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Extending the CO2 Dependency of Isoprene Emission by Vegetation from Current-and-Above to Pre-Industrial and Glacial CO2 Concentrations
Possell, M., Hewitt, C.N. and Beerling, D.J. 2005. The effects of glacial atmospheric CO2 concentrations and climate on isoprene emissions by vascular plants. Global Change Biology 11: 60-69.

What was done
The authors performed multiple three-week-long experiments with two known isoprene-emitting herbaceous species (Mucuna pruriens and Arundo donax), which they grew in controlled environment chambers that were maintained at two different sets of day/night temperatures (29/24C and 24/18C) and atmospheric CO2 concentrations characteristic of glacial (180 ppm), pre-industrial (280 ppm) and current (366 ppm) conditions, where canopy isoprene emission rates were measured on the final day of each experiment.

What was learned
Possell et al. report what they describe as "the first empirical evidence for the enhancement of isoprene production, on a unit leaf area basis, by plants that grew and developed in [a] CO2-depleted atmosphere," which results, in their words, "support earlier findings from short-term studies with woody species (Monson and Fall, 1989; Loreto and Sharkey, 1990)." Combining their emission rate data with those of Rosenstiel et al. (2003) for Populus deltoides, Centritto et al. (2004) for Populus x euroamericana, and Scholefield et al. (2004) for Phragmites australis, they developed a single downward-trending curve that stretches all the way from 180 to 1200 ppm CO2, where it asymptotically approaches a value that is about an order of magnitude less than what it was at 180 ppm.

What it means
These findings are extremely significant, for isoprene is responsible for the production of vast amounts of tropospheric ozone (Chameides et al., 1988; Harley et al., 1999), which (1) is a debilitating scourge of both plant and animal life and (2) lengthens the lifetime of atmospheric methane, one of the world's most powerful greenhouse gases. Hence, it can be appreciated that rising atmospheric CO2 concentrations enhance both plant and animal health via their isoprene-modulated negative impact on tropospheric ozone, while they mitigate some of their own global warming potential via the same mechanism.

Centritto, M., Nascetti, P., Petrilli, L., Raschi, A. and Loreto, F. 2004. Profiles of isoprene emission and photosynthetic parameters in hybrid poplars exposed to free-air CO2 enrichment. Plant, Cell and Environment 27: 403-412.

Chameides, W.L., Lindsay, R.W., Richardson, J. and Kiang, C.S. 1988. The role of biogenic hydrocarbons in urban photochemical smog: Atlanta as a case study. Science 241: 1473-1475.

Harley, P.C., Monson, R.K. and Lerdau, M.T. 1999. Ecological and evolutionary aspects of isoprene emission from plants. Oecologia 118: 109-123.

Loreto, F. and Sharkey, T.D. 1990. A gas-exchange study of photosynthesis and isoprene emission in Quercus robur L. Planta 182: 523-531.

Monson, R.K. and Fall, R. 1989. Isoprene emissions from Aspen leaves. Influence of environment and relation to photosynthesis and photorespiration. Plant Physiology 90: 267-274.

Rosentiel, T.N., Potosnak, M.J., Griffin, K.L., Fall, R. and Monson, R.K. 2003. Increased CO2 uncouples growth from isoprene emission in an agriforest ecosystem. Nature advance online publication, 5 January 2003 (doi:10.1038/nature 01312).

Scholefield, P.A., Doick, K.J., Herbert, B.M.J., Hewitt, C.N.S., Schnitzler, J.-P., Pinelli, P. and Loreto, F. 2004. Impact of rising CO2 on emissions of volatile organic compounds: isoprene emission from Phragmites australis growing at elevated CO2 in a natural carbon dioxide spring. Plant, Cell and Environment 27: 393-401.

Reviewed 23 March 2005