A thesis submitted to the Faculty of Science,
University of the Witwatersrand, Johannesburg,
in fulfilment of the requirements for the degree
of Doctor of Philosophy.
Johannesburg 1992. / The aim of the study was to develop understanding of the relationships between
vegetation types of the grassland biome of South Africa and the environment, with
an emphasis on structural and functional characteristics.
The grassland biome in South Africa has traditionally been divided into 'pure'
grasslands, assumed to be climatically determined, and 'false' grasslands of recent
anthropogenic origin. A review of literature from several disciplines including
palaeobotany, archaeology, ecology and biogeography indicates that this is not a valid
distinction. It is clear that the distribution of the grassland biome as a whole is poorly
understood, but the general correlation between the distribution of biomes and climate
elsewhere in the world suggests that this warrants more detailed investigation.
A water balance approach was used to develop climatic incices that both predict the
distribution of grasslands, and are easy to interpret biologically. The indices are the
mean. number of days per annum when moisture is available for plant growth, tbe
mean temperature on days when moisture is available for plant growth (wet season
temperature),. and the mean temperature when moisture is not available for plant
growth (dry season temperature). Based on these three.indices the grassland biome
in South Africa call be distinguished from neighbouring biomes. The fynbos and
succulent karoo biomes have rainfall in winter. The grassland, nama-karoo and
savanna biomes have' rainfall in summer. The forest biome experiences rainfall
throughout the year. Of the summer rainfall biomes, the quantity of water available
in the grassland biome b greater than in the nama-woo, similar to savanna, but less
than forest. Grasslands experience cooler dry season temperatures than savannas.
The localised distribution of woody plants within the. grassland biome suggests that
it is the effect of climate on the fire regime that may be of overriding importance h'l
determining the distribution of the biome as a whole. Woody elements are restricted
to sites that are either protected from fire, or experience fires of lower intensity than
sites that support- grassland, The unifying feature of the grassland biome is its
proneness to fire. The presence of a warm, moist season promotes plant production
and leads to a high standing crop close to the ground. The prolonged dry season
causes vegetation to dry out annually, rendering it flammable. More arid biomes
have plants more widely spaced, making it difficult for fire to spread. In more mesic
biomes where rainfall is less sea.sonal than in the grasslands or savannas, fuels do not
dry out sufficiently to ignite, A number of additional climatic features may promote
burning in the grassland biome, It has the highest lightning density of all South
Africa's biomes. 'tVarm, dry 'berg' winds desiccate fuels and 1 omote burning in the
more mesic grasslands, The 'curing' of the grass sward due to dry season frost and
temperature drop is important in establishing early dry season flammability. Savanna
trees are fire tolerant, but they appear sensitive to the cold temperatures prevaient in
the grassland biome in. the dry season,
The relationship between the distribution of functional characters of grassland plants
and environmental conditions was investigated. The distincrion between sweetveld,
mixed veld and sourveld was recognised as one of the most Important functional
features of South Africa's grasslands, The distribution of these vegetation types was
examined in detail. Sweetveld occurs In warm, dry areas; sourveld in cool, moist
areas. There Is overlap between these tyP.Js that Is dependant on soil nutrient status.
Sweetveld that occurs in climatic conditions that would be expected to support mixed
veld and sourveld, is on soils derived from basic parent material, including basalt,
dolerite, gabbro and norite. Similarly, sourveld that occurs in areas that climatically
would be expected to support sweetveld, is on soils derived from acid parent material
such as sandstone and quartzite ..
Soil nutrients that are most highly correlated to the occurrence of these three veld
types are phosphoms availability and an index of nitrogen mineralization potential.
'l'here is an increase in bot; available phosphorus and the index of readily
mineralizable nitrogen from sourveld to mixed veld to sweetveld. These features am
inc01).10111tedinto a conceptual model that relates the distribution of these grassland
types to carbon and nitrogen metabolism, with the role of phosphorus either similar
to nitrogen, or else it may act indirectly by affecting the. rate of nitrogen
mineralization, Nitrogen mineralization OCcursat lower water availability than carbon
assimilation, and its temperature optimum is higher than that of carbon assimilation.
Where nitrogen mineralization is favoured ielative to carbon assimilation, sweetveld
is likely to (}C(.1\Xr. Where carbon assimilation is. favoured relative to; nitrogen
mineralization, sourveld is likely to occur ....Soil texture affects the balance between
these two processes in the degree to wm.r;h it protects soil organic matter, and
thereforv the size of the nitrogen and ph_QSPllO_rOll.S pools.
Changes in the rlj,stribution of South Africa's b~\omesfor a scenario of climate change
are predicted using the biome model developed in this study. This illustrates the
value of developing predictive models. / MT2017
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/22708 |
| Date | January 1992 |
| Creators | Ellery, William Nolan |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (211 leaves), application/pdf |
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