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CO2-Induced Increases in Grassland Productivity: Do They Disappear Over Time?
Ainsworth, E.A., Rogers, A., Blum, H., Nosberger, J. and Long, S.P. 2003. Variation in acclimation of photosynthesis in Trifolium repens after eight years of exposure to Free Air CO2 Enrichment (FACE). Journal of Experimental Botany 54: 2769-2774.

According to the authors of this article, who analyzed data obtained from what has become the longest-running FACE experiment ever to be conducted anywhere in the world, it has been speculated that "elevated CO2 may partition resources away from leaves and, through increased production, sequester nutrients into organic matter causing deficiencies which indirectly cause decreased photosynthetic capacity." In this regard, they cite the theoretical study of these considerations conducted by Luo and Reynolds (1999), who they say "predicted that the initial stimulation of photosynthetic production in grasslands would be lost within nine years of a step increase in CO2, as imposed in FACE experiments." Utilizing real-world data obtained over nearly a decade of careful experimentation, they decided to test this claim.

What was done
The group of five scientists studied white clover (Trifolium repens L. cv. Milkanova), which was grown in monoculture in the Swiss FACE array at Eschikon, Switzerland - as described by Hebeisen et al. (1997) - three of the 18-m-diamter circular plots of which were maintained at the ambient atmospheric CO2 concentration of approximately 360 ppm and three of which were maintained at an elevated concentration of approximately 600 ppm. Specifically, they characterized the photosynthetic responses of the plants within the FACE array to the extra 240 ppm of CO2 delivered to the elevated CO2 plots in the spring and autumn of the eighth, ninth and tenth years of the experiment.

What was learned
Ainsworth et al. determined there was no acclimation or down regulation of T. repens photosynthetic capacity in the spring of the year, even after eight full seasons of exposure to elevated CO2 concentrations. In the autumn, however, there was a down regulation of approximately 20%; but it occurred "late in the growing season, when the 24-hour mean temperature had dropped below 10C, and nightly frosts were occurring," under which conditions they say "shoot growth is limited and the sink for carbohydrate is small, and acclimation of photosynthesis to elevated CO2 would be expected." Yet in spite of that acclimation and the stress of those cold conditions, the average photosynthetic rate of the CO2-enriched plants at that time of year was still 37% greater than that of the ambient-treatment plants.

What it means
The five scientists state that their results "do not support the prediction that the response of grassland species to elevated CO2 will be short-lived as the demand for nutrients increases," in clear contradiction of the claim of Luo and Reynolds, as well as the similar claims of others (see our Editorial of 10 December 2003); for as they reiterate in the concluding sentence of their paper, "contrary to the belief that the response of grassland species to elevated CO2 will be short-lived, stimulation of photosynthesis in T. repens remained after eight years of exposure to elevated CO2."

Hebeisen, T., Luscher, A., Zanetti, S., Fischer, B., Hartwig, U.A., Frehner, M., Hendrey, G.R., Blum, H. and Nosberger, J. 1997. Growth response of Trifolium repens L. and Lolium perenne L. as monocultures and bi-species mixture to free air CO2 enrichment and management. Global Change Biology 3: 149-160.

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.

Reviewed 4 February 2004