Chapter 3: The Study Area
3.2 Biophysical Description of Study Area 3.1.2.1 Climate
The study area falls in the summer rainfall region of South Africa, with the resultant characteristic warm, wet summers and cold, dry winters. Figures 3.2.1 and 3.2.2 provide the detailed distribution of temperature and rainfall. Mean annual rainfall varies from 800 mm to 1200 mm, which is either associated with frontal weather patterns or summer thundershowers. With thunderstorms being a primary summer weather phenomenon, a lightening flash density of 7 to 8 ftashes/km" per annum is experienced. Winter precipitation events are sometimes associated with snow in the higher lying areas in the Howick area (Snyman, 2002).
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Climatic Data - Greytown Area
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Figure 3.2.1 Climatic Data - Greytown area. (Snyman, 2002)
Temperatures range between 24°C to 26°C in summer but drop to between 11°C and 14°C in winter for most areas (see Figures 3.2.1 and 3.2.2). Frost is a regular occurrence, apart from the Iow-lying easterly areas.
Climatic Data - Howick Area
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Figure 3.2.2 Climatic Data - Howick area. (Snyman, 2002)
3.1.2.2 Geology and Soils
The Greytown area straddles two major geological zones, with the eastern section underlain by intercalated arenaceous (sandstone) and argillaceous (clay) strata. An assemblage of tillites and shales underlies the western section. Dolerite outcrops also occur (WRC, 1995).
Argillaceous (clay) strata dominate the geology of the Howick area, with the outer
(WRC, 1995). Dolerite sills and dykes are common, in association with areas of the
"red" Hutton and Inanda soil forms.
Soils in the Greytown area are generally well-weathered fine-sandy-c1ay-loam humic topsoils, underlain by yellow or red apedal subsoils, with the dominant soil forms being the Sweetwater, Inanda, Kranskop and Magwa forms. Hutton, Clovelly and Oakleaf forms occur in the lower lying areas, and the drier northern slopes. High (>1.8%) topsoil carbon contents are common, while clay contents vary between 25%
and 35% in topsoil horizons, but reach levels of up to 55% in deeper subsoils.
Mispah and Glenrosa forms (Iithosols) commonly occur on steep slopes, particularly in the drier, warmer areas towards the Tugela River. Hydromorphic soils of the Tukulu form generally occur along riparian areas (Snyman, 2002).
In the Howick area, fine-sandy-c1ay humic topsoils, underlain by yellow or red apedal subsoils, characterise the soils. Dominant soil forms are Inanda and Magwa, with Hutton being the subdominant soil form. Topsoil horizon clay contents vary from 34% to 35%, reaching levels of 60% in deeper subsoils. Hydromorphic soil forms, such as Tukulu and Katspruit, occur along riparian zones, with peat soils such as the Champagne form and organic variants of the Katspruit and Westleigh forms occurring in wetlands, especially in the upper reaches of the Umvoti River (Snyman, 2002).
3.1.2.3 Topography
The Greytown area is characterised by an undulating plateau with moderate to steep slopes in places. The eastern and northern boundaries of the area are characterised by steeply incised river valleys (such as the Tugela River) leading into more rugged terrain below the plateau area. Altitude varies from 750 m to 1570 m above mean sea level (amsl).
In the Howick area, the topography is more varied, ranging from undulating terrain interspersed with prominent hills having moderate to steep slopes, to the Gilboa area, which forms a high plateau, dominated by Mount Gilboa. Steep scarps are present along the perimeter of this plateau. Altitudes in the Howick area vary from 750 m to the peak of Mount Gilboa at 1768 m amsl.
3.1.2.4 Hydrology
The study area within the Greytown area is part of the U primary drainage basin, falling mainly into the U40 and U20 secondary catchments (i.e. the hydrological catchment areas defined by the Water Research Commission (WRC, 1995)). The mean annual precipitation (MAP) is 860 mm, with a median annual simulated run-off of 126 mm. The average groundwater depth is 25 m, with an annual recharge rate of 60 mm. The main river draining through the area is the Umvoti River (WRC, 1995).
The Howick area falls within the U and V primary drainage basins, with the secondary catchments being U20 and V20, and is an important catchment for the major river of the area, the Umgeni River. MAP is 980 mm, and a median annual simulated run-off of 144 mm. The average depth to groundwater is less than 20 m, with an annual groundwater recharge rate of 75 mm (WRC, 1995).
3.1.2.5 Natural Vegetation
In both study areas the predominant natural vegetation type is a grassland biome, associated with the Short Mistbelt vegetation type in the Greytown area, and Moist Upland Grassland vegetation type in the Howick area (Low and Rebelo, 1996).
The main type in the Greytown area is the Natal Mist Belt 'Ngongoni Veld (Acocks veld type 45, (Acocks, 1988)). This occurs as a transitional stage between the Highveld Sourveld of the higher lying plateau regions and the 'Ngongoni Veld proper (Acocks veld type 5) of the lower lying regions.These grasslands are Sourveld, with the main species being Themeda triandra where the sward is relatively undisturbed, with significant communities of Monocymbium ceresiiforme, Trachypogon spicatus and Tristachya leucothrix. Where intensive agriculture, especially overgrazing has occurred,there is a marked increase inAristida junciformis (Camp, 1997).
Patches of indigenous forest and thickets occur in the more sheltered river valleys, with dominant species being Rapanea melanophloeos, Cryptocarya woodii and Syzygium gerrardii. Other species present include shrubs and climbers such as Dalbergia obovata andUvaria caffra.
In the Howick area, the main natural vegetation ranges between Southern Tall Grassveld (Acocks veld type 65) and Natal Mist Belt 'Ngongoni Veld (Acocks veld type 44). Grassland areas are classified as sourveld, with Themeda triandra, Heteropogon contortus, Tristachya leucothrix, Eragrostis curvula and Elionurus muticus being the main species. Hyparrhenia hirta (thatch grass) is also characteristic of these areas. Much of the area's grassland is in a poor condition, and exists as a secondary vegetation status. Incorrect grazing management has led to an increase in unpalatable species and the intrusion of herbaceous weeds (Camp, 1997).
In sheltered sites and rocky outcrops, woodland and thickets occur, with the dominant tree species being Maytenus heterophylla, Zanthozylem capense, Ziziphus mucronata, Rhus rehmanniana and Acacia sieberana. Forest pioneer species such as Rapanea melanophloeos, and Fynbos species such as the Cliffortia species occur in areas protected from fires (Camp, 1997).
Both of these areas fall predominantly into the Bioresource Group 5, Moist Midlands Mistbelt, as defined by Camp (1997).
3.1.2.6 Land Use
By definition, the study area was located in a commercial plantation forestry area.
However, the predominant land use throughout the study area is commercial forestry, together with sugar cane and arable cropping (generally maize) as well as some beef and sheep farming activities. Plantation forestry activities are undertaken by large forestry companies (e.g. Mondi, Sappi), and many private farmers in this area.
The plantation forests consist of exotic hardwood and softwood species that are grown primarily for the production of pulp and paper, although there is a smaller market handling saw timber for the furniture and construction industries. Softwood species are of the genus Pinus, and consist predominantly of three species, P.
patula, P. taeda and P. elliottii. Hardwood species are of the genera Eucalytpus (commonly called Gum) and Acacia (generally known as Wattle). There is only one species of Acacia widely cultivated, A. mearnsii, which is utilised for the production
of tannin extract from its bark, as well as the timber being used in the pulp and cellulose industries.
The Euealytpus species fall into two categories, defined by their wood density.
These are called soft (or sub-tropical) gums, grown in the warmer areas, and hard (or cold-tolerant) gums, grown in the higher lying, more temperate areas. The hard gums have a higher wood density than the soft gums. Main species in the soft gums are E. grandis and E. saligna, while the main species of hard gums include E.
maearthurii, E. dunnii, E. nitens and E. smithii. A great deal of afforestation is now done using clonal hybrids of these species (e.g.E. grandisxE.nitens).
For the pulp and paper industry, gum and wattle are grown on short rotations of 8 to 10 years, while pines are grown for 15 years. The forest industry is run on the principle of the European Normality Model (Von Gadow and Bredenkamp, 1992).
This model allows for sustainable use, whereby the forest areas are managed on a rotational basis, such that all felled areas are replanted, and the area felled at any one time is equivalent tothe total afforested area divided by the rotation length (e.g.
if 100 ha is afforested on a 10 year felling cycle, or rotation, only 10 ha is felled and replanted annually, thus creating a continual production cycle).
Having found sites suitable for this study, the chosen methodologies could then be applied as the next step in this research. The materials, methods and results of applying classification, textural analyses and change detection techniques to medium and high resolution imagery for monitoring plantation forestry operations are described in detail in the next two chapters.