Assessment of Woody Species Diversity and Carbon Stock Estimation along Altitudinal Gradient of Kulkal Ber Natural Forest in Maksegnet District, Northwestern Ethiopia

Abstract

Forest ecosystems play a vital role in mitigating climate change and conserving biodiversity. Assessing their woody species diversity and carbon stocks along altitudinal gradient is essential for informing forest management strategies that enhance carbon sequestration in both vegetation and soil. This study aimed to establish baseline data on woody species diversity, structural attributes, and carbon stock potential along altitudinal gradient of the Kulkal Ber Natural Forest in Maksegnet District, Northwestern Ethiopia. Data collection was conducted using a stratified systematic sampling approach. Six transects, spaced 500 m apart along the altitudinal gradient, were established. A total of 60 plots (20 m × 20 m) were systematically placed at 200 m intervals along the transect lines. Additionally, five 5 m × 5 m subplots (positioned at the corners and center of the main plot) were used for sampling shrubs. Within each 5 m × 5 m subplot, a single 1 m × 1 m nested subplot was used to collect ground herbaceous layer (GHL) and soil samples. Woody species diversity was assessed using the Shannon-Wiener diversity index (H'), while hierarchical cluster analysis was performed to classify plant community types. Above and belowground biomass was estimated using general allometric models, and soil organic carbon (SOC) and grass, herb, and litter (GHL) carbon content were determined through laboratory analysis. A total of 36 woody species belonging to 26 families and 31 genera were identified. The overall Shannon-Wiener diversity index (H') was 1.801, with an evenness value of 0.48. The six most abundant species, in descending order of density, were Dodonaea angustifolia, Combretum molle, Rhus glutinosa, Millettia ferruginea, Rhus vulgaris, and Cordia africana. The forest’s basal area was 6.53 m² ha⁻¹. ANOVA results indicated that altitudinal variation had no significant effect on species diversity. However, species composition, evenness value, and structural attributes highlighted the dominance of few species, with low importance value indices. Hierarchical cluster analysis identified three distinct plant community types, such as Ficus thonningii–Dodonaea viscosa, Olea europaea–Clausena anisata, and Myrsine africana–Euphorbia tirucalli. The total mean carbon stock of the forest was 150.76 t C ha⁻¹, with aboveground carbon (AGC) of 57.29 t C ha⁻¹, belowground carbon (BGC) of 15.47 t C ha⁻¹, grass, herb and litter (GHL) carbon of 3.21 t C ha⁻¹, deadwood carbon (DWC) of 2.86 t C ha⁻¹, and soil organic carbon (SOC) of 71.93 t C ha⁻¹. While AGC, BGC, and SOC showed no significant variation with altitude, GHL carbon was significantly higher at lower altitudes, suggesting that altitude influences ground herbaceous layer carbon accumulation. The forest’s population structure exhibited a reversed J-shaped height class distribution in the two altitudinal gradients, dominated by small trees and shrubs, indicating good regeneration potential. These findings emphasize the importance of conservation measures and sustainable management strategies to safeguard the genetic resources and carbon sequestration capacity of Kulkal Ber Natural Forest