Woody Species Composition, Distribution and Structural Characteristics in Yabo Area, Sokoto State, Nigeria

摘要

Woody species in the study area were assessed in order to determine their composition, distribution and structural characteristics. A total of 32 km2 around Yabo town in Sokoto State, Nigeria was earmarked for the study within which, a total of 50 sample points were randomly generated using IDRISI ANDES Software. On each of the sample points, a quadrat of 50x50 m dimension was laid and all woody species thereon were identified and enumerated. The study examined the prevailing land use/land cover of the study area and also measured vegetation parameters which include height, diameter at breast height, basal area, density and biomass. In addition, dominance at both species and family levels were determined by the Importance Value Index (IVI). Seven major land use types were identified and a total of 671 woody stems which belonged to 40 species, 35 genera and 12 families were identified and enumerated. The highest number of stems (271) was recorded in the savannah woodland and the farmland (137) whilst the lowest was recorded over the built-up area (15) and bare ground (24). Analysis of the importance value index revealed that the dominant species in the area are Azadirachta indica, Balanites aegyptiaca, Senna sieberiana, Combretum micranthum, Guiera senegalensis, Mimosa pigra, Parkia biglobosa and Piliostigma reticulatum which together account for about 67.2% of the species. The extrapolation of the results obtained to the entire study area revealed that overall, 171,600 (54 stands per hectare) woody stands abound. Result of ANOVA and DMR tests show that biomass is the most variable parameter amongst the land uses while species density is the least. The observed species and actual number of tree stands in the area are generally low and are a reflection of severe land degradation going on in the area. Factors that affect the composition, distribution and structural characteristics of woody species in the area were primarily found to be human activities that include unsustainable use of natural resources as manifested in agricultural and grazing activities and wood extraction. However, poor physical conditions in terms of soils, vegetation and inherent extreme variability of climate also play contributory roles. The urgent need to halt the degradation trend and improve upon the state of woody species in the area was stressed.