Forest structure and carbon stock dynamics under traditional coffee farming system in Diga District, western Ethiopia

Abstract Human-dominated landscapes are becoming more important for biodiversity conservation and climate change mitigation. As deforestation and forest degradation continue to reduce carbon stock density of the natural forest in the tropics, traditional coffee agroforestry systems, where Coffea arabica is cultivated under shade trees, are recognized for their structural complexity and capacity to store substantial biomass and soil carbon. However, the conversion of natural forests into simplified coffee systems through progressive thinning of shade trees can significantly diminish their ecological value. This study assessed forest structure and carbon stock dynamics in Natural Forest (NF) and Semi-Forest Coffee (SFC) systems in Diga District, western Ethiopia. To the people of this district, coffee forms a major source of livelihood. Forty sample plots (20 in each forest type) were systematically laid to record floristic composition, stand structure, basal area, and biomass of woody species. Species-specific and generalized allometric equations were used to estimate carbon stocks of trees and shrubs in both NF and SFC. Results revealed that NF had higher woody species richness (52 species) than SFC (25 species), asserting the impacts of traditional shade-tree management via selective removal of specific species and size classes. SFC had significantly lower basal area (8.41 m² ha⁻¹) and biomass carbon (41.59 t ha -1 ) than NF (18.47 m² ha⁻¹) and 99.98 t ha -1 , respectively, demonstrating greater structural complexity. Albizia schimperiana was ecologically dominant in both systems, though with much larger basal area in NF. In both forest type, size-class distributions exhibited reverse J-shaped curves, suggesting active regeneration in NF but also selective removal of larger trees in SFC. Overall, forest structural integrity and carbon storage was markedly reduced due to traditional coffee management activities, emphasizing the need for improved shade-tree selection and retention strategies to improve ecosystem services in coffee-growing landscapes.