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Australian wattle species in the Drakensberg region of South Africa – An invasive alien or a natural resource?
AGRICULTURAL SYSTEMS Agricultural Systems 85 (2005) 216–233 www.elsevier.com/locate/agsy
Australian wattle species in the Drakensberg region of South Africa – An invasive alien or a natural resource? Andreas de Neergaard a,*, Christopher Saarnak a, Trevor Hill b, Musa Khanyile c, Alicia Martinez Berzosa a, Torben Birch-Thomsen d a
Department of Agricultural Sciences, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark b Discipline of Geography, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, Pietermaritzburg, South Africa c Geography Department, University of KwaZulu-Natal, Private Bag X 54001, 4000 Durban, South Africa d Institute of Geography, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen, Denmark
Abstract An investigation into the spread of two alien wattle species (Acacia mearnsii and Acacia dealbata) in rural parts of the Drakensberg region of South Africa and the importance of the trees to the livelihoods of the local communities was carried out. With the aid of aerial photography two plots near the village were selected. Wattle aerial cover increased from 7% and 20% in the two plots, respectively, in 1953, over 21% and 33% in 1975, to 48% and 58% in 2000. In 1995, a government-sponsored national programme, ‘‘Working for Water’’ was established, with the expressed purpose of employing people described as being from the poorest and disadvantaged rural communities to clear areas of invasive alien species from river catchments and water courses. Whilst the programme provides an income to thousands of families in rural areas, it may also be jeopardising the livelihoods of the same communities.
*
Corresponding author. Tel.: +45 3528 3499; fax. +45 3528 3468. E-mail address: [email protected] (A. de Neergaard).
0308-521X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.agsy.2005.06.009
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The wattle is an important resource for village households; virtually all households used it as their primary heat source and for building materials. Other uses included medicine extraction and 20% of the interviewed households gained income from selling firewood. From the rural communitiesÕ perception, the greatest concern regarding the alien species is its high water consumption (although this did not affect them directly), and the fact that it provides cover to thieves and criminals. Consequently, most villagers wanted either a reduction of the wattle stands (58% and 86% in the 2001 and 2003 study, respectively), or that they remained as at present (42% and 10%, respectively). Leaves from woody legumes have demonstrated large potential as green manures and fodder providers in many agroecosystems. The abundance of the wattle in the study area suggested an exploration of alternative beneficial uses in the low input farming systems practiced. The wattleÕs potential as green manure was evaluated in a decomposition trial. In all treatments, the wattle litter immobilised nitrogen from soil, in spite of a high N content of the litter. Plant available P increased in soil after 4–5 months of incubation. We argue that the current practice of externally funded wattle control is not sustainable in the long term. We discuss alternative approaches with a higher level of community participation and ownership, and conclude that the current underutilization of land may, ironically, be one of the largest constraints for a viable solution to the spread of wattle. 2005 Elsevier Ltd. All rights reserved. Keywords: Acacia; Green manure; Land management; Trees; Encroachment
1. Introduction Two invasive, alien wattle species, Acacia dealbata Link (Silver Wattle) and Acacia mearnsii De Wild. (Black Wattle) have widespread distribution in the foothills of the Drakensberg Escarpment of the Eastern Cape and KwaZulu-Natal provinces of South Africa. The trees, native to Australia, were introduced to the country in the 19th century and have been grown in plantations mainly for the production of tannins and timber. With recent shifts in the timber industry the species now have less economic importance; however, they remain a valuable rural resource as firewood and building materials for many people residing in the marginalized rural areas. The trees are fast-growing pioneer species, which, combined with their relatively high water consumption capability, has led to serious concerns regarding the dispersal and encroachment of the species into grassland and arable land. Alien invasives cover 6.5% of the Former-Homeland region known as Transkei and 8.3% of Southern KwaZulu-Natal, which approximates the national average of 8.1% (Le Maitre et al., 2000). Black wattle is among the Ôtop 10Õ invading species in South Africa, growing wild on 2,500,000 ha, whilst Silver wattle occurs on the Ôtop 25Õ list (Le Maitre et al., 2000; Nel et al., 2004). There is a dual concern regarding the potential impact of these two species: the threat to the indigenous biodiversity of an affected region and the issue of water loss. Moreover, communally managed areas used for cattle grazing are being converted to bushland by the encroaching wattle that excludes grasses and herbs. The present study area lies within a region of high
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and threatened biodiversity and experiencing increasing environmental pressure due to expansion of the tourism industry and environmental degradation due to rural communities residing in this fragile ecosystem (Cowling and Hilton-Taylor, 1994). Although the mapping of distribution and intensity of invading species in South Africa is quite detailed (e.g. Nel et al., 2004), the knowledge on the historical spread of the wattle species and its ecological implications is much less developed and in high demand (Macdonald, 2004; Richardson and van Wilgen, 2004). Understanding and connecting land use changes to shifts in plant communities is a prerequisite for developing viable management strategies. Historical aerial photographs provide an unique opportunity of quantitatively assessing vegetation cover changes over decades. The Black and Silver wattle rank first and third in water use by invading species, totalling 25% of the total amount, and are estimated to reduce mean annual runoff by 7% (Le Maitre et al., 2000). The species often form dense stands, maintain a high green leaf area throughout the year, and replace seasonally dormant grasslands, permitting continuously high rates of total evaporation (Dye and Jarmain, 2004). Reductions in the long-term mean annual runoff have been in the order of 100–300 mm yr1, but all the experimental catchments on which this finding is based were in high-rainfall zones (>1100 mm yr1). The initial national estimate of water use by invasive alien plants put the annual reduction at 3300 million m3 (Gorgens and van Wilgen, 2004). A national government-sponsored programme, ‘‘Working for Water’’, was initiated in 1995 by the Department of Water Affairs and Forestry. The primary focus is social upliftment and poverty alleviation through employment opportunities that are created as a consequence of activities adopted to control invasive species. The large-scale programme is active in all of South AfricaÕs nine provinces, most intensively in KwaZulu-Natal, Western Cape and Eastern Cape. The annual budget exceeds 400 million Rand (50 million US$), of which the majority is spent on poverty relief by employing disadvantaged workers for clearing land of invasive alien species. By the end of the 2001/02 financial year, the programme had invested R1.59 billion in clearing during its first seven years of existence, making it arguably the largest environmental programme on the African continent (van Wilgen, 2004). Presently, more than 20,000 people are employed in this programme, almost half of these in KwaZulu-Natal and Eastern Cape. For local rural communities the wattle serves as a valuable resource providing firewood, building materials and, in some instances, although not within the study region, cash income via the South African Wattle Growers Union. Although the species may encroach upon grazing land, the problems related to the spreading of the wattle such as reduced stream flow, competition with indigenous species etc., are often not a particularly pertinent issue to rural communities who seek to use this resource as part of their livelihood strategy. There is a potential conflict between the perceived interests of society (control of the wattle) and local communities (a continued resource of woody species). Furthermore, there is a conflict between creating employment based on invasive alien plant control operations and the wish to complete the same control operations as rapidly as possible.
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Woody legumes have been widely promoted as nutrient providers and animal fodder in various agroecosystems (Palm and Sanchez, 1991). Several advantages are offered by diversifying the system to include woody legumes. Nitrogen fixation may be significant, often contributing 60% to 80% of total N (reviewed by Giller, 2001). Deeper roots may extract nutrients from soil layers inaccessible to arable crops. In this particular case, nutrients in the litter are imported to the farmland from surrounding areas, hence concentrating nutrients on the arable land. Finally, leafy legumes can provide a fodder outside the main crop season, when access to biomass may be limited. The abundance of the wattle in the study area suggested an investigation of alternative uses such as green manure. The high polyphenol content suggests a low release rate of N due to complexation of N with reactive polyphenols (Palm et al., 2001). However, even a minor fertilizing effect can be valuable in the low input, nutrientdeprived maize cropping systems that characterize the study area, usually yielding below 20% of the potential production (Anonymous, 1995). This study aimed at quantifying the spread of wattle and evaluating the importance of wattle for households in the research area through the use of aerial photography, direct observations and semi-structured interviews with affected community members. A decomposition experiment was performed in order to evaluate the wattleÕs potential as green manure. Possible land management strategies with and without external support are discussed.
2. Study site and methodology 2.1. Area description The rural community of Madlangala is positioned in the foothills of the Drakensberg Escarpment near the Lesotho border in the north-eastern part of the Eastern Cape Province (3011 0 S; 2835 0 E). The village is part of the Maluti district, Eastern Cape. Before democratisation in 1994, the village was part of the Transkei homeland – a social construct of the former apartheid government of South Africa that ÔgrantedÕ independence to certain regions of South Africa that were often severely marginalized and under-resourced. Madlangala consists of three hamlets, Pepele, Makomereng and Goxe (Fig. 1), each consisting of approximately 100 households. The population are mainly Xhosa and Sotho speaking. The village is positioned at 1600 m.a.s.l., with an annual precipitation of 710 mm concentrated between October and March. Mean annual temperature is 15 C ranging from 9 C in June to 20 C in January. Frosts occur frequently during winter. There is no electricity in the village, other than from a few privately owned solar panels. Potable water is supplied from several communal taps in each hamlet. The village is placed in a typical rural setting: most residents are subsistence farmers, job opportunities in the area are very limited, and there are strong economic forces pulling the economically active away from the region and towards urban
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Fig. 1. Aerial photograph (2000) of Madlangala, consisting of the three hamlets; Goxe, Pepele and Makomereng. Squares indicate sub-scenes used for further analysis (area 1 – upper left, 650 · 1300 m, area 2 – lower right, 1000 · 1800 m).
centres. Most households have traditionally depended on remittances from migrant workers, although with mine closure and a recent trend in national retrenchments unemployment is increasing and is now approximately 30%. According to the World Bank (Republic of South Africa, 1995), 75% of the countryÕs poor live in rural areas, with the majority concentrated on the 13% of the land that constitutes the former Homelands. Most of the cultivated land is cropped with maize, occasionally bean, potatoes or pumpkin. Most households have home-gardens with vegetables, fruit trees and small plots of maize. On average, each household has a few cattle and goats, although 50% of the households do not have any livestock. Livestock graze on the slopes surrounding the villages or in the mountains towards Lesotho during the cropping season and on the fields during the dry season. 2.2. Data collection and interviews The villages were visited several times between August 2001 and August 2003. Information on livelihoods, agriculture, livestock, land use and natural resource management were collected by interviews, community meetings and biophysical analysis. Sixty semi-structured interviews with villagers specifically addressing local wattle use and control by the Working for Water programme were conducted, as well as additional follow-up interviews with key persons, such as Working for Water staff and contractors.
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Fig. 2. Vegetation change in sub-scene 1, an 85 ha area east of Pepele. The top row shows aerial photos from 1953, 1975 and 2000 (left to right). The bottom row shows the corresponding classification of the imagery into woody/non-woody vegetation. Each image depicts a 650 · 1300 m surface area.
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2.3. Classification of vegetation change Through the supervised classification of three aerial photographs spanning the past 50 years (1953, 1975 and 2000) the change in vegetation pattern and the spread of wattle in the study area have been determined. The imagery was provided in hard copy, and was scanned and later rectified to UTM coordinates with a pixel resolution
Fig. 3. Vegetation change in sub-scene 2, an 180 ha area east of Makomereng. See Fig. 2 for description of image contents. Each image depicts a 1000 · 1800 m surface area.
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of 1 m. The photographs cover the entire study area, but two sub-scenes were chosen to highlight the trend in vegetation cover change on sites that were centres of wattle invasion. Location of the sub-scenes in the study area is shown in Fig. 1. For the two sub-scenes a corresponding Boolean classification into woody/non-woody vegetation was undertaken (see bottom part of Figs. 2 and 3). The classification was made using a box class classification in the digital image processing software program Chips. 2.4. Decomposition trial Fresh leaves from Silver and Black wattle were collected in January 2003. The leaves were air dried and transported to Denmark. In an attempt to lower the polyphenol content of the leaves, a subsample was placed on a fine mesh and kept outdoors exposed to sun and rain for eight weeks from early July to late August. This sample is referred to as the ‘‘exposed’’ treatment, the original plant material as ‘‘intact’’. Plant samples were analysed for (15)N and C on a mass spectrometer (Anca, 20-20, Europa scientific). Polyphenol content was measured using the Folin– Ciocalteu method (Makkar et al., 1993), digestibility of litter was measured according to van Soest (1963). Characteristics of the plant materials are shown in Table 1. The soil used in the experiment was a loamy clay with the following characteristics: 22% clay; 21% silt; 37% fine sand; 20% course sand; 1.31% total C; 0.13% total N; pH 6.6; and water holding capacity 33% of dry weight. The plant material (0.5 g) was incubated at 15 C in moist soil (40% of water holding capacity) corresponding to 100 g dry soil. A control soil with no plant material was included in the experiment. The soil was placed in air tight storage jars including a beaker containing NaOH for passive alkali trapping of CO2. Mineralised CO2 was determined by back titration of the alkali with HCl after precipitation of carbonates with BaCl2. Soils were destructively sampled after 1, 2, 3, 4 and 5 months of incubation. Soils were extracted with 2 M KCl for 45 min and filtered. Extracts were analysed for nitrate and ammonium using flow injection analysis (Lachat 8000 series). Available phosphorous (Olsen P) was measured in 0.5 M NaHCO3 extracts as described by Olsen and Sommers (1982). Table 1 Litter characteristics of the wattle leaves used in the decomposition experiment
%N C:N ratio Polyphenol (% of dry weight) Water soluble (% of dry weight) NDFb (% of dry weight) Hemicellulose (% of dry weight) Lignocellulosea (% of dry weight)
Black wattle
Black wattle exposed
Silver wattle
Silver wattle exposed
2.63 21.2 19 25 18 7 50
2.38 23.9 14 12 15 3 70
2.53 21.5 15 31 26 7 36
2.43 21.7 7 29 17 6 48
a Separate determination of the cellulose and lignin fraction in samples with high polyphenol contents is not practical. b NDF: Neutral Detergent Fraction; soluble fraction of plant material as defined by van Soest (1963).
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2.5. Soil erosion Indications of soil erosion were performed by inserting 1 m long open gutters in the soil surface, perpendicular to the slope (8–10). After one intensive rainfall event (108 mm during 6 h), the soil in the gutters was collected, dried and weighed. The measurements were performed in triplicate on three land uses: wattle stands, grassland, and wattle stands cleared six months prior to measurement.
3. Results and discussion 3.1. Classification of vegetation change In Fig. 2 the first sub-scene is displayed in close-up – an 85 ha area east of the village of Pepele – with the aerial photographs from 1953, 1975 and 2000 in the top row. It is clear that the darker wooded areas (trees and shrub vegetation) have expanded especially when comparing the situation in 1975 and 2000. Several small patches in 1975 can be seen to have extended into a continuous, wooded area. The second sub-scene is displayed in Fig. 3. The presentation of images corresponds to Fig. 2. This area is situated east of the village of Makomereng and has traditionally been used as grazing land. The bright patch in the middle of the subscene used to house a large farm, which can be detected in the 1953 image. As for Fig. 2, there is a marked gradual expansion of the wattle growing in the area. Statistics from the result of the classification into woody/non-woody vegetation are displayed in Table 2. 3.2. Wattle as a resource For the community of Madlangala, wattle provides a valuable ÔnaturalÕ resource. Household interviews conducted in 2001 revealed that wattle was used for construction of kraals, buildings, fences and furniture (preferably silver wattle) by all households; 90% used it as their primary heat source (preferably black wattle), for the remaining households it was a secondary heat source. Nineteen percent of the households derived cash from selling wattle as firewood. In 2003, when additional households were interviewed, similar trends were evident; in addition a new finding was that 45% of these households stated that they used the bark of black wattle for Table 2 Proportion of woody and non-woody vegetation in the two sub-scenes from the Madlangala area Sub-scene 1
Sub-scene 2
1953
1975
2000
1953
1975
2000
Pasture, road, other, ratio Trees, bush (wattle), ratio
0.93 0.07
0.81 0.19
0.52 0.48
0.80 0.20
0.58 0.42
0.36 0.64
Total
1.00
1.00
1.00
1.00
1.00
1.00
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extraction of medicine against diarrhoea or toothache. Although the tree is not native to the area, it has been effectively assimilated into the rich palette of medicinal plants used in the villages, totalling more than 100 species. During the interviews in 2003 the households were asked to express any problems encountered in relation to the presence of the wattle stands. Forty-five percent mentioned high water use as the largest problem (one may question whether this is a real perception or one ÔborrowedÕ from the Working for Water programme). A more ÔrealÕ concern, as stated by 41%, was that it was a hiding place for thieves (cattle theft by people from the neighbouring Lesotho was frequently mentioned as a problem), and 52% feared walking in the forest, in particular females, who are often those entrusted with collection of firewood. Other concerns include the difficulty of controlling the wattle, encroachment of the wattle onto grazing land, and competition with indigenous species. Consequently, most villagers wanted a reduction of the wattle stands (58% in the 2001 study and 86% in 2003), or that the status quo remained (42% and 10%, respectively). Only one respondent mentioned that he wished the wattle to be removed in its entirety. In 1999 a local NGO in Matatiele performed a survey of the villagersÕ opinions on the Working for Water programme (McLeod, 1999). The greatest benefits of the Working for Water programme were identified as: (1) Job creation and eradication of poverty; (2) Reduction of crime and (3) Increasing grazing and residential land. This highlights the fact that the social benefits of the Working for Water programme are perceived as very important, both by officials and local people. The question still remains: what happens once the programme is completed in a particular region, in terms of both environmental impact and also, more importantly, in terms of the ethos of the programme, the social implications? In the study region public involvement in the Working for Water programme appeared very limited, illustrated by the fact that approximately half of the respondents were ignorant or confused about the objectives of the programme (Anonymous, 1999). At a public meeting arranged by a local NGO, strong voices recommended a greater degree of community involvement in the planning of activities and selection of employees for the programme (Anonymous, 1999). 3.3. Alternative uses An alternative use of the wattle might be as green manure. Both wattle species growing in the area are nitrogen fixing leguminous plants, and worldwide woody legumes have been introduced in various intercropping/agroforestry designs to increase nutrient inputs – mainly nitrogen – and build soil fertility. Samples of leaf material from the area have nitrogen contents between 2% and 3% of dry weight (Table 1). The 15N ratio indicates that between 60% and 80% of the plantsÕ nitrogen is derived from fixation of atmospheric nitrogen (data not shown), which is consistent with the generally low fertility of the soil in the area and consistent with data from A. dealbata plantations in Victoria, Australia (May and Attiwill, 2003). However, due to the high polyphenol content of the leaves – up to 20% of dry weight in collected samples (Table 1) – decomposition, and hence release of nutrients is
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40 30 20 Silver wattle Silver wattle exposed Black wattle Black wattle exposed
10 0 0
25
50 75 100 125 150 175 Days of incubation
40
-1
Olsen P (µg P g soil)
a
Mineral soil N (µg N g-1 soil)
C mineralization (% of added C)
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Silver wattle Silver wattle exposed Black wattle Black wattle exposed Control soil
50 40 30 20 10 0 0
1
b
2 3 4 Months of incubation
5
Silver wattle Silver wattle exposed Black wattle Black wattle exposed Control soil
30
20 0
c
60
2
3
4
5
Months of incubation
Fig. 4. (a) Accumulated carbon mineralization from plant litters decomposing in soil at 15 C. The accumulated mineralization is expressed at percent of initially added C in the plant materials. (b) Soil mineral nitrogen after incorporation of plant materials to soil. The control treatment is soil with no added plant litter. Error bars show Standard Error (n = 3). (c) Plant available P in soil (Olsen P) after incorporation of plant litters. The control treatment is soil with no added plant litter. Error bars show Standard Error (n = 3).
severely restricted. Certain villagers do add limited amounts of wattle to their composts, and mix it with cow dung and plant residues. However, the knowledge and use of composts, green manures and other low input fertilisers is limited and restricted to the more resourceful farmers. The decomposition trial with wattle leaves demonstrates a relatively fast C mineralization from the litter, releasing 27–35% of the initial C content during 5 months of decomposition. Silver wattle decomposed faster than black wattle (Fig. 4(a)). For both species, C mineralization from the litter exposed prior to incubation was slower than from the original plant material (intact treatment). Data on soil mineral nitrogen (Fig. 4(b)) show that all litter immobilised nitrogen for the duration of the incubation, in spite of high N content and correspondingly low C:N ratios. Most likely, the high reactive polyphenol content severely limited the availability of N from the litter by protein binding, resulting in N immobilisation from the soil. The exposure-treatment of the wattle was included as a low-tech means of lowering the reactive polyphenol content, and hence increasing N release. Hove et al. (2003) demonstrated that sun and oven drying reduced the protein binding
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capacity of fodder leaf extracts compared with cooler drying. For Silver wattle the exposure treatment led to a reduction of polyphenol content by 50%, and also a decreased immobilisation. Black wattle, however, immobilised more N from the exposed treatment in spite of a lower polyphenol content. However, C mineralization was slower from the exposed Black wattle, indicating that easily available C and N substrates were also lost from the plant material during drying. This is supported by the van Soest fractionation of the Black wattle litter (Table 1). The results indicate that litter quality cannot be easily improved by sun drying. A comparison of nitrogen mineralization from six hedgerow species showed that black wattle supplied nitrogen at the lowest rate (Hui et al., 2002; Kitou and Yoshida, 1994). A second experiment using black wattle as mulch confirmed its limited abilities as a nutrient provider (Hui et al., 2002; Kitou and Yoshida, 1994). Thus the optimum use of such a material is in composts or mulches, not as a fast working green manure (Palm et al., 2001). This use has been recognised as something that could be improved upon in the future through communication and possible involvement of an agriculture extension officer that should service the region. Villagers expressed a strong wish to receive advice on agricultural matters, since the local extension service had stopped servicing the village some 10 years ago. Although the wattle is not a high quality green manure, it is an abundant resource that may provide a much-needed input to help build soil fertility. Analysis of soil P status during decomposition showed no effect of the plant litters for the first months (Fig. 4(c)). After 4–5 months of incubation, extractable P in soil increased for all litter. Some farmers in the area claim that wattle amendments improve plant production, but the basis of this is not clear from this study. Although litter addition may improve soil fertility in the long term, it is unlikely that this will be attributed to wattle amendments by local farmers. N and P release patterns may be different under natural conditions and in the presence of plant roots than in our laboratory incubation. Moreover, effects other than nutritional, such as pathogen control, may play a role under field conditions. 3.4. Sustainable wattle control Although the Working for Water programme is providing much needed employment opportunities for the local community, we argue that this process is not sustainable in its present form. Once the wattle is removed, or funding runs out, involvement in the area will cease, leaving people with no alternatives for income and a depleted resource base. The high wages of the Working for Water programme (30–45 Rand per day, depending on skills) may also be a threat to other income generating activities in the area. In line with conclusions made elsewhere in Eastern Cape regarding the Working for Water programme (Ainsley, 1999) a local NGO suggested that this may have caused people to leave other community projects in the village that subsequently had collapsed (Anonymous, 1999). The large emphasis placed on the programme, as well as the local reliance on the wattle as a resource, has led the local NGO to suggest a community managed woodlot plantation, as well as building small businesses, mainly processing and selling firewood, in the wake of
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the Working for Water programme. A pilot project was carried out in 1999, aimed at processing, distributing and selling wood that had been cut down by the Working for Water employees. Although the pilot study was partly successful, it was economically unsustainable with a monthly cost of 5000 Rand that exceeded income by at least 50%. Suggestions for a managed plantation on the cleared land include both wattle and other species. Seventy-three percent of the households interviewed preferred establishment of fruit orchards on the land, whilst 59% suggested other non-specified tree plantations. No one suggested new wattle plantations, although it is the most likely species to be tested first, according to the local NGO and Working for Water representatives. Only a few respondents suggested re-establishing grazing land on the cleared areas. This corresponds well to information on the number of cattle provided by the veterinarian responsible for the dipping station showing a 40% decline from 2001 to 2003. Villagers expressed strong concerns about cattle theft, and identified this as the main reason for the declining number of cattle in the village. As great an environmental threat as that posed by the presence of the invasive wattle species is the removal of the species and the resulting surface and rill erosion on the exposed soils created by loss of vegetation cover as the wattle stands effectively exclude virtually all vegetation beneath them. The Working for Water programme is aware of this problem, and acknowledges that rehabilitation, predominantly through replanting, after removal is essential in order to prevent erosion and reintroduce desired species that can help control or elevate impact of the wattle. However, in the Madlangala area, no such replanting has commenced and the community is not aware of any such opportunities. The soils in the region are generally highly erosive due to their mineral composition, and erosion control has been a concern since the 1960s when all arable land was terraced with governmental support. Our indicative erosion measurements demonstrated clear differences in topsoil erosion between land uses. On grassland 0.74 ± 0.28 g (SD; n = 3) of soil was collected in each of the 1 m gutters; in the wattle stands 42 ± 4 g (SD; n = 3); and on the cleared land 203 ± 82 g (SD; n = 3). Although these data should not be used to quantify soil loss, they clearly indicate a much increased risk of erosion from the cleared areas. Severe soil erosion has occurred on some of the slopes from which the wattle has been removed. This leads to exposure of the infertile subsoil, restricting colonisation of indigenous species, in particular grasses that could aid in stabilising the soil. The community do recognize the severity of the soil erosion problem exacerbated by wattle removal and have suggested that future clearings be better secured against erosion (McLeod, 1999). Aside from the problems of soil erosion on the newly cleared land, the failure to re-habilitate cleared land with indigenous species, which can help control the wattle, is a concern. After clearing, the stumps are not always killed, and a high number of sprouting or coppicing stumps can be observed. Moreover, wattle produces large quantities of viable seeds that are fire-tolerant, which, combined with the almost complete lack of under-story vegetation under wattle stands, ensures that it is predominantly wattle colonising the bare ground once clearing has been completed.
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Large parts of the area depicted in sub-scene 2 (Fig. 3) have been cleared between 2001 and 2003. Today, the entire area is again covered by emerging wattle plants, up to several meters tall, as no follow-up treatments have been performed. Based on our observations, we conclude that the most likely vegetation to emerge after the removal of the wattle will be new wattle plants, if no measures are taken to re-establish other, more appropriate vegetation. Moreover, the increased soil loss caused by wattle clearance is severely jeopardising the quality and productivity of the soil, and hence the stability of the ecosystem. Biological control of wattle species has been tested since 1993, but is yet inconclusive (Zimmermann et al., 2004). The Acacia seed weevils (Melanterius maculates) from Australia show some potential, and have the advantage that they act by feeding on seeds, hence limiting germination, but not damaging trees. As for many other South African invasives, the fact that the same species often have economic importance under managed conditions, has led to strong conflicts of interest, and apprehension towards biological controlling agents. Biological control has many times demonstrated its advantages in the South African environment (van Wilgen et al., 2004), and alternatives to the weevils, such as the stump colonizing fungus Cylindrobasidium leave have also been tested. Biological controlling agents have high priority under the Working for Water research programme (van Wilgen, 2004), but it is too early to state whether these approaches will be effective (Zimmermann et al., 2004). 3.5. Community involvement and future outlook It is clear from the aerial photos, representing a period of more than 47 years that the invasive wattle species are spreading dramatically in parts of the Madlangala area. The current wattle control programme has yet to prove that it will be able to keep pace with the invasion of the wattle species, and biological control has not yet been introduced to the area. Unfortunately, based on our aerial photographs we cannot determine the trend during the past 5 years; however, on-site observations indicate that the Working for Water initiative is having little success in reducing wattle cover. Even if clearing the wattle stands were successful and re-invasion prevented, the vegetation cover will still be in an unstable state, unless fundamental changes in land management are initiated. The ultimate goal of the Working for Water programme is to make itself superfluous, by achieving its objectives. However, if management of the land is not handed over to the local inhabitants, the regulation of alien species will forever be dependent on external funding at a considerable environmental, economic and social cost. In the Madlangala area, the pressure on land is diminishing as cattle numbers have decreased during the past 10 years, partly due to cattle theft and diseases, and much of the agricultural land is left uncultivated as economic and social pressures force the youth to leave the region and seek ÔbetterÕ opportunities elsewhere. According to the villagers, lack of support from the Department of Agriculture as well as declining remittances from migrant workers have caused a de-intensification of farming in the area. Consequently, wattle is allowed to spread unchecked, as there is no pressure to procure land for agricultural use. Ironically, a greater pressure on
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the resource base (land) appears to be what is needed in order to place more pressure on the successful control of wattle. A paradox here is the notion of HardinÕs ‘‘tragedy of the commons’’ (Hardin, 1968) in reverse and the Boserupian idea of ‘‘more people better land practice’’ (Tiffen et al., 1994): at this study site we advocate that greater pressure for arable and/or grazing land could result in greater concern, care and, ultimately, control of the invasive species. Establishment of managed plantations of wattle or other woody species may satisfy the villagerÕs needs for firewood and building material, and also provide a much needed means of occupation and production in the rural area once the Working for Water programme ceases. However, it is yet to be seen if there are human and economic resources to establish such plantations and whether these initiatives will be economically viable. On the surface there appears to be an interest from both villagers and NGOs, although the basis for this interest is unclear. However, this initiative only solves part of the problem, which relates to securing the villagers supply of wood, but will not remove the threat of reinvasion by the same or new alien species. Of course it is only a threat if perceived as such and there is a fascinating story to be told regarding what transpires when an environmental scientistÕs problem is a rural communityÕs livelihood.
4. Conclusions Our investigation clearly demonstrates a significant increase in wattle cover over the past 50 years, leading to changes in landscape, environment and availability of natural resources. The wattle has many beneficial uses for the local communities, but the potential as green manure appears limited due to the high content of polyphenols, limiting N release from the litter. Should wattle control succeed, testing and introduction of less invasive woody legumes suited to the climate, and providing both firewood and higher-quality litter, must be considered in order to ensure supply of these resources. In order to solve the problem of the spread of the wattle, economically and ecologically sustainable land management or land stewardship founded on ownership, involvement and management by local communities must be developed. This may appear a highly unrealistic suggestion on the basis of the current resource base in the area, but it may not be any more unrealistic than the current approach, and should serve as an overall objective towards which ongoing initiatives should work. It needs to be recognised that the Working with Water programme is a pioneering environmental conservation initiative in that its implementation successfully combines ecological concerns and social development benefits. The programme has significantly contributed to poverty alleviation and social upliftment while controlling invasive alien plants, although the success of the latter in communally owned rural areas may be questioned. The development of a viable Ôexit strategyÕ (that is, interventions to maximize the chances of workers either securing gainful employment after leaving the programme or successfully setting up their own businesses) is the focus of on-going research and is recognised as a point of future consideration.
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Preliminary findings in 2002 revealed that most workers and contractors leaving the programme were not fully equipped to secure work elsewhere, although this situation also reflects the general economic depression in rural areas (Magadlela and Mdzeke, 2004). The involvement of the Working for Water programme in the Madlangala area has provided much needed job opportunities to the villagers, but it must be recognised that the project is highly unsustainable in the sense that it is entirely dependent on external funding, and that it does not offer any solutions to the long term management of invasive alien species, but merely attempts to remove the stands. Moreover, the job opportunities provided by the programme are mainly short term gains, creating dependence and no long-term opportunities. Furthermore, incentives for the villagers to clear the land are not obvious. Aside from the threat of villains hiding in the plantations, the wattle stands do not constitute a big problem for the community, due to the abundance of land and the fact that any increase in water runoff will have no direct or indirect benefit to the community. It is ironic that in a country such as South Africa, where land rights are one of the hottest political and practical issues, it is the apparent abundance and underutilisation of the land as a resource that appears to be one of the largest constraints in developing a viable way of controlling invasive species. In our research area, in the former homeland of Transkei, lack of inputs and human resources results in agricultural activities being perceived as un-rewarding and little attention is paid to this sector, a pattern referred to as de-agrarianization (Bryceson, 2002). We believe that until the productivity of this land, either as grazing land, arable land or natural habitat, is restored, control of invasive species will be dependent upon a continued effort and resource flow from external organisations and institutions.
Acknowledgements The data from the field site were collected during visits by staff and MasterÕs students from University of KwaZulu-Natal, University of Swaziland, University of Botswana, University of Copenhagen, Roskilde University and the Royal Veterinary and Agricultural University, and are jointly owned by these institutions. The field trips were performed as part of the SLUSE consortia activities, and funded by Danida. We thank and pay tribute to the kindness and hospitality that we were afforded by the Madlangala community.
References Ainsley, A., 1999. When communities is not enough: Managing common property natural resources in rural South Africa. Development Southern Africa 16, 375–401. Anonymous, 1995. The Bioresource Units of KwaZulu-Natal. KwaZulu-Natal Department of Agriculture. Cedara Report No. N/A/95/32. Anonymous, 1999. Understanding the impact of the Working for Water programme on rural communities. EDA Trust Report. EDA Trust & Care SA, Matatiele.
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Bryceson, D.F., 2002. The scramble in Africa: Reorienting rural livelihoods. World Development 30, 725– 739. Cowling, R., Hilton-Taylor, C., 1994. Patterns of plant diversity and endemism in southern Africa: an overview. In: Huntley, B. (Ed.), Botanical Diversity in Southern Africa. National Botanical Institute, Pretoria, pp. 19–50. Dye, P., Jarmain, C., 2004. Water use by black wattle (Acacia mearnsii): implications for the link between removal of invading trees and catchment streamflow response. South African Journal of Science 100, 40–44. Giller, K.E., 2001. Nitrogen Fixation in Tropical Farming Systems. CAB International, Wallingford. Gorgens, A.H.M., van Wilgen, B.W., 2004. Invasive alien plants and water resources in South Africa: current understanding, predictive ability and research challenges. South African Journal of Science 100, 27–33. Hardin, G., 1968. The tragedy of the commons. Science 162, 1243–1248. Hove, L., Ndlovu, L.R., Sibanda, S., 2003. The effects of drying temperature on chemical composition and nutritive value of some tropical fodder shrubs. Agroforestry Systems 59, 231–241. Hui, S., Ya, T., Qiguo, Z., 2002. Nitrogen mineralisation of prunings of six N2-fixing hedgerow species in a dry valley of the Jinsha river. Pedosphere 12, 21–25. Kitou, M., Yoshida, S., 1994. Mulching effect of plant residues on soybean growth and soil chemical properties. Soil Science and Plant Nutrition 40, 211–220. Le Maitre, D., Versfeld, D., Chapman, R.A., 2000. The impact of invading alien plants on surface water resources in South Africa: A preliminary assessment. Water S. A. 26, 397–408. Macdonald, I.A.W., 2004. Recent research on alien plant invasions and their management in South Africa: a review of the inaugural symposium of the Working for Water programme. South African Journal of Science 100, 21–26. Magadlela, D., Mdzeke, N., 2004. Social benefits in the Working for Water programme as a public works initiative. South African Journal of Science 100, 94–96. Makkar, H.P.S., Bluemmel, M., Borowy, N.K., Becker, K., 1993. Gravimetric determination of tannins and their correlations with chemical and protein precipitation methods. Journal of Science of Food and Agriculture 61, 161–165. May, B.M., Attiwill, P.M., 2003. Nitrogen-fixation by Acacia dealbata and changes in soil properties 5 years after mechanical disturbance or slash-burning following timber harvest. Forest Ecology and Management 181, 339–355. McLeod, N., 1999. Community based wattle management: Harmonising WfW with communal realities. In: Community Based Land Management Seminar, September 1999, Hammanskrall, South Africa, pp. 1–9. Nel, J.A., Richardson, D.M., Rouget, M., Mgidi, T.N., Mdzeke, N., Le Maitre, D., van Wilgen, B.W., Schonegevel, L., Henderson, L., Neser, S., 2004. A proposed classification of invasive plant species in South Africa: towards prioritizing species and areas for management action. South African Journal of Science 100, 53–64. Olsen, S.R., Sommers, L.E., 1982. Phosphorus. In: Page, A.L. (Ed.), Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties. American Society of Agronomy and Soil Science Society of America, Madison, WI, pp. 403–430. Palm, C.A., Gachenco, C.N., Delve, R.J., Cadisch, G., Giller, K.E., 2001. Organic inputs for soil fertility management in tropical agroecosystems: application of an organic resource database. Agriculture Ecosystems and Environment 83, 27–42. Palm, C.A., Sanchez, P.A., 1991. Nitrogen release from the leaves of some tropical legumes as affected by their lignin and polyphenolic contents. Soil Biology and Biochemistry 23, 83–88. Republic of South Africa, 1995. Rural development strategy of the Government of National Unity. Government Gazette No. 16679, 3 November 1995, Pretoria. Richardson, D.M., van Wilgen, B.W., 2004. Invasive alien plants in South Africa: how well do we understand the ecological impacts. South African Journal of Science 100, 45–52. Tiffen, M., Mortimore, M., Gichuki, F., 1994. More People, Less Erosion. Environmental Recovery in Kenya. ACTS Press, Nairobi.
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van Soest, P.J., 1963. Use of detergents in the analysis of fibrous feeds. Journal of Association of Agricultural Chemistry 46, 825–835. van Wilgen, B.W., 2004. Scientific challenges in the field of invasive alien plant management. South African Journal of Science 100, 19–20. van Wilgen, B.W., de Wit, M.P., Anderson, H.J., Le Maitre, D., Kotze, I.M., Ndala, S., Brown, B., Rapholo, M.B., 2004. Costs and benefits of biological control of invasive alien plants: case studies from South Africa. South African Journal of Science 100, 113–122. Zimmermann, H.G., Moran, V.C., Hoffmann, J.H., 2004. Biological control in the management of invasive alien plants in South Africa, and the role of the Working for Water programme. South African Journal of Science 100, 34–40.
Australian wattle species in the Drakensberg region of South Africa – An invasive alien or a natural resource? Andreas de Neergaard a,*, Christopher Saarnak a, Trevor Hill b, Musa Khanyile c, Alicia Martinez Berzosa a, Torben Birch-Thomsen d a
Department of Agricultural Sciences, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark b Discipline of Geography, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, Pietermaritzburg, South Africa c Geography Department, University of KwaZulu-Natal, Private Bag X 54001, 4000 Durban, South Africa d Institute of Geography, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen, Denmark
Abstract An investigation into the spread of two alien wattle species (Acacia mearnsii and Acacia dealbata) in rural parts of the Drakensberg region of South Africa and the importance of the trees to the livelihoods of the local communities was carried out. With the aid of aerial photography two plots near the village were selected. Wattle aerial cover increased from 7% and 20% in the two plots, respectively, in 1953, over 21% and 33% in 1975, to 48% and 58% in 2000. In 1995, a government-sponsored national programme, ‘‘Working for Water’’ was established, with the expressed purpose of employing people described as being from the poorest and disadvantaged rural communities to clear areas of invasive alien species from river catchments and water courses. Whilst the programme provides an income to thousands of families in rural areas, it may also be jeopardising the livelihoods of the same communities.
*
Corresponding author. Tel.: +45 3528 3499; fax. +45 3528 3468. E-mail address: [email protected] (A. de Neergaard).
0308-521X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.agsy.2005.06.009
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The wattle is an important resource for village households; virtually all households used it as their primary heat source and for building materials. Other uses included medicine extraction and 20% of the interviewed households gained income from selling firewood. From the rural communitiesÕ perception, the greatest concern regarding the alien species is its high water consumption (although this did not affect them directly), and the fact that it provides cover to thieves and criminals. Consequently, most villagers wanted either a reduction of the wattle stands (58% and 86% in the 2001 and 2003 study, respectively), or that they remained as at present (42% and 10%, respectively). Leaves from woody legumes have demonstrated large potential as green manures and fodder providers in many agroecosystems. The abundance of the wattle in the study area suggested an exploration of alternative beneficial uses in the low input farming systems practiced. The wattleÕs potential as green manure was evaluated in a decomposition trial. In all treatments, the wattle litter immobilised nitrogen from soil, in spite of a high N content of the litter. Plant available P increased in soil after 4–5 months of incubation. We argue that the current practice of externally funded wattle control is not sustainable in the long term. We discuss alternative approaches with a higher level of community participation and ownership, and conclude that the current underutilization of land may, ironically, be one of the largest constraints for a viable solution to the spread of wattle. 2005 Elsevier Ltd. All rights reserved. Keywords: Acacia; Green manure; Land management; Trees; Encroachment
1. Introduction Two invasive, alien wattle species, Acacia dealbata Link (Silver Wattle) and Acacia mearnsii De Wild. (Black Wattle) have widespread distribution in the foothills of the Drakensberg Escarpment of the Eastern Cape and KwaZulu-Natal provinces of South Africa. The trees, native to Australia, were introduced to the country in the 19th century and have been grown in plantations mainly for the production of tannins and timber. With recent shifts in the timber industry the species now have less economic importance; however, they remain a valuable rural resource as firewood and building materials for many people residing in the marginalized rural areas. The trees are fast-growing pioneer species, which, combined with their relatively high water consumption capability, has led to serious concerns regarding the dispersal and encroachment of the species into grassland and arable land. Alien invasives cover 6.5% of the Former-Homeland region known as Transkei and 8.3% of Southern KwaZulu-Natal, which approximates the national average of 8.1% (Le Maitre et al., 2000). Black wattle is among the Ôtop 10Õ invading species in South Africa, growing wild on 2,500,000 ha, whilst Silver wattle occurs on the Ôtop 25Õ list (Le Maitre et al., 2000; Nel et al., 2004). There is a dual concern regarding the potential impact of these two species: the threat to the indigenous biodiversity of an affected region and the issue of water loss. Moreover, communally managed areas used for cattle grazing are being converted to bushland by the encroaching wattle that excludes grasses and herbs. The present study area lies within a region of high
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and threatened biodiversity and experiencing increasing environmental pressure due to expansion of the tourism industry and environmental degradation due to rural communities residing in this fragile ecosystem (Cowling and Hilton-Taylor, 1994). Although the mapping of distribution and intensity of invading species in South Africa is quite detailed (e.g. Nel et al., 2004), the knowledge on the historical spread of the wattle species and its ecological implications is much less developed and in high demand (Macdonald, 2004; Richardson and van Wilgen, 2004). Understanding and connecting land use changes to shifts in plant communities is a prerequisite for developing viable management strategies. Historical aerial photographs provide an unique opportunity of quantitatively assessing vegetation cover changes over decades. The Black and Silver wattle rank first and third in water use by invading species, totalling 25% of the total amount, and are estimated to reduce mean annual runoff by 7% (Le Maitre et al., 2000). The species often form dense stands, maintain a high green leaf area throughout the year, and replace seasonally dormant grasslands, permitting continuously high rates of total evaporation (Dye and Jarmain, 2004). Reductions in the long-term mean annual runoff have been in the order of 100–300 mm yr1, but all the experimental catchments on which this finding is based were in high-rainfall zones (>1100 mm yr1). The initial national estimate of water use by invasive alien plants put the annual reduction at 3300 million m3 (Gorgens and van Wilgen, 2004). A national government-sponsored programme, ‘‘Working for Water’’, was initiated in 1995 by the Department of Water Affairs and Forestry. The primary focus is social upliftment and poverty alleviation through employment opportunities that are created as a consequence of activities adopted to control invasive species. The large-scale programme is active in all of South AfricaÕs nine provinces, most intensively in KwaZulu-Natal, Western Cape and Eastern Cape. The annual budget exceeds 400 million Rand (50 million US$), of which the majority is spent on poverty relief by employing disadvantaged workers for clearing land of invasive alien species. By the end of the 2001/02 financial year, the programme had invested R1.59 billion in clearing during its first seven years of existence, making it arguably the largest environmental programme on the African continent (van Wilgen, 2004). Presently, more than 20,000 people are employed in this programme, almost half of these in KwaZulu-Natal and Eastern Cape. For local rural communities the wattle serves as a valuable resource providing firewood, building materials and, in some instances, although not within the study region, cash income via the South African Wattle Growers Union. Although the species may encroach upon grazing land, the problems related to the spreading of the wattle such as reduced stream flow, competition with indigenous species etc., are often not a particularly pertinent issue to rural communities who seek to use this resource as part of their livelihood strategy. There is a potential conflict between the perceived interests of society (control of the wattle) and local communities (a continued resource of woody species). Furthermore, there is a conflict between creating employment based on invasive alien plant control operations and the wish to complete the same control operations as rapidly as possible.
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Woody legumes have been widely promoted as nutrient providers and animal fodder in various agroecosystems (Palm and Sanchez, 1991). Several advantages are offered by diversifying the system to include woody legumes. Nitrogen fixation may be significant, often contributing 60% to 80% of total N (reviewed by Giller, 2001). Deeper roots may extract nutrients from soil layers inaccessible to arable crops. In this particular case, nutrients in the litter are imported to the farmland from surrounding areas, hence concentrating nutrients on the arable land. Finally, leafy legumes can provide a fodder outside the main crop season, when access to biomass may be limited. The abundance of the wattle in the study area suggested an investigation of alternative uses such as green manure. The high polyphenol content suggests a low release rate of N due to complexation of N with reactive polyphenols (Palm et al., 2001). However, even a minor fertilizing effect can be valuable in the low input, nutrientdeprived maize cropping systems that characterize the study area, usually yielding below 20% of the potential production (Anonymous, 1995). This study aimed at quantifying the spread of wattle and evaluating the importance of wattle for households in the research area through the use of aerial photography, direct observations and semi-structured interviews with affected community members. A decomposition experiment was performed in order to evaluate the wattleÕs potential as green manure. Possible land management strategies with and without external support are discussed.
2. Study site and methodology 2.1. Area description The rural community of Madlangala is positioned in the foothills of the Drakensberg Escarpment near the Lesotho border in the north-eastern part of the Eastern Cape Province (3011 0 S; 2835 0 E). The village is part of the Maluti district, Eastern Cape. Before democratisation in 1994, the village was part of the Transkei homeland – a social construct of the former apartheid government of South Africa that ÔgrantedÕ independence to certain regions of South Africa that were often severely marginalized and under-resourced. Madlangala consists of three hamlets, Pepele, Makomereng and Goxe (Fig. 1), each consisting of approximately 100 households. The population are mainly Xhosa and Sotho speaking. The village is positioned at 1600 m.a.s.l., with an annual precipitation of 710 mm concentrated between October and March. Mean annual temperature is 15 C ranging from 9 C in June to 20 C in January. Frosts occur frequently during winter. There is no electricity in the village, other than from a few privately owned solar panels. Potable water is supplied from several communal taps in each hamlet. The village is placed in a typical rural setting: most residents are subsistence farmers, job opportunities in the area are very limited, and there are strong economic forces pulling the economically active away from the region and towards urban
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Fig. 1. Aerial photograph (2000) of Madlangala, consisting of the three hamlets; Goxe, Pepele and Makomereng. Squares indicate sub-scenes used for further analysis (area 1 – upper left, 650 · 1300 m, area 2 – lower right, 1000 · 1800 m).
centres. Most households have traditionally depended on remittances from migrant workers, although with mine closure and a recent trend in national retrenchments unemployment is increasing and is now approximately 30%. According to the World Bank (Republic of South Africa, 1995), 75% of the countryÕs poor live in rural areas, with the majority concentrated on the 13% of the land that constitutes the former Homelands. Most of the cultivated land is cropped with maize, occasionally bean, potatoes or pumpkin. Most households have home-gardens with vegetables, fruit trees and small plots of maize. On average, each household has a few cattle and goats, although 50% of the households do not have any livestock. Livestock graze on the slopes surrounding the villages or in the mountains towards Lesotho during the cropping season and on the fields during the dry season. 2.2. Data collection and interviews The villages were visited several times between August 2001 and August 2003. Information on livelihoods, agriculture, livestock, land use and natural resource management were collected by interviews, community meetings and biophysical analysis. Sixty semi-structured interviews with villagers specifically addressing local wattle use and control by the Working for Water programme were conducted, as well as additional follow-up interviews with key persons, such as Working for Water staff and contractors.
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Fig. 2. Vegetation change in sub-scene 1, an 85 ha area east of Pepele. The top row shows aerial photos from 1953, 1975 and 2000 (left to right). The bottom row shows the corresponding classification of the imagery into woody/non-woody vegetation. Each image depicts a 650 · 1300 m surface area.
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2.3. Classification of vegetation change Through the supervised classification of three aerial photographs spanning the past 50 years (1953, 1975 and 2000) the change in vegetation pattern and the spread of wattle in the study area have been determined. The imagery was provided in hard copy, and was scanned and later rectified to UTM coordinates with a pixel resolution
Fig. 3. Vegetation change in sub-scene 2, an 180 ha area east of Makomereng. See Fig. 2 for description of image contents. Each image depicts a 1000 · 1800 m surface area.
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of 1 m. The photographs cover the entire study area, but two sub-scenes were chosen to highlight the trend in vegetation cover change on sites that were centres of wattle invasion. Location of the sub-scenes in the study area is shown in Fig. 1. For the two sub-scenes a corresponding Boolean classification into woody/non-woody vegetation was undertaken (see bottom part of Figs. 2 and 3). The classification was made using a box class classification in the digital image processing software program Chips. 2.4. Decomposition trial Fresh leaves from Silver and Black wattle were collected in January 2003. The leaves were air dried and transported to Denmark. In an attempt to lower the polyphenol content of the leaves, a subsample was placed on a fine mesh and kept outdoors exposed to sun and rain for eight weeks from early July to late August. This sample is referred to as the ‘‘exposed’’ treatment, the original plant material as ‘‘intact’’. Plant samples were analysed for (15)N and C on a mass spectrometer (Anca, 20-20, Europa scientific). Polyphenol content was measured using the Folin– Ciocalteu method (Makkar et al., 1993), digestibility of litter was measured according to van Soest (1963). Characteristics of the plant materials are shown in Table 1. The soil used in the experiment was a loamy clay with the following characteristics: 22% clay; 21% silt; 37% fine sand; 20% course sand; 1.31% total C; 0.13% total N; pH 6.6; and water holding capacity 33% of dry weight. The plant material (0.5 g) was incubated at 15 C in moist soil (40% of water holding capacity) corresponding to 100 g dry soil. A control soil with no plant material was included in the experiment. The soil was placed in air tight storage jars including a beaker containing NaOH for passive alkali trapping of CO2. Mineralised CO2 was determined by back titration of the alkali with HCl after precipitation of carbonates with BaCl2. Soils were destructively sampled after 1, 2, 3, 4 and 5 months of incubation. Soils were extracted with 2 M KCl for 45 min and filtered. Extracts were analysed for nitrate and ammonium using flow injection analysis (Lachat 8000 series). Available phosphorous (Olsen P) was measured in 0.5 M NaHCO3 extracts as described by Olsen and Sommers (1982). Table 1 Litter characteristics of the wattle leaves used in the decomposition experiment
%N C:N ratio Polyphenol (% of dry weight) Water soluble (% of dry weight) NDFb (% of dry weight) Hemicellulose (% of dry weight) Lignocellulosea (% of dry weight)
Black wattle
Black wattle exposed
Silver wattle
Silver wattle exposed
2.63 21.2 19 25 18 7 50
2.38 23.9 14 12 15 3 70
2.53 21.5 15 31 26 7 36
2.43 21.7 7 29 17 6 48
a Separate determination of the cellulose and lignin fraction in samples with high polyphenol contents is not practical. b NDF: Neutral Detergent Fraction; soluble fraction of plant material as defined by van Soest (1963).
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2.5. Soil erosion Indications of soil erosion were performed by inserting 1 m long open gutters in the soil surface, perpendicular to the slope (8–10). After one intensive rainfall event (108 mm during 6 h), the soil in the gutters was collected, dried and weighed. The measurements were performed in triplicate on three land uses: wattle stands, grassland, and wattle stands cleared six months prior to measurement.
3. Results and discussion 3.1. Classification of vegetation change In Fig. 2 the first sub-scene is displayed in close-up – an 85 ha area east of the village of Pepele – with the aerial photographs from 1953, 1975 and 2000 in the top row. It is clear that the darker wooded areas (trees and shrub vegetation) have expanded especially when comparing the situation in 1975 and 2000. Several small patches in 1975 can be seen to have extended into a continuous, wooded area. The second sub-scene is displayed in Fig. 3. The presentation of images corresponds to Fig. 2. This area is situated east of the village of Makomereng and has traditionally been used as grazing land. The bright patch in the middle of the subscene used to house a large farm, which can be detected in the 1953 image. As for Fig. 2, there is a marked gradual expansion of the wattle growing in the area. Statistics from the result of the classification into woody/non-woody vegetation are displayed in Table 2. 3.2. Wattle as a resource For the community of Madlangala, wattle provides a valuable ÔnaturalÕ resource. Household interviews conducted in 2001 revealed that wattle was used for construction of kraals, buildings, fences and furniture (preferably silver wattle) by all households; 90% used it as their primary heat source (preferably black wattle), for the remaining households it was a secondary heat source. Nineteen percent of the households derived cash from selling wattle as firewood. In 2003, when additional households were interviewed, similar trends were evident; in addition a new finding was that 45% of these households stated that they used the bark of black wattle for Table 2 Proportion of woody and non-woody vegetation in the two sub-scenes from the Madlangala area Sub-scene 1
Sub-scene 2
1953
1975
2000
1953
1975
2000
Pasture, road, other, ratio Trees, bush (wattle), ratio
0.93 0.07
0.81 0.19
0.52 0.48
0.80 0.20
0.58 0.42
0.36 0.64
Total
1.00
1.00
1.00
1.00
1.00
1.00
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extraction of medicine against diarrhoea or toothache. Although the tree is not native to the area, it has been effectively assimilated into the rich palette of medicinal plants used in the villages, totalling more than 100 species. During the interviews in 2003 the households were asked to express any problems encountered in relation to the presence of the wattle stands. Forty-five percent mentioned high water use as the largest problem (one may question whether this is a real perception or one ÔborrowedÕ from the Working for Water programme). A more ÔrealÕ concern, as stated by 41%, was that it was a hiding place for thieves (cattle theft by people from the neighbouring Lesotho was frequently mentioned as a problem), and 52% feared walking in the forest, in particular females, who are often those entrusted with collection of firewood. Other concerns include the difficulty of controlling the wattle, encroachment of the wattle onto grazing land, and competition with indigenous species. Consequently, most villagers wanted a reduction of the wattle stands (58% in the 2001 study and 86% in 2003), or that the status quo remained (42% and 10%, respectively). Only one respondent mentioned that he wished the wattle to be removed in its entirety. In 1999 a local NGO in Matatiele performed a survey of the villagersÕ opinions on the Working for Water programme (McLeod, 1999). The greatest benefits of the Working for Water programme were identified as: (1) Job creation and eradication of poverty; (2) Reduction of crime and (3) Increasing grazing and residential land. This highlights the fact that the social benefits of the Working for Water programme are perceived as very important, both by officials and local people. The question still remains: what happens once the programme is completed in a particular region, in terms of both environmental impact and also, more importantly, in terms of the ethos of the programme, the social implications? In the study region public involvement in the Working for Water programme appeared very limited, illustrated by the fact that approximately half of the respondents were ignorant or confused about the objectives of the programme (Anonymous, 1999). At a public meeting arranged by a local NGO, strong voices recommended a greater degree of community involvement in the planning of activities and selection of employees for the programme (Anonymous, 1999). 3.3. Alternative uses An alternative use of the wattle might be as green manure. Both wattle species growing in the area are nitrogen fixing leguminous plants, and worldwide woody legumes have been introduced in various intercropping/agroforestry designs to increase nutrient inputs – mainly nitrogen – and build soil fertility. Samples of leaf material from the area have nitrogen contents between 2% and 3% of dry weight (Table 1). The 15N ratio indicates that between 60% and 80% of the plantsÕ nitrogen is derived from fixation of atmospheric nitrogen (data not shown), which is consistent with the generally low fertility of the soil in the area and consistent with data from A. dealbata plantations in Victoria, Australia (May and Attiwill, 2003). However, due to the high polyphenol content of the leaves – up to 20% of dry weight in collected samples (Table 1) – decomposition, and hence release of nutrients is
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40 30 20 Silver wattle Silver wattle exposed Black wattle Black wattle exposed
10 0 0
25
50 75 100 125 150 175 Days of incubation
40
-1
Olsen P (µg P g soil)
a
Mineral soil N (µg N g-1 soil)
C mineralization (% of added C)
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Silver wattle Silver wattle exposed Black wattle Black wattle exposed Control soil
50 40 30 20 10 0 0
1
b
2 3 4 Months of incubation
5
Silver wattle Silver wattle exposed Black wattle Black wattle exposed Control soil
30
20 0
c
60
2
3
4
5
Months of incubation
Fig. 4. (a) Accumulated carbon mineralization from plant litters decomposing in soil at 15 C. The accumulated mineralization is expressed at percent of initially added C in the plant materials. (b) Soil mineral nitrogen after incorporation of plant materials to soil. The control treatment is soil with no added plant litter. Error bars show Standard Error (n = 3). (c) Plant available P in soil (Olsen P) after incorporation of plant litters. The control treatment is soil with no added plant litter. Error bars show Standard Error (n = 3).
severely restricted. Certain villagers do add limited amounts of wattle to their composts, and mix it with cow dung and plant residues. However, the knowledge and use of composts, green manures and other low input fertilisers is limited and restricted to the more resourceful farmers. The decomposition trial with wattle leaves demonstrates a relatively fast C mineralization from the litter, releasing 27–35% of the initial C content during 5 months of decomposition. Silver wattle decomposed faster than black wattle (Fig. 4(a)). For both species, C mineralization from the litter exposed prior to incubation was slower than from the original plant material (intact treatment). Data on soil mineral nitrogen (Fig. 4(b)) show that all litter immobilised nitrogen for the duration of the incubation, in spite of high N content and correspondingly low C:N ratios. Most likely, the high reactive polyphenol content severely limited the availability of N from the litter by protein binding, resulting in N immobilisation from the soil. The exposure-treatment of the wattle was included as a low-tech means of lowering the reactive polyphenol content, and hence increasing N release. Hove et al. (2003) demonstrated that sun and oven drying reduced the protein binding
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capacity of fodder leaf extracts compared with cooler drying. For Silver wattle the exposure treatment led to a reduction of polyphenol content by 50%, and also a decreased immobilisation. Black wattle, however, immobilised more N from the exposed treatment in spite of a lower polyphenol content. However, C mineralization was slower from the exposed Black wattle, indicating that easily available C and N substrates were also lost from the plant material during drying. This is supported by the van Soest fractionation of the Black wattle litter (Table 1). The results indicate that litter quality cannot be easily improved by sun drying. A comparison of nitrogen mineralization from six hedgerow species showed that black wattle supplied nitrogen at the lowest rate (Hui et al., 2002; Kitou and Yoshida, 1994). A second experiment using black wattle as mulch confirmed its limited abilities as a nutrient provider (Hui et al., 2002; Kitou and Yoshida, 1994). Thus the optimum use of such a material is in composts or mulches, not as a fast working green manure (Palm et al., 2001). This use has been recognised as something that could be improved upon in the future through communication and possible involvement of an agriculture extension officer that should service the region. Villagers expressed a strong wish to receive advice on agricultural matters, since the local extension service had stopped servicing the village some 10 years ago. Although the wattle is not a high quality green manure, it is an abundant resource that may provide a much-needed input to help build soil fertility. Analysis of soil P status during decomposition showed no effect of the plant litters for the first months (Fig. 4(c)). After 4–5 months of incubation, extractable P in soil increased for all litter. Some farmers in the area claim that wattle amendments improve plant production, but the basis of this is not clear from this study. Although litter addition may improve soil fertility in the long term, it is unlikely that this will be attributed to wattle amendments by local farmers. N and P release patterns may be different under natural conditions and in the presence of plant roots than in our laboratory incubation. Moreover, effects other than nutritional, such as pathogen control, may play a role under field conditions. 3.4. Sustainable wattle control Although the Working for Water programme is providing much needed employment opportunities for the local community, we argue that this process is not sustainable in its present form. Once the wattle is removed, or funding runs out, involvement in the area will cease, leaving people with no alternatives for income and a depleted resource base. The high wages of the Working for Water programme (30–45 Rand per day, depending on skills) may also be a threat to other income generating activities in the area. In line with conclusions made elsewhere in Eastern Cape regarding the Working for Water programme (Ainsley, 1999) a local NGO suggested that this may have caused people to leave other community projects in the village that subsequently had collapsed (Anonymous, 1999). The large emphasis placed on the programme, as well as the local reliance on the wattle as a resource, has led the local NGO to suggest a community managed woodlot plantation, as well as building small businesses, mainly processing and selling firewood, in the wake of
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the Working for Water programme. A pilot project was carried out in 1999, aimed at processing, distributing and selling wood that had been cut down by the Working for Water employees. Although the pilot study was partly successful, it was economically unsustainable with a monthly cost of 5000 Rand that exceeded income by at least 50%. Suggestions for a managed plantation on the cleared land include both wattle and other species. Seventy-three percent of the households interviewed preferred establishment of fruit orchards on the land, whilst 59% suggested other non-specified tree plantations. No one suggested new wattle plantations, although it is the most likely species to be tested first, according to the local NGO and Working for Water representatives. Only a few respondents suggested re-establishing grazing land on the cleared areas. This corresponds well to information on the number of cattle provided by the veterinarian responsible for the dipping station showing a 40% decline from 2001 to 2003. Villagers expressed strong concerns about cattle theft, and identified this as the main reason for the declining number of cattle in the village. As great an environmental threat as that posed by the presence of the invasive wattle species is the removal of the species and the resulting surface and rill erosion on the exposed soils created by loss of vegetation cover as the wattle stands effectively exclude virtually all vegetation beneath them. The Working for Water programme is aware of this problem, and acknowledges that rehabilitation, predominantly through replanting, after removal is essential in order to prevent erosion and reintroduce desired species that can help control or elevate impact of the wattle. However, in the Madlangala area, no such replanting has commenced and the community is not aware of any such opportunities. The soils in the region are generally highly erosive due to their mineral composition, and erosion control has been a concern since the 1960s when all arable land was terraced with governmental support. Our indicative erosion measurements demonstrated clear differences in topsoil erosion between land uses. On grassland 0.74 ± 0.28 g (SD; n = 3) of soil was collected in each of the 1 m gutters; in the wattle stands 42 ± 4 g (SD; n = 3); and on the cleared land 203 ± 82 g (SD; n = 3). Although these data should not be used to quantify soil loss, they clearly indicate a much increased risk of erosion from the cleared areas. Severe soil erosion has occurred on some of the slopes from which the wattle has been removed. This leads to exposure of the infertile subsoil, restricting colonisation of indigenous species, in particular grasses that could aid in stabilising the soil. The community do recognize the severity of the soil erosion problem exacerbated by wattle removal and have suggested that future clearings be better secured against erosion (McLeod, 1999). Aside from the problems of soil erosion on the newly cleared land, the failure to re-habilitate cleared land with indigenous species, which can help control the wattle, is a concern. After clearing, the stumps are not always killed, and a high number of sprouting or coppicing stumps can be observed. Moreover, wattle produces large quantities of viable seeds that are fire-tolerant, which, combined with the almost complete lack of under-story vegetation under wattle stands, ensures that it is predominantly wattle colonising the bare ground once clearing has been completed.
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Large parts of the area depicted in sub-scene 2 (Fig. 3) have been cleared between 2001 and 2003. Today, the entire area is again covered by emerging wattle plants, up to several meters tall, as no follow-up treatments have been performed. Based on our observations, we conclude that the most likely vegetation to emerge after the removal of the wattle will be new wattle plants, if no measures are taken to re-establish other, more appropriate vegetation. Moreover, the increased soil loss caused by wattle clearance is severely jeopardising the quality and productivity of the soil, and hence the stability of the ecosystem. Biological control of wattle species has been tested since 1993, but is yet inconclusive (Zimmermann et al., 2004). The Acacia seed weevils (Melanterius maculates) from Australia show some potential, and have the advantage that they act by feeding on seeds, hence limiting germination, but not damaging trees. As for many other South African invasives, the fact that the same species often have economic importance under managed conditions, has led to strong conflicts of interest, and apprehension towards biological controlling agents. Biological control has many times demonstrated its advantages in the South African environment (van Wilgen et al., 2004), and alternatives to the weevils, such as the stump colonizing fungus Cylindrobasidium leave have also been tested. Biological controlling agents have high priority under the Working for Water research programme (van Wilgen, 2004), but it is too early to state whether these approaches will be effective (Zimmermann et al., 2004). 3.5. Community involvement and future outlook It is clear from the aerial photos, representing a period of more than 47 years that the invasive wattle species are spreading dramatically in parts of the Madlangala area. The current wattle control programme has yet to prove that it will be able to keep pace with the invasion of the wattle species, and biological control has not yet been introduced to the area. Unfortunately, based on our aerial photographs we cannot determine the trend during the past 5 years; however, on-site observations indicate that the Working for Water initiative is having little success in reducing wattle cover. Even if clearing the wattle stands were successful and re-invasion prevented, the vegetation cover will still be in an unstable state, unless fundamental changes in land management are initiated. The ultimate goal of the Working for Water programme is to make itself superfluous, by achieving its objectives. However, if management of the land is not handed over to the local inhabitants, the regulation of alien species will forever be dependent on external funding at a considerable environmental, economic and social cost. In the Madlangala area, the pressure on land is diminishing as cattle numbers have decreased during the past 10 years, partly due to cattle theft and diseases, and much of the agricultural land is left uncultivated as economic and social pressures force the youth to leave the region and seek ÔbetterÕ opportunities elsewhere. According to the villagers, lack of support from the Department of Agriculture as well as declining remittances from migrant workers have caused a de-intensification of farming in the area. Consequently, wattle is allowed to spread unchecked, as there is no pressure to procure land for agricultural use. Ironically, a greater pressure on
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the resource base (land) appears to be what is needed in order to place more pressure on the successful control of wattle. A paradox here is the notion of HardinÕs ‘‘tragedy of the commons’’ (Hardin, 1968) in reverse and the Boserupian idea of ‘‘more people better land practice’’ (Tiffen et al., 1994): at this study site we advocate that greater pressure for arable and/or grazing land could result in greater concern, care and, ultimately, control of the invasive species. Establishment of managed plantations of wattle or other woody species may satisfy the villagerÕs needs for firewood and building material, and also provide a much needed means of occupation and production in the rural area once the Working for Water programme ceases. However, it is yet to be seen if there are human and economic resources to establish such plantations and whether these initiatives will be economically viable. On the surface there appears to be an interest from both villagers and NGOs, although the basis for this interest is unclear. However, this initiative only solves part of the problem, which relates to securing the villagers supply of wood, but will not remove the threat of reinvasion by the same or new alien species. Of course it is only a threat if perceived as such and there is a fascinating story to be told regarding what transpires when an environmental scientistÕs problem is a rural communityÕs livelihood.
4. Conclusions Our investigation clearly demonstrates a significant increase in wattle cover over the past 50 years, leading to changes in landscape, environment and availability of natural resources. The wattle has many beneficial uses for the local communities, but the potential as green manure appears limited due to the high content of polyphenols, limiting N release from the litter. Should wattle control succeed, testing and introduction of less invasive woody legumes suited to the climate, and providing both firewood and higher-quality litter, must be considered in order to ensure supply of these resources. In order to solve the problem of the spread of the wattle, economically and ecologically sustainable land management or land stewardship founded on ownership, involvement and management by local communities must be developed. This may appear a highly unrealistic suggestion on the basis of the current resource base in the area, but it may not be any more unrealistic than the current approach, and should serve as an overall objective towards which ongoing initiatives should work. It needs to be recognised that the Working with Water programme is a pioneering environmental conservation initiative in that its implementation successfully combines ecological concerns and social development benefits. The programme has significantly contributed to poverty alleviation and social upliftment while controlling invasive alien plants, although the success of the latter in communally owned rural areas may be questioned. The development of a viable Ôexit strategyÕ (that is, interventions to maximize the chances of workers either securing gainful employment after leaving the programme or successfully setting up their own businesses) is the focus of on-going research and is recognised as a point of future consideration.
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Preliminary findings in 2002 revealed that most workers and contractors leaving the programme were not fully equipped to secure work elsewhere, although this situation also reflects the general economic depression in rural areas (Magadlela and Mdzeke, 2004). The involvement of the Working for Water programme in the Madlangala area has provided much needed job opportunities to the villagers, but it must be recognised that the project is highly unsustainable in the sense that it is entirely dependent on external funding, and that it does not offer any solutions to the long term management of invasive alien species, but merely attempts to remove the stands. Moreover, the job opportunities provided by the programme are mainly short term gains, creating dependence and no long-term opportunities. Furthermore, incentives for the villagers to clear the land are not obvious. Aside from the threat of villains hiding in the plantations, the wattle stands do not constitute a big problem for the community, due to the abundance of land and the fact that any increase in water runoff will have no direct or indirect benefit to the community. It is ironic that in a country such as South Africa, where land rights are one of the hottest political and practical issues, it is the apparent abundance and underutilisation of the land as a resource that appears to be one of the largest constraints in developing a viable way of controlling invasive species. In our research area, in the former homeland of Transkei, lack of inputs and human resources results in agricultural activities being perceived as un-rewarding and little attention is paid to this sector, a pattern referred to as de-agrarianization (Bryceson, 2002). We believe that until the productivity of this land, either as grazing land, arable land or natural habitat, is restored, control of invasive species will be dependent upon a continued effort and resource flow from external organisations and institutions.
Acknowledgements The data from the field site were collected during visits by staff and MasterÕs students from University of KwaZulu-Natal, University of Swaziland, University of Botswana, University of Copenhagen, Roskilde University and the Royal Veterinary and Agricultural University, and are jointly owned by these institutions. The field trips were performed as part of the SLUSE consortia activities, and funded by Danida. We thank and pay tribute to the kindness and hospitality that we were afforded by the Madlangala community.
References Ainsley, A., 1999. When communities is not enough: Managing common property natural resources in rural South Africa. Development Southern Africa 16, 375–401. Anonymous, 1995. The Bioresource Units of KwaZulu-Natal. KwaZulu-Natal Department of Agriculture. Cedara Report No. N/A/95/32. Anonymous, 1999. Understanding the impact of the Working for Water programme on rural communities. EDA Trust Report. EDA Trust & Care SA, Matatiele.
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Bryceson, D.F., 2002. The scramble in Africa: Reorienting rural livelihoods. World Development 30, 725– 739. Cowling, R., Hilton-Taylor, C., 1994. Patterns of plant diversity and endemism in southern Africa: an overview. In: Huntley, B. (Ed.), Botanical Diversity in Southern Africa. National Botanical Institute, Pretoria, pp. 19–50. Dye, P., Jarmain, C., 2004. Water use by black wattle (Acacia mearnsii): implications for the link between removal of invading trees and catchment streamflow response. South African Journal of Science 100, 40–44. Giller, K.E., 2001. Nitrogen Fixation in Tropical Farming Systems. CAB International, Wallingford. Gorgens, A.H.M., van Wilgen, B.W., 2004. Invasive alien plants and water resources in South Africa: current understanding, predictive ability and research challenges. South African Journal of Science 100, 27–33. Hardin, G., 1968. The tragedy of the commons. Science 162, 1243–1248. Hove, L., Ndlovu, L.R., Sibanda, S., 2003. The effects of drying temperature on chemical composition and nutritive value of some tropical fodder shrubs. Agroforestry Systems 59, 231–241. Hui, S., Ya, T., Qiguo, Z., 2002. Nitrogen mineralisation of prunings of six N2-fixing hedgerow species in a dry valley of the Jinsha river. Pedosphere 12, 21–25. Kitou, M., Yoshida, S., 1994. Mulching effect of plant residues on soybean growth and soil chemical properties. Soil Science and Plant Nutrition 40, 211–220. Le Maitre, D., Versfeld, D., Chapman, R.A., 2000. The impact of invading alien plants on surface water resources in South Africa: A preliminary assessment. Water S. A. 26, 397–408. Macdonald, I.A.W., 2004. Recent research on alien plant invasions and their management in South Africa: a review of the inaugural symposium of the Working for Water programme. South African Journal of Science 100, 21–26. Magadlela, D., Mdzeke, N., 2004. Social benefits in the Working for Water programme as a public works initiative. South African Journal of Science 100, 94–96. Makkar, H.P.S., Bluemmel, M., Borowy, N.K., Becker, K., 1993. Gravimetric determination of tannins and their correlations with chemical and protein precipitation methods. Journal of Science of Food and Agriculture 61, 161–165. May, B.M., Attiwill, P.M., 2003. Nitrogen-fixation by Acacia dealbata and changes in soil properties 5 years after mechanical disturbance or slash-burning following timber harvest. Forest Ecology and Management 181, 339–355. McLeod, N., 1999. Community based wattle management: Harmonising WfW with communal realities. In: Community Based Land Management Seminar, September 1999, Hammanskrall, South Africa, pp. 1–9. Nel, J.A., Richardson, D.M., Rouget, M., Mgidi, T.N., Mdzeke, N., Le Maitre, D., van Wilgen, B.W., Schonegevel, L., Henderson, L., Neser, S., 2004. A proposed classification of invasive plant species in South Africa: towards prioritizing species and areas for management action. South African Journal of Science 100, 53–64. Olsen, S.R., Sommers, L.E., 1982. Phosphorus. In: Page, A.L. (Ed.), Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties. American Society of Agronomy and Soil Science Society of America, Madison, WI, pp. 403–430. Palm, C.A., Gachenco, C.N., Delve, R.J., Cadisch, G., Giller, K.E., 2001. Organic inputs for soil fertility management in tropical agroecosystems: application of an organic resource database. Agriculture Ecosystems and Environment 83, 27–42. Palm, C.A., Sanchez, P.A., 1991. Nitrogen release from the leaves of some tropical legumes as affected by their lignin and polyphenolic contents. Soil Biology and Biochemistry 23, 83–88. Republic of South Africa, 1995. Rural development strategy of the Government of National Unity. Government Gazette No. 16679, 3 November 1995, Pretoria. Richardson, D.M., van Wilgen, B.W., 2004. Invasive alien plants in South Africa: how well do we understand the ecological impacts. South African Journal of Science 100, 45–52. Tiffen, M., Mortimore, M., Gichuki, F., 1994. More People, Less Erosion. Environmental Recovery in Kenya. ACTS Press, Nairobi.
A. de Neergaard et al. / Agricultural Systems 85 (2005) 216–233
233
van Soest, P.J., 1963. Use of detergents in the analysis of fibrous feeds. Journal of Association of Agricultural Chemistry 46, 825–835. van Wilgen, B.W., 2004. Scientific challenges in the field of invasive alien plant management. South African Journal of Science 100, 19–20. van Wilgen, B.W., de Wit, M.P., Anderson, H.J., Le Maitre, D., Kotze, I.M., Ndala, S., Brown, B., Rapholo, M.B., 2004. Costs and benefits of biological control of invasive alien plants: case studies from South Africa. South African Journal of Science 100, 113–122. Zimmermann, H.G., Moran, V.C., Hoffmann, J.H., 2004. Biological control in the management of invasive alien plants in South Africa, and the role of the Working for Water programme. South African Journal of Science 100, 34–40.