Corinna Riginos
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Corinna Riginos
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The Succulent Karoo is a biome in western South Africa with an exceptionally high diversity of plant species, especially succulent shrubs. These shrubs range in size from the tip of your pinkie finger to a small tree, with an equally diverse array of shapes and forms. The Succulent Karoo is the world's only arid area to qualify as a 'Hotspot of Biodiversity,' but the region's diversity is threatened by heavy livestock grazing and climatic changes. From 2000 to 2002, I conducted two studies in the Succulent Karoo aimed at understanding some of these impacts on shrub population and community dynamics.
I conducted this study in collaboration with Timm Hoffman, Director of the Institute for Plant Conservation at the University of Cape Town. This study took place in the communal area of Paulshoek, in the Namaqualand region of the Northern Cape province. Twenty-five percent of Namaqualand is communally managed, and overgrazing in these communal areas is considered one of the main threats to the floral diversity of this part of the Succulent Karoo.
Contrast between communal area (left) and commercial ranch (right) in Paulshoek, Namaqualand. After forty years of heavy grazing, many shrub species have become locally extinct within the communal area, while the unpalatable Galenia africana (unmistakable in its light green color) has increased in abundance . The lightly grazed commercial range is home to a much wider diversity of plant species. We set out to study the impacts of the traditional practice of stockposting on the succulent flora of Paulshoek. Herders in the communal area guide their herds of sheep and goats around by day and return the animals to a stockpost, or kraal, at the end of the day. This creates a center of intense grazing around the stockpost, while the intensity of grazing decreases further away from the stockpost.
We first established that grazing intensity decreases with distance from the stockpost. We then used this gradient to examine how the population dynamics of two important shrub species, Ruschia robusta and Cheiridopsis denticulata, change with grazing intensity. Both species tend to disappear from the landscape in areas that are heavily grazed. Our aim was to identify the stages of their life cycles that are most limited by grazing.
Although we found high adult mortality of these shrubs close to the stockpost, mortality was low at intermediate and farther distances (400 m-2 km). Seedling mortality due to grazing was low over the entire gradient. However, we found that mature fruit production was severely impacted by grazing. This suggests that flower production and/or fruit set is the stage of these species' life cycles most limited by grazing.
This individual of Cheiridopsis denticulata has been lightly grazed or not grazed at all. It has been able to produce many fruits. This individual of Cheiridopsis denticulata has been heavily grazed and trampled, leaving no fruits and many damaged leaves. The results of this study demonstrate that a semi-permanent stockpost can have significant impacts on the surrounding vegetation, suggesting that stockposts should be moved regularly. Moreover, we showed that the main mechanism limiting the populations of two key succulent species is failure to produce a sufficient number of fruit. This suggests that areas might benefit from periodic 'resting' from grazing during the shrub reproductive season. It remains to be seen whether the low fruit production is primarily due to florivory or failure to produce flowers in the first place.
I conducted this study in collaboration with Sue Milton, Department of Conservation Ecology, University of Stellenbosch, as well as Thorsten Wiegand, Department of Ecological Modelling, UFZ, Leipzig. We set out to understand how the interactions between seedlings and adult shrubs differs between two sites that differ in mean annual rainfall and between two habitats that differ in soil nutrients within each site.
Seedlings in arid and semi-arid environments have been widely documented to benefit from neighboring 'nurse' plants. Current facilitation theory predicts that this facilitation will be more improtant in a more stressful abiotic context, whereas competition from adult shrubs should be more important in a more benign abiotic context. Few studies, however, have examined these dynamics across more than one gradient of abiotic stress.
We examined seedling-adult interactions at two sites in the Western Cape: the Tierberg Karoo Research Centre, near Prince Albert (lower rainfall site) and the Worcester Veld Reserve, Worcester (higher rainfall site). Within each site, we studied seedling-adult interactions both in the matrix vegetation and on nutrient-enriched mounds overlying the underground termitaria. We used a combination of spatial pattern anlysis, a greenhouse study, and a field study involving multiple species per site.
Underground termite nests create patches of nutrient-enriched soil, locally known as 'heuweltjies' (little hills). They can be identified here as areas of lighter-colored vegetation. This photo was taken at the Tierberg Karoo Research Centre. Our results were surprising and intriguing. At the higher rainfall site, seedling-adult interactions were similar — sometimes positive, sometimes negative — in both the mound and off-mound habitats. At the lower rainfall site, seedling-adult interactions were very different between the two habitats. In the lower nutrient matrix, there was no strong evidence for facilitation or competition from adult shrubs. In the higher nutrient mound habitat, however, there was consistent evidence for strong competitive effects of adult shrubs on seedlings.
Number of leaves for two species of seedlings from each site that were planted into both on- and off-mound habitats and two microsites within each habitat: areas that had been cleared of vegetation, or in the canopies of adult neighbors. These results demonstrate that a simple gradient of overall abiotic 'harshness' is not sufficient to predict the outcome of plant-plant interactions. We suggest that adult shrub life-history characteristics may determine whether an adult shrub makes a good 'nurse' or not. In the mound habitat at the low rainfall site, shrubs are relatively short-lived and fast-growing — an adaptation, we suspect, to the low rainfall and high animal disturbance that the mound vegetation suffers. In the other three habitats, shrubs are longer-lived and slower-growing, especially in the low rainfall matrix habitat. This supports Goldberg and Novoplansky's (1997) argument that competition in arid environment plants can be strong where plant fitness is primarly determined by opportunistic utilization of pulsed resources (e.g. soil water and nutrients that are only available after rain), but not when plant fitness is most determined by survival during interpulse periods.
I collaborated with Shane Heschel and Johanna Schmitt on part of their larger suite of studies on drought stress tolerance in the New England understory herb, Impatiens capensis. My own research had to do with the maternal effects of drought stress and inbreeding. Maternal effects can be detrimental, or they can buffer offspring against negative genetic or environmental effects. I examined the potential interactions between environmental (drought stress) and genetic (inbreeding) maternal effects and their consequences for progeny fitness. Inbreeding depression has often been observed to be exacerbated by stressful environmental conditions. We wanted to see if this was true for maternal inbreeding and stress.
Individual lines of I. capensis from two populations were inbred for six generations, then either inbred for a seventh generation or outcrossed. Plants were also either subjected to a drought stress, or watered continuously. Seeds were collected from this maternal generation and grown in the greenhouse. Progeny were also either subjected to a drought stress or watered continuously. We examined progeny emergence time, biomass, and physiological traits.
We found evidence for negative effects of maternal inbreeding, but these effects were not exacerbated by maternal drought stress. In general, the effects of maternal environment and inbreeding depended on the history of stresses experienced by the population as well as the conditions experienced by the progeny generation.
