Comparative Analysis of Carbon Stocks Across Forest Degradation Gradients in Ondo and Osun State Forests

Quantifying how forest degradation affects carbon stocks is essential for Nigeria to accurately report national greenhouse gas inventories and access results-based payments under international climate agreements, yet systematic comparative data on carbon across degradation gradients in southwestern Nigerian forests remain limited. This study compares above-ground biomass carbon, below-ground carbon, litter carbon, and soil organic carbon stocks across five forest condition categories ranging from intact closed forest to heavily degraded open woodland in forests of Ondo and Osun States. Forty sample plots per condition category were inventoried, with tree measurements used to calculate biomass through validated allometric equations. Litter biomass was determined by quadrat-level collection and oven-dry weighing. Soil carbon was quantified to 30-centimetre depth using Walkley-Black titration. Total ecosystem carbon declined monotonically from 312 tonnes of carbon per hectare in intact forest to 41 tonnes in heavily degraded forest, representing an 87% reduction across the full degradation gradient. The steepest per-unit-area carbon loss occurred at the transition from lightly to moderately degraded forest, identifying this threshold as the critical intervention point for carbon-motivated conservation investment. Below-ground carbon showed greater relative persistence across degradation stages than above-ground stocks. Soil carbon at the surface horizon was negatively correlated with canopy openness. These findings provide state-specific carbon stock coefficients and degradation emission factors applicable to Nigeria's forest reference emission level calculations under REDD-plus. Keywords: carbon stocks, forest degradation, REDD-plus, Ondo State, Osun State