Dynamics and succession of coastal dune vegetation in the Eastern Cape, South Africa

Abstract

Mobile or partially vegetated dunefields dominate the Eastern Cape coastline, with dune slacks being conspicuously well vegetated. Earlier studies on the dynamics of these habitats suggested that greater soil moisture, reduced sand movement and salt spray favoured the establishment of more mesic species. Since an understanding of these successional processes has important management implications, more detailed studies to firstly describe and then interpret the successional relationships between various plant communities were undertaken. From a survey of 152 relevés at 62 sites, seven coastal communities were recognised by TWINSPAN. This separation appeared to be based on the location, habitat and successional stages of the communities, and ordination by DECORANA showed that in any one area these communities were linked to a succession gradient from pioneer to “climax”. A more detailed investigation of the role played by dune slacks in this succession was undertaken at Kleinemonde. Six communities were separated by TWINSPAN. Pioneer communities of the foredunes had low cover and diversity, being dominated by herbs. Young dune slacks had low cover but moderately high diversity, with sedges and grasses dominating. Old dune slacks had a greater total mean cover, and shrubs were more abundant. Bushclumps were physiognomically more complex than dune slacks, with greater total mean cover, diversity and life form composition. Thicket margin communities were structurally similar to bushclumps, but floristically more complex. Dune thicket was the most physiognomically and floristically complex community sampled. Multivariate analysis revealed a distinct gradient of plant communities with increasing distance (age) from the sea, namely from Pioneer to Young Slack, Old Slack, Bushclump, Thicket Margin and Dune Thicket communities. With age, species richness, cover, diversity, number of life forms and structural complexity of the communities all increased. Importance value curves for 12 diagnostic species highlighted the distribution of these key species along the chronosequence. Pioneer species were restricted to the beginning, and dune slack species the central portion of the chronosequence. Hardy species had a wide distribution, but dune thicket species only appeared later. These trends appear to represent directional changes along a successional sequence as proposed in an earlier model, and the mode of transition between them relies heavily on autogenic changes. Dune slacks play an important role in this facilitation by acting as centres of diversity within the sparsely vegetated dunefield, and the foredunes by protecting dune slacks from salt spray and sand movement. Therefore, the role of foredunes and dune slacks must be considered in the management of the South African dune landscape, as they contribute significantly to the dynamic equilibrium of partly vegetated dunefields. Management should therefore be aimed at protecting these habitats.