Woody vegetation change over more than 30 years in the interior duneveld of the Kalahari Gemsbok National Park
Background and objectives: Long-term studies of woody plants in South Africa are scarce. This study, initiated in the late 1970s, therefore aids understanding of vegetation dynamics in the southern Kalahari by investigating woody vegetation
change at and away from a watering point.
Methods: At three sites, all woody individuals were counted by species in plots 0.5 or 1 ha in size. Seedlings were noted separately from the >0.2 m group of individuals.
Results: Vachellia erioloba and shrub density decreased over time whereas dwarf shrub species’ numbers fluctuated markedly. Additionally, no increase in density of known bush encroaching species (e.g. Grewia flava, Rhigozum trichotomum
and Senegalia mellifera) was found in this large conservation area.
Discussion and conclusion: The changes in density of the woody species seem to point to the importance of particular rainfall patterns or sequences of events over different years that are responsible for these changes in the southern Kalahari, and the evident lack of bush encroachment in this conservation area supports the notion that bush encroachment in arid savannas is driven primarily by land-use practices and not by elevated carbon dioxide levels that are sometimes provided as cause for encroachment.
Barnes, M.E., 2001b, ‘Effects of large herbivores and fire on the regeneration of Acacia erioloba in Chobe National Park, Botswana’, African Journal of Ecology 39, 340–350, DOI: 10.1046/j.1365-2028.2001.00325.x.
Chambers, L.E., Barnard, P., Poloczanska, E.S., Hobday, A.J., Keatley, M.R., Allsopp, N. & Underhill, L.G., 2016, ‘Southern
hemisphere biodiversity and global change: data gaps and strategies’, Austral Ecology 421, 20–30, http://dx.doi.
Haase, P., Tonkin, J.D., Stoll, S., Burkharde, B., Frenzel, M., Geijzendorffer, I.R., Häuser, C., Klotz, S., Kühn, I., Mc-Dowell, W.H., Mirtl, M., Müller, F., Musche, M., Penner, J., Zacharias, S. & Schmeller, D.S., 2018, ‘The next generation of site-based long-term ecological monitoring: Linking essential biodiversity variables and ecosystem integrity’, Science of the Total Environment 613–614, 1376–1384, https://doi.org/10.1016/j.scitotenv.2017.08.111.
Joubert, D.F., Smit, G.N. & Hoffman, M.T., 2013, ‘The influence of rainfall, competition and predation on seed production,
germination and establishment of an encroaching Acacia in an arid Namibian savanna’, Journal of Arid Environments
91, 7–13, DOI: 10.1016/j.jaridenv.2012.11.001.
Lindenmayer, D.B. & Likens, G.E., 2009, ‘Adaptive monitoring – a new paradigm for long-term studies and monitoring’,
Trends in Ecology and Evolution 24, 482–486, DOI:10.1016/j.tree.2009.03.005.
Lindenmayer, D.B. & Likens, G.E., 2010, ‘The science and application of ecological monitoring’, Biological Conservation
143, 1317–1328, DOI: 10.1016/j.biocon.2010.02.013
Magurran, A.E., Baillie, S.R., Buckland, S.T., Dick, J.M., Elston, D.A., Scott, E.M., Smith, R.I., Somerfield, P.J. & Watt, A.D., 2010, ‘Long-term datasets in biodiversity research and monitoring: assessing change in ecological communities through time’, Trends in Ecology and Evolution 25, 574–582, DOI: 10.1016/j.tree.2010.06.016.
Moleele, N.M., Ringrose, S., Matheson, W. & Vanderpost, C., 2002, ‘More woody plants? the status of bush encroachment
in Botswana’s grazing areas’, Journal of Environmental Management 64, 3–11, DOI: 10.1006/jema.2001.0486.
Moore, A., Van Eck, J.A.J., Van Niekerk, J.P. & Robertson, B.L., 1988, ‘Evapotranspirasie in drie plantgemeenskappe van ‘n Rhigozum trichotomum habitat te Upington’, Journal of the Grassland Society of southern Africa 5, 80–84, https://
Moustakas, A., Wiegand, K., Getzin, S., Ward, D., Meyer, K.M., Guenther, M. & Mueller, K.-H., 2008, ‘Spacing patterns of an Acacia tree in the Kalahari over a 61-year period: How clumped becomes regular and vice versa’, Acta Oecologica 33, 355–364, DOI: 10.1016/j.actao.2008.01.008.
Nackley, L.L., Betzelberger, A., Skowno, A., West, A.G., Riley, B.S., Bond, W.J. & Midgley, G.F., 2018. ‘CO2 enrichment does not entirely ameliorate Vachellia karroo drought inhibition: A missing mechanism explaining savanna bush encroachment’, Environmental and Experimental Botany 155, 98–106, DOI: 10.1016/j.envexpbot.2018.06.018.
Parr, T.W., Ferretti, M., Simpson, I.C., Forsius, M. & Kovács-Láng, E., 2002, ‘Towards a long-term integrated monitoring
programme in Europe: network design in theory and practice’, Environmental Monitoring and Assessment 78, 253–290, DOI: 10.1023/a:1019934919140.
Peters, D.P.C., Loescher, H.W., SanClements, M.D. & Havstad, K.M., 2014, ‘Taking the pulse of a continent: expanding site-based research infrastructure for regional- to continental scale ecology’, Ecosphere 5, 1–23, http://dx.doi.org/10.1890/ES13-00295.1.
Seymour, C., 2008, ‘Grass, rainfall and herbivores as determinants of Acacia erioloba (Meyer) recruitment in an African
Savanna’, Plant Ecology 197, 131–138, DOI: 10.1007/s11258-007-9366-x.
Seymour, C. & Milton, S., 2003. ‘A collation and overview of research information on Acacia erioloba (camelthorn) and
identification of relevant research gaps to inform protection of the species’, Department of Water Affairs and Forestry, Pretoria.
Shadwell, E. & February, E., 2017, ‘Effects of groundwater abstraction on two keystone tree species in an arid savanna national park’, Peer J 5, e2923, DOI 10.7717/peerj.2923.
Silvertown, J. & Charlesworth, D., 2001, ‘Introduction to plant population biology’, 4th edn., Blackwell Science.
Steenkamp, C.J., Vogel, J.C., Fuls, A., Van Rooyen, N. & Van Rooyen, M.W., 2008, ‘Age determination of Acacia erioloba
trees in the Kalahari’, Journal of Arid Environments 72, 302–313, DOI: 10.1016/j.jaridenv.2007.07.015.
Tews, J., Schurr, F., Jeltsch, F. & Skarpe, C., 2004, ‘Seed dispersal by cattle may cause shrub encroachment of Grewia
flava on southern Kalahari rangelands’, Applied Vegetation Science 7, 89–102, DOI: 10.1111/j.1654-109X.2004.tb00599.x.
Van der Merwe, H., Van Rooyen, N., Bezuidenhout, H., Bothma, J. du P. & Van Rooyen, M.W., 2019, ‘Vachellia erioloba dynamics over 38 years in the Kalahari Gemsbok National Park, South Africa’, Koedoe 61(1), a1534, DOI:10.4102/koedoe.v61i1.1534.
Van Rooyen, N. & Van Rooyen, M.W., 1998, ‘Vegetation of the South-western arid Kalahari: an overview’, Transactions
of the Royal Society of South Africa 53, 113–140, https://doi.org/10.1080/00359199809520381.
Van Rooyen, N., Bredenkamp, G.J., Theron, G.K., Bothma, J du P. & Le Riche, E.A.N., 1994, ‘Vegetational gradients around artificial watering points in the Kalahari Gemsbok National Park’, Journal of Arid Environments 26, 349–361, https://doi.org/10.1006/jare.1994.1037.
Van Rooyen, N., Bezuidenhout, H., Theron, G.K. & Bothma, J du P., 1990. ‘Monitoring of the vegetation around artificial
watering points (windmills) in the Kalahari Gemsbok National Park, Koedoe 33, 63–88, https://doi.org/10.4102/ koedoe.v33i1.453.
Van Rooyen, M.W., Van Rooyen, N., Bothma, J. du P. & Van den Berg, H., 2008, ‘Landscapes in the Kalahari Gemsbok
National Park, South Africa’, Koedoe 50, 99–112, DOI:10.4102/koedoe.v50i1.154.