Assessment of Tree Species Diversity, Biomass and Carbon Stock in the Nyandungu Eco-Park Restored Forest

Abstract

Rehabilitated urban wetlands remain a critically underquantified carbon sink in sub-Saharan Africa. This study provides the first systematic, plot-based assessment of tree species diversity, biomass, and carbon stock in the Nyandungu Eco- Park Restored Forest (50 ha; Kigali, Rwanda), rehabilitated between 2016 and 2019. Twenty-five stratified random plots (20×20 m; 2% sampling intensity) were established; DBH and height were measured for all stems 10 cm; above-ground ≥ biomass (AGB) was estimated using the Chave et al. (2014) allometric equation, below-ground biomass (BGB) via the IPCC (2006) root-to-shoot model, and carbon stock using the IPCC carbon fraction of 0.47. Diversity–carbon relationships were assessed through OLS regression and Spearman correlation. Across 22 analysed plots, 25 tree species were recorded (mean richness: 3.27 ± 1.35 plot⁻¹; H' = 0.995; D = 0.616), indicating moderate early-successional diversity. Vachellia sieberiana dominated structurally (IVI = 95.58), followed by Spathodea campanulata (IVI = 48.10) and Afrocarpus falcatus (IVI = 42.92). Total biomass was 63,599.11 kg, yielding 29,889.18 kg C (109,701.67 kg CO₂e). Mean carbon stock was 34.0 Mg C ha⁻¹ (range: 2.8–118.1 Mg C ha⁻¹). Species diversity showed a weak, non-significant relationship with carbon stock (R² = 6.43%, p = 0.255); mean DBH was the dominant predictor (ρ = +0.712, p < 0.01), confirming structural maturity as the primary driver of carbon accumulation. Nyandungu Eco-Park provides Rwanda's first IPCC Tier 2- compliant carbon baseline for national GHG inventory and REDD+ MRV reporting, advancing benchmarks for wetland restoration planning across sub-Saharan.

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