Invasive alien trees and water resources in South Africa: case studies of the costs and benefits of management
Introduction
Natural forests in southern Africa are surprisingly limited in their extent. Areas that receive in excess of 800 mm of rainfall annually, at altitudes below 2000 m, are covered by grasslands or shrublands, while similar areas in other parts of the world would support natural forests (Holdridge et al., 1971, Schulze and McGee, 1978). One of the main reasons for this disparity is repeated fires which prevent the establishment of forest species (Moll et al., 1980, Manders, 1990, Manders and Richardson, 1992). In areas which are sheltered from regular fires, forests can and do develop (Geldenhuys, 1994), and forests tend to have fuel properties that do not promote fire (van Wilgen et al., 1990), ensuring their survival in the fire-prone landscape. Nonetheless, natural forests in southern Africa cover less than 0.25% of the landscape (Low and Rebelo, 1996, Midgley et al., 1997).
The lack of a natural source of fast growing timber trees led to the establishment of plantations of alien (introduced) species, beginning in the late 19th century (King, 1943, Le Maitre, 1998a). Plantations of alien trees, primarily pines and eucalypts, now cover 1.52 million ha in South Africa (FOA, 1998). These plantations have brought many benefits. Plantation forestry contributes US$ 300 million, or 2%, to the GDP and employs over 100,000 people. Downstream industries, based on forestry, produce products worth a further US$ 1.6 billion, much of which is exported, earning valuable foreign exchange (FOA, 1998).
However, the establishment of these plantations has not been without cost. The negative impacts of afforestation include significant reductions in surface streamflow (Van Lill et al., 1980, Bosch and Hewlett, 1982, Bosch and von Gadow, 1990). Commercial plantations are estimated to have reduced surface runoff by about 1.4 billion m3 per year or 3.2%, at a national scale (Le Maitre et al., 1997, Scott et al., 1998a). These reductions are important because South Africa has a mean annual rainfall of only 490 mm and less than 10% of this becomes surface runoff (Alexander, 1985). Commercial plantation forestry is the only land-use which is restricted because its impacts of afforestation on streamflow (Van der Zel, 1995). Plantations also have substantial impacts on biodiversity and the functioning of natural ecosystems (Armstrong and van Hensbergen, 1996, Allen et al., 1997).
The National Water Act recognises that a portion of the available water needs to be reserved for basic human needs and to sustain natural ecosystems; this places additional constraints on the water available for other uses (Anon., 1970, DWAF, 1986, Walmsley and Davies, 1991, DWAF, 1996). Water is the primary resource that will ultimately limit development in South Africa and efficient management and allocation of water resources is a national imperative (DWAF, 1996).
Unfortunately, many of the plantation species have become major invaders, spreading the negative impacts far beyond the afforested areas. Foremost amongst these are several species of pine (Pinus) and wattle (Acacia). The forestry industry recognises these problems and subscribes to a code of conduct which, among other things, requires that riparian zones and non-afforested areas within the forest estates are kept clear of invading alien plant species (FIEC, 1995). The forest industry has also recognised that the impacts of plantation trees on streamflow are far greater in the riparian zone than outside it (Scott and Lesch, 1995, Scott et al., 1999), and follow a policy of non-afforestation of such zones (FIEC, 1995). The industry also actively supports the government’s alien plant control programmes by funding control operations, providing expertise and forming partnerships with Working for Water.
Commercial forestry based on alien trees is a well-established feature of the South African landscape and economy, but the invasions that are associated with it are going to have to be managed to minimise conflicts around scarce resources, especially water. As a first step, a clear understanding of the magnitude of the problem needs to be developed, especially in order to establish whether the costs of clearing invaded areas can be justified. Studies to quantify the relative benefits of clearing have been carried out in the Western Cape Province, where mountain catchment areas covered with fynbos shrublands have been invaded by pines and wattles (Le Maitre et al., 1996, van Wilgen et al., 1996, van Wilgen et al., 1997). These areas are subjected to fires at about 15-year intervals; the fires trigger the spread of alien trees, initially from adjacent plantation areas, and subsequently from invaded areas in the catchment.
As the problem of invasive alien trees and their impacts was not restricted to the Western Cape Province (Henderson, 1995, Dye and Poulter, 1995), a study was commissioned to estimate the predicted impact on a national level. This study found that about 10.1 million ha, or 6.8% of South Africa has been invaded to some degree (Versfeld et al., 1998). These invasions were estimated to be using almost 6.7% of the country’s runoff, and would cost an estimated US$ 0.86 billion to clear over 20 years (Le Maitre et al., 2000). While these results were based on very crude estimates of the extent of invasion they were nonetheless convincing enough for the national government to launch an extensive and ambitious control programme aimed at minimising the effects on water resources (van Wilgen et al., 1998).
This paper summarises the findings of management plans that were developed for alien plant control in four representative catchments in South Africa. The aim of these plans was two-fold—firstly to test, at a finer scale, the broad-scale predictions of significant benefits from clearing programmes, and secondly to provide project managers with reasonable estimates of the extent of the task that they faced. In this paper, we compare the composition, extent and impacts of invasive alien trees in the four catchments and provide estimates of the costs and benefits of control operations.
Section snippets
Study sites
We selected four catchments, representative of a cross-section of the catchments in South Africa in terms of their climate, natural ecosystems, kinds of land-use and the composition of the invading plants (Table 1, Fig. 1).
Mapping of invaders
Different methods were used to map invaded areas in the different catchments. Field mapping was used for the Sonderend and Sabie-Sand catchments and for parts of the other two. Mapping followed the guidelines developed by Le Maitre and Versfeld (1994) which provide for a range of density classes from rare (invaders known to be present but canopy cover less than 0.01%) to closed (100% canopy cover). The spatial extent of the invasions was recorded on acetate overlaid on maps from the standard
Land-use
The Sonderend catchment has the greatest area under cultivation with 53% under annual crops and deciduous fruit orchards and the smallest area under plantations (0.3%) (Table 2). Less than 30% of the renosterveld is left, mostly in small isolated remnants. Most of the vegetation in the Keurbooms catchment is still relatively natural, only 16% has been transformed. About 10% of the fynbos and about 50% of the indigenous forest have been transformed. About 25% of the grasslands in the Upper Wilge
Invasion patterns
The invaded state of the four catchments reported on here is typical of those in the higher rainfall areas of South Africa. These catchments have a relatively high agricultural potential and the natural vegetation on the arable areas has been transformed or modified by use for extensive grazing. The extensive invasions in the Sonderend and Keurbooms catchments are typical of those of mountain catchments in the Western Cape (Le Maitre et al., 2000). Fynbos is known to be peculiarly susceptible
Acknowledgements
This study and the development of the management plans for the different catchments was funded by the South African Department of Water Affairs and Forestry, through it’s Working for Water Programme.
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