OFR 151.pdf - CRC LEME

vegetation. Moreover, individual taxa are usually less resilient to environmental stresses than
the communities in which they co-exist (cf. Montuire et al. 1998).
1.1.3 Climatic regions
The earth naturally experiences a wide range of climatic conditions although these can be
grouped into broad climatic types or ‘regions’, many of which appear to be associated with
characteristic suites of landforms (climatic geomorphology) or vegetation types (bioclimates).
At the most general level, climates are classified into (1) continental climates, which are
characterised by a relatively wide range of maximum and minimum temperatures, and (2)
maritime climates where the temperature range is moderated by the oceanic influences. For
this reason changes in relative sea level (eustatic or tectonic) influence the regional climates
along the continental margins. Evidence is increasing that vegetation per se also can have a
profound effect on the climates of inland regions, e.g. via aerosols (Kavouras et al. 1998).
1.1.5 Classification of world climates
The most widely used classification of global climates is the empirical Köppen System
(Christopherson 1997). A modification of this system, the Köppen-Geiger System,
categorises world climates using mean monthly temperatures, mean monthly precipitation and
total annual precipitation to designate climatic zones by latitude from the equator to the pole.
Deserts are unique in being classified primarily on precipitation and, like highland and polar
regions, are considered to be special cases. Other criteria may be used within individual
regions.
1.1.6 Classification of Australian climates
Gentilli (1972, 1986) uses synoptic climatology to subdivide the continent into climatic
‘response’ zones. Warner (1986) uses the ratio of precipitation to evapo-transpiration to
identify the hydrologic regimes that are believed to enhance geological weathering.
1.2 Weathering
1.2.1 Weathering processes
Weathering is the process by which surface and subsurface rocks disintegrate (physical
weathering) or dissolve (chemical weathering).
Rates of weathering are influenced by the composition (lithology) and fabric (bedding,
jointing) of the rock as well as by climate. The fragmented material that overlies unaltered
bedrock is termed regolith whether it is in situ or transported (Ollier 1991). Rocks which are
weathered in situ but still retain evidence of the original fabric are termed saprolite. Soil,
which usually consists of a number of discrete horizons, is the uppermost ‘layer’ of the
regolith. Chemical weathering can produce hard, chemically precipitated deposits
collectively known as duricrust (bauxite, calcrete, silcrete, ferricrete), which cap igneous
rocks, lithified sediments and soil profiles across the continent (Ollier 1991, Anand et al.
1994, Eggleton and Taylor 1998).
Lithological factors aside, the depth of weathering is controlled by annual precipitation,
temperature (especially freeze-thaw cycles), fluctuations in the depth of the water table,
topography (especially slope orientation) and rates of stripping (erosion). Not surprisingly,
physical weathering processes predominate in drier, cool to cold climates and chemical
weathering predominates in wetter, but not necessarily warmer, climates.
34