South African Journal of Botany 73 (2007) 391–401 www.elsevier.com/locate/sajb Vegetation of the central Kavango woodlands in Namibia: An example from the Mile 46 Livestock Development Centre ⁎ B.J. Strohbach a, , A. Petersen b a National Botanical Research Institute, Private Bag 13184, Windhoek, Namibia b Institute for Soil Science, University of Hamburg, Allende-Platz 2, 20146 Hamburg, Germany Received 22 November 2006; accepted 7 March 2007 Abstract No detailed vegetation descriptions are available for the Kavango woodlands — recent descriptions have all been at the broad landscape level without describing any vegetation communities. With this paper the vegetation associations found at and around the Mile 46 Livestock Development Centre (LDC) are described. Two broad classes are recognised: the Acacietea are represented by three Acacia species-dominated associations on nutrient-richer eutric Arenosols, whilst the Burkeo–Pterocarpetea are represented by three associations dominated by broad-leafed phanerophytic species on dystri-ferralic Arenosols. The, for the Kavango woodlands typical, Pterocarpus angolensis–Guibourtia coleosperma bushlands and thickets are further divided into four variants. Fire has been found to be an important factor in determining the structure of the vegetation — exclusion of fire on the LDC itself seems to lead to an increase in shrub (understory) density. © 2007 SAAB. Published by Elsevier B.V. All rights reserved. Keywords: Arenosols; Fire; Kavango woodlands; Land-use impact; Namibia; Zambesian Baikiaea woodlands ecoregion 1. Introduction terms. Yet pressure on the land is increasing — Mendelsohn and el Obeid (2003) illustrate this very clearly in their profile on the Very few phytosociological studies have been undertaken in Kavango Region. Whereas in 1943, only 26,140 ha of land had the past in Namibia (Burke and Strohbach, 2000; Strohbach, been cleared for crops along the Kavango River terrace, this 2001). This, linked with a similar situation regarding soil increased to 72,100 ha in 1972, following main river beds into surveying, essentially means there is a lack of baseline data for the hinterland, and to 194,550 ha in 1996, following all land-use planning. With increasing demands for land, proper available access routes into the hinterland in the search of land-use planning becomes an urgent need of national productive soils. This amounts to an average increase in importance. With this in mind, a project was launched to map cropped fields of 3.9% per year. and properly describe the vegetation of Namibia in the late The rural population not only uses land for cropping, but is 1990s (Strohbach, 2001). highly dependant on vegetation resources for their liveli- Baseline information on the vegetation of the Kavango hoods. Cattle are grazed; timber is harvested for construction Region is as limited. An early description by Page (1980) was purposes as well as carvings; thatch-grass is harvested; and based on Forestry Resource maps, whilst later descriptions by numerous other resources are used from the wild (Gelden- De Sousa Correira and Bredenkamp (1986) and Burke (2002) huys, 1996). These land-use pressures are exacerbated by describe the vegetation only in broad, reconnaissance-level frequent fires. It is estimated that between 30 and 50% of the total area of the Kavango Region is burned annually from anthropogenic causes (De Sousa Correira and Bredenkamp, 1986; Mendelsohn and el Obeid, 2003). These frequent fires ⁎ Corresponding author. are regarded as one of the main causes of vegetation change E-mail address: [email protected] (B.J. Strohbach). in the region (Burke, 2002). 0254-6299/$ - see front matter © 2007 SAAB. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.sajb.2007.03.002 392 B.J. Strohbach, A. Petersen / South African Journal of Botany 73 (2007) 391–401 In order to study the long-term changes in biodiversity as a Administration for Kavango as a demonstration and breeding result of human land-use, climate variations and other habitat farm. Since Independence, the farm has not been fully utilised. influences, the BIOTA Southern Africa project was launched The few cattle on the farm have been kept on the southern part of in 2001. For the purpose of long-term monitoring, a pair of the station close to the homesteads. Since 2003 a livestock standardized “biodiversity observatories” was established at breeding programme has been re-implemented, which is the Mile 46 Livestock Development Centre (LDC) in the currently hampered by dilapidated fencing infrastructure. Kavango Region, as part of a series of similar observatories The building of the trunk road between Grootfontein and extending from the Cape Peninsula in South Africa to the Rundu in the 1970s leads to a systematic settlement along this Kavango Region in northern Namibia. The Mile 46 LDC pair main road. This was further exacerbated by the clearing of the of biodiversity observatories are paired across a land-use vegetation along the road for security reasons by the South border in the form of the border fence of the LDC — within African Defence Force — the local population used these the LDC light rotational grazing is practised, whilst the second clearings for fields. In time a number of villages were observatory is situated outside the LDC in communal grazing established between the trunk road and Mile 46 LDC, of areas of the Mutompo village to the east of the LDC. As these which Mutompo is one of the more important and also included two biodiversity observatories (each 1 km2 in size) only in the socio-economic studies conducted under BIOTA southern provide a limited representative sample of the dominant Africa. For this paper, the study area was chosen to encompass vegetation types, a baseline survey was undertaken of the Mile 46 LDC in the west as well as the various villages up to, various vegetation types in and around the Mile 46 LDC and and including the trunk road. The entire study area covers adjacent communities. 84,314 ha, whilst Mile 46 LDC covers ca. 4600 ha within this study area (J Els, Chief Researcher: Large Stock, pers. comm.). 2. Methods 2.2. Climate 2.1. Study area The climate has been described as a hot, arid steppe The Mile 46 LDC lies at 19°13′00ʺS, 18°16′30ʺE and 19°15′ according to Koeppen's classification system (De Sousa 19ʺS, 18°23′37ʺE at an altitude of approximately 1160 m Correira and Bredenkamp, 1986). At Mile 46 LDC between (±20 m) above sea level within the Kavango Region, to the 500 and 550 mm rain falls annually during the summer months northeast of the trunk road between Rundu and Grootfontein (NARIS, 2001; Botha, 1998/99)(Fig. 1). This results in an (Fig. 1). It was established in 1984 under the pre-independence average growing period of 91 to 120 days, being 80% reliable Fig. 1. Location of the study area within the Kavango Region, north-eastern Namibia. Also indicated on this map are the long-term average annual rainfall isohyets, with their values indicated to the right. B.J. Strohbach, A. Petersen / South African Journal of Botany 73 (2007) 391–401 393 (De Pauw et al., 1998/99). As part of the BIOTA project, an sources only inventorise the soil units without regionalisating automated weather station was erected at the LDC. Using the these. available data (3 years), a climate diagram following Leith et al. Typically, the soils are deep, very pure sands with little (1999) was constructed, as well as a wind rose depicting the nutrients. Heavier textured soils have formed only in the dominant wind directions during the year (Fig. 2). It should be omiramba (singular – omuramba – a wide, flat watercourse noted that the rainfall during these measured years was below with no visible gradient — King, 1963). A number of these average, possibly also resulting in above average high omiramba cross the study area in an east–west direction, temperatures. contributing to the Mpuku Omuramba. These omiramba, because of their more nutrient-rich soils, are sought-after for 2.3. Topography, geology and soils fields by the local population (Graz, 1999; Burke, 2002). The study area falls within the extensive Kalahari sand 2.4. Vegetation basin. These aeolian sands were finally deposited ca. 30,000 years ago as longitudinal, east–west orientated dunes The Kavango falls within the Zambesian Baikiaea wood- (Hegenberger, 1986). Through erosion, the topography at the lands ecoregion as delimited by the World Wildlife Fund study area was flattened to less than 10 m relative difference (WWF) (Vetter, 2001a). As such, it is dominated by open in relief, although the remnants of the dune fields are still woodlands with numerous hardwood species, including Bai- clearly visible in satellite images (Graz, 1999). The depth of kiaea plurijuga, Pterocarpus angolensis and Guibourtia the sand cover is estimated to be between 20 and 40 m, coleosperma. Although named after Baikiaea plurijuga,itis although the entire Kalahari sequence is approximately 150 m pointed out in the WWF description (Vetter, 2001a) that this deep (Hegenberger, 1986; Geological Survey, 1980). The Mile species is fire-sensitive and used extensively for timber, thus 46 LDC is crossed by the upper Mpuku Omuramba, which becoming rare. Giess (1998) includes the study area in the incises deeply into the sand plateau (Graz, 1999; De Sousa Northern Kalahari Woodland savanna. Correira and Bredenkamp, 1986). Soil information on the De Sousa Correira and Bredenkamp (1986) described a broader study area is provided by the FAO soil map (1997) series of land-use units within the Kavango Region, mostly and the Agro-Ecological Zoning Programme of the Ministry of being dependant on the depth of the sands. According to Agriculture, Water and Forestry (AEZ) (NARIS, 2001). The their classification, the study area falls within the C2 land- area is dominated by cambic Arenosols, albic Arenosols, use unit, described as flat to slightly undulating plains calcic Xerosols (FAO, 1997, revised legend).