Composition of the soil seed bank in alien-invaded grassy fynbos: Potential for recovery after clearing

Indigenous soil seed banks play a very important role in facilitating the natural recovery of indigenous fynbos vegetation after clearing invasive alien vegetation. In densely invaded areas, there is a reduction in fynbos cover and seed production, and these ecosystems rely heavily on the remaining soil seed bank as a reservoir of plant propagules. This study used the seedling emergence approach to assess recovery potential based on the soil seed banks of riparian and hillslope grassy fynbos communities that had been densely invaded for three decades, with Acacia longifolia as the dominant alien species. Forty-eight species (of which five were aliens), representing 30 genera and 18 families, emerged from the soil seed bank, with Asteraceae and Cyperaceae being best represented. The mean density of indigenous seedlings for the study area was 1582 seedlings/m2. Senecio rigidus exhibited the highest density, with 274 seedlings/m2, followed by Chironia baccifera (151 seedlings/m2) and Rumohra adiantiformis (136 seedlings/m2). Forbs were the most numerous growth form. Two other alien species exhibited comparable seedling densities, i.e. Solanum nigrum (181 seedlings/m2) and Conyza canadensis (98 seedlings/m2). A. longifolia seed densities of up to 4528 seeds/m2 were found by sieving the soil. Results revealed that alien-invaded grassy fynbos had a diverse and viable soil seed bank with relatively high seed densities. Pioneer species were well represented, as well as graminoids in the riparian zone. Species representing some guilds were missing; e.g. serotinous species from the genera Leucadendron and Protea, and few geophytes were present in the hillslope soil seed bank, although ericoid shrubs were well represented. Riparian species such as Cliffortia graminea and common Cyperaceae and Restionaceae species were also not represented in the seed bank. It would appear from this data that the soil seed bank would be adequate to enable a functional cover of indigenous vegetation to re-establish after clearing. In order to improve vegetation structure and composition, the addition of some missing guilds would facilitate restoration, provided that post-clearing follow-up treatments do not prevent or hinder the establishment of these indigenous species.