Carbon sequestration by forests and agroforests: a reality check for the United States

Abstract

Climate change is a major global threat affecting food security and sustainability. Land use systems involving trees have the potential to positively impact climate change by reducing atmospheric carbon dioxide (CO₂) and providing long-term carbon (C) storage. This review evaluated the C sequestration potential of two major land use systems of the United States (US) involving trees, forests and agroforests, which can also provide other ecosystem services.The estimated total forest C stock on forest land in the US in 1990 was 50,913 Tg and another 1885 Tg remained in harvested wood and discarded wood products. From 1990 to 1995, total C stock rose by 2%, and from 2000 to 2005, it rose by 1.7%. The US forests collectively lose (flux) about 200 Mg C y^-1 from disturbance and harvesting. Currently, about 12% of the conterminous US forest land is at high or very high risk of wildfire. Annually, insects and diseases could transfer ~ 21 Tg of live aboveground biomass to litter and woody debris pools. A scenario that targets an afforestation policy for rural landowners in the eastern US and a reforestation policy targeting understocked federal forest lands in the western would improve US annual sequestration compared to the baseline of 323 Tg CO₂ eq yr^-1 in 2015 to 469 Tg CO₂ eq yr^-1 in 2050.Agroforestry offers greater potential to increase C sequestration of predominantly agriculture-dominated landscapes than monocrop agriculture by storing C in above- and belowground biomass, soil, and living and dead organisms and further extending the duration of C in soils. The estimated total C sequestration of current alley cropping (211,938 ha), riparian buffers (640,732 ha), silvopasture (34 Mha), and windbreak (2.37 Mha) practices is 219 Tg C yr^-1. The total C sequestration would be 240 Tg C yr^-1 with 5% of the US cropland converted to alley cropping (3.7 Tg yr^-1), 15-m wide riparian buffers on both sides of 5% of the total stream length (4.75 Tg yr^-1), 34 Mha converted to silvopasture (207 Tg yr^-1), and windbreaks on 5% (7.45 Mha) of the cropland (25 Tg yr^-1). Despite many limitations including uncertainty of land areas under agroforestry, lack of standardized estimation protocols, and lack of accountability on various C stocks (source-sink services, detritus C, insect/pest damages, etc.), we believe these new accrual rates and the land areas under each practice are much more realistic as new information became available over the last decade.The total C sequestration by forests (776) and agroforests (219) is 995 Tg yr^-1 and represents approximately 15% of the US CO₂ emissions. This review highlights the importance of sustainable management of forests and integration of agroforestry on agricultural lands to mitigate climate challenges further while meeting society’s need for food and a healthy environment.

Keywords

Alley cropping / riparian buffers / silvopasture / windbreaks

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