Investigating hydrological and geomorphological functioning of the mysterious endemic Palmiet wetlands in South Africa.

Palmiet wetlands in South Africa perform a range of supporting, and regulating ecosystem services, including hydrological benefits such as flood attenuation and sustaining baseflows. Despite their importance in regulating the highly seasonal and somewhat flash flood type nature of the region’s hydrology, there remains uncertainty around the reasons for their occurrence in such a limited geographical extent (Eastern and Western Cape). Prionium serratum (Palmiet) is a robust perennial plant that is endemic to wetlands in the sandstones and quartzites of the Table Mountain Group (TMG). Palmiet wetlands are collapsing across the distributional range of this species, largely as a consequence of gully erosion that may or may not have been initiated by human activities.

Despite large investments by the State in restoration, serious knowledge gaps in our understanding of Palmiet wetland structure and function remain. This project identified that palmiet wetlands are sustained by significant amounts of sub-surface water (groundwater and interflow) moving through preferential flow paths in the alluvial fans, which are in turn sustained by groundwater discharge from the surrounding sandstones of the TMG. This conceptual model indicates that a consistent water supply is a palmiet system driver and a key component of palmiet wetland formation. The wetlands clearly retain a significant amount of water, leading to the maintenance of prolonged flows, and a larger baseflow, however the wetland is present due to the existence of the low flows and is not the primary driver of prolonged flows in the system. The work further identified the palmiet plants ability to control water loss through stomatal control, with highly variable but predictable responses to the localised climate systems apparent.

Geomorphology is another clear system driver. This aspect of the study showed that gully erosion leads to longitudinal slope reduction and valley widening in ways that can lead to wetland formation. Dating of sediment from these gullies identified sediment ages of 470 and 7060 BP indicating that these gullies were a part of system functioning prior to the introduction of European farming. The erosion was identified as key to establishing new wetland habitats downstream of the eroded wetland reach.