Northern forests cover more than 42 percent of the region and are enormous reservoirs of carbon. Through photosynthesis, live trees emit oxygen in exchange for carbon dioxide they pull from the atmosphere. As a tree grows it stores carbon in wood above and below ground, and sequestered carbon comprises about half of its dry weight. Dead trees and down logs are also reservoirs of carbon. Forest soils sequester additional carbon in the form of incorporated organic matter. In temperate northern forest ecosystems, roughly as much carbon is sequestered in forest soils as is sequestered as live biomass. Forests that are converted to other land uses release the carbon stored in the trees. Trees growing in newly established forests (afforestation) can sequester additional carbon.