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CO2-Enriched Plants Follow Frugal Dictum of "Waste Not, Want Not" with Respect to Valuable Captured Carbon
As the air's CO2 content continues to rise, earth's plants are becoming more and more productive, thanks to the aerial fertilization effect of atmospheric CO2 enrichment that increases their photosynthetic prowess and helps drive the great "greening of the earth" that is documented by modern satellite studies (Myneni et al., 1997; Zhou et al., 2001).  This phenomenon can readily boost plant growth rates by 30 to 50% in response to a doubling of the ambient CO2 concentration (Idso and Idso, 1994); and it enables earth's plants to remove from the atmosphere greater quantities of what to them is a most highly-prized resource (CO2), ultimately allowing them to sequester more of that trace gas's valuable carbon in their tissues and the soils in which they grow.

In a somewhat comparable situation, when we humans make more of what we highly prize, i.e., money, we often become less efficient - or more wasteful - in our utilization of that resource.  In the case of plants, however, their enhanced acquisition of CO2 (and their associated banking of carbon) seems to engender within them a greater appreciation for what they have; and they expend an even smaller proportion of what they earn in maintaining their life style.

For example, in a review of the pertinent plant science literature, Drake et al. (1999) found that a doubling of the atmospheric CO2 concentration (about a 350 ppm increase) reduced by approximately 17% the mean respiration rate of the plants studied.  This reduction in the rate at which stored carbon is used to drive growth processes and maintain tissue viability - which is measured by the rate of CO2 emitted from plant foliage, stems and roots - is quite substantial, as the twelve scientists calculate it equates to an extra six to seven Gt of sequestered carbon annually over the whole earth.

That this frugality in vegetative carbon economy is likely ubiquitous is suggested by the fact it has been observed in plants ranging from trees to peat moss.  Karnosky et al. (1999), for example, measured a 24% decrease in the dark respiration rate of deciduous trembling aspen leaves exposed to a 200 ppm increase in atmospheric CO2 concentration over a period of one year; while Jach and Cuelemans (2000) measured a 33% decrease in the dark respiration rate of evergreen Scots pine needles exposed to a 400 ppm CO2 increase over a similar time frame.  And in the case of hydroponically-grown peat moss, Van der Heijden et al. (2000) measured dark respiration rate reductions ranging from 40 to 60%, depending upon solution nitrogen concentration, in response to an atmospheric CO2 increase of 350 ppm maintained over a period of six months.

In all of these situations, plant carbon storage is increased at both ends of the CO2 exchange process: as the air's CO2 content rises, more carbon is captured via photosynthesis while less is lost via respiration, which is but one more example of the many ways (see earlier installments in this series) by which atmospheric CO2 enrichment enhances biological carbon sequestration.  Hence, we can be assured that as global CO2 emissions rise in the future, so will the carbon sequestering prowess of the biosphere rise in like manner, helping to reduce the rate at which the air's CO2 content would otherwise increase.

Earth's plants clearly know a good thing when they see it; and they apparently want to hold on to as much of the carbon they capture as they possibly can for as long as they can.  Perhaps we could learn a thing or two from them.  They are bankers extraordinaire!

Dr. Sherwood B. Idso Dr. Keith E. Idso

References
Drake, B.G., Azcon-Bieto, J., Berry, J., Bunce, J., Dijkstra, P., Farrar, J., Gifford, R.M., Gonzalez-Meler, M.A., Koch, G., Lambers, H., Siedow, J. and Wullschleger, S.  1999.  Does elevated atmospheric CO2 inhibit mitochondrial respiration in green plants?  Plant, Cell and Environment 22: 649-657.

Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: A review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.

Jach, M.E. and Ceulemans, R.  2000.  Short- versus long-term effects of elevated CO2 on night-time respiration of needles of Scots pine (Pinus sylvestris L.).  Photosynthetica 38: 57-67.

Karnosky, D.F., Mankovska, B., Percy, K., Dickson, R.E., Podila, G.K., Sober, J., Noormets, A., Hendrey, G., Coleman, M.D., Kubiske, M., Pregitzer, K.S. and Isebrands, J.G.  1999.  Effects of tropospheric O3 on trembling aspen and interaction with CO2: results from an O3-gradient and a FACE experiment.  Water, Air, and Soil Pollution 116: 311-322.

Myneni, R.C., Keeling, C.D., Tucker, C.J., Asrar, G. and Nemani, R.R.  1997.  Increased plant growth in the northern high latitudes from 1981 to 1991.  Nature 386: 698-702.

Van der Heijden, E., Verbeek, S.K. and Kuiper, P.J.C.  2000.  Elevated atmospheric CO2 and increased nitrogen deposition: effects on C and N metabolism and growth of the peat moss Sphagnum recurvum P. Beauv. Var. mucronatum (Russ.) Warnst.  Global Change Biology 6: 201-212.

Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback, D., Shabanov, N.V. and Myneni, R.B.  2001.  Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981 to 1999.  Journal of Geophysical Research 106: 20,069-20,083.