Impact of selective logging on plant diversity, natural recovery and vegetation carbon stock: the case of Bobiri Forest Reserve

This research examined the effect of selective logging on the floristic composition, structure and the vegetation carbon stock recovery of a moist semi-deciduous forest in Ghana. The research was carried out in the Bobiri Forest Reserve, in the moist semi-deciduous southeast sub-type, in southern Ghana. Sites were selected to reflect the following chronosequential manner of 50, 43, 30, 21, 10 years’ post-logged-sites, as well as recently post logged site (ranging between 6 and 12 months) and a strict nature reserve (SNR-unlogged site). Plots that had been used for Tropical Shelterwood System (TSS) were also selected to depict an extreme case of logging.Ten Temporary Sample Plots (TSPs) were established in each of the selected compartments using an identified stump from previous logging as the centre in the logged forest while a tree of exploitable size was used as the centre for the plots in the SNR and the TSS. The main plot size was 50 m by 50 m (2500 m2) which was divided into four sub-plots of size 25 m by 25 m (625 m2) and the sub-plots were further subdivided into sub-sub plots of size 12.5 m by 12.5 m (156.25 m2), forming a nested plot design. The main plots were used for the assessment of trees ≥ 20 cm diameter at breast height (dbh) of 1.3 m, as well as standing dead trees of similar size. A sub-plot of 25 m x 25 m was used for the assessment of trees < 20 cm dbh, and a sub-sub-plot of 12.5 m x 12.5 m was used to assess plants ≤ 10 cm dbh but with height not less than 2.0 m. Five quadrats of 1.0 m x 1.0 m were laid in a ‘Z- shape’ in the main plot for the assessment of undergrowth vegetation and forest floor litter. Phytomass of individual trees was estimated from stem dbh and was used to compute the biomass of various compartments. The herbaceous and litter biomass was calculated from the data obtained from the quadrat. Carbon (C) content values were used to convert the biomass of the various plant functional types to C equivalent. The study revealed that tree basal area (BA) increased with increasing post-logging years with a mean BA of 32.95 m2 ha-1. Trees contributed the highest C-stock, followed by litter with the herbs contributing the least. Total carbon stock (Mg C ha-1) increased in the order of the following: Year1 (165.63) < Year10 (171.08) < SNR (181.57) <Year21 (206.60) < Year43 (220.34) < Year30 (245.24) < TSS (257.14) <Year50 (268.99). The mean carbon stock of the sampled plots was 214.57 Mg C ha-1.The < 20 cm dbh class accounted for 89 % of the stocking density at Bobiri Forest Reserve and contributed 13 % of the biomass carbon to the ecosystem while the classes above 20 cm dbh accounted for 11 % of the stocking density and contributed 87 % biomass carbon. The study indicated that Bobiri Forest Reserve comprised 38.63 % ± 1.08 non-pioneer light demanders (NPLDs), 15.67 % ± 1.12 Pioneers (P) and 37.32 % ± 1.14 shade-bearers (ShB) with the others constituting 8.39 % ± 0.77. The NPLDs accounted for 42.31 % ± 2.01 of the biomass carbon with Ps and ShBs accounting for 31.07 % ± 1.53 and 25.98 % ± 1.77 respectively, while the others contributed less than 1 %. Whereas diameter distribution curve for stocking density showed negative exponential curve, the diameter distribution curve for carbon stock showed positive exponential curve. The study further revealed that Scarlet, Red and Pink Star species though constituted 15 % of the plant population of Bobiri Forest Reserve, they accounted for 51 % of the carbon stock. However, the Green Star species which constituted 83 % of the plant population accounted for 44 % of the carbon stock. The study showed a build-up of biomass carbon, years after selective logging to a peak (at an asymptotic value of 298.5 Mg C ha-1), which remained generally constant. The Bobiri forest therefore followed the normal (Sigmoid) growth pattern during recovery with respect to carbon stocks. Logging appeared to enhance both plant species diversity and carbon stock accumulation though plant species composition differed at different post-logging sites.

Source: KNUST