Syntaxonomy and synecology of the grasslands of the southern parts of the Eastern Cape

Abstract

This study in the Eastern Cape was part of ongoing research on the Grassland Biome Project. The aims of the study were to produce a formal phytosociological inventory of the grassland communities of the Eastern Cape, relate the identified floristic units to environmental factors, predict the distribution of these based on linear modelling techniques and analyse the floristic data to determine biogeographic relationships and establish diversity patterns. As an introduction the physiography, geology, climate, soils and land use of the region were described. This environmental framework highlights the transitional nature of the sub-region and the habitat diversity. For the phytosociological study standard Braun-Blanquet procedures were followed, which resulted in a classification containing 4 dry grassland associations, 5 mesic grassland associations, 2 shrubland associations, 2 savanna associations, 1 grassy fynbos association, 5 karroid vegetation associations and 6 wetland and drainage line associations. These syntax.a were characterised on the basis of constituent floristic elements, habitat and distribution criteria and the relationship to other (published and unpublished) descriptions. Where names of syntax.a were formalised, type releves were identified. The study revealed remarkable variability in the described vegetation, but in many cases the floristic units showed a strong relationship to those from neighbouring regions. Gradients between floristic units were strong and were attributed to strong environmental gradients. These were strong altitude and precipitation-related gradients to explain floristic variability within grasslands, substrate and geomorphological gradients between grassland and woody vegetation, geological and rainfall season gradients to grassy fynbos, and precipitation, altitude and rainfall seasonality gradients to karoo vegetation. Wetland and drainage line communities were azonal to a large extent and depended on local moisture and drainage conditions to affect their character. Linear modelling techniques were used to predict the distribution of some of the main associations based on classes of median annual rainfall and elevation. Diversity patterns were established by examining species richness and dominance, within-association heterogeneity, species turnover along rainfall and surface rockiness gradients, and species turnover between geographical regions within the study area. Diversity in the grasslands and grassland-related vegetation of the Eastern Cape could be attributed to ecological factors (rainfall and surface rockiness gradients), disturbance (e.g. grazing), structural complexity in some vegetation types, and high species turnover between geographical regions. In addition, high dominance by a low number of species appeared to restrict diversity to a large extent.