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Elevated CO2 Increases Leaf Longevity, Giving Plants Extra Time to Deposit More Carbon in Earth's Soil Bank System
There are a number of different ways in which the ongoing rise in the air's CO2 content enables plants to sequester more carbon than they would do under conditions of static or declining atmospheric CO2 concentration.  Most notable of these stimulatory phenomena is the fundamental aerial fertilization effect of atmospheric CO2 enrichment, which increases the photosynthetic rates of plant leaves.  Another important way in which CO2-enriched leaves may remove greater amounts of carbon dioxide from the air is by simply living longer than they do at the current or ambient atmospheric CO2 concentration.

One of the first studies to probe the connection between atmospheric CO2 concentration and leaf longevity was that of Idso et al. (1990), who grew water lilies out-of-doors in sunken metal stock tanks located within clear-plastic-wall open-top chambers maintained at either the ambient atmospheric CO2 concentration or ambient plus 300 ppm CO2.  Over the five-month growing season of their experiment, the water lily leaves exposed to the extra CO2 extended their lifespan by 17.5%.

More recently, Craine and Reich (2001) described an experiment in which they grew monocultures of ten different grassland species and used Free-Air CO2 Enrichment (FACE) technology to increase the CO2 concentration of the air surrounding half of their plants by 200 ppm.  Over the four-month period of their experiment, they observed that leaves of C3 grasses lived 3.3% longer in the CO2-enriched air of the FACE treatments, while C3 forbs exposed to the extra CO2 increased the lifespan of their leaves by 11.7%.  For a 300 ppm increase in atmospheric CO2 concentration such as Idso et al. employed in their study, these results correspond to leaf longevity increases on the order of 5% and 17.5%, respectively.

Another study of CO2 effects on leaf longevity was conducted by Knapp et al. (1999), who used open-top chambers in a Kansas grassland to determine the response of the dominant C4 grass to a doubling of the ambient CO2 concentration (a daytime increase of 335 ppm).  In this experiment, leaf lifespan was extended by 20%, which for a 300 ppm increase in atmospheric CO2 concentration would correspond to an increase of 18%.

What do these findings portend for biological carbon sequestration in the expected high-CO2 world of the future?  Knapp et al. note that greater leaf longevity combined with increased canopy leaf area, which is also a common consequence of atmospheric CO2 enrichment, should allow grasslands to respond more opportunistically to the variable precipitation patterns they typically experience.  They also state that the "CO2-mediated delay in leaf senescence may significantly alter current seasonal patterns in carbon gain, particularly in the autumn," or as Craine and Reich state it more succinctly, "all other things equal, the increase in leaf longevity due to elevated CO2 would lead to greater ecosystem carbon gain."

Of course, all other things are generally not equal under conditions of atmospheric CO2 enrichment.  For one thing, leaf nitrogen concentrations are often reduced as the air's CO2 content rises; but as Craine and Reich discovered via soil nitrogen manipulations in their experiment, leaf longevity tends to increase under this condition as well, which also leads to increased ecosystem carbon gain.

There is thus good reason to believe that as the air's CO2 content continues to rise, earth's terrestrial ecosystems will direct ever larger amounts of carbon into the soil beneath them as the leaves of their plants remain active for a longer period of time late in the growing season.  Add to this observation the fact that the aerial fertilization effect of atmospheric CO2 enrichment concurrently enhances leaf photosynthesis above what it would be under ambient conditions, and you have a formula for even greater carbon gains by the plants of tomorrow.

Clearly, money invested in biological carbon sequestration programs will pay ever-increasing dividends as time progresses and earth's plants deposit ever-increasing amounts of carbon in the planetary soil bank.

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

References
Craine, J.M. and Reich, P.B.  2001.  Elevated CO2 and nitrogen supply alter leaf longevity of grassland species.  New Phytologist 150: 397-403.

Idso, S.B., Allen, S.G. and Kimball, B.A.  1990.  Growth response of water lily to atmospheric CO2 enrichment.  Aquatic Botany 37: 87-92.

Knapp, A.K., Bargmann, N., Maragni, L.A., McAllister, C.A., Bremer, D.J., Ham, J.M. and Owensby C.E.  1999.  Elevated CO2 and leaf longevity in the C4 grassland-dominant Andropogon gerardiiInternational Journal of Plant Science 160: 1057-1061.