Links between lateral riparian vegetation zones and flow

[摘要] ENGLISH ABSTRACT: Riparian vegetation communities that occur along perennial rivers are structured in lateralzones that run parallel to river flow. This dissertation investigated the structure of SouthAfrican riparian vegetation communities along perennial, single-thread headwater streams.The central assumption was that lateral zones result from differential species' responses tochanging abiotic factors along a lateral gradient up the river bank. It was first necessary toestablish the pattern of zones and whether this pattern occurs repetitively and predictably ondifferent rivers in different biomes. Since the flow regime is considered to be the mastervariable that controls the occurrence of lateral zones, the link between flow as the majorabiotic driver and the distribution of plants in zones was determined. Predictions were madewith respect to how variable flow may influence phenological traits, particularly with respectto 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 wasexplored and their vegetation characteristics were described. Plant distribution was relatedto bank slope, as defined by elevation and distance from the wetted channel edge duringsummer (dry season) low flow, indicating a direct link to river bank hydraulics. Whether ornot the same zonation patterns occur in riparian communities in other parts of South Africawas explored next. The four zones described for Fynbos Riparian Vegetation were evidentat all of the other rivers tested, despite major differences in geographic location, vegetationcommunity type, climate and patterns of seasonal flow. The four lateral zones could beseparated from each other using a combination of flood recurrence and inundation duration.Functional differences were investigated between three tree species that occur in FynbosRiparian Vegetation. Functional differences were apparent with respect to timing of seeddispersal, growth in branch length versus girth and three physiological measures of toleranceto 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 afterclearing, the physical rules devised to help delineate zones were used to locate lateral zonesthat 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 dynamiczone, where adult and sapling A. mearnsii were most abundant. In un-invaded systems, thiszone was the least densely vegetated of the four zones, the most varied in terms ofinundation duration and the frequency of inter- and intra-annual floods, and was an area ofactive recruitment comprised mainly of recruiting seedlings and saplings.An understanding of the functional differences between lateral zones was a common threadat each riparian community that was linked to the annual frequency of inundation and theperiod, when inundated.

[发布机构] Stellenbosch University