The impression one gets from reading these reports, as evidenced by the very titles of some of them ("Tree-planting no defense against global warming" (6) and "Trees no savior for global warming" (7)) is that planting trees and allowing them to grow does extremely little in terms of removing CO2 from the atmosphere and sequestering its carbon in long-lived plant tissues and soils.  Nothing, however, could be further from the truth.

Almost all of us have seen with our own eyes, and can thus vouch for the fact, that little seedlings typically grow into much larger trees; and almost half of the dry mass of those trees comes from the carbon of the CO2 they extract from the air, as is explicitly indicated in Reference 1.  Hence, if it's a plant and it's growing, you can be assured it's removing CO2 from the atmosphere and tucking away much of that CO2's carbon in its tissues, as well as a portion of it in the soil in which the plant is rooted.

What the two Nature studies describe is something quite different.  By means of FACE technology, they investigate the degree to which extra CO2 in the air enables trees to produce extra biomass that removes an additional amount of CO2 from the atmosphere above and beyond the large and visibly-obvious amount trees are currently removing from the air.  Hence, to say, as another report does, that "two new studies are challenging the idea that planting forests could be a cheap way to absorb emissions of carbon dioxide" (8), is to miss -- or misconstrue -- the more limited message of the findings of the new papers, which only address the question of the incremental enhancement of carbon sequestration caused by an incremental enrichment of the atmosphere's CO2 concentration.  Even without any CO2-induced enhancement of their growth rates, for example, the current productivity of earth's trees is clearly large enough that "planting forests" can have (and indeed does have) a tremendous impact on the air's CO2 concentration.

Yet even within this more limited domain of applicability, there are important questions that must be addressed before the findings of the commotion-causing studies can be considered to be robust.  In the Oren et al. (1) report, for example, the woody biomass of the trees' trunks, branches and roots was calculated from trunk-diameter-based allometric equations derived for the particular loblolly pine trees of the forest ecosystem they studied, which is a valid approach to take.  But the equations they employed were derived from data obtained only on trees growing in ambient air, which could well be a serious misstep; for the same equations may not apply to trees growing in CO2-enriched air.

In an analogous open-top-chamber study (but of well-watered and fertilized sour orange trees growing in air of either 400 or 700 ppm CO2) Idso and Kimball (9) determined that a single trunk-diameter-based equation did indeed accurately describe the above-ground woody biomass (actually biovolume) of the trees of both CO2 treatments.  But it is by no means certain that the same would be true for loblolly pine trees, especially if the trees were rooted in soil of low nutrient content that is sometimes saturated with water and at other times quite deficient in this essential resource (1,2).

Then there's the question of root biomass.  In the case of the well-watered and fertilized sour orange trees studied by Idso and Kimball (10), the percentage growth stimulation of the CO2-enriched trees was essentially identical both above- and below-ground, which would indeed imply that the one-equation-fits-all-plant-parts approach employed by Oren et al. (1) for calculating the whole-plant (trunk, branch and root) biomass of CO2-enriched trees would indeed be appropriate for well-watered and fertilized sour orange trees.  But, again, it may not be true for loblolly pine trees, especially when growing in soil of low nitrogen content, which generally leads to greater CO2-induced increases in below-ground biomass than in above-ground biomass (11).

In a study of the effects of a 300 ppm increase in atmospheric CO2 concentration on yellow poplar trees growing in a soil that was significantly deficient in nitrogen, for example, Norby et al. (12) could detect no significant biomass increases in any above-ground plant parts after 2.7 growing seasons, which is an even more extreme finding than that reported by Oren et al. (1) for the latter part of their experiment.  Nevertheless, Norby et al. did detect a 37% increase in tap root biomass and a 119% increase in fine root biomass.  And in the very same ecosystem studied by Oren et al. (1), Matamala and Schlesinger (13) documented an 86% increase in loblolly pine tree fine-root biomass in response to but a 200 ppm increase in atmospheric CO2 concentration, which would roughly correspond to a 129% biomass increase for a 300 ppm increase in atmospheric CO2 concentration, such as that employed in the experiment of Norby et al. (12).  Hence, it is very possible, if not highly probable, that both the fine and the larger-than-fine roots of the CO2-enriched trees studied by Oren et al. (1) did indeed experience significantly greater increases in biomass than did the trees' trunks and branches, contrary to what the researchers assumed in their paper.

Another concern about the Oren et al. analysis is that the woody biomass values they reported were derived from their calculated biovolume results by multiplying the latter numbers by experimentally-derived wood density (mass per volume) values, which they indicate were nearly 8% less in the CO2-enriched trees than in the ambient-treatment trees.  But in studies of the very same species (Pinus taeda L.), two groups of researchers found no detectable differences in the wood densities of ambient and CO2-enriched trees (14,15), while two other groups actually reported increases that were as high as 15% in the wood density of the CO2-inriched trees (16,17).  Likewise, in studies of another pine species (Pinus radiata D. Don.), one research group again found no differences in the densities of the wood of ambient-grown and CO2-enriched trees (18), while another group found the wood density of the CO2-enriched trees to be 5 to 6% greater (19).  And in studies of still other species of trees, researchers have found either no wood density differences (16,20) or increases of 4% (21), 6% (21), 7% (22), 8% (23), 13% (20) and 33% (14).  Hence, Oren et al.'s "lone report" of a CO2-induced decrease in wood density seems highly suspect.

What are the potential consequences of these observations?  Merely assuming atmospheric CO2 enrichment to have had no effect on the wood density of the loblolly pine trees studied by Oren et al. would raise their reported 6 to 7% CO2-induced increases in woody biomass over the last few years of their study to 15 to 16% increases.  Assuming a conservative 5% increase in the wood density of the CO2-enriched trees would further elevate the CO2-induced biomass increases to 21 to 22%, which for the more-commonly-employed experimental CO2 increase of 300 ppm would roughly correspond to woody biomass increases of 32 to 33%.

Additionally, these CO2-induced biomass increases should probably be raised even higher, in light of the likelihood that the CO2-enriched trees of the Oren et al. study produced significantly more root tissue than what their ambient-treatment-derived allometric equations predicted, due to the low soil fertility of their experimental site, which generally favors greater below-ground growth than above-ground growth under conditions of atmospheric CO2 enrichment.  In the study of Jach et al. (24), for example, another species of pine (Pinus sylvestris L.) growing on a nutrient poor soil had its root biomass increased by fully three times more than its trunk biomass as a consequence of atmospheric CO2 enrichment experienced over a period of three years.

The soil carbon data of the study of Schlesinger and Lichter (2) may also be interpreted in a significantly different manner from that employed by its authors.  The total carbon content of the uppermost 30 cm of the soil profile (which according to the authors includes "nearly all of the root biomass") can be determined from their data to have been 15.5% greater in the CO2-enriched plots than in the ambient control plots after the first three years of their experiment.  Although this result was noted by the authors to not be significant in a strict statistical sense, the difference is nevertheless substantial.  Adjusted to a 300 ppm enrichment of the air's CO2 content, for example, it amounts to an increase of roughly 23%.

Much more important, of course, are the changes in soil carbon content experienced in the ambient and CO2-enriched plots since the start of the differential CO2 treatments.  In fact, this is the only true measure of the impact of the experimental elevation of the air's CO2 content upon the soil's ability to sequester carbon.  The only reported data that can be used to investigate this phenomenon, however, are the percent carbon (%C) values for the top 15 cm of the soil profile, which at the start of the study were determined to be 1.432% in the control plots and 1.542% in the CO2-enriched plots, with the difference between them being claimed by Schlesinger and Lichter (2) to be "not significantly different."  After three years, however, the %C in the control plots was reduced to 1.31%, while the %C in the CO2-enriched plots was increased to 1.59%; and in this case the difference between the latter two values was determined to be statistically significant.

Taking these four numbers at their face values, we can calculate the relative changes in the %C of the top 15 cm of soil in the ambient and CO2-enriched plots over the first three years of the experiment.  For the ambient plots, the result is a relative decline of 8.5%; while for the CO2-enriched plots, the result is a relative increase of 3.1%.  Since the initial %C values of the two treatments were deemed to be not significantly different from each other, however, we could also have calculated the relative changes over the course of the experiment from the average of the two pre-treatment %C values, which is 1.487%.  Redoing the math then gives a relative ambient-treatment %C decline of 11.9% and a relative CO2-enriched %C increase of 6.9%.

Viewed in this light, the importance of atmospheric CO2 enrichment to soil carbon sequestration is immediately obvious.  Under the site-specific conditions of the study in question, the soils of the forest plots growing in ambient air were actually losing carbon, i.e., they were carbon sources; while the soils of the plots exposed to the extra 200 ppm of CO2 were gaining carbon, i.e., they were carbon sinks.

So what's the bottom line?  In terms of trees, just a little less than half of the biomass that comprises their tissues is carbon that was acquired from the air.  Hence, trees are indeed substantial carbon sinks; and the more of them there are, and the faster they grow, the more CO2 they remove from the atmosphere.  In terms of carbon sequestration in soils, there are many complex and competing factors that come together to determine what is occurring in this underground realm.  In general, however, the greater the productivity of the forest, the greater will be the input of organic matter to the soil and the greater will be the potential for carbon sequestration therein.

Superimposed upon these general "rules of thumb" are the effects of atmospheric CO2 enrichment, which typically enhances the carbon sequestering prowess of both trees and soils.  And in the case of soils, as is evident from the data of Schlesinger and Lichter (2), extra atmospheric CO2 can sometimes turn a soil that is losing carbon into a soil that is gaining carbon.

As to why the press stories generated by the Nature papers were so negative, and why the scientists who produced the papers wrote so disparagingly about the potential for forests to sequester carbon, is anybody's guess.  There was, of course, great pressure upon the United States to forsake this approach to reducing the rate-of-rise of the air's CO2 content during the last round of Kyoto Protocol negotiations; and those sentiments still prevail among many national governments (particularly in Europe), as well as within both pseudo and serious environmental organizations worldwide.  Consequently, the papers in question appeared within a highly-charged political context that may not have been conducive to a dispassionate discussion of the data they contained.  Even if the world was not running hot, political passions were; and they may well have gotten in the way of rational thinking.

2. Schlesinger, W.H. and Lichter, J.  2001.  Limited carbon storage in soil and litter of experimental forest plots under increased atmospheric CO2.  Nature 411: 466-469.

3. Kirby, A.  23 May 2001.  Carbon sinks may not help much.  BBC News.

4. Verrengia, J.B.  23 May 2001.  Global warming carbon experiments.  Associated Press.

5. Reaney, P.  23 May 2001.  Scientists query future power of carbon sinks.  Reuters.

6. Anonymous.  23 May 2001.  Tree-planting no defense against global warming: studies.  AFP.

7. Spotts, P.N.  25 May 2001.  Trees no savior for global warming.  The Christian Science Monitor.

8. Revkin, A.C.  24 May 2001.  Studies challenge role of trees in curbing greenhouse gases.  The New York Times.

9. Idso, S.B. and Kimball, B.A.  1992.  Aboveground inventory of sour orange trees exposed to different atmospheric CO2 concentrations for 3 full years.  Agricultural and Forest Meteorology 60: 145-151.

10. Idso, S.B. and Kimball, B.A.  1991.  Effects of two and a half years of atmospheric CO2 enrichment on the root density distribution of three-year-old sour orange trees.  Agricultural and Forest Meteorology 55: 345-349.

11. Waring, R.H. and Schlesinger, W.H.  1985.  Forest Ecosystems: Concepts and Management.  Academic Press, Orlando, FL.

12. Norby, R.J., Gunderson, C.A., Wullschleger, S.D., O'Neill, E.G. and McCracken, M.K.  1992.  Productivity and compensatory responses of yellow-poplar trees in elevated CO2.  Nature 357: 322-324.

13. Matamala, R. and Schlesinger, W.H.  2000.  Effects of elevated atmospheric CO2 on fine root production and activity in an intact temperate forest ecosystem.  Global Change Biology 6: 967-979.

14. Rogers, H.H., Bingham, G.E., Cure, J.D., Smith, J.M. and Surano, K.A.  1983.  Response of selected plant species to elevated carbon dioxide in the field.  Journal of Environmental Quality 12: 569-574.

15. Telewski, F.W. and Strain, B.R.  1987.  Densitometric and ring width analysis of 3-year-old Pinus taeda L. and Liquidambar styraciflua L. grown under three levels of CO2 and two water regimes.  In: Jacoby, G.C. and Hornbeck, J.W. (Eds.) Proceedings of the International Symposium on Ecological Aspects of Tree Ring Analysis.  U.S. Dept. of Energy DOE/CONF 8608144, pp. 494-500.

16. Doyle, T.W.  1987.  Seedling response to CO2 enrichment under stressed and non-stressed conditions.  In: Jacoby, G.C. and Hornbeck, J.S. (Eds.) Proceedings of the International Symposium on Ecological Aspects of Tree Ring Analysis.  U.S. Dept. of Energy DOE/CONF 8608144. National Technical Information Service, Springfield, VA, pp. 501-506.

17. Telewski, F.W., Swanson, R.T., Strain, B.R. and Burns, J.M.  1999.  Wood properties and ring width responses to long-term atmospheric CO2 enrichment in field-grown loblolly pine (Pinus taeda L.).  Plant, Cell and Environment 22: 213-219.

18. Donaldson, L.A., Hollinger, D., Middleton, T.M. and Souter, E.D.  1987.  Effect of CO2 enrichment on wood structure in Pinus radiata D. Don. International Association of Wood Anatomists Bulletin, New Series 8: 285-295.

19. Conroy, J.P., Milham, P.J., Mazur, M. and Barlow, E.W.R.  1990.  Growth, dry matter partitioning and wood properties of Pinus radiata D. Don. after 2 years of CO2 enrichment.  Plant, Cell and Environment 13: 329-337.

20. Cuelmans, R.  1998.  Responses of trees to climate change.  In: Peter, D., Maracchi, G. and Ghazi, A. (Eds.) Course on Climate Change Impact on Agriculture and Forestry, Office for Official Publications of the EC, Luxembourg, pp. 507-517.

21. Tognetti, R., Johnson, J.D., Michelozzi, M. and Raschi, A.  1998.  Response of foliar metabolism in mature trees of Quercus pubescens and Quercus ilex to long-term elevated CO2.  Environmental and Experimental Botany 39: 233-245.

22. Norby, R.J., Wullschleger, S.D. and Gunderson, C.A.  1996.  Tree responses to elevated CO2 and implications for forests.  In: Koch, G.W. and Mooney, H.A. (Eds.) Carbon Dioxide and Terrestrial Ecosystems. Academic Press, New York, NY, pp. 1-21.

23. Hattenschwiler, S., Schweingruber, F.H. and Korner, C.  1996.  Tree ring responses to elevated CO2 and increased N deposition in Picea abies.  Plant, Cell and Environment 19: 1369-1378.

24. Jach, M.E., Laureysens, I. and Ceulmans, R.  2000.  Above- and below-ground production of young Scots pine (Pinus sylvestris L.) trees after three years of growth in the field under elevated CO2.  Annals of Botany 85: 789-798.