increased (29%) the root-to-shoot ratio of P. repens than their absence. Since the legacy of altered soil chemistry did not have a direct negative effect on P. repens germination and growth; we conclude that restoration efforts do not always have to manage altered soil chemistry after clearing invasive A. saligna.