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Koedoe

versión On-line ISSN 2071-0771
versión impresa ISSN 0075-6458

Koedoe vol.53 no.1 Pretoria ene. 2011

 

ORIGINAL RESEARCH

 

The Grassland vegetation of Platberg, eastern Free State, South Africa

 

 

Robert F. BrandI; Leslie R. BrownI; Pieter J. du PreezII

IApplied Behavioural Ecology and Ecosystem Research Unit, University of South Africa, South Africa
IIDepartment of Plant Sciences, University of the Free State, South Africa

Correspondence to

 

 


ABSTRACT

Platberg is an inselberg that presents a refuge for indigenous plants and animals. Uncontrolled human access to this area threatens this sensitive ecosystem. The vegetation of Platberg was investigated to obtain an inventory of the different plant species and communities present in this area. A hierarchical classification, a description and an ecological interpretation of the grassland communities of Platberg are presented. A total of 169 sample plots were placed on a stratified random basis within the study area. From a TWINSPAN classification a total of 27 different plant communities, which can be grouped into two major community types, nine communities, 18 sub-communities and six variants, were identified. A significant difference in species richness was found between the two major communities, with the higher-altitude communities having a higher species richness than the communities on the lower-lying slopes. A total of 26 endemic or near endemic Drakensberg Alpine Centre species were recorded.
CONSERVATION IMPLICATIONS: Anthropogenic influences are felt globally on ecosystems. Highaltitude habitats and organisms will be affected first. Inselbergs have high levels of endemic organisms and are reservoirs and refugia for unique genetic material. This grassland plant community survey of Platberg provides valuable information on inselberg ecology for conservation planning.


 

 

Introduction

The Grassland Biome of southern Africa is continually under threat from anthropoidal activities such as crop production, overgrazing, residential and industrial development, and invasion by alien plant species. These pressures threaten the biodiversity of this sensitive biome (Van Wyk & Smith 2001). The prevention of large-scale loss of biodiversity is a daunting challenge facing conservationists worldwide (ed. Huntley 1994). In order to conserve our natural resources it is important that ecosystems are studied to compile an inventory of the different plant and animal species present.

Platberg (near Harrismith in the eastern Free State) is an inselberg located within the Grassland Biome. The vegetation comprises a mosaic of woody and grass-dominated vegetation units with short to tall sour grasses constituting the majority of the species composition (Mucina & Rutherford 2006). The Korannaberg near Excelsior (also in the eastern Free State) is the only other inselberg where a phytosociological study was performed, by Du Preez (1991, 1992), Du Preez and Bredenkamp (1991) and Du Preez, Bredenkamp and Venter (1991). Studies of the grassland vegetation of inselbergs in Africa are limited to studies in west Africa by Porembski and Barthlott (1995), Porembski and Brown (1995), Porembski et al. (1996) and Porembski et al. (1998), in central Atlantic Africa by Parmentier et al. (2006), and in Namibia by Burke (2001, 2004). The lack of data of inselberg vegetation can be attributed to the physical difficulty of accessing inselbergs. This inaccessibility also contributes to the unique biodiversity found on these high-altitude areas (Van Wyk & Smith 2001).

Owing to the proposed commercial development on and around Platberg, as well as the aims of the Maloti Drakensberg Transfrontier Project, it was considered necessary to undertake a more detailed plant ecology survey to describe and classify the vegetation of Platberg. This paper aims to classify and describe the grassland vegetation present on the Platberg plateau and slopes.

Study area

Platberg (29º10'S, 28º16'E) is located along the N3 highway between Johannesburg and Durban. It forms a distinctive flat-topped, L-shaped plateau with clear-cut contact between the dolerite plateau and the Cave Sandstone of the Clarens formation (Figure 1). The soil is generally shallow and weakly developed lithosols, with plains soil being mostly sandy loam containing boulders and gravel (Hoare & Bredenkamp 2001; Mucina & Rutherford 2006).

 

 

Altitudes range from 1900 m.a.s.l. to 2394 m.a.s.l. with a surface area of approximately 3000 ha for the study site. The slopes are steep, with numerous vegetated gullies and boulder scree slopes below the vertical cliffs. A permanent stream drains off the southern side and flows off the escarpment in a steeply incised gully (Figure 2).

Rainfall is seasonally, with the most rain falling between November and March. Mean annual precipitation for the Drakensberg is 739 mm, with Platberg's eastern summit receiving up to 1200 mm (SA Weather Services 2007). The area has a cold temperature regime (June and July being the coldest months), with a mean annual temperature of 9.6 ºC. Minimum and maximum temperatures range from -2 ºC (July) to 24 ºC (January), respectively.

 

Materials and methods

The scope of the study was to sample above the 1800-m contour in order to work within the limits set by Killick (1963, 1978), who regarded the region in the Drakensberg above 1800 m as a distinct floristic region called the Afro-Alpine Region.

Plot sizes were fixed at 16 m2 (Du Preez & Bredenkamp 1991; Malan 1998). After a recognisance of the study area, a total of 169 sample plots were placed on a stratified-random basis within representative stands of vegetation to exclude as much heterogeneity as possible with regard to floristic composition, structure and habitat. The general position of the sample plots was pre-determined according to aerial photographs but each section within the identified stand had the same chance of being selected. Chosen sample plots found not to be representative of the general vegetation were moved to be more representative, in accordance with the requirements for Braun-Blanquet surveys (Mueller-Dombois & Ellenberg 1974; Westhoff & Van der Maarel 1980).

All species present in sample plots, except bryophytes and lichens, were recorded. Taxon names conform to those of Germishuizen and Meyer (2003). Over 1500 floristic collections were made and identified at either the Geo-Potts Herbarium (University of the Free State, Bloemfontein) or the National Herbarium in Pretoria. The specimens are currently housed at the Geo-Potts Herbarium at the University of the Free State. The cover or abundance of each species was estimated using the modified 9-degree Braun-Blanquet sampling scale (Keddy 2007; Kent & Coker 1992). The surveys were conducted over a period of 24 months (February 2005 to January 2007).

Environmental data collected included geology (Johnson et al. 2006), soil type (Soil Classification Working Group 1991), aspect, exposure, size of rocks, altitude, percentage of the area covered by rock, and topography. Erosion and slope were estimated as described by Brand, Du Preez and Brown (2008). Additional notes were made on management and utilisation of the area.

Floristic data were captured on the floristic database TURBOVEG (Hennekens 1996a, 1998). The data were then exported to MEGATAB (Hennekens 1996b), followed by the use of TWINSPAN (a multivariate classification technique) to classify the different plant communities. Further refinement was achieved by the repositioning of species and relevés. Plant communities were recognised by using diagnostic and/or characteristic species as defined by Westhoff and Van der Maarel (1980) and Kent and Coker (1992). Species were considered to be diagnostic if they were present in at least 40% of the relevés of a specific plant community, with a constancy of 20%or less in other communities. In exceptional cases, species with a constancy of between 30%and 40%were included if they had a constancy of less than 5% in other communities. The results are presented in a phytosociological table (see online supplement). In order to compare the mean species abundance of the two major communities, a parametric t-test for homogeneous variances was used as the community species variances were not significantly different (F = 1.111, p > 0.05). The nonparametric Kruskal-Wallis analysis of variance (ANOVA) by ranks was used to compare species abundance within major communities. Data were analysed using Statistica version 9.0 (STATISTICA 2009).

 

Results

Vegetation classification and description

The hierarchical classification compiled from 169 relevés resulted in the identification of 27 different plant communities that can be grouped into two major community types, nine communities, eighteen sub-communities and six variants, as listed below. The data set contains 6 trees, 62 shrubs, 55 grasses, 23 succulents, 19 sedges, 85 geophytes, 16 ferns, and 310 forb plant species. A total of 26 Drakensberg Alpine Centre (DAC) endemics and 22 alien plant species were found to be present (Brand, Du Preez & Brown 2010). The variants are discussed in detail in Brand (2007) and are only listed in this paper. All phytosociological descriptions in this paper refer to the table provided the online supplement mentioned earlier.

1. Themeda triandra – Cyanotis speciosa major community

1.1 Cheilanthes eckloniana – Crassula sarcocaulis community

1.2 Hyparrhenia hirta – Tristachya leucothrix community

1.2.1 Hyparrhenia hirta – Leonotis ocymifolia subcommunity

1.2.2 Selago galpinii – Hyparrhenia hirta subcommunity

1.2.3 Felicia filifolia – Diospyros austro-africana subcommunity

1.2.4 Hyparrhenia hirta – Polygala hottentotta subcommunity

1.3 Eragrostis caesia – Ipomoea crassipes community

1.4 Cyanotis speciosa – Leonotis ocymifolia community

2. Agapanthus campanulatus – Aristida monticola major community

2.1 Andropogon appendiculatus – Helictotrichon longifolium community

2.1.1 Cineraria eriodioides – Senecio inornatus subcommunity

2.1.1.1 Senecio inornatus variant

2.1.1.2 Ficinia stolonifera variant

2.1.2 Tristachya leucothrix – Koeleria capensis subcommunity

2.1.3 Andropogon appendiculatus –Pennisetum sphacelatum subcommunity

2.1.3.1 Nemesia caerulea variant

2.1.3.2 Tristachya leucothrix variant

2.1.4 Watsonia lepida – Helichrysum pallidum subcommunity

2.1.5 Ledebouria cooperi – Selago galpinii subcommunity

2.1.6 Andropogon appendiculatus –Senecio paucicalyculatus subcommunity

2.1.7 Lotononis lotononoides – Hesperantha baurii subcommunity

2.1.7.1 Aster bakeranus variant

2.1.7.2 Erica alopecurus variant

2.1.8 Habenaria lithophyla – Eragrostis racemosa subcommunity

2.2 Muraltia saxicola - Helichrysum chionosphaerum community

2.2.1 Muraltia saxicola - Aristea woodii subcommunity

2.2.2 Muraltia saxicola - Helichrysum aureum subcommunity

2.2.3 Ficinia stolonifera - Muraltia saxicola subcommunity

2.3 Andropogon distachyos - Helichrysum aureum community

2.4 Passerina montana - Melinis nerviglumis community

2.5 Cyperus semitrifidus - Digitaria monodactyla community

2.5.1 Digitaria monodactyla - Selaginella caffrorum subcommunity

2.5.2 Eriospermum ornithogalum - Digitaria monodactyla subcommunity

2.5.3 Digitaria monodactyla - Crassula sarcocaulis subcommunity

The grassland vegetation is characterised by the presence of the perennial bunch grasses Themeda triandra, Heteropogon contortus and Eragrostis curvula and the forbs Commelina africana and Oxalis obliquifolia (species group AM), with constancies higher than 50% throughout the study area. Other prominent species include the short perennial grasses Tristachya leucothrix (species group AD), Eragrostis racemosa and Harpochloa falx, and the geophyte Watsonia lepida (species group AK).

1. Themeda triandra - Cyanotis speciosa major community

This major community is found on the hot, dry northern and western aspects of Platberg, at relatively low altitudes (1939 m.a.s.l. - 2078 m.a.s.l.).

This community is characterised by the presence of species from species group I (online supplement) and is dominated by the grasses Eragrostis curvula and Themeda triandra (species group AM). Other prominent species include the grass Tristachya leucothrix (species group AD) and the forb Cyanotis speciosa (species group I).

1.1 Cheilantes ecklonia - Crassula sarcocaulis community

This community is found on a north-western aspect on very steep slopes ( > 26º) in full sun at an altitude of approximately 2000 m.a.s.l. Rock cover varies from 5% to 50% with moist, shallow soils (20 mm - 180 mm deep). The geology is sheet rock outcrops on Clarens Formation sandstone and dolerite dykes. The community shows either no or moderate levels of trampling or erosion.

The community is characterised by species from species group A and includes the succulents Crassula sarcocaulis and Crassula lanceolata ssp. transvaalensis, the sedge Pycreus nigricans, the forb Pelargonium bowkerii and the geophytes Hypoxis filiformis and Ornithogalum tenuifolium. The community is poorly represented by species from other groups with a paucity of grasses. It has one dominant species - the xerophytic fern Cheilanthes eckloniana (species group H) - and significant cover abundance from the grass dwarf succulent shrub C. sarcocaulis and the sedge P. nigricans (species group A), the grass Melinis nerviglumis (species group H), the forb Bidens pilosa (species group I) and the geophyte Hypoxis costata (species group Y). The endemic geophyte Merwilla plumbea (species group AD) is also present. There is an average of 20.6 species per 30 m2.

1.2 Hyparrhenia hirta - Tristachya leucothrix community

This community is located at relatively low altitudes (1926 m - 2078 m) on the warm northern or north-western aspect in full sun. The slope varies from moderate to very steep (8º - 45º). Rock cover varies from 0% to 40%, with an average of between 5% and 10%.

It is characterised by species from species group E and consists of only two grasses, namely Hyparrhenia hirta and Brachiaria serrata. The vegetation is dominated by the perennial grasses Tristachya leucothrix (species group AD) and Themeda triandra (species group AM), with significant cover abundance of Heteropogon contortus, Eragrostis curvula (species group AM), Eragrostis racemosa and Harpochloa falx (species group AL), the forb Felicia filifolia (species group AJ), the dwarf shrub Searsia discolor (species group AD), and the fynbos sub-shrub Selago galpinii (species group AD). The community has 32.41 species per 30 m2.

The Hyparrhenia hirta - Tristachya leucothrix community has four sub-communities:

1.2.1 Hyparrhenia hirta - Leonotis ocymifolia subcommunity

This subcommunity is located on the warm, dry northern side of Platberg on very steep (26º - 45º) slopes. Soils are dry and shallow (30 mm - 120 mm), with little rock cover (< 10%). Moderate to high levels of trampling and erosion observed are due to vegetation use by cattle.

This subcommunity is characterised by the forbs Leonotis ocymifolia and the geophyte Asparagus cooperi from species group B. It is dominated by the perennial grasses Hyparrhenia hirta (species group E), Themeda triandra and Eragrostis curvula (species group AM), and Harpochloa falx (species group AL).

1.2.2 Selago galpinii - Hyparrhenia hirta subcommunity

This subcommunity is located on the northern moderate to steep (8º - 26º) stony hill slopes on the sandstone of the Clarens Formation. Soils are shallow (40 mm - 100 mm) and dry and display moderate to high levels of trampling and erosion. Low sclerophyllous shrubs (< 2 m) provide limited canopy cover (< 25%).

There is no diagnostic species group for this subcommunity and it is defined by the bunch grass Hyparrhenia hirta (species group E) and the sub-shrub Selago galpinii (species group AD). It is dominated locally by the perennial grasses Tristachya leucothrix (species group AD), Themeda triandra and Heteropogon contortus (species group AM). Other species that provide significant cover abundance include Felicia filifolia (species group AJ) and the forb Acalypha punctata (species group AD).

1.2.3 Felicia filifolia - Diospyros austro-africana subcommunity

The Felicia filifolia - Diospyros austro-africana subcommunity is located at an altitude of 2000 m on the hot, dry northern aspect. The slope is between 16º and 45º. The subcommunity is located on Cave Sandstone sheet rock, with dry shallow soil (40 mm - 100 mm deep). No trampling or erosion was recorded. Canopy cover of between 5% and 15% was observed, provided by the low (< 2 m) shrub Diospyros austro-africana.

Species group C defines this subcommunity and includes the shrubs D. austro-africana, Euclea crispa, Osyris lanceolata and Searsia dentata, the grass Trachypogon spicatus, the forb Helichrysum aureonitens and the fynbos shrub Cliffortia ramosissima.

The subcommunity is dominated by the perennial grasses Themda triandra, Eragrostis curvula (species group AM) and Eragrostis racemosa (species group AL), the low sedge Cyperus semitrifidus (species group AJ), and the dwarf shrub D. austro-africana (species group C). Other species locally dominant include the fynbos shrub C. ramosissima, the tree E. crispa (species group C) and the short grass Brachiaria serrata (species group E).

1.2.4 Hyparrhenia hirta - Polygala hottentotta subcommunity

The subcommunity is located between 1992 m.a.s.l. and 2078 m.a.s.l. on the hot, dry northern or north-eastern aspect. The slope varies from moderate to very steep (16º - 45º). Rock size varies from stones to boulders (50 mm to more than 500 mm). Soils are dry, mixed Ib type and of medium depth (120 mm - 200 mm) on Cave Sandstone. Levels of erosion and trampling are moderate to high. Only one relevé had canopy cover (5%) for shrubs lower than 2 m. The vegetation comprises mostly tall, perennial grasses, with the shrub Diospyros austro-africana present in some areas.

Two plants from species group D define this community. They are the erect forb Polygala hottentotta and the uncommon succulent Aloe maculata. There is a well-developed grass layer that dominates the community and includes the perennial grasses Hyparrhenia hirta (species group E), Tristachya leucothrix (species group AD), Eragrostis racemosa (species group AL), Themeda triandra and Heteropogon contortus (species group AM). Other species of significance are the geophyte Hypoxis rigidula (species group F) and the grasses Cymbopogon dieterlenii (species group AJ) and Eragrostis curvula (species group AM).

1.3 Eragrostis caesia - Ipomoea crassipes community

This community is found on the farm Platberg on most aspects on the northern flank of Platberg. The slope is steep to very steep (16º - 45º) on scree or boulder beds, below the Cave Sandstone at an altitude of approximately 2000 m.a.s.l. Rock size exceeds 500 mm, with exposure between 10% and 30%. Soils are dry and shallow (30 mm - 120 mm) and of mixed Ib type. No or only moderate trampling and erosion were observed.

This community is poorly developed and is characterised by the single grass species Eragrostis caesia (species group D). The community has no dominant species, but the grasses Themeda triandra, Eragrostis curvula, Heteropogon contortus (species group AM) and Tristachya leucothrix (species group AD) provide significant cover abundance. An average of 27.57 species per 30 m2 was recorded.

1.4 Cyanotis speciosa-Leonotis ocymifolia community

This community is located on basalt scree and boulder rubble on and below Cave Sandstone outcrops at altitudes between 2000 m.a.s.l. and 2100 m.a.s.l. It is located on the farm Platberg on the hot, dry northern or north-western grassy slopes (16º - 45º). Soils are shallow (40 mm - 120 mm) with good drainage and not more than 15%rock cover. No or only moderate erosion and trampling due to livestock usage were observed.

This community has no single defining species group; it is characterised by the forbs Leonotis ocymifolia (species group B) and Cyanotis speciosa (species group I). Vegetativley it is poorly developed and dominated by a single grass, Themeda triandra, with significant presence of the perennial grasses Eragrostis curvula (species group AM), Harpochloa falx (species group AL) and Tristachya leucothrix, the dwarf shrub Sear