Assessment of land-cover changes and carbon sequestration potentials of tree species in j4 section of Omo Forest Reserve, Ogun State, Nigeria

Abstract

We evaluated carbon stock accumulation on potential of tree species in five forest-types in Omo Forest Reserve of western Nigeria. This included four forest plantations and a natural forest of mixed species. The reserve was stratified into Gmelina arborea, Tectona grandis, Pinus carebeae and Nauclea diderrichii plantations as well as natural forest. Each forest-type was assessed using circular plot method. Plot points were pre-determined using remote sensing. For each point, two circular plots were established, the main plot with a radius of 12.61 m (500 m² ) and the subplot with a radius of 5.64 m (100 m² ). In each plot, tree Dbh and height were measured for trees with Dbh ≥10 cm, while only trees with Dbh <10 cm but >2 cm (i.e. 2 cm ≤ Dbh < 10 cm) were considered in the sub-plot. Soil samples were also collected at 10 locations with 2 each in the north, south, east, west and at the plot centre, in each forest-type. The samples were analysed to obtain soil organic carbon. Above- and belowground biomass and carbon stocks were estimated using the appropriate allometries formulated for the tropics. All empirical relationships were included in the existing allometries with Dbh and height as predictors. Landsat images of the reserve in 1991, 2000, 2014 and 2019 were processed and analysed to assess forest degradation. The results revealed that 23-year-old Pinus caribaea plantation  sequestered more carbon (35.78±2.73 tons/ha) than 35-year-old Gmelina arborea (18.96±1.82 tons/ha), 43-year-old Tectona grandis (17.75±2.13 tons/ha) and 43-year-old Nauclea diderrichii (17.36±1.87 tons/ha) plantations and natural forest (21.98±2.38 tons/ha). The study showed that stand density influences carbon stock accumulation of forest. It was observed that individual stems of Pinus caribaea were better carbon accumulators than Gmelina arborea, Tectona gransdis and Nauclea diderrichii. The same trend was observed for CO₂ captures as Pinus caribaea captured 131.31±10.02 tons/ha with Nauclea diderrichii the least, having a value of 63.71±6.9 tons/ha.

Keywords: Biomass, Carbon stock, Degradation, Forest-type, Stand density