Facilitating the recovery of natural evergreen forests in South Africa via invader plant stands
© The Author(s). 2017
Received: 17 July 2017
Accepted: 25 September 2017
Published: 2 November 2017
Contrary to general belief, planted and naturalized stands of introduced species facilitate the recovery of natural evergreen forests and their diversity. Forest rehabilitation actions are often performed at great cost: mature forest species are planted, while species with adaptations to recover effectively and quickly after severe disturbance are ignored; or stands are cleared of invasive alien species before native tree species are planted. By contrast, cost-effective commercial plantation forestry systems generally use fast-growing pioneer tree species introduced from other natural forest regions. Such planted tree stands often facilitate the recovery of shade-tolerant native forest species. This paper provides a brief overview of disturbance-recovery processes at landscape level, and how pioneer stands of both native and introduced tree species develop from monocultures to diverse mature forest communities. It uses one example of a study of how natural forest species from small forest patches of 3 ha in total invaded a 90-ha stand of the invasive Black wattle, Acacia mearnsii, over a distance of 3.1 ha at Swellendam near Cape Town, South Africa. The study recorded 329 forest species clusters across the wattle stand: more large clusters closer to and more smaller clusters further away from natural forest patches. The 28 recorded forest species (of potentially 40 species in the surrounding forest patches) included 79% tree and 21% shrub species. Colonizing forest species had mostly larger fleshy fruit and softer small seeds, and were dispersed by mostly birds and primate species. Maturing forest trees within developing clusters in the wattle stand became a source for forest regeneration away from the clusters, showing different expansion patterns. Four sets of fenced-unfenced plots in the wattle stand showed the impact of browsing by livestock, antelope, rodents and insects on the successful establishment of regenerating forest species, and the dramatic effect of excluding browsing. The results support the approach to rather selectively manipulate than clear invader plant stands in the natural forest environment. This approach invests in the natural succession process rather than planting. It protects developing seedlings against browsing by stacking invader plant debris around them, rather than protecting them by means of costly fencing. This forest recovery process through nurse stands of invasive species can be managed, with additional benefits: Indigenous tree species provide for better streambank stability; and the practice provides for local job creation over a 10-year period for harvesting poles and firewood from the manipulative conversion process.
Globally there is much concern, emotion and varied perspectives around plant invasions into natural vegetation systems and the threat they pose to the natural biodiversity of the systems (Macdonald et al. 1986; Henderson 2001; Kohli et al. 2009; Boy & Witt 2013). The Working for Water programme in South Africa is considered to be one of the most successful integrated land management programmes in the world (Hobbs 2004). The programme was initiated in 1995 with a modest annual budget of US$4 million. Since then, it has grown substantially; it has successfully cleared significant areas of invasive alien plant infestations, created a large number of jobs and had significant positive economic and social effects, increased water run-off into streams and reduced a major threat to the extraordinary biodiversity found in South Africa. The general aim of this programme is to clear invasive alien plants. However, the costs of controlling the plant invaders are too high to achieve the long-term goal of removing the invasive alien plants. The applied control systems often do not differentiate between the types of invader plants and the specific vegetation systems from which they need to be cleared. For instance, the same approach to clearing the invasive alien plant stands in the fire-prone grasslands and Fynbos shrublands is also followed in the shade-tolerant natural forest environment.
Contrary to general belief, planted and naturalized stands of introduced species facilitate the recovery of natural evergreen forests and their diversity (Knight et al. 1987; Geldenhuys 1992, 1996a, 1997; Parrotta 1995; Loumeto & Huttel 1997; Parrotta & Turnbull 1997; Geldenhuys & Delvaux 2007). Each natural forest area has species that function optimally in different development stages of that forest system, from pioneer stands after a major disturbance, up to mature forest (Geldenhuys 2011a). For example, the pioneer species are light-demanding, fast-growing species forming mono-specific, even-aged stands. They become gradually colonized and eventually replaced by increasingly more shade-tolerant, slower-growing species and develop towards mixed-species, mixed-age mature forest. In forest rehabilitation, we often plant mature forest species but ignore those species with adaptations to recover effectively after severe disturbance and degradation. We clear stands of invasive alien species before we plant native tree species, all at great costs, to rehabilitate natural forest. By contrast, cost-effective commercial plantation forestry systems generally use fast-growing pioneer tree species introduced from other natural forest regions.
Different situations require different solutions. What would be the ecological basis for following different approaches and practices in invasive alien plant control in natural evergreen forest, grassland or shrubland? The taller-growing invasive alien plant stands shade out the light-demanding grassland and shrubland species; they need to be removed entirely from the landscape to enable the grasslands and shrublands to recover. This is contrary to the situation in natural evergreen forest systems. The disturbance-recovery processes in natural evergreen forests resulted in different species adaptations in terms of their regeneration and establishment ecology, relative to the shady conditions of mature forest. Recovery from severe natural forest disturbances follows a successional sequence in which the early shade-intolerant pioneers in the recovery process nurse the establishment of gradually more and more shade-tolerant forest species, i.e. increases species richness of forest species. Natural forest systems have a diverse range of natural forest species that form such pioneer stands, facilitating natural forest recovery. Such systems can be sources of diverse products for livelihood needs while facilitating recovery of natural forest diversity and productivity (Geldenhuys 2013a; Mala et al. 2009). One could therefore expect differences in behavior in terms of forest recovery via shade-intolerant invasive alien plant stands. Several studies, globally, have shown the role of planted tree stands of both introduced and native species stands to facilitate or nurse the establishment of natural forest species under their canopies (Knight et al. 1987; Geldenhuys 1992, 1996a, 1997; Parrotta 1995; Loumeto & Huttel 1997; Parrotta & Turnbull 1997; Geldenhuys & Delvaux 2007). Forests also provide diverse timber, wood and non-wood products that are essential components of the livelihood needs of rural societies in many areas. Invasive alien plant stands in the natural forest environment often provide alternative resources for products in short supply from the natural forests. Clearing such invasive alien plant stands may restart the pressure on natural forest species (for poles, fuel wood, etc.) (Geldenhuys 1986, 1992, 1996a, 2002; Geldenhuys et al. 2016).
The question is therefore whether invasive alien plants pose a problem in natural evergreen forests, and if so, which particular species are causing these problems. Furthermore, if they do not pose serious ecological problems to the natural forest systems, how can their facilitation of natural forest recovery be managed? How can the options presented by the recovery process be used to develop cost-effective forest rehabilitation strategies and sustainable small business?
There may be different reasons for rehabilitating degraded or lost natural forests. One reason may be to restore their diversity per se because many floral and faunal components depend on a functioning forest ecosystem. Another reason may be to provide various resource use options for particular users who may require specific forest products, or to restore essential environmental services (nutrient cycling, carbon sequestration, soil and water conservation), or a combination of such products and services. Within a global context, there are two major concerns that underline the need for cost-effective approaches to rehabilitate natural forests. Firstly, there is the concern that natural forests are degraded through clearing for slash-and-burn traditional agriculture, charcoal production, uncontrolled timber-harvesting, and clearing for commercial agriculture, forestry, infrastructure development and mining (FAO 2003; Palm et al. 2005). Secondly, there is the concern that invasive alien plant species threaten the biodiversity and key functions of natural vegetation systems, including natural forests (Macdonald et al. 1986; Henderson 2001; Kohli et al. 2009; Boy & Witt 2013). Many invasive alien plant species are pioneer species in their native environments, and tend to function in their new environments in a similar way as the native pioneer forest species. However, they may also disrupt some ecological processes in the natural forests in their new environments, such as changing the natural regimes of gap formation (Geldenhuys 1996a), or changing the behavior of fruit/seed dispersers (Dean 1985).
Pioneer tree communities, including those composed of invasive alien species, represent specific stages in the natural development towards mixed-species, mixed-age mature forest communities (Parrotta & Turnbull 1997). The natural transition from mono-specific, even-aged stands to mixed-species, multi-age stands, includes a process of self-thinning, which may extend over long periods of time. Restoration is generally understood as anticipating a defined species composition, but this is often an unrealistic goal, because many disturbing factors are impossible to control (Aronson et al. 1993). However, the process can be facilitated to progress more rapidly. Our approach was to understand how to facilitate the rehabilitation process to advance more rapidly towards a more stable system of mixed-species, mixed-age, under various assumptions and rehabilitation goals.
The purpose of this paper is to review relevant literature on (i) the natural evergreen forests in South Africa and associated disturbance-recovery processes that determine their location patterns in the landscape, (ii) types of invasive alien plant species in the South African natural evergreen forest environment, and (iii) how stands of introduced tree species facilitate the recovery of natural evergreen forests. The paper then summarizes results from a recent study on how natural forest species invade an extensive stand of the invasive Black wattle (Acacia mearnsii) and how browsing by different types of animals limit the establishment success of the natural forest species. Finally, we present a decision-tree for the planning of invasive alien plant rehabilitation actions in a landscape that also includes natural evergreen forest.
Natural forest communities represent the smallest, most widespread yet highly fragmented vegetation formation in South Africa (Mucina & Geldenhuys 2006). The total area covered by forests is estimated at 3000 to 4000 km2, i.e. less than 0.1% of the country. Most forest patches are very small (<10 ha) to small (<100 ha), occurring along the Great Escarpment, mountain ranges and coastal lowlands of eastern and southern South Africa, from 22°40′S (inland mountains) and 27°S (coast) in the northeast to 34°S in the southwest. They are embedded within a matrix vegetation of fire-prone and fire-adapted grassland, shrubland (fynbos) or deciduous woodland (Cooper 1985; Geldenhuys 1991; Berliner 2009). The forest communities contain a high plant diversity and are considered to be the second richest biome in South Africa, containing 5.35% of all South African plant species with a relatively rich 0.514 species∙km−2, with many widespread but also endemic/fragmented species (Mucina & Geldenhuys 2006). The use value of the forests is high in terms of timber and non-timber forest products (Lawes et al. 2004). Most forests occur in areas of high population density. Forests surrounded by affluent societies expand with little impact from infrastructural development (Geldenhuys & MacDevette 1989). Forests surrounded by developing poor rural societies are often degraded by traditional subsistence practices to satisfy social livelihood needs for building material, fuel wood, food and medicine, and other household goods (Geldenhuys & MacDevette 1989; Lawes et al. 2004; Geldenhuys & Cawe 2011).
It is important to note that the natural evergreen forests are not inherently resistant to fire. Occasionally during controlled block-burns in catchments of grassland and fynbos, the wind direction may change, and a forested fire-shadow area may become exposed to a fire-pathway. The mountain forests in the Southern Cape Afrotemperate forests are generally in an advanced regrowth stage, dominated by pioneer and early regrowth fire-adapted tree species (Geldenhuys 1993, 1996b). The strong, gusty Bergwinds may throw burning Protea flower heads, or recently also cones of invader pine stands, over distances of more than 4 km ahead of an approaching fire (Geldenhuys 2010). This may cause burnt spots inside the forest, but such spots may also be caused by lightning (Geldenhuys et al. 1994), creating a specific understory community of the fern Rumohra adiantiformis (Geldenhuys 1993). Charcoal of forest trees was collected below the litter layer in more than 30% of 250 plots sampled throughout the interior of the Southern Cape Afrotemperate forests (Geldenhuys 1993).
If a natural forest expands, or is rehabilitated beyond its natural ‘fire shadow area’, then it is likely to be destroyed during some extreme natural fire event. This happened after rehabilitation of natural forests during a 20-year observation period in the Groenkop forest near George in the Southern Cape, South Africa. This particular rehabilitation was done to extend the forest boundary to a road in order to facilitate boundary management. In one night during 1998, a Bergwind-driven fire burnt the actively rehabilitated forest back to its natural long-term boundary. The general rule for forest rehabilitation is therefore to stay outside the fire pathways (Geldenhuys 2013b). In addition, grassland and Fynbos shrubland species developed specific diverse adaptations to persist with regular fires in the fire pathways. If natural forest is allowed to expand into the fire zones, with fire protection, then such grassland and Fynbos shrubland species may be lost (with the natural forest functioning in similar ways as an invader plant community).
When fire is excluded from the landscape, or cooler fires are applied during the early dry season, trees and other woody plants establish themselves within the landscape in areas potentially suitable for tree growth. Most of these trees are of pioneer-like species, often invasive alien plant species which are forming dense stands. Natural forest species also become established in areas where they did not previously grow, as can be seen on old aerial photographs dating back to the period 1938–1942. Such colonisation by natural forest species in former fire pathways has been referred to as ‘naturalizing natural forest’, but forest colonisation in areas where forest was cleared during recent periods has been called ‘regrowing natural forest’ (Geldenhuys 2011b). The distinction is important because of the legal implications for clearing of regrowing natural forest. The current regulations applied in terms of the National Forests Act of 1998 requires that no stands composed of natural forest species may be cleared, irrespective of whether they recover from destroyed forest stands or expanded into natural grassland and Fynbos shrubland by naturalization (Geldenhuys 2011b).
Invasive alien species in the evergreen forests
Number of introduced species observed in the natural evergreen forests in South Africa and their invasive status, as adapted from Geldenhuys et al. (1986) and updated with current information
a) Species occurring only along forest margins and in large forest gaps
b) Species occurring along forest margins, in small and large gaps
1) 25 tree species (for some genera, individual species were not counted): Mostly passive invaders, or just being seen. More active invaders include Acacia decurrens, A. saligna*, Albizia lophantha*, Eucalyptus spp.*, Melia azedarach, Morus alba*, Pinus spp.*, Populus spp., Psidium cattleianum & Schinus terebinthifolius
2) 28 Woody shrub and liana species: Mostly passive invaders, or just being seen. More active invaders include Acacia cyclops, Caesalpinia decapetala*, Chromolaena odorata*, Lantana camara*, Rubus niveus & Solanum mauritianum*
3) 10 Herb & soft shrub species: All passive invaders, or just being seen
1) 14 tree species: Active invaders, only in large gaps and forest margins, include Acacia longifolia, A. mearnsii*, A. melanoxylon* & Psidium guajava*
Active invaders, also under the forest canopy, include Cestrum laevigatum*, Cinnamomum camphora, Ligustrum japonicum* & Pittosporum undulatum
2) 7 Woody shrub and liana species:
Mostly passive invaders. Rubus cuneifolius* is an active invader, but only in large gaps and forest margins. Dolichandra unguis-cati & Pereskia aculeata* are active invaders also on trees under the canopy
3) 1 soft shrub species: Just being seen
Most of the perceived invasive alien species are pioneer trees, shrubs or climbers in their natural environments that establish themselves on disturbed sites, in large forest gaps and along the forest margin (Table 1, Geldenhuys et al. 1986; Geldenhuys 1996a, 1997). They are generally shade intolerant and thus unable to get established under their own canopies. The fruits/seeds of many invader tree species are either small, wind-dispersed (eucalypts, pines and poplars) or have long-living hard-coated seeds (acacias, some of which are dispersed by birds, e.g. Acacia melanoxylon; see Geldenhuys et al. 1986; Geldenhuys 1997). By contrast, the majority of the natural forest tree species have short-lived seeds, are bird or mammal dispersed and are tolerant of shade (Geldenhuys 1996c). Several of the perceived invasive alien species have three or more uses (Geldenhuys 1986). Many are introduced commercial timber species, used in plantation forestry such as the pines, eucalypts and acacias. Several of these species have considerable use value in rural households and contribute to the conservation of the natural forest by providing essential products, such as poles, laths and firewood (Geldenhuys & Cawe 2011; Geldenhuys et al. 2016). Most of these invasive alien species facilitate establishment and growth of shade-tolerant forest species (Geldenhuys 1996a, 1997; Geldenhuys & Delvaux 2007). Invader plant control was considered to be most effective through the management of the succession process (Geldenhuys et al. 1986).
Alien communities may facilitate natural forest establishment
Planted and naturalizing stands of shade-intolerant introduced tree species of pines, eucalypts, acacias and other species facilitate the establishment of shade-tolerant native forest species, with the development of alien plant communities (Knight et al. 1987; Geldenhuys 1992, 1996a, 1997; Parrotta 1995; Loumeto & Huttel 1997; Parrotta & Turnbull 1997; Geldenhuys & Delvaux 2007). These studies show variation in species composition and density with different nurse stand species, density of the nurse communities and development over time, and in response to the distance from the seed source of a natural forest stand. Such nurse communities may buffer the more variable and extreme conditions of regularly disturbed grassland, shrubland and agricultural cropland, and add litter to the site that ameliorates the nutrient and moisture content of the upper soil layer. This is a typical succession process which could take many years towards recovery of the mixed species mature forest community (Geldenhuys 1994). The forest succession process through the nurse communities can be manipulated through selective thinning to speed up the recovery towards mixed-species natural forests at relatively low cost (Fig. 3; Geldenhuys & Bezuidenhout 2011; Geldenhuys 2013b).
Case study: Buffeljags River
Two specific ecological studies were conducted at Buffeljags River, near Swellendam, South Africa, on the rehabilitation of alien invaded riparian zones and catchments using indigenous trees (Everson et al. 2016).
Assessing the invasion and establishment potential
Distribution of small, intermediate and large clusters of natural forest species in the nearby, intermediate and distant zones (Fig. 4) of the Black wattle (Acacia mearnsii) stands at Buffeljag River study area
Acacia meanrsii zone
Total number of clusters
Cluster size (stems of natural forest species)
Small (1–3 stems)
Medium (4–9 stems)
Large (10+ stems)
Number of clusters
Of the 40 tree and shrub species recorded in the forest patches, 28 species belonging to 20 families were recorded in the forest clusters. Five tree species were common throughout the wattle stand up to its western end: canopy species White stinkwood (Celtis africana), Cape beech (Rapanea melanophloeos) and White ironwood (Vepris lanceolata), and sub-canopy species Turkey berry (Canthium inerme) and Spike thorn (Gymnosporia buxifolia). Riparian forest was the primary seed source of establishing forest clusters, but moist and dry forest patches also contributed species to forest cluster development. Some larger clusters included large trees of common species, suggesting that they could have been part of remnant forest patches from an earlier forest clearing (Atsema-Edda 2014).
Dominant species in the surrounding regeneration had an adult tree within a cluster, such as Celtis africana, Rapanea melanophloeos, Vepris lanceolata or Maytenus acuminata;
Dominant species in a cluster was not dominant or was absent in the surrounding regeneration;
Some species were present in the regeneration zone outside a cluster and were absent from the tree cluster
decreased with increasing distance away from cluster;
increased with increasing distance away from cluster;
showed no pattern away from cluster.
Assessing the impact of browsing
In fire-adapted systems, the invasive alien tree and shrub species compete directly with the shade-intolerant grassland or shrubland species. Here the invasive alien species need to be cleared or eliminated before the natural vegetation systems and species can recover. A different approach is required in the natural forest environment because such clearing of the alien cover will keep the stand development in stages 1 and 2 (Fig. 3), i.e. perpetuate the invasion problem. A manipulative selective thinning of the stems of invasive alien plant stands has been advocated as a more cost-effective and long-lasting approach to the conversion of such stands to natural forest (Geldenhuys & Bezuidenhout 2011; Geldenhuys 2013b). The cover of the nurse stand systems (particularly the alien tree cover) can be removed from the system through gradual thinning of the nurse stand (Stages 2 & 3 in Fig. 3) until no more alien trees are present (stage 4 in Fig. 4). This management approach is possible because most alien ‘invaders’ are shade intolerant (nurse stands) but most natural forest species are shade tolerant. The nurse stand therefore facilitates the recovery of the natural forest system and species.
Clearing alien invaders and subsequent planting of indigenous tree species to rehabilitate natural forest is expensive. Personal practical experience of such a planting action in different areas has shown that natural regeneration outgrew the planted seedlings. Planting of indigenous tree or other species within the regrowing forest system will only be necessary for species that would not easily disperse there.
The areas with stands of invasive shade-intolerant species are usually grassland/shrubland that became invaded by alien tree stands because of a lack of fire, with possibly some areas of a degraded forest. Some of the invader plant stands have to be restored to productive land use, grassland or shrubland (mostly in the fire zone) and in the forest environment have to be restored to forest (in the fire-shadow areas). The rehabilitation area needs to be managed in four steps (Geldenhuys & Bezuidenhout 2011; Geldenhuys 2013b; Everson et al. 2016).
Step 2: Zone the ‘forest’ and riparian sites identified for rehabilitation to regrowth natural forest, by stand development stages (see Fig. 3). The criteria used for the differentiation include stands of different overhead composition (species) and ‘age’(size) and development stages of the indigenous woody vegetation in the understorey. The following criteria have been developed for assessment of the stand development stages in the natural forest zone (Geldenhuys & Bezuidenhout 2011; Geldenhuys 2013b):
Development stage 1: Dense, young nurse stands with many small stems of the nurse stand species, including stunted and suppressed stems. Gradually, as this stand grows taller, many stems die because of competition from the faster-growing stems. Very few to no indigenous tree species, or scattered individuals that existed before, may be present. Manual stand manipulation needs to be delayed until the stand develops through self-thinning. Where forest tree seedlings and saplings do appear, some invader plants can be removed (hand pulled, cut or ring-barked). The stands need to be inspected from time to time.
Development stage 2: The nurse stand is still relatively dense, but taller. Forest tree seedlings and saplings and species of other growth forms start to establish in the understorey, sometimes in small clusters. These species often include shade-tolerant tree species from a nearby natural forest, but they are small and develop slowly because of the relatively low light conditions, even though they are tolerant of some shade. The focus in stand manipulation should be on selective removal of invader plants around establishing natural forest seedlings to facilitate their growth into saplings and poles.
Development stage 3: Natural stand thinning is in progress. More stems of the nurse stand die back to create more space in the understorey, with more light. This allows the development of more forest tree clusters and the development of the establishing natural forest tree species into young tree stems amongst the fewer nurse stand trees. Tree clusters expand laterally with new plants establishing away from the cluster core. The focus in stand manipulation should be to remove all nurse stand plants within the clusters and to create just enough open space adjacent to the cluster to facilitate expansion of the cluster.
Development stage 4: This is the advanced stage of forest recovery, with most or all of the nurse plant stems removed (or dead). The forest structure is developing towards a continuous forest canopy and tree stems are found in a range of stem diameters as more species become established and the young stems grow into trees and into the canopy. At this stage the stand conditions are such that shade-intolerant invader plant species cannot establish, even if they are present in small numbers in the understorey. The focus in stand manipulation should be to remove all invader plants in a gap, or on the forest edge. It is a low-intensity activity that requires an occasional inspection to remove alien trees that may establish in gaps. Such stands could be used as a norm for recovery of stages 1 to 3.
The focus should be on the establishing forest species clusters;
A relatively closed nurse stand canopy should be maintained to prevent regeneration and regrowth of the nurse stand species. It should be opened above forest species clusters to help the forest seedlings, saplings and poles to grow stronger, and to flower and fruit earlier to enable fruit/seed dispersers to help with the process;
Small suppressed or stunted sub-canopy stems of the nurse stand species should be removed to provide more growing space for the regeneration and growth of forest species in the understorey;
The ground cover (herbaceous plants, litter, dead branches, etc.) should be kept intact to prevent invader or pioneer species from establishing. However, when the ground cover becomes too dense and high to exclude seedling establishment and growth, and becomes a shelter for rodents, it should be carefully removed.
Tree debris should not be burnt on the rehabilitation site: the generated heat will a) kill seeds of forest species that may be present in the organic and/or litter layers; b) destroy the soil organic layer and associated micro-organisms involved in the decomposition and nutrient cycling processes (sterilize the soil); and c) stimulate hard-coated seed of particularly invasive Acacia species to regenerate in mass. This will push the recovery process back for several years.
Woody stems should be stacked around developing seedlings to protect them from being browsed by livestock and antelope. The stacks should be at about 1 m or more from the developing seedlings and saplings to prevent antelope to reach the forest plant.
This general process facilitates the conversion of an alien community to a natural forest through selective removal of the invading nurse trees, when the nurse trees have relatively small stem diameters. However, sometimes the nurse stand is composed of large, branchy and useful trees of one or more alien tree species that could be used for timber, poles, fuelwood or charcoal (Geldenhuys & Bezuidenhout 2011). The first priority would then be to remove all utilizable stems before the rehabilitation actions are implemented. The non-harvestable tree debris could be left on site but in sections not longer than 2 m (as bird perch sites, as micro-sites for seedling establishment and small animals and micro-organisms, to protect seedlings against browsers, and as a slow-release ‘fertilizer’ through natural wood decomposition). However, if such an alien tree stand grows inside the riparian zone or on the banks of a river, then the first priority would be the safety and security of the river in terms of people’s lives, infrastructure (bridges, pipelines, water pumps, etc.), and ecotourism activities on and along the river. Then all alien trees should be removed in one action to prevent stream blockage during flooding, by drifting branch wood and deformed non-harvestable main stems, which may cause associated increased flood levels and infrastructure damage. If good native forest regeneration were present in such an alien tree stand, then care should be taken to minimize damage to the native tree regeneration by the falling cut nurse stand trees. But even if the native tree regeneration is damaged by the cut trees, then most of the regenerating woody species will recover through relatively fast vegetative regrowth. In this latter case of removing large tree debris along the river, the non-harvestable tree debris could be burnt on site with care to keep the burnt spot footprint small and to prevent scorch of the regenerating plants of native woody species.
Step 4: Planting of forest seedlings should be limited to spots in the stand with no forest seedlings or to specific species that do not disperse easily. Seedlings could be collected from clusters of seedlings (maximum 50% of the seedlings in dense clusters) from the nurse stand, drainage lines along local roads, or the nearby natural forest. Such planting should be done during misty or rainy weather to ensure success. Where tree debris was burnt, it is necessary to wait until the ash bed has cooled down, and cover the burnt site with sods of top soil and small plants from the areas around the burnt spot to ensure rapid recovery.
Natural pioneer and introduced plantation/invader species can become a partner in forest rehabilitation towards recovery of biodiversity and productivity. The natural succession process with the invasive alien plants as the pioneer nurse stand can be used to reduce costs. The aliens provide a rapid ground cover, which prevents erosion, adds litter to a degraded site which would help to restore nutrient cycling and increase the moisture content of the growing medium. The nurse stand provides shade and reduces strong wind to facilitate the establishment of shade-tolerant natural forest species and thereby recover the biodiversity as in any native pioneer stand. The results from the various rehabilitation actions according to this particular manipulative approach have shown that a pioneer or planted or naturalized nurse stand can be converted to natural forest, cost-effectively, if it is not in the normal fire zone. The manipulative ‘interference’ through selective thinning facilitates and speeds up the development towards a natural forest community. A practice of repeated clearing of invader plants, for example in the riparian zone, can be turned into a system of regrowing forest with highly reduced maintenance costs. The removed stems could be used for laths, poles, firewood, timber, depending on the development stage of the nurse stand. Medicinal plant crops and other forest species could be harvested from such stands (Geldenhuys & Delvaux 2007). The practice therefore provides options for small business development parallel to the rehabilitation process. This new approach requires a different mind-set providing a variety of benefits to the owner of the land, as well as adjacent communities in many areas.
Shade-tolerant invasive alien plants require different approaches (Geldenhuys 2011c). For such species, it is important to remove the seed source by ring-barking the mature trees. In the case of the climbing cactus, Pereskia aculeata, the entire host tree has to be removed, and all material of this species needs to be burnt on site to prevent vegetative regrowth. The basal parts of the climber Dolichandra unguis-cati need to be cut so that the stems in the crowns die. It is a much more complex action, but in most cases the problem is relatively confined to groups of trees.
I wish to express my appreciation for the invitation to participate in the “traveling workshop” organized by the Forest Ecosystems editorial team in China during May 2017.
CJG developed the concept over time and guided the specific research on the experimental part reported on. AA-E did the study on the forest species invading the invader plant stand and completed her MSc study on it. MWM did the study on the browsing impacts and is still busy with writing up her MSc. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- Aronson J, Floret C, Le Floc’h E, Ovalle C, Pontanier R (1993) Restoration and rehabilitation of degraded ecosystems in arid and semi-arid lands. I. A view from the south. Restor Ecol 1:8–17View ArticleGoogle Scholar
- Atsame-Edda A (2014) Regeneration dynamics of natural forest species within a stand of the invasive alien Acacia mearnsii along the Buffeljagsrivier, Swellendam. South Africa. Unpublished MSc thesis, Stellenbosch University, p 122Google Scholar
- Berliner DD (2009) Systematic Conservation Planning for South Africa’s Forest Biome: An assessment of the conservation status of South Africa’s forests and recommendations for their conservation. PhD thesis. In: University of Cape Town. Rondebosch, South AfricaGoogle Scholar
- Boy G, Witt A (2013) Invasive alien plants and their management in Africa. Synthesis Report of the UNEP/GEF Removing Barriers to Invasive Plant Management in Africa (RBIPMA) Project. Gutenberg Press, Malta, CABI Africa, p 177Google Scholar
- Cooper KH (1985) The conservation status of indigenous forests in Transvaal, Natal and Orange Free State, South Africa. Wildlife Society, Durban, South AfricaGoogle Scholar
- Dean WRJ (1985) The utilization of Solanum mauritianum woodlands by birds in the north-eastern Transvaal. In: Bunning LJ (ed) Proceedings of the Symposium on Birds and Man. Witwatersrand Bird Club, Johannesburg, pp 89–97Google Scholar
- Edwards M (2011) Introduction. In: Bredenkamp BV, Upfold SJ (eds) South African forestry handbook, 5th edition. Southern African Institute of Forestry, Pretoria, pp 13–17Google Scholar
- Everson CS, Scott-Shaw BC, Geldenhuys CJ, Starke A, Atsema-Edda A, Schutte S, Mupemba Mwamba R (2016) Rehabilitation of alien invaded riparian zones and catchments using indigenous trees: An assessment of indigenous tree water-use. Volume 1: Research Report. WRC Report No 2081/1/16, Water Research Commission, Pretoria, pp 240Google Scholar
- Everson CS, Starke A, Geldenhuys CJ, Atsema-Edda A, Scott-Shaw BC (2016) Rehabilitation of alien invaded riparian zones and catchments using indigenous trees: An assessment of indigenous tree water-use. Volume 2: The potential for natural forest regeneration within stands of invasive alien trees: Forest rehabilitation guidelines. WRC Report No TT 677/16, Water Research Commission, Pretoria, pp 24Google Scholar
- FAO (2003) Proceedings of Workshop on tropical secondary forest management in Africa: Reality and perspectives. Nairobi, Kenya, 9-13 December 2002. Geldenhuys C, Castañeda F, Savenije H, Kuzee M (eds) FAO, Rome, pp 390Google Scholar
- Feely JM (1986) The distribution of Iron Age farming settlement in Transkei: 470 to 1870. University of Natal, Pietermaritzburg, MA thesis, p 224Google Scholar
- Geldenhuys CJ (1986) Costs and benefits of the Australian Blackwood, Acacia melanoxylon, in South African Forestry. In: Macdonald IAW, Kruger FJ, Ferrar AA (eds) The ecology and management of biological invasions in Southern Africa. Oxford University Press, Cape Town, pp 275–283Google Scholar
- Geldenhuys CJ (1991) Distribution, size and ownership of the southern Cape forests. South Afr Forest J 158:51–66View ArticleGoogle Scholar
- Geldenhuys CJ (1992) Plantation forestry can contribute to the conversion of open areas to rainforest. Annals of FOREST'92: 2nd International Symposium on Environmental Studies of Tropical Rainforests, Rio de Janeiro, Brasil. May 1992Google Scholar
- Geldenhuys CJ (1993) Composition and dynamics of plant communities in the southern Cape forests. Report FOR-DEA 612. Division of Forest Science and Technology, CSIR, Pretoria, p 56Google Scholar
- Geldenhuys CJ (1994) Bergwind fires and the location pattern of forest patches in the southern Cape landscape, South Africa. J Biogeogr 21:49–62View ArticleGoogle Scholar
- Geldenhuys CJ (1996a) The Blackwood Group System: its relevance for sustainable forest management in the southern Cape. South Afr Forest J 177:7–21View ArticleGoogle Scholar
- Geldenhuys CJ (1996b) Forest management systems to sustain resource use and biodiversity: examples from the southern Cape, South Africa. In: Van der Maesen LJG, Van der Burgt XM, Van Medenbach de Rooy JM (eds) The Biodiversity of African Plants. Proceedings of the XIVth AETFAT Congress, Wageningen, The Netherlands. Kluwer Academic Publishers, Dordrecht, pp 317-322Google Scholar
- Geldenhuys CJ (1996c) Fruit/seed characteristics and germination requirements of tree and shrub species of the southern Cape forests. Report FOR-DEA 954, Division of Forest Science and Technology, CSIR, PretoriaGoogle Scholar
- Geldenhuys CJ (1997) Native forest regeneration in pine and eucalypt plantations in Northern Province, South Africa. Forest Ecol Manag 99:101–115View ArticleGoogle Scholar
- Geldenhuys CJ (2002) Acacia melanoxylon in South Africa: commercial and conservation issues in resource management. In: Brown AG (ed) Blackwood management: Learning from New Zealand. International Workshop, Rotorua, New Zealand, pp 28–35Google Scholar
- Geldenhuys CJ (2009) Managing forest complexity through application of disturbance-recovery knowledge in development of silvicultural systems and ecological rehabilitation in natural forest systems in Africa. J. Forest Res 15:3–13View ArticleGoogle Scholar
- Geldenhuys CJ (2010) Expert opinion on issues related to damage by lightning-caused wildfire(s) from the Tsitsikamma Mountains to the MTO Timber plantations in that area during October–November 2005. FW-05/10. Forestwood cc, Pretoria, p 26Google Scholar
- Geldenhuys CJ (2011a) Disturbance and recovery in natural forests and woodlands in Africa: Some concepts for the design of sustainable forest management and rehabilitation practices. In: Geldenhuys CJ, Ham C, Ham H (eds) Sustainable Forest Management in Africa: Some Solutions to Natural Forest Management Problems in Africa. Proceedings of the Sustainable Forest Management in Africa Symposium. Stellenbosch, 3–7 November 2008, pp 61–70Google Scholar
- Geldenhuys CJ (2011b) Natural forests, conservation and development. SA Forestry Magazine February 2011:29–30Google Scholar
- Geldenhuys CJ (2011c) Most invasive alien plants facilitate natural forest recovery – how is that possible? SAPIA News 18:2–5Google Scholar
- Geldenhuys CJ (2013a) c by natural ecological processes through native or alien tree stands. In: Beau N, Dessein S, Robbrecht E (eds) African plant diversity, Systematics and Sustainable development. Proceedings of the XIXth AETFAT Congress, Antananarivo, Madagascar, 26-30 April 2010. Scripta Bot Belg 50: 21-32Google Scholar
- Geldenhuys CJ (2013b) Converting invasive alien plant stands to natural forest nature’s way: Overview, theory and practice. In: Jose S, Singh HP, Batish DR, Kohli RK (eds) Invasive plant ecology, CRC Press, Taylor & Francis Group, Boca Raton, pp 217–237Google Scholar
- Geldenhuys CJ, Bezuidenhout L (2011) Rehabilitation of natural forests using stands of alien trees of plantations or invasions as allies. In: Bredenkamp BV, Upfold SJ (eds) South African forestry handbook, 5th edition. Southern African Institute of Forestry, Pretoria, pp 585–604Google Scholar
- Geldenhuys CJ, Cawe SG (2011) Matching resource use needs with resource status and population dynamics of target species in Transkei Coastal Forests to sustain resource use, Port St. Johns Forest Estate, South Africa. In: Geldenhuys CJ, Ham C, Ham H (eds) Sustainable Forest Management in Africa: Some Solutions to Natural Forest Management Problems in Africa. Proceedings of the Sustainable Forest Management in Africa Symposium. Stellenbosch, 3–7 November 2008, pp 264–279Google Scholar
- Geldenhuys CJ, Delvaux C (2007) The Pinus patula plantation … A nursery for natural forest seedlings. In: Bester JJ, Seydack AHW, Vorster T, Van der Merwe IJ, Dzivhani S (eds) Multiple use management of natural forests and woodlands: Policy refinement and scientific progress. Natural Forests and Savanna Woodland Symposium IV, Port Elizabeth, South Africa, 15-18 May 2006, pp 94-107Google Scholar
- Geldenhuys CJ, Funda O, Aromaye T, Mugure MW (2016) Evaluation of natural forests in the Ntabelanga quaternary catchments in the Maclear area in relation to resource use management. Report Number FW-04/16. Forestwood cc, Pretoria, p 76Google Scholar
- Geldenhuys CJ, Le Roux PJ, Cooper KH (1986) Alien invasions in indigenous evergreen forest. In: Macdonald IAW, Kruger FJ, Ferrar AA (eds) The ecology and management of biological invasions in Southern Africa. Oxford University Press, Cape Town, pp 119–131Google Scholar
- Geldenhuys CJ, MacDevette DR (1989) Conservation status of coastal and montane evergreen forest. In: Huntley BJ (ed) Biotic diversity in southern Africa: concepts and conservation. Oxford University Press, Cape Town, South Africa, pp 224–238Google Scholar
- Geldenhuys CJ, Van der Merwe CJ, Jacobs CJ (1994) Lightning: a disturbance factor in the mixed evergreen forests of the southern Cape. Report FOR-DEA 833, Division of Forest Science and Technology, CSIR, PretoriaGoogle Scholar
- Henderson L (2001) Alien weeds and invasive plants: A complete guide to declared weeds and invaders in South Africa. Plant Protection Research Institute Handbook No. 12, Agricultural Research Council, Pretoria, South Africa: Plant Protection Research Institute, pp 300Google Scholar
- Hobbs RJ (2004) The Working for Water programme in South Africa: the science behind the success. Divers Distrib 10:501–503View ArticleGoogle Scholar
- Knight RS, Geldenhuys CJ, Masson PH, Jarman ML, Cameron MJ (eds) (1987) The role of aliens in forest edge dynamics: a workshop report. Occasional Report No 22, Ecosystem Programmes. FRD, CSIR, Pretoria, p 41Google Scholar
- Kohli RK, Jose S, Singh HP, Batish DR (eds) (2009) Invasive plants and forest ecosystems. CRC Press, Boca Raton, FL, p 437Google Scholar
- Lawes MJ, Obiri JAF, Eeley HAC (2004) Indigenous forests and woodlands in South Africa: Policy, people and practice. In: Lawes MJ, Eeley HAC, Shackleton CM, Geach BGS (eds) University of KwaZulu-Natal Press. Scottsville, South Africa, pp 227–273Google Scholar
- Loumeto JJ, Huttel C (1997) Understorey vegetation in fast-growing tree plantations on savanna sols in Congo. For Ecol Manag 99:65–81View ArticleGoogle Scholar
- Macdonald IAW, Kruger FJ, Ferrar AA (eds) (1986) The ecology and management of biological invasions in southern Africa. Proceedings of the National Synthesis Symposium on the ecology of biological invasions. Cape Town. South Africa, Oxford University Press, p 324Google Scholar
- Mala AW, Geldenhuys CJ, Prabhu R (2009) Local conceptualization of nature, forest knowledge systems and adaptive management in southern Cameroon. In: Parrotta JA, Oteng-Yeboah A, Cobbinah J (eds) (2009) Traditional forest-related knowledge and sustainable forest management in Africa. IUFRO World Series Vol 23, pp102–110Google Scholar
- Mucina L, Geldenhuys CJ (2006) Afrotemperate, subtropical and azonal forests. In: Mucina L, Rutherford MC (eds) The vegetation of South Africa, Lesotho and Swaziland. Strelitzia 19, pp 584–614Google Scholar
- Palm CA, Vosti SA, Sanchez PA, Erickson PJ (eds) (2005) Slash-and-burn agriculture: The search for alternatives. Columbia University Press, p 463Google Scholar
- Parrotta JA (1995) Influence of overstory composition on understory colonization by native species in plantations on a degraded tropical site. J Veget Sci 6:627–636View ArticleGoogle Scholar
- Parrotta JA, Turnbull JW (1997) Special Issue: Catalyzing native forest regeneration on degraded tropical lands. For Ecol Manag 99:1–290View ArticleGoogle Scholar
- Rutherford MC, Westfall RH (1986) Biomes of southern Africa: an objective categorisation. Memoirs of the Botanical Survey of South. Africa 54:1–98Google Scholar
- Von Maltitz G, Mucina L, Geldenhuys CJ, Lawes MJ, Eeley H, Aidie H, Vink D, Fleming G, Bailey C (2003) Classification system for South African Indigenous Forests. An objective classification for the Department of Water Affairs and Forestry. Unpublished report, No. ENV-P-C 2003–017. Environmentek, CSIR, Pretoria, p 275Google Scholar