A comparison of Mopaneveld vegetation in South Africa , Namibia and Zimbabwe

Data from fifteen phytosociological studies were merged and classified to describe and compare the vegetation of geo­ graphically separated and climatically different Mopanev eld types in South Africa. Namibia and Zimbabw e. Seven v egetation types and ten major plant communities were identified using TW INSPAN. Vegetation types were separated according to geo­ graphical regions. There were significant floristic affinities even though there w ere geological and climatic differences between the regions. Plant communities were described according to vegetation structure, habitat and floristic composition. Although environmental data were not adequate for a detailed ordination. DECORANA reflected the distribution of vegetation types and major plant communities along environmental gradients. Limitations of large phytosociological syntheses were also addressed. Species richness (alpha diversity) was calculated for each geographical region. The Musina (Messina) region north of the Soutpansberg. South Africa, has the highest species richness, and Kaokoland. Namibia, the lowest Due to irregular annual rainfall patterns in semi-arid Mopaneveld, it is suggested that variance in species richness is associated with temporal vegeta­ tion states induced by rainfall events. Species richness of Mopaneveld was further compared w ith other sav anna types.


INTRODUCTION
Colophospennum mopane (Kirk ex Benth.)Kirk ex J.Leonard, commonly termed Mopane. is a xeric woody savanna species of south tropical Africa where it occurs in a wide range of vegetation types (Timberlake 1995).collectively referred to as Mopaneveld or Mopani Veld.As the name suggests, C. mopane is the dominant, as well as the character species of this extensive veld type (Winterbach et a i 2(XX)).Floristically, Mopaneveld occurs w ithin the Zambezian Regional Centre of Endemism (Z), Karoo-Namib Regional Centre of Endemism (KN) and Kalahari-Highveld Regional Transition Zone (KH) (White 1983).According to the structural classification by White (1983), Mopaneveld is considered as Wood land (Z) and Scrub Woodland (Z & KH). as well as Karoo Riparian Scrub Forest and Bushland (KN).
Colophospennum mopane can tolerate extreme environ mental conditions (Timberlake 1999).In the southwest ern limits of its distribution range in Namibia.Mopane veld occurs in areas receiving 150 mm or less per annum, while in its southeastern distribution range it receives more than 800 mm per annum in some areas.These non specific sets of environmental conditions reflect the dis tribution of a single species.C. mopane.but little is known of the associated vegetation w ithin the distribu tion range of Mopaneveld (Acocks 1953).However, since 1967.small-scale phytosociological studies have contributed to the know ledge of vegetation in some parts of the southern Mopaneveld (Table 1).Fifteen data sets were available for this study, and considering the dis tance between east and west, this w as still insufficient for the typification of Mopaneveld across its range.How ever.the available information was regarded as baseline data whereby the associated vegetation of Mopaneveld could be compared for the first time.
The main objective of this study w as therefore to clas sify, describe and compare the major plant communities of Mopaneveld in three geographically separated and cli matically different areas of its distribution range to gain a better understanding of the species assemblages and richness o f this vegetation type.These three regions include (1) arid to semi-arid Namibian.(2) semi-arid to sub-humid South African, and (3) sub-humid Zimbab wean Mopaneveld types.

METHODS
The first step in the synthesis of Mopaneveld vegeta tion was to initiate and create a database w ith compatible phytosociological data sets from South Africa.Namibia and Zimbabwe.All the available data sets that were con sidered reliable were included in the database.Reliable data sets had to consist of a detailed floristic survey of both the woody and herbaceous strata.Adequate envi-  ronmental data for each data set should have been a cri terion for inclusion.However, due to limited environ mental data available from the selected studies, this could not be implemented.
Compatible vegetation data on Mopaneveld were obtained from 15 phytosociologieal studies (Table 1).A total of ten data sets were selected from South Africa (1 761 releves), four from Namibia (295 releves) and one from Zimbabwe (230 releves) (Figure 1).The phytosociological data, consisting of 2 286 releves (of equal size.± 2(X) m2) and 1 465 species, were incorporated into a vegetation data base created in TURBOVEG (Hennekens 1996a).Due to taxonomic disaccord in the acceptance of infraspecific taxa.only generally used subspecies and varieties were included in the data set.Infraspecific taxa not generally used were combined under the relevant species name.
The first approximation of a vegetation classification, based on this total floristic data set.was obtained by the application of Two-Way Indicator Species Analysis (TWINSPAN) (Hill 1979a) at a single division level in MEGATAB (Hennekens 1996b).Lowest TWINSPAN cutlevels (0-5-50 option) were optimal for separating dis tinct vegetation units in the data set.Azonal vegetation (e.g.wetlands) in the data set was separated from Mopane veld vegetation by this single division procedure.This procedure was repeated until all azonal types were iden tified.Forty azonal releves were omitted from the data set and stored in a separate database for possible future analysis.TWINSPAN was applied to the remaining 2 246 releves (0-5-25-50 cutlevels.6 levels of division).Forty-three vegetation clusters were separated by TWINSPAN.A synoptic table was constructed to facili tate refinement of the table by means of Braun-Blanquet procedures in MEGATAB (Hennekens 1996b).A species was excluded if it had a frequency of less then 10%, and a synreleve was excluded if it consisted of less than 5 releves.The refinement resulted in 29 synreleves, group ed into ten noda (Table 2).which represent seven vege tation types and ten major plant communities.A vegeta tion type and a major community probably represent svntaxa on order or alliance levels respectively.The hierar chical relationships between the vegetation units are illustrated in Figure 2. The final synoptic table (Table 2) contains the constancy values of the species given in per centages.As higher syntaxa cannot be typified before the lower syntaxa are formally described, no attempt was made here to fix syntaxon names according to the International Code for Syntaxonomical Nomenclature.
Probable environmental gradients were determined by applying Detrended Correspondence Analysis (DC'A) to the floristic data set in the DECORANA computer pro gramme (Hill 1979b).DCA was applied to 29 synreleves without data transformation (this was done before reduc ing the synreleves to 10 noda).Rare species were downweighted.Due to inadequate environmental data avail able from the selected data sets, interpretation of the results could not be quantified.
A basic floristic analysis was undertaken to investi gate species richness and is presented as mean species number per releve for each region (Table 3).Species richness of Mopaneveld was also compared w ith other savanna vegetation types: 1. microphyllous thomveld (Acacia tortilis-dom m ated); 2. mixed bushveld: and 3, broad-leaved savanna (Com bretum spp.-dominated).This data was obtained from the savanna vegetation data base housed at the University of Pretoria.

RESULTS AND DISCUSSION
Application of TW INSPAN resulted in the following hierarchical classification of the selected data sets into 10 noda: 1. D igitaria milanjiana-Colophosperm um mopane Vege tation Type  (Timberlake 1999).The Mopaneveld in South Africa and southeastern Zimbabwe is clearly separated from the Namibian Mopaneveld by the presence of species m species groups K and M (Table 2).Species of signifi-
This vegetation type represents southeastern Zim babwean Mopaneveld of the Save River Valley.TWIN-SPAN distinctly separated it from the South African and Namibian Mopaneveld (Table 2; Figure 2).This type is associated with areas receiving ± 530 mm rainfall per annum.A detailed classification and description of this vegetation type was prepared by Hin (2000).Diagnostic species are listed in species group B (Table 2).High con stancy values in species group C resulted in a division of this vegetation type into two major communities: the Justicia flava-C olophosperm um mopane Major Com munity (Type 1.1) on deep, alluvial soils, and the Setaria sphacelata-Colophosperm um mopane Major Community (Type 1.2) on shallow soils of rocky outcrops and inselbergs.Species richness of this plant community is low in comparison with other Mopaneveld regions (Table 3).

Justicia flava-C olophosperm um mopane Major Com munity
Vegetation of this community is confined to valleys and depressions, typically those found in the Save River Valley, Zimbabwe.This tall valley bushveld on clayey alluvium is characterized by woody species such as Zanthoxylum capense and Boscia mossamhicensis (species group C).The tree layer is well developed (75% cover in certain areas) with individuals of Colophospermum mopane (species group A) reaching heights of 16-20 m (Hin 2000).The shrub layer is less conspicuous.Herbaceous cover is high with dominant grass species such as Sporoholus nitens and Enteropogon monostachys (species group C. Table 2).Diagnostic species are listed in species group C (Table 2) and consist mostly of herbaceous species.

Setaria sphacelata-Colophosperm um mopane Major Community
This major community is associated with disturbed land, rocky outcrops and inselbergs on well-drained, shal low.coarse sandy soils derived mainly from gneiss.The shrub layer is better developed than that of the Justicia Although this community is not characterized by a strong diagnostic species group, its existence is sup ported by the very low constancy of species character istic of the Justicia fla v a -C olophosperm um mopane Major Community (species group C) and a high fre quency of species such as Setaria sph acelata (species group D).Some diagnostic species for this community, such as the shrub Phyllanthus reticulatus (species group D). is representative of riparian habitats.Floristic relationships between savanna vegetation of rocky hills and riverbanks have been recorded before in the arid Lowveld vegetation of South Africa (Bredenkamp & Deutschliinder 1995).
This vegetation type is associated with South African and Zim babw ean riparian vegetation on alluvium, although it does not represent typical azonal (riparian) vegetation due to the high abundance of terrestrial plant species such as C. mopane (species group A).Mopane is known to grow on a wide variety of soils, including 'w et' soils of alluvial origin (Van Rooyen 1978;Biggs 1979;O 'C onnor & Campbell 1986).This type therefore repre sents a transition between true terrestrial and riparian vegetation.Annual rainfall varies between 350 and 800 mm.The tree layer is well developed and often forms tall, closed woodland (Van Rooyen 1981).Woody plant species of tloodplains and riverbanks, such as Croton m egalobotrys, Ficus sycom orus, Hyphaene coriaceae.Phoenix reclinata and Spirostachys africana (species group E) are abundant.Grass species adapted to moist conditions, such as Sporoholus fim briatus (species group E), characterize this vegetation type.
Diagnostic species are listed in species group E (Table 2).This vegetation type shows little relationship with the western (Namibian) Mopaneveld.
A large number of releves (1 375) were classified under this vegetation type, which is predominantly found in the South African Lowveld Mopaneveld, covering an area of ± 7 250 km2 (Gertenbach 1987).Most of the releves of this vegetation type were taken from studies in the Kruger National Park.South Africa (e.g.data sets 3. 5. 6 & 14.Table 1).It is comparable to Broad-Sclerophyll Arid Bushveld (Werger & Coetzee 1978) with an annual rainfall of 350-600 mm (Gertenbach 1980).
The structure of this community varies according to geology-from tall woodland (on shale) to dwarf shrub (on basalt).Diagnostic species are listed in species group F (Table 2).Other species commonly associated with this vegetation type include Combretum apiculatum (species group R), Grewia bicolor.Commiphora africana (species group A), and Schmidtia pappophoroides (species group V) in Mopane Bushveld, and Acacia nigrescens (species group M), Dalbergia melanoxylon (species group F), Combretum imberbe (species group R ).Themeda triandra and Bothriochloa radicans (species group F) in Mopane Shrubveld (Low & Rebelo 1996).Species richness varies between values of 25 in the northern, sandy areas to 41 in the central district on clayey soil (Table 3).This vegetation type has a poor floristic affinity with the Namibian Mopane veld (Table 2; Figure 2) due to higher annual rainfall and differences in geological substrates (Figure 2).
This vegetation type is confined to the Mopaneveld north of the Soutpansberg in the Limpopo River Valley, South Africa.The vegetation of the Messina Experi mental Farm (Dekker & Van Rooyen 1995.Data set 2) is well represented in this vegetation type.This low.open to closed woodland type covers an area of 2 037 km 2 between 300 and 780 m altitude (Louw 1970) and re ceives ± 350 mm rainfall per annum.The geology of this area comprises mosaic formations of metamorphic rocks belonging to the Archaean Complex.
Several Commiphora species are known to be diag nostic for this Mopaneveld (Louw 1970), of which C. tenuipetiolata.C. edulis (species group I), C. mollis (spe cies group J) and C. africana (species group A) are abun dant.Another conspicuous feature of this vegetation type is the scattered stands of Adansonia digitata (species group I) on sandy, undulating plains derived from gran ite and gneiss (Dekker & Van Rooyen 1995).Diagnostic species are listed in species group I (Table 2).The high est species richness values in the study area of Musina (Messina) (Table 3) were recorded for this vegetation type.Floristically it is related to the Cissus co m ifo lia -C o lo phosperm um mopane Vegetation Type (species group J), although it has a more diverse floristic composition, es pecially in the woody component.Although this vegeta tion type occurs under the most arid conditions for M o paneveld in South Africa, it is not similar to the Namibian Mopaneveld.
Releves delineating this vegetation type were sampled in areas that were overgrazed, used for military training and as dumping sites (Beck 1998).In addition, some releves were sampled during sustained drought condi tions.Based on general climatic conditions and location, it was expected that these data sets (data sets I. 8 & 15. Table 1) would be classified under the Ptycholobium contortum -C olophosperm um mopane Vegetation Type, but due to harsh environmental conditions it represents serai communities in semi-arid South African M opane veld.Diagnostic species are listed in species group L (Table 2).Especially grass species are conspicuous and include Panicum natalense (species group L), Enneapogon scoparius (species group R), Stipagrostis uniplumis (species group V), Enneapogon cenchroides, M elinis repens (species group W), A ristida adscensionis and Eragrostis trichophora (species group AA).These species are generally unpalatable grasses, typically asso ciated with disturbed areas.

Boscia foetida-Colophospermum mopane Vegetation Type
This vegetation type represents the semi-arid to arid Mopaneveld of Namibia.It is strongly associated with harsh environments on mainly sand, gravel and calcrete of the Kalahari Group and dolomites, limestone, shale, quartzite and conglomerate of the Damara Sequence.This shrubveld to open tree savanna is characterized by species group N (Table 2).The conspicuous tree, Boscia foetida, which is known for its association with semi-arid environments, is diagnostic for this community.
Although this community is differentiated only by three species (species group O), it comprises elements of extreme habitats such as Mopaneveld of the Cuvelai Delta on aeolian sands of the Kalahari Group (Owamboland, Namibia) and the arid Kaokoland (northern Namibia).Soils are gen erally sandy with a clayey or calcareous subsoil and include mopane shrubveld (Owamboland, 500 mm rainfall/annum) and open tree/ shrub savanna (Kaokoland, 200 mm rain fall/annum).Releves from the Honnet Nature Reserve, north of the Soutpansberg, South Africa (Visser et al. 1996) are more associated with this community than with its near est neighbour, the Ptycholobium contortum-Colophospermum mopane Vegetation Type (type 4).
Tree species such as Boscia albitrunca (species group V) and Terminalia prunioides (species group W) dom i nate the tree layer, whereas Stipagrostis uniplumis (species group V), Enneapogon cenchroides (species group W) and Eragrostis trichophora (species group AA) are domi nant grass species.
This community needs refinement on a smaller scale, because heterogeneous combinations could not be clear ly expressed by TWINSPAN procedures on the large scale of this study.Species richness of the Kaokoland parts of this community is the lowest for the study area (i.e. 12, Table 3) and moderate to low (18, Table 3) in the Owamboland region.
This dry.deciduous tree savanna (300-450 mm annual rainfall) is found in the Etosha National Park and sur rounding areas in Namibia and occurs on calcareous ridges and plains of the Kalahari Group.Diagnostic species are listed in species group P (Table 2), including the prominent Leucospluiera bainesii, which is known to be associated with calcareous soils.Colophospermum mopane individuals on these sodium-rich soils are usually only 2-6 m tall and are associated with a poorly developed herbaceous layer (Le Roux 1980;Timberlake 1995).On very shallow lithosols of calcrete substrates, C. mopane is accompanied by Acacia reficiens (species group U) and Terminalia prunioides (species group W) in the tree stra tum, and Boscia foetida (species group N ), Monechma genistifolium and Petalidium engleranum (species group P) in the shrub stratum.In sites where aeolian sands cover calcrete boulders, C atophractes alexandri, O toptera burchellii (species group U), Rliigozum brevispinosum and Mundulea sericea (species group Z) become prominent.The herbaceous layer is well developed and includes species such as Anthephora schinzii (species group N).Enneapogon desvauxii, Stipagrostis liirtigluma (species group P) and Enneapogon cenchroides (species group W).Lithosols derived from andesites are relatively fertile and produce a heterogeneous vegetation type on this hilly landscape.The herbaceous stratum is perennial with Eragrostis nindensis (species group P) being very promi nent (Le Roux et al. 1988).
This community is distinctly separated from com m u nity 6.1 and shares a number of species with the Philenoptera nelsii-Colophosperm um mopane Major Com munity (Type 7.1) (species group U).Species richness for this community is moderate to high (i.e.28 species per releve.Table 3).

Bauhiniapetersiana-C olophosperm um mopane Vege tation Type
This vegetation type is confined to deep Kalahari sands that are mainly of aeolian origin.Annual rainfall varies between 300 mm and 400 mm.This sandy, dry bushveld is best represented in the sand veld areas of Etosha National Park.Namibia.Diagnostic species are listed in species group S (Table 2).Although Colopho spermum mopane is often associated with heavier, clayey soils in higher rainfall areas, it is also well represented within this vegetation type (species group A).
This community represents vegetation associated with Kalahari sands of aeolian origin within the arid Nami bian Mopaneveld.Several species indicative of soils containing a high sandy content characterize this com munity (species group T) and include Pliilenoptera nelsii, Acanthosicyos naudinianus, Requienia sphaerosperma and Harpagophytum procumbens.Habitats typical of this community include Kowares Sandy Mopane Shrub veld (Kaokoland section, Etosha National Park) and the Sandy Shrub M opaneveld (Sandveld areas.Etosha National Park), often overlying calcrete (Le Roux 1980).The floristic component of calcareous substrates links this community to the Leucosphaera bain esii-C olophospermum mopane Major Community (species group U).Species in species group V links this community to the South African Mopaneveld types.Species richness for this community is 35 (Table 3), which is moderate to high when compared to other types.
This unique community of only 10 releves represents the moister northeastern Namibian Mopaneveld.adja cent to the Caprivi.These mopane woodlands lie in an area of old river drainage lines, which are covered by aeolian sand deposits (Mendelsohn & Roberts 1997).This dry, early-deciduous savanna woodland includes species that prefer deep sandy soils, such as Requienia pseudosphaerosperm a, Hyphaene petersiana, Harpagophytum zeyheri and Dichapetalum cxmosum (species group Y).Other diagnostic species are listed in species group Y.It shows a strong floristic affinity with the Philenoptera nelsii-C olophosperm um mopane Major Community (Type 7.1).The environmental conditions of this community are different from any other vegetation type or major plant community.Although it represents moister Namibian Mopaneveld.moisture conditions are still low and erratic, which probably relate it to the Philenoptera nelsii-C olophosperm um mopane Major Community.Species richness is low (i.e. 18, Table 3), especially when compared with community 7.1.

Evaluation o f vegetation types
Although the Zimbabwean data set from the Save River Valley provided baseline information for the iden tification of the D igitaria m ilanjiana-Colophosperm um mopane Vegetation Type and a comparison with other Mopaneveld types, comprehensive vegetation studies of other types in Zimbabwean Mopaneveld would need to be included for a more detailed account.
The Croton m egalobotrys-C olophosperm um mopane Vegetation Type does not include riparian vegetation from Namibia.In Namibia.Mopaneveld is restricted to the upper clayey soils where the rivers are deeply in cised.Shallow rivers tend to dry out seasonally, which consequently gives C. mopane the ability to inhabit these dry, sandy washes.In the Cuvelai Delta, northern Namibia, isolated patches of Mopaneveld are often asso ciated with upland islands within the broad, sandy, cal careous shores.Local-scale studies on Namibian Mopane veld could separate these discontinuous Mopaneveld patches w ithin the Cuvelai Delta.

The C issus corn ifolia-C oloph osperm u m m opane
Vegetation Type is more diverse than what is obvious in Table 2.These 1 375 releves of the South African Lowveld Mopaneveld were classified independently (Du Plessis 2001) The area north of the Soutpansberg is associated with a diversity of geological substrates.However, the Ptycholobium contortum -Colophosperm um mopane Vegetation Type was not separated into lower syntaxa.A further classification of Mopaneveld vegetation data from this region should reveal the identification of different plant communities based on geology.More data sets would be needed for a detailed synthesis.
As a serai vegetation unit, it may be questioned whether the Enneapogon scoparius-C olophosperm um mopane Vegetation Type carries sufficient weight to be treated as an independent vegetation type.Serai com m u nities are temporal variations of 'true" communities and can therefore be regarded as a variant of such com m uni ties.On a scale as large as the Mopaneveld. it can.how ever.be valued as a vegetation type since it is likely to be repeated spatially.On a local scale, it should rather be considered a variant.
Colophospermum /no/w/i^-dominated vegetation of Namibia is more differentiated than Giess (1998) sug gested (Table 2).Mopane savanna in Namibia comprises elements of Dry Early-Deciduous Shrub Savanna (Wild & Barbosa 1967).elements of the Early-Deciduous Savanna Woodland and an Intermediate Deciduous Savanna (Timberlake 1995) (Werger & Coetzee 1978).The herbaceous stratum is poorly developed, with Schmidtia kalihariensis (species group Z) the dominant grass.The strange grouping of releves from Honnet Nature Reserve (South Africa) with the Eragrostis viscosa-C olophosperm um mopane Major Community (Namibia) can probably be explained by the dry conditions under which sampling were undertaken.At the time of sampling (1995), the vegetation of the Honnet Nature Reserve was in a degraded state, especially the herbaceous component, which relates it to a certain state in the semi-arid/arid Namibian Mopaneveld.
The Bauhinia petersiana-C olophosperm um mopane Vegetation Type is different from all other Mopaneveld types in that it is associated with sandy, rather than clayey soil.The Philenoptera nelsii-C olophosperm um mopane Major Community, Type 7.1, is floristically linked to other types across the Mopaneveld range (species groups V, W & X) due to its calcareous sub strate.Deep Kalahari sand on which the A sparagus nel sii-C olophosperm um mopane Major Community, Type 7.2, occurs, makes it is floristically poorly related to other Mopaneveld types.Its relationship w ith Namibian Mopaneveld (species group Z) is due to similar climatic conditions.Despite these poor relationships and the high sand content of the soil, Colophospermum mopane has a frequency of 80% in this type, which suggests a sandy topsoil, underlain by clayey subsoil.

Ordination
Vegetation types and major plant communities along the first and third axes of a Detrended Correspondence Analysis (DECORANA) scatter diagram is shown in Figure 3. Due to insufficient environmental data avail able, no clear explanation could be found for the distribu tion of the vegetation types along environmental axes.The ordination, however, supports the geographical and climatic (mean annual rainfall) separations between Zimbabwe (far left).South Africa (middle) and Namibia (far right) (Axis 1, Figure 3).The distribution of vegeta tion types and major plant communities along the verti cal axis from bottom to top (Axis 3) probably follows a decrease in soil depth and an increase in clay content (Figure 3).Soil moisture availability is a major factor that determines the distribution of Mopaneveld vegeta tion types (Timberlake et al. 1993).Although all inter acting factors determining soil moisture availability were not assessed (i.e.rainfall, topography, soil texture and depth, drainage and rooting habit), the interaction of soil depth, soil texture and annual rainfall had a significant influence on the distribution of vegetation types along a soil moisture availability gradient (Figure 3).

Species richness
There are distinct differences in species richness within Mopaneveld of different regions (Table 3).Despite its high er rainfall.Zimbabwean Mopaneveld has lower species richness than South Africa.Kaokoland (Namibia) has the lowest species richness in Mopaneveld.whereas Musina (Messina) has the highest.Etosha has the highest species richness in Namibia, probably due to the diversity in land scapes.Species richness in the South African Mopaneveld varies considerably (from 15 to 45).However, the data sets selected for species richness calculation of Mopaneveld in South Africa were sampled during different rainfall condi tions.For instance, the area north of the Soutpansberg nor mally receives ± 350 mm rainfall per annum, but Honnet Nature Reserve (15, Table 3) was surveyed during a drought year (rainfall less than 1(X) mm/annum), and Musina (Messina) (45, Table 3) was surveyed during a wetter year after a drought (>2(X) mm/annum).It has been shown that perennial herbaceous species disappear after a drought event in a semi-arid savanna, but are replaced by annuals after the first rainfall event (O'Connor 1999).According to Oelofse et al. (2(XX)), Mopaneveld vegetation follows a 'state-and-transition' model for vegetation change, which suggests that the herbaceous layer dies back after an event such as overgrazing, fire or drought, but responds rapidly to an event such as rainfall.The response to rainfall is usually a dense cover of many different annual species, which tem porarily induces an increase in plant species richness.Furthermore, species richness in Mopaneveld is often dependent on the cover of Colophospermum mopane.High cover of C. mopane results in low species richness, where as a higher species richness is noted in areas with low C. mopane cover (O'Connor 1992).It can therefore be sug gested that high annual rainfall and high tree cover (e.g.Zimbabwean Mopaneveld) do not induce species richness, but rather unpredictable rainfall events and low tree cover in semi-arid areas such as the Musina (Messina) region.
When species richness of other savanna types are com pared with Mopaneveld types.Mopaneveld appears to be richer than expected (Table 3).This comparison of species richness (alpha-diversity), however, does not suggest high species diversity.According to Timberlake (1995) Mopaneveld has a low gamma diversity due to typically associated species being common and present in most veg etation types across its range.

Limitations
Broad-scale phytosociological syntheses have limita tions.which should not be ignored: 1. Adequate phytosociological data sets were limited for Zimbabwe and Namibia.These regions are therefore weak ly represented in this comparative study.Consequently, this study only touches on differences and associations between the geographically separated Mopaneveld regions and is not a detailed account of the region.
2. Limited environmental data were available from the selected studies, which influenced the interpretation of results (e.g.ordination).Dealing with this constraint emphasized the need for the collection of detailed env i ronmental data, which include, amongst others.Global Positioning System (GPS) readings for each sample plot.
3. Mopaneveld is considered an event-driven system (Du Plessis 2(H) 1) and is characterized by highly dynam ic, unstable vegetation states.Vegetation classification of such systems is intricate due to temporal and spatial rela tionships between communities.This dynamic character of Mopaneveld vegetation, especially in the field layer, causes a major constraint in phytosociological syntheses in that plant communities are irregularly separated or combined by TWINSPAN procedures.Plant community descriptions are therefore not accurate and it is suggest ed that plant com munity descriptions in semi-arid regions should focus only on perennial herbaceous and woody species rather than a total floristic composition.
4. Although the objectives of this study were to iden tify major vegetation types, too little variation could be derived from TWINSPAN classification on the regional scale.It therefore became evident that detailed phytoso ciological syntheses should also be undertaken on a local scale.For instance.Becker & Jurgens (2(XX)) identified a total of four major vegetation units along a decreasing moisture gradient in Kaokoland.This kind of variance is easily overlooked on a regional scale.5. Differences in mean annual rainfall appears to be one of the major driving forces on a regional scale, but on a local scale soil character plays an important role.The Zimbabwean vegetation type was separated into two major communities based on soil and topography.In the Namibian Mopaneveld. the Boscia foetida-Colophospermum mopane Vegetation Type, as w ell as the Bauhinia petersiana-Colophospermum mopane Vegetation Type, was subdivided into two major types based on soil type.The Eragwstis viscosa-Colophospermum mopane Major Community.Type 6.1.occurs on clayey soil with a thin sand deposit, whereas the Leuco spluiera bainesii-Colophospennum mopane Major Com munity.Type 6.2. is characterized by calcareous substrates.Although the South African Lowveld Mopaneveld was not separated during the classification of the entire data set.sepa rate classification procedures revealed distinct major plant communities according to soil type (Du Plessis 2001).
6. Vegetation types represent broad units with some variation in environmental conditions, which therefore constitute different habitats, w ith different plant communi ties of lower rank.Certain species are confined to these plant communities (habitats), though will not have any influence on a synoptic table, as these communities are all consolidated into the single sv nreleve.Such species of lim ited distribution often have low frequency values and may not be included in the synoptic table.The vegetation types may therefore be floristically and environmentally much more diverse than indicated by the table and descriptions.

CONCLUSIONS
Despite the limitations associated with a phytosocio logical synthesis, this classification and description re vealed a discernible difference between Mopaneveld vegetation of South Africa.Namibia and Zimbabwe.Although Mopaneveld vegetation varies between differ ent geographical regions, there is a relationship between Z im babw ean and South African M opaneveld.The Namibian Mopaneveld displays few relationships with the eastern Mopaneveld.although the dynamics of the herbaceous layer in Mopaneveld vegetation may induce temporal shifts in plant communities towards spatial affinities.Species richness in Mopaneveld is therefore a weak indication of species diversity due to the dynamic shifts in the field layer.This study makes Mopaneveld floristically and on plant community levels far more extensive than was previously thought.

FIGURE 1 .-
FIGURE 1.-Colophospermum mopa/ie-dominated vegetation types in southern Africa (from Mapaure 1994) and approximate location of data sets in the study area.
, and revealed the identification of at least four different major plant communities on different geo logical substrates, namely the (a) Terminalia se rice a -Colophospermum mopane Community on sandy soils derived from alluvium, shale, andesite and the Malvemia Formation; (b) A cacia n igrescen s-C oloph osperm um mopane Community on heavy clays derived from basalt and gabbro; (c) Euclea divinorum -Colophosperm um mopane Community on clayey soils derived from shale of the Ecca Group; and (d) Combretum apiculatum -Colophospermum mopane Community on coarse, welldrained.sandy soils derived from granite and gneiss.Differentiation in geological parent material is responsi ble for the distinct physiognomical variance typically associated with the South African Lowveld Mopaneveld; Mopane Shrubveld and Mopane Bushveld (Low & Rebelo 1996.types 9 & 10).Mopane Shrubveld occurs on flat plains of vertic or near-vertic clays derived main ly from igneous gabbro and basalt.The shrubveld type is generally dominated by a stunted and multi-stemmed shrubby growth of Colophospermum mopane.In contrast w ith Mopane Shrubveld.Mopane Bushveld is character ized by a fairly dense grow th of C. mopane trees occur ring on undulating landscapes derived from basalt, shale, solonetzes and coarse, sandy soils derived from granite (Van Rooyen & Bredenkamp 1998).
. According to descriptions of C olo phosperm um m opane-dom inated vegetation in Namibia (Giess 1998).the Boscia foetida-C olophosperm u m m o pane Vegetation Type represents the intermediate lower, sparser Dry Deciduous Mopane Savanna.The Eragrostis viscosa-Coloplwspermum mopane Major Community.Type 6.1.comprises communities that are not reflected at this scale; Owamboland and Kaokoland.Owamboland (northern Namibia) is a broad plain about 1 1(K) m above sea level.Aeolian Kalahari sands of vary ing depth cover the area w ith scattered patches of cal careous substrates.Oshanas are seasonally flooded watercourses of the Cuvelai Delta in Owamboland.Mopaneveld occurs as interfaces on slightly elevated ter races between the oshanas.Dominant trees include Colophospermum m opane, several species of A cacia, Combretum and Com m iphora, the palm Hyphaene p e te rsiana, Adansonia digitata, Terminalia prunioides and 71 sericea.However, in the dry, central parts of the Kaoko land escarpment, an open tree savanna predominates at an altitude between 700 m and 1 100 m.Being the domi nant woody species for this open savanna, Colopho spermum m opane occurs here as a small tree (height of 2.5 m).Accompanying species in this savanna type include Catophractes alexandri, Terminalia prunioides, Combretum apiculatum .Euphorbia dam arana.Ceraria longipedunculata, Com miphora m ultijuga, C. virgata.C. africana, M aerua schinzii and Sesamothamnus guerichii FIGURE 3.-Ordination diagram of axes 1 and 3 illustrating the distribution of Mopaneveld vegetation types along environmental gradients

TABLE 2 .
-Abbreviated synoptic table of Mopaneveld vegetation types in the study area(cont.)

TABLE 2
.-Abbreviated synoptic table of Mopaneveld vegetation types in the study area(cont.)

TABLE 3 .-Species richness for Mopaneveld and other savanna types presented as the mean number of species per releve Species richness
Vegetation types proved sufficiently homogeneous at the scale of this study to be regarded as single units.The descrip tion of the seven proposed Mopaneveld vegetation types follows as an amplification of the suggested Commiphora mollis-Colophospermetea mopani of the Central Savanna Biome.South Africa (Winterbach et al. 2(XX)).
Major Community.Tree species reach heights of 15-25 m. making it closed wood land.The herbaceous layer is well developed with domi nant grass species such as Digitaria milanjiana (species group B) and Setaria sphacelata (species group D).
FIGURE 2.-Dendrogram depicting the TWINSPAN division of Mopaneveld vegetation in the study area R. annual rainfall: G. major rocks.flava-Colophospermummopane