Zonal and azonal vegetation revisited: How is wetland vegetation distributed across different zonobiomes

Abstract

AbstractIn ‘zonal’ vegetation, climatic factors are the main influence on growth and performance and the climate determines the vegetation type completely, which makes this vegetation dominant in the landscape. If vegetation is ‘azonal’ however, local stresses are assumed to have an overwhelming influence on plant performance and climatic influences will be minimal; typically, this vegetation occurs only in small patches in the landscape. In this study I ask whether wetland plant communities, as they are described for South Africa, are evenly distributed among different terrestrial vegetation types, to test whether they are zonal or azonal. Three contingency tables were construed based on the counts of wetland vegetation records, defined on three hierarchical levels (Main Clusters, Community Groups and Community) and their occurrence in the country (at the level of Biome, Bioregion and terrestrial vegetation type). An ‘azonality index’ was calculated as the sum of all Chi‐square values for each wetland vegetation type divided by the total number of records. The overall correlation between hydroperiod and the azonality index was very weak. At the finest level, terrestrial vegetation types were clustered on the basis of having similar combinations of wetland community types. Eighteen different ‘wetland ecoregions’ have been defined, on the basis of wetland vegetation types occurring within them. Instead of regarding wetland vegetation as azonal, it should rather be regarded as ‘intrazonal’, meaning that climate does have an impact but many vegetation types are widespread across climatic regions. The reason why community types in wetlands are widespread is due to the monodominance of a single widespread, often clonal, species. The different wetland ecoregions do not correspond to terrestrial biomes, so it is expected that wetland vegetation responds differently to climate than terrestrial vegetation.