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Trees Spend More Time Sequestering Carbon with More CO2 in the Air
One of the current hot topics in the global climate change debate is whether the planting of trees is a viable mechanism for offsetting anthropogenic carbon dioxide emissions.  Many politically-active environmental groups, claim this approach won't work.  Indeed, they say that using trees to sequester carbon in lieu of reducing CO2 emissions is immoral and akin to committing criminal acts against the earth.

What's wrong with this picture?  If you can recall what you learned in high school biology, you will remember that plants remove CO2 from the air and convert it into sugars that are used to produce substances needed to sustain their growth and development.  Many of these CO2-derived products, particularly lignin and cellulose, are present in large quantities within the woody tissues of trees and shrubs.  Hence, as long as these plants are alive and growing, they actively remove carbon from the air around them.  Moreover, even after their biological activities cease, trees continue to retain the carbon they sequestered during their lifetimes within their woody tissues; and the products that mankind develops from them retain this carbon over their lifetimes as well.  Thus, trees and other woody plants, aided by human ingenuity, possess an enormous potential to sequester vast amounts of carbon for very long periods of time.  For many species, in fact, this time may amount to hundreds of years; and for several others it may even exceed a millennium (Chambers et al., 1998).

An important point hardly ever mentioned by opponents of carbon sequestration by trees is the fact that their ability to remove CO2 from the air will only grow stronger with each passing year, as the air's CO2 content continues to rise, due to the well-known aerial fertilization effect of atmospheric CO2 enrichment.  In a recent analysis of the results of 176 separate studies of this phenomenon in all types of tree seedlings, for example, Idso (1999) found that a 300 ppm increase in the air's CO2 concentration increased seedling biomass production by fully 50%; while Saxe et al. (1998) put the biomass increase for such a CO2 increase at 130% for conifers.

These biomass increases are very large; and many people have wondered how they could possibly be sustained by virtue of the aerial fertilization effect alone.  Now comes a study that helps to answer that question by illuminating a closely-related way in which elevated levels of atmospheric CO2 increase the carbon sequestering power of trees.

In this experiment, Marek et al. (2001) constructed open-top chambers around 30-year-old mature oak (Quercus ilex) trees growing in perennial evergreen stands in central Italy, through which they forced continual flows of air having CO2 concentrations of either 350 or 700 ppm for five full years.  As expected, the aerial fertilization effect of the extra CO2 increased the photosynthetic rates of sun-exposed and shaded leaves on the CO2-enriched trees by 68 and 59%, respectively.  In addition, the authors discovered that the light compensation point, i.e., the light level at which net photosynthesis changes from negative to positive as light intensity increases in the morning, or at which it changes from positive to negative as it decreases towards evening, was reduced by 24 and 30% in sun-exposed and shaded leaves, respectively, in the CO2-enriched trees.

What are the implications of these observations?  First of all, they demonstrate that the aerial fertilization effect of atmospheric CO2 enrichment operates on mature trees as well as on seedlings.  Second, because elevated CO2 significantly reduces the light level at which the trees can successfully remove CO2 from the atmosphere, the trees can begin sequestering carbon earlier after sunrise each morning, and they can continue doing so closer towards sunset each evening.  Hence, not only do the CO2-enriched trees possess greater power to remove CO2 from the air at any given time, they exercise that power for a longer period of time each day, greatly enhancing their carbon sequestering capacities.

The notion that the planting of trees is not a viable mechanism for the long-term sequestering of carbon is ludicrous.  Trees clearly possess a number of mechanisms - of which we here discuss but two - that allow them to sequester great quantities of carbon and significantly offset anthropogenic CO2 emissions.  Planting them for this purpose is a reasonable and prudent action that could be used successfully to measurably reduce the rate of rise of the air's CO2 concentration.

Dr. Craig D. Idso Dr. Keith E. Idso

References
Chambers, J.Q., Higuchi, N. and Schimel, J.P.  1998.  Ancient trees in Amazonia.  Nature 391: 135-136.

Idso, S.B.  1999.  The long-term response of trees to atmospheric CO2 enrichment.  Global Change Biology 5: 493-495.

Marek, M.V., Sprtova, M., De Angelis, P. and Scarascia-Mugnozza, G.  2001.  Spatial distribution of photosynthetic response to long-term influence of elevated CO2 in a Mediterranean macchia mini-ecosystem.  Plant Science 160: 1125-1136.

Saxe, H., Ellsworth, D.S. and Heath, J.  1998.  Tree and forest functioning in an enriched CO2 atmosphere.  New Phytologist 139: 395-436.