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Ten Years of Photosynthetic Response to Atmospheric CO2 Enrichment in Perennial Ryegrass
Volume 6, Number 33: 13 August 2003

In the first sentence of the abstract of the paper describing this longest of atmospheric CO2 enrichment studies ever to be conducted on a grassland species, Ainsworth et al. (2003) note that "photosynthesis is commonly stimulated in grasslands with experimental increases in atmospheric CO2 concentration, a physiological response that could significantly alter the future carbon cycle if it persists in the long term."  However, they further note that "an acclimation of photosynthetic capacity suggested by theoretical models and short-term experiments could completely remove this effect of CO2."  The prevailing view, in their words, is that "perennial systems will respond to elevated CO2 in the short term, but that the response for grasslands will be short-lived (Roumet et al., 2000)," and they cite Luo and Reynolds (1999) as suggesting an effective CO2-induced stimulatory period of less than ten years for both high- and low-productivity grasslands.

As was the case with trees [see our Editorial of 5 March 2003], where it was also long believed that the stimulatory effect of elevated CO2 on photosynthesis and growth would gradually waste away over time, the only way to resolve the same issue with respect to grasslands would appear to be the conducting of a long-term experiment - such as the sour orange tree study of Idso and Kimball (2001) - which is exactly what the Ainsworth et al. team of American, British, Italian and Swiss scientists did in their ten-year study of perennial ryegrass (Lolium perenne L. cv. Bastion).

The record-setting study was conducted at Eschikon, Switzerland, within three replicate blocks of two 18-m-diameter Free-Air CO2 Enrichment (FACE) rings maintained at either 360 or 600 ppm CO2 throughout each growing season of the entire 10-year period.  The experimental plots, which were established in 1993 on a field of perennial ryegrass that had been planted in August of 1992, were further subdivided into low and high nitrogen fertilization treatments; and the plants grown within them were periodically harvested several times a year.  In addition, in the words of the authors, "more than 3000 measurements characterized the response of leaf photosynthesis and stomatal conductance to elevated CO2 across each growing season for the duration of the experiment."

So what was learned in this measurement-intensive study?  Ainsworth et al. report that "over the 10 years as a whole, growth at elevated CO2 resulted in a 43% higher rate of light-saturated leaf photosynthesis and a 36% increase in daily integral of leaf CO2 uptake."  Interestingly, the 36% increase in daily CO2 uptake was, in their words, "almost identical to the 38% increase seen on the first day of measurements in August 1993 and the 39% stimulation on the last day of measurements in May 2002."

There was also a seasonal trend in the CO2-induced increase in the daily integral of CO2 fixation, which ranged from 25% in the spring to 41% in the summer and 48% in the fall.  The scientists say this finding "is consistent with theoretical expectation, where because of the differing sensitivities of Rubisco oxygenase and carboxylase activity, the proportionate stimulation of photosynthesis by a given increase in CO2 will rise with temperature (Long, 1991)."  This phenomenon has also been observed in a number of other plants as described in our major report on The Specter of Species Extinction.

Ainsworth et al. additionally note that "the percentage increase in photosynthetic carbon uptake in the first 20 days following a harvest (45%) was nearly double the percentage increase later in the regrowth cycle (23%)."  This finding is indicative of the fact that CO2-induced growth stimulation is greatest when plant source:sink ratio is small, i.e., when there are few photosynthesizing leaves and many photosynthate-storing roots, so that the CO2-induced enhancement of photosynthesis need not immediately decline for lack of a sufficiently large repository to deposit the fruits of its labors, so to speak.

Summing up, the international team of scientists said the CO2-induced photosynthetic stimulation "was maximal following harvest, at the warmest times of year and with a high supply of nitrogen."  Most important of all, however, was their ultimate conclusion: "this open-air field experiment provides no support for the prediction that stimulation of photosynthesis under elevated CO2 is a transient phenomenon," or as they phrased it in the abstract of their paper, "in contrast with theoretical expectations and the results of shorter duration experiments, the present results provide no [evidence of] significant change in photosynthetic stimulation across a 10-year period, nor greater acclimation ? in the latter years in either nitrogen treatment."

Simply put, the aerial fertilization effect of atmospheric CO2 enrichment is both real and enduring.  It operates now and will continue to operate in the future.

Sherwood, Keith and Craig Idso

Ainsworth, E.A., Davey, P.A., Hymus, G.J., Osborne, C.P., Rogers, A., Blum, H., Nosberger, J. and Long, S.P.  2003.  Is stimulation of leaf photosynthesis by elevated carbon dioxide concentration maintained in the long term?  A test with Lolium perenne grown for 10 years at two nitrogen fertilization levels under Free Air CO2 Enrichment (FACE).  Plant, Cell and Environment 26: 705-714.

Idso, S.B. and Kimball, B.A.  2001.  CO2 enrichment of sour orange trees: 13 years and counting.  Environmental and Experimental Botany 46: 147-153.

Long, S.P.  1991.  Modification of the response of photosynthetic productivity to rising temperature by atmospheric CO2 concentrations: has its importance been underestimated?  Plant, Cell and Environment 14: 729-739.

Luo, Y.Q. and Reynolds, J.F.  1999.  Validity of extrapolating field CO2 experiments to predict carbon sequestration in natural ecosystems.  Ecology 80: 1568-1583.

Roumet, C., Garnier, E., Suzor, H., Salager, J.-L. and Roy, J.  2000.  Short and long-term responses of whole-plant gas exchange to elevated CO2 in four herbaceous species.  Environmental and Experimental Botany 43: 155-169.