CONSERVATION AND USE OF TRIPLOCHITON SCLEROXYLON IN MOIST WEST AFRICA

by
A.B.I. Igboanugoⁱ and P. Iversenⁱⁱ

INTRODUCTION

Triplochiton scleroxylon K. Schum (Sterculiaceae) is a large, deciduous African forest tree. It is distributed in the semi-deciduous moist forests from Sierra Leone to Congo. The highest concentration of the tree is associated with altitudes below 500 m, mean annual rainfall from 1 100 to 1 800 mm, a seasonal and two-peaked rainfall distribution, average temperatures 20-35 °C and well drained ferruginous soils (Hall and Bada, 1979). It may sometimes be found in the dry forest zone, where it occurs in clusters, either within the dry forests or at its boundaries with the moist forest. Trees found in the dry forest zone have smaller leafs, narrower crowns and shorter bole lengths (Jones, 1975). Within the natural range, three sections are generally recognized, namely: Sierra Leone to Togo, Benin to Nigeria and Cameroon to Congo (Hall and Bada, 1979; Figure 1).

Figure 1: Approximate range of Triplochiton scleroxylon in West Africa

Where cross hatching is bounded by a broken line, the location of Triplochiton scleroxylon is debated.

Triplochiton scleroxylon is an important timber tree and in the FAO database REFORGENⁱⁱⁱ, 12 countries list it as a priority species. The tree is native to 11 of the countries listed, while the last country, Solomon Islands, use it as an exotic. In 8 of the 12 countries the species is used in plantations. Côte d’Ivoire lists it as endangered and 4 countries report either in situ or ex situ conservation programmes. Three countries have mentioned tree selection or improvement programmes on the species.

OVER EXPLOITATION

The species has been exploited in all the countries within the native range to such a degree that most natural forests have become depleted; selective logging has also probably eroded the native gene pools (Ladipo et al., 1992) and continued exploitation from natural forests is increasingly difficult due to the scarcity of the resource. It used to be the commonest export timber of the countries bordering the Gulf of Guinea, comprising nearly half of all timber production by volume, but in Nigeria it has been almost eliminated from the forest due to over-exploitation (Lowe, 1997).

PLANTATIONS

T. scleroxylon has been tested for cultivation in plantations because it is fast growing (Table 2), self pruning and well established in the timber marked (FAO, 2002). There are however pest problems affecting the species. Leading shoots are often killed by aphids. Almost annually (in August-September), heavy defoliation by the caterpillar of Anaphe venata occurs, although increment loss is not apparent (FAO, 2002).

Stumps of T. scleroxylon are also liable to severe or fatal termite attack if soil is not moist at the time of planting (Taylor, 1962).

The species has been used both for plantation establishment and in enrichment planting in Ghana and Nigeria. Lamb (1967), in FAO (2002), inspected line-planting in Ghana and Nigeria and he reported that
T. scleroxylon was one of the species that grew fast enough to become established successfully. He noted also the benefit of side shade provided by line planting for early growth. Igboanugo (1990) also observed that medium shade enhanced growth more than dense shade and full light. These practices are believed to be contributing to curtail foliar insect infestation and enhance growth.

Table 2. Estimated Mean Annual Increment (MAI) in Ghana for selected species.

FAO (2002) - FST: forest savanna transition, ME=moist evergreen,
MS= moist semi-deciduous, S: savanna, WE: wet evergreen

SEED AND GERMPLASM PROCUREMENT

T. scleroxylon has irregular seed years with unpredictable quantities (Jones, 1975 and 1976) and furthermore the seeds are short-lived and cannot be relied upon for supplying plantation requirements (Foli and Ofosu-Asiedu, 1977; FAO, 2002). This has stimulated considerable research into vegetative propagation techniques and juvenile leaf cuttings have been propagated successfully (Leakey et al., 1982; Leakey, 1985; Leakey and Mohammed, 1985).

Lowe (1997) mentioned that based on his experience in Nigeria, and despite the success and comparative ease of the methods used to propagate T. scleroxylon, he never observed this method being used in routine management. Peprah (1999) also mentioned that vegetative propagation techniques, which would have allowed the establishment of clonal trials to identify the extent of genotypic variation, have not been fully utilized in Ghana.

TREE SELECTION & IMPROVEMENT PROGRAMMES

According to information stored in REFORGEN, three countries, Cameroon, Côte d’Ivoire and Nigeria, have tree improvement programmes for T. scleroxylon including provenance, progenies and clonal trials. Siaw (2001) reports that Ghana has carried out provenance trials and according to Ladipo et al. (1992) clonal evaluations have resulted in better understanding of genetic variability and an overall gain in stem volume of over 30 percent has been achieved for T. scleroxylon. Studies on the early growth have included the relationship between branching frequency and apical dominance, as well as measurements of photosynthesis, as aids to early selection of desirable genotypes. Developments from these research efforts have enhanced reforestation, particularly in Côte d'Ivoire and Nigeria.

Unwin (1920) described T. scleroxylon growing in the forest canopy as being slightly shade bearing at first and becoming light demanding as it matures. In this case it might be possible that due to physiological ageing, cuttings from mature trees may exhibit a different response to light intensity than plants obtained from seeds.

DISCUSSION

Unreliable seed supplies and pest management problems have been major obstacles to larger utilization in plantation; the situation may worsen since dwindling the resource in natural forests threatens future seed supply, in both quantitative and qualitative terms. Nevertheless, vegetative propagation methods seem to be a short-term way to overcome the problem of supplies of good planting material and provide new options for tree selection and improvement programmes.

Plantation forestry relying on vegetative (clonal) propagation requires the conservation of a comprehensive and representative gene pool, storing material from all known sources of genetic variation within the distribution range. Such long-term gene banks actually condition the long term of any intensive tree planting programme. Selection and marking of plus trees followed by establishment of provenance trials in a number of sites which at the same time could act as tree improvement programmes and as a part of a conservation programmes should be initiated.

For such a programme to be successful, government agencies need to work with local communities to protect selected stands and relevant institutions need capacity building in vegetative propagation methodology. Considering the cross boundary pattern of the distribution area, reproductive materials should be exchanged between countries in the region.

REFERENCES

Siaw, D.E.K.A. 2001. State of Forest Genetic Resources in Ghana. Forest Genetic Resources Working Papers, FGR/17E. Forest Resources Development Service, Forest Resources Division. FAO, Rome.

FAO. 2002. Hardwood plantations in Ghana by F. Odoom. Forest Plantations Working Paper 24. Forest Resources Development Service, Forest Resources Division. FAO, Rome.

Hall, J.B. & Bada, S.O. 1979. The distribution and ecology of Obeche (Triplochiton scleroxylon). Journal of Ecology, 67: 543-564.

Igboanugo, A.B.I. 1990. Adaptation of shoot growth and leaf production of laboratory grown cuttings of Triplochiton scleroxylon K. Schum in response to changes in irradiance. For. Ecol. Man. 43 (1-2). 35-42.

Jones, N. 1975. Observation of Triplochiton scleroxylon (K Schum). Flower and fruit development. Sympososium on variation and breeding systems of T. scleroxylon, Fed. Dept. For. Res. Ibadan, Nigeria, 12 pp.

Jones, N. 1976. Some biological factors including seed setting in Triplochiton scleroxylon (K Schum). In Burley, J and Styles, B.T. (eds.). Tropical Trees: Variation, Breeding and Conservation. pp. 125-134.

Ladipo, D.O., Britwum, S.P.K., Tchoundjeu, Z., Oni, O. & Leakey, R.R.B. 1992. Genetic Improvement of West-African tree species: past and present. In Tropical trees: the potential for domesication and the rebuilding of forest resources. Eds. R.R.B.Leakey and A.C.Newton. ITE Symposium No. 29, ECTF Symposium No. 1.

Leakey, R.R.S. 1985. The capacity for vegetative propagation in trees. In Cannell, M.G.R. and Jackson, J.E. (eds.) Attributes of trees as Crop Plants. Proceedings of IUFRO meeting, Edinburgh. pp. 110-133.

Leakey, R.R.S., Chapmain, V.P. & Longman, K.A. 1982. Physiological studies for tropical tree improvement and conservation. Factors affecting root initiation on cuttings of Triplochiton scleroxylon (K Schum). Forest Ecology and Management, 4: 53-66

Leakey, R.R.S. & Mohammed, H.R.S. 1985. Effects of stem length on root initiation in sequential single – node cuttings of Triplochiton scleroxylon (K Schum), Journal of Horticultural Science, 60: 431-432.

Lowe, R. G. 1997, Letter to the editor, Tropical Forest Update, Vol. 7: 4. ITTO.

Peprah, T. 1999. National report on forest genetic resources in Ghana. Training workshop on FGR, 6^th -11^th December 1999, Nairobi, Kenya.

Taylor, C.J. 1962. Synecology and Silviculture in Ghana. Thomas Nelson and Sons, Edingburgh. 102 pp.

Unwin, A. H. 1920. West African Forest and Forestry, Fisher Unwin Ltd., London. 153 pp.

ⁱ Forestry Research Institute of Nigeria, Moist Forest Research Station, P.O. Box 2444, Benin City, Nigeria

ⁱⁱ Forest Resources Development Service, FAO, Rome, Italy; now at the Danish Forest and Nature Agency, Haraldsgade 53, 2100 Copenhagen, Denmark

ⁱⁱⁱ REFORGEN can be accessed at http://www.fao.org/forestry/fgr