zietsman.qxd 2006/04/17 08:48 PM Page 33 Dune vegetation and coastal thicket plant communities in threatened limestone fynbos of Andrew’s Field and Tsaba-Tsaba Nature Reserve, Struisbaai, Western Cape M.M. ZIETSMAN and G.J. BREDENKAMP Zietsman, M.M. and G.J. Bredenkamp. 2006. Dune vegetation and coastal thicket plant communities in threatened Limestone Fynbos of Andrew’s Field and Tsaba-Tsaba Nature Reserve, Struisbaai, Western Cape. Koedoe 49(1): 33–47. Pretoria. ISSN 0075- 6458. The coastal thicket and dune vegetation of Andrew’s Field and Tsaba-Tsaba Nature Reserve was classified using Braun-Blanquet procedures and TWINSPAN. The vegeta- tion was sampled using 74 randomly stratified sample plots. The floristic composition, cover-abundance of each species, and several environmental variables were recorded in each sample plot. Six plant communities were identified, namely, Rhus glauca - Euclea racemosa low to tall closed thicket community; Chrysanthemoides monilifera - Solanum africanum low closed dune shrub community; Chrysanthemoides monilifera - Ehrharta villosa var. maxima low to high closed dune shrub community; Ehrharta vil- losa var. maxima low to short closed dune grassland community; Ammophila arenaria low to short closed dune grassland community; and Arcthotheca populifolia - Thinopyrum distichum low to short open beach community. These were subdivided into eight sub- communities and four variants. All communities, sub-communities and variants were described and ecologically interpreted. The distribution of the communities, sub-com- munities and variants can mainly be ascribed to differences in landform, rockiness of the soil surface the degree of protection / exposure of the vegetation to the dominating winds of the area. Key words: coastal thicket, conservation area, endemic plants, Limestone Fynbos, phy- tosociology, plant communities, TWINSPAN, Western Cape M.M. Zietsman and G.J. Bredenkamp, African Vegetation and Plant Diversity Research Centre, Department of Botany, University of Pretoria, Pretoria, 0002 Republic of South Africa. Introduction level of organisation (Bredenkamp & Brown 2001). Studying the vegetation of an area A sound knowledge of the ecology of the area is an essential prerequisite for the estab- allows the identification, description and lishment of efficient wildlife management classification of plant communities. On the programmes and compilation of conservation basis of the plant communities as vegetation policies for any area (Edwards 1972). Vege- units, management units can be delineated. tation science has been applied in the fields Vegetation studies also allow the identifica- of nature conservation for years, but recent tion of ecologically sensitive areas, bush developments relate to the application of encroached areas or areas infested with alien plant ecological knowledge to environmental plants, degraded areas, habitats of rare or management (Scheepers 1983). Bredenkamp endangered plant species, and habitats of & Brown (2001) emphasise the use of plant specific animals. communities as a reliable basis for any eco- The Andrew’s Field and Tsaba-Tsaba logical planning and management. Reserve is situated in the Bredasdorp/Rivers- To obtain knowledge of the ecology of the dale Centre of Endemism (Cowling 1992). study area, a study of the vegetation of the This centre of endemism refers to a well- area should be made at the plant community defined group of plants, confined to the lime- ISSN 0075-6458 33 Koedoe 49/1 (2006) zietsman.qxd 2006/04/17 08:48 PM Page 34 stone of the Bredasdorp Formation and asso- namely Strandveld and Waenhuiskrans for- ciated colluvial deposits (Heydenrych 1994). mations are found in the study area (Malan et According to Hilton-Taylor & Le Roux al. 1994). Beach and terrace deposits, not (1989) Limestone Fynbos is one of the most formally named as a formation, but forming threatened vegetation types in the Cape part of the Bredasdorp Group, are found in Floristic Region. Factors threatening Lime- the form of roll-stones on the coastal beach stone Fynbos vegetation include alien plants (Malan et al. 1994). Previous changes in sea (considered the biggest problem), land clear- level resulted in the presence of roll-stones ing, resort development, inappropriate fire of Table Mountain Sandstone. management and over-harvesting of flowers The Strandveld Formation is restricted to the (Heydenrych 1994). coast. It consists of white to light-grey dune Thicket encroachment into the adjacent sands with a high percentage shell frag- Limestone Fynbos and Renosterveld is ments. Partial cementing of sands is a result therefore a serious threat to the species-rich of the high calcium carbonate content in the Fynbos with many endemic and red data sand. The lithology of this formation is species. The species richness of coastal described as white dune sand, strand sand thicket in the reserve is very low compared with finely divided shell and alluvial stones to those of the adjacent Limestone Fynbos (Malan et al. 1994). and Renosterveld communities (Zietsman & The Waenhuiskrans Formation forms out- Bredenkamp in press) and the true thicket crops adjacent to the current coastline. The species like Rhus glauca and Euclea race- Waenhuiskrans stratiotype is 12.4 m deep, is mosa are neither rare nor endemic. Regard- locally overlain with 1 m thick calcretes and ing the importance of the conservation of consists of medium-grained cross-layered Limestone Fynbos in areas where the thicket calcarenite with well-rounded quartz and a communities occur, the effective manage- few glauconite grains. Large-scale aeolic ment of the area becomes all the more cross-layers are characteristic of the unit. important, and fully justifies a detailed vege- The lithology of this formation is described tation study of the coastal thicket. as partially calcified dune sand (Malan et al. 1994). Study area The study area is situated on the Agulhas Plain, a coastal lowland. The majority of the The reserve is situated in the Bredasdorp dis- area falls below the 10 m contour (Jefferey trict, Western Cape, between Struisbaai 1996). Coastal dunes and dune plains are North in the south and De Mond State Forest typical in this area, with the highest point on in the north. It is bordered in the west by the a ridge 31 m above sea level. Bredasdorp/Struisbaai road, and in the south by the coastline. The study area is situated The soil can be described as shallow sandy close to the coastal town of Struisbaai. The soil, overlying limestone, or shallow to deep Andrew’s Field (129 ha) and Tsaba-Tsaba sandy soil, overlying clay, silt and gravel. Nature Reserve is approximately 979 ha in Four different soil forms have been distin- extent. Approximately one third of the study guished in the area (MacVicar 1991): area consists of the coastal vegetation, which - Coega: Orthic A on hard bank carbonate is dealt with in this report. The remainder of horizon; family Marydale: lime containing A-horizon. This soil form is found on the the study area consists of inland plains and limestone hills and shallow-soil limestone hills, described by Zietsman & Bredenkamp plain. (in press). - Immerpan: Melanic A on hard bank car- The dominant substrate in the study area is bonate horizon; family Kalkpan: lime con- the Bredasdorp Group (Malan et al. 1994). taining A-horizon. This soil form is found Two formations of the Bredasdorp Group on the proteoid dominated limestone plain Koedoe 49/1 (2006) 34 ISSN 0075-6458 zietsman.qxd 2006/04/17 08:48 PM Page 35 A = Altitude, B = Mean Annual Temperature, C = Mean Annual Rainfall, D = Mean Daily Minimum (Coldest Month), E = Mean Daily Maximum (Hottest Month) Fig. 1. A Walter climatic diagram for Andrew’s Field and Tsaba-Tsaba Nature Reserve. where the soil is deep. The study area is situated in the transitional - Brandvlei: Orthic A on soft carbonate zone between the winter-rainfall region in horizon; family Kolke: signs of wetness in the west and the non-seasonal rainfall region carbonate horizon. The marsh area is char- in the east (Mustart et al. 1997). The south acterised by this soil form. coast has a warm temperate climate with all- - Namib: Orthic A on regic sand; family seasons and bimodal equinoctial rainfall Beachwood: containing lime within 1500 (Strydom 1992). The average annual precip- mm from soil surface. This soil form is itation of the area is 444 mm. Precipitation is found on the dune plain and the deep-sand mostly rain, but it also occurs in the form of plains. fog. The maximum precipitation is in June, and the minimum during February and According to Tinley (1985) two distinct sand December. According to Heydorn & Tinley characteristics occur in dune fields. Bare (1980) the study area is situated in a low dune sands, yellow in colour, absorbs all rain rainfall region, within an arid belt protruding and consequently there is no surface runoff. into the interior from the coast. The low rain- Grey sands, stained by humus and covered fall appears to be the result of cold inshore with woody vegetation, have a water repel- waters that inhibit shoreline rains. A Walter lent layer near the surface, beneath the litter, climatic diagram was compiled (Fig. 1) from causing a massive surface runoff when data obtained from the Agulhas weather sta- heavy rains follow a dry period (Tinley tion. 1985). The average annual maximum and minimum The main factors affecting the climate are temperatures for the area are 6 ºC and 13.3 ºC the contrasting sea surface temperatures of respectively. The maximum temperature for the two major ocean currents and the inshore the area was 36.1 ºC, obtained in February. circulation (Tinley 1985). According to the The minimum temperature for the area is geographical division of Heydorn & Tinley 3.9 ºC, obtained in June (Zietsman & Bre- (1980), the reserve is situated on the south denkamp in press). coast, which is a transitional zone between The wind along the south coast is bidirec- tropical and temperate waters.