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Dark Respiration of Leaves and Needles: How Is It Influenced by Elevated CO2?
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.

"Averaged across many previous investigations," in the words of the authors, "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."

What was done
Using a technique that avoids the potential artifacts of prior attempts to resolve the long-standing question of short- and long-term effects of atmospheric CO2 enrichment on leaf and needle dark respiration, 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 plus the response of seven species of plants to long-term elevation of the air's CO2 content in four different field experiments.

What was learned
The group of seven scientists report 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 response; but it was small (7% on a leaf mass basis), and it was positive, which, in their words, "contrasts sharply with the average 18% decrease reported by Wang and Curtis (2002)."

What it means
Most earlier studies of this subject have suggested that the prior apparent CO2-induced decrease in plant dark respiration would increase the potential of terrestrial vegetation to sequester carbon.  Now, however, that assumption appears to be invalid.  In fact, the long-term field studies analyzed by Davey et al. appear to suggest the likelihood of a small CO2-induced decrease in the potential of terrestrial vegetation to sequester carbon via the positive effect of atmospheric CO2 enrichment on dark respiration.  Fortunately for the biosphere, what happens in the light is much more important, as the positive effect of elevated CO2 on photosynthesis is typically vastly greater than its positive effect on dark respiration.  Consequently, the net effect of higher levels of atmospheric CO2 on earth's plants does indeed increase their ability to sequester carbon, as may be seen by perusing the Plant Growth Data (Biomass) section of our website.

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.

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.

Reviewed 16 June 2004