TRIPLOCHITON SCLEROXYLON: ITS CONSERVATION AND FUTURE IMPROVEMENT

by

M.R. Bowen¹, P. Howland¹, F.T. Last²
R.R.B. Leakey² and K.A. Longman²

¹ U.K. Ministry of Overseas Development, Department of Technical Co-operation at Forestry Research Institute of Nigeria, P.M.B. 5054, Ibadan, Nigeria.

² Institute of Terrestrial Ecology, Bush Estate, Penicuik, Midlothian, EH26 OQB, Scotland, U.K.

INTRODUCTION

Judged by the way in which it has been commercially exploited, Triplochiton scleroxylon (‘Obeche’) has been a valued natural resource in many West African countries stretching from Zaire through Cameroon, Nigeria, Ghana and the Ivory Coast to outliers in Sierra Leone and Guinea (Fig. 1).

As is only too common, indiscriminate felling has rarely been balanced by energetic attempts to ensure regeneration, whether natural or artificial. But it must be granted that, in the past, such attempts would have been halted by many apparently insuperable technical difficulties, e.g. the irregularity of fruiting, the inability to maintain seed viability during protracted periods of storage - an essential requirement if a continous programme of planting were to be maintained between mast years. However, foresters, following the example set by horticulturists, are now more and more considering the use of rooted cuttings for planting programmes (Hinds and Krugman, 1974; Kleinschmit and Sauer, 1976). But the impact of vegetative propagation is not restricted to the straight-forward production of planting stocks; it could greatly influence our approach to selection and inevitably there is a need to clarify our methods of separating genotypic from phenotypic responses.

Rooting is a reflection of hormonal balances; so presumably is flowering. If cuttings of T. scleroxylon can be made to root, is it possible, subsequently, to induce them to flower precociously, on relatively small trees on which controlled crosses could be made conveniently, and where the incidences of pests and pathogens could be effectively minimized? This is virtually impossible in the field, where flowering usually occurs only when trees are 20 m or more in height.

Following observations made by our former colleague, Mr. Norman Jones, who successfully rooted a few cuttings, the U.K. Ministry of Overseas Development arranged an extensive programme of T. scleroxylon research with the cordial co-operation of Mr. D.E. Iyamabo and Dr. S.T. Olatoye, successive Directors of the Forestry Research Institute of Nigeria (formerly the Federal Department of Forest Research). With their encouragement, the help of their colleagues and the provision of glasshouse and field plot facilities, a four part programme has been developing during the last 6 years. It concerns:

1. Viability of seed during storage
2. Rooting of cuttings and the management of stockplants
3. Clonal selection, and
4. Flower induction.

Figure 1: Approximate Range of Triplochiton scleroxylon in West Africa

Note: Where the cross hatching is bounded by a broken line, the location of T. scleroxylon is questionable.

Table 1. Effects of storing green undried fruits at different temperatures on the time taken for seeds of T. scleroxylon to lose their ability to germinate.

N.B. - 93% of seed were initially viable

Table 2. The germinability of T. scleroxylon seeds taken from fruits dried to 14% moisture and kept at different storage temperatures for 18 months.

(1) Viability of seed during storage

In Nigeria plantings of T. scleroxylon have, perforce, been limited to mast years. Recently, however, seed has also been collected to support an experimental programme testing effects of different factors operative during storage. Decreasing temperatures from 40°C to 0°C appreciably delayed the loss of germinability from 6 to 41 days (Table 1), but subsequent experiments showed that this time-scale could be very greatly extended, from weeks to years. Viability could be prolonged if weevil (Apion spp.) infestations and smut (Mycosyrinx sp.) infections were minimized; if seeds were taken when their fruits were at the right stage of maturation; and if the moisture content of seeds was subsequently slowly lowered to c. 8%, at a recommended loss of weight during drying of 1% per hour. Prepared in this way, 39% of seeds retained the ability to germinate after 18 months storage at 0°C, but at -18°C there was no detectable loss from the 79% germinability at the start of the experiment (Table 2, Bowen in litt.).

(2) Rooting of cuttings and the management of stockplants

Very few cuttings of T. scleroxylon were successfully rooted before 1972 (Britwum, 1970; Okoro, 1974). But since then large numbers have been rooted at Ibadan and in glasshouses at Edinburgh using single-node, leafy, ‘soft-wood’ cuttings taken from young plants and kept in humid atmospheres using automatic misting equipment or polythene covers.

It has been possible to produce more than 60 000 rooted cuttings for (i) a series of replicated field experiments and clonal trials; and (ii) the establishment of a gene bank, but there is, understandably, still room for improvement and a greater understanding of the physiology of adventitious root initiation. Why is the retention of some leaf lamina tissue (e.g. 5 cm²) necessary to ensure survival and rooting? An effective chemical substitute, the identification of a co-factor (Hess, 1969), might facilitate the use of less spacedemanding leafless cuttings; it might also aid the rooting of ‘difficult’ hard-wood cuttings from mature trees.

Clones of T. scleroxylon differ in their rooting ability; clones from some seedlots are consistently more difficult to root than others. Interestingly, the number of cuttings successfully rooting seems unaffected by the application of hormones in Ibadan but in Edinburgh it is appreciably increased, significant clone x hormone interactions being detected (Howland, 1975; Leakey, Chapman and Longman in litt.). For example clone 8036 responded similarly to indole butyric acid (IBA) and naphthalene acetic acid (NAA) whereas clone 8021 responded only to the former (Fig. 2).

Horticulturists are only too well aware of the importance of stockplant management - differences in management may explain the differing responses to hormones in Ibadan and Edinburgh. An effective programme of propagation essentially depends on an abundant supply of uniformly produced cutting material. In this tree species, with strong apical dominance, the development of axillary shoots can be encouraged in many ways, the simplest being main-stem decapitation. Decapitation stimulates axillary shoot outgrowth but because of the reimposition of dominance, the yield of material suitable for cuttings is not greatly increased.

Stockplant management must strike the right balance between using young seedlings, which yield few easy-to-root cuttings and older plants giving many more cuttings which however may root less readily. When T. scleroxylon seedlings are allowed to grow unchecked so as to increase the availability of cutting material, a loss of uniformity and rootability is readily detectable (see Komissarov, 1969). Thus, when cutting material was taken some weeks after decapitating vertically growing seedlings 1 m high, it was found that the rootability of the basal axillary shoots was c. 70% compared with 15% for apical shoots (Fig. 3). The differences in rootability of basal and apical shoots decreased greatly if the decapitated stockplants were grown horizontally or at an angle of 45° when, however, a significant proportion of rooted apical shoots grew plagiotropically (Leakey & Longman, 1976).

CLONE 8021	CLONE 8036

Figure 2: The differing rooting responses (%) of 2 clones (8021 and 8036) of Triplochiton scleroxylon to different amounts of indolebutyric acid (IBA) and naphthalene acetic acid (NAA).

Figure 3: The rootability (%) of axillary shoots taken from seedlings of Triplochiton scleroxylon grown horizontally, vertically or at 45° (angled) after being decapitated. (The axillary shoots were divided into two groups - basal (•) and apical (o).)

Management is clearly a complex matter and many facets remain to be investigated, e.g. the repeated trimming (plucking) to produce multi-stemmed ‘tea bushes’, the possible effects of phase-change and ageing operating through a succession of young cuttings, the effects of nutrition …. The adoption of standard horticultural techniques (Howland, 1975) has, however, already enabled significant progress; the techniques are nearing the stage when commercial plantings might be considered. But, is it possible to predict how cuttings from young seedlings will perform in mature stands?

(3) Clonal selection

The exploitation of vegetative propagation could have far-reaching effects on forestry because the availability of genetically uniform planting stock, without the reassortment of characters resulting from sexual reproduction, enables the choice of genotypes most suited for particular locations or for particular end-uses (Longman, 1976). But equally the loss of variation might prove hazardous - how would a genetically uniform stand respond to the pressures of competition when available light and/or nutrients become limiting? Would the use of a single clone increase the risk of damaging attacks by pathogens? If ‘yes’, how many different clones should be inter-planted?

Work with T. scleroxylon is in its infancy, but already a 30 ha site at Onigambari, near Ibadan, has been planted with 230 clones of 27 half-sib seedlots, included in experiments:

1. testing seedlots (provenances) collected from a diverse array of sites representing, where appropriate, the different rainfall zones of Nigeria, and

2. comparing seedlings and cuttings from the same and differing seedlots - at this early stage there is, in some instances, a surprising difference between clones taken from the same parent but some of these may prove attributable to ‘c’ effects (Burdon and Sweet, 1976). Together these experiments form a complex of hardwood plantings which is probably unique in the tropics; an asset that will continue to increase in value as the plantings become older when it should provide a more critical opportunity for:

   1. assessing the range of variation within T. scleroxylon,

   2. studying the extent to which early growth characteristics can be used to predict form at maturity, and

   3. comparing the growth of clones and seedlings.

These experiments are very labour intensive, with continuing detailed observations of many facets on marked trees. They include not merely traditional assessments of height and diameter but also detailed observations of branch numbers, positions and sizes, internode lengths, the incidence of pest damage …. Already plans have been made to supplement Nigerian material with collections made in Liberia, Ivory Coast, Ghana and Cameroon; additionally, critical comparisons between selected clones and unselected seedlings are being prepared.

Field experiments are essential, but they tend to be long-term. Experiments done with yound seedlings have suggested, however, that decapitation treatments induce a response which is strongly inherited and may have relevance to field performance. These responses, in which numbers of axillary buds actively producing shoots after decapitation differ greatly from one clone to the next, are being investigated.

(4) Flower induction

Flowering in forests rarely occurs on trees less than 20 m in height, when it is already becoming impractical to envisage extensive programmes of controlled pollinations. However, there is hope because flower buds have been produced, albeit somewhat unexpectedly, on a number of young trees (not more than 3 years old) propagated from seedlings grown in glasshouses in Edinburgh. This has, at the least, allowed the identification of clones with a propensity to flower. Meanwhile at Ibadan, eight potted grafts, made with scion material from mature trees, have flowered prolifically, producing nearly a thousand flowers successively over periods of months, as happens in forest conditions. Some progress towards controlled flowering has been achieved at Edinburgh, where ‘reproductive-shoots’ have been stimulated by a combination of various techniques.

Contrary to expectations, flowers of T. scleroxylon seem to be self sterile. With the Ibadan flowering grafts, it was possible to arrange a series of self- and cross-pollinations. Whereas the ‘selfs’ failed, many of the cross-pollinations produced viable seed.

With (i) the identification of precocious clones, (ii) the ability to obtain flowers in some grafts from mature trees, and perhaps also young plants, and (iii) the near-certain knowledge that T. scleroxylon depends on cross-pollination, many of the essential features of a breeding programme are becoming less problematic. But much remains to be done before flowers can be produced reproducibly and at will. Using stocks that are known to have a propensity to flower, it is now essential to examine critically the environmental and endogenous factors inducing flowering.

IMPLICATIONS

In attempting to (a) arrange readily available supplies of uniform rooted cuttings, (b) conserve the gene-pool of T. scleroxylon that still exists and (c) use cuttings and seedlings to effect tree improvement, it was decided to bring T. scleroxylon into the ‘laboratory’. By applying tried and tested methods of horticulture it was possible to propagate leafy soft-wood cuttings vegetatively, but the essential need of lamina tissues remains unexplained; hard-wood cuttings are still problematic. The performances of cuttings derived from seedlings from many West African seed collections are now being compared and at the same time it is hoped to further our understanding of juvenile/mature relations. Recognizing the possible dangers of using populations of homogenous cuttings, which of course could be minimized by planting mixtures of clones, the team is exploring the possibility of tree improvement by controlled breeding. These possibilities have become a reality as a result of 6 years intensive study of T. scleroxylon. Are there similar prospects for other tropical trees, many of which have been rooted at Edinburgh and Ibadan (Table 3)?

Table 3. Tropical trees rooted as leafy stem cuttings at Ibadan, Nigeria and/or Penicuik, Scotland, 1973–76

LITERATURE CITED

Bowen, M.R. (in litt.). Physiological studies for tropical tree improvement. I.

Britwum, S.P.K. 1970 Vegetative propagation of some tropical forest trees. Tech. Newsletter, F.P.R.I., Kumasi, Ghana, 4, 10–15.

Burdon, R.D. and Sweet, G.B. 1976 The problem of interpreting inherent differences in tree growth shortly after planting. In Tree Physiology and Yield Improvement, ed. by M.G.R. Cannell and F.T. Last, Academic Press, London, pp. 483–502.

Hess, C.E. 1969 Internal and external factors regulating root initiation. In Root Growth, ed. by W.J. Whittington, Plenom Press, New York, pp. 42–52.

Hinds, H.V. and Krugman, S.L. (Eds.). 1974 Vegetative propagation. (Proc. IUFRO mtg. Rotorua, 1973). N.Z.J. Forestry Sci., 4 (2), 119–458.

Howland, P. 1975 a - Vegetative propagation methods for Triplochiton scleroxylon K. Schum.;
b - Variation in rooting of stem cuttings of Triplochiton scleroxylon K. Schum.;
c - Current management techniques for raising Triplochiton scleroxylon K. Schum. Proc. Symp. on Variation and Breeding of Triplochiton scleroxylon K. Schum, Ibadan, Nigeria.

Kleinschmit, J. and Sauer, A. 1976 Variation in morphology, phenology and nutrient content among Picea abies clones and provenances and its implications for tree improvement. In Tree Physiology and Yield Improvement, ed. by M.G.R. Cannell & F.T. Last, Academic Press, London, pp. 503–517.

Komissarov, D.A. 1969 Biological basis for the propagation of woody plants by cuttings. Israel Programme Sci. Transl., Jerusalem.

Leakey, R.R.B., Chapman, V.R. and Longman, K.A. (in litt.). Physiological studies for tropical tree improvement. II. Factors affecting root initiation in cuttings of Triplochiton scleroxylon K. Schum.

Leakey, R.R.B. and Longman, K.A. 1976 Root and Bud Formation in West African Trees. 2nd Annual Report, 1975.

Longman, K.A. 1976 Conservation and utilization of gene resources by vegetative multiplication of tropical trees. In Tropical Trees: Variation, Breeding and Conservation, ed. by J. Burley and B.T. Styles, Academic Press, London, pp. 19–24.

Okoro, O.O. 1974 A preliminary investigation of rooting of stem cuttings of Triplochiton scleroxylon. Fed. Dept. Forest Res., Ibadan, Res. Paper (Forest Series) 28, pp. 5.

Photo I: Solitary tree of Triplochiton scleroxylon	Photo II: Rooted and establishedjuvenile cutting of T. scleroxylon
Photo III: Flower and immature fruits of T. scleroxylon	Photo IV: Ripe fruits of T. scleroxylon.