ASSESSMENT OF DIVERSITY AND ABUNDANCE OF INDIGENOUS FRUIT TREES / SHRUBS AND THEIR AGROFORESTRY POTENTIALS IN TARABA STATE, NIGERIA

Abstract

The study was carried out to assess the diversity and abundance of indigenous fruit trees/shrubs and their agroforestry potentials in Northern Guinea Savanna, Taraba State, Nigeria. Three transects with a distance of 500 m apart were used for the study, sample plots of 25 m x 25 m in dimension were laid in alternate positions along each transect at 250 m intervals. All the trees/shrubs encountered in each of the sample plots were identified by their botanical names. Biodiversity indices such as; Shannon-Wiener diversity index, Species evenness, Simpson’s diversity index, Margalef’s richness index and Menhinick’s diversity index, were estimated using appropriate formulas. Identification of indigenous fruit trees/shrubs in the reserves was carried out. To assess farmers’ most preferred fruit trees/shrubs, data was derived from the use of pretested questionnaire. A multistage and stratified sampling technique was used in the selection of the responding farmers. The data collected were analyzed using descriptive statistics; inference was drawn using t-test and analysis of variance. For determination of nutritional quality, stock of the most preferred fruits were harvested at maturity from healthy tree and composite samples were analyzed for detailed nutritional and anti-nutritional content as well as their mineral composition using the standard methods. The data were subjected to Analysis of Variance (ANOVA) while Duncan’s Multiple Range Tests was used for the comparison of means. Statistical analysis was performed using the Statistical Package for Social Science (SPSS) version 17. The result of floristic diversity assessment showed that the two forest reserves were able to conserve trees/shrubs species diversity while the diversity of the farmlands was relatively low. This was reflected by the value obtained for diversity indices for the two land use types: Shannon- Weinner (3.29, 3.08), species evenness (0.47, 0.49), Simpson (0.95, 0.93), Margalef (6.83, 6.49), Minhinck’s index (2.09, 1.79) in Sonkpa and Jabwanje forest reserves respectively. While, Shannon- weinner diversity indeces was (1.99, 1.82), species evenness (0.78, 0.73), Simpson index (0.92, 0.90), Margalef’s index (3.37, 2.41), Minhinck’s index (2.22, 2.02) for the two farmland in close proximity to the forest reserves. The density of fruit trees/shrubs encountered in the two forest reserves were (279, 414) which accounted for (47% and 56%) of the total woody population in Sonkpa and Jabwanje forest reserves respectively. Among the prominent fruit trees/shrubs encountered were Nauclea latifolia, Gardenia aqualla, Annona senegalensis, Prosopis africana, Vitellaria paradoxa, Vitex doniana, Strychnos innocua. Farmers ranked the fruit trees/shrubs based on food value, medicinal value, marketability, ecosystem services, and ease of regeneration. Species were ranked in this order of preferences; Parkia biglobosa>Vitellaria paradoxa>Prosopis africana>Vitex doniana>Adansonia digitata. The proximate, antinutrients and mineral contents of the five most preferred fruits/seeds showed that the moisture content of the fruits ranged from 4.88 to 76.82% in Vitellaria paradoxa pulp and Vitex doniana pulp respectively. Parkia biglobosa seeds had highest levels of protein (25.03%) while the least (4.84%) was in Vitex doniana fruit. The highest ash (4.57%) and fibre (16.46%) content was found in Vitellaria paradoxa pulp. Carbohydrate content of the fruit ranged (9.37-72.06 %) in Vitex doniana fruit pulp and Adansonia digitata pulp respectively. The fruits/seeds equally contained phytate, ranged (0.58 - 3.79mg/g) in Vitex doniana pulp and Adansonia digitata seed. Oxalate level was low ranging from 0.23mg/g -1.40mg/g in Vitex doniana pulp and Vitellaria paradoxa pulp. Phenol and Tannins levels of the fruits were low (0.009-0.0255mg/g and 0.005-0.028mg/g), respectively. Adanasonia digitata seeds had highest Saponin content (9.7%) while the lowest (3.9%) was in Vitex doniana pulp. Cyanide content also ranged from 6.91-76.94mg/kg in Vitellaria paradoxa pulp and Adansonia digitata seed. The predominant mineral element in the samples were potassium (K) and calcium (Ca) while the highest concentration of Ca, Mg, P, Zn, Mn, and Fe were found in Parkia biglobosa pulp. Adansonia digitata seed had the highest concentration of Potassium (55.55mg/kg) while iron concentration ranged (0.12-1.74mg//kg) in the fruit/seed samples. The indigenous fruits/seeds are rich in their nutritional and mineral composition therefore; regular consumption of these fruits can raise the nutritional profile and health status of the local communities. The antinutrient level of the fruits and seeds were at safe levels and can also be reduce through adequate processing techniques. However, the absence of Irvingia gabonensis and lower density of Afzelia africana was an indication that some important species are already going to extinction while others are endangered. Hence, the State government should, intensify the management of Savanna forest by controlling the intensity of tree harvesting, stop illegal felling and encourage enrichment planting of the prioritized species. Further research on genetic improvement through germplasm collection and propagation studies should be carried out on the prioritized species in order to develop cultivars with superior trait and fruits.