Journal Archives > BGCNews > The Experience of Foresters in Re-establishment and Habitat Restoration
The Experience of Foresters in Re-establishment and Habitat Restoration
Volume 2 Number 2 - July 1993
A major requirement traditionally placed upon foresters has been to maximise timber production in order to attempt to satisfy growing demands for wood and wood-based products. As such, their interest in species re-establishment has tended to be limited to where it has appeared to be the most efficient approach to producing wood. Even in these cases, however, the forest structure has generally been manipulated to favour the merchantable species, often at the expense of the native flora of the site. The concept of completely restoring forest habitats for their ecological value is relatively new, although interest in it appears to be growing.
This paper is an attempt to draw on the experience of foresters in the area of natural forest management in order to identify key points which need to be considered when attempting to re-establish plant species and restore habitats.
Approaches to Manipulation of Forest Structure
The simplest form of timber exploitation relies on clear felling without replanting. While this approach is taken in some areas, increasing knowledge of the silvicultural requirements of many species has provided foresters with opportunities to develop systems of crop management aimed at providing a sustained yield of timber. Plantation silviculture using exotic species represents the most extreme approach to habitat modification to achieve this result, but other approaches involving more subtle, although often more complex, adaptations of natural ecosystems have been widely practiced and provide examples of foresters' experiences with manipulation of forest structure to provide favourable conditions for certain species.
There are two approaches to natural forest management of particular relevance to this subject; management to favour natural regeneration, and enrichment planting. Factors which influence the choice of one of these approaches include, the objective of forest management, the level of damage incurred by the forest during felling and extraction, the level of knowledge of the regeneration and silvicultural requirements of the species in question, the costs involved, and the level of management expertise available.
In order for any species to become established, or re-established, in an ecosystem it must be able to reproduce itself successfully. Foresters have consequently developed a series of ecologically sustainable management techniques designed to favour natural regeneration. These approaches were developed in temperate regions (the most common being clear cutting following seedfall and various types of shelterwood systems) and later modified for the tropics. A key feature of all of these systems is a thorough knowledge of the requirements for successful seed production (ITTO, 1989), and the ecological requirements for germination and establishment of seedlings of the species.
Coupled with these management techniques is a need to protect the young trees from competing weeds and browsing animals, which may involve employing either physical or chemical barriers (Yellappa Reddy et al, 1987). Unfortunately, timber demands have led to increased pressures on natural forests, particularly in the tropics, with the result that, in some cases, the more ecologically sensitive techniques used in the past which allowed regeneration of the timber crop have been phased out and replaced by systems which produce a greater yield in the short term but which appear to be less sustainable from an ecological point of view (Chin, 1989).
Natural regeneration should eventually become the system of renewal to sustain any re-established species and, if any doubt exist regarding management techniques, this approach should be considered as it is undoubtably of lower adverse ecological impact than many other systems of stand establishment (Synnott and Kemp, 1976).
It is not always possible to restore a species or habitat through immediate attempts at natural regeneration. From the forestry perspective, the species already on the site and most successful at propagating themselves may not be the ones required for timber production, and it may therefore be necessary to artificially manipulate the stand composition in order to encourage growth of a population of the desired species.
The most common approach to this situation is employment of a silvicultural system known as enrichment planting. This technique, sometimes known as line planting, strip planting, group planting, gap planting or diffused planting, involves planting trees which will form the final crop within gaps created artificially in the natural forest. Enrichment planting has been applied to most rainforest countries, although no absolutely successful method has been devised (Evans, 1982). It is, however, the best way of introducing a selection of species characteristic of ancient undisturbed forest into degraded secondary forest, to speed up their succession (T.J. Synott, Oxford Forestry Institute, pers. comm.), and can be particularly useful where natural regeneration has been uneven (Bustomi and Soemarna, 1986).
At its most extreme, large scale replanting follows soon after felling; a system which is preferred over natural regeneration in many situations because it is much quicker. Examples where this has been successful include replanting teak on cut over teak forests in Asia; Eucalyptus regnans in Tasmania; and Aucoumea, Terminalia, and Triplochiton in West Africa. On the other hand, examples of comparative failures include species in which pests and diseases of limited importance on scattered trees in multi-specific natural forests cause catastrophic losses or damage in mono-specific close plantations, eg. Phytolima lata (the psyllid gall-maker) on Chlorophora excelsa (R.L. Willan, Forestry Consultant, Oxford, pers. comm.).
Most examples of enrichment planting are, however, concerned with planting small areas such as lines or groups in canopy openings within an existing matrix of trees, and there are numerous examples throughout this century of management designed to achieve these results.
As the aim of enrichment planting is to ensure the continued or increased production of chosen timber species, rather than preservation of the forest ecosystem, the management regime has to reflect this and consequently has a strong influence on the structure of the forest. This impact is, however, much less severe than many other approaches to forest regeneration and flora can recover to give the visual impression of natural forests. Although native species are generally not employed in this system, there are examples where they have been used with great success, eg. enrichment with mahogany in logged-out forests in South America (R.L. Willan, pers. comm.), and in Queensland, Australia, with native species in both rain forest and wet-sclerophyll forest areas (Conservator of Forests, Queensland Forestry Service, Brisbane, pers. comm.). In these cases not only has enrichment planting with species already naturally present on the site proved to be the most productive method of forest management, but also the continued regeneration, and hence conservation, of the species is guaranteed.
Factors Influencing Success with Enrichment Planting
The success or failure of enrichment planting depends partly upon the ecological situation (some species and forest systems respond better than others), and partly on management efficiency. Factors which have been shown to influence success include suitable age of transplants, provision of the correct amount of shade, adequate supervision at planting, sufficient follow up (ITTO, 1989), availability of local species and provenances, and control of competing vegetation (Synnott and Kemp, 1976; Willan, 1989) and browsing animals (this is particularly important with small plants, and in island communities where alternative browse may be scarce).
Enrichment planting is most successful when using species characteristic of early stages of succession, but they must also be fast growing, particularly in height, straight stemmed, and self-pruning (Willan, 1989; from Dawkins, 1958). It is better if browsing animals are excluded from the site, but if that is not possible, in some cases the lower leaves of the trees should be removed until they are out of their reach (Sharma, 1979). Success has also been improved when planting is concentrated in lines or bands, rather than when felling, and consequently replanting, is restricted to only a few stems (Willan, 1989). Seedling size and gap size in the canopy can be important determinants in seedling survival, with larger seedlings and smaller gaps providing greatest success in one experimental area in Borneo (Royal Society, 1991).
The choice of species for use in enrichment planting has tended to rely largely on experience, but the number of failed plantings tends to suggest that there is great scope for trials to evaluate the performance of indigenous species in relation to light, moisture and nutrient levels (Carvalho and Stohr, 1978; Whitmore and Bowen, 1983).
Although the cost of enrichment planting is generally high, if properly managed it can produce good survival rates, which is vital in stabilizing the ecosystem and reducing the loss of other resources from the site (Molas and Pretzsch, 1989; Synnott and Kemp, 1976).
Information Required Before Attempting Re-establishment
The reasons for loss of the species from the site in question must be understood before any attempts are made at re-establishment as the findings may well influence the approach taken, and the level of success which can be expected. The possible reasons for extinction of trees from sites are many; some are the result of natural forces while others involve human beings directly. For example, species may have problems in maintaining numbers due to disease (in which case resistant strains may be required when attempting re-establishment (Burnham, 1988), the surrounding habitat may have been changed to such a degree that key survival factors are altered for the species (for example changing stand structure can reduce the number of seed-bearing trees (Rakotonirina and Prelaz, 1982), natural regeneration may be impossible due to browsing pressure, or the species may simply have been logged out of existence.
If it is decided that a re-establishment programme can proceed then the following factors need careful consideration:
a) Size of the re-establishment area
It is desirable to restore as much of the original community of plants and animals as possible in order to produce maximum robustness in the ecosystem. There are few reported examples of attempts to re-establish tree species within a framework of ecological restoration, but those that there are emphasise the necessity to have a broad genetic base for the introduced population, not only to provide the new population with a wide range of alleles to allow for adaptability and change, but also to provide the numbers of individuals required to produce a viable breeding population in order that natural regeneration can sustain the species in the future. The actual size of the area required will vary with species, depending on factors such as its breeding system, its pollinators and the number of individuals which will make up a robust gene pool for the site. Examples which illustrate these points are given below but it is worth noting that they all highlight the importance of a thorough knowledge of the ecology of the forest system into which planting is to take place before one can answer the question of what area can be restored.
b) Situation of the re-establishment area
An important consideration when planning an area for re-establishment is the surrounding environment. Unless the area is isolated from the influences of flora and fauna in surrounding habitats, it is quite likely that invasion of the re-establishment area by the surrounding vegetation will occur, sometimes producing a mixed stand quite different from the one intended (Burdon and Chilvers, 1977). In many cases, due to financial or land ownership constraints, this is an inevitable situation which can either be tolerated and encompassed in the management plan or moves can be made to eradicate altogether such exotics as occur. This can take various approaches, from situating the area within a buffer zone in order to manage the surrounding land to control weeds and pests (Greenwood and Skipworth, 1979) to physical removal of regenerating exotics or planting of native species within the re-establishment area to shade them out (Anon., 1990).
c) Modification of the site
The physical characteristics of the site may need modifying before the habitat is capable of sustaining the desired species. Tropical foresters have long been involved in the reclamation and stabilization of depleted soils utilizing a great variety of techniques, from civil engineering of terraces and retaining walls, to contour lines of grasses and trees and the direct sowing and planting of understorey vegetation. Even with these techniques, however, the soil degradation which follows clearance of tropical forests generally requires site recapture with suitable species to stop the decline continuing. The most effective approach is to attempt to establish species which will develop a rapidly recycling surface organic pool of nutrients, thereby reintroducing the pattern of nutrient dynamics that is evident in native forests. It has been found that deep rooted tree species appear to be most effective to achieve this result (Charley, 1983; Woo and Lee, 1989).
d) The successional nature of site recapture
Management of the site to support the chosen species may involve promoting or introducing other species which will eventually assist in creating a suitable environment for the desired species to establish and reproduce. Site recapture can, however, be greatly speeded up by use of appropriate management techniques, eg. controlled grazing of exotics (Anon, 1990), or planting of grassland with aggressive or nitrogen fixing species which will provide the conditions needed to allow succession by other selected woody species (Whitmore, 1984).
It is noteworthy that increasing pressures on land, heightened by increases in population, will inevitably lead to the conversion of the more fertile soils to agricultural use. This will lead to forestry, and hence any forest restoration projects, being focused more and more on infertile and marginal sites. In these instances, the role of nitrogen-fixing trees as soil improvers playing a part in the successional nature of re-establishment will undoubtably become more important (Duboux and Dommergues, 1985).
If the site already has exotic trees growing on it, it is possible through careful stand manipulation to favour regeneration of native species. This may take many years but can, if managed correctly, utilize the cover provided by the exotic trees to benefit the restoration of the native flora (Gilmour et al, 1990; Otto, 1985; Pimm, 1979).
e) Ecology of the area and the silvicultural requirements of the species
The importance of a thorough understanding of the ecology of the area and the silvicultural requirements of the species in question cannot be underestimated in any attempts to re-establish trees. Successful forest plantations in both temperate and tropical areas have only been established following detailed knowledge of both the plant succession of the factors influencing regeneration, although this may take some time to acquire (Gradwohl and Greenberg, 1988; Poore, 1989; J. Wyatt-Smith, Forestry Consultant, Oxford, pers. comm.).
The plant and animal relationships on the site in question must be given careful consideration if the habitat is to become self-sustaining. An obvious example is the need to accommodate populations of pollinators and seed dispersers, whether these are mammals, birds (Clout, 1984), bats or insects, by the provision of appropriate food plants, cover, and breeding and roosting sites. Other, perhaps more subtle, relationships (for example those between pests and predators) should also be examined. It must, however, be appreciated that in tropical forests, the incredible complexity and diversity of the ecosystems, and the lack of associated knowledge, make restoration particularly difficult (Gradwohl and Greenberg, 1988). In contrast, some ecosystems are more simple and hence re-establishment of the native flora is a more straightforward exercise. For example, Corylus avellana and Tilia spp. mixed forests in eastern England have already withstood one rotation of Pinus nigra var. maritima and appear to be present in such numbers that there would be little problem in their re-establishment if the pine was removed (J.E.J. White, U.K. Forestry Commission, pers. comm.).
An example of the value of knowledge of the ecological history of a site is to be found in West Africa where foresters worked for over two decades to achieve the success of their Malayan counterparts with management of the tropical rain forest. They attempted to restore Meliaceae to dominance in the coastal rain forest and largely failed, using similar techniques to those employed in Malaysia. Results from eco-physiological experiments indicated that the foresters had not taken into account that the forests were of much more recent origin to that which they had assumed, the original forest having been modified greatly by local human populations, and therefore required different approaches to regeneration (J.R. Palmer, Forestry Consultant, Oxford, pers. comm.).
f) Propagation facilities and their situation
Foresters are well aware of the value of healthy plants in successful establishment and have consequently placed great emphasis on knowledge relating to seed pretreatment and all aspects of nursery practices for each species grown, from use of correct growing media to providing the seedlings with the correct amount of light. The importance of locating nurseries as close as possible to the planting site is well recognised and not only provides the seedlings with a similar climate to grow in as the nursery but also reduces the time in transit between nursery and planting site when the plants are likely to suffer various degrees of physical damage. Adherence to these principles was a major reason for the success in re-establishing the ebony (Trochetiopsis melanoxylon) on St Helena (Q.C.B. Cronk, Dept. of Plant Sciences, Univ. of Oxford, pers. comm.).
Genetic Composition of the Re-established Population
When re-establishing a species it is of paramount importance to consider the genetic composition of the new population. It is advisable to use seed of the local provenance as this has the best possibility of being adapted to local conditions, thereby reducing ecological disruption during re-establishment. This is, unfortunately, an ideal which is rarely achievable due to the very fact that the local population is generally no longer present on site in restoration projects. In some cases, it may be possible to obtain seed of the local provenance either from seed banks or from population growing in other countries, but in the latter case caution has to be applied as the exotic environment may have exerted selection pressures on the population subsequently altering its allelic frequencies to such an extent that any germplasm collection for re-establishment may be quite different to that which was found originally on the site. For example, Delonix regia is a leguminous tree which, although thought to be extinct in its native Madagascar until 1932, is a fairly common street tree in the tropics. This large ex situ conservation resource would be potentially useful if re-establishment was ever required, but any land races which may have developed may not re-establish easily (R.L. Willan, pers. comm.). If seed from a local provenance is not available it is wise to attempt to use seed from an area which has similar ecological characteristics.
The composition of the re-established population should ideally be broad enough to give the greatest possibility of having captured suitable alleles for the population to survive and regenerate successfully. This requires collecting seed from as many independently crossing individuals as possible from the trees chosen to form the basis of the re-established population. Populations arising from small collections may be prone to disease, insect attack, or simply unable to survive in different habitats due to their narrow genetic base.
In a stand where native populations have become mixed with introduced provenances, management to favour local provenances can be employed, in theory, if they can be identified by morphological methods (Pederick, 1976), or by biochemical means. This situation is unfortunately rarely attainable and gene pools of native trees are frequently contaminated with exotic provenances to such an extent that separation of the local population is impossible. This situation has become increasingly commonplace in the U.K. with movement of seed of hardwood species around the country and more recently, with large numbers of trees being imported from continental Europe. As growth rates and phenology are likely to vary between populations, this mixing of provenances has implications not only for the intrinsic value of any associated re-establishment programme, but also in its ability to support associated flora and fauna in an ecosystem similar to the original. Likewise, the potential for any re-established population crossing with other species or sub-species within pollination distance must also be considered.
Another important point to consider in any attempt at re-establishment is the genetic history of the species or population in question. Many tree species have suffered contraction of their ranges in the past due to climatic forces and have either become isolated as species or formed small isolated populations. When this occurs the population is liable to randomly fix non-adaptive genes due to inbreeding and therefore exhibit distinctive characteristics. Such genetic drift is suspected as being the cause of the unusual behaviour of several species of southern spruce, where they appear to have no set combination of characteristics indicative to adaptation to southern climates (Wright, 1976). Whilst the elimination of many deliterious genes may appear to be of benefit to the species or population these groups are in fact in a precarious situation due to their lack of adaptiveness caused by their uniformity. Not only are such populations particularly threatened by climatic changes but they could also prove extremely difficult to re-establish in their natural habitat should the need arise. This situation is likely to be of less concern with tropical trees than with temperate ones as the climate is more stable and therefore adaptability for climatic extremes is less important, furthermore, trees are adapted to self-fertilization due to the relative low density of trees of the same species in tropical forests in comparison with temperate forests.
Involvement of Local People
The involvement of the local human population can be instrumental in the success of re-establishment projects. If care and consideration is taken regarding the projected impact of a re-establishment project and local people are involved in the process and perceive a value, whether that be commercial, social or cultural, then they are much more likely to take active steps in the protection of the area (Chetty, 1989). As far as trees are concerned, this may mean maintenance of multiple use of the area, or production of multiple products from it (Gilmour et al, 1990). Re-establishment of forest trees through natural regeneration can also be encouraged if local people see it as a viable alternative to buying planting stock. Management of restoration sites could be taken over by local trusts, but in this case the question of access to a potentially ecologically sensitive site by trust members has to be addressed (Greenwood and Skipworth, 1979).
Examples of Successful Forest Restoration Projects
Guanacaste National Park Project, Costa Rica
Since 1986, the Guanacaste National Park Project has attempted to restore a large area of degraded land back to dry forest. The project has three objectives: firstly, to use leftover fragments of intact dry forest to restore an area of about 700 sq. km. to a condition where it is able to sustain all of the flora and fauna present in Costa Rica at the time the conquistadors arrived; secondly, to be economically useful to the people of the surrounding area; and, thirdly, to re-establish the cultural links of the local people to the land. They have attempted to achieve these goals by employing a freeze on agricultural development in the area, purchasing land for the project, designing a management plan for the whole park, carrying out relevant research, and instigating educational programmes for local people and tourists.
A major objective of the restoration project is to restore the large amount of land within the park which is currently covered with African grasses back to dry forest. Fires in the park have been identified as a major handicap in attempts to restore the natural woody vegetation and great effort has consequently focused on reducing their incidence. Ironically, it has been found that cattle grazing can have a positive effect on attempts at restoration due to their ability to reduce the amount of grass which could burn should a fire start, and a policy of controlled grazing has therefore been instigated.
Control of fires would not, in the short term, lead to recreation of the dry forest, as, although it assists woody plant regeneration, the species colonising the regenerating areas are predominantly wind borne dispersers and these only account for about one-fifth of dry forest species. Furthermore, wind dispersal is not necessarily associated with early successional species. To encourage seed dispersal by birds of such species, they have planted large trees and hedgerows which they hope willl assist in the development of a forest of fruit-bearing trees and bushes.
Although positive aspects of restoration have already been achieved, the managers of the park do not expect natural high forest to reappear for at least 300 years, and even then they accept that it may take even longer to completely restore the natural ecosystem again (Anon., 1990; Gradwohl and Greenberg, 1988).
Lomas Barbudal, Costa Rica
Lomas Barbudal is also a dry forest area where many of the same problems and approaches to park management which have arisen in Guanacaste are being encountered. An outstanding feature of Lomas Barbudal, however, is the approach taken to education and involvement of local people in park affairs. This begins at school where the park runs education programmes in association with outside funding agencies. One of the objectives of such programmes is to identify and encourage outstanding students who can then become more closely associated with the restoration work in the park. They have also improved the educational materials present in their visitor centre and established a trail system running through the park. A management group has been established which works closely with local landowners on a variety of activities and capacities, and also uses local people whenever possible in park management, particularly for their knowledge of fire control. The management committee also undertakes programmes which are of benefit to both the park and local communities, eg. for road repairs and the establishment of a tree nursery. All of these actions have not only led to an appreciation of the restoration project by local people but also gained their support in achieving its objectives (Anon., 1989).
The multinational mining company Minera‡ao Re-establishmentio do Norte instigated a programme designed to reclaim open cast mining areas using a wide variety of native species. Their approach was to plant seedlings in a mixture of leguminous ground cover and exotic nurse trees. The trees were then tended by weeding, elimination of leaf-cutting ants and some replanting. Protection and care in the first year was found to be essential for the success of the plants. They found that this approach was successful with the trees responding well to the fast growing nurse trees and leguminous ground cover. The fast growing species appeared to have a particularly beneficial influence on soil microclimate. In areas of intensive treatment, unplanted forest species have since reseeded and a humus layer is developing, although this has been hampered somewhat by the approach to mining which did not leave islands of trees to act as a source for natural regeneration.
Problems which have arisen include insect damage to saplings planted close to existing forest, rapid colonisation by leaf cutting ants, toxicity of the soil for some species, and lack of knowledge on how to break seed dormancy and silvicultural requirements for some species (Gradwohl and Greenberg, 1988).
Nonsuch Island, Bermuda
The small size of the Bermudan islands, coupled with human population pressures has led to a depletion of much of the native flora. For example, 96% of the indigenous population of Juniperus bermudiana, Bermuda cedar, was killed between 1946 and 1951 due to the accidental introduction of 2 scale insect species. A recent attempt to re-establish native plants as part of a restoration project focused on one of the smaller islands. The programme proceeded by three steps: firstly, elimination of all exotic flora (and banning of importation of any exotics); secondly, native plants have been imported, both from the main island and abroad, and major reafforestation has taken place: and lastly, additional habitats and ecological niches have been created for birds, using artificial techniques. Complete elimination of exotic flora has not been totally successful due to financial constraints and seeds being carried in bird droppings from the main island, but great success have been obtained in restoration of the native flora due to an emphasis on weeding and vigilance. In conjunction, a programme of elimination of exotic fauna which posed a threat to re-establishment was carried out, although it is worthy of note that it was impossible to make this completely successful due to the difficulty of eradicating one particular species of lizard (which was in part due to the improvement in forest cover which the project achieved), and the occasional toad or rat swimming from the main island. In this example, re-establishment of native flora and fauna were mutually supportive.
Two exotic species were planted to act as nurse crops to the native cedar. These exotics were carefully chosen in order that they would not self-seed in Bermuda so they could easily be eliminated when the time came. Initial re-establishments were based on many species known to be native to the islands in pre-colonial times but not the cedar, due to its known susceptibility to scale insect attack. Later introduction of a tolerant strain coupled with biological control meant that the cedar could be re-established.
It would appear that three reasons for the success of the project were that, firstly, the project involved the restoration of the whole habitat, secondly, that it dealt with an island community which provided a level of isolation, and, lastly, that it was based on a thorough knowledge of the ecosystem in question (Wingate, 1990).
Caledonian Region, Scotland
A programme initiated by the Forestry Commission in the 1980s aimed at encouraging appropriate management of native pinewoods in Britain offered grants to plant Scots pine (Pinus sylvestris) of local origin in its native upland areas in Scotland (Forestry Commission, 1989).
The apparent success of the project can, in part, be attributed to the wealth of knowledge surrounding the species and its requirements, the large area of existing remnant pinewoods which served as an example, the low possibility of genetic contamination from adjacent trees, and the experience gained during the programme. The agreement of local landowners to be involved in the project has also been of fundamental importance. This has been achieved not only because of the substantial planting incentives put forward, but also the relatively low pressure for alternative use of the land.
The pinewood re-establishment project can also owe some of its success to the relative simplicity of the ecosystem involved which has made recovery easier than could be expected with re-establishment of trees in many other situations (A.H.A. Scott, Forestry Commission, Edinburgh, pers. comm.).
Ma Da Forest Farm, Vietnam
The catastrophic ecological effects of almost 30 years of continual war between 1945 and 1975 included the defoliation and subsequent death of extensive areas of forest which subsequently became invaded by grasses. Early attempts at replanting indigenous tree species largely failed due to a combination of low humidity, poor soil, high temperatures and light intensities, and grassland fires during the dry season. A subsequent approach to re-establishment of native forest species involved preparation of the site by using fast growing trees such as Indigofera tenesmani, Acacia auriculiformis and Eucalyptus tereticornis to form shade under which native trees were then planted. At present there are over 1,000 hectares of such plantings. It is hoped that eventually the forest will re-establish and attract native birds and mammals which will assist re-establishment by acting as seed dispersal agents (Vo Quy, In press). Although the process has been labour intensive and slow, and many mistakes have been made, Ma Da serves as an excellent example of how even the most severely disrupted environments can, with human assistance, once again support natural vegetation.
The development and implementation of species recovery plans and programmes provide integrated conservation strategies for wild plants. These often involve a combination of methods to enable their recovery from the brink of extinction.
The Threat Posed by Habitat Loss and Degradation
Habitat loss is the primary cause of species loss at local, regional and global scales. It is estimated that habitat destruction from human activity is the primary cause of risk for 83% of endangered plant species.