Range of variation in wetland vegetation on a subcontinental scale in Southern Africa

The post provided by Erwin Sieben

This Behind the paper post refers to the article Components of plant species diversity along environmental gradients at various spatial scales in wetland environments of southern Africa by Erwin Sieben et al. (2021), published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.13097).

During the 2000s, it was clear that the South African Government started to place more value on aquatic ecosystems as they understood that South Africa is a water-scarce country.  Various decision-making tools were developed to evaluate wetland ecosystems and the services they deliver.  These were ‘quick’ assessments based on a single field visit that helped managers to make decisions about wetland restoration measures, prioritization of how important a wetland was within the wider catchment and what the causes of degradation in a wetland were.  Most of this type of decision-making tools were designed to use very little data as there was very little information on biodiversity in wetlands on a national level.  As a group of wetland scientists, we started worrying that for many politicians and decision-makers, these tools would be the final story.  We thought that while these decision-making tools were being used to protect and restore wetlands, at the same time, we should also develop a database that would go in greater detail to provide actual data on wetlands and wetland vegetation so that better decisions for reference conditions and environmental monitoring could be developed in the future.  It all started at a workshop organized in August 2009.  Here we looked at what vegetation data already existed across the country and how we should bring it all together, based on what were considered the minimum data requirements.  Two exemplary studies that were carried out at that time were Fynn Corry’s work on the use of vascular plants as bio-indicators on the Cape coastal forelands and Nacelle Collins’ work on the wetlands of the Free State Province, where he also sampled soils in great detail.  Both studies involved the sampling of soils in the same place where vegetation samples were collected from, and they collected soils for every single plot (in the case of the Free State plots, even for every diagnostic horizon within the soil profile).

We settled on a set of minimum data requirements and started to get to work on compiling a database on South African wetland vegetation, based on historical studies.  Wetland plots had been sampled in the past, but in South Africa, the focus had always been on terrestrial vegetation with a few wetland plots in each study included as ‘azonal vegetation’.  Only few areas where wetlands were very abundant or where there were very large wetlands of special significance, had studies that focused in their entirety on wetland vegetation, such as on the coastal flats of Northern KwaZulu-Natal, near Lake St. Lucia.  In those studies, special focus was directed towards the measurements of soil factors that were important to understand the context and the ecology of the wetland in itself.  Most importantly, the hydroperiod, which is the time period that an area is saturated with water, could be assessed by looking at soil hydromorphic features (which worked in most cases but proved tricky in some situations, for example, on coastal sands).  For this reason, a soil auger was used to dig a hole up to 1 m deep in most wetlands.  It became clear that the studies done by Fynn Corry and Nacelle Collins, who extended these methods over much larger regions, were really the first of their kind, but we wanted to do this type of work to cover the entire country, so a proposal was submitted to the Water Research Commission for funding to do that work.

When the funding became available, it led to a few of the most busy field seasons that we have experienced during our careers, and every free week that we could find was spent on touring the country, visiting wetlands, and taking vegetation samples.  In the summer of 2010/2011, Erwin Sieben left the Free State, where he was working at the time, to travel the Western Cape before the December holidays, meeting with several fellow wetland ecologists, traveled up the coast towards the East, took a short holiday somewhere along the coast for the Christmas holidays, and then went on to Durban in early January to pick up a student with whom he would then sample the wetlands of the Wild Coast, just south of Durban.  These and many other memorable field trips led to a large number of plant collections that required identification and soil samples that needed to be dried and sent out for analysis in the laboratory of the Agricultural Research Council in Pretoria.  By now, we sampled only a single soil plot per wetland, unlike previous studies where soil samples were taken in every vegetation plot of the wetland.

The previously mentioned Wild Coast area in the Eastern Cape formed one of the most interesting and most inaccessible areas to visit, and there were certainly some botanical treasures to be found there.  Generally, the wetlands that were found in nutrient-poor substrates such as those on quartzites or in peat on coastal sands proved to be the richest in species and among the most interesting wetlands in terms of species composition.  Another area that was very rich in species was the Maloti-Drakensberg area, but this had already been extensively visited during the surveys that were done for the Maloti-Drakensberg Transfrontier Park in 2006,  and the standard sampling protocol developed in 2009 was not yet applied there.

For this reason, the study that is now published in the Journal of Vegetation Science would not have been possible without the contribution of vegetation plots in Lesotho, which represents the upper reaches of the biggest rivers in South Africa, but is politically an independent nation.  So when Peter Chatanga eventually joined Erwin’s research team in 2017 to survey wetlands in Lesotho, we could finally sample vegetation plots in the mountainous regions with the standard sampling protocol and including soil samples.  The plots in Lesotho made the environmental gradient from low to high altitude wetlands complete, and therefore the inclusion of plots from this country made for a narrative that makes ecological sense.  Ecologically speaking, South Africa, Lesotho, and Eswatini (formerly Swaziland) should be regarded as one region, as they share river catchments, geological units, and climatic zones, and it is clear that political borders are quite arbitrary when it comes to understanding the large-scale ecology of a region.  Eswatini has not been included in this study, but that is not a serious problem as similar altitudes and climatic zones have been sampled extensively in neighbouring South Africa, whereas Lesotho represents a unique climatic zone.

Brief personal summary: Erwin Sieben is a Senior Lecturer at the University of KwaZulu-Natal where he teaches biogeography, environmental science and research methodology.  His research concentrates on wetland ecology, conservation and restoration.