Agroforestry Systems 48: 273–288, 2000. 2000 Kluwer Academic Publishers. Printed in the Netherlands.
Evidence for anthropic selection of the Sheanut tree (Vitellaria paradoxa) P. N. LOVETT1 and N. HAQ2, * 1
The Shea Project, COVOL Uganda, P.O. Box 833, Lira, Uganda (E-mail:
[email protected]); Environmental Research Group, International Centre for Underutilised Crops, The University of Southampton, Highfield, Southampton, SO17 1BJ, UK (*Author for correspondence: E-mail:
[email protected])
2
Key words: tree domestication, unconscious selection, Vitellaria paradoxa Gaertn. Abstract. The Sheanut tree (Vitellaria paradoxa Gaertn.), a multi-purpose species highly valued for the oil obtained from its seeds, is commonly maintained in the semi-arid parklands of subSaharan West Africa. An inventory in the West Gonja District, Northern Region, Ghana, revealed that on intensively farmed land this species constituted 79.7 ± 7.2% (Basal area = 2.19 ± 0.64 m2 ha–1) of the woody biomass, on low intensity farmland 84.2 ± 10.0% (2.16 ± 0.57 m2 ha–1) and only 10.2 ± 3.3% (0.92 ± 0.23 m2 ha–1) in unmanaged woodland, with similar environmental characteristics. No significant differences were found between total Sheanut tree densities on different land use intensities, although as a proportion of all trees surveyed, large trees were more common on farmed land. Participatory surveys revealed that these populations are a direct result of anthropic selection as local farmers eliminate unwanted woody species on farmland, leaving only those Sheanut trees that meet criteria based on spacing, size, growth, health, age and yield. Characteristics that could affect population dynamics during traditional management and harvesting including short viability seeds and cryptogeal germination are also discussed with reference to unconscious selection. Tree improvement is currently constrained, as true to type varieties are difficult to propagate. It is proposed that Sheanut trees on farmland are semi-domesticated having been subject to long-term anthropic selection during cycles of traditional fallow and crop cultivation.
Introduction The Sheanut tree, Vitellaria paradoxa Gaertn. (Family – Sapotaceae, syn. Butryospermum paradoxum), is a major component of the woody flora of the Sudan and Guinea savannah vegetation zones of sub-Saharan Africa. The species range forms an almost unbroken belt approximately 5,000 km long by 500 km wide from Senegal to Uganda (Bonkoungou, 1987). Vitellaria paradoxa is an important source of many non-timber products – the nutritious fruits are eaten, the bark is used for traditional medicines and the latex is used for making glue. The wood is used for charcoal, construction, for furniture and as pounding mortars (Dalziel, 1937; Abbiw, 1990). It is also of note that the pestles for use with these mortars, are poles from the so-called ‘male’ Sheanut tree (Lophira lanceolata Van Tiegh. ex Keay), with which Vitellaria is sometimes confused. The most valued product (usually harvested, processed and sold by women) is the Shea butter (francophone: le beurre de Karité)
274 extracted from the dried kernels, which is the main oleaginous product in many areas of this species semi-arid range (Lamien et al., 1996). The oil is widely utilised for local domestic purposes such as cooking, as a skin moisturiser or as an illuminant. Shea butter is also utilised commercially as an ingredient in cosmetic, pharmaceutical and edible products (Abbiw, 1990). Due to the importance of this species, Vitellaria has been included on the priority list of African forest genetic resources by FAO (FAO, 1977, 1984, 1988). As a commodity of trade, Shea butter has had a very long history in West Africa, described by Ibn-Batutta, an Arab traveller in the 14th Century (Mainy, 1953) and by Labat in 1728 (Dalziel, 1937). In 1797, the Scottish explorer Mungo Park noted that traditional management was to selectively maintain Sheanut trees on land cleared for cultivation (Park, 1799). This form of management has probably been practised for many centuries prior to these observations, as recent evidence suggests that at a site in Northern Burkina Faso, ca. 1000 AD, Sheanut trees were being maintained on cultivated land (Neumann et al., 1998). The Sheanut tree has a sacred status where utilised and has often been described as a major component of the indigenous yam, sorghum or millet based farming systems practised in West Africa (Clark, 1980; Boffa, 1995; Harlan, 1992; Pullan, 1974; Kessler, 1992). Although the best specimens are commonly found on farmed land, available evidence suggests that Sheanut trees have been managed in traditional farming systems for centuries. This species has usually been described as ‘wild’ as opposed to being a domesticated species (Abbiw, 1990; Baker, 1962; Hall et al., 1996). Archaeological evidence for the domestication of plants in West Africa has been scarce and dates of ca. 5,000 yrs bp are usually offered (Blumler, 1996), however, linguistic evidence has suggested dates at least 8,000 yrs bp (Ehret, 1984). What is certain, is that many annual crops, including African Rice, Yam, Sorghum, Millet and Tef, originated in sub-Saharan Africa, a location that Harlan (1971) refers to as a ‘non-center’ of agricultural origins due to the large expanse of this area. It has also been suggested, following long term associations with man, that a number of African tree species are also in an intermediary state of becoming domesticated, but since this is an ongoing and iterative process there is no clear point at which a species can be said to be no longer wild. These include the Boabab (Adansonia digitata L.), the White-barked Acacia (Faidherbia albida (Del.) A.Chev.), the West African oil palm (Elaeis guineensis Jacq.), the Cabbage tree (Moringa stenopotala Bak.F. Cuf.) and the Locust bean tree (Parkia biglobosa (Jacq.) Don.) (Harlan, 1992; Jahn, 1991; Z. Teklehaimanot, pers. com., 1999). In Northern Ghana, tradition dictates that the Sheanut tree is rarely planted and most individuals result only from natural regeneration. Vitellaria also has recalcitrant seeds, a widely out-crossing breeding system and a lengthy maturation period (up to 15–20 years). Therefore, tree improvement through ‘methodical’ selection is a laborious option and has probably not been attempted by local farmers who also describe this species as wild. This does
275 not preclude other types of ‘human-induced change’ as reports describe Vitellaria populations where larger ‘superior’ individuals are common on farmland but regeneration abundant only on fallow land (Abbiw, 1990; Hall et al., 1996). Assuming that the maintenance of healthy trees in a man-made environment, following the removal of unwanted individuals during cyclical fallow clearance, offers an analogous domestication mechanism to that when seeds of annual crops are reserved for planting the following season. These studies offer evidence for the domestication of the Sheanut tree following ‘unconscious’ or ‘natural’ selection in environments controlled by anthropic activities. The majority reports compare Vitellaria populations on cultivated land with those on fallow land previously disturbed by anthropic activities, as opposed to ‘wild’ populations in completely unmanaged areas (Boffa, 1995; Schreckenberg, 1996; Osei-Amaning, 1996). The lack of small diameter trees has also been given as evidence for the degradation of West African parklands (Gijsbers et al., 1993). Therefore, this study aimed to use a quantitative inventory to compare Vitellaria populations, in an environmentally homogenous area, on sites with different intensities of landuse (high, medium and low). Using participatory interview methods with local farmers, this study further aimed to investigate the role that anthropic activities have had on the management, conservation and possible domestication of Vitellaria paradoxa in the farmed parklands of Northern Ghana.
Materials and methods Tree inventory Using Landsat thematic images (January 1986), an area encompassing different landuse intensities was selected in the West Gonja District, Northern Region, Ghana (Figure 1). Following discussions with long-term residents, it was also established that few changes had occurred in the previous ten years (apart from the introduction of mechanised ploughing and fertiliser use near Bole). The intensities of landuse were defined as follows: low – unmanaged woodland south of the hamlet of Jintigi, medium – scattered farmland around the small village of Mandari and high – intensively farmed land around the more densely populated district capital of Bole. Using soil maps (FAOUNESCO, 1977; FAO, 1968), 1:50,000 maps (Survey of Ghana, 1966) and information from a local geological survey (S. Meadows-Smith, pers. comm., 1997), it was determined that all locations had similar soil types (FAO soil unit Lf31-a, petric phase), altitude (ca. 300 m above sea level) and geology (pre-Cambrian granites). Using data from Meteorological Services Department, Legon (C. Adams, pers comm., 1995), climatic similarity was assumed due to the close proximity (within 25 km) of all locations to Bole (1092 ± 72.9 mm yr–1, 33 to 21 °C).
276
Lighter areas indicate vegetation clearance (cloud cover minimal) Figure 1. Landsat thematic image of study area (18th January 1986) in the Northern Region, Ghana.
1 square hectare plots were surveyed on randomly selected farms, between May and September 1997, after obtaining permission from local landowners on medium intensity farmland (around Mandari village) and on high intensity farmland (near Bole town). To prevent potential confusion with farmed land that had been left fallow and areas of unknown management history, plots were located only on land currently being cultivated. Due to high tree density and transport limitations, smaller 0.25 ha plots were located in the low intensity area (south of Jintigi hamlet, in woodland about twenty miles west of Bole) using random positions on a geological survey grid. All woody stems inside these plots (≥ 7 cm diameter at breast height, dbh) were measured and identified by species. Participatory questionnaires With the aim of assessing traditional agroforestry practices in Northern Ghana, and to determine whether the West Gonja study sites were representative of Ghanaian parklands, participatory interviews were held with 130 people during 39 interview sessions in three regions of Ghana. Since gender was an important factor with regard to Sheanut related activities, women were also asked
277 to express their views. Women made comments at 59% of interviews although they were the sole or main respondents at only 15%. Information was obtained on the farm and tree management systems employed in the area (e.g. farm size, crops grown, fallow duration, utilisation of tree species, favoured Vitellaria traits and selection at land clearance). Care was taken to ask questions without suggestion and to crosscheck answers. Answers were also confirmed by local knowledge gained during two years of field research in the area. Tree selection on farmed parklands was similar throughout Northern Ghana and almost pure stands of Sheanut trees were frequently seen in fields planted with yams, millet, sorghum, maize, cassava or legumes. Most farmers still practised the traditional method of hand ploughing, but deep mechanised ploughing was more frequent near larger habitation centres in intensively farmed areas. Animal traction was, however, the preferred power source in the Upper-West region of Ghana. Farms were generally similar, although the distances from the home to farm in the study area were significantly greater than at other locations (P < 0.01). The other notable difference was that farms in the Upper West region were generally larger and fallow periods were shorter (P = 0.07–0.10), indications of a higher intensity of landuse than that occurring in the study area (Table 1). Data analysis To compare species diversity, Simpson’s diversity index was calculated. This index provides an integrated value for the two components of α-diversity, variety and evenness (Hamilton and Perrott, 1981). The results are presented in the form 1-D per plot, where values approaching 1 indicate areas with many species, each at low density, and a value of 0 indicates an area with only one species. For two areas with an equal number of species, higher values for 1-D indicate evenness. The percentage contribution to woody biomass was calculated as the total basal area (BA) per hectare of each species divided by the total BA per hectare of all species. BA [m2 ha–1] = α(dbh in cm/200)2 × 10,000/plot size in m2 The Sheanut tree population structure was examined by plotting the mean density of stems occurring at each location in each 7 cm dbh size class. Using SPSS for Windows ver. 8.0, ANOVA was used to determine significant differences between quantitative site variables and post-hoc pairwise multiple comparisons were performed using Tukey’s honestly significant difference test.
278
Table 1. Participatory interviews and farm descriptors in the Northern Region, Ghana. Region
Location
Villages
Interviews
Men
Women
Total
Farm size Acres
House to farm miles
Time farmed yrs
Time fallow/ yrs
Brong Ahafo Northern Upper West
Wenchi Bole Nandom/Wa
3 5 8
10 21 08
18 41 14
17 31 09
35 72 23
5.1 ± 2.1 5.3 ± 1.4 6.4 ± 1.9
2.0 ± 0.5 3.6 ± 0.7 1.5 ± 0.6
2.9 ± 0.7 3.0 ± 0.3 3.9 ± 0.7
8.1 ± 2.7 7.2 ± 2.0 4.3 ± 1.7
279 Results Woody species composition of sites A significant variation in the total number, and abundance of each tree species occurred between sites. In particular, species commonly utilised for their edible or other non-timber products, were found in greater numbers on farmland. Parkia biglobosa (Don) was not found in any woodland plots and large individuals of Diospyros mespiliformis (Hochsetter ex A. de Condelle) – a species locally taboo to cut – were more common near Bole (Table 2). The number of individual trees, total woody biomass, species number and diversity was also significantly lower in farmed areas (Table 3). Results for Vitellaria only, show no significant variation in numbers of individuals (dbh ≥ 7 cm) between sites. The basal area and total number of large individuals (dbh > 20 cm), however, were significantly greater on high intensity farmland near Bole than in natural woodland (Table 4). It was evident that few small (dbh < 13 cm) individuals of Vitellaria were maintained on land cleared for cultivation, particularly in farms mechanically ploughed, and ring barking was observed as a method used to kill occasional mature trees. In woodland, the population structure showed many similarities to other ‘natural’ systems, with abundant small, probably immature individuals and few large trees (Figure 2). At this site, 82.6% of all Sheanut stems sampled were small trees (dbh < 20 cm) although some plots contained only trees greater than this size class. All mature trees in farmland, greater than 20 cm dbh, were seen to bear at least some fruit during the harvest season. In woodland, even after extensive searching of the whole area during the same period, only one individual with fruit was found (dbh = 18 cm). Mature fruit collection from this individual required vigorous shaking and viability was high even four weeks after collection. On farmed land, fruit fell to the ground when mature (April–June) but all collections lost viability after three weeks. Indigenous agroforestry management When farmers were questioned whether they perform selective management on Sheanut trees, the following results were obtained (as a mean of all three regions): • 72 ± 37% practised some form of selection during land clearance for cultivation. • 37 ± 37% admitted to having ever cut any productive mature trees. • 58 ± 24% protected immature trees on cultivated land. • The majority of farmers said they would leave dense stands and cultivate elsewhere. This confirms field observations described above since male farmers said that although cutting of Sheanut trees was locally taboo, they still removed
280
Table 2. Top ten woody species at sites in North Ghana. Bole (high intensity farmland)
Mandari (med. intensity farmland)
Unmanaged Woodland
Species
Use1
%2
Species
Use
%
Species
Use
%
Vitellaria paradoxa Parkia biglobosa Diospyros mespiliformis Azadirachta indica Daniellia oliveri Hannoa undulata Pericopsis laxiflora Sterculia setigera Lannea acida
EBFM EF EFM M SM FMC F M EF
79.7 07.1 04.4 03.3 01.6 00.7 00.6 00.5 00.4
Vitellaria paradoxa Hannoa undulata Sterculia setigera Parkia biglobosa Daniellia oliveri Diospyros mespiliformis Vitex doniana Lannea acida Bombax costatum
EBFM FMC M EF SM EFM E EF C
84.2 03.6 02.4 02.0 01.8 01.6 00.9 00.7 00.7
Berlinia grandiflora Unknown species #15 Vitellaria paradoxa Pericopsis laxiflora Isoberlinia doka Lannea acida Afzelia africana Combretum molle Terminalia avicennioides
BF – EBFM F BF EF BM F F
32.2 12.2 10.2 10.1 08.0 06.3 04.4 02.8 02.5
1 2
Local utilisation of products (E – Edible, B – Building, F – Fuel, S – Soil improver, M – Medicine, C – carving). Average percentage contribution to average basal area at each site.
281 Table 3. Comparison of biomass at sites in North Ghana – all woody species ≤ 7 cm dbh. Site (intensity)
No. of plots
Trees ha–1
Total BA m2 ha–1
Species1 plot–1
1-D
Bole (high) Mandari (med.) Woodland (low)
15 06 06
029 ± 13a 061 ± 24a 353 ± 61b
2.7 ± 0.7a 2.6 ± 0.6a 9.6 ± 2.0b
04 ± 2a 08 ± 3ab 13 ± 2b
0.30 ± 0.13a 0.37 ± 0.19a 0.81 ± 0.03b
Means by column with no similar superscript letters indicate Tukey’s honestly significant difference (P < 0.01). 1 Not possible to give species per hectare due to different plot sizes.
both unwanted immature and mature Sheanut trees from the land they wished to cultivate. The high confidence limits are probably indicative of farmers being afraid of admitting to a stranger that they cut this valuable economic species since traditional agroforestry practices often conflict with that of the state forestry laws. Of those farmers who practised any form of selection, 77% described the criteria used to keep an individual tree were usually based on age and fruit productivity, stating that mature trees were kept unless unproductive due to old age or poor health. 19% of ‘selective’ farmers revealed that they would kill trees that significantly affected annual crops, by reducing light availability, and if at high density, the healthiest, most productive trees were preferentially maintained. 36% of farmers protecting regenerating saplings stated that they would maintain those that were taller than 1 m at land clearance, providing they were healthy, fast growing and well spaced with relation to other trees. When describing the ‘best’ trees, farmers had a wide range of preferences and often gave more than one character. Good health, low competitive effects on crop yield, large sweet fruit, fast growth and resistance to mistletoe were described as preferred traits. Trees with sweet fruit and high yield were equally the most popular traits and both were mentioned by 45% of respondents. Some confusion existed as to the preferred seed (whole nut) size, 38% of male farmers said that small seeds yielded more oil whereas 28% said large seeds were better. Farmers in the Upper West Region also described a ‘Black’ variety of Sheanut tree, which supposedly had high yields and dark buds. All women, apart from one interviewee, said that small seeds would yield more oil than the same volume of large seeds. Fat analysis of seeds collected from different mother trees throughout Northern Ghana (n = 140 trees, four seeds per tree) revealed that no correlation (r2 < 0.01, P > 0.33) existed between mean fat content per tree (range = 29 to 62% by dry kernel weight) and mean seed size per tree (from 1.74 to 3.74 cm long) (Lovett and Haq, in prep.). The only woman who did not prefer small seeds said that she was a lazy market trader, and as Sheanuts were sold by volume, it was easier to fill a basin with large seeds.
282
Table 4. Comparison of biomass in sites in North Ghana – Vitellaria paradoxa only. Site (intensity)
No. of plots
Trees ha–1
Stems ha–1
% Stems > 20 cm dbh
BA m2 ha–1
% contribution to total BA
BA Tree–1
Bole (high) Mandari (med.) Woodland (low)
15 06 06
21 ± 6a 44 ± 11a 41 ± 28a
24 ± 7a 48 ± 12a 46 ± 30a
72.0 ± 9.4a 53.1 ± 15.1ab 40.2 ± 32.9b
2.19 ± 0.64a 2.16 ± 0.57ab 0.92 ± 0.23b
79.7 ± 7.2a 84.2 ± 10.0a 10.2 ± 3.3b
0.11 ± 0.02a 0.05 ± 0.01b 0.05 ± 0.04b
Means by column with no similar superscript letters indicate Tukey’s honestly significant difference (P < 0.05).
283
Points without similar lower case letters, by size class, indicate Tukey’s honestly significant difference (P < 0.05) Figure 2. Size Class distribution of Vitellaria paradoxa, Northern Region, Ghana.
Local knowledge on the Sheanut tree was extensive and 88% of respondents said they knew where the best (the healthiest and most productive) trees grew. 40% of these farmers described topographic situation as the most important influence and the best trees were always said to be on higher land. 28% said that surrounding vegetation and soil type were equally important and the best individuals were said to be found on cultivated land with well-drained stoney or loamy soils. Only 4% mentioned that fire had any detrimental effect on Sheanut trees, stating that if vegetation was burnt during the flowering season (November–January) the fruit yield would be reduced. Women and children are the main collectors of mature Sheanut fruits and
284 are given priority rights to harvest from land currently being farmed by family members. Open access rights are granted in fallow or unmanaged areas, although women often described that it was easier and less hazardous to collect fallen fruit from cleared land. During germplasm collections, it was noted that few viable Sheanuts remained on cleared land as women gather everything they find in the early morning. Harvested seeds are depulped and immediately boiled before sun drying. This processing stage is said to improve yields and oil quality by preventing germination and fungal infection of seeds. Despite this extensive knowledge and intense local competition for Sheanuts, a traditional two-week ban on all harvesting was strictly enforced once fruit abscission had commenced. The reasons behind this were explained by local residents and a number of alternatives offered. One viewpoint suggested that it freed up human resources during a busy cultivation period. Another explanation was that the ban allowed land pacification ceremonies to take place and ensured good rainfall. The last reason, described by an influential member of the community, was that the strictly enforced fines allowed all women to have an equal share of the harvest when the ban was lifted.
Discussion Traditional anthropic agroforestry practices have clearly had a significant impact on both the woody species composition and, on the population structure of trees on farmland in the study area. It is also clear that the majority of trees recorded on cultivated land are maintained because their non-timber products are locally valued. The most frequent species on farmed land was the Sheanut tree (Vitellaria paradoxa), which often formed over 80% of the woody biomass. Field observations revealed that the main selection stage occurs when fallow or woodland is cleared, as most immature individuals are removed and only certain large trees are maintained on cultivated land. Over time, this has led to significant changes in the Vitellaria population structure as compared to that expected in the ‘natural’ environment, and a much larger proportion of mature productive trees (dbh > 20 cm) are present on farmed land than in unmanaged woodland. Similarities of the medium intensity landuse site (around Mandari) with unmanaged areas, in terms of total species numbers, Vitellaria biomass and population structure may be an artefact of small sample sizes or different plot sizes. Alternatively, it may indicate that landuse is either better managed, at an intermediary stage of intensity with longer fallow periods or that farmland has been expanding into recently unmanaged woodland. Important considerations for Vitellaria conservation and management are implied by the differences between the high (Bole) and medium (Mandari) intensity farmland because deep ploughing reduces regeneration and damages mature tree root systems. The local elders near Mandari strictly enforce traditional methods and it is apparent that this site has a healthy population
285 structure. Farmland around Bole, however, has a much higher incidence of tractor ploughing and also lacks regeneration. Fallow length also appears to be decreasing at this site and many fields are now permanently farmed with the use of fertilisers. Field observations and interviews in other regions also support this view since a similar Vitellaria population structure was observed on farmland near Nandom in the Upper West region of Ghana, an area where cattle ploughing is widespread and fallows are shorter. Interviews with local farmers, revealed that male farmers are actively selecting specific Sheanut trees when land is cleared for cultivation from fallow or woodland. The selection criteria used for mature trees are based primarily on fruit productivity (as a function of age, health and size) and competitive effects with annual crops (determined by tree size, leaf density and spacing). These views are further supported by similarities in the mean total woody biomass and Vitellaria biomass between farmland sites, which suggest that farmers are actively maintaining a careful balance between crop and tree productivity. Unwanted mature trees are usually killed by ring barking and allowed to dry before being cut for household uses (building poles, mortars and charcoal). New recruits for farmland Sheanut populations are selected from regeneration by not cutting and then protecting from fire, during the cyclical clearing of fallow land (uncultivated for 5–10 years in the study area). The criteria used to select immature Sheanut trees were based on spacing, health and speed of growth (only saplings > 1 m kept). Apart from characters given above, others preferred traits included fruit palatability, seed size and resistance to mistletoe, but since this species is not planted, these were not obviously part of the selection process. Men stated that they like large sweet fruit but were not convinced as to which seed size was better for oil production. Women, who are the sole processors of Shea butter in the area, clearly stated that small seeds give a higher yield of oil by volume. Laboratory analysis (Lovett and Haq, in preparation) showed no correlation of seed size with oil content (percentage by dry weight), supporting a preference for seeds with a high mass to volume ratio. It is also conceivable that oil is more easily extracted using traditional methods from certain seed types and local views deserve further research. Other ‘unconscious’ anthropic selection pressures on Vitellaria are also possible following intensive harvesting and land cultivation during the germination period. This is particularly relevant in the study area since OseiAmaning (1996) suggests that most immature trees in fallow around Bole result from coppice. Although further research is required to determine the exact period when seedlings actually become established during the farming cycle, it is suggested that the traditional two week ban, coupled with thorough collection of viable seeds, would encourage survival of seeds which abscise on maturity and germinate immediately. It is also of note that traditional bans on early harvesting are a common occurrence in many areas of West Africa (J-M. Boffa, pers. com., 1999). Mature seeds that remain on the tree or had long viability would be more likely to be collected for oil production since
286 germinated seeds are usually discarded. Cryptogeal germination of Vitellaria also occurs during the cultivation period and new shoots do not appear above ground until planting and weeding has been completed four to eight weeks later. Since the land is usually burnt four to six months later, this unusual characteristic, which probably originally evolved in response to fire (Jackson, 1974), may now be protecting seedlings from the intensive but shallow hand cultivation commonly practised throughout the region. As with other cultivated crops, it is therefore possible that natural dispersal mechanisms of Vitellaria have adapted to survive in environments which alternate between intensive cultivation and fallow land. The length of time for ‘domesticated’ adaptations to evolve in a population varies between species, though it has been postulated that populations of cultivated wild lentils could develop 100% indehiscence in as little as twenty generations (Blumler and Bryne, 1991). The Gonja tribe conquered the indigenous Vagla and Safalba tribes near Bole, after migrating from Mali in the sixteenth century. Agriculture was probably already well established at this point as the latter tribes are still given a degree of control over land and tree management. It is also likely that population densities, though not as high as present, were quite large up until recently as there has been considerable military activity and tribal migrations in the area (Goody, 1969). Assuming an average farm cycle of 15 years and similar agricultural practices, this period would have allowed at least thirty Vitellaria generations to have established since the arrival of the Gonja. Evidence presented confirms that Vitellaria populations, in the West African farmed parklands, are a direct result of anthropic selection following many centuries of traditional cultivation and fallow. Although ‘methodical’ selection has probably been limited as preferred varieties are not planted or specially protected. It is proposed that the maintenance of healthy, productive trees well suited to these agroforestry systems, coupled with intensive harvesting and cultivation across the species range, has led to Vitellaria paradoxa becoming semi-domesticated following ‘unconscious’ and ‘natural’ selection mechanisms. In recent years, much attention has been paid to those woody plants termed as ‘Cinderella’ species which provide economic or environmental benefits through traditional utilisation but have been often been neglected by mainstream domestication (Leakey and Newton, 1994). The focus of this study is just such a species, with regard to current agroforestry requirements, Vitellaria paradoxa still has many undesirable characteristics. Trees take up to 20 years to mature, the seeds are recalcitrant and the heterozygous populations produce a highly variable crop in terms of quantity and quality. Unfortunately, superior ‘true-to-type’ individuals are difficult to maintain as the species has an outcrossing breeding system and vegetative propagation methods have only recently been developed (Bonkoungou, 1987; Grolleau, 1989; Lovett et al., 1996). Analysis of diversity followed by identification and multiplication of individuals with superior characteristics (from both domesticated and wild
287 populations) will allow ‘methodical’ selection and continuation of a tree improvement process first started by African farmers thousands of years ago.
Acknowledgements We express our gratitude to: staff at the Cocoa Research Institute of Ghana, SheaGold Ltd, the Botany Department, University of Ghana, the many farmers and other Ghanaians, for their kind assistance during fieldwork and to Dr JeanMarc Boffa for comments on earlier drafts of this paper.
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