Submitted to: Global Change Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/24/2008
Publication Date: 2/19/2009
Citation: Wilkinson, M., Monson, R.K., Trahan, N., Lee, S., Brown, E., Jackson, R.B., Polley, H.W., Fay, P.A., Fall, R. 2009. Leaf isoprene emission rate as a function of atmospheric CO2 concentration. Global Change Biology. 15:1189-1200. Interpretive Summary: Plants release several organic compounds into the atmosphere. Some of these compounds react with other chemical constituents in air to regulate the production and lifetime of atmospheric ozone and methane, gases that contribute to what is commonly known as global warming. Isoprene is among the most influential of the volatile compounds released by plants. Via complex chemical reactions, isoprene may increase ozone levels in air and extend the lifetime of methane. However, the rate at which isoprene is emitted from plant leaves may depend on the concentration of carbon dioxide gas (CO2) in the atmosphere. Atmospheric CO2 concentration is rising as a result of fossil fuel consumption by humans. We grew trees of two species at lower-than-present and at elevated CO2 concentrations and trees of two additional species at elevated CO2 to determine how recent and anticipated increases in atmospheric CO2 concentration affect isoprene emission rates. Increasing CO2 above the current concentration reduced isoprene emissions from leaves of sweetgum (Liquidambar styraciflua), eucalyptus (Eucalyptus globulus), aspen (Populus tremuloides), and cottonwood (Populus deltoids) trees. Isoprene emissions were greater at low CO2 levels representative of the pre-Industrial period than at the current CO2 concentration for eucalyptus, but not for sweetgum. Our results demonstrate that isoprene emission rates may be significantly overestimated if direct effects of rising CO2 concentration on emissions are ignored. By slowing emissions of isoprene, atmospheric CO2 enrichment may reduce the concentration and lifetime of trace gases like ozone and methane that contribute to global warming.
Technical Abstract: There is considerable interest in modeling isoprene emissions from terrestrial vegetation, since these emissions exert a principal control over the oxidative capacity of the troposphere, influencing the production of ozone, organic nitrates, organic acids, and affect the atmospheric lifetime of methane, a principle greenhouse gas. Here, we demonstrate that isoprene emissions from the leaves of four tree species are reduced after growth in elevated atmospheric CO2 concentrations. In one set of studies, we used a unique field-based experiment in which the CO2 concentration could be adjusted from 240 ppmv to 520 ppmv to demonstrate that isoprene emissions in Eucalyptus globulus were enhanced at the lowest CO2 concentration (which was similar to the estimated CO2 concentrations during the last Glacial Maximum), compared to the current atmosphere. Leaves of Liquidambar styraciflua did not show an increase in isoprene emission at the lower-than-current CO2 concentration. However, isoprene emission rates from both species were lower for trees grown at 520 ppmv CO2 compared to trees grown at 380 ppmv CO2. When grown in environmentally-controlled chambers, trees of Populus deltoides exhibited a 40% reduction in isoprene emission rate when grown at 800 ppmv CO2, compared to 400 ppmv CO2. Trees of Populus tremuloides exhibited a 30% reduction in isoprene emission rate when grown at 800 ppmv CO2, compared to 400 ppmv CO2, and a 33% reduction when grown at 1200 ppmv CO2, compared to 600 ppmv CO2. Using mathematical models from past studies we demonstrated that significant errors occur in predicting the isoprene emission rate from leaves if the direct effects of the growth CO2 regime are not taken into account. We present a new model to describe the response of isoprene emission to changes in atmospheric CO2 concentration that is based on assumed competition between cytosolic and chloroplastic processes for pyruvate, one of the principal substrates of isoprene biosynthesis. The model provides a good fit to the observed CO2 response for P. tremuloides leaves when grown and exposed to different CO2 concentrations.