The vegetation ecology of the Eastern Transvaal Escarpment in the Sabie area . 1 . Physical environment

The physiography, geology, soil and climate of a broad transect of the Eastern Transvaal Escarpment in the Sabie area are described. The transect extends from Hazyview (530 m elevation) to Mount Anderson (2 280 m elevation). The description provides a contextual framework for the subsequent vegetation classification.


INTRODUCTION
The Eastern Transvaal Escarpment, representing Acocks's (1975) Veld Types 8 (North-Eastern Mountain Sourveld) and 9 (Lowveld Sour Bushveld), is an area subject to land-use conflict.The need to produce food and timber comes into increasing conflict with the need to protect mountain catchments, to conserve natural ecosystems, and to preserve scenic landscapes.Rational land-use planning is required to resolve such conflict (F e rra ri al. 1988).
In the past, Acocks's (1975) veld types have provided useful information at the scale of broad landscape types.However, to meet the present needs for regional and subregional planning, attention must be focused on sec tions of these landscapes (Van der Meulen & Scheepers 1978).A more detailed classification of vegetation into ecologically interpretable vegetation units is therefore necessary.A description of the physical environment is fundamental to this task.

STUDY AREA
This study is confined to a broad transect located in the Sabie area of the Eastern Transvaal Escarpment.The transect is situated between the Olifants River in the north and the Crocodile River in the south.Spanning both the Pilgrims Rest and the Nelspruit Districts, it is bounded by latitudes 25°00' and 25° 10' south and longi tudes 30°30' and 31°10' east (Figure 1).It is 65 km long and 20 km broad, covering approximately 1 300 km2.
The village of Sabie (1 050 m) is centrally situated in the transect, whilst Hazyview (530 m) and Mount Anderson (2 280 m) are situated at the eastern and western extrem ities respectively.Its orientation is perpendicular to that of the Escarpment, which has a roughly north-south orientation in this area.PHYSIOGRAPHY Following the rationale of Scheepers (1978) and with certain modifications, the study area may be divided into the following broad physiographic belts (Figure 2): 1, the Subalpine Belt consisting of the rolling plains, level terraces and prominent peaks of the mountain sum mits (above 1 900 m); 2, the steeply sloping Montane Belt of the mountain slopes to the west of the Escarpment Plateau (± 1 2 00-1 900 m); 3, the steeply sloping, much dissected Submontane Belt of the escarpment slopes to the east of the Escarp ment Crest, and including the Escarpment Plateau (± 9 0 0 -1 400 m); 4, the gently sloping, undulating Upland Belt of the foothills (± 500-900 m).
A finer subdivision of these physiographic belts leads to the identification of ten physiographic zones (Figure 2).The main criteria for physiographic zonation are geomorphology and altitude.
Since geomorphology does not always vary con sistently with altitude, the altitudinal limits of the phy siographic zones are arbitrary.

(d)
Escarpment Upper Slopes (± 1 100-1 300 m).This is a steeply sloping zone immediately below, and to the east of the main Escarpment Crest.The slopes are intersected by numerous deeply-incised gorges, kloofs and valleys.
(e) Escarpment Lower Slopes (± 900-1 100 m).This zone is due east of the Escarpment Upper Slopes.Valleys are wider and less deeply incised.
(f) Upper Foothills (± 700-900 m).This and the following zone represent a transition from the Escarp ment Slopes to the Lowveld.The Upper Foothills are characterized by broken undulating terrain.
(g) Lower Foothills (± 500-700 m).This zone lies between the Upper Foothills and the Lowveld.Valleys are wide and flat-bottomed, and slopes are gentle.
In Kayser's (1983) morphogenetic classification of south-eastern Africa our Foothills and Escarpment Slopes would be described as comprising 'peneplains, clusters of residual peneplains, regions with mountains and knobs'.His concept of 'peneplains on the lower escarpment level with moderate degree of dissection' finds equivalence in our Lower Foothills, whereas 'peneplains on the upper escarpment level with high degree of dissection' corresponds to our Escarpment Slopes and Upper Foothills.The Escarpment Plateau matches 'marginal zones on the inland plateau level with strong relief (Kayser 1983).The Plateau Crest Zone is identified as a particularly high benchland scarp, steep and divided into sections (presumably co-incident with the Black Reef Quartzite Formation).The transition from Escarpment Plateau to Mountain Slopes is marked by a medium-height single benchland scarp (presumably co-incident with the Rooihoogte Formation).The Moun tain Slopes and Summit would be described as 'high, closed benchland highland' (Kayser 1983).
The main drainage systems eastward are the Sabie and Mac-Mac Rivers (Figure 1).The Sabie River has its source in the Mount Anderson range and flows eastnorth-east to the Lowveld.The Mac-Mac River drains the dolomite country southwards from Driekop near Graskop until it swings abruptly east at Mac-Mac Falls and then continues its course to the Lowveld.

GEOLOGY
A major geological survey resulted in the publication of a map of the area south of Sabie in 1960 (Visser & Verwoerd 1960).Since then surveys and lithostratigraphic studies have been undertaken on the Transvaal Supergroup (Zietsman 1964;Button 1973a), the Wolk berg Group (Button 1973b), the Malmani Subgroup (Button 1973c), the Timeball Hill Formation (Eriksson 1973) and the Archaean granite basement (Robb 1978;Lageat & Robb 1984).A great deal of confusion sur rounding the lithostratigraphy of the area was cleared up by the recent handbook published by the Geological Sur vey (SACS 1980).
The whole Escarpment region forms the eastern rim of the Bushveld Igneous Complex of the central Transvaal.The stratigraphy and field nomenclature are shown in Table 1.The varying relief of the Escarpment is deter mined largely by the underlying geological structure.Essentially, four different geological systems are repre sented.They are traversed by a network of diabase in trusions.Contacts between the systems are roughly parallel and in a north-south direction.

Nelspruit Granite
Nelspruit Granite, an intrusive of Swazian age, is the oldest rock formation (Table 1).It comprises the biotitebearing gneissose granites and migmatites forming the undulating terrain of the Escarpment Slopes and Foot hills (Figure 2).
Overlying the granite, and reaching from the Escarp ment Plateau to the Summit, are the stratified rocks of Vaalian age, namely the Transvaal Sequence, repre sented mainly by the Wolkberg, Chuniespoort and Preto ria Groups (Table 1; Figure 2).

The Wolkberg Group
In the study area, the Wolkberg Group is represented mainly by the Sekororo, Selati Shale and Black Reef Quartzite Formations (Table 1).The Black Reef Quartzite Formation, with its extreme resistance to weathering, is largely responsible for the formation of the Escarpment Plateau with its numerous associated waterfalls, viz.Sabie, Mac-Mac, Lisbon and Berlin Falls.Accelerated weathering along joints results in the characteristic 'pillar and passage' topography with its weird weathering forms ('gendarmes').The Black Reef Quartzite Formation rarely exceeds 10 m in thickness south of Sabie, but reaches nearly 100 m near Mariepskop in the north.It has a westerly dip of 5°.Shale is always present, particularly near the top, close to the contact with the overlying dolomite (SACS 1980) (Table 1).North-south flowing streams often follow the course of this shale, thus forming a natural division between the Black Reef Quartzite and the dolomites.

The Chuniespoort Group
Overlying the Black Reef Quartzite Formation and rising into the Mountains from the Escarpment Plateau, are the chemical sediments of the Chuniespoort Group, including dolomite, shale, limestone and chert (Table 1; Figure 2).The dolomite is mostly fine-grained and bluegrey in colour with a black weathered surface (owing to manganese oxides) and a wrinkled texture resembling the skin of an elephant, thus deriving the name 'Olifantsklip'.
The incorporation of carbonaceous shale and quartzite with the overlying dolomite produces the dark dolomite of the Oaktree Formation.The Monte Christo, Lyttelton, Eccles and Frisco Formations are characterized on the basis of chert content (Table 1).Dolomite rock is not often exposed within the high rainfall zone owing to the solubility of the limestone.Exceptions are Spitskop and the cliff line of the Eccles Formation, more resistant to weathering because of the high -chert content.The solubility of the limestone has resulted in the formation of caves, usually more visible within the Eccles Formation.

The Pretoria Group
The dolomites mark the transition to the overlying Pretoria Group, which consists predominantly of quartzite and shale in the Rooihoogte, Timeball Hill and x ïa The Timeball Hill Formation comprises a shale zone up to 1 600 m thick.Two bands of quartzite, the Klapperkop Quartzite Member, run through the middle of the zone, giving rise to broken cliff faces with waterfalls.The lower band averages 10 m in thickness whereas the upper band reaches up to 60 m.Its resistance to weather ing has resulted in the formation of the Summit Plateau (e.g. at Hartebeestvlakte).
The shales and mudstones of the Timeball Hill Forma tion are carbonaceous, thinly bedded, highly jointed, fissile and dark in colour.They weather to pale colours.In the Sabie area there are anomalous dips due to slump ing into solution cavities in the dolomite beneath, and further tilting of the strata sometimes occurs in the vicin ity of diabase dykes.
The Boshoek Formation forms a small, prominent cliff above the Timeball Hill Shale.It consists mostly of quartzite, conglomerate, shale and pyroclastics.
In the Hekpoort Andesite Formation flows of basaltic andesite are the most prominent rock types.The Forma tion reaches a thickness of 200 m.

Transvaal Diabase
Transvaal Diabase, an intrusive of Mokolian age, criss-crosses the entire area in the form of dykes and interbedded sills (Table 1).It is rare in the Wolkberg Group, but most common in the Pretoria Group, particu larly in the form of sills.Other basic rocks also make up these sills, those in the upper Chuniespoort Group and Pretoria Group being composed of pyroxenitic material.Sills occur most frequently between the two bands of Klapperkop Quartzite and within the Boshoek Forma tion.Dykes run mostly in a north-north-easterly direc tion.

SOILS
Von Christen (1959Christen ( , 1964) ) studied 40 soil profiles in the Escarpment area in an attempt to gain insight into nutrient cycling and podzolization processes under pine stands.Subsequently a land type survey was conducted by Schoeman et al. (1980), which included the samples of 10 profiles.
The description which follows is based largely on the study of 439 profiles covering the widest possible range in soil and site conditions (Schutz in prep.).Soil proper ties are strongly influenced by parent material and geo logical substrate (Table 2).
Nelspruit Granite.The soils derived from the granites are deep, well drained, apedal, red, ferrallitic, and highly leached.Well decomposed granite saprolite is quite frequently encountered within the soil profile below a depth of 800 mm.Quartz grit, often in the form of a broken stoneline, is commonly found in the upper B horizon.Coarse sand is prevalent, mainly towards the surface, whereas the subsoil, or decomposed saprolite is siltier.Although the clay content of the B horizons of these soils is usually high (sandy clay loam to sandy clay), the water-holding capacity of the topsoil is low.In bottom-land positions and river terrace areas, hydromorphic soils tend to develop.These soils can be extremely wet during summer.
Organic carbon content in the topsoil of granitic soils is far lower than that of other soils in the area (2,3 %).This is probably owing to accelerated decomposition of organic substances occasioned by high temperatures (Franzle 1984).Some pockets of organic rich or humic topsoils (Oakleaf Form) do occur in narrow drainage lines or concave footslopes.Otherwise topsoils are rela tively pale in colour (usually Hutton soils).
Selati Shale.Soils derived from Selati shale are not extensive, occurring in a narrow band along the Escarp ment Crest, where the Black Reef Quartzite Formation has been eroded.Parent materials are shale and quartzite.
These are the shallowest of all the soils in the area (740 mm) and the bedrock is usually overlain by a layer of hardpan ferricrete.In summer, the water table is generally high.In winter, however, slow but efficient drainage is facilitated both by the sandy texture of the soil and by the slight westerly dip of the strata (Figure 2).These soils can thus be particularly dry in winter.Such fluctuations of the water table together with eluviation are conducive to the formation of bleached E horizons.Soils derived from Selati shale are usually sandy clay loams with a coarse sandgrade in the A horizon.Stonelines are common when the parent material is shale.On the shallow soils of the Escarpment Crest the dominant soil form is the Glenrosa.Profiles of the Hutton, Oakleaf and Griffin Forms develop on the deeper soils.
Black Reef Quartzite.Soils on the Black Reef Quartzite Formation extend further south than Selati soils, but are also not extensive.South of Sabie all sandy soils along the Escarpment Crest are derived from Black Reef material.North of Sabie they occur in a band parallel with Selati soils, but generally further west down the dipslope.Soils on Black Reef Quartzite are mostly very shallow over solid bedrock.Under grassland, however, fairly deep soils have developed.Of all soils in the area they have the deepest A horizons (450 mm).Soils on Black Reef Quartzite are sandy loams, with the highest percentage sand (78 %) of all the soils.They also have the darkest surface soils, with lowest values and chromas.Organic, peaty topsoils are common, reac hing a maximum of 9,3 % organic carbon.
The dominant soil form is the Hutton, but Mispah, Clovelly, Inanda and Oakleaf Forms are also common.Less common soil forms include Champagne (under marshy conditions) and Houwhoek and La Motte (under highly leached conditions).
Oaktree.Towards the Plateau Interior, the Oaktree soils begin to appear.They are distinguished from Black Reef sands by an increase in clay content of the subsoil.
A horizons are shallow but the solum is moderately deep (about 1 090 mm).The topsoils have the highest percentage of fine sand of all the soils (together with Timeball Hill Formation).They are generally clay loams except in cases where B horizons are overlain with a colluvial, dark sandy A horizon derived from Black Reef Quartzite.B horizons are classified as clay.Some of the wetter B horizons tend to have massive structure.Stone content of the B horizon is low (11 %).
Surface horizons are usually dark when associated with Black Reef Quartzite soils.Yellow-brown apedals over red apedals abound.The B horizon has the highest colour value of all the soils.Dolomite.Soils derived from dolomite are deep, and well drained.Only on ridge tops and steep slopes does the unweathered rock come close to the surface.The presence of many chert stones and manganese concre tions suggests that these soils are transported.
Of all soils in the area, dolomite soils have the highest pH and lowest exchange acidity, probably owing to their limestone origin.Although P levels are low, the sum of basic cations is high, making them the most fertile of soils in the area.Often, there is such colluvial mixing of unconsolidated rock with the red soil matrix that classifi cation of the soil becomes difficult.Otherwise, Hutton and Griffin Forms are usually dominant.
Timeball Hill.Because they are developed in the dis sected terrain and steep slopes of the Mountain and Sum mit Zones, the soils derived from the Timeball Hill For mation tend to be shallow, except on terraces and valley bottoms.The A horizons are shallower than those of other soils in the area.Pockets of silty clay loam 400-500 mm deep are nevertheless prevalent on the slopes, and root penetration is further facilitated by cracks in the shale.Where the shale strata dip (in asso ciation with diabase intrusions), weathering is usually deeper than where strata are horizontal.Soils are well drained clay loams with a fine sandgrade in the topsoil.Subsoils are high in clay content, but can be quite sandy in areas below the two Klapperkop Quartzite cliff lines.The high subsurface stone con tent (47 %) is typical of these soils.The stones may be loose, or compacted into stonelines, particularly where there is shale colluvium over diabase.
Transvaal Diabase.Soils derived from Transvaal Diabase are not easily recognizable as they are always buried below colluvial topsoil from the adjacent country rock, from which they are separated by a stoneline at 300 to 500 mm depth.
Diabase soils therefore exist only as subsoils and their position as an important subdivision of soils is question able.

CLIMATE
The Eastern Transvaal Escarpment lies in a seasonally arid, subtropical region, with hot wet summers and cool dry winters.It forms a transition area between the warmer Lowveld to the east and the climatically more extreme Highveld plateau to the west.Summer to winter shifts in the position of the high pressure system of the southern hemisphere play a major role in the constitution of weather (Fabricius 1988).
A climatic gradient facilitates the recognition of two major climatic belts based on altitudinal distribution of mist.These belts are especially meaningful for identify ing gross climatic variability.Following Scheepers (1978), there is the Mistbelt of the high, cooler altitudes as opposed to the Low Country of the low, warmer alti tudes (Figure 2).The boundary between these two belts is irregular and poorly defined in the study area, but appears to fluctuate between 900 and 1 100 m elevation.This zone of fluctuation may constitute a third climatic belt, namely Transitional Mistbelt.Its range conforms to Scheepers's (1978) arbitrary lower limit of about 1 050 m elevation.A fourth climatic belt, derived from the subdivision of Mistbelt into 'moist' and 'dry' sectors (Humid and Subhumid Mistbelt respectively, Figure 2), may also be justified (Deall 1985).The Subhumid Mist belt (represented by the Summit Zone), is a distinctly cooler and drier sector of the Mistbelt (Figures 3 & 4).It is frequently misty up to about 1 800 m elevation and completely clear on the Summit above.The boundary between Humid and Subhumid Mistbelt is therefore assumed to co-incide with the transition from Montane to Subalpine Belt (Figure 2).Fabricius's (1984Fabricius's ( , 1988) )  H°T ' -   Fabricius' (1984Fabricius' ( , 1988) ) classification.
According to Schulze & McGee (1978), light (insola tion), temperature, and moisture (precipitation) are the most significant climatic factors in vegetation develop ment.These operate together to produce 'homogeneous environments in which certain plant communities attain importance'.

INSOLATION
Insolation may be measured directly in terms of the quantity of incoming solar radiation, and indirectly by means of sunshine duration.Maps of incoming solar radiation patterns based on solar radiation measurements in southern Africa have been compiled by Schulze & McGee (1978).In addition, the Weather Bureau (1950) has compiled maps showing broad, countrywide patterns of sunshine duration and cloud cover based on a number of stations in South Africa.Schulze & McGee's (1978) maps show that incoming radiation is subject to seasonal variation.For instance, radiant flux densities for the study area in summer are apparently 80 x 10s Jm -2 day-1 higher than in winter.The magnitude of seasonal variation is supposedly tem pered by the interposing effect of cloud cover which, during summer in the study area, reduces the average duration of bright sunshine by 2 0 -3 0 % (Weather Bureau 1950).
There is also geographic variation of sunshine dura tion within the study area.For instance, the average annual duration of bright sunshine in the Mistbelt is less than 60 % of the possible sunshine, whereas it is 60 -70 % in the Low Country.Similarly, the Mistbelt experiences less 'bright' days (days with 9 0 -1 0 0 % of possible sunshine) than the Low Country (Weather Bureau 1950).This trend of increasing cloudiness is broken in places on the Summit (Subhumid Mistbelt) however, where, owing to rain-shadow effects, the duration of daily sunshine may be greater than in the Low Country.Thus throughout the year, Long Tom (2 118 m) in the Sub humid Mistbelt experiences more sunshine than Hazy view (530 m) in the Low Country (Table 3).
Besides seasonal and geographic variability, there is also physiographic variability of insolation owing to slope and aspect.Daily incoming radiant flux densities on sloping terrain as a function of slope, aspect, and season have been presented for cloudless days in South    Africa for the latitudinal range 20°S to 35°S by Schulze (1975).The following general trends for the study area (at 25°S) are derived from these data.
In midsummer, steep slopes receive less radiation than gentle slopes, regardless of aspect.Steep slopes with a south-facing aspect receive greater radiation than those with a north-facing aspect.Incoming radiation on gentle slopes, however, is apparently unaffected by aspect.
At the equinoxes and in midwinter, north-facing aspects receive greater radiation on steep slopes than they do on gentle slopes.The converse applies to southfacing aspects.Finally, regardless of steepness, slopes experience decreasing radiation as they tend more towards south-facing aspects.TEMPERATURE Although temperature alone may not be a significant factor in determining major regional vegetation forma tions, it does play a part in determining floristic varia tions on a meso-and micro-scale (Schulze & McGee 1978).Such variations result from differential effects of temperature on plant growth rates, seed germination, seedling survival and flowering phenology.
Temperature data for several stations in the study area are given in Tables 4 and 5 and in Figure 3.These data will be discussed comparatively with respect to the dif ferent climatic belts.

Subhumid Mistbelt
Long Tom station on the Summit Plateau is considered to be representative of the Subhumid Mistbelt.Mean monthly temperatures range from 8,9°C in June to 16,6°C in January (Table 4E).Average summer and winter tem peratures are differentiated by 7,7°C.Extreme daily maxima are greatest in early summer (November: 29,1°C), and extreme daily minima are least in winter (June: -4,0°C ) (Table 4A, B; Figure 3A).Both the mean and the extreme daily ranges are greatest in September (11,3°C and 29,5°C respectively) (Table 4C, D).Although data are lacking, frost is assumed to be a signi ficant factor in the climate of the Subhumid Mistbelt.

Humid Mistbelt
Data for the stations Sabie and Tweefontein on the Escarpment Plateau are considered to be representative of the Humid Mistbelt.Mean monthly temperatures in the Humid Mistbelt range from 12,1°C in the coolest month (July) to 21,1°C in the warmest month (January) (Table 4E).Average summer and winter temperatures are differentiated by 9,0°C.Extreme daily maxima are greatest in late spring (October: 37,5°C), and extreme daily minima are least in winter (July: -4,2°C ) (Table 4A, B; Figure 3B).Although the mean daily range is least in February (9,9°C) and greatest in July (17,3°C) (Table 4D), the extreme daily range can reach up to 37,5°C in early spring (August to September).
Severe frost (less than 0°C in Stevenson screen) is prevalent in the winter months (June to July), but has been known to occur as early as May and as late as September (Table 5).Its average duration, however, is only 31 days.The prevalence of extremely high day temperatures in the absence of rain in late spring could be inhibitory to many species, especially those on ex posed, north-facing (xeric) sites.Likewise, frosty condi tions would tend to exclude species which inhabit the lower frost-free climatic belts.

Transitional Mistbelt
Temperature conditions at Bergvliet (Escarpment Lower Slopes) are considered representative of the Transitional Mistbelt.Mean monthly temperatures range from 14,4°C in the coolest month (July) to 21,5°C in the warmest month (January) (Table 4E).Seasonal differen tiation of mean monthly temperature is only 7 ,1°C.
As in the Humid Mistbelt, extreme daily maxima are greatest in late spring (October: 38,3°C), and extreme daily minima are least in winter (July: 0,0°C) (Table 4A, B).Extreme daily maxima, and especially extreme daily minima, are maintained at higher levels than in the Humid Mistbelt.The nett effect is a narrowing of the extreme daily range, its magnitude being greatest in October (33,0°C) and smallest in March (23,9°C) (Table 4C).The greatest mean daily range, however, occurs in August (13,0°C) and the smallest in February (9,2°C) (Table 4D).
Severe frost (less than 0°C in Stevenson screen) has never been recorded in 15 years of observation (Table 5).Isolated pockets of frost may occur, however, in areas of poor air drainage such as valley bottoms.
As in the Humid Mistbelt, the desiccating conditions of late spring are probably inhibiting to many plants.This is enhanced in the Transitional Mistbelt where the dampening effect of mist is less prevalent.Conversely, frost is no longer a factor limiting to plants in the Transi tional Mistbelt.

Low Country
Temperature characteristics for this climatic belt are exemplified by the stations Emmett and Hazyview (Lower Foothills).Mean monthly temperatures range from 15,6°C in winter (July) to 24,6°C in summer (January) (Table 4E).As in the Humid Mistbelt, sea sonal temperatures differ by 9,0°C.The greatest extreme daily maximum for Emmett (40,4°C) occurred in Sep tember, a month earlier than in the Humid and Transi tional Mistbelts and two months earlier than in the Subhumid Mistbelt.Conversely, the lowest extreme daily minimum for Emmett (1,8°C) occurred in August, a Bothalia 19.1 (1989) month and two months later respectively (Figure 3; Table 4A, B).Thus, the Low Country apparently warms up sooner and cools down later than the Mistbelt as a whole.It therefore has a longer warm-hot season than the latter (Figure 3).
As in the Humid Mistbelt, the greatest extreme daily range occurs in September (36,6°C).The smallest extreme daily range, as in the Transitional Mistbelt, occurs in March (20,8°C) (Table 4C).These extreme ranges co-incide largely with the mean daily range for Emmett, which is greatest in September (13,4°C) and smallest in January (10,5°C) (Table 4D).This suggests that temperatures in the Low Country are seasonally more consistent than in the Humid and Transitional Mistbelts.Frost (less than 0°C in Stevenson screen) is not an important factor in the Low Country although it may occur in isolated pockets, as in the Transitional Mistbelt.
As for the other climatic belts, the Low Country experiences hot dry conditions in late spring prior to the rainy season, and with the absence of mist, conditions become quite arid.Compared to the Mistbelt, the long warm-hot season of the Low Country means an extended growing season for plants, without the inhibiting effect of frost.Furthermore, the greater predictability of temperature regimes in the Low Country probably mode rates demands on the adaptive capability of plants.
The Mistbelt as a whole is consistently cooler than the Low Country (Figure 3).Mean monthly temperatures decrease with increasing altitude, Long Tom being approximately 7,5°C and Sabie 3,5°C cooler than Hazy view throughout the year (Table 4E).Likewise, but with the exception of anomalies caused by local geomorpho logy, the temperature range generally narrows with in creasing altitude.For example, the annual average of the mean daily range is 13,6°C at Hazyview (530 m) and only 9,8°C at Long Tom (2 118 m) (Table 4D).

Wind
Although wind data for stations in the area are lack ing, wind conditions at nearby stations are considered fairly representative and are therefore presented in Table 6.South-easterly to easterly to north-easterly winds pre dominate during summer (January).These winds blow from the Indian Ocean and are often associated with anticyclonic systems, but can also be associated with cyclonic systems.Their persistence is often the har binger of the steady rains, drizzle and mist so typical of Escarpment weather in summer.
Winds which blow from the southerly to south-west erly sectors during early summer, especially in the afternoon, are often associated with thunderstorms.Dur ing winter, these same winds are associated with cold fronts which are sometimes attended by mist and drizzle.
Violent bergwind conditions, such as the 115 kph westerly wind gust recorded in Pietersburg one Septem ber, may occasionally occur (Weather Bureau 1960).These winds become heated by compression as they drop over the Escarpment from the Highveld Plateau and are known to cause considerable physical damage to timber plantations.Indigenous vegetation does not apparently suffer much physical damage, but the low humidity of such winds in the presence of high spring temperatures is sure to have adverse physiological effects.Furthermore, such winds may seriously increase the risk of fire in the dry months of early spring.

Precipitation
Soil moisture is derived from precipitation mainly in the form of rainfall, mist (and fog), dew, hail and snow. (a) Rainfall.The study area falls within the summer rainfall zone where the bulk (85 %) of annual rainfall occurs between November and March.During this period, rain is precipitated mainly in the form of afternoon thunderstorms and instability showers caused by convection in, and convergence of, tropical air masses (Weather Bureau 1965a).The convergence is often occasioned by a particularly pronounced trough of low pressure over Botswana-Namibia (Fabricius 1988).Light orographic rainfall (drizzle) without thunder, and associated with advection, is also prevalent in the sum mer months, especially on the windward sides of the Mountain and Escarpment Slopes.The small proportion of winter rainfall is derived solely from orographic preci pitation, since conditions favouring thunderstorms and instability showers do not persist in the winter months (Weather Bureau 1965a).
On average, the Escarpment region experiences a maximum of over 140 days with measurable rainfall per annum, including 60 to 80 thunderstorms occurring early in the rainy season (Weather Bureau 1965a).Prolonged periods of rain, usually in the form of drizzle are fairly common.For instance, periods with seven consecutive rainy days are encountered on about two occasions per year, and periods with four consecutive rainy days may be encountered on as many as 10 occasions per year (Weather Bureau 1965a).
Rainfall along the eastern Escarpment is generally reliable.In 58 years of recording, 78 % of the annual falls lie within about 20 % of the normal rainfall.A further 10 % of annual falls may be regarded as 'wet' years (120-140 % of normal), and the remaining 12 % as 'dry' years (6 0 -8 0 °k of normal) (Weather Bureau 1965a).
The regional climate is of the monsoon type in which three seasons can be recognized: 1, the rainy season of summer and late summer (i.e.November to March); 2, the cool dry season of 'autumn to early spring (i.e.April to August); 3, the warm dry season of spring and early summer (i.e.September to October).
The strongly seasonal nature of the rainfall at three stations in the study area is illustrated in Figure 4. (Cau tion should be exercised in comparing A and C because of the latter's relatively short period of recording).Maxi mum rainfall occurs during the warmer months, from November to March.In the Low Country and Humid Mistbelt, November to March are 'superhumid' months in which the monthly rainfall consistently exceeds 100 mm (Walter 1971).In the Subhumid Mistbelt, however, the superhumid period is limited largely to January and February.Also, there is a general increase in rainfall with altitude; the mean monthly January rainfall in the Humid Mistbelt being approximately 52 mm more than in the Low Country.This trend is interrupted on the Summit, however, where rainfall is considerably less than in the Low Country (Figure 4A), and where the humid period is considerably shorter.
The average number of rainy days in both the Mistbelt and Low Country shows much the same type of annual ' ariation as the rainfall amount (Figure 4B, C).Rainfall is nevertheless consistently heavier in the Humid Mist belt than in the Low Country.For example, for the same number of rainy days in December, Tweefontein in the Humid Mistbelt experiences 45 mm more rainfall than Emmett in the Low Country (Figure 4B, C).The heavier rainfall in the Humid Mistbelt may be attributed to the greater influence of orographic rainfall in this climatic belt.
Rainfall data for specific stations within and around the study area are given in Table 7. Stations are grouped according to the physiographic zones they occupy, and this facilitates the computation of a 'composite' mean or index for each zone (except the Escarpment Upper Slopes).This value broadly represents the mean annual rainfall of each zone.Zonal variability can be explained largely in terms of the prevailing physiography.Thus it is clear from Table 7 that rainfall increases steadily from the Lower Foothills (904 mm) up to a first maximum on the Plateau Crest (1 607 mm).This increase is due to the fact that elevations of land force air currents to rise and cool, and hence to precipitate moisture.Furthermore, even in the absence of general atmospheric movements, mountain regions are known to have local ascending currents that would enhance the precipitation process (Killick 1963).From the Plateau Crest, rainfall decreases into the Plateau Interior (1 200 mm).The effect of the Plateau Crest in creating a local rain-shadow may be responsible for this decrease.From the Plateau Interior, rainfall again increases steadily up to a second maximum on the Upper Mountain Slopes (1 853 mm).Above this zone, rain-shadow effects and falling tempe ratures probably cause significant reductions in the vapour content of the rising air masses, and rainfall con sequently decreases to 774 mm on the Summit (Table 7).(b) M ist* Precipitation from mist (and fog) supple ments rainfall precipitation quite significantly.Prelimi nary results from 'fog-catchers' in the Eastern Transvaal have yielded precipitation figures that exceed those of standard rain gauges by between 105 and 280 % (Fabricius 1969).Where mist or fog is carried through the crown of trees, fine droplets of water collect on foliage and branches.These droplets coalesce and eventually become large enough to drip to the ground, thus contri buting directly to soil moisture.This phenomenon, known as 'fog drip', is most significant when mist or fog is moving through tall vegetation.Even when mist or fog is stagnant or when it moves through short vegetation, it contributes indirectly to soil moisture by reducing evapotranspiration.Conversely, stagnant fogs may prevent the precipitation of dew by their inhibiting effect on radiation.
In the Mistbelt, mist is experienced extensively.It also occurs occasionally in the Low Country, on wind ward, mesic slopes during very wet periods in summer.Cold-air drainage on calm winter nights results in the formation of stagnant valley fogs.In the Low Country, these fogs do not persist beyond early morning, being soon dissipated by the heat of the sun.
(c) Dew.Radiation of heat at night causes surface temperatures to fall and, under relatively humid condi tions, this results in precipitation of moisture in the form of dew.River valleys are especially subject to dew pre cipitation.
Dew contributes to soil moisture by reducing the rate and duration of evapotranspiration.Such reduction is accomplished when, with the evaporation of dew, rapid temperature-rises are checked and humidities raised.Dew precipitation may thus serve to ease, or even posi tively counteract, drought conditions.
(d) Hail.Precipitation by hail does not contribute very significantly to soil moisture and neither does it appear to cause much damage to indigenous vegetation.This is mainly because it occurs so infrequently, being prevalent during only four or five spring thunderstorms annually (Weather Bureau 1965a).
(e) Snow.Snowfall results from subpolar air masses advancing from the south of the study area (Fabricius 1988).Apart from increasing soil moisture during winter, snow acts as an insulating blanket protecting plants from excessively cold temperatures and prevent ing the ground beneath from freezing (Killick 1963).Owing to the dryness of the winter, snowfalls in the study area are not particularly frequent.Between 1969 and1983, only four instances of snow were recorded, usually in early spring (August-September) (Mrs D. Livingstone pers.comm.).The snow is confined to the Summit and Mountain Slopes, where slopes as low as 1 600 m elevation may be affected.Snow in early spring may therefore be particularly beneficial to high-altitude plants whose soil moisture is largely depleted after the prolonged dry conditions of winter.* In this (ext.the term 'mist' is used to indicate suspended moisture droplets condensed by the cooling of saturated air masses rising against sloping ground In contradistinction, the term fog is used to denote such suspended moisture droplets condensed from saturated air cooled at night by radiation and temperature inversion, being restricted mainly to low-lying and level terrain.

Humidity
Data for Hazyview in the Low Country and Long Tom in the Subhumid Mistbelt are presented in Table 8.As expected, humidity levels are correlated with precipita tion, temperature, and wind; the highest mean values being recorded in A pril-M ay in the Low Country and in February-M arch in the Subhumid Mistbelt.These periods are associated with decreasing autumn tempera tures (Figure 3A, C) near the close of the rainy season, when fairly high levels of soil and atmospheric moisture still prevail (Figure 4A, C).The lowest mean humidity values were recorded in June-July in both the Low Country and the Subhumid Mistbelt.This is a period of low rainfall and low temperatures (Figures 3 & 4).In addition, prevailing winds tend to be dry south-westerly (Table 6).
The period July-A ugust in the Subhumid Mistbelt may be subject to large fluctuations in humidity.For example, the extreme minimum humidity may be as low as 2%, whereas the extreme maximum may be as high as 96% (Table 8).The low minima may arise on occasions when hot, dry south-westerly 'bergwinds' blow.Condi tions in the Low Country are not as dry, the lowest extreme minimum humidity being 11% in September.
Table 8 shows the situation at the extremes of the study area only.Humidities for the intermediate Humid and Transitional Mistbelts are obviously expected to be higher than those for either the Subhumid Mistbelt or the Low Country.

CONCLUSION
The foregoing treatment has been necessary for estab lishing a basic environmental context for vegetation classification (cf.Deall et al. 1989).Environmental attributes are largely responsible for determining plantspecies distributions.Discussion of the environment, therefore, draws attention to the potential determinants of vegetation composition, vegetation structure, and vegetation distribution.The Eastern Transvaal Escarp ment is a region with a great diversity of natural habitats.It is this diversity which elicits the vegetation response reflected in the numerous syntaxa described elsewhere (cf.Deall etal. 1989).Bothalia 19,1 (1989) FIGURE 1.-Location of the study area (inset), showing a hypothetical profile (above) and the detailed topography (below).Based on S. A. 1:250 000 Topographical Sheet 2530 Barberton.
hygric classification of south-eastern Africa, based on a humidity index (the ratio of precipitation to potential evapotranspiration-Papadakis 1966), concurs fairly well with the climatic

TABLE 3 ,
-Annual m arch o f mean daily sunshine (hours) for two stations in the Sabie area o f the Eastern Transvaal Escarpment* * Based on climatological reports ot the Soil and Irrigation Research Institute, Private Bag X79, Pretoria 0001.

TABLE 5 .
-Occurrcncc o f severe frost (less than 0°C in Steven son screen) at two stations in the Sabie area of the Eastern * Compiled from W eather Bureau, S.A. (1954a).

TABLE 6 .
-Seasonal variation in wind direction lor three stations in the vicinity o f the study area*

TABLE 7 .
-Mean annual and absolute m axim um and m inim um rainfall for 22 stations in the Sabie area o f the Eastern Transvaal * Compiled from miscellaneous records of the Weather Bureau, Pretoria; from W eather Bureau, S.A. (1954b): and from climatological reports o f the Soil and Irrigation Research Institute, Private Bag X79, Pretoria 0001.**Rainfallfiguresadjusted to the nearest millimetre.+Correctedaltitude.\Bothalia19,1 (1989)

TABLE 8 .
-Annual m arch o f mean and extrem e m axim um and m inimum hum idity for tw o stations in the Sabie area of the Eastern Transvaal Escarpm ent*