As can be seen in the above section outlining work in process, there are a number of institutions in the Latin American region working alone or in association with national and international research institutions, universities and NGOs, on a variety of themes related to Swietenia and Cedrela species.
There are projects related to the taxonomy of the Meliaceae, to reproductive biology, to flowering and phenology, pollination and pollenvectors, seed dispersal, natural regeneration, genetic variation, resistance to Hypsipyla, to name only a few.
While collaborative schemes are common, there are other cases where similar work is being carried out simultaneously in several countries without evidence of collaboration between institutions or individuals. For example, in some of the countries which provided information for the present study, there are groups of researchers working on sampling over a range of provenances to quantify genetic variation, through molecular markers and/or progeny and provenance trials, relating, in some cases, morphological and taxonomic characteristics and molecular level variation. Similarly there are many independent studies on plantation management, genetic improvement and strategies for the conservation of genetic resources of Meliaceae.
There are also many groups of researchers studying the dynamics of natural populations, natural regeneration, their phyto-sociological relationships and the inter-relationships between sympatric species. Growth and yield models are also commonly studied and through the development of models in which various factors are included, strategies are sought for the sustainable management of tropical forests in which mahogany and cedar grow.
In the following sections, which do not attempt to cover all the problems affecting the conservation and sustainable management of these species, some possible actions are proposed for the region in fields relating to genetic resources, the solutions of which would help underpin the sustainable management of natural forests and the establishment and management of large-scale plantations on a sustainable basis, while at the same time facilitating the conservation of genetic resources of neotropical mahogany species.
From the review carried out for the present study, it is evident that there are a number of taxonomic studies which cover the Swietenia and Cedrela species and some of their natural and induced hybrids. The broad ecological and geographic distribution of both genera points to the occurrence of high levels of genetic variation. Present studies include only parts of the ranges and more systematic studies with a wider coverage are needed. Such studies could with advantage be supported by International Research Centres and FAO, in close coordination with National Research Centres.
It is clear that the growing world-wide demand for timber of Swietenia and Cedrela must, in the future, increasingly come from plantations established in areas deforested in the past in tropical America, Asia and Africa.
To achieve this, it will be necessary to resolve some problems that impede the growth and normal development of the species. Collaborative research will no doubt enable the acceleration of developments in this regard. The problems caused by Hypsipyla grandella, for example, are under study by CATIE, in efforts focused on the search of genetic resistance. Here, there is an opportunity for collaboration to solve a problem of common interest, and other institutions could complement such work, particularly in the sampling of populations throughout the natural distribution ranges of concerned species, and through the establishment of coordinated field trials in which potentially resistant genetic materials could be included.
Plant propagation seems to be sufficiently covered, as is plantation establishment and management in general.
There are however some gaps in our knowledge of plantation management which merit further research. These include determination of optimal plantation density, species behaviour in pure and mixed plantations, management of associated vegetation, and certain issues related to yield and harvest. The planting of trees associated with agricultural crops as an alternative to reduce the length of time which the producer who invests in mahogany plantations will have to wait to recover part of his/her capital, also merits further attention.
In several countries work is under way on the genetic improvement of Swietenia and Cedrela species. Common strategies used include the establishment of seed orchards for the production of seed of high genetic quality for use in plantation establishment. Considering the importance of these species, such efforts should be greatly stepped up; attention should be paid, in addition, to high genetic quality in general, and to resistance to Hypsipyla grandella.
The Meliaceae shoot borer, Hypsipyla grandella, is one of the most avidly studied tropical forest pests. Research has focused on biological control; the use of insecticides; the application of novel silvicultural techniques; and various other alternatives for control and combat. In spite of these efforts, Hypsipyla continues to be the principal obstacle to the establishment of large-scale plantations of Meliaceae species.
As noted in the corresponding section, it has been found that certain Swietenia species are less susceptible to the attack of the shoot borer, H. grandella than others. For example, in Puerto Rico, S. mahagoni is less susceptible than S. macrophylla, and their hybrid shows intermediate resistance (Whitmore and Hinojosa, 1977). It has also been noted that C. odorata is more susceptible to the shoot borer than are Swietenia species.
Newton et al (1994), commenting on the evidence of existing mechanisms of resistance in the various Meliaceae species, indicate that there is little available information on intra-specific variation in resistance to shoot borer attack. They suggest a domestication strategy for mahoganies focused on the selection of resistant genotypes, subsequent vegetative propagation, and the use of adequately resistant materials in managed plantations in which appropriate silvicultural methods are systematically applied to optimise control of insect attack.
It is a well-known fact that large-scale use of resistant clones will assert a very strong selection pressure in the Hypsipyla populations to break resistance to attack, and reduce the positive impacts of selection and breeding programmes. In addition to taking this factor into account in improvement strategies, it will therefore be necessary to develop complementary strategies to increase the sustainability of effort.
Kageyama (1996) notes that one of the silviculture systems which shows potential in Brazil is the establishment of mixed plantations of Cedrus and Swietenia, associating them with other species belonging to different taxonomic groups, representing various stages of natural succession (pioneers: early secondary, late secondary and climax, species).
In summary, there are vast possibilities for cooperation among countries, institutions and individuals in the field of minimising shoot borer attack. In this regard, countries are linked by the common interest to make the establishment of Meliaceae plantations possible, thus reducing the pressure on natural forests while satisfying the demand for timber of these species.
Loss of genetic resources within and among populations caused by selective harvesting and loss of populations due to changes in land use, are critical in assessing status and trends in conservation of Meliaceae genetic resources in the neotropics.
One of the primary concerns of the scientific community is the necessity to prevent the genetic erosion which some species, including S. mahagoni and S. Macrophylla have suffered. The former species lost its best individuals in the past for a number of reasons. The populations which remain today, which are descendants of those trees which were left behind and which thus provided seed and propegules for the next generation, show a high proportion of forked and malformed branchy trees.
There is a great variation in S. macrophylla and other Meliaceae in the morphology of leaves, fruits, and wood properties. This suggests that the species is variable over its geographic distribution range and that variation also exists between isolated occurrences, caused by natural barriers or fragmentation due to anthropogenic or natural causes.
The above might suggest that each provenance in essence sub-divides the species into groups of adapted biotypes, which correspond to a range of different habitats. In particular, there seems to be evidence that characteristics such as the form of the trees and levels of resistance to the shoot borer H. grandella are highly heritable. This underlines the importance of genetic conservation at the level of individuals and provenances, with special reference to these specific characteristics, which could be lost by neglect if the present tendencies of deforestation and selective harvesting of the best phenotypes in the forests are allowed to continue.
Cutting the best individuals in terms of growth and form constitutes a de facto dysgenic, negative selection (Newton et al 1996). As a result, trees in the next generation will be of lesser quality; this development is called “genetic erosion”.
In view of the discussions by CITES (1994) to include S. macrophylla in Appendix II, surprisingly few investigations have been carried out on the genetic variation of the species, in spite of its economic importance. Newton et al (1992) agree that little priority has been given to clarifying the levels of genetic variation and variation patterns found in the natural populations of this species.
Better understanding of genetic variation in Swietenia and Cedrela species is needed to define sound strategies for genetic improvement, silvicultural management and conservation of genetic resources. Likewise, the search for genetic resistance to Hypsipyla also requires more knowledge of existing variation throughout the distribution ranges of species in these genera.
Work is in progress to review variation between provenances and within populations in practically all the countries of the sub-region, however, only part of the ranges of the species in question are covered and included in on-going research. For this reason it will be useful to promote, cooperative, coordinated studies among concerned institutes in the neotropics, aimed at complementing and broadening the present coverage of provenances under study.
In spite of the fact that there are numerous projects on the conservation in situ and ex situ of Swietenia and Cedrela species in the sub-region, increased efforts are needed to adequately conserve the Meliaceae and many other tropical species.
For sound planning of conservation management it is essential to have available basic information on genetic variation and variation patterns and information on factors such as: geographic distribution, levels of disturbance or threat to the species and its populations, number of individuals occurring per hectare, dispersion of seed and pollen, growth, phenology, reproductive biology, pollination systems and pollinators, seed characteristics, extent of seed and propagule predation, and the dynamics of natural regeneration.
These factors taken together allow for the re-adjustment of objectives and goals. They also support sampling and the siting and determination of size of conservation areas; facilitate development of strategies for the management, genetic improvement, and collection of seeds; and the use of the species in plantations.