Fog is a key source of moisture to the diverse coastal Namib Desert biota, delivering five times more moisture than rain. Apart from the importance of fog as a source of water for plants, it is also associated with particulates that may contain essential nutrients for plants. Furthermore, dry deposition can be an important input of nutrients to many ecosystems, but without water, dust deposited on leaves or on soil is inaccessible for plant uptake. In other studies of coastal ecosystems (e.g. Strandveld), it has been found that this combined deposition of nutrients represents a major source of nutrients to terrestrial ecosystems. In the case of the Namib Desert, the range of Arthraerua leubnitziae is limited to those areas where fog occurs. This study was carried out at five meteorological stations in the gravel plains of the Namib Desert, along an east-west transect increasing in elevation inland. I hypothesised that marine-derived deposition contributes to moisture and nutrient supply of Arthraerua leubnitziae, an endemic shrub restricted to the fog zone of the central Namib Desert, and consequently determines its distribution. To test this hypothesis, two sub-hypotheses were developed and tested independently. The first sub-hypothesis was that fog contributed to the distribution range of A. leubnitziae in the central Namib Desert and the second was that fog deposition has a significant potential to supply moisture and nutrients to A. leubnitziae in the central Namib Desert. To test the first sub-hypothesis, I measured fog and rain volume, and plant morphological characteristics at the five sites. In addition, a fog map was derived using climatic variables from the five sites and used with climate and edaphic variables in MaxEnt model of the probability of occurrence of A. leubnitziae. The occurrence of A. leubnitziae was found to coincide with areas with high fog occurrence with fog contributing 36% to the modelled distribution of A. leubnitziae alongside precipitation, elevation and isothermality. In order to test the second sub-hypothesis, I measured nutrients deposited in fog water derived from wet and dry deposition (Ca, K, Mg, Na, Cl, Br, NO₃, PO₄ and SO₄) and plant essential nutrients in plant and soil samples (N, P, K, Ca, Mg, Mn, Fe, Cu and Zn). I also determined the nutrient uptake by stems of A. leubnitziae. During the period of Sep 2015 to Aug 2016, fog provided 92% of the yearly water input across the study sites. Over the course of the sampling cycle, the total annual nutrient content of fog and dust was dominated by Ca and Na. Most of the nutrients (K, Mg, Na, and S) were of marine origin. However, Ca enrichment factors (relative to seawater) were higher than 1, suggesting an input from dust. A. leubnitziae was found to be able to directly intercept and absorb some of the nutrients in the dust and fog via their stems. Even though plant and soil nutrients did not match each other, a contribution to plant nutrition from dust and fog was evident. The low nutrient concentrations in the soils of the Namib Desert and significant inputs from dust and fog suggests deposition is an important source of nutrients for A. leubnitziae. Although work is still required to better understand the importance of fog water uptake for nutrient provision, I have identified that water, nutrients and environmental stress alleviation should not be considered separately in studying the role of fog as a determinant of plant distributions.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/27949 |
| Date | January 2018 |
| Creators | Gottlieb, Tunehafo Ruusa |
| Contributors | Eckardt, Frank, Cramer, Michael D |
| Publisher | University of Cape Town, Faculty of Science, Department of Environmental and Geographical Science |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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