The Biology of Carbon Sequestration

By Thembi Borras

In response to the G8 meeting in Scotland of which global warming was one focus, I will explore the relationship between global warming and forests in the next several posts.

Greenhouse gases contribute to global warming, of these greenhouse gases, carbon dioxide is emitted in the greatest quantity. The bulk of the carbon dioxide is let into the atmosphere when fossil fuels are burned.

Carbon dioxide passes into the tree through the stomata, which are openings in the "skin" of the leaves and needles. Light energy, stored in chlorophyll, triggers photosynthesis, the reaction that changes inorganic carbon (carbon dioxide) into organic carbon (carbohydrate). Organic carbon is moved downward (translocated) in vascular tissue called phloem from a source, such as a mature leaf, to a sink such as roots, the tree bole, and developing fruits.

Organic carbon is used by the tree to increase biomass and provides the energy to build and maintain that biomass. To obtain this energy, trees respire consuming approximately 1/2 of the organic carbon assimilated during photosynthesis releasing carbon dioxide back into the atmosphere.

The carbon a tree gains is a function of the balance between carbon uptake by photosynthesis and carbon lost by respiration. Carbon sequestration is the term used for this net gain of organic carbon.

Although younger rapidly growing forests are more productive in terms of carbon dioxide uptake from the atmosphere and respiration is more efficient, the younger forest lacks the accumulated organic carbon from many decades of growth. When older forests are replaced by younger forests, they store less carbon. More influential is when forests are converted to non timber uses, such as roads, housing and agriculture and the capacity to store carbon is further decreased.

A portion of this post was gleaned from Introduction to Plant Physiology by William G. Hopkins and Forest Carbon in the United States: Opportunities and Options for Private Lands a publication offered by The Pacific Forest Trust.