Soil seed banks after Acacia dealbata Link invasion and removal: implications for restoration

Abstract

Background: Acacia dealbata Link is a problematic invasive alien tree in South Africa, with severe environment effects such as native plant displacement and soil nutrient manipulation (nitrogen fixation). Most studies on A. dealbata invasion are on aboveground vegetation effects, with a few studies on belowground soil seed bank impacts associated with both invasion and removal for restoration purposes. This study assessed the impact of A. dealbata invasion and removal on soil seed banks in grasslands targeted for ecological restoration in the upper Tsitsana communal area of Eastern Cape Province, South Africa. Soils were collected from A. dealbata invaded, cleared, and uninvaded treatments and germinated in an ambient greenhouse over 11 months. We measured species diversity, density, frequency of occupancy, and identified all the seedlings that germinated in trays measuring 20 cm wide × 20 cm long × 15 cm deep. Results: Species richness and abundance were significantly (P < 0.001) lower in the cleared and invaded treatments compared to the uninvaded treatment, with this trend being more pronounced for forbs and graminoids than for trees and shrubs, indicating that soil seed bank impacts are growth-form specific. Species density for all species was significantly (P < 0.001) lower in the invaded treatment compared to the cleared and uninvaded treatments. The removal of A. dealbata increased species composition in the cleared treatment with woody species such as Diospyrus and Searsia species occurring in the cleared than invaded treatment. Conclusions: The study concludes that A. dealbata invasion reduces soil seed bank community diversity and composition in upper Tsitsana grasslands. However, seed banks in the cleared treatment appear adequate to facilitate grassland recovery following the removal of A. dealbata.