Aboveground Species Diversity and Carbon Stocks in Smallholder Coffee Agroforestry in the Highlands of Uganda

Abstract

Types of agroforestry systems and their capacity to sequester carbon vary globally, and the extent of carbon sequestered greatly depends on environmental conditions and system management. This study aimed at investigating the species composition and determining the aboveground carbon stocks of coffee agroforests at low (1240–1320 m a.s.l.) and medium (1321–1504 m a.s.l.) elevations of Manafwa District in Uganda. For each elevation, the agroforest structures were described and the aboveground carbon (AGC) stocks estimated using allometric models for all measured shade and coffee trees. Two coffee varieties were cultivated with SL-14 extending up to 40 years, while LWIL-11, a more recently introduced variety, extended up to 7 years only. Therefore, the estimated AGC stocks were significantly greater for the SL-14 (0.250–2.317 tons ha−1) than LWIL-11 (1.044–2.099 tons ha−1) and were significantly higher at the medium versus the low elevation. The analysis for shade trees indicated no significant differences in the species diversity for the elevation sites, but with significant variations in mean DBH and thus AGC stocks. Farms at low elevation were characterized by smaller (2.037 ± 0.131 tCO2e ha−1) and significantly high (2.037 ± 0.131 tCO2e ha−1) mean AGC stocks per unit area for coffee and shade trees, respectively, as compared to the medium elevation farms. While the variation in the coffee trees within the elevation sites could be attributed to the uneven distribution within the age groups, the AGC stocks in the shade trees were attributed to the generally large sizes of the trees that dominated. Irrespective of the differences in elevation attributes, coffee agroforests can potentially provide carbon sinks and thus contribute to climate change mitigation.