Abstract
Plantation forests can capture and retain carbon in their biomass and soil over time. However, an unregulated harvest of a plantation would result in biomass and carbon removal from the site. So, appropriate management of plantation forests is important to enhance the carbon sink and hence, climate change mitigation. The general objective of this study was to investigate the effects of thinning frequency on biomass and soil organic carbon stocks of Cupressus Lusitania plantations in Central highland, Ethiopia.
Two plantations attributing: - once thinned (at Kofele site) and twice thinned (at Shashemene site) were purposively selected for this study. Twenty-two main sample plots (12 from once thinned and 10 from a twice thinned site) were laid own for data collection. Nested plots of size 20m * 20m as main plots were systematically established and used for tree inventory. Three 1m * 1m sub-sample plots within the main plots were selected for soil and litter sampling. Data of trees whose DBH (≥ 5 cm) and total tree height were measured in the main plot using diameter caliper and hypsometer respectively. The litter was collected from three sub-sample plots 1 m x 1 m laid randomly within the main plots. To analyze the total biomass carbon stock was analyzed using locally developed biomass allometric equation for Cupressus lusitanica at WGCF-NR was used for determining above-ground biomass. This model was selected because the current study sites were located in the vicinity of WGCF-NR. Soil samples for carbon content determination were collected from three randomly selected sub-sample pots 1m * 1m in the main plots from the soil depth 0 - 20 cm, 20 - 40 cm, and 40-60 cm using auger method. Similarly, soil samples were taken from 0 - 60 cm soil depth (0 - 20 cm, 20 - 40 cm, and 40 - 60 cm in layers) to determine soil bulk density using the core method. The result indicated that the average basal area (m2/ha) in the once thinned site was 33.75 ± 6.50 whereas 24.25 ± 3.25 in the twice thinned. The total mean carbon stock density of the once thinned site was 207.48 ± 54.12 was higher than 156.80 ± 12.53 t/ha in the twice thinned site. In a twice thinned site, relatively the largest carbon stock was observed in soil organic carbon pool (55.69 %). However, it was 50.05 % once thinned. In both sites, the contribution of litter biomass carbon stocks to the total (ecosystem) carbon stock was less than 1 %. The result of this study showed that as thinning frequency increased from once thinned to twice thinned, the total biomass carbon (t/ha) decreased by 33 % and by 15.90 % in soil organic carbon pools. This much biomass and soil organic carbon variation or loss were due to differences in the thinning frequency of the two sites. Therefore, plantation management should minimize thinning frequency to complement climate change mitigation strategy in the tropics to benefit from carbon financing.
Keywords: - Carbon sequestration, litter, organic carbon, silviculture.