1032-B1
Oghenekome U. Onokpise[1], Reiner Finkeldey and Hans H. Hattemer
The loss of genetic resources of tree species in tropical forest ecosystems through selective logging, subsistence agriculture, especially shifting cultivation, and forest fragmentation, has led to calls for preservation, conservation and restoration of these gene pools. Understanding the gene flow patterns of these tropical forests will enhance tree improvement and gene conservation efforts. Gene flow is mediated by pollen, seed and seedling dispersal, and factors affecting this gene flow include phenology, spatial distribution, population structures, seed predation, sexual and mating systems as well as physical and biological barriers to gene flow.
Gene conservation needs to be conducted for six forest management types practised: conventional or traditional forestry, agroforestry, plantation forestry, short rotation forestry, urban and community forestry, and international forestry. Gene Conservation Zones (GCZs) have been recommended for the forest reserves and national parks of several countries in sub-Saharan Africa. These GCZs will need effective and operational seed nurseries similar to the one currently funded by the KORUP National Park in Cameroon. A major constraint in the development of gene conservation programs in sub-Saharan Africa is the lack of coordinated effort and willingness and the lack of financial resources from national governments or private organizations to strengthen gene conservation in the management of tropical forests. While research activities continue on silvicultural management, ecological and evolutionary processes, and biodiversity conservation of African forest species, very little attention is given to enhancing forest tree genetic resources that are central to gene conservation in the region. Therefore, a massive long-term gene conservation effort (of 15 to 20 years) with international funding should be initiated immediately with many of the hardwood species using knowledge of gene flow mechanisms. Private organizations and national governments could then be responsible later. Invariably, gene conservation and forest management must take into account the social, cultural and economic needs of the local population.
Over the past two and half decades, there has been an increased concern for the disappearance of tropical forest ecosystems. In Africa, especially sub-Saharan Africa where much of its tropical rain forests exist, shifting cultivation is said to be responsible for more than 70 % of the deforestation (FAO, 1982; Onokpise, 1999). Dixon et al.(1996) provide a long list for the causes of forest loss and degradation in Cameroon and Ghana, from economic factors to cultural/historic reasons and government investment policies to the traditional shifting cultivation. More recently, industrial oil and gas exploitation in the oil producing countries of Angola, Cameroon, Congo, Gabon and Nigeria have been implicated for such deforestation and/or degradation (ANWORD, 1999). In Nigeria for example, oil spills and fires in the Niger Delta could be responsible for almost 35% of deforestation in that region (Dada, 2001).
For practicing foresters, gene flow must be related to the type of forestry that we practice. In this regards, we can identify six forestry types practised worldwide (Onokpise, unpublished lecture notes): Conventional Forestry, Agroforestry Systems, Plantation Forestry, Urban and Community Forestry, Short Rotation Forestry, and International Forestry.
The challenge for forest managers is to make gene flow relevant to these forestry practices. Bawa and Krugman (1991), suggested that 40 years of gene flow studies had little impact with regards to tropical forests because they were unrelated to forest policies and forest practices in the tropics. Of what use are genetic markers of Khaya ivorensis or Khaya senegalensis if these markers cannot be used to enhance management at the silvicultural level or a policy of non-export of mahogany timber. If we know how seeds are dispersed and yet cannot capture the seedflow pattern to enhance seed germination and subsequent seedling growth and management, then such gene flow is only academic. Therefore, the goal of this paper is to relate the concepts of gene flow to the different forestry practices, and then examine in situ and ex situ gene conservation that utilize gene flow in what is left of the tropical forests of sub-Saharan Africa.
Hattemer (1995) defined gene conservation as the preservation of gene resources in a condition allowing for their regeneration. This definition subsumes that genetic resources for preservation are living rather than dead regardless of the duration for which these resources are preserved. Two types of gene conservation are known and widely used in literature: ex situ and in situ. Ex situ conservation refers to the management of living organisms outside of their natural habitats (Asfaw, 2000). This type of gene conservation is also called static conservation in which tree seeds, pollen and "cultures" particularly of endangered species are stored in seed gene banks, pollen banks, or in vitro tissue culture (Eriksson and Ekberg, 2001). Also related to this are the incipient ex situ conservation in which individuals and small tree growers and institutions exchange seeds among themselves in a network of collective and individual seed maintenance systems. In situ gene conservation involves a strategy of managing living organisms in their natural state and within natural habitats. Also called dynamic conservation, it is a system for maintaining genetic resources with due consideration of the natural ecological system to ensure the continuation of co-evolutionary processes. Related to this type of conservation is the on-site-in situ preservation undertaken by farmers who raise crops on their lands while retaining the trees on these lands for many generations. Similar on-site-in situ preservation also takes place with regards to home gardens.
The ultimate goal of gene conservation is to preserve as many alleles and allelic combinations as possible, both in situ and ex situ. Specific objectives for gene conservation will be dependent on the potential uses of the conserved genes. In an environment such as the tropical forests of sub-Saharan Africa, in which much of the natural forests is now limited to Forest Reserves (West Africa) and National Parks (Central Africa), reduced gene flow frequencies can become significant in managing gene conservation regardless of the forestry type or management that is being practised.
Gene flow determination is dependent on the use of genetic markers. And as previously discussed, these markers allow for inference of the genotype of individuals at one or several Mendelian loci from their variable phenotypes. Thus, electropherograms of proteins, isozymes, and increasingly, DNA based techniques, are invaluable tools in the framework of gene conservation studies and practical applications in tree species. Hosius et al., (2000), used isozyme analysis as a concept for the establishment of seed orchards in the genetic conservation of Silver fir (Abies alba. Mill). The trait emphasized in their plus tree selection was vitality with yield and wood quality being given second priority, because the most important purpose for the seed orchard was gene preservation and not wood production. Such studies and concepts are yet to be applied or have been applied to a very limited extent to the gene conservation for tropical forests in sub-Saharan Africa. As a start, we propose a model in Figure 1 that comprehensively looks at gene flow in natural populations and how this can be used for gene conservation given the type of forest management practised. Genetic markers are included as means of identification of gene flow patterns among and within f
Given the difficulty in establishing successful plantations of indigenous tropical timber species (Pannell, 1989; Evans, 1999), much of the gene conservation for these species will be done in situ (Figure 1). Under these circumstances, conventional forestry practices that encourage natural regeneration of logged areas along side fragmented natural patches will have to be encouraged. This will allow for gene flow from fragmented patches into deforested areas. It will make for seed dispersal agents like birds, monkeys and other animals to repopulate the previously deforested areas (Pannell, 1989; Hamilton, 1999). Then on a regular basis, forest geneticists can use genetic markers to confirm such migration of seeds and pollen into the naturally regenerated areas (Boshier et al., 1995). But care must be taken to avoid contamination of selected gene pools.
Using allozymes for studies on Ancistrocladus korupensis in the tropical rain forests of Cameroon, Foster and Sork (1997) found low genetic variation among the populations and subpopulations. In order not to lose rare alleles of this economically important medicinal species, the current protected areas should be extended to the additional areas that have the highest density of A. korupensis outside the Cameroon Korup National Park. Similar studies are urgently needed for the gene conservation of the economically important tree species, some threatened, that have been largely neglected in sub-Saharan Africa.
Gene conservation for agroforestry systems present interesting perspectives. Much of the tree species used for agroforestry remain in the wild and are used simultaneously with cultivation of agronomic crops while providing fuelwood or fodder. Therefore, in situ conservation of forest genetic resources under these circumstances is highly recommended (Figure 1). In other cases, land races or exotic introductions are evaluated as provenances for the eventual establishment of seed orchards. Hence ex situ preservation of selected genotypes is recommended. And yet in other cases, trees are used in home gardens over many generations. So, preserving the genetic resources of these home garden trees should be done on-site-in situ (Figue 1).
Gene conservation for SRF also presents interesting perspectives. For species that show high coppicing ability, these can be preserved in situ. Gene flow should be non-restricted and so in situ preservation should continue to provide sufficient genetic variability for future harvests. For species that have low coppicing ability, it may be necessary to undertake ex situ (Figure 1) preservation either in seed orchards, seed gene banks or in vitro cultures as is currently done with poplar and willows in temperate countries where energy plantations are in existence.
Although the concept of firewood plantations may appear alien to sub-Saharan Africans whose culture of firewood gathering is well established, such plantations may be one of the easiest ways to undertake ex situ gene conservation of rapidly growing firewood species that are non-coppicing. Also, some of the species could be evaluated for their micropropagation potential. Results could then be used for in vitro ex situ gene preservation of these species (Figure 1). Additionally, seeds of these species as well as their plantlets from in vitro cultures could be used to establish firewood seedling and clonal nurseries that can become the source of plant genetic materials for SRF tree improvement programs in the region.
Much of PF in sub-Saharan Africa is dominated by exotic tree species (Gmelina aborea, Tectona grandis, Eucalyptus camaldulensis, etc.). Ex situ gene conservation should be practiced for PF in the region (Figure 1). This in fact should be the case if clonal nurseries can be successfully established for the relevant species within the region. However, in the native or natural habitat of the species from which genetic materials have been obtained, in situ gene conservation is highly recommended. Once provenance tests are completed in sub-Saharan Africa, then ex situ gene conservation in clonal orchards and nurseries can be undertaken. In vitro cultures are suggested as an alternative method of gene conservation of these species. If in the future, native species such as T. scleroxylon become important in PF activities, then both ex situ and in situ gene conservation of selected plus trees and seeds are recommended for sub Saharan Africa.
Because many tree species used in urban forestry as shade trees or brightly colored ones for aesthetics stay in the same location for their entire lifespan, on-site in situ gene conservation is recommended. The same could also be said for community forestry with increased use of trees as home gardens. However, tree species used in UCF are often raised and maintained in large seedling and clonal nurseries by the nursery industries. A lot of times these nursery operatives obtain planting materials locally or from far way places. Under these circumstances, ex situ gene conservation in these commercial nurseries is most appropriate (Figure 1). Similarly ex situ conservation in seed gene banks and in vitro cultures are highly recommended especially because micropropagation techniques are very well established for a number of the urban forestry tree species. A major problem in maintaining such gene banks and clonal nurseries in sub-Saharan Africa is the often the total neglect or unwillingness to carry out arboricultural practices. This can lead to the genetic erosion of the few species used in urban forestry.
When IF simply meant the exchange of plant genetic material for provenance testing and establishment of plantation forests, gene conservation was regarded as mostly ex situ (Figure 1). In circumstances where this is still the practice as with many Eucalyptus species or where a species is threatened or endangered as with T. scleroxylon, such ex situ gene conservation should be maintained (Figure 1). However, in many instances today, international forestry is much more than the exchange of plant genetic materials. Large hectares of indigenous species are being recommended for preservation. The Rio Convention on Biodiversity not only emphasizes the need for such genetic conservation, in some cases it is mandated. And it has become a tool in the hands of politicians to undertake what is now regarded as "Biodiversity Conservation for debt relief." Interestingly, those who make these recommendations seldom understand gene flow patterns in these ecosytems or generally do not commit the relevant resources to understanding the gene flow necessary for the recommended in situ gene preservation of forest genetic resources.
For gene conservation where maximum genetic variation is critical, and as demonstrated by Finkeldey et al. (1999) that levels of selfing would not result in inbreeding depression and not seriously impact the conservation of a given tree species, then in situ gene conservation of forest genetic resources is highly recommended. But if fragmentation of forests occurs and makes it difficult for gene flow mediation as shown in the studies of Hamilton (1999) and the comments of Janzen and Vazquez-Yanes (1991) then both ex situ and in situ gene flow are highly recommended (Figure 1). Forster and Sork (1997) have already suggested the expansion of in situ gene conservation of A. korupensis into areas with large genetic diversity of the species that is currently outside the KORUP National Park in Cameroon. In all these cases, gene conservation interests appear to extend beyond national boundries. As such, in situ or ex situ preservation of forest tree genetic resources becomes a matter for the international community.
The use of genetic markers in determining gene flow and subsequently recommending gene conservation is now well established for forest genetic resources. The question becomes the effectiveness of such genetic markers in given forestry types. For CF, where tree replacement for reforestation activities and where specific forest reserves and national parks are well demarcated, genetic markers will be useful in ensuring that the identified ecosystem is conserved through the use of a genetic marker data bank, e.g. on silver fir (Hosius et al. 2000). Perhaps ex situ preservation in seed orchards and clonal nurseries will be more appropriate for the other forestry types when it comes to the use of gene markers and gene conservation. These static preservation methods assume a continuous renewal of preserved materials either in vivo or in vitro.
With limited funding from their national and state/provincial governments that see tree genetics and gene conservation program as too long term, these institutions find it difficult (and will continue to do so) to establish gene conservation programs. Therefore, it is incumbent on international organizations like FAO, IUFRO and others to initiate a massive forest genetics and gene conservation program for sub-Saharan Africa located within the region perhaps at different sites. This will entail recruiting nationals (local and external) native to the region, as well as interested international forest geneticists to form the nucleus of this massive effort. It will be important to fund a 15-20 year program to enable the scientists to provide the much needed continuity for a younger generation, rather than the usual short term project of three to five years. Many tropical tree species don’t even flower until ten years old.
Specific recommendations in non-chronological order are: 1) Study biology of native species including seed germination and seedling performance under nursery conditions; 2) Establish gene conservation zones (GCZs) in Forest Reserves and National Parks; 3) Include forest geneticists in forest management practices and forest policy determinations; 4) Recruit sub-Saharan African forest geneticists from all over the world and give a mandate to accomplish well defined gene conservation goals within a certain timeframe, 5) Establish a multimillion dollar African Tree Seed Center (ATSC) similar to the Australian tree seed center, 5) Research seed orchards strategies, 6) Encourage regional gene conservation programs as currently envisaged by EU countries, 7) Make forest genetics and related sciences integral parts of forest management practice in sub-Saharan Africa.
We thank Sigrid Schmaltz of the Institute of Forest Genetics and Tree Breeding, and the staff of the faculty library of the Faculty of Forest Sciences and Forest Ecology, University of Gottingen, for their tremendous assistance in searching literature and providing other reading materials to the senior author. Thanks are also extended to Dr. Martin Ziehe and Dr. Heide Glock for assistance with Figures 1. This paper was written as part of a sabbatical leave activities of the senior author. Funds provided through a faculty fellowship from the German Academic Exchange program (DAAD) for a three-months research study at the Institute of Forest Genetics and Tree Breeding, University of Göttingen, Germany, are gratefully acknowledged. The senior author also wishes to thank Florida A&M University, Tallahassee, Florida, for granting him a sabbatical leave for the 2001/2002 academic year. This paper has been adapted from an earlier paper on gene flow that was presented at the Festkolloquium zu Ehren von Professor Dr. Hans H. Hattemer at the University of Gottingen, Gottingen, Germany.
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[1] Professor, Forestry and
Natural Resources Conservation, College of Engineering Sciences, Technology and
Agriculture (CESTA), Florida A&M University, Tallahassee, Florida 32307,
U.S.A. |