Browsing by Author "Reinecke, Karl"

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    Can hydrological niche segregation explain species distributions in an aquifer-dependent wetland?
    (2025) Havhi, Mpho; West, Adam; Reinecke, Karl; Van Blerk, Justin
    Aim: To investigate post-fire succession in fynbos wetland plant communities, test whether these species segregate along fine-scale hydrological gradients and to identify species potentially most sensitive to groundwater abstraction. Intro: Hydrological gradients, which form between permanently inundated wetlands and adjacent dry habitats, are key drivers of vegetation community composition and structure, promoting opportunities for adaptive speciation and community coexistence. In the seasonally arid fynbos region of South Africa, the occurrence of permanently inundated, groundwater-fed wetlands suggests that fine-scale hydrological gradients may significantly shape floristic diversity in this region. The extent to which fine-scale hydrological gradients influence community composition and species segregation requires further testing in fynbos but has important implications for understanding the potential impacts of groundwater abstraction in these fire-prone systems as well as identifying potentially sensitive species. Methods: I analyzed plant community composition and abundance data, along with soil volumetric water content collected across three transects in an aquifer-fed wetland, from 2011 to 2022. Non-parametric multivariate tests were used to quantify spatial and temporal changes in community composition, highlighting general patterns of community clustering across transects and changes related to post-fire succession. The hydrological niche space was quantified across an aquifer-dependent wetland by representing soil moisture data along two hydrological axes: the sum of exceedance values for flooding (SEVa) and drought (SEVd). Using a subset of persistent species, species locations were fitted to these hydrological axes based on their proximity to soil moisture probes. Species were ranked in accordance with their position on this gradient, indicating a preference for flooded or drier sites on the wetland. Results: Successional changes were observed in the vegetation at different ages post-fire as well as shifts in early post-fire community composition between fire events. A robust test of the hydrological niche concept using several species that persisted over the post-fire successional trajectory shows that wetland community composition is spatially structured along hydrological gradients. It was also determined that species display some degree of hydrological niche segregation, with preference for either waterlogged or dry niches within the wetland. Discussion: The results suggest that species segregate along hydrological gradients defined by the number of days under flooding and dry conditions and can thus be ranked according to their preference for inundated or dry hydrological niches. The occurrence of several species with a strong preference for, or dependence on, permanently inundated niches, highlights the potential impacts of groundwater abstraction on species composition, abundance and distribution, which may affect this hydrological niche. The study takes proactive steps in monitoring ecologically sensitive ecosystems before groundwater abstraction, to ensure that appropriate vegetation management and conservation strategies are in place and can be implemented when necessary.
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    Riparian trees as common denominators across the river flow spectrum: are ecophysiological methods useful tools in environmental flow assessments?
    (2014) Schachtschneider, Klaudia; Reinecke, Karl
    Riparian tree species, growing under different conditions of water availability, can adapt their physiology to maximise their survival chances. Rivers in South Africa may flow perennially, seasonally or ephemerally (episodically). Different riparian species are adapted to survive under each of these different flow regimes by making use of surface, ground, soil, rainwater, or some combination of these. These water sources are available to varying degrees, depending on local climatic, hydrological, geohydrological and geomorphological conditions. This paper tests physiological differences among trees along rivers with varying flow regimes. In this study 3 parameters were selected and tested, namely wood density, specific leaf area and water use efficiency through stable carbon isotope measurements. All three parameters are quick, simple and cheap to determine and as such their value for standard-procedure river monitoring programmes or environmental flow requirement procedures was tested. Acacia erioloba is an arid-adapted riparian tree along the ephemeral Kuiseb (Namibia) and Kuruman (South Africa) Rivers that shows decreasing specific leaf area and increasing wood density correlating with deeper groundwater levels. Intraspecific changes for specific leaf area and carbon isotope values were demonstrated for Acacia mellifera and Croton gratissimus at varying distances from the active channel of the seasonal Mokolo River (South Africa). No significant differences in physiology were noted for Salix mucronata, Brabejum stellatifolium and Metrosideros angustifolia, growing along the perennial Molenaars and Sanddrifskloof Rivers (South Africa) under reduced flow conditions. Only the measurement of specific leaf area recurrently showed that significant physiological differences for trees occurred along rivers of the drier flow regime spectrum (seasonal and ephemeral). As such, this physiological measurement may be a valuable indicator for water stress, while the other measurements might provide more conclusive results if a larger sampling size were used. Specific leaf area, in conjunction with other carefully picked water stress measurement methods, could be considered for monitoring programmes during environmental flow assessments, river health monitoring exercises and restoration projects. This would be particularly valuable in rivers without permanent flow, where there is little species-specific knowledge and where current monitoring methods are unsuited.