Forests: a CO2 Reservoir, not a "Sponge"!

Much has been made over the purported ability of forests to act as a "sponge" removing carbon dioxide from the atmosphere.  Politicians have jumped on this bandwagon, as have those selling and promoting so called "carbon-offsets".   However, basic biology dictates otherwise.

To understand the reality, first consider an individual tree.  This tree, like all plants utilizes solar energy to combine water and carbon dioxide, resulting in the production of carbohydrates (sugars).  We call the combination of chemical reactions involved in the process "photosynthesis".   Photosynthesis requires the presence of the green pigment chlorophyll, and thus occurs predominately in the leaves, or needles, of trees: and of course, only during the daylight hours.  Like all living organisms the tree requires energy for life processes, which it obtains by oxidizing some of the carbohydrates manufactured by photosynthesis.   This process of "respiration" is chemically the reverse of photosynthesis, occurs throughout the life of the tree, and results in the release of carbon dioxide into the atmosphere. (http://arewetoast.com/energy-biology-chemistry-and-physics.html)

Plants typically manufacture far more carbohydrate than is utilized during respiration; some of which is stored in the living cells, while most is converted into cellulose which provides the rigid structure (wood) of the tree, both above ground and below ground (roots).  Growth of the tree represents the accumulation of carbohydrates and typically occurs more rapidly in young trees  and then slows as the tree matures.  Throughout the life of the tree, foliage is shed and decays, and the smaller roots die and decay as the root system expands.  Eventually the tree dies, and also decays.  The process of decay, conducted by fungi and other micro-organisms is again the process of oxidation, breaking down the accumulated carbohydrates and releasing carbon dioxide into the atmosphere.  Thus when the tree is completely decayed we have a balance, the carbon dioxide removed from the atmosphere during photosynthesis is eventually returned to the atmosphere by the oxidative processes of respiration and decay.  The net result is zero, no carbon dioxide permanently removed from the atmosphere and none added.  The tree has been a CO₂ reservoir, accumulating and storing CO₂ during its lifetime, and then returning CO₂to the atmosphere as it respires,and ultimately decays.

If we view a forest as simply a collection of trees we can better relate to the forest as a reservoir.  A young forest will typically consist of a number of young, rapidly growing trees, with their growth made possible by the accumulation of carbohydrates manufactured using atmospheric CO₂.  A forester calls this stage a period of high "productivity".  However, as the forest ages, or becomes mature, growth will slow and individual trees will start to die and return their accumulated CO₂ to the atmosphere.  Eventually the forest will essentially come into equilibrium with its environment and over time maintain a relatively constant amount of accumulated carbohydrate per unit of area (i.e., "standing crop" or "biomass").  Like a reservoir, or lake,  with the amount of water flowing in equal to the amount of outflow, the amount of CO₂ removed from the atmosphere by the forest remains equal to the amount of CO₂ returned to the atmosphere. Just as the amount of water in a reservoir may vary between seasons, or from year to year, the accumulated CO₂ (biomass) of the forest may vary by season and from year to year, but in both cases, over time the net accumulation is zero.

There is no doubt that our forest reservoir can hold a vast amount of carbon, with catastrophic events such as fire, disease, or pests resulting in the return of significant amount of CO₂ to the atmosphere.  For example, the Mountain Pine Beetle is currently devastating the lodgepole pines in western North America.  According to the CBC (http://tinyurl.com/yguojjr) the Canadian Forest Service estimates that by 2013 the damaged forest will release almost one billion megatonnes of CO₂(eq) into the atmosphere, an amount equal to the total 5-year emissions from Canada's transportation sector.  Likewise, deforestation by humans causes the release of vast amounts of CO₂ into the atmosphere, and is indeed one of the major sources of CO₂ emissions on a global basis.  Deforestation is analogous to emptying the reservoir into the atmosphere.  Given enough time the reservoir may refill to its original capacity, however it will most likely take centuries for a stable ecosystem to return.

When balanced over the globe, and over time, the analogy of forests as CO₂ reservoirs, not steadily accumulating  sponges, holds true. Thus any claim that forests can be used to offset CO₂ emissions should be viewed with skepticism.