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University of Cape Town (2018)

Determining the potential drivers of invasive C4 grasses at De Hoop Nature Reserve, South Africa

Munyai, Nthabeliseni Meryling

Titre : Determining the potential drivers of invasive C4 grasses at De Hoop Nature Reserve, South Africa

Auteur : Munyai, Nthabeliseni Meryling

Université de soutenance : University of Cape Town.

Grade : Master of Science in the Department of Biological Sciences. 2018

Résumé
Grassland and shrubland distribution in SA is associated with rainfall seasonality. In grassland the vegetation is exclusively C4 while the shrub and tree component is generally C3. Shrublands, in contrast, are predominantly C3, for both woody and herbaceous species, though there can be a small C4 sedge and grass component. The C4 grassy biomes dominate the warm season rainfall region while C3 grasses predominate in the shrublands of the cool season rainfall regions. The C4 grasses are poorly competitive in cold climates. There are however anomalous patches of C4 grasses in cool season rainfall regions dominated by fynbos shrublands such as those at Potberg, De Hoop Nature Reserve, Overberg region, South Africa. Although the southern Overberg region receives rainfall year-round, more than half of the rain falls in the cooler months of the year. These C4 grassland patches appear to be invading the fynbos even on the nutrient poor podsols. The main objective of this study was to gain a better understanding of the ecology of these anomalous C4 grasslands in a fynbosdominated region. I first explored the role of roads as a conduit for grass invasion by sampling sites adjacent to and further away from the road verge. This was done by identifying all species in plots 5 m (Roadside pots) and 100 m (Adjacent plots) from the road verge every 500 m along a 14 km management road at Potberg. My results show that the most common C4 grasses were Cynodon dactylon and Eragrostis curvula whilst the most common C3 grass was Merxmuellera disticha. Both Roadside and Adjacent plots had more C4 grass species than C3 grasses. The frequency distribution of both C3 and C4 grass species was significantly higher on the Roadside plots than on the Adjacent plots. Although there was a higher incidence of C4 grass occurrence on the Roadside plots I conclude that, roadsides are not the conduit for C4 grass invasion into fynbos as several large patches of the C4 grass Eragrostis curvula and Imperata cylindrica can be found several kilometres away from the roadside and there are fewer C4 grasses away from the road verge. I then explored the impacts of C4 grasses on fynbos species diversity by identifying all the plants in 100 plots, half in fynbos-dominated vegetation and half in grassy patches. My results show that C4 grasses had a negative effect on fynbos species richness.

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