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Respiration (Response to CO2 - Woody Plants: Multiple Tree Studies) -- Summary
Nearly all of earth's plants respond favorably to increases in the air's CO2 concentration by increasing their net carbon assimilation and biomass production rates. Is part of this positive response due to CO2-induced reductions in respiratory carbon losses? In this summary, we review what has been learned about this question from several analyses of multi-species data sets.

Amthor (2000) measured dark respiration rates of 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, whereupon their respiration rates were measured for 30 minutes, after which the CO2 concentration in the leaf chamber was raised to 800 ppm for 15 minutes and respiration data were again recorded for the same leaves. This protocol revealed that elevated CO2 had little effect on leaf dark respiration rates. In fact, the extra 400 ppm of CO2 within the measurement cuvette decreased the median respiration rate by only 1.5% across the nine tree species. This observation thus led Amthor to state that the "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." Upon this premise, he thus 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."

Amthor's conclusion, however, is debatable, for it is based upon the extrapolation of the short-term respiratory responses of individual leaves, exposed to elevated CO2 for only an hour or two, to that of entire trees, many of which will experience rising CO2 levels for over a century or more during their lifetimes. Trees are long-lived organisms that should not be expected to reveal the ultimate nature of their long-term responses to elevated atmospheric CO2 concentrations on as short a time scale as that employed by Amthor. Indeed, their initial respiratory responses may change significantly with the passage of time, as they acclimate and optimize their physiology and growth patterns to the gradually-rising CO2 content of earth's atmosphere, as evidenced by the findings of the following two studies.

Wang and Curtis (2002) conducted a meta-analysis of the results of 45 area-based dark respiration (Rda) and 44 mass-based dark respiration (Rdm) assessments of the effects of an approximate doubling of the air's CO2 concentration on 33 species of plants (both herbaceous and woody) derived from 37 scientific publications. This effort revealed that the mean leaf Rda of the woody plants they analyzed was unaffected by elevated CO2. However, there was an effect on mean leaf Rdm, and it was determined to be time-dependent. The woody plants exposed to elevated CO2 for < 100 days, in the reviewing scientists' words, "showed significantly less of a reduction in leaf Rdm due to CO2 enrichment (-12%) than did plants exposed for longer periods (-35%, P < 0.01)." Hence, for conditions of continuous long-term atmospheric CO2 enrichment, the results of Wang and Curtis' analysis suggest that woody plants may well experience an approximate 35% decrease in leaf Rdm.

Drake et al. (1999) also conducted a comprehensive analysis of the peer-reviewed scientific literature to determine the effects of elevated atmospheric CO2 concentrations on plant respiration rates. In doing so, they found that atmospheric CO2 enrichment typically decreased respiration rates in mature foliage, stems, and roots of CO2-enriched plants relative to rates measured in plants grown in ambient air; and when normalized on a biomass basis, they determined that a doubling of the atmosphere's CO2 concentration would likely reduce plant respiration rates by an average of 18%. To determine the potential effects of this phenomenon on annual global carbon cycling, which the twelve researchers say "will enhance the quantity of carbon stored by forests," they input a 15% CO2-induced respiration reduction into a carbon sequestration model, finding that an additional 6 to 7 Gt of carbon would remain sequestered within the terrestrial biosphere each year, thus substantially strengthening the terrestrial carbon sink.

More recently, however, Davey et al. (2004) have reversed our view of the subject yet again. "Averaged across many previous investigations," as they put it, "doubling the CO2 concentration has frequently been reported to cause an instantaneous reduction of leaf dark respiration measured as CO2 efflux." However, as they continue, "no known mechanism accounts for this effect, and four recent studies [Amthor (2000); Anthor et al. (2001); Jahnke (2001); Jahnke and Krewitt (2002)] have shown that the measurement of respiratory CO2 efflux is prone to experimental artifacts that could account for the reported response."

Using a technique that avoids the potential artifacts of prior attempts to resolve the issue, Davey et al. employed a high-resolution dual channel oxygen analyzer in an open gas exchange system to measure the respiratory O2 uptake of nine different species of plants in response to a short-term increase in atmospheric CO2 concentration, as well as the response of seven species to long-term elevation of the air's CO2 content in four different field experiments. In doing so, they found that "over six hundred separate measurements of respiration failed to reveal any decrease in respiratory O2 uptake with an instantaneous increase in CO2." Neither could they detect any response to a five-fold increase in the air's CO2 concentration nor to the total removal of CO2 from the air. They also note that "this lack of response of respiration to elevated CO2 was independent of treatment method, developmental stage, beginning or end of night, and the CO2 concentration at which the plants had been grown." In the long-term field studies, however, there was a respiratory response; but it was small (7% on a leaf mass basis), and it was positive, not negative.

So what is one to ultimately conclude about the matter? Perhaps the most reasonable conclusion would be that atmospheric CO2 enrichment may either increase or decrease woody-plant respiration, but not to any great degree, and that in the mean, the net result for the conglomerate of earth's trees would likely be something of little impact, one way or the other.

References
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.

Amthor, J.S., Koch, G.W., Willms, J.R. and Layzell, D.B. 2001. Leaf O2 uptake in the dark is independent of coincident CO2 partial pressure. Journal of Experimental Botany 52: 2235-2238.

Davey, P.A., Hunt, S., Hymus, G.J., DeLucia, E.H., Drake, B.G., Karnosky, D.F. and Long, S.P. 2004. Respiratory oxygen uptake is not decreased by an instantaneous elevation of [CO2], but is increased with long-term growth in the field at elevated [CO2]. Plant Physiology 134: 520-527.

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.

Jahnke, S. 2001. Atmospheric CO2 concentration does not directly affect leaf respiration in bean or poplar. Plant, Cell and Environment 24: 1139-1151.

Jahnke, S. and Krewitt, M. 2002. Atmospheric CO2 concentration may directly affect leaf respiration measurement in tobacco, but not respiration itself. Plant, Cell and Environment 25: 641-651.

Wang, X. and Curtis, P. 2002. A meta-analytical test of elevated CO2 effects on plant respiration. Plant Ecology 161: 251-261.

Last updated 2 August 2006