ABSTRACT
A quantitative description of the lowland rainforest of Okomu National Park, Nigeria, was carried out using vegetation attributes of woody plant species composition and richness, woody plant species diversity, stem basal area, percentage contribution of five most abundant woody plant species to overall species enumerated, girth size-class distribution of woody plants, woody plant density and soil pH, organic matter and physical characteristics to assess the extent of variability in the vegetation of the area.
At about 10% sampling intensity, nine compartments each measuring 2.56 km2 were randomly selected out of the 78 compartments in Okomu National Park. Six of these were from 'Dry Area’ (Section A) relatively remote from creeks and streams while the remaining three were from ‘Wet Area’ (Section B) close to creeks and streams. A total of 18 temporary sample plots–two from a compartment and each measuring 25 m x 25 m (0.0625 ha.) were randomly located. Total enumeration and measurements at breast height (1.3m) and over bark of all woody plant species > 1m in height were carried out. Soil samples collected from two randomly chosen locations in each plot at depths of 0-45cm were composited, air-dried and passed through a 2mm sieve and analyzed for pH, organic matter and physical characteristics.
There were no significant differences (p>0.05) in species composition and also in species richness index of Margalef between 'Dry Area' and 'Wet Area'. Shannon-Weiner’s species diversity indiex showed high values in all plots ranging from 1.88 in Plot 1 in Compartment 110 to 3.06 in Plot 2 in Compartment 50 (both in Section A-Dry Area). Simpson’s index had a range of 0.06 in Compartment 113 (Section B-Wet Area) to 0.24 in Compartment 36 (Section A). Shannon-Weiner’s species diversity index varied significantly (p>0.05) between plots in Compartments 36, 50, 110 (Section A) and in 55, and 113 (Section B-Wet Area) and also between the 'Dry and Wet Areas' (i.e. Sections A and B). Stem basal area values between plots in Compartments 36, 50, 82, 110 ('Dry Area'), 55, 88 and 113 (‘Wet Area’) and between 'Dry and Wet Areas' respectively showed significant variability (p>0.05). Amongst the five most abundant woody plant species, Diospyros dendowas most frequently occurring, accounting for 17.1% of the total of 2086 individuals enumerated. Other frequently occurring species were Strombosia oblonga(9.3%), Rinorea dentata(7.2%), Strombosia grandifolia(7.1%) and Sterculia oblonga(3.8%). Stand structure regression models varied significantly (p>0.05) in Plots 1 and 2 in Compartment 50and in Plot 2 in Compartment 82. Also, Compartments 50 and 110 (Dry Area) varied significantly when totals of each compartment and sections were tested. Coefficient of determination (r2) values ranged from 0.24 in Plot 2 in Compartment 36 to 0.46 in Plot 2 in Compartment 50. Plant density did not indicate significant differences (p>0.05) between 'Dry and Wet Areas'. The soils were generally sandy and acidic. However, significant variations (p>0.05) were recorded for organic matter, silt and sand for entire data and for 'Dry Area'. The vegetation of the lowland rainforest of Okomu National Park, Nigeria, is heterogeneous and exhibits variability when examined closely on a small scale.
