Takeuchi et al. (2001) grew quaking aspen (Populus tremuloides) trees for a period of two years within 30-meter-diameter free-air CO2 enrichment or FACE plots near Rhinelander, Wisconsin (USA), which were maintained at atmospheric CO2 concentrations of either 360 or 560 ppm. In doing so, they found that elevated CO2 stimulated the growth and closure of the trees' canopies, as indicated by light intensities measured near the seedlings' lowermost branches that were only 17 and 9% of values observed near their uppermost branches in the ambient and CO2-enriched environments, respectively, which led to the light availability for photosynthesis decreasing with canopy depth, but more so for the CO2-enriched seedlings. Nevertheless, seasonal photosynthetic rates were always greater in the CO2-enriched seedlings, although the growth stimulation was much greater in the upper compared to the lower canopy (26 and 3%, respectively). Likewise, photosynthetic acclimation also occurred in the CO2-enriched seedlings in a depth-dependent manner, with less acclimation occurring in the upper as opposed to the lower canopy, as indicated by decreases in foliar rubisco content of 28 and 50%, respectively, in those two locations. Yet in spite of this significant acclimation, the elevated CO2 still led to a greater total net carbon uptake; and the CO2-enriched seedlings grew 18% taller than the seedlings exposed to ambient air.

Working at the EuroFACE facility near Viterbo in Central Italy, Wittig et al. (2005) grew stands of closely-spaced (1 m x 1 m) individuals of three Populus species -- white poplar (P. alba), black poplar (P. nigra) and robusta poplar (P. x euramericana) -- from the time of planting through canopy closure to coppice (aboveground tree harvest) at atmospheric CO2 concentrations of 370 and 550 ppm for three full years. Based on measurements of leaf area index and various photosynthetic parameters made at regular intervals throughout the entire period, the photosynthetic rates of different leaf classes were determined for monthly intervals and summed to obtain annual canopy photosynthesis or gross primary production (GPP) in each of the three years of the study.

This work revealed, in their words, that "significant stimulation of GPP driven by elevated CO2 occurred in all 3 years, and was greatest in the first year (223-251%), but markedly lower in the second (19-24%) and third years (5-19%)." Interestingly, this decline in CO2-induced growth stimulation was not due to photosynthetic acclimation, but was simply a consequence of canopy closure and the increased shading of leaves that accompanied it.

Averaged across all species and plots, the CO2-induced stimulation of annual GPP was 234%, 22% and 11% in 1999, 2000 and 2001, respectively, while averaged over the entire three-year period, the GPP enhancements for P. alba, P. nigra and P. x euramericana were, respectively, 17%, 17% and 25%; and the group of ten scientists notes that these results "were consistent with independent measurements of net primary production, determined independently from biomass increments and turnover." Thus, Wittig et al. say their results suggest that "with selection, nutrient and moisture supply, coppice managed plantation poplars have the potential for large and sustained increases in GPP [italics added]," in response to atmospheric CO2 enrichment.

Over the following year, as the trees sent up new sprouts and renewed their growth after coppicing, Davey et al. (2006) measured total daily photosynthetic carbon assimilation together with a number of related physiological parameters and processes. In doing so, they found that "diurnal photosynthesis in poplar trees grown in elevated CO2 over four growing seasons showed a sustained increase in photosynthesis of between 35 and 60% prior to coppicing," and that "this increase in daily photosynthesis is maintained during the re-growth following coppicing in P. x euramericana."

These observations and their other data indicate, as the seven scientists describe it, that "no long-term photosynthetic acclimation to CO2 occurred in these plants," and that "poplar trees are able to 'escape' from long-term, acclamatory down-regulation of photosynthesis through a high capacity for starch synthesis and carbon export." Also of great importance, they note that "Wittig et al. (2005) showed that the canopy photosynthetic carbon gain in these species is proportional to wood increment, implying that the increased photosynthesis will result in more carbon in wood." Therefore, the seven scientists say their findings "show that the acclamatory loss of the initial increase in photosynthetic rate under elevated CO2 is not inevitable," and that "poplar species, selected for rapid growth, may be well suited to a future elevated CO2 environment and particularly suited to afforestation projects aimed to increase carbon uptake into wood in the near term."

Throughout the same year, as well as over one additional year, Liberloo et al. (2005) studied the regrowth of the trees under unfertilized and fertilized conditions; and during those first and second growing seasons after coppicing, they found that the elevated CO2 treatment significantly increased the trees' leaf area index, with relative differences between the CO2-enriched and control trees ranging from +1.7 to +38.7%, +4.7 to +38.5% and +3.9 to +45% for P. alba, P. nigra and P. x euramericana, respectively, for unfertilized and fertilized conditions, respectively. In addition, they report that the increased leaf area index "supported increased aboveground biomass production," but only in the fertilized treatment. And in light of these findings, they remark that if the CO2-induced growth enhancement after canopy closure continues to hold true in subsequent years, it "will have important implications for the carbon balance of terrestrial ecosystems, because forests could behave as a larger carbon sink under future atmospheric conditions."

Well, one year later, Liberloo et al. (2006) reported results for the second three-year growth rotation of the trees; and, interestingly, they found that fertilization did not affect the growth of the second-rotation trees, "likely because of the high rates of fertilization during the previous agricultural land use." In contrast, in their words, "elevated CO2 enhanced biomass production by up to 29%, and this stimulation did not differ between above- and below-ground parts." They also report that the net rate of carbon assimilation was "on average for all species stimulated up to 30% during the third year of the second rotation," and that "after six years of fumigation, measurements of photosynthetic parameters along the canopy profile could not detect any clear sign of acclimation to elevated CO2" for any of the three species. Thus, they concluded that "poplar trees are able to optimally profit from future high CO2 concentrations, provided that they are intensively managed, planted in regions with high incident radiation and supplied with sufficient nutrients and water." Such "high-density poplar coppice cultures," in their opinion, "offer possibilities to mitigate the rise of atmospheric CO2 by producing renewable bio-energy in an economically feasible way, whereby the elevated CO2 stimulation might sustain over several rotation cycles."

Working at the same site, but studying only robusta poplar that had been growing there for a period of five years, Calfapietra et al. (2005) measured the trees' photosynthetic responses to an approximate 200-ppm increase in the air's CO2 concentration in mid-July of the study's fifth year, comparing their results with what was observed at the beginning of the experiment, both with and without supplemental nitrogen fertilization; and as they describe their findings, "even after such a long period of exposure, leaves of Populus x euramericana have not shown clear signs of photosynthetic acclimation." What is more, they report that CO2 enrichment "significantly decreased stomatal conductance both on upper and lower canopy leaves," which together with the CO2-induced stimulation of photosynthesis implies a significant sustained increase in leaf water use efficiency throughout the trees' canopies. Thus, in the concluding sentence of their paper, they say their results "suggest that the photosynthetic acclimation of poplar plantations is unlikely to occur in an atmosphere enriched in CO2 and thereby will not influence the response of poplar plantations to increasing atmospheric CO2 concentrations either over the long term or under conditions of nitrogen deposition."

Returning to where we commenced this Summary -- the Aspen FACE site near Rhinelander, Wisconsin (USA) -- Kets et al. (2010) measured diurnal changes in light-saturated net photosynthesis (Pn) rate under both ambient and elevated atmospheric CO2 and/or ozone (O3) concentrations over wide ranges of stomatal conductance, water potential, intercellular CO2, leaf temperature, and vapor pressure difference between leaf and air in two clones (271 and 42E) of quaking aspen (Populus tremuloides Michx.) trees that differed in their sensitivity to ozone and had been growing under the aforementioned experimental conditions for seven to eight years. This work revealed that Pn was typically enhanced by 33-46% in the CO2-enriched treatment over the course of the study, and that there was a small increase in leaf chlorophyll concentration as well.

Noting that "previous Aspen FACE studies have reported 25-36% increases in Pn (Noormets et al., 2001; Takeuchi et al., 2001; Sharma et al., 2003; Ellsworth et al., 2004)," the six scientists emphasize that the aerial fertilization effect of atmospheric CO2 enrichment on Pn observed in their study "has rather been increasing in time than decreasing," stating that this phenomenon may be caused by the "slight but significant increase in leaf chlorophyll content per leaf area, which is rather positive acclimation in photosynthetic apparatus than negative acclimation [italics added]," in support of which conclusion they also cite the studies of Centritto and Jarvis (1999) and Eichelmann et al. (2004). Hence, their experimental persistence demonstrates that some of the benefits of elevated atmospheric CO2 concentrations may actually increase with the passage of time.

Last of all, we come to the study of Darbah et al. (2010), who analyzed photosynthesis data that they and others had collected at the Aspen FACE site over a period of eleven years for the same two quaking aspen clones, which were exposed to all combinations of ambient and elevated (560 ppm) CO2 and ambient and elevated (1.5 times ambient) ozone (O3). And as an added bonus, they studied leaf stomatal conductance under the same conditions. This work revealed, as they describe it, "no long-term photosynthetic and stomatal acclimation to elevated CO2, O3 or CO2 + O3 in aspen trees exposed to elevated CO2 and/or O3 gases for 11 years," adding that the aspen trees "have sustained their maximum instantaneous photosynthesis stimulation for over a decade." And in discussing their findings, Darbah et al. say they support the observations of (1) Liberloo et al. (2007), who measured a 49% increase in net photosynthetic rate in poplar trees after six years of exposure to elevated CO2, (2) the findings of Sholtis et al. (2004), who reported a 44% stimulation of net photosynthesis in sweetgum trees after three years of exposure to elevated CO2, (3) Crous and Ellsworth (2004), who found a photosynthetic enhancement of 51-69% in Pinus taeda trees after six years of exposure to elevated CO2, as well as (4) Davey et al. (2006) and (5) Paoletti et al. (2007), of whose work Darbah et al. state "there was no photosynthetic acclimation (down-regulation) occurring in Quercus ilex under long-term CO2 enrichment." In addition, they remark that (6) even in white clover (Trifolium repens), Ainsworth et al. (2003) found that photosynthetic stimulation "remained after nine years of exposure to elevated CO2."

Thus, as ever more long-term experiments are conducted on long-lived woody plants growing out-of-doors and rooted in the earth, where their roots are not artificially confined to a limited volume of soil, it is becoming abundantly clear that they generally do not experience a complete cessation of the initial photosynthetic stimulation provided them by the extra CO2 to which they are exposed in CO2 enrichment studies. In fact, they often show very little long-term reduction in CO2-induced growth stimulation, and sometimes no reduction at all, as is evidenced in these several studies of aspen/poplar trees.

References
Ainsworth, A.E., Rogers, A., Blum, H., Nosberger, J. and Long, S.P. 2003. Variation in acclimation of photosynthesis in Trifolium repens after eight years of exposure to free air CO2 enrichment (FACE). Journal of Experimental Botany 54: 2769-2774.

Calfapietra, C., Tulva, I., Eensalu, E., Perez, M., De Angelis, P., Scarascia-Mugnozza, G. and Kull, O. 2005. Canopy profiles of photosynthetic parameters under elevated CO2 and N fertilization in a poplar plantation. Environmental Pollution 137: 525-535.

Centritto, M. and Jarvis, P.G. 1999. Long-term effects of elevated carbon dioxide concentration and provenance on four clones of Sitka spruce (Picea sitchensis). II. Photosynthetic capacity and nitrogen use efficiency. Tree Physiology 19: 807-814.

Crous, K.Y. and Ellsworth, D.S. 2004. Canopy position affects photosynthetic adjustments to long-term elevated CO2 concentration (FACE) in aging needles in a mature Pinus taeda forest. Tree Physiology 24: 961-970.

Darbah, J.N.T., Kubiske, M.E., Nelson, N., Kets, K., Riikonen, J., Sober, A., Rouse, L. and Karnosky, D.F. 2010. Will photosynthetic capacity of aspen trees acclimate after long-term exposure to elevated CO2 and O3? Environmental Pollution 158: 983-991.

Davey, P.A., Olcer, H., Zakhleniuk, O., Bernacchi, C.J., Calfapietra, C., Long, S.P. and Raines, C.A. 2006. Can fast-growing plantation trees escape biochemical down-regulation of photosynthesis when grown throughout their complete production cycle in the open air under elevated carbon dioxide? Plant, Cell and Environment 29: 1235-1244.

Eichelmann, H., Oja, V., Rasulov, B., Padu, E., Bichele, I., Pettai, H., Mols, T., Kasparova, I., Vapaavuori, E. and Laisk, A. 2004. Photosynthetic parameters of birch (Betula pendula Roth) leaves growing in normal and in CO2- and O3-enriched atmospheres. Plant, Cell and Environment 27: 479-495.

Ellsworth, D.S., Reich, P.B., Naumburg, E.S., Koch, G.W., Kubiske, M.E. and Smith, S.D. 2004. Photosynthesis, carboxylation and leaf nitrogen responses of 16 species to elevated pCO2 across four free-air CO2 enrichment experiments in forest, grassland and desert. Global Change Biology 10: 2121-2138.

Kets, K., Darbah, J.N.T., Sober, A., Riikonen, J., Sober, J. and Karnosky, D.F. 2010. Diurnal changes in photosynthetic parameters of Populus tremuloides, modulated by elevated concentrations of CO2 and/or O3 and daily climatic variation. Environmental Pollution 158: 1000-1007.

Liberloo, M., Calfapietra, C., Lukac, M., Godbold, D., Luo, Z.-B., Polle, A., Hoosbeek, M.R., Kull, O., Marek, M., Raines, C., Rubino, M., Taylor, G., Scarascia-Mugnozza, G. and Ceulemans, R. 2006. Woody biomass production during the second rotation of a bio-energy Populus plantation increases in a future high CO2 world. Global Change Biology 12: 1094-1106.

Liberloo, M., Dillen, S.Y., Calfapietra, C., Marinari, S., Luo, Z.B., De Angelis, P. and Ceulemans, R. 2005. Elevated CO2 concentration, fertilization and their interaction: growth stimulation in a short-rotation poplar coppice (EUROFACE). Tree Physiology 25: 179-189.

Liberloo, M., Tulva, I., Raim, O., Kull, O. and Ceulemans, R. 2007. Photosynthetic stimulation under long-term CO2 enrichment and fertilization is sustained across a closed Populus canopy profile (EUROFACE). New Phytologist 173: 537-549.

Noormets, A., Sober, A., Pell, E.J., Dickson, R.E., Podila, G.K., Sober, J., Isebrands, J.G. and Karnosky, D.F. 2001. Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and O3. Plant, Cell and Environment 24: 327-336.

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

Sharma, P., Sober, A., Sober, J., Podila, G.K., Kubiske, M.E., Mattson, W.J., Isebrands, J.G. and Karnosky, D.F. 2003. Moderation of CO2-induced gas exchange responses by elevated tropospheric O3 in trembling aspen and sugar maple. Ekologia 22 (S1): 304-317.

Sholtis, J.D., Gunderson, C.A., Norby, R.J. and Tissue, D.T. 2004. Persistent stimulation of photosynthesis by elevated CO2 in a sweetgum (Liquidambar styraciflua) forest stand. New Phytologist 162: 243-254.

Takeuchi, Y., Kubiske, M.E., Isebrands, J.G., Pregitzer, K.S., Hendrey, G. and Karnosky, D.F. 2001. Photosynthesis, light and nitrogen relationships in a young deciduous forest canopy under open-air CO2 enrichment. Plant, Cell and Environment 24: 1257-1268.

Wittig, V.E., Bernacchi, C.J., Zhu, X.-G., Calfapietra, C., Ceulemans, R., DeAngelis, P., Gielens, B., Miglietta, F., Morgan, P.B. and Long, S.P. 2005. Gross primary production is stimulated for three Populus species grown under free-air CO2 enrichment from planting through canopy closure. Global Change Biology 33: 644-656.