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Long-Term Studies (Woody Plants - Oak Trees) -- Summary
How do oak trees respond to long-term increases in the air's CO2 concentration?

After burning to the ground a natural scrub-oak ecosystem comprised of Quercus myrtifolia, Q. chapmanii and Q. geminata, which was located on an island just off the coast of central Florida, USA, Ainsworth et al. (2002) set about trying to answer this question by erecting sixteen open-top chambers on the site and fumigating them with air of either 380 or 700 ppm CO2.  In the third and fourth years of this experiment, they found that the extra CO2 increased photosynthetic rates in regenerating Q. myrtifolia and Q. chapmanii trees by as much as 150% without inducing any degree of photosynthetic acclimation or down regulation.  Q. geminata, on the other hand, did exhibit signs of acclimation; but after three years of exposure to elevated CO2, the three species still exhibited an average increase of 53% in their combined mean rate of photosynthesis.

In a subsequent analysis of other data obtained from the same experiment, Dijkstra et al. (2002) evaluated the effects of elevated CO2 on the growth of the three oak species by means of allometric relationships between stem diameter and aboveground biomass (AGB), which they derived from destructive measurements made on trees growing on an adjacent site.  In doing so, they found, in their words, that "increased AGB in elevated CO2 was apparent after eight months (44%), and the relative stimulation increased over time, from 55% at the end of 1997, 66% at the end of 1998, to 75% at the end of 1999."  They also report that at the time of the last measurement, the AGB of the dominant Q. myrtifolia had increased by 73%, while the AGB of the subdominant Q. geminata had increased by only 23%, but that the AGB of the subdominant Q. chapmanni had risen in excess of 150%.  With respect to individual years, they note that in spite of the fact that the mean increase in AGB during the drought year of 1998 was 51% lower than it was during 1997 and 54% lower than it was in 1999, "elevated CO2 significantly increased annual increment in AGB by 122% during the drought year 1998, compared to a 65% increase in 1997 and a 116% increase in 1999."

In yet another study of the Florida scrub-oak ecosystem, Hymus et al. (2003) report that throughout the experiment the extra CO2 supplied to the CO2-enriched chambers increased maximum net ecosystem exchange of CO2 (NEE) and the apparent quantum yield of NEE during the photoperiod, noting further that the magnitude of the stimulation of maximum NEE, expressed per unit ground area, "was seasonal, rising from 50% in the winter to 180% in the summer," in accord with what is known about the interactive effects of atmospheric CO2 enrichment and daily, seasonal and multi-year warming (see Growth Response to CO2 With Other Variables (Temperature) in our Subject Index).  Hymus et al. additionally note that their study was the largest to show "the effects of elevated CO2 on NEE measured in situ, and is the first to be carried out in a woody ecosystem," where the beneficial effects of atmospheric CO2 enrichment are "still evident after 6 years regeneration in the elevated CO2."

Another way of studying the long-term effects of atmospheric CO2 enrichment on trees and shrubs has been pioneered by researchers in Italy, where many natural springs emit copious quantities of CO2 into the air, raising atmospheric CO2 concentrations over modest tracts of land by various amounts.  By measuring the air's CO2 content at different places around these "CO2 springs" over the course of long-term experiments conducted there, mean canopy-level atmospheric CO2 concentrations can be determined; and woody plants growing at those locations are typically assumed to have lived their entire lives at the measured CO2 concentrations.

In a study of Quercus ilex trees, some growing close to, and others distant from, certain of these CO2 springs, Paoletti et al. (1998) found that in moving from an atmospheric CO2 concentration of 350 ppm to 750 ppm, leaf stomatal frequency dropped by a factor of nearly 1.5, but that there were no further reductions in this parameter as the air's CO2 concentration rose as high as 2600 ppm.  However, they also determined that the amount of wax comprising the leaf cuticle was increased by nearly three-fold between 750 and 2600 ppm CO2, but that between 350 and 750 ppm CO2 there was no difference in this leaf property.  The net effect of these several responses was thus a continuous decline in water loss from the trees as the air's CO2 content rose ever higher, which led to a concomitant continuous increase in their water use efficiencies.

In another study conducted in Italy in the vicinity of natural CO2 springs, Stylinski et al. (2000) worked with Quercus pubescens trees that had been grown in ambient air and at an atmospheric CO2 concentration of approximately 700 ppm throughout the entire 40 to 50 years of their existence.  They observed the CO2-enriched trees to exhibit photosynthetic rates that were 36-77% greater than those of the trees growing in ambient air; and they could detect no signs of any photosynthetic down regulation in the CO2-enriched trees.  In fact, they could find no differences between the CO2-enriched and ambient-treatment trees in terms of rubisco activity and content, total nitrogen content, chlorophyll content and carotenoid content.  As a result, they concluded that "enhanced leaf photosynthetic rates at the CO2 springs could increase carbon sequestrating and productivity of whole tree canopies," and that "higher carbon acquisition by Q. pubescens and other species could slow the rise in atmospheric CO2."

Blaschke et al. (2001) also studied gas exchange in mature Q. pubescens and Q. ilex trees that had been exposed to atmospheric CO2 concentrations of approximately 370 and 700 ppm for their entire lives; and in their study the average net photosynthetic rates of the CO2-enriched trees were, respectively, 69% and 26% greater than those of the trees growing in ambient air.  In addition, they determined that the stomatal conductances of the CO2-enriched Q. pubescens trees were approximately 23% lower than those of trees of the same species growing in ambient air, while the CO2-enriched Q. ilex trees displayed no stomatal response to elevated CO2.  Nevertheless, both species exhibited significant CO2-induced increases in water use efficiency.

One less-than-ideal aspect of the Italian CO2 springs is that they emit higher-than-normal concentrations of the major phytotoxic air pollutants H2S and SO2 (Schulte et al., 1999).  This fact can be used to advantage, however, as it makes the springs perfect settings in which to study the relative strengths of two competing phenomena: the growth-promoting effect of elevated CO2 and the growth-retarding effect of elevated H2S and SO2.

Capitalizing upon this situation, Grill et al. (2004) analyzed various properties of leaves and acorns produced on Q. ilex and Q. pubescens trees growing at double-to-triple normal atmospheric CO2 concentrations near the CO2 springs, as well as the same characteristics of leaves and acorns growing on similar trees located some distance away in ambient-CO2 air.  In addition, they analyzed several characteristics of seedlings they sprouted from acorns produced by the CO2-enriched and ambient-treatment trees; and they used chromosome stress tests "to investigate whether alterations in sulphur-regime have negative consequences for seedlings."

In reporting their findings, Grill et al. say that "acorns from CO2 springs contained significantly higher sulphur concentrations than controls (0.67 vs. 0.47 mg g-1 dry weight in Q. ilex cotyledons and 1.10 vs. 0.80 in Q. pubescens)," indicative of the fact that the trees were indeed affected by the H2S and SO2 contained in the air in the vicinity of the CO2 springs.  They also report that Q. ilix seedlings grown from CO2-spring acorns showed elevated rates of chromosomal aberrations in their root tips, which is also suggestive of the presence of a permanent pollution-induced stress.  Nevertheless, as demonstrated by the results of several other studies conducted near the springs, the CO2-enriched air - even in the presence of phytotoxic H2S and SO2 - significantly enhanced the trees' photosynthetic prowess: by 26-69% in the study of Blaschke et al. (2001), by 36-77% in the study of Stylinski et al. (2000), and by a whopping 175-510% in the study of Tognetti et al. (1998).

Still in Italy, but in a study that did not make use of natural CO2 springs, Marek et al. (2001) constructed open-top chambers around 30-year-old Q. ilex trees growing in perennial evergreen stands, and they continuously exposed them to atmospheric CO2 concentrations of 350 and 700 ppm for five more years.  Throughout this period, the extra CO2 increased rates of net photosynthesis in sun and shade leaves by 68% and 59%, respectively; and photosynthetic acclimation was not apparent in any of the CO2-enriched trees' leaves.  In addition, the light compensation point, i.e., the light intensity at which photosynthetic carbon uptake is equivalent to respiratory carbon loss, was 24% and 30% lower in the sun and shade leaves of the CO2-enriched trees than it was in the corresponding leaves of trees growing in ambient air, which suggests that Q. ilex trees growing in CO2-enriched air should exhibit net carbon gains earlier in the morning and maintain them later into the evening than trees exposed to ambient air.  Together with the stimulatory effect of higher CO2 concentrations on photosynthesis, this observation further suggests that carbon sequestration by this tree species will likely be much greater in a higher-CO2 world of the future.

Although all of the reports described above imply nothing but good about the ongoing rise in the air's CO2 content as it pertains to the long-term growth and health of oak trees, Gartner et al. (2003) were concerned that the wood of the trees might be more vulnerable to embolism in a CO2-enriched atmosphere.  Hence, they investigated this question with Quercus ilex seedlings that had been grown for over a year in climate-controlled greenhouses in either ambient air or air enriched to twice the ambient concentration of CO2; but contrary to what they had initially thought, they found that the "plants grown in elevated CO2 did not differ significantly in vulnerability to embolism or kS [specific conductivity] from plants grown in ambient CO2."  In addition, they report that "Tognetti et al. (1999) found no significant effect of elevated CO2 on vulnerability to embolism or kS of branch samples from Q. ilix trees growing near CO2 vents compared with trees growing at normal ambient CO2."

In one final study that bests all the others in terms of being conducted under totally natural real-world conditions, Waterhouse et al. (2004) determined the intrinsic water use efficiency (IWUE) responses of three tree species growing across northern Europe - one of which was pedunculate oak (Quercus robur) that grew at three sites in England and two sites in Finland - to the increase in atmospheric CO2 concentration experienced between 1895 and 1994, using parameters derived from measurements of stable carbon isotope ratios of trunk cellulose.  They report that "all species at all the sites show a long-term increase in their values of IWUE during the past century," noting that "the main cause of this common behaviour is likely to be the increase in atmospheric CO2 concentration."

Linearly extrapolating these responses (which occurred over a period of time when the air's CO2 concentration rose by approximately 65 ppm) to what would be expected for the more common 300-ppm increase employed in the majority of atmospheric CO2 enrichment experiments, the IWUE increase Waterhouse et al. observed for Q. robur amounted to 158 14%, as best we can determine from the graphs of their results.  This response is huge, and is probably not due to rising CO2 alone, but to the positive synergism that occurs when atmospheric CO2 and temperature rise together (see Interactive Effects of CO2 and Temperature on Plant Growth (Trees) in our Subject Index), as these parameters have done over the past century or so, which clearly demonstrates that the "twin evils" of the climate-alarmist crowd (high temperatures and CO2 concentrations) are just what plants love.

In conclusion, the various findings of the several papers reviewed above point to the likelihood that oak trees of all species will grow ever more productively as the air's CO2 content continues to climb, and that they will be better able to withstand droughty conditions and be more effective in sequestering carbon in the years and decades ahead.

References
Ainsworth, E.A., Davey, P.A., Hymus, G.J., Drake, B.G. and Long, S.P.  2002.  Long-term response of photosynthesis to elevated carbon dioxide in a Florida scrub-oak ecosystem.  Ecological Applications 12: 1267-1275.

Blaschke, L., Schulte, M., Raschi, A., Slee, N., Rennenberg, H. and Polle, A.  2001.  Photosynthesis, soluble and structural carbon compounds in two Mediterranean oak species (Quercus pubescens and Q. ilex) after lifetime growth at naturally elevated CO2 concentrations.  Plant Biology 3: 288-297.

Dijkstra, P., Hymus, G., Colavito, D., Vieglais, D.A., Cundari, C.M., Johnson, D.P., Hungate, B.A., Hinkle, C.R. and Drake, B.G.  2002.  Elevated atmospheric CO2 stimulates aboveground biomass in a fire-regenerated scrub-oak ecosystem.  Global Change Biology 8: 90-103.

Gartner, B.L., Roy, J. and Huc, R.  2003.  Effects of tension wood on specific conductivity and vulnerability to embolism of Quercus ilex seedlings grown at two atmospheric CO2 concentrations.  Tree Physiology 23: 387-395.

Grill, D., Muller, M., Tausz, M. Strnad, B., Wonisch, A. and Raschi, A.  2004.  Effects of sulphurous gases in two CO2 springs on total sulphur and thiols in acorns and oak seedlings.  Atmospheric Environment 38: 3775-3780.

Hymus, G.J., Johnson, D.P., Dore, S., Anderson, H.P., Hinkle, C.R. and Drake, B.G.  2003.  Effects of elevated atmospheric CO2 on net ecosystem CO2 exchange of a scrub-oak ecosystem.  Global Change Biology 9: 1802-1812.

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.

Paoletti, E., Nourrisson, G., Garrec, J.P. and Raschi, A.  1998.  Modifications of the leaf surface structures of Quercus ilex L. in open, naturally CO2-enriched environments.  Plant, Cell and Environment 21: 1071-1075.

Schulte, M., Raiesi, F.G., Papke, H., Butterbach-Bahl, K., van Breemen, N. and Rennenberg, H.  1999.  CO2 concentration and atmospheric trace gas mixing ratio around natural CO2 vents in different Mediterranean forests in central Italy.  In: Raschi, A., Vaccori, F.P. and Miglietta, F.  (Eds.).  Ecosystem Response to CO2: The Maple Project Results.  European Communities, Brussels, Belgium, pp. 168-188.

Stylinski, C.D., Oechel, W.C., Gamon, J.A., Tissue, D.T., Miglietta, F. and Raschi, A.  2000.  Effects of lifelong [CO2] enrichment on carboxylation and light utilization of Quercus pubescens Willd. examined with gas exchange, biochemistry and optical techniques.  Plant, Cell and Environment 23: 1353-1362.

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 CO2Environmental and Experimental Botany 39: 233-245.

Tognetti, R., Longobucco, A. and Raschi, A.  1999.  Seasonal embolism and xylem vulnerability in deciduous evergreen Mediterranean trees influenced by proximity to a carbon dioxide spring.  Tree Physiology 19: 271-277.

Waterhouse, J.S., Switsur, V.R., Barker, A.C., Carter, A.H.C., Hemming, D.L., Loader, N.J. and Robertson, I.  2004.  Northern European trees show a progressively diminishing response to increasing atmospheric carbon dioxide concentrations.  Quaternary Science Reviews 23: 803-810.

Last updated 16 February 2005