Springer 2006
Wetlands Ecology and Management (2006) 14:329–348 DOI 10.1007/s11273-005-4990-5
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Impact of land use on vegetation composition, diversity, and selected soil properties of wetlands in the southern Drakensberg mountains, South Africa D.J.J. Walters1,*, D.C. Kotze2 and T.G. O’Connor3 1
Mondi Wetlands Project, Wildlife and Environment Society of South Africa and WWF-South Africa, P.O. Box 493, Merrivale 3291, South Africa; 2Centre for Environment, Agriculture and Development, University of KwaZulu Natal, Private Bag X01, Scottsville, 3209, South Africa; 3Centre for African Ecology, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, P.O. WITS, 2050, South Africa; *Author for correspondence (e-mail:
[email protected]; phone: +27-33-3305831) Received 21 July 2003; accepted in revised form 25 October 2005
Key words: Carbon, Communal, Cultivation, DCA, Invasives, Oxbows, Plant diversity, South Africa
Abstract Wetlands provide the ecosystem services of enhancing water quality, attenuating floods, sequestrating carbon and supporting biodiversity. In southern Africa, the pattern and intensity of land use is influenced by whether land tenure is public (state), private (individual ownership), or communal (shared agricultural and grazing resources). The influence of land tenure and its associated use on service provision was compared for communal tenure (grazing, maize production), wildlife conservation, and commercial agriculture (grazing, planted pastures) in the southern Drakensberg. Ordination analyses revealed that oxbow marshes, hill slope seepages and hygrophilous grasslands, the main hydro-geomorphic units, supported distinct plant communities that differed in their response to land use because of wetness or slope. Oxbows, uncultivated because of wetness, were inherently species poor with few exotics. Composition of hill slope seepages, uncultivated because of saturated slopes, varied among tenure types most likely in relation to grazing pressure. Seepages were threatened by the exotic invasive Rubus cuneifolius. Eighty-five percent of hygrophilous grassland had been cultivated by 1953, most of which was subsequently abandoned to secondary grassland. Primary hygrophilous grassland and hill slope seepages were the main repository for indigenous plant diversity, while communal maize fields supported a diverse mixture of mainly exotic species. Soil carbon concentrations decreased from oxbows to pastures, seepages, primary hygrophilous grassland, secondary grassland, and maize on former grassland (7.0, 4.1, 4.0, 3.5, 2.4, and 1.7%, respectively). The pattern for total soil nitrogen and sulphur were the same. Cultivation of hygrophilous grassland was estimated to have reduced soil carbon stocks to 69% of pre-settlement levels by 1953 (150 years BP). Stocks then increased by 8% to 2001 following crop abandonment. Cultivation has impaired water quality enhancement and flood attenuation because of greater amounts of bare ground and shorter vegetation. Further improvement of ecosystem services will depend on the influence of socio-economic factors on communal cropping.
Introduction Wetlands are of inestimable value for the supply of goods and services to society (Begg 1987; Kotze
and Breen 1994; Mitsch and Gosselink 2000), but they are threatened globally (Maltby 1991). In eastern South Africa, approximately 50% of wetlands have been lost or degraded (Kotze et al.
330 1995), most commonly a result of modification by commercial or subsistence agriculture (Kotze et al. 1995). Global climate change and invasion of exotic plants are additional impacts. Scientific insight regarding land-use impacts on wetlands is based mainly on Euro-American study, with a dearth of African research having been undertaken (Kotze and Breen 1994). Foremost among the critical ecosystem services of wetlands are enhancement of water quality, erosion control and flood regulation, all of which depend on the ability and capacity of wetland vegetation to attenuate storm flows (Mitsch and Gosselink 2000). Their contribution to carbon sequestration is highlighted by the fact that wetlands occupy 4–5% of the land area of the globe yet hold approximately 20% of the carbon in the terrestrial biosphere (Roulet 2000). In South Africa and presumably elsewhere, wetlands are essential for biodiversity conservation as many threatened taxa are dependent on them (Goodman 1987). Vegetation structure, in particular, can be critical for the success of many avifaunal (MacLean 1985) and mammalian (Smithers 1983) species. In terms of goods, they provide fodder for livestock and plants used for crafts and medicinal purposes. The value of medicinal plants to South Africa is emphasized by the fact that 27 million consumers trade US$60 million of plants per annum (Mander 1998). Some wetlands are well suited to agriculture because of fertile soils and water availability. Their transformation by cultivation will obviously impact on biodiversity but it is less clear how it will impact on aspects of ecosystem functioning. A potential conflict of interest in the Drakensberg foothills arises from the demand of this area for agriculture and its importance as a water catchment. South Africa is a semi-arid country with a mean annual rainfall of only 450 mm (Schulze 1997). This is half the amount received by the Drakensberg, emphasizing its importance for water delivery. The delivery of high quality water is, in part, assured by wetlands that cover 5–20% of the area (Begg 1989; Enviromap 1996). Reliable rainfall and relatively fertile soils have attracted settlement (Hilliard and Burtt 1987). Indigenous peoples have lived under a system of communal tenure since the 1850’s, in which land is shared (no private ownership). Settlers have occupied land since the early 1900’s under private tenure (indi-
vidual ownership), while the state has administered land for conservation. The three types of tenure impact wetlands through livestock or wildlife grazing and cultivation of crops or pastures. Communal areas differ from their commercial counterparts in that livestock stocking rates can be three-fold greater (Tapson 1993) and that cultivation has limited inputs of fertilizer and lacks mechanized harvesting. Public bodies are charged with administration of conservation lands, on which only light grazing by indigenous wildlife occurs (Short et al. 2003). Differences between the types of land tenure thus result in differences in land use. Wetlands, on account of being transitional between terrestrial and aquatic systems, are characterized by prolonged flooding or saturated soils (Cowardin et al. 1979; Mitsch and Gosselink 2000). Hydro-geomorphological settings of wetlands can be extremely diverse and can be a primary determinant of wetland structure and functioning (Brinson 1993; Higgins et al. 1996; Kotze and O’Connor 2000). It is therefore anticipated that both the nature of use and response to use of a wetland could vary in response to hydrogeomorphological characteristics. The aim of this study was to investigate the impact of land tenure and use on the structure and composition of wetlands in the southern Drakensberg and on the potential services and goods provided by these wetlands. Specifically, we investigated impact of land tenure on plant community composition, plant richness, vegetation structure, flood attenuation, and provision of habitat. We also measured soil carbon concentrations because of the role that wetlands play in assimilation of pollutants and carbon sequestration to counter effects of global change. Soil nitrogen and sulphur were measured because of their importance for plant growth and potential to function as pollutants. On account of the strong influence that hydro-geomorphological units have on wetland functioning, the impact of land tenure and use was investigated separately for each unit.
Study area and land uses The study was conducted in the Underberg district of the southern Drakensberg (2941–60¢ S; 2925– 37¢ E) on properties (Figure 1) ranging in altitude
331 from 1480 to 1580 m.a.s.l. Mean annual precipitation is 900–1200 mm per annum; most precipitation falling during summer (September–April) as rainfall. Climate is temperate: mean daily maximum and minimum temperatures at Cobham (station 13466, Institute of Soil, Climate and Water, Pretoria) were 16.2 and 11.7 C for January and 16.8 and 0.2C for July. More than 100 frost nights can occur per winter (Schulze 1997). Topography varies from undulating to rugged hills with rivers and wetlands in the valleys. The dominant soils are generally deep (1 m), leached, acid, ferralitic soils derived from dolerite dykes and sills and Beaufort series sandstones of the Karoo system (Hurt 1989; Metroplan 2001). The area lies within the Grassland Biome (Rutherford and Westfall 1986). Sampling was undertaken on two state-owned conservation properties, two communal areas and three private properties, which occurred along two high order (>4) rivers meandering across flat valley bottoms differing in their dominant land-uses (Figure 1). The Ngwangwana River flowed through a commercial livestock property, Coleford Nature Reserve, and then the Ndwana communal area,
while the Pholela river flowed through a commercial dairy farm, a commercial livestock farm, The Swamp Nature Reserve, and Reichenau communal area. Physiography of the valley basin was judged reasonably uniform along each sampled length of river and between the two rivers. Communal areas supported livestock grazing of rangeland and cultivation of predominantly maize. Commercial producers used wetlands for livestock grazing and for the planting of high quality pastures, mainly for dairy production. Communal maize fields receive some fertilizer at rates of application much less than those applied on commercial farms. Conservation properties 1. Coleford Nature Reserve (CNR; 1272 ha) was acquired in 1958 for wildlife conservation, prior to which it was a private farm with cultivation practised on wetlands. Mean stocking density over the past 15 years was 0.07 (up to 0.12) animal units (AU) ha 1 (AU defined as the metabolic mass equivalent of a 455 kg steer) (Short et al. 2003). Wetlands were extensively drained and cultivated whilst the property was used for agriculture, but
Figure 1. The location of the two study rivers and the properties on which sampling occurred in the southern Drakensberg, South Africa.
332 wetlands were re-established in the mid-1980’s by plugging canals (KZN Wildlife, unpublished reports). 2. ‘The Swamp’ was acquired by conservation in 1984. It was previously a ‘tenant’ farm, with a settlement of approximately 206 dwellings adjacent to the wetland visible in the 1953 aerial photographs. This community cultivated the wetlands extensively before they were relocated under apartheid policy. Both conservation properties therefore have a history of intensive cultivation of wetlands but have been protected from agriculture for a period of time. Private (Commercial) Properties. The southern Drakensberg was opened to ‘white’ agriculture under private tenure (hereafter termed ‘commercial’ for convenience) about a century ago (Hilliard and Burtt 1987). Most farms (±1000 ha) practised a mixture of extensive livestock ranching and dairy production. Indigenous grassland is grazed mainly from spring through to early winter (end May) by livestock at a stocking density of 2.5 ha per AU. The properties sampled were Scafell, Bergview and Petersfield (Figure 1). 1. Scafell supports a dairy enterprise. Approximately 60% of its floodplain (hygrophilous flats) was planted to ryegrass (Lolium spp.) and cocksfoot (Dactylis glomerata) pastures, which have been fertilized with nitrogen, phosphorus and potassium, and intensively grazed. 2. Bergview has not been an agricultural production system for some time, so its wetlands have been only lightly grazed. 3. Petersfield was a beef enterprise but a number of pastures have been abandoned. Communal areas 1. Reichenau Mission is a 600 ha, 120-year-old communal area situated among private farms. Stocking density on 4 September 2001 was about 0.4 animal units per hectare (defined as the metabolic mass equivalent of a 450 kg steer). Cultivation covered more than 60% of the floodplain (hygrophilous grasslands) in 1953, and was reasonably extensive up until the 1980s, but was confined to less than 5% of the floodplain in 2002. 2. Ndwana, an unbounded area of about 3000 ha, is an historical communal area. Stocking density on
the 22 August 2001 was about 0.5 animal units per hectare. Cultivation of wetlands is confined predominantly to the better drained portions of the floodplain adjacent to the river channel. The area cropped has decreased in recent time and covered approximately 20% of the floodplain in 2002. Cropping areas are considered to have previously supported a mosaic of hygrophilous grassland and mesic non-wetland soils. According to local farmers, cultivated fields are rarely flooded by the river. Based on the 1953 aerial photographs, 85% of the hygrophilous grasslands in the study area were estimated to have once been cultivated but many have been abandoned over the past few decades, resulting in extensive secondary grasslands.
Methods Sampling strategy Three hydro-geomorphological units were sampled: (1) oxbow marsh, permanently moist areas at the lowest elevation of valley bottoms; (2) hygrophilous grasslands, areas of temporary saturation in valley bottoms at higher elevations than oxbows; and (3) hill slope seepage, seasonally saturated areas extending from the outer margin of the valley bottom up the adjacent hill slope. Lateral input from surface run-on, seepage and tributaries appeared to be more important in maintaining these wetlands than bank over-spill from the rivers. Maize cultivation and planted pastures were confined to hygrophilous grasslands furthermore, these land uses where found exclusively in areas of communal and private tenure, respectively whilst historically cultivated and noncultivated sites where common to all land tenure types. A total of seven individual properties were sampled, ensuring an equitable sampling effort across hydro-geomorphological units and land use/tenure types (Table 1).
Plant community composition and richness, and soils For each area sampled, 3–15 sampling plots were located per wetland unit, with sampling intensity
333 Table 1. The distribution of the number of sampling sites across tenure type, land use, and hydro-geomorphological unit. Tenure/land use
Hydro-geomorphological unit Hygrophilous flats – developed
Conservation Communal livestock grazing Private livestock grazing Planted pasture Maize
Hygrophilous flats – secondary grassland
Hygrophilous flats – primary grassland
Oxbow marsh
Hill slope seepage
11 20
13 11
9 (3 rehabilitated) 10
16 14
7
14
6
14
– –
– –
– –
12 7
related to vegetation heterogeneity per unit. A total of 161 plots were sampled during the growing season (December 2001 to January 2002), at each of which the following was described. Botanical composition. Aerial cover of vascular species within a circular plot (2 m radius) was estimated using the Domin scale (Jager and Looman 1987). A reconnaissance confirmed previous studies (e.g., Downing 1966; Thompson 1985) that graminoids dominate wetlands in southern Africa, all taxa of which were thoroughly sampled. Several non-graminoid taxa (notably Iridaceae and Orchidaceae) are markedly seasonal and may only be identifiable in spring or autumn (Downing 1966) and were thus not well sampled, but all other nongraminoid species were. Derived measures were the number and aerial cover of indigenous and exotic vascular species. Nomenclature follows Arnold and De Wet (1993). Vegetation structure. Percentage cover of surface litter and vegetation cover were estimated. Litter of oxbow marshes could not be measured because of inundation. Standing height of the vegetation was measured. Degree of soil wetness. This descriptor of the long-term hydrological regime was described using soil morphological features (chroma, intensity and depth of soil mottling) following Kotze et al. (1994, 1996). In brief, a core is sampled to a depth of 0.5 m using a Dutch screw auger, the core is compared with a soil wetness system in order to categorize the site as one of four wetness classes: permanently/semi-permanently wet, seasonally wet, temporarily wet and non-wetland. This measure integrates the many factors affecting wetlands, such as duration and timing of soil saturation.
Soil texture. Soil texture was estimated at 10 and 30 cm depth, based on a ‘finger test’ of the soil’s tactile and visual properties when worked between the fingers. This is a reliable method for undertaking field-based assessments of soil particle size composition. (Foth 1984; Baize 1993). To minimize error, all samples were manipulated with sufficient water to a state of maximum plasticity, and were undertaken by the same experienced individual. Soil chemistry. Three soil samples to a depth of 0–10 cm in each plot were pooled and analysed for organic carbon, nitrogen and sulphur. Total C, N and S are analyzed by the Automated Dumas dry combustion method using a LECO CNS 2000 (Leco Corporation, Michigan, USA; Matjovic 1996). Carbon stocks for each unit were calculated assuming a uniform bulk density of 1.5 tonnes m 3 based on values reported in the Soil Classification Working Group (1991) for clay loams and clays such as those found in the study area. In order to gauge the effect of land use on carbon stocks, scenarios were developed based on calculated stocks and the historical change in the surface cover of each hydrogeomorphic unit. The latter was estimated using aerial photographs taken in 1953 and 2001 (Surveyor General, Mowbray, South Africa). The three scenarios were for 1953, 2001, and for pre-settlement (no cultivation).
Data analysis Composition Compositional variation amongst land uses and wetland units was examined with ordination techniques using the CANOCO package (Ter
334 Braak and Smilauer 1998). A preliminary DCA of all sites revealed that oxbow marshes were completely distinct from the other two vegetation units, while hill slope seepages and hygrophilous grasslands were distinct despite a degree of overlap. Eigenvalues of the first four axes ranged from 0.929 to 0.525, indicating complex compositional gradients related to strong abiotic effects (primarily degree of wetness) that would have obscured the main aim of examining the influence of land tenure and use. It was therefore deemed appropriate to undertake separate ordinations for each hydrogeomorphic unit. DCA was chosen as the most appropriate ordination technique for all three hydrogeomorphic units as gradient lengths were about 4 s.d. or greater (Ter Braak 1987). Unconstrained ordination was used to describe compositional variation and its relation to environmental variables, while constrained ordination was used to test whether included environmental variables, in particular land use, had a significant effect on compositional variation, using a Monte Carlo permutation test with 199 permutations (Ter Braak and Smilauer 1998). The significance of the regression coefficients of the detrended correspondence analysis were tested using Students t-test. The environmental variables included were; topsoil texture (the vector Tt in Figure 2), wetness (the vector Wt in Figure 2) and each land tenure/use as a dummy variable. Hygrophilous grasslands occurred on all three tenure types and were subject to a variety of
uses, including grazing on primary grassland and secondary grassland, maize cultivation and planted pastures. Pastures occurred only under private tenure and maize cultivation occurred only under communal tenure, whereas secondary and primary grassland occurred on all three types of tenure. During an initial exploratory ordination communal tenure proved to be collinear with maize cultivation and pastures with private tenure thus it was decided to do separate ordination analysis for land tenure and land use. This differentiation allowed us to explore the vegetation data more thoroughly with regards to the impact that specific land uses have on community composition and recovery. Primary grassland and private land tenure were collinear with the other land use variables and was therefore deleted. A tenure type or land use was represented in an ordination diagram by an envelope, which shows 90% of the distribution around a mean. The rehabilitated oxbow marshes were classified post analysis within CANODRAW and demarcated within the diagram with an X (see Figure 2a). Individual variables For individual variables of vegetation structure, species richness, cover of exotics (aliens), and soil chemistry, differences among combinations of land tenure and use were examined with one-way analyses of variance using the GLM procedure of SAS (SAS 1989). For comparison of means, it was deemed more appropriate to control for comparic
Figure 2. The first two axes of a Detrended Correspondence Analysis ordination for the (a) sites and (b) species of oxbow marshes; (c) sites and (d) species of hillslope seepages; and (e) sites and (f) species of hygrophilous grasslands. Key to environmental variables: Com – communal tenure; Sg – secondary grassland; Pg – primary grassland; Pr – protected (conservation) area; Pri – private tenure; Cm – cultivated maize; Cp – cultivated pasture; Tt – topsoil texture; Wt – soil wetness. Key to species (*is exotic): Oxbow marsh. Cyperus fastigiatus, Cfas; Carex acutiformis, Cacu; Utricularia vulgaris, Uvul; Lagarosiphon maio, Lmaio; Leersia hexandra, Lhex; Polygonum salicifolium, P,sali; Paspalum distichum, Pdist; Potamogeton thunbergii, Poth; Persicaria hydropiper*, Phyd; Eleocharis limosa, Elim; Schoenoplectus brachyceras, Sbra; Helichrysum auronitens, Haur; Hemarthria altissima, Halt; Eleocharis dregeana, Edre; Lobelia filiformi, Lfili; Pycreus macranthus, Pmac. Hillslope seepage. Fuirena pubescens, Fpub; Leersia hexandra, Lhex; Scleria welwitschii, Swel; Pennisetum thunbergii, Pthu; Rubus cunneifolius*, Rcunn; Carex acutiformis, Cacu; Paspalum dilatatum*, Pdila; Hyparrhenia dregeana, Hdreg; Arundinella nepelensis, Anep; Eleocharis dregeana, Edre; Helichrysum auronitens, Haur; Eragrostis plana, Epla; Helichrysum mundii, Hmun; Gunnera perpensa, Gperp; Kyllinga pauciflora, Kpauci; Eragrostis planiculmis, Eplan; Hemarthria altissima, Halt; Aristida junciformis, Ajunc; Miscanthus capensis, Micap; Pycnostachys reticulata, Pret; Conyza pinnata, Cpinn; Verbena bonariensis*, Vbon; Eragrostis curvula, Ecurv; Imperata cylindrica, Icyl; Scleria dietelenii, Sdiet; Senecio inornatus, Sinor; Bulbostylis schoenoides, Bscho; Juncus tenuis*, Jten; Scleria sp., SclSP; Setaria pallide-fusca, Sp-f. Hygrophilous flats. Eragrostis planiculmis, Eplan; Eragrostis plana, Epla; Hemarthria altissima, Halt; Eragrostis curvula, Ecurv; Hyparrhenia dregeana, Hdreg; Leersia hexandra, Lhex; Paspalum dilatatum*, Pdila; Andropogon appendiculatus, Aappen; Arundinella nepelensis, Anep; Senecio inornatus, Sinor; Gunnera perpensa, Gperp; Lolium multiflorum*, Lmul; Carex acutiformis, Cacu; Dactylis glomerata*, Dglo; Pennisetum thunbergii, Pthu; Plantago lanceolata*, Planc; Trifolium repens, Trep; Tristachya leucothrix, Tleuc; Helictotrichon turgidulum, Hturg; Themeda triandra, Ttria; Cyperus esculentus, Cesc; Senecio isatideus, Sisa; Agrostis gigantean*, Agig; Harpochloa falx, Hfal; Agrostis eriantha, Aerian; Conyza pinnata, Cpinn; Helichrysum auronitens, Haur; Scleria welwitschii, Swel; Acalypha punctata, Apunc; Bromus catharticus*, Bcart.
335
son-wise than experiment-wise error rate because sample size differed markedly among combinations (e.g., n = 6 for maize; n = 44 for seeps),
which usually obscures pair-wise differences when experiment-wise error is controlled (Sokal and Rohlf 1981). As the key focus was on which ten-
336
Figure 2. Continued.
337
Figure 2. Continued.
ure-use combinations differed, means were therefore compared using least significant differences. Data transformation was not required.
Bare ground was confounded with litter cover (r = 0.55; df = 139; p < 0.0001), vegetation cover (r = 0.46; df = 139; p < 0.0001) and
338
Figure 2. Continued.
with vegetation height (r = 0.38; df = 139; p < 0.0001), so it is not considered further. Vegetation height and litter cover were not related
(p > 0.5) while vegetation cover and litter cover were only weakly related (r = 0.17; df = 139; p < 0.04), so each is considered.
339 Results Composition Oxbow marshes Oxbow marshes occurred on all tenure types and none had been cultivated, although one had been drained and subsequently rehabilitated in the 1980’s (indicated by X in Figure 2a). Oxbow marshes have almost all remained under natural vegetation and were accessible to grazing by livestock or wildlife. Although oxbow marshes shared several species such as Carex acutiformis, Leersia hexandra and Hemarthria altissma with hygrophilous grasslands or hill slope seepages (Table 2) they did differ significantly in both composition and structure (Table 6). The DCA ordination of oxbow marshes captured most of the floristic variation on the first axis (Table 3; Figure 2a, b). A DCCA ordination Table 3) did not reveal any influence of land use (p > 0.05) and the appreciably smaller eigenvalues of the DCCA than the DCA ordination indicated that other environmental variables were responsible for the recorded variation. The rehabilitated marsh (indicated by X in Figure 2a) did not differ in composition from the others, indicating that compositional recovery of oxbow marshes following rehabilitation can occur within 15 years. The absence of an effect of land use in the DCCA illustrates that grazing regime had not influenced composition. Most oxbow marshes were dominated by Cyperus fastigiatus, which had a higher local abundance than any other species (Table 2). Floristic variation across the first axis of the DCA (Figure 2b) was from sites characterized by Carex acutiformis to those characterized by Leersia hexandra and the exotic Persicaria hydropiper, respectively associated with clay rich and sandier soils. The second axis identified an association among Pycreus macranthus, Eleocharis dregeana, Lobelia filiformis, Hemarthria altissima and Helichrysum aureonitens which are species showing a preference for seasonal wetness rather than permanent inundation (Kotze and O’Connor 2000). Submerged aquatic plants, notably Utricalaria vulgaris, were only encountered in oxbow marshes. Hill slope seepages Hill slope seepages occurred on all land tenure types but had remained uncultivated because of
their relatively steep slope and seasonally saturated soils. The first axis of a DCA ordination described a gradient from communal to private tenure (Table 3; Figure 2c), with an associated pronounced floristic gradient (Figure 2d). The DCCA ordination, whose first canonical axis and overall ordination were significant, confirmed this effect. Although tenure types differed in composition, overlap of envelopes indicated private and protected areas were similar (Figure 2c). Hill side seepages, that are apparently maintained by groundwater discharge (Brinson et al. 1995; Stein et al. 2004), supported a mixture of hydric grasses and short-growing sedges. The two most abundant species were the sedge Fuirena pubescens which seems to show a preference for hill slope areas and the grass Leersia hexandra, which was also the most ubiquitous species in terms of frequency and cover abundance across the whole study site (Table 2). The first ordination axis identified two associations of species most conspicuous on communal areas, the one including Paspalum dilatatum, Leersia hexandra and Hermarthria altissima, the other Eleocharis dregeana and Helichrysum mundii. A species grouping characteristic of private tenure was that of Aristida junciformis, Scleria welwitschii and Helichrysum aureoniten. The only other notable cluster of species in the ordination was Eliocharis limosa, leersia hexandra, Persicaria hydropiper and Paspalum distichum, with the latter two species being alien plants. Leersia hexandra commonly colonizes disturbed areas (Imeokparia 1989; Oko 1989) and the two alien species would appear to respond similarly. An effect of topsoil texture was apparent on the second axis (Table 2), with sandier soils characterized by Fuirena pubescens and heavier soils characterized by Carex acutiformis and Imperata cylindrica. Topsoil texture and soil wetness appear to poorly correlated and have a similar influence on their axis.
Hygrophilous grasslands The land tenure ordinations exposed a gradient from private tenure to communal and protected tenure (Figure 2e). There was considerable variation within private and communal hygrophilous grasslands probably because of the variation of land use within these tenure types. The envelopes for land tenure class (Figure 2f) show substantial
340 Table 2. The 30 most abundant species for each of three wetland hydrogeomorphic units in the southern Drakensberg with the exception of oxbow marshes which only had 16 species present. Mean local abundance (cover %)a
Frequency (%)
Oxbow marshes n = 23 Cyperus fastigiatus Carex acutiformis Utricularia vulgaris Lagarosiphon maior Leersia hexandra Polygonum salicifolium Paspalum distichum Potamogeton thunbergii Persicaria hydropiper* Eleocharis limosa Schoenoplectus brachyceras Helichrysum auronitens Hemarthria altissima Eleocharis dregeana Lobelia filiformis Pycreus macranthus
56.23 7.79 6.25 2.92 2.63 2.35 0.69 0.29 0.27 0.25 0.19 0.13 0.13 0.04 0.02 0.02
100 25 33.3 12.5 41.6 41.6 8.3 16.6 12.5 8.3 16.6 4.1 4.1 8.3 4.1 4.1
Hill side seeps n = 44 Fuirena pubescens Leersia hexandra Scleria welwitschii Pennisetum thunbergii Rubus cunneifolius* Carex acutiformis Paspalum dilatatum* Hyparrhenia dregeana Arundinella nepelensis Eleocharis dregeana Helichrysum auronitens Eragrostis plana Helichrysum mundii Gunnera perpensa Kyllinga pauciflora Eragrostis planiculmis Hemarthria altissima Aristida junciformis Miscanthus capensis Pycnostachys reticulata Conyza pinnata Verbena bonariensis* Eragrostis curvula Imperata cylindrical Scleria dietelenii Senecio inornatus Bulbostylis schoenoides Juncus tenuis* Scleria sp. Setaria pallide-fusca
7.54 5.10 3.96 3.78 2.83 2.70 2.66 2.59 2.57 2.47 2.09 2.00 1.71 1.53 1.50 1.38 1.38 0.99 0.92 0.91 0.87 0.56 0.52 0.50 0.46 0.46 0.44 0.44 0.44 0.43
53.3 57.8 15.6 46.7 20 17.8 33.3 37.8 26.7 31.1 28.9 28.9 15.6 6.7 13.3 37.8 24.4 15.6 13.3 11.1 24.4 31.1 13.3 8.9 11.1 24.4 8.9 8.9 8.9 6.7
Hygrophilous grassland n = 94 Eragrostis planiculmis Eragrostis plana
4.61 3.42
38.3 46.8
Hydrogeomorphic unit
Table 2. Continued. Hydrogeomorphic unit
Mean local abundance (cover %)a
Frequency (%)
Hemarthria altissima Eragrostis curvula Hyparrhenia dregeana Leersia hexandra Paspalum dilatatum* Andropogon appendiculatus Arundinella nepelensis Senecio inornatus Gunnera perpensa Lolium multiflorum* Carex acutiformis Dactylis glomerata* Pennisetum thunbergii Plantago lanceolata Trifolium repen*s Tristachya leucothrix Helictotrichon turgidulum Themeda triandra Cyperus esculentus Senecio isatideus Agrostis gigantean* Harpochloa falx Agrostis eriantha Conyza pinnata Helichrysum auronitens Scleria welwitschii Acalypha punctata Bromus catharticus*
3 2.94 2.65 2.31 1.95 1.83 1.74 1.45 1.42 1.3 1.16 1.16 1.15 1.13 1.12 1.12 1.07 1.03 0.98 0.98 0.96 0.86 0.85 0.84 0.78 0.75 0.72 0.72
36.1 34 26.6 30.8 35.1 11.7 19.2 34 5.3 8.5 7.5 6.4 11.7 28.7 10.6 10.6 45.7 11.7 16.0 14.9 7.4 18.1 4.3 26.6 19.1 4.3 18.1 6.4
All hydrogeomorphic units n = 161
Mean abundanceb
Frequency (%)
Cyperus fastigiatus Leersia hexandra Eragrostis planiculmis Carex acutiformis Eragrostis plana Fuirena pubescens Hyparrhenia dregeana Hemarthria altissima Paspalum dilatatum* Eragrostis curvula Arundinella nepelensis Pennisetum thunbergii Scleria welwitschii Gunnera perpensa Andropogon appendiculatus Helichrysum auronitens Senecio inornatus Utricularia vulgaris Rubus cunneifolius* Lolium multiflorum* Conyza pinnata Tristachya leucothrix
8.33 3.13 3.06 2.58 2.54 2.35 2.26 2.14 1.86 1.85 1.72 1.72 1.53 1.25 1.09 1.05 0.97 0.93 0.82 0.81 0.73 0.72
14.8 39.5 32.7 13.0 34.6 24.1 25.9 27.8 29.0 23.5 18.5 19.8 6.8 4.9 8.6 19.1 26.5 4.9 8.6 4.9 22.2 9.9
341 Table 2. Continued. All hydrogeomorphic units n = 161
Mean abundanceb
Frequency (%)
Eleocharis dregeana Dactylis glomerata* Helictotrichon turgidulum Plantago lanceolata* Trifolium repens* Themeda triandra Senecio isatideus Cyperus esculentus
0.70 0.67 0.67 0.67 0.65 0.64 0.59 0.57
12.3 3.7 33.3 18.5 6.2 9.3 13.0 9.3
Exotic species denoted by an asterisk. a Sampling units in which it occurred. b Across all sampling units.
overlap and their centroids are positioned closely to one another most likely due to the land uses shared by all three tenure types. Topsoil texture and wetness are closely correlated with topsoil texture have marginally more influence on the gradient. Even though the first and overall axis of the DCCA for land tenure were significant (p = 0.005) the eigenvalues of the DCCCA were appreciably lower than those of the DCA indicating that some important variables had been omitted. The first axis of the land use DCA ordination, described a disturbance gradient from pastures through recently cultivated maize, then secondary grassland (historically cultivated) to primary grassland (Figure 2g). Envelopes indicated cultivated pastures were compositionally distinct from other types, and were characterized by the exotics pasture grass species, Lolium multiflorum and Dactylus glomerata, the exotic dicotyledon Trifolium repens together with the exotic ruderal species Plantago lanceolata and Bromus catharticus (Figure 2h). Recently cultivated maize fields overlapped in composition with secondary grasslands but were distinct from primary grasslands and pastures (Figure 2g). A characteristic species association of maize fields included the graminoids Cyperus esculentus, Paspalum dilitatum, Agrostis eriantha, and Agrostis gigantean. Secondary grasslands were more variable in composition (greatest spread along the first axis) than the other types, and were characterized by the grasses Hermathria altissima, Eragrostis plana and Eragrostis planiculmis, and the dicotyledon Helichrysum aureon-
itens. These species, as well as Helictotrichon turgidulum and Hyparrhenia dregeana, also occurred in primary grasslands. These primary grasslands varied considerably in composition (spread along second axis). This is probably in response to edaphic factors with topsoil texture strongly related to the second axis (Table 3). Species conspicuous on heavier soils were Ledebouria cooperi, Andropogon appendiculatus and Scleria welwitschii. For sandier soils, a species association including Acalypha punctata, Themeda triandra, Tristachya leucothrix and Harpochloa falx was characteristic of mesic conditions, whereas Senecio inornatus, Phalaris arudinacea, Arundinella nepalensis, Pennisetum thunbergii and Carex austro-africana were conspicuous on hydric areas. For non-cultivated grassland, both hydric species (e.g., Hemarthria altissima, Eragrostic planiculmis) and ‘dryland’ species (e.g., Themeda triandra, Tristachya leucothrix) were conspicuous. Gunnera perpensa and Carex acutiformis occurred as isolated clumps only within primary grasslands (and seepage slopes) (Table 2). Hygrophilous grasslands and hill slope seepages shared many of their most abundant species (Table 2). The above interpretation of the DCA ordination was supported by the DCCA ordination, for which both the first axis and the overall ordination were significant (p < 0.05), and for which the influence of environmental variables was similar (Table 3). The similarity of the two suggests that many of the important environmental variables had been accounted for, at least for the main pattern of variation.
Soils Variation of soil carbon concentration was closely mirrored by that of nitrogen (r = 0.98; df = 151; p < 0.0001) and sulphur (r = 0.86; df = 151; p < 0.0001). All three differed markedly among land use and geomorphic units in essentially the same pattern (Table 4). Mean soil carbon varied four-fold in concentration (1.66–6.96%), with that of oxbows (mean = 6.96%) significantly higher than any other tenure or land-use types (Table 4). The second highest concentrations of soil carbon and nitrogen were found in hygrophilous grass-
342 Table 3. Summary of the ordination analyses conducted for each of the three hydrogeomorphic units. DCA
Oxbow marsh Eigenvalues Lengths of gradient Species–environmental correlations Cumulative percentage variance Of species Of species–environment relation Regression/canonical coefficients Commercial Communal Topsoil texture Hillslope seepages Eigenvalues Lengths of gradient Species–environmental correlations Cumulative percentage variance Of species Of species–environment relation Regression/canonical coefficients Topsoil texture Wetness Commercial Communal Hygophilous grasslands (tenure) Eigenvalues Lengths of gradient Species–environmental correlations Cumulative percentage variance Of species Of species–environment relation Regression/canonical coefficients Topsoil texture Wetness Protected Communal Private Hygophilous grasslands (land use) Eigenvalues Lengths of gradient Species–environmental correlations Cumulative percentage variance Of species Of species–environment relation Regression/canonical coefficients Topsoil texture Wetness Cultivated maize Cultivated pasture Secondary grassland Primary grassland
DCCA
Axis 1
Axis 2
Axis 3
Axis 4
Axis 1
Axis 2
Axis 3
Axis 4
0.515 2.746 0.557
0.148 1.469 0.21
0.024 1.024 0.348
0.017 1.175 0.344
0.182 0.895 0.61
0.02 0.388 0.435
0 0.536 0.491
0.398 2.727 0
31.9 66
41.1 66.9
42.6 0
43.7 0
11.3 73.7
12.5 87.4
12.5 0
37.2 0
0.054 0.054 0.237*
0.081 0.620 0.035
0.095 0.053 0.014
0.119 0.069 0.035
0.049 0.264 0.199
0.148 0.113 0.048
0.051 0.175 0.099
0 0 0
0.759 5.485 0.492
0.583 4.792 0.582
0.441 4.171 0.571
0.282 4.46 0.13
0.531 2.388 0.915
0.22 2.363 0.799
0.073 1.3 0.503
0.018 0.696 0.366
8.9 0
9.1 0
8.2 12.5
14.5 26.7
19.3 0
22.3 0
5.7 34.8
8.1 53.1
0.133 0.238 0.230 0.768***
0.407** 0.281 0.084 0.338*
0.031 0.248 0.471* 0.258*
0.068 0.016 0.047 0.008
0.064 0.074 0.155 0.878
0.019 0.401 0.283 0.229
0.224 0.116 0.086 0.151
0.032 0.158 0.032 0.008
0.828 6.839 0.412
0.709 5.199 0.683
0.584 4.393 0.241
0.415 4.169 0.346
0.578 3.008 0.915
0.247 2.147 0.660
0.161 2.019 0.739
0.024 1.227 0.566
6.8 0.0
6.9 0.0
5.7 8.3
10.6 30.2
14.6 0.0
17.4 0.0
4.0 37.3
5.7 53.3
0.139 0.231 0.590** 0.144 0
0.759*** 0.1864 0.216 0.346** 0
0.088 0.123 0.212 0.173 0
0.164 0.1169 0.323** 0.099 0
0.352 0.494 0.686 0.451 0
0.222 0.133 0.367 0.545 0
0.447 0.332 0.497 0.009 0
0.131 0.164 0.310 0.113 0
0.828 6.829 0.833
0.709 5.199 0.625
0.584 4.393 0.419
0.415 4.169 0.475
0.739 4.346 0.927
0.416 3.407 0.884
0.147 1.617 0.641
0.049 1.250 0.526
9.0 0.0
9.3 0.0
0.263 0.440 0.080 0.403 0.1359 0
0.133 0.362 0.050 0.009 0.742 0
5.7 22.1 0.415*** 0.327*** 0.664*** 1.026*** 0.510*** 0
10.6 33.9
14.6 0.0
0.601*** 0.238 0.110 0.188 0.265* 0
*Denotes p = 0.05; **Denotes p = 0.005; ***Denotes p = 0.001.
0.192 0.238 0.188 0.0176 0.313 0
17.4 0.0 0.002 0.133* 0.284 0.047 0.140** 0
5.1 25.8 0.420 0.093 0.586 0.760*** 0.473 0
7.9 46.2 0.387 0.372 0.073 0.206 0.376 0
343 Table 4. Mean (S.E.) concentration of soil variables according to combinations of wetland type and land tenure/use occurring in the study area. Soil variables
Oxbow marshes (n = 23)
Hygrophi lous flats – virgin grassland (n = 38)
Hygroph ilous flats – secondary grassland (n = 38)
Hygrophi lous flats – pasture (n = 12)
Hygrophi lous flats – maize (n = 6)
Hillslope seepages (n = 44)
F
P
Total carbon (%) Total sulphur (%) Total nitrogen (%)
6.96a 0.06a 0.55a
3.45b 0.03c 0.29b
2.42c 0.02d 0.22c
4.14b 0.04b 0.37b
1.66c 0.01d 0.17c
4.02b 0.03c 0.32b
13.99 29.23 19.19
0.0001 0.0001 0.0001
0.989 0.007 0.058
0.214 0.001 0.015
0.156 0.001 0.011
0.207 0.002 0.013
0.281 0.002 0.016
0.344 0.002 0.022
Means within a row sharing the same superscript do not differ significantly (a = 0.05).
Table 5. Total mass of organic carbon contained within topsoil (uppermost 10 cm of soil) expressed on a tonnes per ha basis for three different land-use scenarios. Percentage dry mass values are those reported in Table 4. Wetland/land-use type
Dry mass (%)
Oxbow marshes Hygrophilous flats – virgin grassland Hygrophilous flats – secondary grassland Hygrophilous flats – maize Hygrophilous flats – pastures Hillslope seepages Total organic carbon (tonnes) ha 1 of wetland a
7.0 3.5 2.4 1.7 4.1 4.0
Carbon (tonnes/ha)a
105.0 52.5 36.0 25.5 61.5 60.0
Proportion of area 1953
2001
No cultivation
0.07 0.10 0.08 0.63 0.10 0.02 38.9
0.07 0.08 0.73 0.05 0.05 0.02 43.39
0.07 0.91 – – – 0.02 56.33
Based on an average soil bulk density of 1.5 tonnes/m3.
lands that were in pasture and were commercially fertilized. The lowest soil C, N and S were in hygrophilous grasslands that were planted in maize (Table 4). The pattern of differences was the same for sulphur concentration except that the concentration for hygrophilous grasslands in pas-
ture was greater than that for indigenous grassland (Table 4). Carbon, N and S values for secondary grasslands were intermediate between those of maize fields and non-cultivated grassland (Table 4), suggesting a degree of recovery of carbon and soil fertility following abandonment of
Table 6. Mean (±S.E.) of variables describing vegetation structure according to combinations of wetland type and land tenure/use occurring in the study area. Vegetation variables
Oxbow marshes (n = 23)
Hygrophi lous flats – virgin grassland (n = 38)
Hygrophi lous flats – secondary grassland (n = 38)
Hygrophilous flats – pasture (n = 12)
Hygrophi lous flats – maize (n = 6)
Hillslope seepages (n = 44)
F
P
Indigenous plant species richnessa Exotic plant species richnessa Height (cm) Aerial cover (%) Litter cover (%) Bare ground (%) Indigenous plant cover (%) Alien plant cover (%)
3.3e 0.2e 143a 58b – – 76a 1d
13.4a 2.1cd 44b 65a 15bc 26 63b 7cd
8.6c 2.4c 35c 54b 12c 34 48c 8c
2.0f 6.8b 26cd 61ab 26ab 15 5d 50a
5.5d 10.0a 18d 33c 8c 63 19d 26b
10.7b 1.6d 47b 61ab 25a 20 56b 4cd
61.70 48.01 129.93 8.55 4.67 17.28 29.06 25.60
0.0001 0.0001 0 0 0.0015 0 0.0001 0.0001
0.35 ±0.12 4.5 2.9 – – ±4.0 ±0.8
0.68 ±0.27 3.3 1.5 2.0 2.3 ±3.5 1.0
0.46 ±0.33 1.9 1.8 1.9 2.3 ±3.2 2.8
0.28 ±0.51 7.4 ±4.1 4.5 2.5 ±1.9 6.0
Means within a row sharing the same superscript do not differ significantly (p = 0.05). a Expressed as average number of species measured in a 2 m radius plot.
0.85 ±1.48 6.5 5.7 3.3 7.0 ±7.7 6.0
0.57 ±0.24 2.8 2.0 3.4 2.3 ±3.3 2.3
344 cultivation. By contrast, pastures had higher concentrations of carbon, sulphur and nitrogen than primary grassland, which is attributed to less frequent tillage and greater application of fertilizer than for maize fields. The scenarios of change in carbon stocks between pre-settlement times and 1953, and between 1953 and 2001 (Table 5) indicate that no changes have occurred for oxbow marshes or hill slope seepages, ostensibly because they have not been transformed. Carbon stocks of hygrophilous grasslands have, however, changed considerably over both these periods as a result of changes in land use. Owing to impact on hygrophilous grasslands, carbon stocks of all wetlands are estimated to have declined to 69% of pre-settlement levels by 1953 (56.33 to 38.9 tonnes ha 1) owing mainly to cultivation of maize, but had recovered to 77% of pre-settlement levels (43.39 tonnes ha 1) by 2001 owing to the development of secondary grassland following abandonment of cultivation. Considering that wetlands occupy 16% of the landscape in the upper Pholela catchment (Begg 1989) and were judged to occupy a similar extent in the Ndwana area, these changes would be of consequence at a landscape level.
transformed cover types. Maize fields were particularly rich in exotic species, their density equivalent to that of indigenous species on seepage grasslands. Despite having been derived from abandoned maize fields, secondary grasslands had an equivalent richness of exotic species to primary grassland, indicating succession eliminates most exotic species, which were predominantly ruderals. Some exotic species were commonplace. Paspalum dilitatum was the most abundant exotic species, being the ninth most abundant species in both hygrophilous grassland and on hill slope seepages. Rubus cuneifolius was the fifth most abundant species on hill slope seepages, where it had a greater frequency than in any other wetland type (Table 2). It was absent from oxbow marshes, maize lands and pastures and occurred infrequently in both secondary and primary hygrophilous grassland. Plantago lanceolata was particularly abundant in cultivated pastures of the hygrophilous grasslands. Although exotic plants were conspicuously less abundant in oxbow marshes than in the other units, Perscicaria hydropiper was nevertheless the ninth most abundant species in this wetland type (Table 2).
Vegetation structure Species richness A total of 212 vascular plant species, 165 indigenous and 47 exotic, were recorded (list obtainable on request from the third author). Of the total species occurrences recorded in the 162 sampling units, 21% were of exotic species and 79% were indigenous. Richness of indigenous species varied six-fold across wetland types and land-uses (Table 6). The greatest richness of indigenous species was in primary hygrophilous grasslands and hill side seepage wetlands and the least in planted pastures. Oxbow marshes were depauperate in terms of species by comparison with even maize, with the latter supporting three-fold greater richness than pasture. Secondary grasslands had recovered to 64% of the richness of indigenous species present in primary grassland. Richness of indigenous species of primary grassland did not vary (p > 0.05) among the three types of tenure. Exotic species were present on all units but their richness varied 48-fold across units (Table 6). They were, as expected, well represented on
Units differed in vegetation structure, with greater cover of both litter and vegetation on non-transformed than transformed areas, with the exception of pastures (Table 6). Maize fields had the lowest values for all three variables, and secondary grasslands the second lowest. Hill slope seepages and primary grasslands were amongst the highest, but were matched in cover by pastures kept short by grazing. Oxbows, by nature of being permanent marsh, were an obvious anomaly. They had tall vegetation of only moderate cover because of inundation. For hill slope seepages, there was three times more bare ground on communal (26%) than on conservation (9%) areas. As expected, cover of exotic species was greatest on transformed vegetation types whereas cover of indigenous species was greatest on indigenous vegetation types (Table 6). Maize fields, however, supported an appreciable cover of indigenous species that was not much less than the cover of exotic species. Although primary grasslands supported a greater cover of indigenous species than
345 secondary grassland, both supported an equivalent cover of exotic species.
Discussion Influences on community composition A key pattern identified in this study was that land tenure had little influence on the composition of oxbow marshes, only a moderate influence on hill slope seepages, whereas land use imposed a strong pattern on hygrophilous grasslands. Thus the influence of land use appears to be constrained by the abiotic regime of a hydrogeomorphic unit, particularly by the degree of wetness and slope. Inundation precluded cultivation of oxbows, unless drained, while seasonal saturation in combination with slope precluded cultivation of hill slope seepages. By contrast, hygrophilous grasslands were particularly suitable for cultivation because of their flat topography and less prolonged periods of saturation. Accordingly, they have been subjected to the greatest range of uses. The invariant response of oxbow marshes to land tenure indicates little effect of grazing. All marshes were dominated by the tall sedge Cyperus fatigiatus, a species associated with valley-bottom flats of mid- to high order streams (Kotze and O’Connor 2000), were uniformly poor in species and of similar structure. Their uniformity is attributed to their relatively stable aquatic environment, which is ensured by their occupation of inward draining depressions on the lowest portion of a floodplain. Their water levels were maintained by a shallow water table and run-on and not by flooding of the main river channel, which has occurred too infrequently (less than one in 5 years). Flooding, soil anaerobic conditions and abundant phytomass can limit plant species richness under specific circumstances (Stromberg and Tiller 1996; Wassen et al. 2002). Widespread drainage of oxbow marshes for the purpose of cultivation has not taken place. Rehabilitation of previously breached oxbows on Coleford Nature Reserve revealed a relatively rapid (
