Carbon Sequestration

There are some questionable media reports and resulting public discussion of the role of forests in carbon storage (“sequestration”) and the balance of carbon stored and carbon released by our forest management practices. Perhaps a review of the variables and the processes will clarify this potentially biased discussion.

All the carbon in trees came from the atmosphere. If all the trees decomposed, their carbon would go back to the atmosphere. That would be carbon neutral but the greenhouse effect and global temperatures would increase.

The simple argument above fails to give insight because it does not consider time. We must deal with rates. The boreal forest is the largest carbon store left on earth and we do not want to release carbon from that forest at a rate that is greater than the rate at which carbon is being stored by tree growth. The rates are critical, not just the tonnage of carbon.

It is important to know that, in many types of boreal forest, there is as much or more carbon stored in the litter and the A-horizon of the soil than there is in the trees. In 11 of 16 measurements, there was more carbon in the litter and soil than in the trees 1. When forest is harvested, commonly only the part of the carbon that is in the tree stems is removed and diverted to lumber and paper products. Tops may be left, as is the litter. But harvesting can cause a lot of the carbon in the litter to be released into the atmosphere. This happens by increasing access of oxygen to the litter, by opening the litter to the sun and by warming the litter. All increase decomposition.

It also is important to note that much of the boreal forest area is not forest but is bog and wetlands. These are not harvested but some of their stored carbon is released by forest harvesting activities such as drainage, road building, and removal of shade.

A large fraction of tree stems harvested from the boreal go into paper. A smaller fraction goes into lumber products. The lumber products may be preserved for 50 or more years in buildings. The paper probably decomposes in about five years releasing its carbon to the atmosphere.

Harvested forest commonly is replanted. Planting often involves scarification of the forest floor to increase seedling survival. This process speeds the decomposition of litter and also causes decomposition of organic matter in the surface soil. The reforested area may grow to be harvestable in 70 years. How long it takes for the litter and soil organic matter to be fully regenerated is unclear as is whether the same amount of carbon is restored in litter by the end of the harvest cycle.

To attain carbon neutrality in harvested boreal forest would require that carbon from the atmosphere be stored in new tree growth and new litter and soil organic matter at the same rate as it is being returned to the atmosphere as a result of harvesting. Simply knowing that the same area is being replanted as is being harvested per unit time does not guarantee this balance.

Carbon neutrality requires that our actions not change the amount of carbon in the atmosphere (usually thought of as carbon dioxide although methane is a more potent greenhouse gas). Considering forests, this means that the rate of release of carbon must equal the rate of storage of carbon in the components of the forest. These rates can be equalized in time (harvest vs. regrowth) or in space (harvested patches vs. regrowing patches). Carbon neutrality is not possible if harvesting causes large releases of carbon dioxide and there is a long waiting period before an equal amount of carbon dioxide is restored in the forest. Carbon neutrality is absolutely not possible if the total amount of forest is inexorably reduced, which is the recent and the current case.

1 Nalder, I.A. 2002. Forest floor dynamics, Ph.D. thesis, University of Alberta