by Stephen Johnson
Over 40% of the world’s terrestrial surface is covered by agricultural activities, and approximately half of that area is agroforestry. Agroforests, agricultural areas that are at least 10% covered by tree shade, run the gamut from areas with a few exotic species to structurally complex, highly diverse ecosystems that mimic natural forests. Woody vegetation biomass is approximately 50% carbon, so incorporating trees into agricultural areas significantly improves the ability of these systems to sequester and store carbon. Given that agroforests cover almost half a billion hectares, they may represent a significant and underestimated carbon sink. The amount of carbon that can be stored is determined by a variety of factors, including the number of trees and the density of their wood. The type of and number of the trees present in turn depends on the individual management of the farm. In Ethiopia, Tadesse et al. (2014) investigated how different management regimes affected the species of trees present and the amount of carbon stored, compared to natural forests. They measured the density and species of trees in smallholder coffee farms, state-owned plantations, and forest fragments, and used these measurements to determine the carbon storage capacity of each forest. They also interviewed farmers to see how and why species are selected for inclusion in plantations. Tadesse et al. found that agroforests can store 50-62% of the carbon that natural forests can store. They also found that farmers tended to prefer and harvest denser-wooded species, though less dense species were used for some limited purposes.
Tadesse et al. established plots in multiple forest fragments, smallholder farms (< 3 Ha), and state-owned plantations. They recorded the height, diameter, and density of all trees larger than 10cm diameter, as well as the species. From these measurements they calculated the aboveground biomass and amount of carbon stored in each type of system. They found that trees in plantations tended to have less dense wood and stored 50-62% of the carbon in natural forests. Natural forests had more samplings than agroforests, and while the wood density of smallholder farm species did not differ from natural forests, species in state-owned farms tended to be less dense. Carbon storage was higher in smallholder farms, where there were more trees (281 per hectare vs. 180 per hectare in state-owned plantations) and the trees tended to have denser wood.
The researchers also interviewed the owners of the small farms. To better understand why each species was selected, they asked about common uses and which species were preferred. They found that wood was commonly used for charcoal, construction, and fuel. Species were selected for a variety of factors, including straightness, easiness to split, flammability, and availability. Species with dense wood were preferred for construction and fuel, with only beehive construction using lighter wood. This resulted in dense-wooded species being heavily harvested in forest fragments and farms, though not in the better-protected state-owned plantations.
Both types of plantation, smallholder and state-owned, are capable of storing a significant amount of carbon. Smallholder plantations are more intensively used, with higher levels of harvesting than the protected state-owned plantations. However, they also have more trees and tend to have denser-wooded species, resulting in a carbon storage capacity that is almost 2/3 that of natural forest. Agroforestry may therefore be highly important in the effort to sequester carbon and mitigate climate change. These systems should be encouraged, but carbon storage considerations are likely to be beyond the concern of small-scale subsistence farmers. Instituting carbon credit programs would allow farmers to place a monetary value on the presence of living trees on their farms, and could provide the impetus for implementing more sustainable farming practices.
Tadesse, G., Zavaleta, E., Shennan, C., 2014. Effects of land-use changes on woody species distribution and above-ground carbon storage of forest-coffee systems. Agriculture, Ecosystems and Environment 197, 21–30. doi:10.1016/j.agee.2014.07.008