Okavango Basin - Vegetation

Okavango Basin - Vegetation The vegetation of the Okavango basin is evergreen tree species Cryptosepalum their leaves for a period of some months shaped by a strong environmental gradient. exfoliatum ssp. pseudotaxus with its during the dry season. The dominance of From the northwest to the southeast mean microphyllous, compound leaves the canopy species depends on local abiotic annual precipitation diminishes strongly dominates extensive areas mostly on poor conditions and changes from Brachystegia and elevation drops from 1,850 m a.s.l. to ferralitic soils. It has been cited to be spiciformis to Julbernardia paniculata just 940 m a.s.l. in the Delta. In the intolerant of fire (Timberlake & together with various other woody species highlands of the Bié Plateau in central Chidumayo 2011) and hence is often (Fig. 4). Angola the topographic positions and associated with spares undergrowth but Miombo forests dominated by geologic substrate have a strong impact on thick moss layers and lianas. The evergreen Julbernardia paniculata: This vegetation vegetation patterns. However, the canopy creates rather moderate and humid unit covers the drier, southernmost belt of landscape in the middle reaches of the basin microclimatic habitat conditions and thus the Angolan Miombo woodlands and is rather flat and dominated by Kalahari facilitates the occurrence of species related represents the southern limit of Miombo sands; species composition here is mostly to central African forests (Fig. 1, 2 and 3). sensus stricto. Altitudes in this vegetation determined by nutrient availability or soil Miombo forests dominated by zone are considerably lower and hence water regime. In the Delta area topographic deciduous tree species: This type of these areas receive less precipitation. microfeatures again become paramount as Miombo is composed of tree species losing Woodlands are dominated by Julbernardia slight differences in altitude and relief position govern sedimentation and erosion processes, flooding frequencies, and flood duration. Furthermore, remnants of the former longitudinal dune system still shape the distribution of current vegetation in the sandy hinterland surrounding the Delta. Large areas of the Okavango basin are remote and still hardly accessible. This holds true especially for the Angolan part of the basin. Therefore, remote sensing data are indispensable in generating a comprehensive overview of the vegetation cover. In the following we discuss the vegetation units resulting from the classification of multitemporal MODIS data (Stellmes et al. 2013). Groundtruthing is based on standardized vegetation plots (see Electronic Appendix) and quick vegetation assessments along the main access routes. Miombo forests and woodlands Miombo forests and woodlands are the dominant vegetation in the upper reaches of the basin. They form part of the tropical dry forest ecosystem stretching over large parts of south-central Africa where precipitation is seasonal and moderate ranging from 710 mm to 1,365 mm per year. The canopy of Miombo ecosystems is characterized by the dominance of Caesalpinoid tree species from the genera Brachystegia , Cryptosepalum, Julbernardia or Isoberlinia, and meso- and microphyllous, compound leaves (Campbell 1996). Fig. 1: Miombo forest with Cryptosepalum exfoliatum ssp. pseudotaxus with dense Miombo forests with Cryptosepalum moss layer on the ground and stems covered by lichens at the core site of Cusseque exfoliatum ssp. pseudotaxus: The (photo: R. Revermann). In: Oldeland, J., Erb, C., Finckh, M. & Jürgens, N. (2013) [Eds.]: Environmental Assessments in the Okavango Region. – Biodiversity & Ecology 5: 29–35. DOI: 10.7809/b-e.00240. 29 paniculata and Brachystegia bakeriana is woodlands are composed of a multitude of Mashare region, often known as “Kavango strongly represented in the sub canopy. Tall tree species such as Burkea africana, woodlands”. Pterocarpus angolensis, a shrubs of Diospyros batocana and Pterocarpus angolensis, Erythrophleum valuable timber species, frequently occurs Copaifera baumiana, amongst others, form africanum, Schinziophyton rautanenii, and here as well as Schinziophyton rautanenii, the understorey (Fig. 5). Guibourtia coleosperma (Fig. 6). the Manketti tree, which is valued for its oil Baikiaea-Burkea woodlands (dense): rich fruits (Fig. 7). Woodlands on Kalahari sand Within the Okavango Basin this vegetation Baikiaea-Burkea woodlands (open): unit has a very restricted spatial extent. In Whilst similar in species composition to the The drier woodlands south of the Miombo the communal area of Savate it is set apart above mentioned unit, these woodlands are belt are classified by Olson et al. (2001) as from the surrounding open woodlands by a dominated by fire tolerant species or the Zambezian Baikiaea Woodlands thicket-like understorey characterized by species adapted to browsing by livestock ecoregion. Vegetation structure changes shrubs and small trees of Croton such as Gymnosporia senegalensis, profoundly, from a rather closed canopy in gratissimus, Combretum celastroides and Diplorhynchus condylocarpon, and Miombo to a more savannah like structure C. engleri, while Baikiaea plurijuga Strychnos cocculoides. The canopy is far with large crowns of individual trees and represents the only emergent canopy tree less closed and hence ground cover by huge gaps in between. Consequently, a species. shrubs and grasses increases. shrub and grass layer with a dense ground Baikiaea-Burkea woodlands (medium Mixed Burkea woodlands (with cover establishes. Although Baikiaea dense): This is the typical woodland of the Terminalia sericaea): Mixed Kavango plurijuga is the eponymous species, these middle reaches and of the Caiundo and woodland, usually resulting from land use or limiting environmental conditions. On sandy soils Terminalia sericea stands form extensive shrublands tolerating droughts and temporary waterlogging (Fig. 8). Other woodlands Mixed woodlands (including Colophospermum mopane): To the east of the Panhandle and the Delta, the Baikiaea woodlands give way to another vegetation unit, the Colophospermum mopane woodlands, which are often associated with comparatively clay rich soils (Fig. 9). Open and degraded woodlands (on sandy soils): This unit contains unspecified woodlands that have a low coverage due to constraining environmental conditions or degradation Fig. 2: Dense Miombo forest north-east of the core site of Cusseque (photo: M. Finckh). by human overexploitation. The central highland in the provinces of Bié and Huambo provide many examples where anthropogenic activities have been intense since early colonial times and lead to degraded woodland remnants. Thornbush savannah dominated by Acacia spp. West of the Delta and to the south of the Kavango Region in Namibia a profound change occurs as the climate becomes more arid and a thornbush savannah develops. Depending on local soil conditions and land use, species composition and vegetation structure change from thornbush savannah with seasonally dense grass layer, to a medium dense Fig. 3: Warneckea sapinii, a typical species of intact Miombo forests in the Angolan thornbush savannah and to a sparse Highlands (photo: M. Finckh). thornbush savannah not further specified 30 Biodiversity & Ecology 5 2013 here. The class sparse shrubland or urban Parinari capensis grasslands (on “Cassamba”. It covers the mid- and and cultivated areas subsumes areas humid sands): These grasslands are footslopes of the tributaries of the upper depicting a high fraction of bare soil (Fig. intermingled with various dwarf shrubs, catchment in the Miombo region especially 10). most prominently Parinari capensis and on ferralitic soils. The drastic change of its Syzygium guineense ssp. huillense. They appearance throughout the season is occur on sandy plains in the larger valleys striking. In the late rainy season these areas Shrub grasslands of southern Bié and northern Cuando resemble grasslands dominated by tall Cubango featuring deep, sandy, leached grasses. However, after the fires of the dry Burkea-Baphia shrublands: Open soils exhibiting hydromorphic features in season, dwarf shrubs resprout and savannah-like stands of Burkea africana the lower soil horizons. Similar grasslands dominate the landscape for several months with Baphia massaiensis, Combretum cover large surfaces in Moxico (e.g. the (Fig. 13). species and further small tree species occur Cameia National Park) and Zambia (e.g. Forest grassland ecotone (with especially on dry Kalahari sands in Liuwa Plains National Park) west of the Cryptosepalum maraviense): This unit northern Cuando Cubango. Zambezi river (Fig. 11 and 12). comprises various types of ecotones Open shrublands on sandy soils: This Cryptosepalum m a r a v i e n s e between the Miombo woodlands and open unit comprises various types of shrublands grasslands: Locally, this vegetation unit is grasslands in the upper basin, frequently of the study area, including in pans. referred to as “Anhares de Ongote” or located on the midslopes. Tree species include Monotes glaber, Uapaca kirkiana, U. nitida and Terminalia brachystemma. The soil is normally covered by C. maraviense (Fig. 14). Wetlands Seasonally flooded grasslands and reedbeds: This unit subsumes the various wetland types along permanently flowing water courses, ranging from seasonally flooded grasslands to permanently flooded Cyperus papyrus and Phragmites swamps and the open water of the channels within the Okavango Delta. In this region vegetation patterns are governed

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