by
M. Corbasson & O. Souvannavong
Division Matériel Végétal - CTFT
45bis, Avenue de la Belle Gabrielle
94736 Nogent-sur-Marne Cédex, France
NATURAL RANGE AND MAIN CHARACTERISTICS OF THE SPECIES 1
Terminalia superba is a timber species found in the upper storey in the moist forests of West and Central Africa. It belongs to the family Combretaceae.
Its natural range is very wide (see distribution map), and extends from Sierra Leone and Guinea in the west to the Central African Republic in the east, and south of the equator down to Cabinda and the part of Zaire which is northwest of Angola. The southern occurrence in Congo, Cabinda (Angola) and Zaire is separated from the rest of the range by the dense Gabon forest areas and the savannah zone of the Bateke plateaus in the Congo. It occurs from sea level up to an altitude of about 1,000 m. It is a gregarious, light-demanding species in semi-deciduous forests under climates with over 1,500 mm annual rainfall, and less than a 4 months' dry season. However, it also occurs in drier areas, but is in that case restricted to gallery forests. It is also sometimes found under climates with less than a 2 months' dry season. However, it is a moisture demanding species which does not tolerate periods of severe drought, but it grows on a fairly wide range of soils.
Map 1. Natural distribution area of Terminalia superba
Source: Corbasson, M. Limba, p. 184. 1986 In: Amélioration génétique des arbres forestiers. Revue forestière Française 38.
Terminalia superba yields a timber of excellent quality, cream-coloured, soft to medium-hard, light, of good stability, which saws and works easily, and is easy to paint and varnish. It is suitable for many uses, and is extensively utilized (indoor joinery, outdoor joinery after protective treatment, plywood). Timber from the northern occurrence (called Fraké in Côte d'Ivoire, White Afara in English-speaking countries, Akom in Cameroon) is less valued than timber from the southern occurrence (called Limba in Congo and Zaire), since its aspect is frequently depreciated by fine inner pinholes. Limba made up a very important part of the Congo timber trade until the 60s, when the stands of South Congo became gradually depleted.
NEED FOR GENETIC IMPROVEMENT
Plantation of Terminalia superba started comparatively early in the Congo (6,500 ha over the period 1949–1960) and in Zaire (7,500 ha planted by 1958).
Large scale plantations have also taken place in Côte d'Ivoire since 1972. Their rate was increased since 1978 by using mechanized techniques, up to 2,000–3,000 ha per year.
These plantations were carried out up to now from locally collected seeds, the only improvement operation being some selection of mother trees among seed-bearing trees.
The use of a genetically improved plant material increasingly appears as a necessity in countries where Terminalia superba plantations are important for timber production. This trend comes within a context of intensive silviculture aiming at high quality products, shortening of the production cycle, and reduction of the amount of time and work spent in tending operations. This intensification approach proved to be one of the means required to ensure long term conservation and development of timber resources in many countries of the humid tropics facing qualitative impoverishment of their forests and high demand of land for agriculture.
The strategy of genetic improvement undertaken by CTFT is based on the following four main directions:
exploiting interprovenance variation 1;
exploiting individual variation, and vegetative propagation;
conservation of genetic resources;
miscellaneous work and longer term prospects of genetic improvement through sexual propagation.
EXPLOITING INTERPROVENANCE VARIATION
Some trials were carried out in the 60s to compare various provenances from the northern and southern parts of the range. The most important was the trial established in 1969 in Côte d'Ivoire, which compared 17 provenances in a statistical design. In spite of difficulties encountered for establishing such trials in dense forest, due in particular to site heterogeneity, difficult evaluation and sometimes inadequate sampling, these early trials revealed significant differences in such characteristics as vigour and growth rate. In all trials, in particular, “Limba” provenances from the southern range proved of little value concerning growth rate. A great variation was also noted for individual performance within provenances.
A major programme for exploration in the natural range and provenance collection was launched late in 1981, and carried out under the responsibility of CTFT (M. Cossalter) in cooperation with the relevant national institutions. It concerned Côte d'Ivoire (14 provenances), the Congo (7 provenances), Cameroon (4 provenances) and the Central African Republic (4 provenances). An ex situ provenance from Burundi, originating from the Zaire Mayombe, was also collected.
30 seedlots of provenances grouping a total of 362 progenies are also available for establishment of international provenance trials 1.
In principle, unless exception, seeds were collected from trees several hundred metres apart, of acceptable form, without always selecting only out-standing individuals. Each tree is identified, and a detailed description is made. Identity of seedlots from each mother tree is maintained throughout the process from seed collection to the constitution of provenance seedlots.
Experience confirmed how essential it is that seedlots are suitably treated already from the moment of collection in order to later obtain high germination rates. If these conditions are met, the controls carried out showed that seeds of good initial quality can be stored satisfactorily at a temperature of 4°C, provided a low humidity can be maintained (airtight containers, room with controlled humidity).
As early as 1982, the first trial plots were established in Côte d'Ivoire. At this stage, they included only provenances from Côte d'Ivoire, since this country was engaged in a major programme of industrial plantations, at this time based essentially on Terminalia superba, and wished to have quick results in order to take the best advantage of its own genetic resources.
EXPLOITING INDIVIDUAL VARIATION, AND VEGETATIVE PROPAGATION
The objective is to take advantage of the existing individual variation along two lines of action, both using vegetative propagation and cuttings as an evaluation tool for individuals to be selected for the constitution of a multiclonal variety, as well as a method of rapid propagation of the selected clones.
In the Congo, the first activities started in 1974. They consisted in exploration of the natural stands of South Congo: 1,000 adult trees (100 for each provenance). 10 provenances were preselected on the basis of their general appearance and on important characteristics regarding timber quality, with a fairly good heritability according to general knowledge on forest species (e.g. stem form, branching, natural pruning).
A secondary selection of 100 trees among the 1,000 trees initially retained (10 for each provenance) was carried out controlling growth rate of the preselected trees by measurement with increment borer (growth rings are annual in Limba).
Mobilization of the selected material consisted in collecting scions intended to form the head of the 100 clones retained, using a rifle, grafting them in clone gardens, and propagating the material thus selected by a chain of cutting propagation in order to obtain a satisfactory rejuvenation.
Establishment of clonal tests to confirm the real value of clones then permits final selection.
The clones finally retained are then mass propagated for dissemination. This approach has the advantage of rapidity. It is likely to be efficient as regards form criteria. Phenotype selection, however, may not be totally efficient concerning growth rate, in view of the widely varied site conditions characteristic of the natural stands.
In Côte d'Ivoire the approach followed since 1982 consisted in carrying out selection at an early stage (young seedling) in F1 generation (10 plants selected in each progeny) in a breeding population with a broad genetic base.
Propagation by cuttings of the F1 generation is used for:
establishment of clonal tests for clone evaluation (each clone being represented by several ramets);
conservation of the clones studied in clonal bank (1,540 clones maintained);
dissemination of clones after final selection.
This approach prevents the hazards of phenotype selection in natural forests and the difficulties of rejuvenation, however, its efficiency eventually depends on the level of juvenile/adult correlations that can be expected in Terminalia superba.
CONSERVATION OF GENETIC RESOURCES - WORK ACHIEVED
Conservation of genetic resources of this species poses particularly acute problems in countries where pressure on land for agriculture is heavy, e.g. in Benin or Nigeria, or in areas where exploitation of natural forests is intense, as in Côte d'Ivoire.
In this last country, the Centre Technique Forestier Tropical de Côte d'Ivoire and the Société pour le Dévelopement des Plantations Forestières (SODEFOR), a state agency in charge of industrial timber plantations, seized the opportunity offered by the seed collection campaigns organized to serve the genetic improvement programmes and international provenance trials to establish, from half of the seeds collected in Côte d'Ivoire, about 1,000 ha of provenance conservation plots in four different localities. In these plots, progenies from a same provenance are mixed, the identity of each provenance established separately being, of course, maintained.
A programme for the conservation and management of gazetted forests is also currently being developed in Côte d'Ivoire, and should contribute to the conservation of genetic resources of this species.
In another field of interest, research is currently undertaken in a laboratory of the French Centre National de la Recherche Scientifique associated with the CTFT (POVAR) to develop long term conservation techniques of Terminalia superba seeds by cryoconservation and lyophilization. The results obtained already proved positive, and the method could well have practical applications in the case of particularly rare or valuable plant material to be conserved over a long period.
OTHER RESEARCH WORK UNDERTAKEN ON TERMINALIA SUPERBA
Beyond provenance trials, and using opportunities offered by the seed collections gathered, a series of research activities concerning natural variation and reproduction of Terminalia superba are currently on the way in the tropics and in France.
Current research include e.g.:
Investigation on enzyme proteins in Terminalia superba by electrophoresis, which has already produced useful data concerning variation in the natural range of the species, the genetic structure of the populations sampled during seed collections for international provenance trials, distances between provenances (for instance the degree of enzyme polymorphism in provenances from Côte d'Ivoire is much higher than in those from the Congo), the probability of obtaining heterosis effect by crossing between provenances, and the distribution of variation. All the alleles present in the material from Côte d'Ivoire and the Congo are also found in investigated provenances from Cameroon and the Central African Republic, which suggests that the latter countries might be the area of origin of the species.
The first field trials established in Côte d'Ivoire, which should at more or less long term supply preliminary information on variation at interprovenance level of Côte d'Ivoire provenances, as well as at half sib family level, on juvenile/adult correlation, on the efficiency of some early selection tests, and on performance to be expected from the first clonal seed orchards established.
Optimization of vegetative propagation techniques: cuttings for the dissemination of selected clones, grafting, in vitro culture to be used as a tool for maintaining useful genotypes (independently from the opportunities offered by seed collection), for rejuvenation of clone heads to be propagated by cuttings, and possibly, if the method proves effective, for large scale propagation of clones.
Floral biology and phenology of Terminalia superba, sexualization of buds, and the possibility of performing controlled crossings, are other subjects of research.
Research work on morphogenesis and growth under strictly controlled conditions is currently carried out in the CNRS plant laboratory (“phytotron”), with various objectives:
to understand modes and determinism of growth in Terminalia superba;
to establish the most favourable site conditions;
to study correlation phenomena liable to occur at development level;
to explore opportunities for establishing criteria for early selection;
to tackle the problem of determinism in flowering, and the ways to induce flowering under artificial conditions so as to overcome the obstacle put to breeding programmes by the long juvenile phase preceding flowering.
While some of these investigations are of prospective nature, others lead to already important applications.
