Also working in Italy with mature holly oak (Quercus ilix) trees that had been growing under the same set of conditions, Polle et al. (2001) collected acorns from trees that had been exposed to ambient and twice-ambient atmospheric CO2 concentrations for their entire lifetimes; and after germinating those acorns, the seedlings they produced were grown for eight months at both atmospheric CO2 concentrations in order to see if atmospheric CO2 enrichment of the parent trees had any effect on their offspring's response to atmospheric CO2 enrichment.

In following this protocol, the researchers found that elevated CO2 increased whole-plant biomass by 158 and 246% in seedlings derived from acorns produced in ambient and twice-ambient atmospheric CO2 concentrations, respectively, and that the final biomass of the CO2-enriched seedlings that had been derived from the acorns produced in the CO2-enriched air was 25% greater than that of the CO2-enriched seedlings derived from the acorns produced in ambient air. In addition, they report that their gas exchange measurements indicated that the CO2-enriched seedlings derived from the acorns produced on the CO2-enriched trees exhibited less-pronounced photosynthetic acclimation to elevated CO2 than did the CO2-enriched seedlings derived from the acorns produced on the trees exposed to ambient air.

Contemporaneously, Blaschke et al. (2001) studied the effects of long-term atmospheric CO2 enrichment on gas exchange in both species of mature oak trees -- Quercus ilix (a strongly drought-tolerant evergreen species) and Quercus pubescens (a less drought-tolerant deciduous species) -- both of which were growing near CO2-emitting springs in Italy, where they made physiological and biochemical measurements on trees that had been exposed to atmospheric CO2 concentrations of approximately 370 and 700 ppm for their entire lifetimes, which ranged from 30 to 50 years in duration. This work revealed that the CO2-enriched Q. pubescens and Q. ilex trees exhibited net photosynthetic rates that were 69 and 26% greater, respectively, than those displayed by control trees exposed to ambient CO2, in spite of CO2-induced decreases of 30 and 15% in their respective foliar rubisco concentrations. In addition, stomatal conductances of CO2-enriched Q. pubescens trees were approximately 23% lower than those of controls, while stomatal conductances of Q. ilex trees displayed no CO2-sensitivity. Nevertheless, both species exhibited increased water use efficiencies in the elevated CO2 environment closest to the CO2-emitting springs; and the totality of Blaschke et al.'s results clearly demonstrates that fully mature trees continue to exhibit enhanced rates of photosynthesis and increases in water use efficiency even after decades of exposure to elevated atmospheric CO2 concentrations.

Also working in Italy around the same time, Marek et al. (2001) employed open-top chambers to boost the air's CO2 content approximately two-fold for a period of five years around 30-year-old Quercus ilex trees growing in perennial evergreen stands, finding in the process that the extra CO2 increased rates of net photosynthesis in sun-exposed and shaded leaves by 68 and 59%, respectively. And after measuring short-term photosynthetic rates at various atmospheric CO2 concentrations, they could find no evidence of photosynthetic acclimation in the leaves of the mature trees. In addition, they determined that the trees' light compensation point -- the light level at which photosynthetic carbon uptake is matched by respiratory carbon loss -- was 24 and 30% lower in sun-exposed and shaded leaves, respectively, of CO2-enriched trees than it was in corresponding leaves of the trees growing in ambient air.

As the atmosphere's CO2 concentration continues to increase, therefore, the work of Marek et al. suggests that its stimulatory effect on oak tree photosynthesis will persist over the long term. In addition, because elevated CO2 significantly lowered the light compensation point in mature oak trees, which would allow them to exhibit net carbon gains earlier in the morning and maintain them later into the evening, the stimulatory effect of elevated CO2 on daily carbon uptake will be further enhanced. And together, these two observations suggest that carbon sequestration by oak trees, and perhaps other tree species, may well be more substantial in future CO2-enriched air than what has typically been projected.

Finally, one year later, we find a study conducted in the United States, where -- after burning a Florida scrub-oak ecosystem (dominated by Quercus myrtifolia, Q. chapmanii, and Q. geminata) to the ground -- Ainsworth et al. (2002) erected open-top chambers on the site and fumigated them with ambient (380 ppm) and CO2-enriched (700 ppm) air to study the effects of elevated CO2 on community regeneration. During the third and fourth years of this study, they found that elevated CO2 consistently increased photosynthetic rates in Q. myrtifolia and Q. chapmanii by as much as 150% without inducing any degree of photosynthetic acclimation, although such was observed in Q. geminata. Nevertheless, after three years of exposure to elevated CO2, all three oak species, taken together, exhibited an average increase in their rates of photosynthesis of 53%. Thus, as the CO2 content of the air continues to rise, regenerating scrub-oak communities will likely exhibit enhanced rates of photosynthesis that will persist throughout canopy closure and maturity. And these increases in photosynthesis will likely enhance community biomass production. In fact, the five researchers concluded that the sustained increases in photosynthesis exhibited by Q. myrtifolia and Q. chapmanii have "translated to increased growth in these species, and there is no suggestion that this trend is changing." Thus, carbon sequestration in regenerating and maturing scrub-oak ecosystems is likely to continue to increase with future increases in the air's CO2 concentration.

Last of all, we return to Italy, where Paoletti et al. (2007) measured rates of net photosynthesis in upper sunlit leaves of mature Quercus ilex trees growing close to (5 m) and further away from (130 m) a CO2-emitting spring, where the trees had experienced lifetime exposure to atmospheric CO2 concentrations of approximately 1500 and 400 ppm, respectively. This was done during a two-week period in June of 2002 at the end of the spring rains, when midday air temperatures rose above 40°C; and the data revealed that the net photosynthetic rates of the leaves on the trees growing closest to the CO2 spring were approximately 250% greater than those of the leaves on the trees growing 125 meters further away, where the air's CO2 concentration was 1100 ppm less than it was in the vicinity of the trees nearest the spring. Thus, the four Italian researchers who conducted the work pretty much stated the obvious when concluding that "the considerable photosynthetic stimulation at the very high CO2 site suggests no photosynthetic down-regulation over long-term CO2 enrichment." And this real-world finding demonstrates the truly amazing potential for large increases in the air's CO2 content to greatly stimulate photosynthesis and significantly enhance the growth and development of earth's trees over the very-long-term.

Clearly, the aerial fertilization effect of atmospheric CO2 enrichment is not a flash-in-the-pan phenomenon. It is here to stay ... and growing bigger by the day.

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.

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., Seufert, G., Della Rocca, G. and Thomsen, H. 2007. Photosynthetic responses to elevated CO2 and O3 in Quercus ilex leaves at a natural CO2 spring. Environmental Pollution 147: 516-524.

Polle, A., McKee, I. and Blaschke, L. 2001. Altered physiological and growth responses to elevated [CO2] in offspring from holm oak (Quercus ilex L.) mother trees with lifetime exposure to naturally elevated [CO2]. Plant, Cell and Environment 24: 1075-1083.

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.