Plant community classification and environmental gradient correlates along the eastern portion of the Mkuze swamps.
The aim of this study was to describe the plant communities within the eastern portion of the Mkuze Wetland, including only those systems that have a substantial input of water from the Mbazwane Stream, and to determine environmental factors that control distribution. The Mbazwane Wetland has a catchment of reworked sandy marine sediments, and its gradient is very shallow, such that water moving downstream through this system is free of suspended sediment and is clear. In addition, it has low conductivity suggesting a low solute load. A total of nine communities were identified in the study, based on the TWINSPAN cluster analysis. These are: Ficus trichopodalScleria angusta Swamp Forest Community; Rubus rigidus Disturbed Swamp Forest; Phragmites australis/Ficus trichopoda Precursor Swamp Forest; Typha capenis/Pycreus mundii/Leersia hexandra Marsh Community; Phragmites australis/Cladium mariscus/Ficus verruculosa Swamp Community; Cyperus papyrus Swamp Community; Echinochloa pyramidalis Floodplain Community; Ischaemum fasciculatum/Centella asiatica Floodplain Community; Paspalum distichum Floodplain Community. These communities can be divided into three broad groups, swamp forest, emergent herbaceous swamp/marsh and grassland floodplain communities. One of the main environmental variables influencing distribution was duration and depth of flooding, which separated the permanently flooded swamp and emergent swamp/marsh communities from the seasonally/ periodically inundated floodplain communities. Environmental variables that account for further division of the communities within these two groups, appear to be disturbances from fire and substratum differences related to sediment deposition from the Mkuze River, which enters the lower Mbazwane Wetland from the west. The permanently flooded swamp forest communities, are largely restricted to the northern parts of the study area, while the emergent swamp/ marsh communities, are restricted to the southern part of the study area. The swamp forest community is fringed to the west by an extremely high and steep dune, while the swamp/marsh communities are fringed to the west by much lower and more gently sloping dunes. It is suggested here that wildfires in conjunction with topography influence the distribution of these two groups of communities. An analysis of diurnal variation in temperature in the winter months (June - August) for winds (greater than l.0m/s) reveals that winds blowing from the west to north-west are associated with extremely high temperatures that persist during the day and well into the night. These are berg wind conditions that have been strongly linked to the desiccation of vegetation and promoting its susceptibility to burning in wildfires, and it is during these conditions that fires are most likely to occur in the study area. Swamp forest is situated in areas that are protected from direct exposure to these winds by the high, steep dune immediately to the west. These are thus likely to be naturally protected from fire. In contrast the herbaceous swamp/marsh communities are not protected from wind or fire by a similar topographic feature. Mature swamp forests were restricted to these 'berg wind shadow' areas, where there is complete protection from fire. Precursor and disturbed swamp forests occur where they are less protected and thus are infrequently exposed to fire. The distribution of the permanently flooded swamp/marsh communities in the areas exposed to fire appears to be related to the input of nutrients. The Cyperus papyrus Swamp Community was rooted in clay rich peat in the area around the Mkuze Delta that receives an input of clay from the Mkuze River during very high floods. In contrast, the Phragmites australis/Cladium mariscuslLeersia hexandra Swamp Community was rooted in peat with low ash content, as there is little or no input of clay from the Mkuze River, even during high floods. The seasonally/periodically- flooded communities included the Echinochloa pyramidalis Floodplain Community, the Ischaemum fasciculatum/Centella asiatica Floodplain Community and the Paspalum distichum Floodplain Community. The Echinochloa pyramidalis Floodplain Community was restricted to seasonally flooded areas receiving an input of clastic sediment from the Mkuze River during high floods, while the other floodplain communities occurred in areas receiving water from the relatively sediment free Mbazwane Stream. The distribution of these two communities appears to be influenced by the duration of inundation, with the lschaemum fasciculatum/Centella asiatica Floodplain Community being at higher elevation and therefore less frequently flooded than the Paspalum distichum Floodplain Community.