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Riparian vegetation management in landscapes invaded by alien plants: Insights from South Africa
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South African Journal of Botany 74 (2008) 397 – 400 www.elsevier.com/locate/sajb
Editorial
Riparian vegetation management in landscapes invaded by alien plants: Insights from South Africa The important roles that intact riparian vegetation play in maintaining ecosystem health and services have been increasingly highlighted in the literature as we acknowledge the degradation of these “critical transition zones” (Ewel et al., 2001). Riparian corridors are naturally disturbance-mediated ecosystems but because they receive inputs of matter and propagules from such large areas, they accumulate, concentrate and exacerbate further human-induced impacts and disturbances. Therefore it is not surprising that they are amongst the most degraded ecosystems globally (Revenga et al., 2005; Dudgeon et al., 2006). Nel et al. (2007), in the first comprehensive assessment of South Africa's main river ecosystems (quaternary catchments) reported that an alarming 54% are critically endangered, with less than 20% of their total original extent intact. One important category of impacts to riparian ecosystems is that associated with plant invasions. Invasive alien plants thrive in frequently disturbed environments (Huston, 2004), and multiple opportunities exist for their establishment and spread as these linear corridors snake through the landscape. Whether invasives are drivers or passengers of change in degraded ecosystems (MacDougall and Turkington, 2005) is a moot point, but those species that significantly alter native composition, structure and function (termed “transformers” sensu Richardson et al., 2000) are now receiving considerable attention as we attempt to manage them and their negative impacts. Following global trends, invasive alien plants are becoming an increasingly large problem in South Africa where mounting evidence links invasive alien plant transformation to declines in ecosystem integrity and services (Richardson and Van Wilgen, 2004). Here, woody invasive riparian transformers have been linked to significant reductions in water supply (Van Lill et al., 1989; Le Maitre et al., 2002; Dye and Jarmain, 2004), leading to one of the world's largest initiatives to clear watersheds of invasive trees - the Working for Water programme (Van Wilgen et al., 1998). Working for Water (WfW), with its joint aims to enhance ecological integrity, water security and social development, has been operating since 1995 under the assumption that its target ecosystems, mostly riparian, would “self-repair” once the main stressor (dense stands of invasive alien trees) had been removed. This assumption has been largely untested until now, and is the focus of this special issue on riparian vegetation management and ecosystem repair in alien plant-invaded landscapes of South Africa.
Resilience, the ability (Wali, 1999) or time taken (Mitchell et al., 2000) for an ecosystem to recover to some acceptable structural or functional reference level, is a challenge to conceptualize in riparian zones because of their inherent dynamic nature (Richardson et al., 2007). Frequent disturbances in riparian ecosystems provide many opportunities for changes to arise in the diversity and relative abundance of component species. If invasive alien species are able to exert disproportional influence on other species, e.g. due to their ability to recruit prolifically and crowd out other species, they have the capacity to compromise resilience. Recovery is expected to be dependent on the type, size and periodicity of the disturbance, as well as the survival strategies of component species. A key question is: when is it necessary to implement restoration management interventions beyond the removal of alien plants? There are many angles to, and components of, this question, and any deliberation must address ecological, social and economic concerns (Aronson et al., 2007). It is interesting to note how few studies on restoration deal with all these aspects - a gap identified in the synthesis paper in this Special Issue, and indeed a gap in the issue itself (Holmes et al., 2008-this issue). There have been an increasing number of articles addressing river restoration ecology over the past twenty years (Nilsson et al., 2007). Of particular interest is a 2007 special feature on restoring riverine landscapes in the journal “Ecology and Society” (Nilsson et al., 2007). This contribution presents articles centred on the three basic themes of setting priorities, identifying techniques and identifying relevant reference conditions against which to gauge progress. Our Special Issue contains very much the same basic elements, while targeting a more focused sub-theme of repair after invasion. It is a culmination of work (plus some additional papers contributing to the theme) from a project commissioned by Working for Water on targets for ecosystem repair in alien-invaded riparian zones. The ultimate aim of this project was to produce guidelines and tools to improve management of these systems. These are summarized in the final paper in the Special Issue by Holmes et al. (2008-this issue). While riparian zones are proportionally small components of the landscape, the three biomes covered in this issue; viz. the Fynbos, Grassland and Savanna biomes make up 67% of the land surface of South Africa, Lesotho and Swaziland (Rutherford et al., 2006). Within all three of these biomes riparian zones are hugely important in fulfilling many important physical, chemical, biological and socio-economic roles (Table 1).
0254-6299/$ - see front matter © 2008 SAAB. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.sajb.2008.01.168
398
Editorial
Table 1 Functions and benefits of riparian vegetation Physical/chemical • Moderates stream temperatures and light levels, thereby functions: influencing habitat suitability for aquatic organisms • Maintains stream-bank integrity (roots) • Influences channel dynamics by contributing woody debris to streams • Aboveground stems of vegetation increase channel roughness during over-bank flow, thereby decreasing erosive action of floods and retaining material in transport • Retains water, thus increasing evapotranspiration and groundwater recharge • Regulates exchange of nutrients and organic material between streams and upland areas, and acts a sink for pollutants Biological • Source of habitat and nourishment for both vertebrates functions: and invertebrates • Harbours biotic diversity (aquatic and terrestrial) • Provides natural fire-breaks • Provides refuges for biota during unfavourable periods • Provides corridors for dispersal and movement of plants and animals Human-use • Provides food, water and other resources for people functions: (e.g., fuelwood and building material) and livestock (e.g., forage, shade) • Provides a place of recreation
Although the papers in this special issue touch on many complexities involved in the management of riparian zones in the face of escalating pressure from invasive species, we identify three critical themes that justify some introductory discussion: issues relating to the setting of priorities at different scales; refining methods for effective removal of the invasive species and subsequent treatments to steer recovery in desired directions; and the problem of reference sites. Each of these is discussed below. 1. Setting priorities Using an economic approach, Marais and Wannenburgh (2008-this issue) explore the extent of riparian alien clearing and analyse the actual WfW management data to investigate the costs and water benefits of this clearing countrywide. An estimated 7% of closed stand invasions in riparian zones have been cleared, yielding significant water yield increases. The cost of clearing high water-using alien species was found to be a good investment in comparison to other water resource options. From an ecological perspective, clearing scattered (b 5% cover) invasions was significantly cheaper than closed stands whilst safeguarding the water resource from future impacts of alien densification. However, short-term cost-efficiency in terms of generating water yield is generally higher for denser alien stands. The high cumulative costs of repeated follow-up treatments in 3% of the denser alien Acacia stands indicate potential viability of active restoration interventions at these sites. 2. Identifying techniques Largely adopting a comparative approach through space (Blanchard and Holmes, 2008-this issue; Vosse et al., 2008-this
issue) and time (Beater et al., 2008-this issue (10 yr); Morris et al., 2008-this issue (1 yr, before and after clearing); Reinecke et al., 2008-this issue (4 yr); Witkowski and Garner, 2008-this issue (10 yr)), and to a lesser extent an experimental approach (Behenna et al., 2008-this issue; Pretorius et al., 2008-this issue (8 yr)), most papers in this issue address the underlying question - are the current alien-clearing practices achieving the ecosystem repair goals set by WfW to restore indigenous riparian vegetation structure, diversity and function? In tackling this question, the aim was to identify best-practice techniques to ensure recovery after alien clearing (Holmes et al., 2008-this issue). A shortfall of the series of studies is that most of them took place over a relatively short time relative to natural ecosystem trajectories, making the assessment of recovery towards a reference site condition rather speculative, based only on early stages of succession. For a more long-term perspective, a monitoring framework is essential (Woolsey et al., 2007), but was found to be lacking in a 2002/3 review of the WfW programme (Holmes et al., 2003). This gap is currently being addressed by consultants appointed by WfW. Without active intervention, and given the short-time frame constraint noted above, recovery was partial and depended on how densely-invaded the systems were prior to clearing. While seed banks have the potential to initiate recovery (Fourie, 2008this issue; Vosse et al., 2008-this issue) in densely invaded Fynbos ecosystems of the Western and Eastern Cape, clearing techniques can either delay or promote recovery (Behenna et al., 2008-this issue; Blanchard and Holmes, 2008-this issue; Reinecke et al., 2008-this issue). In savanna and grassland ecosystems, largescale disturbances associated with flood events add complexity to indigenous recovery patterns as well as alien recruitment patterns (Foxcroft et al., 2008-this issue), thus requiring more strategic and focused management attention (Morris et al., 2008-this issue). On a worrisome note, the two longer-term studies in grassland and savanna systems indicated that after a decade of clearing, invasion had not been reduced; rather the nature of the problem had shifted from one dealing with a few large trees to one now dealing with a deadly cocktail of smaller Invasive Alien Plants (IAPs) occurring in larger numbers (Beater et al., 2008-this issue; Witkowski and Garner, 2008-this issue). Above all, the critical importance of careful follow-up clearing was stressed (Morris et al., 2008-this issue; Pretorius et al., 2008-this issue; Reinecke et al., 2008-this issue). The need for active restoration after clearing may be apparent shortly after clearing (Reinecke et al., 2008-this issue), but guidelines for large-scale, ecologically- and economicallyappropriate restoration techniques for various riparian systems are still lacking. The only restoration experiment reported in this issue was limited to a local scale and did not include economic considerations (Pretorius et al., 2008-this issue). Its strength is that it reports on a time sequence, with data indicating that indigenous sowing treatments can succeed in improving native species richness and abundance providing sufficient follow-up control is maintained. Targeted planting might well be an option where key species are missing from the seed bank, but to optimize success, detailed knowledge of their ecology and function is needed, such as that provided by Swift et al. (2008-this issue). Finally, the focus on techniques in this issue was largely at the biotic level, with no
Editorial
emphasis on abiotic-level restoration such as control of erosion or alteration of geohydrology. Attempts at restoration without such interventions are futile in certain cases (Richardson et al., 2007). 3. The problem of identifying reference sites When assessing or monitoring the success or failure of a particular intervention such as alien clearing and restoration, a key challenge is to identify sites against which one can compare the treatment. It is natural to consider “pristine” sites with similar environmental attributes as good reference sites. In many areas, degradation is so pervasive that such sites no longer exist. Even where well-matched reference sites can be found, riparian systems are so dynamic that they are always in some trajectory of postdisturbance recovery, which greatly complicates comparisons between systems (Richardson et al., 2007) Nonetheless, largelyuninvaded, relatively-intact sites do exist in some parts of South Africa, and detailed work to describe reference sites such as that by Sieben and Reinecke (2008-this issue) in mountain streams of the Fynbos Biome provide some idea of a baseline state, even if it is limited to certain habitats. In this issue, Blanchard and Holmes (2008-this issue) and Vosse et al. (2008-this issue) effectively use reference sites to compare above- and below-ground (seed bank) treatments and to identify appropriate management interventions after alien clearing. Carefully selected comparative studies like these, in other parts of the country would help to improve our understanding of the dynamics of these systems. Given the critical role that riparian vegetation plays in maintaining ecosystem function and services, the degree to which these systems have been invaded and degraded and the priority given to them by WfW, we hope that this Special Issue will stimulate further research into restoration of riparian zones. Acknowledgements The authors of papers in this issue for their contributions and the many reviewers for their role in improving the manuscripts. Financial support from the DST-NRF Centre for Invasion Biology and the Department of Water Affairs and Forestry, in collaboration with Working for Water (Targets for Ecosystem Repair in Riparian Ecosystems in Fynbos, Grassland and Savanna Biomes) is gratefully acknowledged. References Aronson, J., Milton, S.J., Blignaut, J.N., 2007. Restoring Natural Capital: Science, Business, and Practice. Island Press, Washington. Beater, M.M.T., Garner, R.D., Witkowski, E.T.F., 2008. Impacts of clearing invasive alien plants from 1995 to 2005 on vegetation structure, invasion intensity and ground cover in a temperate to subtropical riparian ecosystem. South African Journal of Botany 74, 495–507 (this issue). doi:10.1016/j. sajb.2008.01.174. Behenna, M., Vetter, S., Fourie, S., 2008. Viability of alien and native seed banks after slash and burn: effects of soil moisture, depth of burial and fuel load. South African Journal of Botany 74, 454–462 (this issue). doi:10.1016/j. sajb.2008.01.179. Blanchard, R., Holmes, P.M., 2008. Riparian vegetation recovery after invasive alien tree clearance in the fynbos biome. South African Journal of Botany 74, 421–431 (this issue). doi:10.1016/j.sajb.2008.01.178.
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Dudgeon, D., Arthington, A.H., Gessner, M.O., Kawabata, Z., Knowler, D.J., Lévêque, C., Naiman, R.J., Prieur-Richard, A., Soto, D., Stiassny, M.L.J., Sullivan, C.A., 2006. Freshwater biodiversity: importance, threats, status and conservation challenges. Biological Review 81, 163–182. 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, 40–45. Ewel, K.C., Cressa, C., Kneib, R.T., Lakes, P.S., Levin, L.A., Palmer, M.A., Snelgrove, P., Wall, D.H., 2001. Managing critical transition zones. Ecosystems 4, 452–460. Fourie, S., Blignaut, J.N., 2008. Composition of the soil seed bank in alien-invaded grassy fynbos: potential for recovery after clearing. South African Journal of Botany 74, 445–453 (this issue). doi:10.1016/j.sajb.2008.01.172. Foxcroft, L.C., Parsons, M., McLoughlin, C.A., Richardson, D.M., 2008. Patterns of alien plant distribution in a river landscape following an extreme flood. South African Journal of Botany 74, 463–475 (this issue). doi:10.1016/j.sajb.2008.01.181. Holmes, P.M., Euston-Brown, D., Richardson, D.M., 2003. External evaluation of the Working for Water programme. Terrestrial ecology theme. Report to Common Ground Consulting. Cape Ecological Services/Institute for Plant Conservation, Cape Town. Holmes, P., Esler, K.J., Richardson, D., Witkowski, E., 2008. Guidelines for improved management of riparian zones invaded by alien plants in South Africa. South African Journal of Botany 74, 538–552 (this issue). doi:10.1016/j.sajb.2008.01.182. Huston, M.A., 2004. Management strategies for plant invasions: manipulating productivity, disturbance, and competition. Diversity and Distributions 10, 167–178. Le Maitre, D.C., Van Wilgen, B.W., Gelderblom, C.M., Bailey, C., Chapman, R.A., Nel, J.A., 2002. Invasive alien trees and water resources in South Africa: case studies of the costs and benefits of management. Forest Ecology and Management 160, 143–159. MacDougall, A.S., Turkington, R., 2005. Are invasive species the drivers or passengers of change in degraded ecosystems? Ecology 86, 42–55. Marais, C., Wannenburgh, A.M., 2008. Restoration of water resources (natural capital) through the clearing of invasive alien plants from riparian areas in South Africa - costs and water benefits. South African Journal of Botany 74, 526–537 (this issue). doi:10.1016/j.sajb.2008.01.175. Mitchell, R.J., Auld, M.H.D., Le Duc, M.G., Marrs, R.H., 2000. Ecosystem stability and resilience: a review of their relevance for the conservation management of lowland heaths. Perspectives in Plant Ecology, Evolution and Systematics 3, 142–160. Morris, T.L., Witkowski, E.T.F., Coetzee, J.A., 2008. Initial response of riparian plant community structure to clearing of invasive alien plants in Kruger National Park, South Africa. South African Journal of Botany 74, 485–494 (this issue). doi:10.1016/j.sajb.2008.01.177. Nel, J.L., Roux, D.J., Maree, G., Kleynhans, C.J., Moolman, J., Reyers, B., Rouget, M., Cowling, R.M., 2007. Rivers in peril inside and outside protected areas: a systematic approach to conservation assessment of river ecosystems. Diversity & Distributions, 13, 341–352. Nilsson, C., Jansson, R., Malmqvist, B., Naiman, R.J., 2007. Restoring riverine landscapes: the challenge of identifying priorities, reference states, and techniques. Ecology and Society 12, 16 [Online] URL: http://www. ecologyandsociety.org/vol12/iss1/art16/. Pretorius, M., Esler, K.J., Holmes, P.M., Prins, N., 2008. The effectiveness of active restoration following alien clearance in fynbos riparian zones and resilience of treatments to fire. South African Journal of Botany 74, 517–525 (this issue). doi:10.1016/j.sajb.2008.01.180. Reinecke, M.K., Pigot, A., King, J.M., 2008. Spontaneous succession of riparian fynbos: is unassisted recovery a viable restoration strategy? Ecological Economics 74, 412–420 (this issue). doi:10.1016/j.sajb.2008. 01.171. Revenga, C., Campbell, I., Abell, R., De Villiers, P., Bryer, M., 2005. Prospects for monitoring freshwater ecosystems towards the 2010 targets. Philosophical Transactions of the Royal Society 360, 397–413. 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.
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Richardson, D.M., Pyšek, P., Rejmánek, M., Barbour, M.G., Panetta, F.D., West, C.J., 2000. Naturalization and invasion of alien plants - concepts and definitions. Diversity & Distributions 6, 93–107. Richardson, D.M., Holmes, P.M., Esler, K.J., Galatowitsch, S.M., Stromberg, J.C., Kirkman, S.P., Pyšek, P., Hobbs, R.J., 2007. Riparian vegetation: degradation, alien plant invasions and restoration prospects. Diversity & Distributions 13, 126–139. Rutherford, M.C., Mucina, L., Powrie, L.W., 2006. Biomes and bioregions of Southern Africa. In: Mucina, L., Rutherford, M.C. (Eds.), The Vegetation of South Africa, Lesotho and Swaziland. . Strelitzia, vol. 19. South African National Biodiversity Institute, Pretoria, pp. 31–51. Sieben, E.J.J., Q12 Reinecke, K., 2008. Description of reference conditions for restoration projects in riparian vegetation from the species-rich Fynbos Biome. South African Journal of Botany 74, 401–411 (this issue). doi:10.1016/j.sajb.2008.01.176. Swift, C., Jacobs, S., Esler, K.J., 2008. Drought induced xylem embolism in four riparian trees from the Western Cape Province: insights and implications for planning and evaluation of restoration. South African Journal of Botany 74, 508–516 (this issue). doi:10.1016/j.sajb.2008.01.169. Van Lill, W.S., Kruger, F.J., Van Wyk, D.B., 1989. The effect of afforestation with Eucalyptus grandis Hill ex Maiden and Pinus patula Schlecht Et Cham on stream flow from experimental catchments at Mokobulaan, Transvaal. Journal of Hydrology 48, 107–118. Van Wilgen, B.W., Cowling, R.M., Le Maitre, D.C., 1998. Ecosystem services, efficiency, sustainability and equity: South Africa's Working for Water programme. Trends in Ecology and Evolution 13, 378. Vosse, S., Esler, K.J.., Richardson, D.M., Holmes, P.M., 2008. Can riparian seed banks initiate restoration after alien plant invasion? Evidence from the Western Cape, South Africa. South African Journal of Botany 74, 432–444 (this issue). doi:10.1016/j.sajb.2008.01.170. Wali, M.K., 1999. Ecological succession and the rehabilitation of disturbed terrestrial ecosystems. Plant Soil 213, 195–220. Witkowski, E.T.F., Garner, K.J., 2008. Seed production, seed bank dynamics, resprouting and long-term response to clearing of the alien invasive Solanum mauritianum in a temperate to subtropical riparian ecosystem. South African Journal of Botany 74, 476–484 (this issue). doi:10.1016/j. sajb.2008.01.173. Woolsey, S., Capelli, F., Gonser, T., Hoehn, E., Hostmann, M., Junker, B., Paetzold, A., Roulier, C., Schweizer, S., Tiegs, S.D., Tockner, K., Weber, C., Peter, A., 2007. A strategy to assess river restoration success. Freshwater Biology 52, 752–769.
Edited by J Van Staden
K.J. Esler Centre for Invasion Biology, Department of Conservation Ecology and Entomology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa Corresponding author. Tel.: +27 21 8084005. E-mail address: [email protected]. P.M. Holmes City of Cape Town, Environmental Resource Management Department, Private Bag X5, Plumstead 7801, South Africa D.M. Richardson Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa E.T.F. Witkowski Restoration and Conservation Biology Research Group, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa 31 January 2008
South African Journal of Botany 74 (2008) 397 – 400 www.elsevier.com/locate/sajb
Editorial
Riparian vegetation management in landscapes invaded by alien plants: Insights from South Africa The important roles that intact riparian vegetation play in maintaining ecosystem health and services have been increasingly highlighted in the literature as we acknowledge the degradation of these “critical transition zones” (Ewel et al., 2001). Riparian corridors are naturally disturbance-mediated ecosystems but because they receive inputs of matter and propagules from such large areas, they accumulate, concentrate and exacerbate further human-induced impacts and disturbances. Therefore it is not surprising that they are amongst the most degraded ecosystems globally (Revenga et al., 2005; Dudgeon et al., 2006). Nel et al. (2007), in the first comprehensive assessment of South Africa's main river ecosystems (quaternary catchments) reported that an alarming 54% are critically endangered, with less than 20% of their total original extent intact. One important category of impacts to riparian ecosystems is that associated with plant invasions. Invasive alien plants thrive in frequently disturbed environments (Huston, 2004), and multiple opportunities exist for their establishment and spread as these linear corridors snake through the landscape. Whether invasives are drivers or passengers of change in degraded ecosystems (MacDougall and Turkington, 2005) is a moot point, but those species that significantly alter native composition, structure and function (termed “transformers” sensu Richardson et al., 2000) are now receiving considerable attention as we attempt to manage them and their negative impacts. Following global trends, invasive alien plants are becoming an increasingly large problem in South Africa where mounting evidence links invasive alien plant transformation to declines in ecosystem integrity and services (Richardson and Van Wilgen, 2004). Here, woody invasive riparian transformers have been linked to significant reductions in water supply (Van Lill et al., 1989; Le Maitre et al., 2002; Dye and Jarmain, 2004), leading to one of the world's largest initiatives to clear watersheds of invasive trees - the Working for Water programme (Van Wilgen et al., 1998). Working for Water (WfW), with its joint aims to enhance ecological integrity, water security and social development, has been operating since 1995 under the assumption that its target ecosystems, mostly riparian, would “self-repair” once the main stressor (dense stands of invasive alien trees) had been removed. This assumption has been largely untested until now, and is the focus of this special issue on riparian vegetation management and ecosystem repair in alien plant-invaded landscapes of South Africa.
Resilience, the ability (Wali, 1999) or time taken (Mitchell et al., 2000) for an ecosystem to recover to some acceptable structural or functional reference level, is a challenge to conceptualize in riparian zones because of their inherent dynamic nature (Richardson et al., 2007). Frequent disturbances in riparian ecosystems provide many opportunities for changes to arise in the diversity and relative abundance of component species. If invasive alien species are able to exert disproportional influence on other species, e.g. due to their ability to recruit prolifically and crowd out other species, they have the capacity to compromise resilience. Recovery is expected to be dependent on the type, size and periodicity of the disturbance, as well as the survival strategies of component species. A key question is: when is it necessary to implement restoration management interventions beyond the removal of alien plants? There are many angles to, and components of, this question, and any deliberation must address ecological, social and economic concerns (Aronson et al., 2007). It is interesting to note how few studies on restoration deal with all these aspects - a gap identified in the synthesis paper in this Special Issue, and indeed a gap in the issue itself (Holmes et al., 2008-this issue). There have been an increasing number of articles addressing river restoration ecology over the past twenty years (Nilsson et al., 2007). Of particular interest is a 2007 special feature on restoring riverine landscapes in the journal “Ecology and Society” (Nilsson et al., 2007). This contribution presents articles centred on the three basic themes of setting priorities, identifying techniques and identifying relevant reference conditions against which to gauge progress. Our Special Issue contains very much the same basic elements, while targeting a more focused sub-theme of repair after invasion. It is a culmination of work (plus some additional papers contributing to the theme) from a project commissioned by Working for Water on targets for ecosystem repair in alien-invaded riparian zones. The ultimate aim of this project was to produce guidelines and tools to improve management of these systems. These are summarized in the final paper in the Special Issue by Holmes et al. (2008-this issue). While riparian zones are proportionally small components of the landscape, the three biomes covered in this issue; viz. the Fynbos, Grassland and Savanna biomes make up 67% of the land surface of South Africa, Lesotho and Swaziland (Rutherford et al., 2006). Within all three of these biomes riparian zones are hugely important in fulfilling many important physical, chemical, biological and socio-economic roles (Table 1).
0254-6299/$ - see front matter © 2008 SAAB. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.sajb.2008.01.168
398
Editorial
Table 1 Functions and benefits of riparian vegetation Physical/chemical • Moderates stream temperatures and light levels, thereby functions: influencing habitat suitability for aquatic organisms • Maintains stream-bank integrity (roots) • Influences channel dynamics by contributing woody debris to streams • Aboveground stems of vegetation increase channel roughness during over-bank flow, thereby decreasing erosive action of floods and retaining material in transport • Retains water, thus increasing evapotranspiration and groundwater recharge • Regulates exchange of nutrients and organic material between streams and upland areas, and acts a sink for pollutants Biological • Source of habitat and nourishment for both vertebrates functions: and invertebrates • Harbours biotic diversity (aquatic and terrestrial) • Provides natural fire-breaks • Provides refuges for biota during unfavourable periods • Provides corridors for dispersal and movement of plants and animals Human-use • Provides food, water and other resources for people functions: (e.g., fuelwood and building material) and livestock (e.g., forage, shade) • Provides a place of recreation
Although the papers in this special issue touch on many complexities involved in the management of riparian zones in the face of escalating pressure from invasive species, we identify three critical themes that justify some introductory discussion: issues relating to the setting of priorities at different scales; refining methods for effective removal of the invasive species and subsequent treatments to steer recovery in desired directions; and the problem of reference sites. Each of these is discussed below. 1. Setting priorities Using an economic approach, Marais and Wannenburgh (2008-this issue) explore the extent of riparian alien clearing and analyse the actual WfW management data to investigate the costs and water benefits of this clearing countrywide. An estimated 7% of closed stand invasions in riparian zones have been cleared, yielding significant water yield increases. The cost of clearing high water-using alien species was found to be a good investment in comparison to other water resource options. From an ecological perspective, clearing scattered (b 5% cover) invasions was significantly cheaper than closed stands whilst safeguarding the water resource from future impacts of alien densification. However, short-term cost-efficiency in terms of generating water yield is generally higher for denser alien stands. The high cumulative costs of repeated follow-up treatments in 3% of the denser alien Acacia stands indicate potential viability of active restoration interventions at these sites. 2. Identifying techniques Largely adopting a comparative approach through space (Blanchard and Holmes, 2008-this issue; Vosse et al., 2008-this
issue) and time (Beater et al., 2008-this issue (10 yr); Morris et al., 2008-this issue (1 yr, before and after clearing); Reinecke et al., 2008-this issue (4 yr); Witkowski and Garner, 2008-this issue (10 yr)), and to a lesser extent an experimental approach (Behenna et al., 2008-this issue; Pretorius et al., 2008-this issue (8 yr)), most papers in this issue address the underlying question - are the current alien-clearing practices achieving the ecosystem repair goals set by WfW to restore indigenous riparian vegetation structure, diversity and function? In tackling this question, the aim was to identify best-practice techniques to ensure recovery after alien clearing (Holmes et al., 2008-this issue). A shortfall of the series of studies is that most of them took place over a relatively short time relative to natural ecosystem trajectories, making the assessment of recovery towards a reference site condition rather speculative, based only on early stages of succession. For a more long-term perspective, a monitoring framework is essential (Woolsey et al., 2007), but was found to be lacking in a 2002/3 review of the WfW programme (Holmes et al., 2003). This gap is currently being addressed by consultants appointed by WfW. Without active intervention, and given the short-time frame constraint noted above, recovery was partial and depended on how densely-invaded the systems were prior to clearing. While seed banks have the potential to initiate recovery (Fourie, 2008this issue; Vosse et al., 2008-this issue) in densely invaded Fynbos ecosystems of the Western and Eastern Cape, clearing techniques can either delay or promote recovery (Behenna et al., 2008-this issue; Blanchard and Holmes, 2008-this issue; Reinecke et al., 2008-this issue). In savanna and grassland ecosystems, largescale disturbances associated with flood events add complexity to indigenous recovery patterns as well as alien recruitment patterns (Foxcroft et al., 2008-this issue), thus requiring more strategic and focused management attention (Morris et al., 2008-this issue). On a worrisome note, the two longer-term studies in grassland and savanna systems indicated that after a decade of clearing, invasion had not been reduced; rather the nature of the problem had shifted from one dealing with a few large trees to one now dealing with a deadly cocktail of smaller Invasive Alien Plants (IAPs) occurring in larger numbers (Beater et al., 2008-this issue; Witkowski and Garner, 2008-this issue). Above all, the critical importance of careful follow-up clearing was stressed (Morris et al., 2008-this issue; Pretorius et al., 2008-this issue; Reinecke et al., 2008-this issue). The need for active restoration after clearing may be apparent shortly after clearing (Reinecke et al., 2008-this issue), but guidelines for large-scale, ecologically- and economicallyappropriate restoration techniques for various riparian systems are still lacking. The only restoration experiment reported in this issue was limited to a local scale and did not include economic considerations (Pretorius et al., 2008-this issue). Its strength is that it reports on a time sequence, with data indicating that indigenous sowing treatments can succeed in improving native species richness and abundance providing sufficient follow-up control is maintained. Targeted planting might well be an option where key species are missing from the seed bank, but to optimize success, detailed knowledge of their ecology and function is needed, such as that provided by Swift et al. (2008-this issue). Finally, the focus on techniques in this issue was largely at the biotic level, with no
Editorial
emphasis on abiotic-level restoration such as control of erosion or alteration of geohydrology. Attempts at restoration without such interventions are futile in certain cases (Richardson et al., 2007). 3. The problem of identifying reference sites When assessing or monitoring the success or failure of a particular intervention such as alien clearing and restoration, a key challenge is to identify sites against which one can compare the treatment. It is natural to consider “pristine” sites with similar environmental attributes as good reference sites. In many areas, degradation is so pervasive that such sites no longer exist. Even where well-matched reference sites can be found, riparian systems are so dynamic that they are always in some trajectory of postdisturbance recovery, which greatly complicates comparisons between systems (Richardson et al., 2007) Nonetheless, largelyuninvaded, relatively-intact sites do exist in some parts of South Africa, and detailed work to describe reference sites such as that by Sieben and Reinecke (2008-this issue) in mountain streams of the Fynbos Biome provide some idea of a baseline state, even if it is limited to certain habitats. In this issue, Blanchard and Holmes (2008-this issue) and Vosse et al. (2008-this issue) effectively use reference sites to compare above- and below-ground (seed bank) treatments and to identify appropriate management interventions after alien clearing. Carefully selected comparative studies like these, in other parts of the country would help to improve our understanding of the dynamics of these systems. Given the critical role that riparian vegetation plays in maintaining ecosystem function and services, the degree to which these systems have been invaded and degraded and the priority given to them by WfW, we hope that this Special Issue will stimulate further research into restoration of riparian zones. Acknowledgements The authors of papers in this issue for their contributions and the many reviewers for their role in improving the manuscripts. Financial support from the DST-NRF Centre for Invasion Biology and the Department of Water Affairs and Forestry, in collaboration with Working for Water (Targets for Ecosystem Repair in Riparian Ecosystems in Fynbos, Grassland and Savanna Biomes) is gratefully acknowledged. References Aronson, J., Milton, S.J., Blignaut, J.N., 2007. Restoring Natural Capital: Science, Business, and Practice. Island Press, Washington. Beater, M.M.T., Garner, R.D., Witkowski, E.T.F., 2008. Impacts of clearing invasive alien plants from 1995 to 2005 on vegetation structure, invasion intensity and ground cover in a temperate to subtropical riparian ecosystem. South African Journal of Botany 74, 495–507 (this issue). doi:10.1016/j. sajb.2008.01.174. Behenna, M., Vetter, S., Fourie, S., 2008. Viability of alien and native seed banks after slash and burn: effects of soil moisture, depth of burial and fuel load. South African Journal of Botany 74, 454–462 (this issue). doi:10.1016/j. sajb.2008.01.179. Blanchard, R., Holmes, P.M., 2008. Riparian vegetation recovery after invasive alien tree clearance in the fynbos biome. South African Journal of Botany 74, 421–431 (this issue). doi:10.1016/j.sajb.2008.01.178.
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Dudgeon, D., Arthington, A.H., Gessner, M.O., Kawabata, Z., Knowler, D.J., Lévêque, C., Naiman, R.J., Prieur-Richard, A., Soto, D., Stiassny, M.L.J., Sullivan, C.A., 2006. Freshwater biodiversity: importance, threats, status and conservation challenges. Biological Review 81, 163–182. 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, 40–45. Ewel, K.C., Cressa, C., Kneib, R.T., Lakes, P.S., Levin, L.A., Palmer, M.A., Snelgrove, P., Wall, D.H., 2001. Managing critical transition zones. Ecosystems 4, 452–460. Fourie, S., Blignaut, J.N., 2008. Composition of the soil seed bank in alien-invaded grassy fynbos: potential for recovery after clearing. South African Journal of Botany 74, 445–453 (this issue). doi:10.1016/j.sajb.2008.01.172. Foxcroft, L.C., Parsons, M., McLoughlin, C.A., Richardson, D.M., 2008. Patterns of alien plant distribution in a river landscape following an extreme flood. South African Journal of Botany 74, 463–475 (this issue). doi:10.1016/j.sajb.2008.01.181. Holmes, P.M., Euston-Brown, D., Richardson, D.M., 2003. External evaluation of the Working for Water programme. Terrestrial ecology theme. Report to Common Ground Consulting. Cape Ecological Services/Institute for Plant Conservation, Cape Town. Holmes, P., Esler, K.J., Richardson, D., Witkowski, E., 2008. Guidelines for improved management of riparian zones invaded by alien plants in South Africa. South African Journal of Botany 74, 538–552 (this issue). doi:10.1016/j.sajb.2008.01.182. Huston, M.A., 2004. Management strategies for plant invasions: manipulating productivity, disturbance, and competition. Diversity and Distributions 10, 167–178. Le Maitre, D.C., Van Wilgen, B.W., Gelderblom, C.M., Bailey, C., Chapman, R.A., Nel, J.A., 2002. Invasive alien trees and water resources in South Africa: case studies of the costs and benefits of management. Forest Ecology and Management 160, 143–159. MacDougall, A.S., Turkington, R., 2005. Are invasive species the drivers or passengers of change in degraded ecosystems? Ecology 86, 42–55. Marais, C., Wannenburgh, A.M., 2008. Restoration of water resources (natural capital) through the clearing of invasive alien plants from riparian areas in South Africa - costs and water benefits. South African Journal of Botany 74, 526–537 (this issue). doi:10.1016/j.sajb.2008.01.175. Mitchell, R.J., Auld, M.H.D., Le Duc, M.G., Marrs, R.H., 2000. Ecosystem stability and resilience: a review of their relevance for the conservation management of lowland heaths. Perspectives in Plant Ecology, Evolution and Systematics 3, 142–160. Morris, T.L., Witkowski, E.T.F., Coetzee, J.A., 2008. Initial response of riparian plant community structure to clearing of invasive alien plants in Kruger National Park, South Africa. South African Journal of Botany 74, 485–494 (this issue). doi:10.1016/j.sajb.2008.01.177. Nel, J.L., Roux, D.J., Maree, G., Kleynhans, C.J., Moolman, J., Reyers, B., Rouget, M., Cowling, R.M., 2007. Rivers in peril inside and outside protected areas: a systematic approach to conservation assessment of river ecosystems. Diversity & Distributions, 13, 341–352. Nilsson, C., Jansson, R., Malmqvist, B., Naiman, R.J., 2007. Restoring riverine landscapes: the challenge of identifying priorities, reference states, and techniques. Ecology and Society 12, 16 [Online] URL: http://www. ecologyandsociety.org/vol12/iss1/art16/. Pretorius, M., Esler, K.J., Holmes, P.M., Prins, N., 2008. The effectiveness of active restoration following alien clearance in fynbos riparian zones and resilience of treatments to fire. South African Journal of Botany 74, 517–525 (this issue). doi:10.1016/j.sajb.2008.01.180. Reinecke, M.K., Pigot, A., King, J.M., 2008. Spontaneous succession of riparian fynbos: is unassisted recovery a viable restoration strategy? Ecological Economics 74, 412–420 (this issue). doi:10.1016/j.sajb.2008. 01.171. Revenga, C., Campbell, I., Abell, R., De Villiers, P., Bryer, M., 2005. Prospects for monitoring freshwater ecosystems towards the 2010 targets. Philosophical Transactions of the Royal Society 360, 397–413. 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.
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Edited by J Van Staden
K.J. Esler Centre for Invasion Biology, Department of Conservation Ecology and Entomology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa Corresponding author. Tel.: +27 21 8084005. E-mail address: [email protected]. P.M. Holmes City of Cape Town, Environmental Resource Management Department, Private Bag X5, Plumstead 7801, South Africa D.M. Richardson Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa E.T.F. Witkowski Restoration and Conservation Biology Research Group, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa 31 January 2008