Seed bank distribution and viability of selected Vachellia and Acacia species in KwaZulu-Natal, South Africa.

Abstract

There is increasing concern regarding the impact of invasive alien plants (IAPs), where their spread is a serious threat to both the structure and functionality of ecosystems, which causes the loss of biodiversity. Approximately 10 million hectares is currently covered by IAPs in South Africa, with programs such as Working for Water (WFW) having been implemented in an attempt to manage them. This research investigated the seed soil distribution and viability of selected indigenous (Vachellia) and exotic invasive Acacia species in KwaZulu-Natal, South Africa. In this study, the seed banks of four tree species were sampled; Vachellia karroo Hayne, Vachellia nilotica (L.) P.J.H. Hurter & Mabb, Acacia mearnsii De Wild. (black wattle) and Acacia dealbata Link (silver wattle). The first two species are indigenous and the latter two are exotic. The selected invasive species are both classified as Category 2 species by the National Environmental Management: Biodiversity Act of 2004. The soil seed banks were determined, using a set sampling strategy of six pits at three depths for twelve selected trees per species. Seeds were removed from the soil, using soil sieves, and they were counted and tested for viability, using the tetrazolium chloride (TTZ) test. The majority of methods replicated those of Witkowski and Garner (2000). There was a high variability in seed numbers between different species and individual trees of the same species. Soil seed densities were greater in the Acacia species, compared to those of the Vachellia species. Acacia dealbata had the largest seed density, with the highest number of seeds in the top layer between 0-2 cm. Soil seed density declined with increasing distance from the trunk and with soil depth. The species with the greatest number of viable seeds in the seed bank was A. dealbata, followed by A. mearnsii. There was no significant difference (α = 0.05) in viability between the depths. The Acacia species had an advantage over the Vachellia species, with a higher soil seed bank density and seed viability. An improved knowledge of the seed banks can assist in providing evidence-based recommendations to improve the effectiveness of current methods for the removal of IAPs, which focus predominantly on ‘above-ground material’.