Nearly all of earth's plants respond favorably to increases in the air's CO2 concentration by exhibiting enhanced rates of net photosynthesis and biomass production. In many cases, observed increases in these parameters (especially biomass production) are believed to be due, in part, to CO2-induced reductions in carbon losses via respiration, which seems only logical (albeit appearances may sometimes be deceiving), for with less carbon being respired under CO2-enriched conditions, rates of net carbon assimilation and its incorporation into plant organic matter would be expected to rise, as they indeed do in most cases when the air's CO2 content is increased. In this summary, therefore, we examine what has been learned about this subject from experiments conducted on coniferous trees.
Jach and Ceulemans (2000) grew three-year old Scots pine seedlings out-of-doors and rooted in the ground in open-top chambers maintained at atmospheric CO2 concentrations of either 350 or 750 ppm for two years; and to make the experiment even more representative of the natural world, they applied no nutrients or irrigation water to the soils in which the trees grew for the duration of the study. After two years of growth under these conditions, dark respiration on a needle mass basis in the CO2-enriched seedlings was 27% and 33% lower in current-year and one-year-old needles, respectively, with the greater reduction in the older needles being thought to arise from the greater duration of elevated CO2 exposure experienced by those needles.
Hamilton et al. (2001) studied the short- and long-term respiratory responses of loblolly pines in a free-air CO2-enrichment (FACE) study that was established in 1996 on 13-year-old trees in a North Carolina (USA) plantation, where the CO2-enriched trees were exposed to an extra 200 ppm of CO2. This modest increase in the atmosphere's CO2 concentration produced no significant short-term suppression of dark respiration rates in the trees' needles. Neither did long-term exposure to elevated CO2 alter maintenance respiration, which is the amount of CO2 respired to maintain existing plant tissues. However, growth respiration, which is the amount of CO2 respired when constructing new tissues, was reduced by 21%.
McDowell et al. (1999) grew five-month-old seedlings of western hemlock in root boxes subjected to various root-space CO2 concentrations (ranging from 90 to 7000 ppm) for periods of several hours to determine the effects of soil CO2 concentration on growth, maintenance and total root respiration. In doing so, they found that although elevated CO2 had no effect on growth respiration, it significantly impacted maintenance and total respiration. At a soil CO2 concentration of 1585 ppm, for example, total and maintenance respiration rates of roots were 55% and 60% lower, respectively, than they were at 395 ppm. In fact, the impact of elevated CO2 on maintenance respiration was so strong that it exhibited an exponential decline of about 37% for every doubling of soil CO2 concentration. The implications of this observation are especially important, since maintenance respiration comprised 85% of total root respiration in this study.
The results of these experiments suggest that both above and below the soil surface, coniferous trees might exhibit reductions in total respiration in a high-CO2 world of the future. Three studies of but three species, however, is no guarantee of anything. Hence, it is important to consider the findings described in this summary within the broader context provided by the results of experiments conducted on other woody plants, as well as herbaceous plants, a large number of which are archived under the other sub-headings of Respiration in our Subject Index.
References
Hamilton, J.G., Thomas, R.B. and DeLucia, E.H. 2001. Direct and indirect effects of elevated CO2 on leaf respiration in a forest ecosystem. Plant, Cell and Environment 24: 975-982.
Jach, M.E. and Ceulemans, R. 2000. Short- versus long-term effects of elevated CO2 on night-time respiration of needles of Scots pine (Pinus sylvestris L.). Photosynthetica 38: 57-67.
McDowell, N.G., Marshall, J.D., Qi, J. and Mattson, K. 1999. Direct inhibition of maintenance respiration in western hemlock roots exposed to ambient soil carbon dioxide concentrations. Tree Physiology 19: 599-605.
Last updated 5 July 2006