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Short-Term Effect of Elevated CO2 on Respiration
Amthor, J.S. 2000. Direct effect of elevated CO2 on nocturnal in situ leaf respiration in nine temperate deciduous tree species is small. Tree Physiology 20: 139-144.

What was done
The author measured dark respiration rates for intact leaves of nine different tree species growing naturally in an American deciduous forest. Within a specially designed leaf chamber, the CO2 concentration surrounding individual leaves was stabilized at 400 ppm for 15 minutes, after which their respiration rates were measured for 30 minutes. Immediately thereafter, the CO2 concentration in the leaf chamber was raised to 800 ppm for 15 minutes, before respiration data were recorded for the same leaves at this higher CO2 concentration. Thus, the author studied the very short-term effects of elevated CO2 on dark respiration rates in leaves of deciduous trees.

What was learned
In this short-term experiment, elevated CO2 was found to have little effect on leaf dark respiration rates. In fact, the transient exposure of leaves to 800 ppm CO2 within the measurement cuvette decreased the median respiration rate by only 1.5% across all nine trees. This observation led the author to state that "rising atmospheric CO2 concentration has only a small direct effect on tree leaf respiration in deciduous forests;" and he calculated that it can be "more than eliminated by a 0.22C temperature increase." Using this premise, the author concluded that "future direct effects of increasing CO2 in combination with warming could stimulate tree leaf respiration in their sum," and that this consequence "would translate into only slight, if any, effects on the carbon balance of temperate deciduous forests in a future atmosphere containing as much as [800 ppm] CO2."

What it doesn't mean
The author's final conclusion is without merit, for it is based upon a weak foundation derived from misleading data. To extrapolate the short-term respiratory response of individual leaves, exposed to elevated CO2 for only an hour or two, to that of entire trees, which will realistically experience rising CO2 levels for over a century or more during their lifetimes, is short-sighted in the extreme.

Trees are long-lived perennial species that do not necessarily reveal the ultimate nature of their responses to elevated CO2 concentrations on such short temporal time scales as the one employed in this paper. Indeed, it is very likely that their initial responses would change over time, as they acclimate and optimize their physiology and growth patterns to elevated CO2 concentrations. Additionally, Drake et al. (1999) recently reviewed the literature in this field and reported a 17% reduction in respiration in response to a doubling of the air's CO2 content. This observation led them to conclude that the ongoing rise in the air's CO2 concentration "will enhance the quantity of carbon stored by forests," increasing the terrestrial carbon sink by an additional 6 to 7 gigatons.

Clearly, the gap between the author's short-term data and his far-reaching conclusion is much too wide to accept his statements about earth's deciduous forests and their ability to influence the global carbon cycle.

Drake, B.G., Azcon-Bieto, J., Berry, J., Bunce, J., Dijkstra, P., Farrar, J., Gifford, R.M., Gonzalez-Meler, M.A., Koch, G., Lambers, H., Siedow, J. and Wullschleger, S. 1999. Does elevated atmospheric CO2 inhibit mitochondrial respiration in green plants? Plant, Cell and Environment 22: 649-657.

Reviewed 15 March 2000