Volume 7, Number 29: 21 July 2004
Hamilton et al. (2004) begin their report of their investigation of this important question by noting that increases in atmospheric CO2 concentration have been shown to stimulate the productivity of essentially all terrestrial ecosystems (Curtis and Wang, 1998; DeLucia et al., 1999; Hamilton et al., 2002); but they also report that most single-species studies of this subject have predicted that increases in atmospheric CO2 will increase herbivory (Bezemer and Jones, 1998; Cannon, 1998; Coviella and Trumble, 1999; Hunter, 2001; Lincoln et al., 1993; Whittaker, 1999). However, because there are so many feedbacks and complex interactions among the numerous components of real-world ecosystems, Hamilton et al. suggest that one ought not put too much faith in these predictions until relevant real-world ecosystem-level experiments have been conducted.
In one such study they conducted at the Duke Forest FACE facility near Chapel Hill, North Carolina, USA, Hamilton et al. "measured the amount of leaf tissue damaged by insects and other herbivorous arthropods during two growing seasons in a deciduous forest understory continuously exposed to ambient (360 ppm) and elevated ( 560 ppm) CO2 conditions." This forest is dominated by loblolly pine trees that account for fully 92% of the ecosystem's total woody biomass. In addition, it contains 48 species of other woody plants (trees, shrubs and vines) that have naturally established themselves in the forest's understory. In their study of this ecosystem, Hamilton et al. quantified the loss of foliage due to herbivory that was experienced by three deciduous tree species: sweetgum (Liquidambar styraciflua L.), red maple (Acer rubrum L.) and winged elm (Ulmus alata Michx.).
So what did they observe? As Hamilton et al. describe it, "we found that elevated CO2 led to a trend toward reduced herbivory in [the] deciduous understory in a situation that included the full complement of naturally occurring plant and insect species." In 1999, for example, they report that "elevated CO2 reduced overall herbivory by more than 40% with elm showing greater reduction than either red maple or sweetgum," while in 2000 they say they observed "the same pattern and magnitude of reduction."
With respect to changes in foliage properties that might have been expected to lead to increases in herbivory, Hamilton et al. report they "found no evidence for significant changes in leaf nitrogen, C/N ratio, sugar, starch or total leaf phenolics in either year of [the] study," noting that these findings agree with those of "another study performed at the Duke Forest FACE site that also found no effect of elevated CO2 on the chemical composition of leaves of understory trees (Finzi and Schlesinger, 2002)."
Hamilton et al. conclude their landmark paper by emphasizing that "despite the large number of studies that predict increased herbivory, particularly from leaf chewers, under elevated CO2, our study found a trend toward reduced herbivory two years in a row." In addition, they note that their real-world results "agree with the only other large-scale field experiment that quantified herbivory for a community exposed to elevated CO2 (Stilling et al., 2003)."
Consequently, and in spite of all the predictions of increased destruction of natural ecosystems by insects and other herbivorous arthropods in a CO2-enriched world of the future, just the opposite would appear to be the more likely outcome, i.e., greater plant productivity plus less foliage consumption by herbivores, "whether expressed on an absolute or a percent basis," as Hamilton et al. found to be the case in their impressive study of this most significant question.
| Sherwood, Keith and Craig Idso |
References
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Cannon, R.J. 1998. The implications of predicted climate change for insect pests in the UK, with emphasis on non-indigenous species. Global Change Biology 4: 785-796.
Coviella, C. and Trumble, J. 1999. Effects of elevated atmospheric carbon dioxide on insect-plant interactions. Conservation Biology 13: 700-712.
Curtis, P.S. and Wang, X. 1998. A meta-analysis of elevated CO2 effects on woody plant mass, form, and physiology. Oecologia 113: 299-313.
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Hamilton, J.G., DeLucia, E.H., George, K., Naidu, S.L., Finzi, A.C. and Schlesinger, W.H. 2002. Forest carbon balance under elevated CO2. Oecologia 131: 250-260.
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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.
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