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CO2-Herbivore-Litter Interactions in a Scrub Oak Ecosystem
Hall, M.C., Stiling, P., Hungate, B.A., Drake, B.G. and Hunter, M.D.  2005.  Effects of elevated CO2 and herbivore damage on litter quality in a scrub oak ecosystem.  Journal of Chemical Ecology 31: 2343-2356.

What was done
Employing open-top chamber technology within a two-hectare scrub oak community at the Kennedy Space Center, Florida, USA (comprised of 76% Quercus myrtifolia, 15% Q. geminata, 7% Q. chapmanii, and 2% all else), the authors studied the effects of an extra 350 ppm of CO2 on litter quality, herbivore activity and their interactions.

What was learned
Over the three years of the study (2000, 2001, 2002), Hall et al. report that "changes in litter chemistry from year to year were far larger than effects of CO2 or insect damage, suggesting that these may have only minor effects on litter decomposition."  The one exception to this finding was that "condensed tannin concentrations increased under elevated CO2 regardless of species, herbivore damage, or growing season," rising by 11% in 2000, 18% in 2001 and 41% in 2002 as a result of atmospheric CO2 enrichment, as best we can determine from the authors' bar graphs.  Also, the five researchers report that "lepidopteran larvae can exhibit slower growth rates when feeding on elevated CO2 plants (Fajer et al., 1991) and become more susceptible to pathogens, parasitoids, and predators (Lindroth, 1996; Stiling et al., 1999)," noting further that at their field site, "which hosts the longest continuous study of the effects of elevated CO2 on insects, herbivore populations decline markedly under elevated CO2 (Stiling et al., 1999, 2002, 2003; Hall et al., 2005)."

What it means
The large and continuous enhancement of condensed tannin concentrations in foliage produced in CO2-enriched air is a good omen for people worried about greenhouse gas-induced global warming, because methane emissions from ruminants feeding on foliage rich in condensed tannins tend to be lower than methane emissions from ruminants feeding on foliage containing lower tannin concentrations.  Also, the marked declines in herbivore populations observed in CO2-enriched open-top chambers bodes well for the ability of earth's vegetation to better resist herbivore attacks as the air's CO2 content continues to climb.

Fajer, E.D., Bowers, M.D. and Bazzaz, F.A.  1991.  The effects of enriched CO2 atmospheres on the buckeye butterfly, Junonia coeniaEcology 72: 751-754.

Hall, M.C., Stiling, P., Moon, D.C., Drake, B.G. and Hunter, M.D.  2005.  Effects of elevated CO2 on foliar quality and herbivore damage in a scrub oak ecosystem.  Journal of Chemical Ecology 31: 267-286.

Lindroth, R.L.  1996.  CO2-mediated changes in tree chemistry and tree-Lepidoptera interactions.  In: Koch, G.W. and Mooney, H,A. (Eds.).  Carbon Dioxide and Terrestrial Ecosystems.  Academic Press, San Diego, California, USA, pp. 105-120.

Stiling, P., Rossi, A.M., Hungate, B., Djkstra, P., Hinkle, D.R., Knott, W.M. and Drake, B.G.  1999.  Decreased leaf-miner abundance in elevated CO2: reduced leaf quality and increased parasitoid attack.  Ecological Applications 9: 240-244.

Stiling, P., Cattell, M., Moon, D.C., Rossi, A., Hungate, B.A., Hymus, G. and Drake, B.G.  2002.  Elevated atmospheric CO2 lowers herbivore abundance, but increases leaf abscission rates.  Global Change Biology 8: 658-667.

Stiling, P., Moon, D.C., Hunter, M.D., Colson, J., Rossi, A.M., Hymus, G.J. and Drake, B.G.  2003.  Elevated CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest.  Oecologia 134: 82-87.

Reviewed 11 January 2006