Standing vegetation and seed bank patterns paint a bleak picture for urban grassland restoration

Abstract

Urbanisation and urban sprawl are major drivers of global habitat transformation and biodiversity loss. Natural vegetation in urban areas is confined to remnant patches and, as a result, the conservation of these patches of vegetation is becoming increasingly important for biodiversity conservation. Globally grasslands experience high rates of transformation and are threatened by expanding urban areas, causing fragmentation, and facilitating the spread of invasive species. This study explores how above- and belowground vegetation communities within remnant grassland patches vary as a function of patch connectivity and patch size in the City of Tshwane, Gauteng, South Africa. We sampled twelve sites from eleven nature reserves within Tshwane to investigate the relationships between urbanisation and vegetation characteristics. Tshwane’s grassland patches have high levels of invasion, with alien species making up a considerable portion of both standing (13% richness and a third of cover) and seed bank vegetation (31% richness and 26% abundance). Furthermore, we found low similarity between standing vegetation and the seed bank (mean ± SD = 0.25 ± 0.06). Neither road density nor reserve size were related to the total cover or richness of vascular plants, nor the cover or richness of alien species. Similarly, neither variable predicted above-belowground similarity. This suggests that the connectivity of remnant grassland patches does not have any significant effect on vegetation characteristics in this disturbed urban environment. Our results indicate that restoration relying only on natural revegetation from the seed bank is unlikely to be effective and we suggest that active restoration interventions, such as reseeding and invasive species control, may be needed to restore these grasslands and improve their long-term conservation value.