The natural resistance of 85 West African hardwood timbers to attack by termites and microorganisms

Published Date
The natural resistance of 85 West African hardwood timbers to attack by termites and microorganisms

© Crown copyright 1979

Extracts from this publication may be reproduced provided that the source is acknowledged

ISBN 0 85135 103 4

Abstract

This work was started during the second phase of a technical cooperation scheme based at the Building and Road Research Institute, Kumasi, Ghana. One or more trees of each of 85 timber species found in Ghana were sampled, test blocks being cut from the sapwood, outer heartwood and inner heartwood zones, where distinguishable. The timbers were tested at a forest site and a savanna site by graveyard tests of buried blocks for 96 weeks, and tests of small blocks between glass plates for 3 months. The natural resistance of each wood zone of each timber to termites and microorganisms was determined by measurement of percentage weight loss, due mainly to termite attack in the graveyard tests and almost wholly to termite attack in the small block tests. Results for each timber are discussed in the text and presented in detail at the end of this bulletin.

Sommaire

Ce travail a été commencé au cours de la deuxième phase d'un projet d'assistance technique ayant pour base l'Institut de Recherche du Bâtiment et des Travaux Publics de Kumasi au Ghana. On a échantillonné un ou plusieurs arbres de chacune des 85 essences de bois d'oeuvre trouvées au Ghana en découpant des blocs des zones d'aubier, de coeur interne et de coeur externe lorsque celles-ci pouvaient être distinguées. Les bois ont été mis … l'épreuve dans deux endroits, I'un en forêt, I'autre en savane, les gros blocs étant enterrés pendant 96 semaines et les petite blocs maintenus entre des plaques de verre pendant 3 mois. On a déterminé la résistance naturelle aux termites et aux microorganismes de chaque zone de bois de chaque essence en mesurant le pourcentage de perte de poids, qui était dû principalement … l'action des termites dans les essais d'enterrement et presque entièrement … l'action des termites dans les essais portent sur les petite blocs. Les résultats obtenus pour chaque essence vent discutés dans le texte et présentés en détail … la fin de ce bulletin.

Prefatory note

The work described here was started during Dr Usher's employment by the Ministry of Overseas Development from August 1971 to June 1973 as Adviser on Termite Research to the Building and Road Research Institute, Kumasi, Ghana. This technical cooperation scheme was initiated in 1968 under the Special Commonwealth African Assistance Plan, and its first phase was described by Williams (1973).

Introduction

For centuries plant materials have been used by human communities for drugs, food, coverings, structural purposes, beverages, fuel, etc. In any area traditions have developed whereby only one species, or just a few species, have been used for a particular purpose. Timber, the plant material of greatest biomass, besides being used as fuel, or sometimes for medicines, is an important structural material. Traditionally, in Ghana, one species has been prized in this role: this species is odum (Chlorophora excelsa, trade name: iroko) since it is relatively durable to attack by insects and microorganisms and relatively easy to work with most tools. These two criteria are important. A constructional timber should not be perishable in the short or medium term due to economic considerations. Also, it should not be difficult to saw, carve or nail a constructional timber for reasons of labour effectiveness and efficiency.

The traditional uses of woody vegetation in West Africa, with special reference to Ghana, have been collected and described by Irvine (1961), and for many timbers he quotes their durability. Taylor (1960) similarly, in describing the gynecology and autecology of the Ghanaian forest tree species, records whether or not the timber is durable. In a survey of 700 African timbers Bolza and Keating (1972) record the durability of the timbers in four classes, ranging from timber of a very high natural durability (class 1) to timber of low durability (class 4).

Thus, for many species of Ghanaian timber trees there are qualitative data on their durability. For a few species there are data of a more quantitative nature, recording the amount of damage that timber sustains from termites, etc., for example Butterworth et a/. (1958) and Williams (1973). There is apparently no evidence on the natural durability of only a few species. The aim of the present study is to give quantitative data on the weight loss of timber exposed to fungal and termite attack.

Three considerations were important in planning this series of tests. Firstly, the study should produce data on as wide a spectrum of Ghanaian timber species as possible. It was important to include in the tests species which were little known, since data on these could have economic importance. However, it was also important to include the well-known species so that the test results could be compared with the literature: discrepancies in these could cast doubts on the accuracy of the results of the little-known species. Secondly, there is a body of evidence, for example Rudman et a/. (1967) and Ocloo (1975), to suggest that there is considerable radial variation in both the fungal and termite resistance of wood, as well as radial variation in attack by marine borers (Barnacle and Ampong, 1975). 

Generally, sapwood is nondurable, while heartwood has least durability near the pith, the durability increasing, not necessarily linearly, with distance from the pith. It therefore seemed appropriate to study different zones in the tree. Thirdly, for the results to be generally applicable in Ghana, the methods of testing should be as diverse as possible. Ghana extends from the forest vegetation zones in the south to the savanna zones in the north, and hence tests have been carried out in both of these major vegetation types. Also, two different testing methods have been used so as to gain insight into the likely behaviour of timber in different situations. More details of these three considerations are given below.

Materials and methods

Timber species

Enumerations of the Ghanaian forest reserves have generally shown between 200 and 250 species of hardwood within any individual forest reserve. Since many species are confined to particular vegetation (or climatic) regions, the total number of indigenous hardwood species within Ghana is in excess of 400. Although it would have been useful to test the whole of this range of species for their resistance against termite attack, several considerations acted to reduce the number included in the tests.

Firstly, many of the species are extremely rare, only occurring with a frequency of 1 or 2 in samples of 100,000 or more trees. Although the persistence of many rare species in tropical forest ecosystems has considerable ecological interest, these rare species are of relatively minor commercial importance because there are insufficient quantities of their timber for effective marketing, and since they are difficult to locate in the forest.

Secondly, there was a practical aspect of collecting timber samples. The authors are indebted to the many sawmills which provided timber, but only 15 species were regularly available from mills, and a further 13 species were obtained (these were infrequently milled, and in general samples from only one or two logs could be obtained). Hence, sawmills could only supply 28 species of hardwood during the year in which samples were being collected.

An attempt was made to list all the more frequent forest species of the major vegetation zones and, where these species were not available from sawmills, to liaise with the Forestry Department so that appropriate trees could be felled. In this manner a total of 85 species have been included in the experiments. Although this represents only about 20% of the hardwood species in Ghana, these species probably account for at least 85% of the trunks above 50 cm diameter in the majority of forest reserves south of 7º30' N.

The original aim was to test timber from at least three different trees of each species included in the tests. With the 15 species regularly available from sawmills this proved possible (the average number of trees per species tested was 6 3). An average of 4 7 trees/species was obtained for the 13 species infrequently obtained from sawmills. However, this level of replication could not be attained with the species felled by ourselves: of these 57 species there is an average of 1 -9 trees/species. Wood from a total of 266 trees has been included in the tests.

Full details of the 85 timber species are given in Table 4 at the end of this bulletin and an alphabetical list of trade and local names is given in Table 1. The scientific names of the species follow the Flora of West Tropical Africa by Hutchinson and Dalziel (1954- 1972), without later revisions, though synonyms in common usage by Taylor 11960), Irvine (1961) and Bolza and Keating (1972) are given in the text. The number of trees of each species included in the tests is shown in Table 4, where it will be seen that 32 of the species are unsatisfactorily represented since wood from only a single tree could be obtained. A further 17 species were represented by wood from only two trees, and the results here are also relatively unsatisfactory.

An index of standard and local trade names of timbers referred to in the text; the page reference is to the description of the timber properties in the text, the figure reference is to the graphs of graveyard test performance

Sample preparation

In sawmills, a centre board was obtained from logs which were being sawn through and through (lengthwise), the length of board obtained being as long as possible. For trees felled in the forest, a disc about 25 to 30 cm long was obtained, and a centre board about 5 cm wide was cut (see Fig. 1). For timber obtained from either source the sample board was obtained from the butt end of the log.

In general, test pieces were cut from three zones of the sample board, sapwood and heartwood (both inner and outer heartwood). The sapwood could usually be recognised by a difference in colour from the heartwood, or by testing for starch with iodine. Outer heartwood samples were cut within 10 cm of the transition from heartwood to sapwood, but not within 2 cm of this transition so that no portion of sapwood would be included in the test pieces. The inner heartwood samples were cut within 10 cm of the pith where this could be distinguished, but care was taken to exclude any pith or rotten timber in the test pieces. The locations of the outer and inner heartwood test pieces in the sample board are shown in Fig. 1.

In some instances, it was impossible to cut test pieces from both inner and outer heartwood zones: in these cases all samples were cut from across the heartwood zone, some towards the sapwood/heartwood transition and some towards the pith, and these were referred to as 'heartwood' in the experiments. In the case of two species, the palms Borassus aethiopum and Elaeis guineensis, there is no differentiation into heartwood and sapwood, and hence test pieces were cut from across the whole sample board.

Two sizes of test pieces were prepared. Larger test pieces were 10 cm along the grain by 4 cm tangentially by 2 - 5 cm radially. It was not possible to prepare all pieces to these dimensions, especially of sapwood samples where the width of sapwood was less than 2 5 cm. In these cases the largest sample possible with a rectangular or square cross-section was prepared; all samples were 10 cm in length. The smaller test pieces were 3 cm along the grain by 1 - 5 cm tangentially by 1 cm radially. Samples from all species and all zones were prepared to these dimensions.

All test pieces were handsawn and planed to the appropriate size, and they were lightly sanded with medium or fine sandpaper so as to remove all rough edges. Test pieces with cracks or other defects were rejected and not included in the experiments.

Figure 1

Testing methods

All experiments were carried out on two sites, one within the forest zone of Ghana and the other in the Guinea savanna woodland zone. The forest site is near the village of Fumesua (6º42' N, 1º31' W), situated about 12 km east of the city of Kumasi. The ecology of the termite species on this test site has been described by Usher (1975), and a list of the species attacking timber is given in Table 2. Before tests were conducted the area was a farm scrub, having been cultivated and abandoned for about 6 years. The scrub re-growth was mostly cut down, leaving scattered trees and bushes so that access to the whole site was simple and so that the majority of the site was shaded from the sun for most of the day. By the time that the experiments were begun there was an almost complete ground cover of grasses, and during the experiments there was little visual change in the test site. Figure 2 shows this site in late 1973.

The savanna test site is situated at the north east corner of Keni Keni Forest Reserve (9º13' N, 1 º55' W), about 3 km west of the town of Larabanga. The termite species seen during the tests are listed in Table 2: those species attacking telegraph poles in the area have been described by Usher and Barnacle (1974). The area is typical of Guinea savanna woodland, with scattered trees up to 20 m in height and an almost closed canopy of bushes and small trees. Grass growth is seasonal, the grass reaching a height of over 1 m (and exceptionally over 3 m) in the wet season (July and August) and dying back in the dry season (December to March). The test site was not burnt during the experimental period as it is within the Forest Reserve.

In the literature there are many methods for testing timbers against termite attack, reviewed for example by Harris (1961). Unlike controlled laboratory experiments with beetles, there are no standard conditions for field tests with termites. The concept of standardisation of tests has long been recognised and discussed in conferences, for example by Purushotham and Mascarenhas (1952) and Becker (1969), but in general the stake sizes suggested are too large for quantitative assessment of damage. It was impossible in Kumasi to cut test pieces to the length suggested by Purushotham and Mascarenhas (1952) (about 60 cm), and if long wood samples had been used there would have been the possibility of their proving attractive to fire-wood hunters. Hence, in graveyard tests much smaller test pieces, 10 cm long, have been used. Although this length is shorter than that advocated by Usher and Ocloo (1974) it was used since it was possible to cut all samples to this length: in some timbers it was impossible to obtain a 15 cm length. A rectangular cross-section, 4 cm by 2 5 cm, was selected since there is evidence that rectangular pieces are more heavily attacked by termites (Usher and Ocloo, 1974).

On the forest test site, wood from all the trees of each of the species was included, replicating each zone of each tree at least twice (two replicates were used for species that were represented by three or more sample trees, three replicates for species represented by two sample trees, and six replicates for species represented by only a single sample tree). The graveyards were established as shown in Fig. 3. The test pieces were selected at random and placed 10 cm apart in rows that were separated from each other by 0 5 m. The whole graveyard was interconnected with baitwood strips of Triplochiton scleroxylon sapwood which were approximately 0 - 6 cm wide and 5 cm deep. Rows of test pieces were joined by baitwood strips every 1 m, these joining strips being staggered as shown in Fig. 3. All test pieces and baitwood strips were buried so that their upper surfaces were flush with ground level: the whole site was then covered with bush or grassland litter so that the experiment was not obvious to any passers-by (or fire wood gatherers). No interference from human sources was recorded on either test site. The test on the forest site was laid down in October and November 1972, while that on the savanna test site was laid down in May 1973.

Figure 3

Despite the fact that oven drying might drive off highly volatile toxicants, it was felt that standard conditioning of the timber before weighing was important for comparative reasons. Thus, the test pieces were oven dried at 85ºC to constant weight. They were then left in the ground for 4 weeks, when they were dug up and both the termite species and termite damage noted. The samples were thoroughly washed and again oven dried. The loss in weight could therefore be calculated by subtraction. Further measurements of the weight loss were made after totals of 12, 24, 48 and 96 weeks buried in the soil. At any of these inspections if more than about 50% of the initial weight of any test piece had been lost that test piece was not replaced in the graveyard. This extent of termite attack indicated that the test piece was far from resistant, and thus that it was of no further interest in establishing the degree of resistance shown by that timber species.

The small test pieces were sandwiched between two glass sheets before exposure to termite attack. Such a method was described and illustrated by Williams (1973) for exposing timber to attack by Coptotermes intermedius and Pseudacanthotermes militaris, and was used on small, chemically treated timber samples by Usher and Ocloo (1975). In the present series of tests 36 small test pieces were selected, comprising timber from five tree species together with controls of obeche and iroko, for each of the five species there were samples of sapwood, outer heartwood and inner heartwood each replicated twice, as well as single samples of the three timber zones of obeche and iroko. The controls, outer and inner heartwood and sapwood, were cut from the same trees of Triplochiton scleroxylon (obeche) and Chlorophora excelsa (iroko), and were used so that the results of the many plates could be compared. The glass plates were amply supplied with sapwood of T. scleroxylon as bait, ensuring that there were sufficient cracks of approximately 5 mm width for termite penetration (Usher, 1974). The plates were laid on the ground surface, approximately 1 m apart, and were interconnected with buried strips of T. scleroxylon sapwood as a bait (Fig. 4). The plates were covered with roofing felt held above the plate with stakes of durable timber so that the termites could live in darkness (Fig. 5). The plates, which were laid down on the forest site in November 1974 and the savanna site in April 1975, were left in position for approximately 3 months, after which they were returned to the laboratory, washed, termite damage noted, and the weight loss of test pieces measured by the difference between the initial and final oven-dry weights of the blocks. It was apparent that a considerable amount of fungal damage had been avoided both by the use of small samples between sheets of glass and by the short duration of the tests.

Results

Introduction

Termites attacked, in some way or other, all graveyard samples. Some samples were completely eaten, while on others, after 2 years, there was only a minimum of nibbling amounting to less than 0 5% of the volume of the sample. Figures 6 - 9 show examples from the Fumesua test site. In discussing the results of these tests eight factors have been considered.

First, there is a considerable amount of literature concerning the properties of African hardwoods. In order to restrict the amount of referencing, Bolza and Keating's review (1972) of 700 species of African timbers has been used as a standard reference except for those few species which are not included in their publication. Two books dealing specifically with Ghanaian timbers have also been used; these are Irvine (1961) and Taylor (1960). In the following text these three books are referred to as 'the standard references'. Although these three references include local and trade names, the non-scientific names used in the text follow the system used by the Ghana Timber Marketing Board (1970) and the Ghanaian Forest Products Research Institute (no date).

Second, in the descriptions which follow the timbers are presented in botanical order, following Hutchinson and Dalziel (1954- 1972). Since there are often similarities between the species of a family, the species are discussed family by family. The scientific name is followed by the standard trade name, if any, and, in brackets, the local trade name where this is different from the standard trade name.

Results

Introduction

Termites attacked, in some way or other, all graveyard samples. Some samples were completely eaten, while on others, after 2 years, there was only a minimum of nibbling amounting to less than 0 5% of the volume of the sample. Figures 6 - 9 show examples from the Fumesua test site. In discussing the results of these tests eight factors have been considered.

First, there is a considerable amount of literature concerning the properties of African hardwoods. In order to restrict the amount of referencing, Bolza and Keating's review (1972) of 700 species of African timbers has been used as a standard reference except for those few species which are not included in their publication. Two books dealing specifically with Ghanaian timbers have also been used; these are Irvine (1961) and Taylor (1960). In the following text these three books are referred to as 'the standard references'. Although these three references include local and trade names, the non-scientific names used in the text follow the system used by the Ghana Timber Marketing Board (1970) and the Ghanaian Forest Products Research Institute (no date).

Second, in the descriptions which follow the timbers are presented in botanical order, following Hutchinson and Dalziel (1954- 1972). Since there are often similarities between the species of a family, the species are discussed family by family. The scientific name is followed by the standard trade name, if any, and, in brackets, the local trade name where this is different from the standard trade name.

Third, the results of the two large graveyard tests, at Fumesua and Keni Keni Forest Reserve, are presented graphically in Figs. 10-21, following Table 4 at the end of this bulletin. The percentage weight losses during the 96 weeks of the test are shown as three different line diagrams, for sapwood, inner heartwood and outer heartwood. The timbers are presented in the same botanical order as the descriptions to facilitate visual comparisons within genera and families.

Fourth, the mean percentage weight losses in both graveyard and small block tests have been given in Table 4. In some instances most of the wood had been eaten by the termites before the end of the experimental period, and hence mean percentage weight losses can only be given for the first few readings. The table gives, for each wood zone used, the data for both test sites. The timbers are given here in alphabetical order of genera and species to facilitate detailed study of the results for any particular timber. The full scientific name, with authorship, is given.

Fifth, as there is insufficient space in a publication to tabulate the complete data obtained during these tests, computer print-outs of the data will be filed in the Building and Road Research Institute (Kumasi, Ghana) and the Centre for Overseas Pest Research (London, U.K.). Enquiries about the availability of the data should be addressed to either of these establishments.

Sixth, analyses of variance have been performed on the data, species by species, comparing, in appropriate instances, the percentage weight losses (in arcsin transformation) of the different trees, of the different wood zones, and of the different times of inspection of the graveyard tests. In the majority of analyses there was sufficient replication for the interaction terms between these main factors to be investigated. There is insufficient space to give analysis of variance tables for all these analyses, but some of the statistically significant results are discussed in the text.

Seventh, in thinking about weight loss it is obvious that this can be attributed to four main sets of causes, namely leaching (of water soluble substances by rainwater, etc.), loss of volatile chemicals (though this was thought to be negligible), microorganisms (fungi were noted to be particularly active colonisers of bare wood), and wood-feeding invertebrates. During the test no wood-boring beetles were observed to attack the samples, and hence this final category can be assigned to the termites. Samples which had completed the full 96-week period in the graveyards were inspected for termite damage. Two people (Mr R. M. C. Williams and Dr M. B. Usher) assessed visually the amount of damage due to termites if the damage was less than an estimated 5%. This visual assessment could then be compared with the observed weight loss during the 96 weeks in the soil. In all 168 blocks thus inspected the weight loss over the 96 weeks was substantially greater than the estimated weight loss due to termites. Hence, for species where such an inspection was possible (the more durable species) it is possible to give an indication of the proportion of the weight loss due to termites and the proportion due to leaching and microorganisms. For the less durable species, the extent of the termite attack was so great that such a division into causes of weight loss was impossible.

Finally, all samples were measured (to the nearest 0 5 mm) after they were oven dried and before they were inserted in their test positions. It is thus simple to calculate the specific gravity of oven-dry wood. Since it is usual to measure the density of wood with a 12% moisture content, for example, see Bolza and Keating (1972) the oven-dry specific gravity data in Table 4 are likely to be rather smaller than might occur elsewhere in the literature though this depends upon the shrinkage of the wood specimens with oven drying. In the text, and in Table 4, the specific gravity is expressed on an oven-dry basis.

Family Myristicaceae

Pyonanthus angolensis, ilomba (otie), is the only common timber tree of this family in Ghana. The standard references regard the timber as non-durable, and Smith (1969) has shown that the timber is nondurable in temperate environments. The results of the graveyard tests show that the majority of the timber is consumed within the first 3 months of exposure to termite attack (Fig. 10, Table 4). Analyses of variance indicated no significant differences in the weight lost by the three zones, sapwood and outer and inner heartwood, and no significant difference between trees. The data in Table 4 indicate that termite attack is faster in the forest than in the savanna region. The mean oven-dry density of Ghanaian timber is towards the lower end of the range of air dry densities indicated by Bolza and Keating (1972).

Family Flacourtiaceae

In Ghana, this is a family of small trees except for the two species of the genus Scottellia, S. coriacea and S. chevalier), odoko (koroko, tiabutuo): the two Scottellia species are difficult to distinguish from each other. Bolza and Keating (1972) include both species in their lowest durability class, whereas both Irvine (1961) and Taylor (1960) record that S. coriacea is not very durable. This qualified statement is demonstrated in the results of the graveyard experiments where it can be seen that there was little termite damage during the first 6 months in the soil (Fig. 10, Table 4) Analyses of variance indicated no significant difference between the trees, but did indicate that the sapwood was significantly less damaged than the two heartwood zones in the Keni Keni graveyard and for the small blocks between glass plates. This species is one of the very few which show a more durable sapwood than heartwood. The densities are consistent with the ranges quoted by Bolza and Keating (1972).

The results for this species are particularly interesting due both to the sapwood phenomenon, and to the almost complete destruction of the samples after exposure for a year. These facts would be consistent either with the gradual leaching of some water-soluble repellent which was initially contained within the wood, or possibly by the destruction of a repellent by microorganisms.

Family Ochnaceae

The two timber species in Ghana are Lophira a/ata, ekki (kaku), a tall tree of the wetter forest associations, and L. Ianceolata, probably with the same trade and local names, a smaller tree of the savanna woodland. Taylor (1960) records that the timber of L. alata is resistant to insects, fungi and marine borers. Bolza and Keating (1972) also put the timber into the most durable class, but they indicate that L. Ianceolata might not be so durable. The results of the graveyard tests indicate that both species are extremely durable, a finding supported by the results of the small blocks tests (Fig. 10, Table 4). Analyses of variance of both species data indicate that there are significant differences between the zones: the sapwood, although relatively resistant compared to other hardwoods, is more heavily attacked than the heartwood, especially with L. Ianceolata. Eight of the 27 samples of L. alata in the graveyards had rather little termite damage (an average of an estimated 1 4% of the block eaten), whereas they had lost on average 8 2% of their weight. This indicates that, over a 2-year period in the soil, leaching, loss of volatiles, and microorganism attack will cause L. alata to lose nearly 7% of its initial dry weight. A similar estimate on the 10 samples of L. Ianceolata indicated that leaching and microorganisms accounted for just over 7% of initial dry weight over the 96-week period. The similarity of these figures suggests that Lophira timber will lose 3 - 4% of its weight per year for the first 2 years when buried in soil. Both timbers are extremely dense, the specific gravities in Table 4 closely agreeing with the data in Bolza and Keating (1972).

Family Lecythidaceae

The only large timber tree in this family in West Africa is Combretodendron africanum, essia (esia). Reports on the natural resistance of this timber to insect attack are contradictory (Bolza and Keating, 1972), although it has been found to be neither very resistant nor very susceptible. In Europe the timber is reasonably durable (Smith, 1969). The results indicate that the species is reasonably resistant, with a consistently smaller amount of damage on the savanna test site than on the forest test site (Fig. 10, Table 4). The sapwood is more susceptible to damage than the heartwood (there being little difference between inner and outer heartwood zones). Analysis of the seven test pieces which had only been slightly damaged by termites indicated that, over a 2-year period, the weight loss due to leaching and microorganism attack would be about 10% with outer heartwood and 23% with inner heartwood. These findings accord with Taylor's (1960) observation that the timber is not very resistant to fungal decay. The specific gravities shown in Table 4 are rather lower than the range (0 - 73 - 1 - 01) quoted by Bolza and Keating (1972).

Family Combretaceae

This is a family of shrubs and small trees. In the forest zone of Ghana two species, Terminalia ivorensis, idigbo (emere) and T. superba, afara (ofram), are large forest trees, while in savanna woodlands one species, Anogeissus leiocarpus, kane, is of timber size. These three species show a variety of susceptibilities to termite decomposition. A. Ieiocarpus is stated by Irvine (1961) to be "impervious to termites", but the data in Fig. 10 and Table 4 show that, beyond 1 year in the ground, both the heartwood and sapwood are considerably damaged by termites. In the absence of fungal activity (small blocks tests, Table 4) it is likely that A. Ieiocarpus timber will be moderately durable. The specific gravities shown in Table 4 are consistent with the data quoted by Bolza and Keating (1972).

Reports of termite resistance of T. ivorensis are variable (Bolza and Keating, 1972), though Irvine (1961) considered that the timber was "fairly resistant to fungi and insects, except...termites". It is, however, generally accepted as reasonably resistant to termite attack (Ocloo, 1975). The data in Fig. 10 and Table 4 shows that the outer heartwood is reasonably resistant to attack, while the inner heartwood is far more liable to attack by termites. This striking difference between inner and outer heartwood, also demonstrated by Ocloo (1975), may account for the variable results summarised by Bolza and Keating (1972). Outer heartwood samples, with only minimal termite damage, indicated that about 7% of the weight had been lost over the 2-year period due to leaching and microorganism activity. The specific gravity of Ghanaian material is rather less than the range of 0 - 51 - 0 - 64 quoted by Bolza and Keating (1972), especially for the inner heartwood (Table 4).

The timber of T. superba is generally accepted as being non-resistant to termite attack, a feature supported by the data in Fig. 10 and Table 4. Analysis of variance indicated no significant differences between the weight lost by heartwood or sapwood. The only unexplainable results concern the small blocks between glass plates, which were almost completely destroyed on the forest test site but which were only slightly damaged on the savanna test site. The reason is uncertain, though this might relate to moisture differences between the habitats. Reports of the specific gravity of the timber are contradictory, ranging from 0 - 41 to 0 - 72, a range which spans the results shown in Table 4.

Family Rhizophoraceae

The only large timber tree in this family in Ghana is Anopyxis klaineana, kokoti. Both Irvine (1961) and Bolza and Keating (1972) record that the timber is generally not very durable; both sources, however, state that the timber is resistant to termite attack. The data show that termite attack can be extensive (Fig. 11, Table 4). In this species the sapwood was significantly less attacked than either the inner or outer heartwood (this difference might not extend beyond 2 years: if the test had extended for three or more years the sapwood might have deteriorated more than the heartwood), and one sample that was only slightly damaged by termites after 96 weeks in the ground had an 11 % weight loss which could be attributed to leaching, volatilisation, and microorganism decomposition. The timber appeared very durable between glass plates, perhaps indicating that a water-soluble termite repellent may exist in the timber (Taylor (1960) records that gums are noticeable in the vessels). The timber is reasonably dense, the specific gravities being towards the lower end of the range quoted by Bolza and Keating (1972).

Family Guttiferae

In this family, which contains a number of small to medium-sized trees, only one species, Mammea africana, African apple (bompegya), reaches really large sizes. The three standard references disagree as to the durability of this timber: Taylor (1960) states that it is durable while Irvine (1961) considers that it is moderately resistant to decay. Bolza and Keating (1972) state that it is moderately susceptible to termites, but not susceptible to borers. This study shows that, during the first 24 weeks of exposure, the timber is strongly resistant to termite attack ( Fig. 11, Table 4). However, on longer exposure the amount of termite attack increased, with sapwood being damaged significantly more than heartwood (there is no significant difference between the damage to heartwoods, which are reasonably resistant). In the non-leaching environment between glass plates, the damage to the heartwood was negligible. After 96 weeks exposure only one sample remained which showed little termite damage: about 3% of the weight lost by this sample can be attributed to leaching and microbial activity. The specific gravities lie within the very wide range of densities recorded by Bolza and Keating (1972).

Family Sterculiaceae

A large number of timber species are included in this family, one genus of which, Co/a, contains many species. Nine species were included in the tests: these are Mansonia altissima, mansonia (aprono), Triplochiton scleroxylon, obeche (wawa), Nesogordonia papa verifera, danta, Pterygota macrocarpa, pterygota (kyere), Tarrietia utilis, niangon (nyankom), Sterculia oblonga, yellow sterculia (ohaa), S. rhinopetala, brown sterculia (wawabima), Co/a gigantea, (watapuo), and C. Iateritia, also referred to locally as watapuo.

The standard references agree that M. altissima timber is resistant to termite attack: the results of the present tests do not confirm these findings ( Fig. 11, Table 4). During the first 24 weeks in the ground the losses of weight by the heartwood were minimal, but thereafter termite attack was intense. The termite species of the forest test site would appear to destroy M. altissima timber faster than those of the savanna test site, which is also the result with the small blocks between glass plates. The observed specific gravities are rather less than the range quoted by Bolza and Keating (1972). In the absence of extensive termite damage, two samples of M. altissima timber lost approximately 7% of their weight over a 96-week period.

The standard references record that the timber of T. scleroxylon is very susceptible to attack by termites, a conclusion confirmed by these studies (Figs. 6 and 11, Table 4). Although not the most extensively damaged timber in these studies, both its susceptibility and its abundance in Ghanaian sawmills have made it an ideal baitwood for termites (Williams, 1973; Usher, 1975). The specific gravities are towards the lower end of the range quoted by Bolza and Keating (1972). In temperate environments the timber would appear to be more durable (Smith, 1969).

Although Irvine (1961) records that the timber of N. papa verifera is resistant to fungus, termites, and other insects, Bolza and Keating (1972) only rank it as moderately durable. The studies here show that the timber can be extensively damaged by termites (Fig. 11, Table 4). There was a significant difference between trees, indicating that some trees are generally more resistant than others. It is also interesting to note that the general level of attack was greater on the savanna test site than on the forest test site, although attack in the savanna on the heartwood did not commence in the first 12 weeks. The specific gravities recorded are less than the range 0 73-0 80 quoted by Bolza and Keating (1972).

The timber of P. macrocarpa is recorded as being of low durability (Irvine, 1961), which is supported by the data in Table 4. On analysis there were no significant differences between trees or between the three zones of wood used. Between glass plates the small blocks were extensively damaged over the 3-month period. The specific gravities listed in Table 4 are very much less than those given by Bolza and Keating (1972) for West African P. bequeertii.

The literature records a variety of results for the durability of T. utilis: Taylor (1960) states that it is not resistant to insects or decay, while Bolza and Keating (1972) list it as having a moderate to high natural durability, and in Europe it is durable (Smith, 1969). It is damaged relatively extensively by termites when in contact with the ground (one sample with only slight termite damage lost about 7% of its weight due to leaching and fungi)(Fig. 12, Table 4). However, the samples between glass plates show that, in these nonleaching conditions, the wood is only very slightly attacked (Table 4). On the forest test site, in particular, termites were slow to start attacking the timber, and it is particularly interesting to note that the inner heartwood was more extensively attacked than the sapwood. The wide discrepancy between the two heartwood zones, and the apparent discrepancy between leached and non-leached timber, would explain the contradictory results in the literature. The specific gravities in Table 4 are in agreement with Bolza and Keating's (1972) range.

Both Taylor (1960) and Bolza and Keating (1972) state that Sterculia oblonga timber is not resistant to termite attack; this is supported by the data in Fig. 11, which shows that the most heavily attacked wood is the outer heartwood. This result, repeated on both the forest and savanna test sites, is unexpected. All three standard references record that S. rhinopetala timber is moderately resistant to termite attack, which is certainly true for both inner and outer heartwood (Fig. 11). Three samples, which were only slightly attacked by termites, showed that about 12% of the weight was lost by leaching and microorganism activity after 96 weeks in contact with the soil. The results for small blocks between glass plates, similar on both test sites, confirm the findings of the graveyard tests (Table 4). The specific gravities are interesting since, for both species, those for sapwood and inner heartwood are at the top of the range quoted by Bolza and Keating (1972), while the specific gravities for the outer heartwood exceed the ranges quoted.

Rather little is known about the termite resistance, or durability, of the Cola species, a large genus of generally small forest trees. Of the two species included in these tests, Bolza and Keating (1972) is the only standard reference to provide information, this being that C. gigantea is susceptible to termite attack. The data in Fig. 12 and Table 4 indicate that both species, C. gigantea and C. lateritia, are extensively damaged by termites. Analyses of variance indicated no significant differences between the wood zones. The ovendry specific gravity data agree with those for C. gigantea quoted by Bolza and Keating (1972) and show that the densities of the two species are similar (Table 4).

Family Bombacaceae

The timber of this family, which contains a few species of massive forest trees, is generally light and non-durable. The three common species, Bombex brevicuspe, (onyinakoben), B. buenopozense, (akata), and Ceiba pentandra, ceiba (onyina), have been included in these tests (Fig. 12, Table 4).

C. pentandra was extensively damaged by termites, being, together with Pycnanthus angolensis, the most susceptible Ghanaian timber to termite destruction; both in contact with the soil, and between glass plates, the majority of the timber had been destroyed within 3 months.

Despite the similarities in the appearance of the timber of the two Bombax species, there are some slight differences in their termite susceptibility. B. buonopozense, which has a smaller specific gravity than B brevicuspe, was extensively damaged by termites, the damage starting soon after exposure to termite attack. Between glass plates the timber was more extensively damaged than that of B. brevicuspe. B. brevicuspe timber was only slightly attacked during the first few weeks of exposure to termite attack, but damage was extensive after 24 weeks. It therefore appears that B. brevicuspe has slightly more resistance than B. buonopozense to attack by termites, though both species could be classified as nonresistant.

The specific gravity data for the three species in this family, shown in Table 4, are consistent with those quoted by Bolza and Keating (1972).

Family Humiriaceae

The only Ghanaian species of this small family, Sacoglottis gabonensis, (tiabutuo, a name shared with other timbers) is confined to the rain forest, where trees develop to a large size. Rather little seems to be known about the termite susceptibility of the timber: Irvine (1961) states that it can be used for railway sleepers, indicating that it might be relatively resistant. This resistance is confirmed by the graveyard tests where it can be seen that, during the first year of exposure, the attack was light (Fig. 12, Table 4). Analysis of variance showed no significant difference between the inner and outer heartwood, though there is an indication that sapwood is less durable. With a species that decays so little in contact with the ground, it is not surprising that it appears so resistant when tested between glass plates (Table 4). The specific gravities recorded are at the lower end of the range quoted by Bolza and Keating (1972).

Family Euphorbiaceae

There is one large forest tree, Ricinodendron heudelotii, erimado (wama), amongst this family of herbaceous and shrubby plants and small trees. All three standard references record that this species is non-durable, as indicated by the data in Fig. 12 and Table 4. It is interesting to note that the timber in the graveyard tests was attacked faster on the savanna than on the forest test site, although on neither site were there differences between the three different zones of timber. The range of densities quoted by Bolza and Keating (1972) is consistent with that given in Table 4.

Family Caesalpiniaceae

This family contains a large number of genera of forest trees: only nine of these genera are included in the present study, though the taxonomic position of one of these genera, Amphimas, is uncertain as it is sometimes included in Papilionaceae. However, A. pterocarpoides, (yaya), is included in this family by Hutchinson and Dalziel (1954-1972). Its timber, despite being relatively dense (the specific gravities recorded in Table 4 are lower than the range quoted by Bolza and Keating (1972)), is considered by the standard references to be non-durable. The results of the graveyard tests show that the timber is slowly decomposed so that the majority of it has been destroyed after 2 years (Fig. 13, Table 4). On analysis of variance of these data there are significant differences between trees as well as between zones of wood on the forest test site only (where the sapwood is less damaged than the heartwood zones).

The termite resistant properties and durability of the timber of Dialium aubrevillei, (duabankye), are less well-known since they are not referred to by any of the standard references. However, this dense timber does show considerable resistance to attack by termites (Table 4). Although the sapwood is damaged, the heartwood lost only about 16% of its weight over a 2-year period, and between glass plates the heartwood was scarcely damaged (Fig. 13, Table 4). Two heartwood samples that were only slightly damaged by termites indicated that about 8% of the weight loss could be attributed to leaching and microorganisms.

The standard references record that the timber of Distemonanthus benthamianus, ayan (bonsamdua), is reasonably durable, and Bolza and Keating (1972) state that it is "resistant to moderately resistant to termites", the properties of the timber often depending upon its silica content. There are very significant differences between trees included in the graveyard tests. These tests show that the heartwood is reasonably durable (Fig. 13, Table 4). It is interesting to note that, on the forest test site, the inner heartwood both of graveyard samples and blocks between glass plates was more severely attacked than the outer heartwood, while on the savanna test site the outer heartwood was more extensively damaged in both tests. Only one graveyard sample was slightly attacked by termites, and this indicated approximately a 5% loss in weight due to leaching and microorganisms. The specific gravity of this species is slightly less than the range quoted by Bolza and Keating (1972).

A timber that was partially resistant to termite attack is Cynometra ananta, ananta and apome (ananta). The timber was hardly attacked by termites either when it was between glass plates or during its first year in the soil. After a year, the sapwood was attacked, but the outer heartwood remained reasonably resistant (Fig. 13, Table 4). These results are consistent with both Taylor (1960) and Bolza and Keating (1972), as are the density data in Table 4. Despite the resistance to termite attack, the only graveyard sample which showed slight termite damage at the end of the test had lost about 13% of its weight over the 2-year period.

Another similar timber with outer heartwood reasonably resistant to termite attack is Afzelia africana, afzelia (papao). The standard references agree that this species of Afzelia, more inclined to the forestsavanna margin than to the rain forest, has a high degree of natural durability (Figs. 7 and 14, Table 4). Unlike Cynometra, however, the sapwood of Afzelia is attacked almost as soon as it is exposed to termites. The density of the sapwood is less than the quoted range of wood density (Bolza and Keating, 1972). Of the four graveyard stakes (all outer heartwood) remaining reasonably undamaged after 2 years, the average weight loss attributable to fungi and other microorganisms was about 9%.

Two species of Daniellia are frequent in Ghana: D. ogea, ogea (hyedua or shedua), in the rain forest communities and D. oliveri, (senya), in the savanna. Although rather little is recorded of the durability and termite resistant properties Bolza and Keating (1972) put both in their non-durable class. Forest Products Research Laboratory (1967) states that the timber of D. oliveri is perishable, and D. ogea has proved to be non-durable in Europe (Smith, 1969). This assessment of the timber is shown in Fig. 13 and Table 4 where not only the sapwood but both heartwood zones were extensively attacked within the first 3 months of exposure to termite attack. Despite this susceptibility to attack, there are significant differences between the two species and between the zones. Thus, with D. ogea between glass plates, all zones were attacked, and on analysis there were no significant differences between zones, but in D. oliveri the inner and outer heartwood, while not different to each other, were attacked significantly less than the sapwood (Table 4). The density of the Ghanaian material is less than the ranges of density quoted by Bolza and Keating (1972).

The three remaining timbers in this family are Guibourtia ehie, ovangkol, ehie (hyeduanini), Bussea occidentalis, (samanta), and Erythrophleum ivorense, missanda (potrodom), all of which are forest species. Bolza and Keating (1972) record that the timber of G. ehie is "only rarely attacked by termites", and has a density in the range 0 73 to 0 90. The first of these statements is not supported by the data in Fig. 14 or in Table 4 and the specific gravity data are below the quoted range. Although termite attack on graveyard stakes is initially rather slow, after about 6 months there is considerable damage to both sapwood and inner heartwood samples (the damage on these two zones is not significantly different, though both are damaged more than the outer heartwood). An interesting feature of G. ehie is the strong tree-to-tree variability (in the analysis of variance, for the difference between trees P<0 - 001). The data for tests between sheets of glass reflect the apparent resistance of this species during the first few months of exposure to termites.

B. occidentalis is a heavy timber about which little is known. Taylor (1960) states that it is durable against termites, a fact supported by the data on heartwood (though the sapwood is extensively damaged in graveyard tests after the first 6 months). Three heartwood samples showed little termite damage after 96 weeks in the ground, but these had lost on average about 6% of their weight due to both leaching and the activity of fungi and other microorganisms.

Despite the fact that the standard references state that the timber of E. ivorense is durable and resistant to termite attack, the results of these tests show that both the heartwood and sapwood can be extensively damaged by termites (analysis of variance showed no significant difference between the means of these two zones). During the first 12 weeks of exposure there was virtually no damage, but thereafter termite damage increased so that, after 96 weeks, all graveyard samples showed considerable to extensive damage. The specific gravity of the tree included in these tests is unusual since the heartwood density is substantially less than that of the sapwood, and the heartwood density is also substantially less than that quoted by Bolza and Keating (1972). The history of growth of the tree is unknown.

Family Caesalpiniaceae contains a large number of timber species, many of which are extremely dense. Although some of the denser species, for example B. occidentalis, are termite resistant and the less dense, for example D. ogea, are very susceptible to attack, it can be seen in the data for this family that resistance and density are not necessarily correlated. If there were such a correlation, then the moderately dense timber of G. ehie and A. pterocarpoides would be expected to be far more termite resistant. The family also demonstrates that the presence of gums does not necessarily confer termite resistance: the three species D. ogea, D. oliveri and G. ehie all produce gums which are used locally for incense (hence the local names of hye, incense and dua, tree).

Family Mimosaceae

This family also contains a number of genera with dense, and frequently durable, timber. Despite a large number of genera of timber-producing trees, many of them are relatively infrequent in the forests, and thus only four genera were included in these studies. These are Pentaclethra macrophylla, ovala (otaa or atewa), Piptadeniastrum africanum, dahoma, Cylicodiscus gabunensis, okan (denya}, and three species of the genus Albizia.

The standard references agree in stating that the timber of P. macrophylla is resistant to termite attack, a fact which is supported by the data in Fig. 14 and Table 4. On analysis of variance, the sapwood lost significantly more weight than either of the heartwood zones, which were not significantly different from each other. Four of the graveyard stakes were only slightly damaged by termites after 96 weeks exposure in the soil: their average weight loss was 9.5% which, when compared with the data in Table 4, indicates that heartwood samples were losing only about 5.5% of their weight due to termite attack over a 2-year period. These results indicate that the dense heartwood of P. macrophylla is one of the most resistant timbers to termite attack.

The timber of P. africanum is variable, since on analysis of variance there were differences between the eight trees tested (0 001<P<0 01). For the data from the Fumesua test site, there are significant differences between the mean weight losses of the three timber zones; the outer heartwood was significantly less attacked than either the inner heartwood or sapwood (Fig. 14, Table 4). However, with the data from the Keni Keni test site there is no significant difference between the three zones. This difference between the two test sites is also shown in the small block tests. Weight loss due to leaching, fungi and other microorganisms averages about 12.5% although it varied from about 6 to 20%. The variability in the timber is referred to by the standard references: Irvine (1961) refers to the controversy of results for Nigeria and South Africa which respectively state that the timber is and is not resistant to termites. The mean density of the Ghanaian material in Table 4 is rather less than the density range 0 65-0 80 quoted by Bolza and Keating (1972).

C. gabunensis is another heavy timber in which the inner heartwood is considerably less dense than the outer heartwood and sapwood. The density effects are reflected in the weight lost by the samples in the graveyard test where, on both test sites, the inner heartwood lost more weight than either the outer heartwood or sapwood (Figs. 8 and 15, Table 4). It is interesting to note that there is a reasonable degree of between tree homogeneity in this timber since there was no significant difference between the weight losses of the five trees included in the graveyard experiments. All three standard references indicate that the timber is durable and resistant to attack by termites. The small blocks between glass plates showed virtually no weight loss over the 3-month period, confirming the results of the graveyard tests. In fact, many of the graveyard samples showed only minor termite damage after 96 weeks in the soil: these samples had average weight losses of 15% (sapwood) and 3% (heartwood) which could be attributable to fungi and other microorganisms. These data, together with the results in Table 4, indicate that outer heartwood samples lost on average only 3 - 4% of their weight due to termite attack over a 2-year period.

The three species of Albizia included in these tests are A. adianthifolia, (pampena), A. ferruginea, West African albizzia, (awiemfo-samina), and A. zygia, okuro. The standard references agree in designating both A. adianthifolia and A. zygia as not resistant to termite attack, but while both Taylor (1960) and Irvine (1961) state that A. ferruginea is very resistant to termite attack, Bolza and Keating (1972) state that the resistance of this species varies in Africa, being highest in Nigerian timber. The results of these three species indicate that the timber of both A. adianthifolia and A. zygia is moderately severely attacked by termites with relatively little difference between the two heartwood zones, and no significant difference between individual trees (Fig. 15, Table 4). Despite the fact that A. zygia timber is denser than that of A. adianthifolia, the densities of the Ghanaian material are at the higher end of the ranges quoted by Bolza and Keating (1972), there is relatively little difference between the weights lost by samples of these two species. However, A. ferruginea, with a density intermediate between the other two species, shows a greater degree of termite resistance, particularly in the outer heartwood. The sapwood is destroyed fairly rapidly by termites when it is in contact with the soil, but between glass plates there was rather little damage to the sapwood and virtually no damage to the heartwood. Four of the outer heartwood graveyard stakes showed only limited termite damage, and their weight losses indicated that about 9% weight loss could be expected over a 2-year period by fungi and other microorganisms. There was no significant difference between the timber from different trees.

Family Papilionaceae

Despite the extensive number of species in this family in West Africa, there are relatively few species that develop into timber producing trees. Only two species of this family have been included in these tests, Afrormosia ( = Pericopsis) elate, afrormosia (kokrodua), from the forest region and Pterocarpus erinaceus, Senegal rosewood, from the savanna woodlands. The results for both timbers are shown in Fig. 15 and Table 4, the density data being within the ranges quoted by Bolza and Keating (1972).

The standard references agree that the timber of A. elate is highly resistant to termite attack, which is supported by the results of both the graveyard and glass plates experiments. In all instances the sapwood was readily attacked by termites, but the outer heartwood, particularly, showed a marked degree of resistance. The five graveyard stakes showing only slight termite damage indicated that about 8% of the initial weight was lost due to leaching, volatilisation, fungi and other microorganisms over a 2-year period.

The only standard reference to discuss the properties of P. erinaceus timber is Bolza and Keating (1972) who state that it is "moderately resistant to very resistant to termites". The difference between heartwood and sapwood durability has been mentioned by Forest Products Research Laboratory (1967). The results of these experiments show that, although the sapwood is rapidly destroyed by termites, the heartwood is reasonably resistant. It is perhaps interesting to note that on the savanna test site its loss in weight was much greater than on the forest test site, perhaps due either to the presence of decomposer organisms capable of attacking this savanna species or to climatic conditions volatilising repellent substances. The heartwood graveyard samples from the forest test site indicated that fungi and other microorganisms accounted for about a 7% loss in weight over a 2-year period.

Family Ulmaceae

The only genus in this family containing large and common forest trees in West Africa is Celtis, a genus posing taxonomic problems not dissimilar to those of Ulmus in temperate climates. A number of species of Celtis are found in Ghanaian forest, but three of these species are particularly common, all being marketed under the trade name celtis. The species are C. adolfi-friderici, (esa-kosua), C. mildbraedii, (esafufu), and C. zenkeri, (esa-koko), all of which are frequently referred to locally as esa. The standard references, although distinguishing the species, hardly distinguish the properties of the timber. It appears from the literature that the timber of all species is suceptible to termite attack, even though poles of esa are used in some rural construction projects.

The results of graveyard tests and tests between glass plates are shown in Fig. 16 and Table 4. From these data it can be seen that the characteristics of C. mildbraediiand C. zenkeri are very similar. Analysis of variance indicated no significant differences between the three zones of wood used in the tests for either species. There were, however, considerable differences between the wood samples taken from separate trees. C. adolfi-friderici, which has the lightest timber of the three species, showed a more rapid loss in weight in the graveyard tests than the other two species. However, in the presence of termite attack between glass plates all three zones of the timber of this species were extensively damaged over a 3-month period. It is thus clear that this species is considerably less resistant to termite attack than the other two species.

Compared to the density ranges quoted by Bolza and Keating (1972) the specific gravity data in Table 4 are variable. Thus the data for C. adolfi-friderici are slightly less dense than the quoted range, while the data for C. zenkeri are greater than the quoted range: C. mildbraedii lies almost at the mid-point of the quoted range of densities.

Family Moraceae

This relatively small family contains a number of medium to large-sized forest trees, as well as the extensive genus Ficus of shrubs, climbers, epiphytes and trees. The five species of timber included in these tests can be divided into two groups: one group showing considerable resistance to termite attack including Morus mesozygia, (wonton) and Chlorophora excelsa, iroko (odum), and the other group, with non-durable species, including Ficus capensis, (doma or odoma), Antiaris africana, antiaris (kyenkyen), and Musanga cecropioides, African corkwood (odwuma). Data for all five species are shown in Fig. 16 and Table 4.

C. excelsa has a local reputation for being resistant to termite attack, and is in demand in Ghana as a constructional timber. Although the standard references all agree that the timber is resistant, the results of the graveyard tests show that the timber is more rapidly destroyed by termites than timber of some other West African trees. All the tests indicate that the outer heartwood is more resistant than the inner heartwood, and that the sapwood is susceptible to termite damage: the results of tests against marine borers are similar (Barnacle and Ampong, 1975). In Fig. 16 and Table 4 it can be seen that in the graveyard experiments there was very little damage to any of the heartwood samples during the first 24 weeks of exposure, but that thereafter the damage increased significantly. There was considerable variability between the timber samples from different trees (P<0 - 001 in the analysis of variance). Nevertheless, the satisfactory performance of this species in construction over many years indicates at least a moderately high resistance to termite attack.

M. mesezygia had a better performance than C. excelsa. Although the sapwood is susceptible to termite attack, the two heartwood zones, which are not significantly different from each other, show a considerable degree of resistance. Taylor (1960) indicated that this species showed a similar resistance to C. excelsa, whereas Bolza and Keating (1972) state that the timber is liable to attack by termites. On analysis of variance of the graveyard data there was no significant difference between the timber samples from different trees. It would appear, therefore, that Ghanaian timber of M. mesozygia is reasonably resistant to termite attack.

The density for C. excelsa shown in Table 4 lies within the range quoted by Bolza and Keating (1972), although the density of M. mesozygia is considerably less than the 0- 81 - 0- 91 range quoted. Some outer heartwood graveyard samples of both C. excelsa and M. mesozygia remained reasonably free from termite attack and these indicated that the weight loss of both species due to leaching, volatilisation, and fungal and other microorganism attack was about 8% over a 2-year period.

The timber of both F. capensis and A. africana was extensively damaged by termites, there being no significant differences between the timber zones of either species. By accident no timber of A. africana was included in the graveyard test at Keni Keni Forest Reserve, but the performance of the timber between glass plates indicates that the timber is unlikely to be any more resistant in the savanna than in the forest zone. The specific gravity of A. africana timber lies within the range quoted by Bolza and Keating (1972), while that of F. capensis timber is rather greater than the maximum of their range (0 - 32)(Table 4).

The very light timber of M. cecropioides has rather unpredictable properties. Thus, Taylor (1960) states that "it is surprising how durable it is…..", and Bolza and Keating (1972) state that it is only rarely attacked by termites. During the first 12 weeks of exposure on the forest test site the graveyard heartwood samples showed very little damage by termites, but thereafter the damage became extensive. On the savanna test site there was relatively little damage after 4 weeks, but at 12 weeks the damage was extensive. The glass plates experiments yielded contradictory results: on the forest test site all samples were totally destroyed, while on the savanna test site there was relatively little termite damage. Analysis of variance indicated considerable variability between the timber samples of different trees. Taking these results in total, together with the statements in the literature, it appears that M. cecropioides timber has a limited resistance against termite attack for a period of up to 3 months, but thereafter it becomes extremely susceptible.

Family Olaceae

This relatively small family contains three genera of forest trees, two of which are represented in this study. Neither Strombosia glaucescens, afina, nor Ongokea gore, (bodwe), develop into large trees, since both have a maximum height of about 27 m.

Irvine (1961) states that the timber of S. glaucescens resists fungal and termite attack, and both he and Taylor (1960) indicate that, due to this durability, the timber is used in the construction of houses and bridges and for posts and telegraph poles. The timber showed relatively little weight loss during the first 24 weeks of the graveyard tests (Fig. 17, Table 4), but thereafter damage to the heartwood became extensive on both of the test sites. There was no significant difference between the timber from different trees. There was relatively little weight loss between glass plates (Table 4). However, these results do not support Bolza and Keating's (1972) conclusion that the timber is immune to termite attack. The Ghanaian material in these tests is considerably less dense than the range quoted by Bolza and Keating, which might account for some of the difference in termite-resistant properties. Only one graveyard stake showed slight termite damage, this having a weight loss of about 10% due to fungal decay.

The timber of O. gore has been used in West Africa for railway sleepers (Irvine, 1961), implying that it is probably reasonably termite resistant. This is confirmed by the results of these experiments in which it can be seen that the sapwood is liable to damage (more on the forest than on the savanna test site), whereas the heartwood is reasonably resistant, there being no significant difference between the weight losses of the two heartwood zones (Fig. 17, Table 4). Graveyard samples showing only limited termite damage indicated that about 7% of the weight would be lost due to leaching, volatilisation, and fungal and other microorganism activity over a 2-year period.

Family Irvingiaceae

This small family, only recently separated from Simaroubaceae, contains only two timber species, one of which, Klainedoxa gabonensis, (kroma), is included in these tests (Fig. 17, Table 4). The small blocks between glass plates lost very little weight during their 3-month exposure to termite attack. However, in the graveyard experiments the timber was extensively damaged over a 2-year period. Analysis of variance of the results indicates that there were differences between the wood zones (0-01<K0-05), with the sapwood being slightly less damaged on both test sites than the heartwood. These findings are in direct contradiction to the standard references: indeed Bolza and Keating (1972) state that the timber is "immune to termite attack". The density of the Ghanaian material is towards the top of the range quoted by Bolza and Keating. K. gabonensis was the densest of the 85 timber species included in these tests.

Family Burseraceae

Canarium schweinfurthii, canarium (bediwunua), is the only large timber tree of this small family in Ghana. Its timber is relatively light (Table 4), and both Irvine (1961) and Bolza and Keating (1972) agree that it is liable to attack by termites. The termite susceptibility of the timber both in graveyard tests and between glass plates is shown in Fig. 17 and Table 4. On analysis of variance of the graveyard data there was relatively little difference between the weight losses of the three zones (for the Fumesua data the significance of the difference was 0 - 01 < P <= 0 - 05, while for the Keni Keni data there was no significant difference between the zones).

Family Meliaceae

This family is the most important in the Ghanaian timber trade since species in five genera are exported: these are all attractive hardwoods used for furniture manufacture and interior work, and hence their durability is unlikely to be of prime importance in their utilisation.

Four species of the genus Khaya (the African mahoganies) have been included in the tests. The species used were K. anthotheca, anthotheca (white mahogany), K. ivorensis, African mahogany (mahogany), K. grandifoliola, (names probably as K. ivorensis), and K. senegalensis, (dry zone mahogany). Due to their essentially interior use the standard references give relatively little information on the termite resistance of these four species. It would appear, however, that while K. anthotheca and K. ivorensis are considered non-resistant, K. grandifoliola is supposed to be resistant (Bolza and Keating, 1972) and K. senegalensis to be very resistant (Forest Products Research Laboratory, 1967). The results of the graveyard tests indicate that both K. anthotheca and K. ivorensis are extensively damaged by termites (Fig. 18, Table 4). Both the graveyard test and the test between glass plates also indicate that the timber of K. grandifoliola is damaged more extensively than that of K. anthotheca or K ivorensis (Table 4). Analyses of variance indicated that there was significant variability between trees, and between zones, of K. anthotheca, with the sapwood being most heavily damaged. There was neither a significant difference between trees nor a significant difference between the three timber zones in K. ivorensis. However, there was a significant difference between heartwood and sapwood of K. senegalensis, the heartwood proving to be very resistant against termite attack. Many of the heartwood samples of this species in the graveyard test had very little termite damage, and these indicated that about 10% of the weight had been lost due to leaching, volatilisation and fungal damage over the 2-year period.

The specific gravities of K. anthotheca and K ivorensis in Table 4 are within the ranges quoted by Bolza and Keating (1972). However, that of K. grandifoliola is considerably lighter than the quoted range, which might be related to the contrary results on termite susceptibility. The density of K. senegalensis is within the quoted range, this being the densest mahogany to occur in Ghana.

Four species of the genus Entandrophragma are important for the timber trade. These are E. angolense, gedu nohor (edinam), E. cylindricum, sapele, E. utile, utile, and E. candollei, omu and heavy sapele (candollei). The standard references indicate that the timber of all four species is moderately resistant to damage, although Bolza and Keating (1972) note that the resistance of E. angolense is variable. The results of the graveyard tests hardly confirm these results since the weight lost by the Entandrophragma species was similar to that lost by the Khaya species (Fig. 18, Table 4). During the first 24 weeks of exposure in the soil there was generally little weight loss; after this the weight loss gradually increased, with the sapwood generally being the most heavily attacked. Analyses of variance on each species indicated significant differences between trees for E. utile (0 - 01 < P <= 0 - 05), E. angolense (0 - 001 < P <= 0 - 01) and E. cylindricum (P<0 - 001), and significant differences between the zones of timber for all four species.

These results are confirmed for three of the species in the tests between glass plates (Table 4) however, E. candollei was extensively damaged on both test sites. In the graveyard tests the amount of damage to this species after 2 years was slightly greater than that of the other three species. It would therefore appear to be the least resistant of the four Entandrophragma species to termite attack.

A few graveyard test samples of all four species of Entandrophragma were only slightly damaged by termites. The weight loss of the samples which can be attributable to leaching, volatilisation and fungi and other microorganisms is approximately 32% for E. cylindricum, 21 % for E. angolense and 8% for both E. candollei and E. utile over a 2-year period. The specific gravity data for these four species in Table 4 conform to the density ranges quoted by Bolza and Keating (1972) except for E. candollei: the Ghanaian material is substantially lighter than the quoted range (0 65- 0 72).

The northern Ghanaian species Pseudocedrela kotschyi, krubeta, proved to be the most resistant timber species to be tested, as judged by the number of graveyard samples showing negligible termite damage after 96 weeks inserted in the ground (Fig. 19, Table 4). Analysis of variance indicated that the sapwood was more susceptible to attack by termites than the heartwood. The majority of samples in the graveyard tests were only slightly damaged by termites after 2 years, and from these it can be estimated that heartwood samples lost about 11 % and 8% of their weight due to leaching, volatilisation and fungal damage on the forest and savanna test sites respectively. Sapwood samples, however, lost about 36% of their weight due to fungal decay. These results conflict with Bolza and Keating (1972) who state that the timber is "mildly prone to termite attack", and who do not assign the timber to the most durable class.

Both Irvine (1961) and Bolza and Keating (1972) suggest that the timber of Lovoa trichilioides, African walnut, is moderately susceptible to termite attack. The results of the graveyard tests show that although in the short term, up to 12 weeks, there is relatively little weight loss from samples of this timber, yet in the longer term it is extensively damaged (Fig. 19, Table 4). Analysis of variance indicated that sapwood lost more weight than either of the heartwoods, which were not significantly different from each other: there were no significant differences between the weight losses of timber from different trees. The densities recorded in Table 4 are rather less than the range quoted by Bolza and Keating (1972).

Two under-storey species of this family, Carapa procera, African crabwood (kwaku-bise), and Trichilia prieureana, (kakadukro), are common within the forests of Ghana. Although neither produces large timber trunks the wood is used locally and is available in large quantities. The wood of C. procera is said to be fairly resistant to termites (Irvine, 1961; Bolza and Keating, 1972), but there appears to be no data on the termite resistance of T. prieuriana. Although both of these species gave encouraging results during the first 12 weeks of the graveyard tests (Fig. 19, Table 4), their performance became successively worse so that after 2 years the majority of the timber had been destroyed. The interesting feature of these results is that the inner heartwood of T. prieuriana shows significantly less damage than the outer heartwood in all tests; with C. procera there is no significant difference between the two heartwood zones. The specific gravities of both species in Table 4 are below the density ranges quoted by Bolza and Keating (1972).

Turraeanthus africanus, avodire, is another species of timber about which rather little seems to be known. Although Taylor (1960) states that it is not durable against decay, Bolza and Keating (1972) consider that it is moderately resistant to termite attack, and Smith (1969) shows that it is reasonably durable in Europe. During the first 6 months of the graveyard tests, T. africanus heartwood showed very little weight loss indicating its resistance both to termites and fungi. However, after 6 months the weight loss increased rapidly so that most of the timber was destroyed after 2 years (after which time a weight loss of about 17% could be attributed to fungi, leaching and volatilisation). Analysis of variance indicated no significant differences either between the zones of timber or between the wood of individual trees.

Although there are two species of the genus Guarea in the Ghanaian forests, only G. cedrata, scented guarea (kwabohoro), is at all common. G. thompsonii, guarea or black guarea, is only met infrequently in the local timber trade. Both species are reported to be moderately resistant to termite attack (Irvine, 1961; Bolza and Keating, 1972), G. thompsonii being perhaps less durable than G. cedrata, though they are similar under European test conditions (Smith, 1969). These statements are supported by the results of the graveyard tests which show that both species are extensively damaged by termites, the damage being greater in G. thompsonii (Fig. 19, Table 4). With the small blocks between glass plates G. cedrata suffered only slight damage (Table 4), while sapwood and inner heartwood of G. thompsonii were severely damaged. Analysis of variance of the graveyard test data indicated a significant difference between trees of G. cedrata, but no significant difference between inner and outer heartwood.

Family Sapindaceae

Although this family is represented by several genera in West Africa, only one genus, Blighia, contains large forest trees, one of which, B. sapida, akee apple (akye), is included in these tests. Irvine (1961) states that timber is "said to be immune to termites", which is partially supported by the results in Fig. 17 and Table 4. Over a few months, either in the graveyard or between glass sheets, there was very little weight loss by either heartwood or sapwood. However, by a year, there was about a 10% weight loss from all three zones of wood, and after 2 years there was appreciable termite damage especially to the sapwood (at this time about 10% of the weight loss of heartwood could be attributed to leaching, volatilisation and fungi). On analysis of variance there was no significant difference between the timber of different trees. This timber, which is not particularly dense, appears to be reasonably resistant to termite attack.

Family Anacardiaceae

Only one species of this small family has been included in the tests. Both Taylor (1960) and Bolza and Keating (1972) state that the timber of Antrocaryon micraster, antrocaryon (aprokuma), is non-durable and susceptible to insect attack. In Ghana, the timber would appear to be one of those most susceptible to termite attack: in the 3 months between glass plates, on both test sites, the majority of the timber was destroyed (Table 4). In the graveyard tests the damage was not quite so complete, but after 12 weeks in the soil the majority of samples showed extensive attack by termites (Fig. 17, Table 4). On analysis of variance there were no significant differences either between timber zones or between the timber samples of different trees. With these termite susceptible properties, the timber of A. micraster would make a suitable baitwood for attracting termites into other test materials.

Family Ebenaceae

The genus Diospyros contains a number of species which are collectively referred to as the African ebonies. The largest of these species which is at all common is D. sanza-minika, kusibiri. Despite the fact that all three standard references indicated that the timber was reasonably durable and resistant to termite attack, this species gave disappointing results on test (Fig. 20, Table 4). On both test sites there was a gradual and continual erosion of the timber samples by termites so that after 2 years the samples had been more or less completely destroyed. There was no significant difference between the heartwood and sapwood samples, nor was there a significant difference between the timber from different trees. The Ghanaian material is rather denser than the range quoted for this timber by Bolza and Keating (1972).

Family Sapotaceae

Although this is a relatively small family it contains a number of forest tree species. Trees of the genus Manilkara, which are only small, have timber which is reputed to be durable and resistant to termite attack (Irvine, 1961). Two species of this genus, M. multinervis, and M. obovata (synonym: M. lacera), both with the local name berekankum, were included in the tests. Both species have very dense timber with a heartwood specific gravity in excess of 1 -0 (Table 4). In the graveyard experiments both species proved to be reasonably durable, suffering only superficial termite damage (except the sapwood of M. multinervis) ( Fig. 20): a number of heartwood samples which were only slightly damaged indicated that the weight loss due to fungi over a 2-year period was about 7% in M. mu/tinervis and 10% in M. obovata. The performance of the graveyard samples is supported by the results of the small blocks between glass plates which showed only minimal weight loss over a 3-month period (Table 4).

The properties of the timber of Gluema ivorensis, (nsudua), are not included in any of the standard references. Despite the lack of attention to this species, it has proved to be one of the most durable and resistant timbers included in these tests. Heartwood samples lost on average only 10% of their weight while inserted in the soil for 2 years: this can be apportioned into approximately 8% due to leaching, volatilisation, and fungi and other microorganisms and 2% due to termites. The performance of the timber between glass plates, unfortunately only on one site due to lack of materials, was also extremely good. This relatively dense timber would therefore seem promising for use, untreated, in situations where resistance to termite attack is important. The timber also appears to be very resistant to attack by marine borers (Ampong and Barnacle, unpublished data).

Tieghemella heckelli, makore (baku), is a very large tree that is recorded as being termite resistant by the standard references. Despite this reputation both the graveyard samples and the small blocks between glass plates were attacked by termites (Fig. 20, Table 4). Analysis of variance of the data from the graveyard tests indicated that there was a significant difference (P<0 - 001) between the wood from different trees as well as differences between the zones (inner and outer heartwood were not, however, significantly different). A few samples suffered only slight termite damage, and these indicated that about 8% of the weight lost by heartwood over a 2-year period could be attributed to leaching, volatilisation and fungal activity.

Another relatively unknown timber was that of Aningeria robusta, (asamfona), the durability properties of which are not recorded in the standard references. The results of these experiments indicate that the timber is extremely susceptible to attack by termites. Many of the samples were more or less completely destroyed within 12 weeks, though it is noticeable that the sapwood proved to be more resistant to termites over this time scale than the heartwood (Fig.20, Table 4). Analysis of variance failed to indicate any inter-tree differences.

The genus Chrysophyllum contains a number of forest tree species. The local names seemed to vary, though at times the name akasaa was applied to at least the two species included in these tests. Since most of these species are not exploited, there are no local trade names, though Bolza and Keating (1972) use the name white star apple for C. albidum, and samfona for C. perpulchrum. Both Irvine (1961) and Bolza and Keating (1972) consider that these timbers are not resistant to termite attack. The results of the experiments indicate that these two species are extensively damaged by termites, but they are not so rapidly destroyed as A. robusta (Fig. 20, Table 4). C. albidum in Ghana had a timber less dense than the quoted range, but its heartwood appeared to be destroyed more quickly than its sapwood. C. perpulchrum had denser timber which showed a more or less linear loss of weight with time in the graveyard experiments. Analyses of variance, as well as indicating differences between the timber zones, indicated that there might be inter-tree differences in C. albidum (0 - 05 >= P > 0 - 01).

Family Apocynaceae

This extensive family of shrubby plants contains only two genera of forest trees, Holarrhena and Alstonia. Both A. boonei, alstonia (sindru), and H. floribunda, (osese or sometimes sese), have relatively light timber and are recorded in the standard references as being susceptible to termite damage. Both the small blocks between glass plates and the graveyard test specimens suffered extensive termite damage (Fig. 21, Table 4), confirming the references. Analysis of variance indicated neither differences between the timber zones nor differences between trees for either species. The density data are more or less in agreement with the ranges quoted by Bolza and Keating (1972), except that the specific gravity of A. boonei in Table 4 is lighter.

Family Rubiaceae

Very few species in this large family are forest trees; however, one species, Nauclea diderrichii, opepe (kusia), which is sometimes separated from Rubiaceae into Naucleaceae, is an important timber species, and the genus Mitragyna contains two closely related species, M. ciliata and M. stipulosa, abura (subaha). The data for these two genera are shown in Fig. 21 and given in Table 4.

The results of the tests of N. diderrichii are particularly interesting. In the forest test site, the graveyard samples of the inner heartwood were more extensively damaged than the sapwood samples, while in the savanna test site the sapwood was most heavily damaged. In the experiment with small blocks between glass plates there were no significant differences between timber from three zones. The analysis also indicated very significant inter-tree variability (P < 0 - 001). These results combine to indicate that the timber of N. diderrichii can vary rather greatly in its termite-resistant properties, and might account for the differing results in marine tests quoted by Barnacle and Bentum (1972). The samples with little termite damage indicated that these graveyard samples might lose about 5% of their weight by leaching, volatilisation and fungal activity over a 2-year period. Taylor (1960) commented on the durability of this species, quoting as an example timber that had been in the ground at Axim undamaged for 30 years. It is generally accepted as a durable timber species (Bolza and Keating, 1972).

However, the three standard references agree that M. stipulosa is non-durable, being readily attacked by termites. The data for the graveyard tests, showing no significant difference between zones of wood, indicates the speed with which termites will attack this species. Between glass plates, the timber, particularly the inner heartwood, was extensively damaged by termites. On analysis there was no significant difference between timber from different trees. For both N. diderrichii and M. stipulosa the density data in Table 4 are within the ranges quoted by Bolza and Keating (1972).

Family Palmae

Although this family of monocotyledons does not form true wood, two species were included in the test. One of these, Borassus aethiopum, agobeam, is known in the timber trade, while the others Elaeis guineensis, (oil palm), is widely cultivated for such products as palm nuts, palm oil and palm wine. The timber of E. guineensis, although not mentioned in any of the standard references, is only moderately damaged by termites (Fig. 21, Table 4). During the first 24 weeks of exposure in the graveyard tests the weight loss was relatively little, indicating that the timber could probably be used above ground in situations where the termite hazard was low. The performance of B. aethiopum is extremely good since about 5% weight loss over a 2-year period by fungal activity was indicated. Irvine ( 1961 ) indicates that the 'wood' of the two sexes of this tree may differ, that of the male palm being denser than that of the female palm: unfortunately the trees included in these experiments were not sexed (Fig. 9). It is noticeable that the timber was attacked more extensively on the forest than on the savanna test site in both the graveyard test and the small blocks test.

Discussion and conclusions

It is usual to rank timbers, for example Fougerousse (1969), or to group timbers into resistance classes, for example Small et a/. (1960), after performing experiments on their durability or termite resistance. This approach has not, however, been attempted here since the tables and illustrations record the results of two separate experiments, each conducted in two separate areas, one experiment over a 3-month period and the other over a 2-year period. Although in many species the results from all four tests are similar, yet in others there are differences either between the two sites or between the two testing methods. It is therefore appropriate to quote the test results in detail, but not to attempt to rank or group the timbers (which has been attempted by Ocloo and Usher, in press). Three general points can be made about Figs. 10-21 which are the results of graveyard tests on small specimens of timber with a large surface to volume ratio.

First, if any heartwood graph remains close to the zero weight loss (horizontal) axis throughout the 96-week period of the test, it is reasonable to assume that it will be extremely resistant to biodegradation by termites and fungi, for example the Lophira species in Fig. 10. Secondly, if any heartwood graph terminates before week 96 on the horizontal axis, then that timber is highly susceptible to termite attack. During the whole of the test no other arthropod species were seen to attack the graveyard samples, and hence elimination of samples from the test can be attributed solely to termites {though fungal decay may have contributed to loss in weight). An example of a highly susceptible species is Pycnanthus in Fig. 10. Thirdly, any heartwood graph that continues to rise during the whole of the 96-week period would indicate reasonably resistant to reasonably susceptible timber species. Thus, in Fig. 10, the graph of Terminalia superba rises throughout the 96-week period and is reasonably susceptible, while T. ivorensis rises much less steeply and is reasonably resistant. In general, in interpreting these series of illustrations, the sapwood should be neglected since it is more termite susceptible than heartwood. However, it is of interest to note that the sapwood was more resistant than the heartwood in Anopyxis klaineana, Diospyros sanza-minika and Klainedoxa gabonensis in the graveyard tests. This should not be interpreted to mean that sapwood would remain more resistant over a period greater than 96 weeks: indeed the graphs of Amphimas pterocarpoides in Fig. 13 indicate that towards the end of the test period the sapwood was attacked.

The weight losses recorded in these experiments might appear alarmingly large, since every sample was at least nibbled by termites. It should be stated that the experiment was carried out under 'accelerated' conditions, with termites led into contact with the samples by baitwood, the samples being rectangular in cross-section and extremely small (and thus with a large surface: volume ratio). It is certain that percentage weight loss would have been considerably less if the samples had been larger, square or round in section, and without baitwood. Although no figure can be put on an acceleration factor, yet it is possible that the damage experienced in these tests might not happen in under a 10-20-year period in actual use. Both the design of the experiment and the treatment of the test sites to maintain large populations of termites were undertaken to maximise the differences between the timber species and the wood zones in the minimum length of time.

Table 3

One of the unknown factors in such field tests on timbers is the role of the chemicals which could act as termite repellents or poisons, and the effects of leaching or microbial activity upon these chemicals. Most species of timber have wood extractives: a list prepared by the Building Research Laboratory is given in Table 3, though the list is probably not complete. Carter et a/. (1975) discuss some of the issues raised in extracting such chemicals from tropical timbers. These extractives are confined mainly to the heartwood, though the sapwood of some species can contain small quantities of extractives although they are not always the same compounds as those found in the heartwood. Within the heartwood the concentration of extractives is generally greatest in the outer heartwood, decreasing across the trunk to the pith. However, variations occur between the different anatomical elements: ray cells frequently contain a high proportion of the extractives, and there are differences between cell-wall and cell-lumen deposition.

Finally, and related to the above, another feature of interpreting results is important and is often neglected in resistance or durability tests. This is the radial position of a heartwood sample. Work in Australia by Rudman (1964) on Eucalyptus and Rudman et al. (1967) and Da Costa et al (1961) on Tectona has demonstrated considerable radial variation in these genera, and Ocloo (1975) has found similar radial variation in the termite resistance of Ghanaian specimens of Terminalia ivorensis. From a commercial point of view, it is difficult to separate most inner and outer heartwood when sawing a tree. Hence, in interpreting the results of these tests, the performance of heartwood, which is affected by its radial position in the tree, will be a function of the difference between 'inner heartwood' and 'outer heartwood' samples. Thus, in Table 4 and Fig. 10, the performance of heartwood of L. alata can be relied on to a much greater extent than the heartwood performance of T. ivorensis in which the outer heartwood is considerably more resistant than the inner heartwood. An engineer or architect who cannot specify the quality or exact radial position of heartwood timber to be used in a construction project will find it preferable to interpret the results in Figs. 10-21 and Table 4 conservatively by using the results for the worst heartwood zone. However, it might be preferable for the engineer or architect to insist on timber quality to utilise locally grown and produced materials to a maximum. A specification of timber quality should, perhaps, be more frequently included in building and other constructional contracts.

Acknowledgements

In carrying out a series of experiments such as these it is inevitable that we were given advice and practical help from a large number of people, and it is impossible to thank them all by name. We should, first, like to thank the Directors of the Building and Road Research Institute and the Forest Products Research Institute who provided facilities; and many of their staff who undertook the tedious tasks of band sawing specimens, cutting and planing the experimental samples, and working on the test sites. In particular we should like to thank Mr J. E. Barnacle, Division of Building Research, C.S.I.R.O., Australia (formerly of Forest Products Research Institute, Ghana), for many helpful discussion and comments on a draft of this paper; Mr F. F. K. Ampong for assistance in collecting timber specimens; Dr W. A. Sands and Mr R. M. C. Williams, Centre for Overseas Pest Research, London, U.K., who discussed aspects of the work with us; Dr R. J. Orsler, Building Research Establishment, Aylesbury, U.K., for providing information on the chemicals in the woods; our technical staff, Miss Mary Dinsey, Mr S. Nyarko and Mr J. K. Appiah Kwarteng, for their help both in the field and the laboratory; and the Director, staff and students of Kumasi Polytechnic who also helped by preparing samples.

The extensive collection of timber samples could not have been obtained without the help and advice of many people in the Forestry and Timber Industries. We should therefore like to thank the staff of the Ghana Forestry Department in Kumasi; Mr F. Cudjoe, Forest Products Research Institute, for help in identifying trees; and the managers and staff of the following sawmills (in alphabetical order):-Africa Timber and Plywood (Ghana) Ltd. (P.O. Box 1, Samreboi); Amakom Sawmill Co. Ltd. (P.O. Box 1381, Kumasi); Anthony Timbers Ltd. (P.O. Box 826, Kumasi); Ashanti Curl and Lumber Producers (P.O. Box 652, Kumasi); Bibiani Lumber and Logging Co. (P.O. Box 170, Kumasi); Daniel and Company Sawmill (P.O. Box 329, Kumasi); Ehwia Sawmills (Ghana Timber Marketing Board, P.O. Box 3813, Kumasi); Ghana Timber Marketing Board (P.O. Box 515, Takoradi); Gliksten (Ghana) Ltd. (Private Post Bag, Sefwi Wiawso); A. Lang Ltd. (P.O. Box 1981, Kumasi); Mim Timber Co. Ltd. (P.O. Box 1950, Kumasi); Nkawkaw Sawmills Ltd. (P.O. Box 59, Nkawkaw); Poku Transport and Sawmills (P.O. Box 2027, Kumasi); Timber and Transport, TAT-(KK) Ltd. (P.O. Box 1925, Kumasi); and Wood Supply (Ghana) Ltd. (P.O. Box 1405, Kumasi).

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