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Effects of Elevated CO2 on Forest Leaf Damage Produced by Insect Herbivores
Volume 8, Number 31: 3 August 2005

Increases in per capita consumption of foliage by insect herbivores in CO2-enriched air have periodically been observed in laboratory and greenhouse studies (Bezemer and Jones, 1998; Coviella and Trumble, 1999; Hunter, 2001), leading to periodic claims that earth's forests will suffer severely at the mandibles of ravenous hordes of leaf-chewing insects in the years and decades to come, unless, of course, something is done to stop the ongoing rise in the atmosphere's CO2 concentration.  However, as Knepp et al. (2005) have recently pointed out, these observations do not necessarily imply there will be increased insect herbivory in native forest communities in a CO2-enriched world of the future, citing the findings of Hamilton et al. (2004) in support of this contention; and they go on to describe the results of a study they conducted in just such a community using free-air carbon dioxide enrichment (FACE) technology to further explore the subject.

In the Knepp et al. study, leaf damage by chewing insects was quantified on saplings of twelve species of hardwood trees growing in the understory of a 17-year-old (in 2000) loblolly pine plantation in the Piedmont region of North Carolina, USA, where three 30-m-diameter FACE plots have been continuously enriched with an extra 200 ppm of CO2 and three identical control plots have been similarly exposed to ambient air since 1996.  Leaf damage was measured on seven species - Acer rubrum L. (red maple), Cercis Canadensis L. (redbud), Liquidambar styraciflua L. (sweetgum), Prunus serotina Ehrh. (black cherry), Quercus alba L. (white oak), Quercus phellos L. (willow oak), and Ulmus alata Michx. (winged elm) - in 2001, 2002 and 2003, while five additional species - Acer barbatum Michx. (sugar maple), Liriodendron tulipifera L. (yellow poplar), Quercus rubra L. (red oak), Quercus velutina Lam. (black oak) and Robinia pseudo-acacia L. (black locust) - were included in 2001 and 2003.

The six scientists involved in the work report that "across the seven species that were measured in each of the three years, elevated CO2 caused a reduction in the percentage of leaf area removed by chewing insects," noting that "the percentage of leaf tissue damaged by insect herbivores was 3.8% per leaf under ambient CO2 and 3.3% per leaf under elevated CO2."  Greatest effects were observed in 2001, when they report that "across 12 species the average damage per leaf under ambient CO2 was 3.1% compared with 1.7% for plants under elevated CO2," which was "indicative of a 46% decrease in the total area and total mass of leaf tissue damaged by chewing insects in the elevated CO2 plots."

What was responsible for these highly positive results?  Knepp et al. say that "given the consistent reduction in herbivory under high CO2 across species in 2001, it appears that some universal feature of chemistry or structure that affected leaf suitability was altered by the treatment."  Another possibility they discus is that "forest herbivory may decrease under elevated CO2 because of a decline in the abundance of chewing insects," citing the observations of Stilling et al. (2002) to this effect and noting that "slower rates of development under elevated CO2 prolongs the time that insect herbivores are susceptible to natural enemies, which may be abundant in open-top chambers and FACE experiments but absent from greenhouse experiments."  In addition, they suggest that "decreased foliar quality and increased per capita consumption under elevated CO2 may increase exposure to toxins and insect mortality," also noting that "CO2-induced changes in host plant quality directly decrease insect fecundity," citing the work of Coviella and Trumble (1999) and Awmack and Leather (2002).

So what's the bottom line with respect to the outlook for earth's forests in a high-CO2 world of the future?  In their concluding paragraph, Knepp et al. say that "in contrast to the view that herbivore damage will increase under elevated CO2 as a result of compensatory feeding on lower quality foliage, our results and those of Stiling et al. (2002) and Hamilton et al. (2004) in open experimental systems suggest that damage to trees may decrease."

Sherwood, Keith and Craig Idso

Awmack, C.S. and Leather, S.R.  2002.  Host plant quality and fecundity in herbivorous insects.  Annual Review of Entomology 47: 817-844.

Bezemer, T.M. and Jones, T.H.  1998.  Plant-insect herbivore interactions in elevated atmospheric CO2: quantitative analyses and guild effects.  Oikos 82: 212-222.

Coviella, C.E. and Trumble, J.T.  1999.  Effects of elevated atmospheric carbon dioxide on insect-plant interactions.  Conservation Biology 13: 700-712.

Hamilton, J.G., Zangerl, A.R., Berenbaum, M.R., Pippen, J.S., Aldea, M. and DeLucia, E.H.  2004.  Insect herbivory in an intact forest understory under experimental CO2 enrichment.  Oecologia 138: 566-573.

Hunter, M.D.  2001.  Effects of elevated atmospheric carbon dioxide on insect-plant interactions.  Agricultural and Forest Entomology 3: 153-159.

Knepp, R.G., Hamilton, J.G., Mohan, J.E., Zangerl, A.R., Berenbaum, M.R. and DeLucia, E.H.  2005.  Elevated CO2 reduces leaf damage by insect herbivores in a forest community.  New Phytologist 167: 207-218.

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