Tree Biomass Carbon in different Land Use Types Managed by Smallholder Farmers Along Altitudinal Gradient in Central Highlands of Ethiopia

Background and Aims Biomass carbon (C) stocks of trees across three common traditional agroforestry practices (i.e. home-garden, farmland, and grazing land) and along altitudinal gradients (i.e. upper, middle and lower) in southern Ethiopia are presented and evaluated. These traditional AF systems are characterized by a high proportion and diversity of trees hence can play a remarkable role in climate change adaptation and mitigation.Methods Data were collected from a total of 89 plots: 41 home-gardens, 29 croplands, and 19 grazing lands using total count and Y-frame transect sampling methods. Above- and belowground biomass of trees were calculated using allometric equations developed somewhere else but proven for application in agroforestry systems. Trees with DBH ≥ 2 cm were considered for biomass carbon stock determination.Results above-ground biomass carbon stock of the three land uses showed the order: Home-garden > grazing land > cropland, but the differences in the carbon stocks were not statistically significant. Soil carbon stocks measured from the studied land uses at middle altitude also followed a similar trend. Similarly, differences among altitudinal gradients were not statistically significant. However, the variations observed reflected differences in species composition among the system as well as altitudinal gradients. Other factors such as wealth and garden size also significantly affected biomass C stocks per ha, where rich household holds higher above-ground biomass than medium and poor households. Overall, this reflects that rich households own a large parcel and large total number of trees, compared to that of medium and poor wealth classes.Conclusions The biomass C stocks of the tree across the three studied AF systems were found to be lower than reported for tropical forests and agro-forestry systems due to the recently agro-forestry intensification in the form of reduction in tree diversity and density. For enhancing their biomass carbon stocks traditional agro-forestry should be integrated in future REDD+ projects. As the biomass C stocks were more determined by socioeconomic factors such as wealth and garden size than the types of agro-forestry practice and climate (elevation) future carbon management strategies should also considered them.