Student Karl Reinecke
Academic institution Stellenbosch University, Faculty of AgriSciences, Department of Conservation Ecology and Entomology
Supervisors Professor Cate Brown (Southern Waters)
Professor Karen Esler (Stellenbosch University)
Professor Jackie King (Water Matters)
Title Links between lateral riparian vegetation zones and flow
Riparian vegetation communities that occur along perennial rivers are structured in lateral zones that run parallel to river flow. This dissertation investigated the structure of South African riparian vegetation communities along perennial, single-thread headwater streams. The central assumption was that lateral zones result from differential species’ responses to changing abiotic factors along a lateral gradient up the river bank. It was first necessary to establish the pattern of zones and whether this pattern occurs repetitively and predictably on different rivers in different biomes. Since the flow regime is considered to be the master variable that controls the occurrence of lateral zones, the link between flow as the major abiotic driver and the distribution of plants in zones was determined. Predictions were made with respect to how variable flow may influence phenological traits, particularly with respect to seed dispersal, and physiological tolerances to drying out and were tested.
The existence of lateral zones at reference sites in the Western Cape of South Africa was explored and their vegetation characteristics were described. Plant distribution was related to bank slope, as defined by elevation and distance from the wetted channel edge during summer (dry season) low flow, indicating a direct link to river bank hydraulics. Whether or not the same zonation patterns occur in riparian communities in other parts of South Africa was explored next. The four zones described for Fynbos Riparian Vegetation were evident at all of the other rivers tested, despite major differences in geographic location, vegetation community type, climate and patterns of seasonal flow. The four lateral zones could be separated from each other using a combination of flood recurrence and inundation duration. Functional differences were investigated between three tree species that occur in Fynbos Riparian Vegetation. Functional differences were apparent with respect to timing of seed dispersal, growth in branch length versus girth and three physiological measures of tolerance to drying out; specific leaf area (cm2.g-1), wood density (g.cm-3) and levels of carbon isotopes (δ13C). In order to determine the impact of invasive alien plants and to monitor recovery after clearing, the physical rules devised to help delineate zones were used to locate lateral zones that had been obliterated after invasion and subsequent clearing. At the sites invaded by A. mearnsii plants, the zone delineations showed that invasion started in the lower dynamic zone, where adult and sapling A. mearnsii were most abundant. In un-invaded systems, this zone was the least densely vegetated of the four zones, the most varied in terms of inundation duration and the frequency of inter- and intra-annual floods, and was an area of active recruitment comprised mainly of recruiting seedlings and saplings.
An understanding of the functional differences between lateral zones was a common thread at each riparian community that was linked to the annual frequency of inundation and the period, when inundated.