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Medium-term vegetation recovery after removal of invasive Eucalyptus camaldulensis stands along a South African river
Abstract
Effective ecological restoration requires detailed monitoring to determine the success achieved through different interventions in achieving objectives. In 2017, we resurveyed riparian sites along the Berg River in the Western Cape, South Africa, that have been cleared of invasive stands of Eucalyptus camaldulensis in 2010 using two clearing methods (fell-and-stackburn and fell-and-remove) and two restoration approaches: passive (where vegetation was allowed to recover without intervention) and active (assisted recovery). A significant increase in vegetation cover (P b .001) and diversity (P b .05) of native riparian species was recorded in passive restoration plots, but an increase in the cover of woody invasive alien plants was also observed. Only four of the nine native species that were planted to fast-track restoration were still present in the active restoration plots, but the abundance of these native species was significantly (P b .001) lower in 2017 than in 2011. We conclude that native vegetation recovery following E.camaldulensis removal seven years ago is following a positive recovery trajectory in both passive and active restoration sites, as shown by the increased occurrence of native trees and shrubs, e.g. Maytenus oleoides, Melianthus major and Searsia angustifolia which were not present before clearing. However, the reinvasion of cleared sites by woody invasive alien plants has the potential to slow down and potentially halt the recovery process. Further management interventions, e.g. removal of reinvading woody invasive alien plants, are required, emphasizing the sustained engagement to ensure restoration in these ecosystems.
... Generally, the programme is regarded as a success, although several challenges have been reported, e.g., having dual objectives, which negatively affect budget prioritisation, inefficiencies associated with having multiple country-wide projects, lack of restoration goals post alien plant clearing, and funding limitations [9,10]. Besides the above-mentioned challenges, little is known regarding how cleared areas recover post alien plant clearing by WfW [11]. Although few studies have been conducted in South Africa to assess ecosystem recovery after invasive alien plant removal by WfW, some studies have shown little vegetation recovery due to secondary invasion and a lack of native species soil seed banks [11,12], whereas others have reported a positive vegetation recovery trajectory [11,13,14]. ...
... Besides the above-mentioned challenges, little is known regarding how cleared areas recover post alien plant clearing by WfW [11]. Although few studies have been conducted in South Africa to assess ecosystem recovery after invasive alien plant removal by WfW, some studies have shown little vegetation recovery due to secondary invasion and a lack of native species soil seed banks [11,12], whereas others have reported a positive vegetation recovery trajectory [11,13,14]. Most of the above-mentioned studies were shortterm monitoring studies that were conducted less than five years after the initial alien plant clearing, thus failing to give a clear picture of the restoration trajectory. ...
... Besides the above-mentioned challenges, little is known regarding how cleared areas recover post alien plant clearing by WfW [11]. Although few studies have been conducted in South Africa to assess ecosystem recovery after invasive alien plant removal by WfW, some studies have shown little vegetation recovery due to secondary invasion and a lack of native species soil seed banks [11,12], whereas others have reported a positive vegetation recovery trajectory [11,13,14]. Most of the above-mentioned studies were shortterm monitoring studies that were conducted less than five years after the initial alien plant clearing, thus failing to give a clear picture of the restoration trajectory. ...
A great deal of effort has been made to clear invasive alien plants in South Africa, yet it remains unclear if the clearing efforts are yielding positive soil and vegetation recovery trajectories. A few short-term studies have been conducted to monitor soil and vegetation recovery after alien plant removal in South Africa, but convincing, long-term monitoring studies are scarce yet needed. We investigated topsoil and vegetation recovery following Eucalyptus grandis removal 14 years ago by Working for Water in Makhanda, Eastern Cape province of South Africa. The detailed topsoil and vegetation surveys were conducted on forty 10 m × 10 m plots that were in paired cleared and natural sites. The results show no significant differences for the measured soil pH, total N, total C, K, Ca, and Na between the cleared and natural sites, an indication that the two sites are becoming similar. Similarly, the gravimetric soil moisture content shows no significant differences between the two sites, although monthly variations are observed. The topsoils in the cleared sites are hydrophobic as compared to those in the natural sites, which are wettable. We observed no significant vegetation diversity differences between the two sites, with native woody species, such as Crassula pellucida and Helichrysum petiolare, frequently occurring in the cleared sites. We recorded low reinvasion by E. grandis and other secondary invaders like Acacia mearnsii and Rubus cuneifolius in the cleared sites. Based on these results, we conclude that 14 years after E. grandis clearing, both topsoil and vegetation recovery are following a positive trajectory towards the natural sites. However, both reinvasion and secondary invasion have the potential to slow down soil and native vegetation recovery. Recommendations such as timeous follow-up clearing and incorporating restoration monitoring in the WfW clearing programme are discussed.
... Monitoring of ecological restoration initiatives is essential for investigating trajectories to recovery, thereby providing crucial information for adaptive management to direct succession as required (Prach et al., 2007). Unfortunately, most restoration projects have been characterised by a lack of such long-term monitoring (Ruwanza et al., 2018). This study assesses the vegetation recovery more than a decade after the initial restoration treatments, seeks to determine whether the additional investment in active restoration was justified, and assesses the scale of the invasive species reinvasion. ...
... It also provides justification for intensifying ecological restoration efforts, which are mostly constrained due to resource limitations. This and other studies clearly show that a crucial step in ecological restoration of fynbos is to stop the reinvasion of the restored sites while the fynbos vegetation slowly recovers following initial clearing (Mucina et al. 2017;Ruwanza et al., 2018;Froeschlin et al. 2022). ...
... For example, introducing fast-growing and drought-resistant pioneer species first, to potentially suppress alien plant regrowth, and adding missing elements such as serotinous proteoid shrubs (which rely on canopy-stored seed and are often lost during longduration plant invasions) later. This form of active restoration has the potential to increase native diversity and reduce problems with competition for resources with alien and native species (Ruwanza et al., 2018). It would be useful to conduct another survey after the next fire on these sites as there has been no fire since the original restoration in 2008. ...
In 2019 we re-surveyed a site on the Agulhas Plain, South Africa, which had been part of a restoration project conducted in 2008. The site was invaded by invasive trees, mainly Australian Acacia species. In the original restoration program, the site had been burned and various treatments (i.e. sowing of competitive, fast-growing native species) were conducted to determine the effectiveness of different restoration strategies. The sowing of native species to assist the recovery process is defined as an active restoration method, whereas passive restoration only removes stressors for the ecosystem (e.g. removal of the invader) and leaves it to recover by itself. For this study, we conducted a vegetation survey at the previously restored sites and compared them with a nearby reference site where no intervention had taken place and which is characterized by near-pristine native fynbos vegetation. Secondly, we compared the new data from 2019 with older data from 2010. However, a decade after the initial restoration effort, both active and passively treated plots had not achieved the same condition (species richness, diversity, evenness) as the reference site. The results also show that active restoration is most likely unnecessary in fynbos restoration projects where there are nearby patches with native vegetation or the existing native seed bank is of sufficient size. It seems that it is more important to prevent the reinvasion of invasive alien plants so that the vegetation can recover over time without further disruption.
... The assessment of invasive alien plant management initiatives requires monitoring over timescales appropriate for evaluating the effectiveness of interventions (Ruwanza et al., 2018). Monitoring of ecological restoration initiatives is essential for investigating trajectories to recovery, thereby providing crucial information for adaptive management to direct succession as required (Prach et al., 2007). ...
... This shows that invasive species still pose a threat on the restored site. Ruwanza et al. (2018) also observed a larger native plant cover and a smaller alien plant cover on their reference sites compared to their restored sites after removal of invasive Eucalyptus trees along a Western Cape river. ...
... They reported a larger number of Acacias on the passively restored site than on the actively restored site. Ruwanza et al. (2018) obtained similar results in their study after the clearing of invasive Eucalyptus trees along a South African river. ...
Alien plant invasions are increasing in extent worldwide and diverse measures are being applied to reduce impacts. Besides removing the invasive plants, follow-up measures are often needed to restore diversity and functionality of invaded ecosystems. Effective restoration requires long-term commitment and monitoring to determine the success achieved through different measures.
In 2019 we resurveyed a site in South Africa's Cape Floristic Region (CFR) which had been used as a Eucalyptus plantation, which consisted of a heterogenous mixture of Eucalyptus conferruminata, E. cladocalyx and E. gomphocephala interspersed with Acacia saligna and Paraserianthes lophantha. It was restored twelve years prior to our survey using “active” (clearing followed by sowing of native species and soil restoration treatments) and “passive” (clearing and burning, only) approaches in separate plots. The last survey that monitored recovery took place in 2010. The aim of our study was to determine how the restored fynbos vegetation had changed since 2010. In 2019 we conducted a vegetation survey on the restored sites and on a reference site with native fynbos vegetation. We tested whether invasive species had been successfully controlled and whether active and passive restoration approaches differed in effectiveness.
Although diversity, evenness and species richness for native taxa had increased significantly since 2010, the decrease in alien species cover was insignificant. Compared to the restored site, the reference site had a significantly greater cover of native plants, and higher species richness, diversity and evenness. There were also significantly more alien species with a significantly greater cover on the restored site, implying that further attention is required to maintain the site free of invaders. The passively restored plots were more diverse than the actively restored plots, largely due to the dominance of one of the sown native species (Searsia laevigata) on the actively restored plots. The vegetation in passively restored plots was dominated by weedy native grasses.
We conclude that the restoration was mostly successful in terms of re-establishing native species diversity, but that there is room for improving restoration techniques. The tenacious invader A. saligna still occurs on restored sites and ongoing efforts will be needed to eliminate this species.
... However, evaluation is usually complicated by the absence of a pristine reference site to measure restoration success against (Prach et al., 2007). Other than largely shortterm (but see Ruwanza et al., 2018) and vegetation studies (e.g. Galatowitsch and Richardson, 2005;Blanchard and Holmes, 2008;Ruwanza et al., 2013a;Kerr and Ruwanza, 2016;Ndou and Ruwanza, 2016;Fill et al., 2018), longer term studies and those that quantify the recovery of other taxa become more common (Golet et al., 2008;Heleno et al., 2010;Magoba and Samways, 2010;Samways et al., 2011; https://doi.org/10.1016/j.actao.2019.103483 ...
... A dearth of knowledge exists regarding the historical communities of the Berg River system and here we use the near-pristine sites as an alternative and pragmatic target community for restoration. The partial restoration of native plant assemblages several years after clearing is similar to Ruwanza et al. (2018) who also reported an incomplete recovery in native plant diversity after seven years following clearing of E. camaldulensis. Although plant and bird assemblages recover to levels comparable to those of near-pristine sites, ten native plant species (37%) and eight bird species (19%) were absent in cleared sites. ...
... Natural recovery of native plant species, through native soil-stored seed banks and recruitment from remnant natural vegetation saves costs and is desirable for any restoration project (Blanchard and Holmes, 2008;Morris et al., 2008). However, if conditions for germination have become unsuitable for some species or no dispersal from nearby areas is possible (Galatowitsch and Richardson, 2005;Holmes et al., 2005;Ruwanza et al., 2013a), secondary invasions will make up a large component of the vegetation as those aliens are better suited to establish after the disturbance on cleared sites (Ruwanza et al., 2018). ...
... For example, inadequate attention is often given to follow-up operations after initial clearing, due to logistical and project-management challenges. Such cases have led to regrowth and re-establishment of invasive E. camaldulensis populations (Figure 4 h), or other woody secondary invaders, requiring additional felling which is expensive and may cause substantial damage to physical properties of the riparian habitats (Geldenhuys and Bezuidenhout, 2008;Ruwanza et al., 2018). Because invasive stands of E. camaldulensis form dense forests of large trees, mechanical clearing requires the use of heavy machinery; this often leads to soil erosion and the destruction of understorey native vegetation, both of which hamper vegetation rehabilitation (Ruwanza et al., 2018). ...
... Such cases have led to regrowth and re-establishment of invasive E. camaldulensis populations (Figure 4 h), or other woody secondary invaders, requiring additional felling which is expensive and may cause substantial damage to physical properties of the riparian habitats (Geldenhuys and Bezuidenhout, 2008;Ruwanza et al., 2018). Because invasive stands of E. camaldulensis form dense forests of large trees, mechanical clearing requires the use of heavy machinery; this often leads to soil erosion and the destruction of understorey native vegetation, both of which hamper vegetation rehabilitation (Ruwanza et al., 2018). Debris from mechanical clearing (trunks, stems, and branches) remaining after such operations can also cause damage to the river channel, even at sites far from where clearing was carried out (Geldenhuys and Bezuidenhout, 2008). ...
... A study by Tererai et al. (2015b) along the Berg River in the Western Cape showed that the soil-stored seed banks in the ecosystem invaded by river red gum still contained sufficient seeds of native species to provide a modest source to facilitate regeneration. Nevertheless, Tererai et al. (2015b) and Ruwanza et al. (2018) also stress that secondary invasions are likely to counteract a re-establishment of natural riparian forest vegetation; careful management is needed to deal with this problem. ...
Eucalyptus camaldulensis can be seen as an iconic tree of superlatives. It is the eucalypt with the widest native range, and one of the most widely planted eucalypts around the globe. In South Africa, it is the most widespread and the most aggressively invasive eucalypt. It has many uses, but also causes major impacts. However, little is known about key aspects of its ecology in South Africa, including its invasion history, invasion processes and dynamics, and people’s perceptions of its positive and negative effects on ecosystems. Such knowledge is crucial for developing robust and defendable guidelines for sustainable management of the species. This paper provides a comprehensive dossier of the species in South Africa. It reviews what is known of its introduction and planting history, its current distribution, its value for commercial forestry and other uses, its impacts as an invasive species, pests and pathogens associated with the species, people’s perceptions of the species and conflicts of interest, and the options for management and restoration. The review reveals that E. camaldulensis is a tree of many contradictions in South Africa, making it a poster-child example of a conflict-generating non-native species. Based on available knowledge, we assess options for improved management. We highlight several knowledge gaps which need to be addressed in more detail through future research. It is hoped that this species profile will serve as a model for the types of information that are needed for developing objective management strategies for non-native tree species in different parts of the world.
... Furthermore, conditions at the forest edge have been found to be more heterogeneous compared to the interior (Ewers and Didham 2006). Previous studies have shown that forest edges influence woody plant species richness and diversity in different ecosystems including in South Africa (Ruwanza et al. 2018), Tanzania (Kacholi 2014) and Brazil (Fontoura et al. 2006;Sampaio and Scariot 2011). However, few studies have investigated the herbaceous layer diversity in response to created edges (Liepa et al. 2020). ...
Since the type of forest influences vegetation patterns from the edge-to-interior forest, site-specific edge studies are needed but there have been few studies in open-canopied forests such as oak savannahs. Our objective was to compare patterns of herbaceous plant diversity along the forest edge-to-interior gradient between open areas and underneath oak trees in the Zagros Forest in Iran. We established eighteen transects from the forest edge to the interior in small and large forest fragments to sample herbaceous species in five 0.25 m² quadrats at 1 m intervals from the base of the tree to the open area at different distances from the forest edge. We analyzed the data using randomization tests for edge influence and generalized linear mixed models. Edge influence had a positive effect on herbaceous species richness and diversity underneath oak trees but a negative effect in open areas. At forest edges, species richness and diversity significantly decreased from the tree base toward open areas, but exhibited the opposite pattern away from the edge. Edge influence extended up to 50 m from the forest edge to the interior. Our findings highlight the importance of considering forest type and stand heterogeneity when studying edge influence on plant diversity. Our results show that edge studies are needed for specific forest types, particularly in heterogeneous landscapes, to ensure appropriate conservation of species diversity. We recommend establishing a 50-m buffer zone along edges in the Zagros Forest in Iran to minimize negative edge influence on herbaceous plant diversity.
... The reason why we mostly have annual graminoids and forbs is probably that their seeds easily spread from surrounding agricultural areas (Table 1). In contrast, Ruwanza et al. (2018) found that although alien herbs and graminoids dominated soon after clearing of Eucalyptus trees (Ruwanza et al., 2013), over time, these were replaced by woody invasive alien plants due to the lack of follow-up clearing. However, the high graminoid component after alien tree clearing might differ depending on the alien invasive tree species being cleared. ...
After the clearing of invasive alien plants, non-target alien species often move in. This is known as secondary invasion and these alien species can suppress native plant species’ recovery. This is a global problem that has only recently received attention in South Africa. But whether secondary invaders in riparian areas have similar effects than those in more terrestrial areas is largely unknown globally and in South Africa. Therefore, here we ask which species and growth forms are associated with secondary invasion in riparian areas in the Fynbos biome, and secondly whether secondary invader species richness changes over time post clearing. We found 13 secondary invader species in a riparian area after clearing of alien invasive Eucalyptus trees. In the first year after clearing, there were only annuals, but thereafter it was a mix of annuals and perennials (shrubs, herbs, and graminoids). Secondary invader richness did not significantly differ with time after clearing, persisting even up to 10 years. In this riparian area, secondary invader species richness was much lower than that for non-riparian Fynbos areas. From this preliminary study we highlight the need for more research in understanding how secondary invaders influence restoration efforts and native species recovery in riparian systems.
... Riparian zones are more commonly invaded by acacia species (e.g. Acacia longifolia, A. mearnsii; Holmes et al., 2008) and in the lowland river segments also by eucalypts (especially Eucalyptus camaldulensis; Tererai et al., 2013;Ruwanza et al., 2018;Hirsch et al., 2020). Eucalypts are profligate water-users and alter local conditions to create situations of water-repellency and allelopathy in soils, which negatively impacts the ability of native species to persist or establish (Ruwanza et al., 2013;Ruwanza et al., 2015). ...
Ecological restoration is a global imperative to reverse widespread habitat loss and degradation, including by invasive alien plants. In South Africa’s Core Cape Subregion, alien tree invasions are widespread and their control continues to be a major undertaking. As funding is limited, active restoration interventions are rarely implemented and the focus is on invader removal – the assumption being that ecosystems will self-repair. This paper reviews research findings from the past three decades to assess in which situations spontaneous succession is a viable strategy for restoring alien-invaded ecosystems. We found that ecosystems can self-repair, provided that key biotic and/or abiotic thresholds have not yet been crossed. Self-repair has been observed in many cases where dense invader stands with short residence times have been cleared and where diverse native plant growth forms survive, either in the above-ground vegetation or in soil seed banks. However, several factors influence this generalisation, including the identity of the invader, the ecosystem type, and the efficacy of alien control. Thresholds are crossed sooner with invasions of alien Acacia and Eucalyptus species than those of Hakea and Pinus species, resulting in lower potential for spontaneous recovery. Lowland fynbos ecosystems are less resilient to invasion, and have a lower capacity for self-repair, than mountain fynbos ecosystems. Poorly implemented alien plant control measures can result in a resurgence of the invader to the detriment of native species recovery. We outline some management principles for optimising spontaneous succession potential and integrating alien control and restoration interventions.
... 19, for similar arguments). Early field results (2-3 years post-intervention) may not accurately reflect the restoration trajectory as measured in subsequent years (Ruwanza et al. 2018). More long-term studies are required that build on, and monitor, earlier shortterm field studies to further analyse the impacts of restoration interventions and to improve restoration guidelines. ...
Invasive alien plant species can be a major cause of ecosystem degradation in South Africa, and ecosystem recovery may require restoration interventions beyond controlling the target alien species. Active restoration interventions are usually required if legacy effects result from the invasion. Legacy effects may induce regime shifts when thresholds to autogenic recovery are breached. In such cases, active restoration interventions will be required to manipulate the ecosystem along a trajectory to recovery. In some cases, alien control measures may be sufficient to restore a structurally and functionally representative ecosystem, provided that implementation occurs early in the invasion process and that the control methods do not hamper spontaneous regeneration. It is important that key stakeholders discuss and set realistic restoration goals at the project planning stage. Studies on the costs and benefits of ecological restoration indicate that when important services are improved, benefits outweigh the costs of alien clearing (assuming spontaneous regeneration of the native ecosystem). The costs of moderate, active restoration interventions are economically viable, whereas the costs of fully restoring ecosystem structure, functioning and composition in highly degraded ecosystems are rarely deemed economically justifiable. Valuations of specific biodiversity components, such as threatened ecosystems and species, remain problematic to assess, and these components could be under-valued in such studies. South African researchers have made significant contributions to the theory and practice of restoration ecology globally and have produced local guidelines for ecological restoration. However, there has been limited uptake in implementing active restoration projects at larger scales. This apparent knowing-doing gap may have three causes: firstly, insufficient co-production by all stakeholders in planning restoration projects, including prioritisation and goal setting; secondly, shifting beyond clearing invasive alien species to restoring ecosystems; and thirdly, insufficient resources to implement active restoration projects at the necessary scale. To achieve Convention on Biological Diversity and the UN Sustainable Development Goals, interventions must shift from controlling invasive alien species alone to restoring native ecosystem structure and functioning.
... Managers frequently monitor target weed responses to management as a surrogate for native asset responses that remain unmeasured (Reid et al. 2009;Foster et al. 2019), and financial constraints often preclude monitoring beyond one or a few growing seasons (Kettenring and Adams 2011). Similarly, reviews and meta-analyses of invasive plant control research have found that while most studies measured the effect of management on the target invasive species, far fewer also assessed the response of native plant species (estimates range from 19% to 30%) (Reid et al. 2009, Kettenring andAdams 2011; for a recent example of native species response over the medium term see Ruwanza et al. 2018). Emphasis on the response of the target invasive species commonly finds expression in project operational milestones, for example area of weed infestation cleared, with an assumption that weed removal benefits resident biodiversity. ...
A substantial body of work underlies the theory and practice of early intervention in the management of invasive alien plants, but less attention has been paid to the strategic management of widespread weeds, especially in the context of natural asset recovery. The assumption lingers amongst some researchers and land managers that removing weeds will automatically lead to positive biodiversity outcomes, with the more weed removed, the better the outcome. However, this is often not the case, particularly for long-established weed species whose dominance has created impoverished communities with little capacity for passive recovery. A common result may be wasted investment in weed control and, in the extreme, net negative impacts upon asset values. We present a conceptual model for the management of weed-impacted assets, plus guidance for its application, with a view to improving asset recovery practice. Weed removal should be calibrated by asset recovery, which may mean not seeking to completely remove a weed at a given spatial scale. Our model focusses on weed removal that is enough to initiate asset recovery, but not more than is necessary to promote maximum expression of asset resilience, particularly in the context of secondary invasions. Optimal management efficiency will involve a proportional allocation of resources to control, monitoring and revegetation activities that is appropriate to the stage of asset recovery, as well as a willingness to revise a management goal if the original one cannot be achieved within existing constraints on resources.
Following the control of an invasive tree species, native forest ecosystem restoration can be prevented by a secondary invasion by another invasive species that inhibits the reestablishment of native trees. In this study, we evaluated the effects of the invasive shrub Leucaena leucocephala on the initial growth of planted seedlings of four native tree species ( Pandanus boninensis , Rhaphiolepis indica , Schima mertensiana , and Terminalia catappa ) after eradication of the invasive tree Casuarina equisetifolia in the Ogasawara Islands in the northwestern Pacific Ocean. In a 24‐month field experiment, we compared the sizes of planted seedlings (diameter at ground level, height, and leaf number) among four treatments in a 2 × 2 design of initial seedling size (large vs. small) × L. leucocephala invasion (sowing L. leucocephala seeds vs. control). More than 75% of the seedlings (excluding S. mertensiana ) survived regardless of the treatments. For all four species, initially large seedlings tended to become larger plants at the end of the experiment than initially small seedlings. We detected significant effects of L. leucocephala invasion on one or more plant size parameters in all four species. Seedlings grown in L. leucocephala invasion quadrats were smaller than those in control quadrats at the end of the experiment. We detected significant interactions between initial seedling sizes and L. leucocephala invasion for three species. Together, these results indicate that secondary invasion by L. leucocephala after controlling C. equisetifolia inhibits the initial growth of native tree seedlings, depending on their size at the time of invasion.
Invasive alien plants negatively impact ecosystems, necessitating intricate management actions. In a critically endangered vegetation type within the fynbos biome of South Africa, a study was performed comparing different management interventions over plots invaded by Acacia saligna. A dynamic modelling approach was designed to analyze field data and simulate effectiveness of several restoration methods. Field data for vegetation recovery rates over the course of two years were fed into the model, which allowed the extrapolation of multiple recovery trajectories over a long time‐span, not possible to obtain from traditional short‐term field surveys. Our model simulations show that different treatments in similarly degraded states at the time of clearing can result in vastly different recovery trajectories. Active seed sowing was initially most expensive but resulted in most successful native shrub recovery, decreasing costs of longer‐term follow‐up acacia clearing. Clearing without burning was cheapest but resulted in limited establishment of both native and acacia cover, providing an opportunity for secondary invasion by alien forbs. In this case, biotic thresholds may have been crossed which prevented recovery of certain vegetation components. Active sowing can partially reverse thresholds by restoring shrub cover but not structural diversity. Therefore even applying this treatment did not resemble vegetation structure of the reference condition after an extended period of 30 years, but does show how restoration can be improved by native seed sowing compared to passive restoration alone. Our model simulations provide a useful tool to support decision‐making by providing management recommendations for optimizing alien plant clearing protocols.
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South African terrestrial ecosystems are invaded by hundreds of alien plant species, and large‐scale clearing based on the passive restoration assumption that cleared areas will recover unaided is underway. This study assessed the recovery of vegetation and soil properties, three years following Eucalyptus grandis clearing using fell‐and‐removal and fell‐and‐stackburn methods at Zvakanaka Farm in Limpopo Province, South Africa. The main aim was to ascertain the extent of vegetation and soil recovery, as well as determining which clearing methods facilitate passive vegetation and soil restoration. Results indicate significantly ( p < 0.001) lower native species diversity, cover and composition in cleared than in uninvaded sites. However, the recorded low species diversity and composition in cleared sites were more pronounced in the fell‐and‐stackburn than in the fell‐and‐removal sites. Measured soil physical properties varied, with compaction being higher in fell‐and‐removal, whereas soils were more repellent in fell‐and‐stackburn sites. The study concludes that vegetation and soil recovery, following E. grandis clearing, is complex and involves several interacting factors, which are linked to invasion history and intensity. Therefore, for vegetation and soil properties to recover, following E. grandis removal, the clearing programme should consider active restoration techniques, for example soil manipulation and native plant seeding.
Urban areas have unique assemblages of species which are governed by novel ecological processes. People living in these environments have specific needs and demands in terms of ecosystem services (ES). Urban ecosystems are transformed in many ways by human activities and their floras comprise a high proportion of alien plant species, many of which were intentionally introduced to provide, augment or restore ES. Urban environments also have novel disturbance regimes and provide colonization sites for the establishment, dispersal and proliferation of alien plant species; such conditions often generate biological invasions which may cause marked changes to ES. We review the roles that alien plants play in providing urban ES and ecosystem disservices (EDS) globally. We identify the main ES and EDS associated with alien plants, and highlight the key species involved. A literature search revealed 335 papers, representing studies in 58 cities or urban areas in 27 countries. These studies recorded 337 alien plant species, contributing to 39 different ES and 27 EDS–310 species were recorded s contributing to ES and 53 species to EDS. A small number of alien plant taxa were frequently recorded as providing multiple ES in many urban ecosystems; the 10 most recorded species accounted for 21% of the ES recorded. Some of these species also result in significant EDS; three species accounted for 30% of the EDS recorded. Cultural services (notably aesthetics) are the most reported ES provided by alien plants in urban areas of developed countries, while provisioning services (notably food production) are most reported in developing countries. The most commonly studied EDS provided by alien plants is the impact on human health (notably allergic reactions). Eighty percent of studies on alien plants and ES and EDS have been done in developed countries. To elucidate the full range of effects of alien plants, more work is needed in developing countries. Urban planners and managers need to be mindful of both the positive and negative impacts of alien plant species to maximise the provision of ES.
The Working for Water programme simultaneously promotes conservation and poverty relief through alien plant control projects. We trace the programme's history and review the factors that led to its success. These included a sound scientific grounding, a clear demonstration that the plants are a serious threat to vital and scarce water resources, and a unique opportunity presented by South Africa's transition to democratic government. The programme built on historical precedents for control, and was able to capitalise on a core of dedicated managers that delivered a good-news story, leading to increased funding. The programme has facilitated advances in biological control, raised levels of awareness, enacted legislation, and promoted research. However, it has only treated a relatively small proportion of the estimated invaded area, and assessments of progress towards ecosystem-scale outcomes cannot be made as they are not monitored. The need to operate in a bureaucratic environment, an emphasis on job-creation and relative neglect of environmental goals, and high levels of political interference are significant obstacles to progress.
High intensity wildfires in semiarid shrub and woodland plant communities can leave ecosystems incapable of self-repair and susceptible to weed invasion. Subsequently, land managers need effective restoration tools to reseed native vegetation back into these degraded systems. In order to develop successful post-fire restoration approaches in these communities, it is critical that we understand the mechanisms that impair reseeding success. Our objective was to quantify the influence of soil water repellency on seedling emergence and plant growth in a greenhouse study using soil cores obtained from beneath burned Juniperus osteosperma trees. Soil cores were seeded with either Elymus wawawaiensis or Agropyron cristatum, and watered with either a high (watered daily) or a low water regime (watered every 5 days). During the first watering event, water repellency was ameliorated in half the cores by adding a wetting-agent comprised of alkylpolyglycoside-ethylene oxide/propylene oxide block copolymers. Results showed that water repellency reduced seedling emergence and seedling survival by decreasing soil moisture availability. Wetting-agents improved ecohydrologic properties required for plant growth by decreasing runoff and increasing the amount and duration of available water for seedling emergence, survival, and plant growth. These results indicate that soil water repellency can act as an ecological threshold by impairing establishment of reseeded species after a fire. Where restoration efforts are limited by soil water repellency, wetting agents have the potential to improve the success of post-fire reseeding efforts. Future work is needed to validate these findings in the field.
This paper reviews the results of recent research on riparian vegetation recovery following the clearance of invasive alien plants. In Fynbos, Grassland and Savanna Biomes, riparian ecosystems were found to have relatively-high ecological resilience to invasion by alien plants, except in some situations of closed alien stands (75–100% aerial cover). Where alien invasion is the primary disturbance at a site, and invasion intensity is low (
The invasion of alien trees is a major threat to the freshwater resources and biodiversity of South Africa. The Working for Water (WfW) Program was initiated in 1995 in order to control the growth and spread of woody alien species in riparian zones, but the extent to which the indigenous vegetation naturally recovers following alien clearance remains poorly understood. In this study spontaneous succession of riparian vegetation following wild fires and alien clearing was monitored over a number of years at two sites on the Cape Peninsula: a pine plantation in an upland plateau and an Acacia spp.-invaded valley floodplain. After clearing, the vegetation at the pine site was successfully recovering along a trajectory towards Afromontane forest and as a result it is suggested that no active restoration is required. By contrast, our results show that areas cleared of Acacia spp. may be less resilient, with extensive regeneration of woody aliens and only a negligible recovery of indigenous trees. We propose that the absence of riparian trees may have been responsible for precipitating the transition to a community dominated by weedy nitrophilous grasses, and find evidence that this may be perpetuated through the continued removal of Acacia spp. It is hypothesized that this grass-dominated state may be resilient to natural restoration and thus represents an additional constraint to the recovery of riparian communities. Under such circumstances, we argue that active restoration would be required in order to re-instate the riparian community.
The upper catchment area of the Berg River in the Western Cape, South Africa, supplies most of Cape Town and its suburbs with freshwater, in addition to providing water for irrigation purposes along the middle and lower reaches of the river. This study investigates the nutrient status of the Berg River and long-term trends therein. It is shown that inorganic nitrogen and phosphorus levels increase downstream by a factor of more than 10, in response to anthropogenic inputs. Similarly, nutrient levels fluctuate seasonally by more than an order of magnitude, in response to input from diffuse and point sources of pollu - tion. These changes of more than 1 000% far exceed the 15% maximum change stipulated by the South African water quality guidelines for aquatic ecosystems. Total phosphorus levels indicate that hypertrophic conditions prevail at least episodically at all of the Berg River monitoring stations and most of the time at some of them. Additionally, river water phosphate levels show a dramatic increase over the past 20 years. There is also strong evidence that the trophic status of the Berg River is very sensitive to reduced river runoff. The implication is that the construction of the new Skuifraam Dam in the upper catchment area of the Berg River will exacerbate the existing situation, threatening ecosystem services, human health and lucrative agricultural activities.
Nurse plants are those that facilitate the growth and development of other plant species (target species) beneath their canopy because they offer benign microhabitats that are more favorable for seed germination and/or seedling recruitment than their surrounding envi-ronment. Nurse plants have been mainly used to restore vegetation in arid and sub-arid zones in recent years. Based on summarizing the definition of nurse plant and target plant, we review the nursing effect mechanisms, ecological factors that influence nursing effect, rela-tionships between nurse plant and ecological restoration. This review also brings forward possible pairs of nurse and target species at lower subtropical areas. Furthermore, we provide the potential tendency in nurse plant research and application.
Aim The biophysical impacts of invasive Australian acacias and their effects on ecosystem services are explored and used to develop a framework for improved restoration practices.
Location South Africa, Portugal and Chile.
Methods A conceptual model of ecosystem responses to the increasing severity (density and duration) of invasions was developed from the literature and our knowledge of how these impacts affect options for restoration. Case studies are used to identify similarities and differences between three regions severely affected by invasions of Australian acacias: Acacia dealbata in Chile, Acacia longifolia in Portugal and Acacia saligna in South Africa.
Results Australian acacias have a wide range of impacts on ecosystems that increase with time and disturbance, transform ecosystems and alter and reduce ecosystem service delivery. A shared trait is the accumulation of massive seed banks, which enables them to become dominant after disturbances. Ecosystem trajectories and recovery potential suggest that there are important thresholds in ecosystem state and resilience. When these are crossed, options for restoration are radically altered; in many cases, autogenic (self-driven and self-sustaining) recovery to a pre-invasion condition is inhibited, necessitating active intervention to restore composition and function.
Main conclusions The conceptual model demonstrates the degree, nature and reversibility of ecosystem degradation and identifies key actions needed to restore ecosystems to desired states. Control and restoration operations, particularly active restoration, require substantial short- to medium-term investments, which can reduce losses of biodiversity and ecosystem services, and the costs to society in the long term. Increasing restoration effectiveness will require further research into linkages between impacts and restoration. This research should involve scientists, practitioners and managers engaged in invasive plant control and restoration programmes, together with society as both the investors in, and beneficiaries of, more effective restoration.
In 1998, a restoration field trial was initiated in a catchment near Wellington (Western Cape, South Africa) to determine whether fynbos riparian scrub vegetation cleared of woody invasive alien trees require post-clearance restoration actions to accelerate indigenous vegetation recovery. The aim was to assess the relative effectiveness of three sowing treatments for restoring indigenous vegetation cover after the widely used “Fell & Burn” method of clearing invasive alien trees. Sowing treatments included non-invasive alien grasses to determine whether they have a negative effect on recovering native vegetation. A summer fire, eight years after trial initiation, provided an opportunity to determine how resilient restoration treatments are to alien re-invasion and fire. Restoring the site after alien clearing by sowing indigenous seeds increased both diversity, by improving species presence and abundance. However, a census done 8 years later (in 2006) revealed that seedlings of woody invasive alien plants dominated all plots, and had also survived the burn by resprouting, indicating the importance of follow-up control to justify initial clearing and restoration costs. Indigenous grass density was significantly reduced in plots where alien grasses were sown, while in the control and fynbos sowing treatment, indigenous grass density increased. By 2006, alien grass density was negligible in all treatments, indicating that the two grass species sown are not persistent or invasive. Active restoration of riparian areas after alien plant clearing has potential to facilitate vegetation recovery, but must be coupled with a long-term plan for adequate follow-up removal of post-clearance and post-fire alien recruits.
The Working for Water programme is tasked with the important role of controlling invasive alien plants with an assumption that indigenous vegetation will recover naturally. This study assessed vegetation composition and structure following alien clearance in closed-stand invasion of riparian areas and a minimum of two years' passive recovery. Three initial clearing treatments — Fell Only, Fell & Remove and Fell & Burn — were compared to uninvaded Reference conditions. The aim was to ascertain the nature of vegetation recovery, as well as to determine which clearing treatment was most successful in promoting recovery. A Detrended Correspondence Analysis revealed that the Fell & Remove treatment most closely approached the Reference condition while Fell Only and Fell & Burn plots had altered composition and structure. All clearing treatments had significantly lower vegetation cover than the Reference and species composition was altered by invasion and clearance. Important growth forms, such as small (3–10 m) trees were suppressed by felled slash and burning. Although burning was the best method to reduce woody alien species, secondary invasion by alien herbaceous species occurred where natural riparian vegetation did not re-establish. The Fell & Remove treatment is recommended as the best to use in promoting indigenous vegetation recovery, and together with continued alien follow-up control, is able to minimize alien re-invasion of riparian ecosystems. Managers are advised to consider active restoration measures in areas where recovery is likely to be protracted.
Riparian zones are complex disturbance-mediated systems that are highly susceptible to invasion by alien plants. They are prioritized in most alien-plant management initiatives in South Africa. The current practice for the restoration of cleared riparian areas relies largely on the unaided recovery of native species from residual individuals and regeneration from soil-stored seed banks. Little is known about the factors that determine the effectiveness of this approach. We need to know how seed banks of native species in riparian ecosystems are affected by invasion, and the potential for cleared riparian areas to recover unaided after clearing operations. Study sites were selected on four river systems in the Western Cape: the Berg, Eerste, Molenaars and Wit Rivers. Plots were selected in both invaded (> 75% Invasive Alien Plant (IAP) canopy cover) and un-invaded (also termed reference, with < 25% IAP canopy cover) sections of the rivers. Replicate plots were established at two elevations (mountain stream and foothill) and in three moisture regimes (dry, wet and transitional bank zones). Soil samples were taken, surveys were done of the aboveground vegetation, and comparisons were made between invaded and non-invaded sites. Seed bank communities were clearly defined by the state of the river (reference or invaded) and moisture regimes (wet and dry bank zones). Comparisons at a landscape scale showed no clear pattern, as the composition of both aboveground and seed bank species assemblages were strongly influenced by site history, especially the extent of invasion and fire frequency. Even after heavy and extensive invasion, riparian seed banks have the potential to initiate the restoration process. However, not all riparian species are represented in the seed bank. Based on these results, restoration recommendations are outlined for alien-invaded riparian zones.
Predictions that invasive alien plants would use significant amounts of water were a major factor in the establishment of South Africa's Working for Water programme, which aims to protect water resources by clearing these plants. The predictions were made by combining the results of hydrological experiments, conducted to assess the effects of afforestation with alien trees on water resources, with an ecological understanding of the spread and establishment of invasive trees. The forecasts were then scaled up to arrive at national estimates of the corresponding water consumption. This paper reviews the approaches that have been used to estimate these consequences at different scales. We propose a framework for assessing the process of knowledge generation, and review the approaches used in South Africa at each level of the framework, the current level of understanding arising from the use of these methods, and significant gaps in understanding. The framework has four levels: fundamental observations from which a detailed understanding of biophysical processes can be developed; applied or predictive research from which an understanding of processes can be scaled up to predict generic outcomes; integrative research where a predictive understanding of hydrology can be combined with information from other disciplines to place the outcomes in a wider context; and research on management support, such that the information can be used to improve management and policy decisions. We conclude that much knowledge exists, but that there are also significant gaps in understanding, and challenges associated with scaling up and down to make appropriate predictions. This is especially true at the management support level, where very different kinds of uncertainties operate in the same comparative framework. Existing knowledge needs also to be used more effectively, to help prioritize clearing operations by targeting areas in terms of water-related benefits.
This paper examines the evidence for the effects of invasive alien plants in natural and semi-natural ecosystems in South Africa. Invasive alien plants are concentrated in the Western Cape, along the eastern seaboard, and into the eastern interior, but there is a shortage of accurate data on abundance within this range. Most information on site-specific impacts comes from the fynbos biome, and is generally poor for other biomes. The consequences of invasions for the delivery of ecosystem goods and services to people are, with the notable exception of their influence on water resources, inadequately studied. The understanding of many of the broader aspects of invasion ecology needs to be enhanced, and identify important challenges for research to address critical gaps in knowledge. Priorities for future research include the development of a predictive understanding of the rates of spread of invasive alien plants, and the development of achievable goals for ecosystem repair after clearing, including measurable criteria for assessing the success of restoration. Climate change could significantly exacerbate problems with invasive species and work is needed to accommodate plausible trajectories in planning and management frameworks. Perhaps the greatest challenge for South African ecologists is to address the twin issues of skills development and social transformation, to ensure that adequate and relevant ecological expertise is maintained to meet future research and management needs. Formal collaboration between organizations to address capacity building and educational transformation in the field of invasion ecology would represent a significant step forward.
The impacts of the widespread invasions by alien plants in South Africa are increasingly recognised. Most of the past concern has been about the impacts on conservation areas, other areas of natural vegetation and on agricultural productivity. The potential impact of invading alien woody plants on water resources was known to be serious but there has been no information available to evaluate the significance of these water losses across the whole country. This paper reports on the results of a preliminary survey aimed at obtaining an overview of the extent, impacts and implications of alien plant invasions at a national and regional level for South Africa and Lesotho. Data on the extent and location of the invaded areas were obtained from a variety of sources including detailed field mapping, mainly at a 1:250 000 scale with some at 1:50 000 and 1:10 000, and generalised information on species and densities. The density class of each species in each polygon was mapped and used to derive the condensed areas (the equivalent area with a canopy cover of 100%). Each of the invading species was classified as a tall shrub, medium tree or tall tree - based on growth form and likely water use - and its biomass was estimated from a function based on vegetation age. The incremental water use (i.e. the additional water use compared with the natural vegetation) was calculated using the following equation: Water use (mm) = 0.0238 x biomass (g/m (2)) which was derived from catchment studies. Alien plants, mainly trees and woody shrubs, have invaded an estimated 10.1 million ha of South Africa and Lesotho, an area larger than the province of KwaZulu-Natal. The equivalent condensed area is 1.7 million ha which is greater than the area of Gauteng Province. The Western Cape is the most heavily invaded at about a third of the total area, followed by Mpumalanga, KwaZulu-Natal and Northern Province. The catchments of the Berg and Breede Rivers are the most heavily invaded followed by the George-Tsitsikamma region, Port Elizabeth coastal region and the Drakensberg escarpment in Mpumalanga. The total incremental water use of invading alien plants is estimated at 3 300 million m(3); of water per year, equivalent to about 75% of the virgin MAR of the Vaal River system. About a third of the estimated total water use, by volume, is accounted for by alien invaders in the Western Cape, followed by KwaZulu-Natal (17%), the Eastern Cape (17%) and Mpumalanga (14%). The greatest reduction as a percentage of MAR was found in the arid Northern Cape (17%), followed by the Western Cape (15%) and Gauteng (10%). For primary catchments, the greatest percentage reductions were in the Namaqualand coast (catchment F, 91%) followed by the Eastern Cape Coast (P, 42%) and the south-western Cape (G, 31%). The extent and density of the invasions and thus the impact on water resources could increase significantly in the next 5 to 10 years, resulting in the loss of much, or possibly even all, of the available water in certain catchment areas. Alien plant control is expensive but it has been shown that control programmes are cost-effective compared with alternative water supply schemes. This preliminary assessment needs to be interpreted with caution because the results are based on a data set that contains some important uncertainties. The water-use estimates also involve some critical assumptions. Nevertheless, the scale of the invasions, the magnitudes of the impacts and the rapid expansion we are observing are such that a national control programme is essential if the country's water resources are to be protected.
Riparian habitats in many parts of South Africa are severely degraded by invasive alien plants, especially trees. These invasions reduce water yields from catchments and affect riverine functioning and biodiversity. Initiatives are under way countrywide to clear alien plants from watercourses and surrounding catchments. Current understanding of key processes that regulate riparian functioning and define options for restoration is rudimentary. We review the impacts of riparian invasions and identify factors limiting the recovery of natural vegetation following alien clearance. We propose a framework of strategic interventions for optimizing restoration success. The framework identifies abiotic and biotic barriers to restoration at the scales of catchments and local reaches. In highly transformed catchments, interventions at the reach scale may fail if important barriers at the catchment scale are not addressed. The extent to which propagule supply and microsite conditions inhibit vegetation recovery is unknown. We also know little of the relative importance of dispersing vegetative propagules, dispersing seeds and soil-stored seed banks in vegetation dynamics, particularly after severe disturbances such as dense invasion by alien plants. The importance of geomorphological and hydrological factors in mediating recovery of riparian vegetation has not been adequately explored for all climatic areas in South Africa. More research is needed to determine the influence of different alien species and clearing treatments on the recovery of riparian vegetation. The literature strongly suggests that in highly alien-transformed catchments, the re-introduction of riparian species is required to promote recovery and suppress re-invasion. However, such interventions are unlikely to be widely implemented unless the cost: benefit ratios are favourable.
Questions
The degree to which renosterveld shrublands are fire‐dependent is currently unclear. To address this issue, the following questions were asked: (1) does smoke stimulate germination of soil‐stored seeds in renosterveld; (2) does recently‐burned renosterveld display changed composition and higher diversity than unburned vegetation; and (3) how do the species compositions of renosterveld soil seed banks and standing vegetation compare?
Location
Swartland, Cape Floristic Region, South Africa.
Methods
Soil seed bank samples from a north‐ and south‐facing slope were smoke‐treated and germinated to test for smoke‐stimulated germination. Burned standing vegetation was surveyed 16 months post‐fire, as was unburned vegetation on the same slopes. Seed bank species richness and density were compared between smoke‐treated and untreated samples within and between slopes. Burned and unburned standing vegetation were compared within and between slopes in terms of species richness, abundance and aerial cover. Compositional similarity of the seed banks and standing vegetation was assessed.
Results
Seed banks were dominated by annuals and graminoids. Smoke treatment had no effect, except for driving significantly higher species richness and seedling density in south‐facing slope perennial shrubs. Species richness and seedling density were significantly higher in seed banks on the south‐facing slope compared to the north‐facing slope. Burned standing vegetation exhibited significantly higher diversity than unburned vegetation. Annuals and graminoids displayed significantly higher species richness and aerial cover in burned renosterveld. The north‐facing slope contained less than half the number of species/m ² compared to the south‐facing slope. The seed banks and standing vegetation showed low to intermediate similarity (Sørensen = 31%–53%), but grouped close together on an NMDS plot, suggesting intermediate similarity overall.
Conclusions
Elevated germination of perennial shrubs in smoke‐treated seed bank samples and increased diversity of post‐fire standing vegetation suggest the renosterveld in this study shows elements of a fire‐driven system. Certain species only recruited in burned sites, suggesting fire‐stimulated germination. Aspect had a major influence on plant community composition, with the mesic south‐facing slope being more diverse than the xeric north‐facing slope. The similarity between the seed banks and standing vegetation was higher than previously shown for renosterveld, and appears to be higher than for fynbos.
The outcomes of ecosystem restoration projects should be periodically monitored to inform subsequent adaptive management decisions. In 2012, a project was begun to remove both invasive alien plants and fish from the Rondegat River in South Africa. Although the initial post-intervention dynamics of aquatic fauna have been documented, the results of the simultaneous clearing of dense riparian stands of alien trees and shrubs have not been reported. We examined native riparian vegetation recovery over 3 years after alien plant clearing. We documented increased cover of native riparian shrubs, but a simultaneous increase of alien and native weedy grass cover. Secondary invasions, especially by grasses, can have strong effects on ecosystem dynamics and achieving the goals of restoration may therefore require additional active management. Our findings provide an initial baseline reference for future monitoring and adaptive management decisions.
There is growing interest in ecosystem disservices, i.e. the negative effects of ecosystems on humans. The focus on disservices has been controversial because of the lack of clarity on how to disentangle ecosystem services and disservices related to human wellbeing. A perspective that considers both services and disservices is needed to inform objective decision-making. We propose a comprehensive typology of ecosystem disservices, and present a framework for integrating ecosystem services and disservices for human wellbeing linked to ecosystem functioning. Our treatment is underpinned by three key assumptions: (1) ecosystem attributes and functions are value-free; (2) the perception of benefits or nuisances are however dependent on societal context, and preferences and actions by societal actors may trigger, enhance or alleviate benefits or nuisances derived from ecosystems; and (3) the notion of disservices must account for the role of human management in assessments of ecosystem values, i.e. the social and technological measures that identify, protect, promote or restore desirable levels of services, and concurrently minimise, mitigate or adapt to disservices. We illustrate our ideas with examples from plant invasions as a complex social-ecological phenomenon.
The impact of alien tree clearing on soil and vegetation recovery remains largely understudied. This study focused on changes in soil and vegetation properties following Acacia removal. The aim was to quantify the long-term consequences of alien clearing. Paired cleared (old – 15 years; medium – 11 years; recent – 6 years) and uncleared sites were selected along the Palmiet catchment in Eastern Cape Province, South Africa. Various soil physico-chemical properties (soil moisture, pH, P, N, C, K, Na, Ca, Mg and soil repellency) and vegetation diversity measures were studied on 10 m × 10 m plots. Results indicate that measured soil nutrients are significantly (P < 0.05) lower in cleared than in uncleared sites. However, comparisons among cleared sites alone indicate that soil properties are recovering with older cleared sites having higher (P < 0.05) nutrients than recent cleared sites. Soils in uncleared sites are more repellent than soil in cleared sites. Vegetation recovery in cleared sites was taking place with older cleared sites having higher native species diversity than recently cleared site. We conclude that the removal of alien plants could have caused a reduction in soil nutrients. However, as native vegetation recovers on cleared sites, soil nutrients are gradually improving.
Indigenous soil seed banks play a very important role in facilitating the natural recovery of indigenous fynbos vegetation after clearing invasive alien vegetation. In densely invaded areas, there is a reduction in fynbos cover and seed production, and these ecosystems rely heavily on the remaining soil seed bank as a reservoir of plant propagules. This study used the seedling emergence approach to assess recovery potential based on the soil seed banks of riparian and hillslope grassy fynbos communities that had been densely invaded for three decades, with Acacia longifolia as the dominant alien species. Forty-eight species (of which five were aliens), representing 30 genera and 18 families, emerged from the soil seed bank, with Asteraceae and Cyperaceae being best represented. The mean density of indigenous seedlings for the study area was 1582 seedlings/m2. Senecio rigidus exhibited the highest density, with 274 seedlings/m2, followed by Chironia baccifera (151 seedlings/m2) and Rumohra adiantiformis (136 seedlings/m2). Forbs were the most numerous growth form. Two other alien species exhibited comparable seedling densities, i.e. Solanum nigrum (181 seedlings/m2) and Conyza canadensis (98 seedlings/m2). A. longifolia seed densities of up to 4528 seeds/m2 were found by sieving the soil. Results revealed that alien-invaded grassy fynbos had a diverse and viable soil seed bank with relatively high seed densities. Pioneer species were well represented, as well as graminoids in the riparian zone. Species representing some guilds were missing; e.g. serotinous species from the genera Leucadendron and Protea, and few geophytes were present in the hillslope soil seed bank, although ericoid shrubs were well represented. Riparian species such as Cliffortia graminea and common Cyperaceae and Restionaceae species were also not represented in the seed bank. It would appear from this data that the soil seed bank would be adequate to enable a functional cover of indigenous vegetation to re-establish after clearing. In order to improve vegetation structure and composition, the addition of some missing guilds would facilitate restoration, provided that post-clearing follow-up treatments do not prevent or hinder the establishment of these indigenous species.
Two portable weirs were used to quantify changes in streamflow following clearfelling of a dense stand of self-sown Pinus patula and Acacia mearnsii along a riparian zone on Kalmoesfontein, a SAPPI forest plantation south-east of Lydenburg, Eastern Transvaal. The weirs were set up at positions 500 m apart on the same stream, and used to monitor streamflow levels before and after the clearing of all trees between the weirs to an average distance of 25 m from the stream. Analysis of streamflow data revealed that the clearing operation resulted in a 120% increase in streamflow at the lower weir, equivalent to 30,5 m3 of water per day. Streamflow at the lower weir was less than at the upper weir before clearfelling, but equalled that at the upper weir after clearfelling. This demonstrates that the intervening trees were responsible for the initial difference in streamflow. Two further lines of evidence point to the adjacent trees exerting a strong influence on streamflow. Firstly, a clear daily fluctuation in streamflow was evident at both weirs. This is a consequence of transpiration by trees taking place only during hours of daylight. Secondly, the occurrence of cloudy, rain-free weather led to an increase in streamflow. Such weather reduces the evaporative demand of the air, causing transpiration rates to decline as well.We conclude that invasive exotic trees should be removed from riparian zones to promote significant streamflow increases from afforested catchments. There is a need for a broader body of information to assess the effects of species, density and age distribution, as well as catchment characteristics, on streamflow responses to clearing invasive trees. This study has demonstrated the suitability of the portable weir technique in capturing such information.
The changes in streamflow following the removal of invasive wattle (Acacia mearnsii and A. longifolia) from riparian zones during the dry summer months in three small catchments in the Western Cape Province are described. Portable steel box weirs, with a 30° compound V-notch and equipped with Belfort water level recorders, were used to measure streamflow. Each of the three streams had a control catchment against which it was correlated during a pre-treatment period. The riparian zones of the treatment catchments were cleared after this period of calibration, and the response of stream-flow after treatment was recorded.
In the three catchments there was a marked increase in streamflow after clearing of the riparian invasive vegetation. The streamflow increases in the three catchments were 8,8, 10,4 and 12 m³/day per ha cleared. The responses measured in these experiments are the result of changing from tall vegetation to minimal cover and represent a maximum response. Streamflow is expected to decrease again as vegetation regrows, but not to the levels that characterised the invaded site. The riparian areas should be kept under short indigenous vegetation, such as grass or fynbos, to sustain the long-term increases in streamflow.
Headwater rivers are the predominant kind of aquatic ecosystem in South Africa’s Cape Floristic Region. Invasion by alien woody taxa (especially Acacia longifolia and Acacia mearnsii) have altered these rivers in recent decades, replacing indigenous vegetation and altering ecosystem functioning. Aliens have been systematically cleared in watercourses since 1995 as part of a national program (“Working for Water”) to increase water production and improve water quality. Cleared sites are mostly left to recover to their pre-invasion state without additional intervention. We compared the vegetation of seven rivers that vary in their extent of invasion and clearing to identify factors limiting recolonization. Dense invasions cleared 3–6 years before the survey differ in vegetation structure and composition from uninvaded or lightly invaded/cleared riparian reaches in the following ways: (1) diminished extent of indigenous trees in the canopy and increased importance of shrubs or reinvading alien trees, (2) reduced species richness, (3) reduced likelihood of occupancy for more than one-third of common, indigenous species, and (4) reduced incidence of indigenous tree regeneration. Overall, indigenous tree regeneration is very low and not disturbance-triggered which will likely result in slow recovery without additional intervention. We recommend focusing active revegetation on common riparian scrub trees (i.e., Metrosideros angustifolia, Brachylaena neriifolia, Brabejum stellatifolium, and Erica caffra). These species tolerate open habitats favored by alien trees, eventually forming closed canopies required by shade-tolerant species. Accelerating establishment of these small trees is likely critical for shifting cleared riparian corridors from a state that favors alien reinvasion. Effective establishment strategies will need to be developed in the context of hydrologic impairment, since alien-invaded rivers in this region typically have reduced flow.
Gum trees, or eucalypts (Eucalyptus species), have been targeted for invasive alien plant clearing programmes in many parts of South Africa. This has caused some dissatisfaction where the species concerned also have useful characteristics, and stakeholders contend that some of these useful species are not invasive. A rapid assessment of the invasive status of Eucalyptus species at 82 sites in South Africa (54 in the Western Cape and 28 in Mpumalanga) indicated that only Red River gum (E. camaidutensis) and flooded gum (E. grandis) are clearly invasive. Surveys were not undertaken in parts of the Western Cape known to be invaded by spider gum (E. lehmannii); the invasive status of this species is well known and is not contested. Red River gum has transformed long stretches of rivers and its importance as a major weed has been underestimated in previous reviews of alien plant invasions in South Africa. Most other species were naturalized. The authors recommend that projects aimed at clearing eucalypts should focus on riparian areas and nature reserves (where all eucalypts have deleterious effects), but that clearing projects outside these areas should only target species known to be invasive until such time as the invasive status of the other eucalypts (notably sugar gum, E. cladocalyx, and karri, E. diversicolor) can be ascertained with a greater degree of confidence.
Tropical forest recovery in pastures is slowed by a number of biotic and abiotic factors, including a lack of adequate seed dispersal and harsh microclimatic extremes. Accordingly, methods to accelerate forest recovery must address multiple impediments. Here, we evaluated the ability of "tree islands" to serve as "recruitment foci" in a two-year study at three sites in northern Honduras. Islands of three sizes (64, 16, and 4 m2) and at two distances to secondary forest (20 and 50 m) were created by planting 2 m tall vegetative stakes of two native species: Gliricidia sepium (Fabaceae) and Bursera simaruba (Burseraceae), each in monoculture. Open-pasture "islands" of equal sizes served as controls. Tree islands reduced temperature and light (PAR) extremes as compared to open pasture, creating a microenvironment more favorable to seedling establishment. Seed-dispersing birds (quantified at one site only) showed an overwhelming preference for islands; 160 visits were recorded to islands compared with one visit to open pasture. Additionally, frugivores visited large islands more often, and for longer time periods, than small islands, thereby increasing the likelihood of a dispersal event there. In total, 144 140 seeds belonging to 186 species were collected in islands; more than 80% were grasses. Tree islands increased zoochorous tree seed rain; seed density and species richness were greater in tree islands than in open pasture, and large islands had greater seed density than smaller islands (Gliricidia only), suggesting that they are more effective for restoration. Distance to forest did not affect seed rain. A total of 543 seedlings and 41 species established in islands; > 85% were zoochorous. Seedling density did not differ among treatments (mean 0.2 seedlings/m2 for islands vs. 0.1 seedlings/m2 for pasture), although an increasing trend in tree islands over the course of two years suggests that seedling recruitment is accelerated there. Lastly, similar seedling densities were censused in the 1 m perimeter surrounding islands, suggesting that islands can expand outward into pasture. Planting vegetative stakes to create tree islands in pastures accelerates forest recovery by overcoming a number of impediments, and presents a simple, broadly applicable alternative for facilitating forest regeneration in abandoned pastures.
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