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Elevated Carbon Dioxide: Does It Boost Forest Growth?
Volume 8, Number 37: 14 September 2005

The title of a brief review posted on the website of Scientific American (sciam.com) on 26 August 2005 says "Researchers Find That Carbon Dioxide Does Not Boost Forest Growth."  The claim is made again at the end of the review's first paragraph: "A four-year study of a forest in Switzerland indicates that additional carbon dioxide does not boost tree growth."  And with that announcement, many radical environmentalists are claiming the case is closed, asking us to turn our backs on literally hundreds of other studies that suggest otherwise.

So what's the problem with the contrary experiment of Korner et al. (2005)?  Why are its findings so different from those of almost every other study that has addressed the subject?  There is probably no single answer to this question that completely resolves the issue; but the study is possessed of enough serious shortcomings to demonstrate the invalidity of the political use that is being made of its conclusions by those who seek to demonize the burning of fossil fuels and the CO2 emissions that arise from that activity.

We begin with a consideration of the CO2-enrichment method employed by Korner et al.  It is a variant of the free-air CO2 enrichment (FACE) technique and was developed for the specific purpose of studying trees that were deemed to be too tall for the more well-established approach.  Dubbed web-FACE, it has been described in detail by Pepin and Korner (2002) within the very context of the Korner et al. study, i.e., the one-and-only setting in which it has ever been employed, which raises the logical concern that it is not yet a "tried and true" method of appropriately enriching the air with carbon dioxide.

The technique, as applied by Korner et al. to twelve 32- to 35-m-tall mature deciduous hardwood trees in a temperate forest in Switzerland, utilizes a system of thin plastic tubes woven into the trees' crowns, to which access is provided by a 45-m tower crane located in the center of the 12-tree segment of the forest.  Between 300 and 1000 m of tubing wind about the branches of each tree's crown (depending on tree size), through which pure CO2 is pumped and released to the air via small (0.5 mm diameter) laser-drilled holes spaced at 30-cm intervals along the lines of tubing, which CO2 release system is operated by a computer-controlled CO2 monitoring system.

Just how good, technically speaking, is the web-FACE system, especially as it was employed in the Korner et al. study?  "Based on performance standards that use CO2 averages over 1 min," in the words of Pepin and Korner, "the CO2 control in web-FACE was less accurate than in conventional forest FACE, with 1-min means being within ▒ 10% of target for 47% of the exposure time (compared with 69% for Duke forest FACE)."  However, as they continue, "76% of the 1-min averages were within ▒ 20% of the target concentration, which is very close to the '80% of the time' defined as an acceptable FACE performance by the scientific community (see Hendrey et al., 1999; Miglietta et al., 2001)."  Nevertheless, the web-FACE system used in the Korner et al. study, by their own admission, fell short of that performance standard, which suggests that its performance was unacceptable.  Their conclusion on this point is that "the web technology is definitely a most promising avenue towards the exposure of tall forest canopies to long-term CO2-enrichment," with which we agree.  However, that promise was not fulfilled in their specific study, which is not at all surprising, seeing that their web-FACE experiment was the first of its kind ever to be conducted.

Even when acceptable FACE performance standards are achieved, however, there is an additional question about this approach to atmospheric CO2 enrichment that needs to be answered.  For a long time, FACE studies of the effects of elevated CO2 on the growth and development of plants were considered to be the most realistic experimental route to determining the biological consequences of the ongoing rise in the air's CO2 content, as it was assumed that it provided minimal alterations to the natural environment of the plants being studied.  Recently, however, this assumption has been questioned, as concerns have been raised about the physiological effects of the rapidly fluctuating CO2 concentrations that occur in response to the over- and under-shooting of target CO2 concentrations as the FACE apparatus continually adjusts to counteract the concentration-perturbing effects of variations in wind speed and direction.

In a study of this potential problem, Holtum and Winter (2003) "tested whether the responses of net CO2 exchange by seedlings or leaves of two tropical tree species, teak (Tectona grandis L. f.) and Pseudobombax septenatum (Jacq.) Dug., to an increase in CO2 concentration from ca. 370 to 600 ppm CO2 are affected by symmetric oscillations around 600 ppm, with half-cycles of considerably less than 1 minute."  Interestingly, they found that in air of constant 600 ppm CO2 concentration, the net CO2 uptake rates of shoots and leaves of seedlings of T. grandis and P. septenatum rose by approximately 28 and 52%, respectively; but in the presence of CO2 oscillations with a half-cycle of 20 seconds and amplitude of 170 ppm about a mean of 600 ppm, "the CO2 stimulation in photosynthesis associated with a change in exposure from 370 to 600 ppm CO2 was reduced by a third in both species."

To the problems associated with the technological issues discussed above must be added a set of problems associated with other less-than-ideal aspects of the Korner et al. study.  In the operational category, we note that "CO2 release occurred during daytime hours only," as reported by Pepin and Korner.  This huge departure from the environmental reality that is expected to exist in the future eliminates any and all effects the elevated CO2 might otherwise have had on the dark-period activities of the trees.  In addition, daytime in their case was defined as the period when a canopy-height quantum sensor measured photon flux densities that were greater than 75 Ámol m-2 s-1.  Their use of this non-negligible cutoff value likely ensured that most of the leaves in the CO2-enriched trees were unable to take advantage of the lowered light compensation point they might otherwise have experienced, which could have enabled them to switch from negative to positive values of net photosynthesis earlier each morning and from positive to negative values later each day; and the resultant CO2-induced lengthening of the period of positive net photosynthesis may well have significantly boosted their daily uptake of CO2 relative to that of the trees growing in ambient air.

While exploring this phenomenon in an open-top chamber study of 30-year-old oak trees, for example, Marek et al. (2001) found the trees' light compensation points to be reduced by 24 and 30% in sun and shade leaves, respectively, when the trees were exposed to a doubling of the air's CO2 content.  Likewise, Hattenschwiler (2001) grew the temperate forest species Abies alba, Acer pseudoplatanus, Fagus sylvatica, Quercus robur and Taxus baccata in open-top chambers maintained at atmospheric CO2 concentrations of 360, 500 and 660 ppm for two growing seasons while exposed to light intensities of only 3.4 and 1.3% of full sunlight.  Even at these extremely low light intensities, all five species exhibited significant CO2-induced increases in total biomass.  At 660 ppm CO2, for example, the total biomass increases recorded for Fagus, Taxus, Acer, Quercus and Abies at 3.4% full sunlight were 17, 16, 50, 31 and 62%, respectively, while at 1.3% of full sunlight Fagus seedlings exhibited a 74% increase in total biomass at both 500 and 660 ppm CO2, while Taxus seedlings displayed 34 and 41% biomass increases at 500 and 660 ppm CO2, respectively.

Also of relevance to the failure of Korner et al. to maintain CO2 enrichment throughout the entire daylight period is the study of Naumburg and Ellsworth (2000), who measured photosynthetic rates in leaves of four hardwood saplings growing beneath the canopy of a Pinus taeda forest, several portions of which were exposed to either ambient or ambient + 200 ppm CO2 concentrations in a two-year FACE study.  Their work revealed that in going from lighted to shaded conditions, elevated CO2 extended the time during which maximal rates of photosynthesis were maintained, which benefit would be significantly reduced if CO2 enrichment were not employed throughout the entire daylight period.  Also, and in the same experimental setting, Naumburg et al. (2001) found that the extra 200 ppm of CO2 enhanced daily photosynthetic carbon uptake in leaves exposed to less than 3% of full sunlight by more than two-fold in three of the four species studied.  They thus concluded that "elevated CO2 could have profound impacts on individual species' performance in marginal understory sites" and that "rising CO2 may benefit plants growing in poor light microsites relatively more than at better microsites," which also means that atmospheric CO2 enrichment will do the same at the two times of day when similar low-light conditions are experienced, i.e., just after sunrise and some time in advance of sunset.

Another operational feature that may have severely hampered Korner et al. was the low level of CO2 enrichment they employed.  Whereas most non-FACE studies generally work with CO2 increases on the order of 300 ppm or more, and whereas many FACE studies employ a differential of 200 ppm, Korner et al. say their initial enrichment was only "about 180-190 ppm above ambient."  Adding to the problem, monetary constraints soon forced them to reduce this already-meager concentration differential by another 50 ppm, which brought their degree of CO2 enrichment down to approximately 135 ppm.  Combining this low enrichment level with the small number of CO2-enriched trees of each species studied - Fagus sylvatica (3), Quercus petraea (3), Carpinus betulus (3), Tilia platyphylla (1), Acer campestre (1) and Prunus avium (1) - greatly increased the difficulty of detecting any statistically significant CO2 effects, which were only observed in two of four years in one species (Fagus sylvatica).

In light of the many technological problems faced by Korner et al., plus the several less-than-ideal operational procedures they employed, it is little wonder they were unable to discern any enduring positive effects of atmospheric CO2 enrichment on the growth of the trees they studied.  Clearly, therefore, their findings are not applicable to forests in general, nor can they be considered to be definitive for the species they studied.  In fact, they likely do not reveal the truth about even the specific trees with which they worked.

Yet even if everything had functioned perfectly and absolutely no corners had been cut, it is likely that the long-term equilibrium responses of the trees Korner et al. studied would still be unknown; for the sour orange study of Idso and Kimball has revealed that considerably more than a decade may be required to obtain this knowledge, while the wetland sedge study of Rasse et al. (2005) has revealed that fully two decades may be needed.  To claim that a single four-year study of any tree or group of woody species, no matter how perfectly it may have been conducted, reveals how all of earth's forests will ultimately respond to the ongoing rise in the air's CO2 concentration - especially if the study's findings run contrary to the findings of many equally long or even longer studies - is naive at best and disingenuous at worst.  In fact, it is almost insane.

Sherwood, Keith and Craig Idso

References
Hattenschwiler, S.  2001.  Tree seedling growth in natural deep shade: functional traits related to interspecific variation in response to elevated CO2Oecologia 129: 31-42.

Hendrey, G.R., Ellsworth, D.S., Lewin, K.F. and Nagy, J.  1999.  A free-air enrichment system for exposing tall forest vegetation to elevated atmospheric CO2Global Change Biology 5: 293-309.

Holtum, J.A.M. and Winter, K.  2003.  Photosynthetic CO2 uptake in seedlings of two tropical tree species exposed to oscillating elevated concentrations of CO2Planta 218: 152-158.

Korner, C., Asshoff, R., Bignucolo, O., Hattenschwiler, S., Keel, S.G., Pelaez-Riedl, S., Pepin, S., Siegwolf, R.T.W. and Zotz, G.  2005.  Carbon flux and growth in mature deciduous forest trees exposed to elevated CO2Science 309: 1360-1362.

Marek, M.V., Sprtova, M., De Angelis, P. and Scarascia-Mugnozza, G.  2001.  Spatial distribution of photosynthetic response to long-term influence of elevated CO2 in a Mediterranean macchia mini-ecosystem.  Plant Science 160: 1125-1136.

Miglietta, F., Peressotti, A., Vaccari, F.P., Zaldei, A., deAngelis, P. and Scarascia-Mugnozza, G.  2001.  Free-air CO2 enrichment (FACE) of a poplar plantation: the POPFACE fumigation system.  New Phytologist 150: 465-476.

Naumburg, E. and Ellsworth, D.S.  2000.  Photosynthetic sunfleck utilization potential of understory saplings growing under elevated CO2 in FACE.  Oecologia 122: 163-174.

Naumburg, E., Ellsworth, D.S. and Katul, G.G.  2001.  Modeling dynamic understory photosynthesis of contrasting species in ambient and elevated carbon dioxide.  Oecologia 126: 487-499.

Pepin, S. and Korner, C.  2002.  Web-FACE: a new canopy free-air CO2 enrichment system for tall trees in mature forests.  Oecologia 133: 1-9.

Rasse, D.P., Peresta, G. and Drake, B.G.  2005.  Seventeen years of elevated CO2 exposure in a Chesapeake Bay Wetland: sustained but contrasting responses of plant growth and CO2 uptake.  Global Change Biology 11: 369-377.