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Acclimation (Tree Species: Pine) -- Summary
Studies of the effects of atmospheric CO2 enrichment on the growth and development of pine trees over the past several years have focused chiefly on three different species: Pinus radiata (Monterey pine), Pinus sylvestris (Scots pine) and Pinus taeda (Loblolly pine). Hence, we will consider what has been learned about the phenomenon of acclimation in these three species in this same alphabetical order in what follows.

Turnbull et al. (1998) grew seedlings of P. radiata in open-top chambers near Bromley, Christchurch (New Zealand) for four years at atmospheric CO2 concentrations of 360 and 650 ppm; while near the conclusion of this time period they measured various photosynthetic parameters in current-year and one-year-old needles at each of the CO2 concentrations to determine the effects of elevated CO2 and leaf age on photosynthesis. This work revealed CO2-induced increases in needle photosynthetic rate of 63 and 31% in current and one-year-old needles, respectively, suggesting that needle age might be an important determinant of photosynthetic acclimation in this species. And as further evidence for this hypothesis, they found that atmospheric CO2 enrichment did not induce changes in rubisco content or activity in current-year needles, but that it did reduce rubisco content and activity by about 40% in one-year-old needles. Thus, after four years of CO2 enrichment, the CO2-induced photosynthetic enhancement persisted in each year's new flush of needles, while it experienced a partial down regulation in older needles.

Reporting further on the same experiment, Griffin et al. (2000) additionally noted a decline in older-needle rubisco content in the elevated CO2 treatment, as well as a 40% reduction in needle stomatal conductance. The first of these changes implies that with the redistribution of nitrogen away from rubisco (which allows it to perform other vital functions), it is likely that P. radiata will maintain high rates of net carbon uptake while using less resources (like nitrogen) in doing so. And coupling these observations with the reduction in water loss due to CO2-induced decreases in stomatal conductance, it would appear that P. radiata seedlings will probably grow more efficiently with less water inputs in future atmospheres containing greater concentrations of CO2.

Last of all, at the conclusion of the four-year study of P. radiata in Christchurch, New Zealand, Greenep et al. (2003) took cuttings from the four-year-old Monterey pines that had experienced lifetime exposure to either ambient or elevated atmospheric CO2 concentrations and grew them for one year in their respective CO2 treatments in open-top chambers, where they were irrigated daily and fertilized every three months. And what did they find? They found that the "photosynthetic rate in young needles during summer, autumn and spring was 34, 43 and 38% higher, respectively, in trees grown at elevated CO2 than in trees grown at ambient CO2," while "in older needles, the corresponding photosynthetic rate increases were 26, 47 and 49%." In addition, water use efficiency was 49% higher in the foliage of the elevated CO2 treatment, although there was no change in needle stomatal conductance.

These responses were comparable to those observed in the parent trees when they were approximately the same size and age as the second-generation trees. However, there were signs of photosynthetic acclimation in the older needles of the parent trees when they were larger (in their 3rd and 4th years) but physiologically younger than the second-generation trees, which by the time of this study (having been derived from 4-year-old trees and grown for an additional year) were in their fifth year of life. Thus, the researchers suggest that the down-regulation of photosynthesis observed in the parent trees in their third and fourth years "was a result of a shift in the proportion of young to old needles as the trees increased in size."

In concluding, they thus hypothesized that "in small trees, close proximity of active sinks, such as developing buds, to a proportionally small reservoir of source tissue (mature foliage) would increase the overall sink strength and reduce the extent of photosynthetic acclimation (down-regulation) at elevated CO2." They note, for example, that "within an immature P. radiata canopy, young needles may represent as much as 4 to 10 times the biomass of older needles," but that "as the canopy matures this ratio approaches equality (Turnbull et al., 1998)." Hence, they concluded that "down-regulation of photosynthesis at elevated CO2 is related to tree size rather than tree age or duration of exposure," and that -- most important of all -- "the capacity for enhanced photosynthesis in trees growing in elevated CO2 is unlikely to be lost in subsequent generations."

Turning next to Pinus sylvestris or Scots pine, Jach and Ceulemans (2000) studied three-year-old seedlings that were rooted in the ground and grown in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 750 ppm for two additional years to determine the long-term effects of elevated CO2 on photosynthesis in this important European timber species. In addition, in order to make the experimental results more representative of the natural world, no nutrients or irrigation waters were applied to the soils during their investigation. So what did they find?

During the second year of atmospheric CO2 enrichment, the photosynthetic rates of the current-year and one-year-old CO2-enriched needles were 62 and 65% greater, respectively, than the rates displayed by the needles present on the seedlings growing in ambient air. However, when photosynthesis was measured at atmospheric CO2 concentrations reciprocal to growth CO2 concentrations, they detected photosynthetic acclimation in the CO2-enriched seedlings, as evidenced by a 21% reduction in their photosynthetic rates. However, the two researchers rightly noted that "the stimulatory effect of elevated CO2 on photosynthesis substantially exceeded the magnitude of down-regulation." Therefore, and in spite of the occurrence of photosynthetic acclimation, the rate of net photosynthesis in the CO2-enriched seedlings was still more than 40% greater than the rate measured in the control seedlings exposed to ambient air.

In a parallel study of Scots pine, Gielen et al. (2000) worked with six-year-old seedlings that were rooted in the ground within open-top chambers maintained at atmospheric CO2 concentrations of 350 and 750 ppm for three years, in order to determine the long-term effects of elevated CO2 on chlorophyll fluorescence and needle characteristics, where once again no nutrients or irrigation water was applied to the soil. At the end of this period, a detailed seasonal analysis indicated that elevated CO2 did not significantly impact the photochemical quantum efficiency of photosystem II; nor did it affect any parameters associated with chlorophyll fluorescence, indicative of the fact that atmospheric CO2 enrichment did not modify the light-dependent reactions of photosynthesis in this species.

With respect to needle characteristics, on the other hand, elevated CO2 reduced needle nitrogen and chlorophyll contents by 33 and 26%, respectively; and these observations suggest that the light-independent reactions of photosynthesis were being modified by long-term exposure to elevated CO2 in a manner indicative of photosynthetic acclimation. But this acclimation of the photosynthetic process allows for the redistribution of limiting resources -- such as nitrogen -- away from what thus becomes a more efficient photosynthetic apparatus, so that this important nutrient can be utilized in other areas of the tree where it is needed more. Thus, this CO2-induced phenomenon can ultimately have a positive effect on growth; for it allows trees to produce more biomass under conditions of low soil fertility than would otherwise be possible under ambient CO2 concentrations, due to the mobilization of nitrogen out of photosynthetically-active leaves and into actively-expanding sink tissues.

Moving on to Pinus taeda or loblolly pines, Maier et al. (2002) constructed open-top chambers around 13-year-old trees growing on an infertile sandy soil and fumigated them for two more years with air containing either 350 or 550 ppm CO2, while half of the trees at each CO2 concentration received supplemental soil fertilization. These procedures led them to discover that the elevated CO2 increased branch needle area by 13%, while soil fertilization increased it by 38%; and applied together, the two treatments enhanced branch needle area by 56%. In addition, the extra CO2 enhanced the trees' net photosynthesis rates by 82%, with the trees showing no signs of photosynthetic acclimation over the two-year duration of the study.

Two years later, Crous and Ellsworth (2004) reported the photosynthetic rates of different-age needles measured at different crown positions on the 19-year-old (in 2002) loblolly pine trees at the Duke Forest FACE facility in the sixth year of that long-term study, comparing them with the results of similar measurements made over the prior five years. Although there was some evidence of moderate photosynthetic down-regulation in one-year-old needles across the fifth to sixth year of CO2 exposure, the two researchers report that "strong photosynthetic enhancement in response to elevated CO2 (e.g., +60% across age classes and canopy locations) was observed across the years."

Fast-forwarding four more years, and looking at the concept of acclimation from a somewhat different perspective, Lichter et al. (2008) wrote that progressive nitrogen limitation (PNL) may "accompany carbon sequestration in plants and soils stimulated by CO2 fertilization, gradually attenuating the CO2 response," after which they describe what they learned about the PNL hypothesis over the following nine years.

First of all, the nine researchers report that their data pertaining to forest-floor carbon pools indicate the existence of "a long-term steady-state sink" of about 30 g C per m2 per year, which represents, in their words, "a substantial increase in forest-floor C storage under elevated CO2 (i.e. 29%)," and which they attribute to "increased litterfall and root turnover during the first 9 years of the study." Secondly, down below the forest floor, they say that of the mineral soil C formed during the past 9 years, "approximately 20% has been allocated to stable pools that will likely remain protected from microbial activity and associated release as CO2."

A third important finding of the research team was "a significant widening of the C:N ratio of soil organic matter in the upper mineral soil under both elevated and ambient CO2," which suggests, as they describe it, that "enhanced rates of soil organic matter decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO2." At the Duke Forest FACE site, for example, Pritchard et al. (2008) say that this CO2-induced increase in productivity amounts to approximately 30% annually; and they add that there is "little evidence to indicate a diminished response through time," citing the analysis of Finzi et al. (2007), who found the same to be true at the long-term forest FACE studies being conducted at Rhinelander, Wisconsin (USA), Oak Ridge National Laboratory (USA), and Tuscania (Italy).

Contrary to the early expectations of many scientists (and most climate alarmists), it would thus appear that many of earth's forests that are thought to have access to less-than-adequate soil nitrogen supplies may indeed be able to acquire the extra nitrogen they need to maintain the sizable increases in their growth rates that are driven by elevated concentrations of atmospheric CO2. In the case of North Carolina's Duke Forest, for example, "even after nine years of experimental CO2 fertilization," as Lichter et al. describe it, "attenuation of the CO2-induced productivity enhancement has not been observed," as has also been noted to be the case by Finzi et al. (2006). And this finding at this location is extremely significant, because the growth of pine-hardwood forests in the southeastern United States often removes so much nitrogen from the soils in which they grow that they induce what Finzi and Schlesinger (2003) have described as "a state of acute nutrient deficiency that can only be reversed with fertilization," which operation, however, was not employed in the Duke Forest FACE study.

In conclusion, and in light of the several discoveries described above, it would appear that even though there may sometimes be a partial acclimation of the photosynthetic process in pine trees in some CO2-enrichment experiments, the down regulation seems never to be complete. In fact, the phenomenon may even play a positive role in shifting much-needed nitrogen from the sites of photosynthesis to the sinks for photosynthates in situations where soil fertility is a limiting factor to primary productivity. Therefore, under almost all conditions imaginable, the ongoing rise in the air's CO2 concentration should prove a boon to the biosphere.

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.

Finzi, A.C., Moore, D.J.P., DeLucia, E.H., Lichter, J., Hofmockel, K.S., Jackson, R.B., Kim, H.-S., Matamala, R., McCarthy, H.R., Oren, R., Pippen, J.S. and Schlesinger, W.H. 2006. Progressive nitrogen limitation of ecosystem processes under elevated CO2 in a warm-temperate forest. Ecology 87: 15-25.

Finzi, A.C., Norby, R.J., Calfapietra, C., Gallet-Budynek, A., Gielen, B., Holmes, W.E., Hoosbeek, M.R., Iversen, C.M., Jackson, R.B., Kubiske, M.E., Ledford, J., Liberloo, M., Oren, R., Polle, A., Pritchard, S., Zak, D.R., Schlesinger, W.H. and Ceulemans, R. 2007. Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2. Proceedings of the National Academy of Sciences, USA 104: 14,014-14,019.

Finzi, A.C. and Schlesinger, W.H. 2003. Soil-nitrogen cycling in a pine forest exposed to 5 years of elevated carbon dioxide. Ecosystems 6: 444-456.

Gielen, B., Jach, M.E. and Ceulemans, R. 2000. Effects of season, needle age and elevated atmospheric CO2 on chlorophyll fluorescence parameters and needle nitrogen concentration in (Pinus sylvestris L.). Photosynthetica 38: 13-21.

Greenep, H., Turnbull, M.H. and Whitehead, D. 2003. Response of photosynthesis in second-generation Pinus radiata trees to long-term exposure to elevated carbon dioxide partial pressure. Tree Physiology 23: 569-576.

Griffin, K.L., Tissue, D.T., Turnbull, M.H. and Whitehead, D. 2000. The onset of photosynthetic acclimation to elevated CO2 partial pressure in field-grown Pinus radiata D. Don. after 4 years. Plant, Cell and Environment 23: 1089-1098.

Jach, M.E. and Ceulemans, R. 2000. Effects of season, needle age and elevated atmospheric CO2 on photosynthesis in Scots pine (Pinus sylvestris L.). Tree Physiology 20: 145-157.

Lichter, J., Billings, S.A., Ziegler, S.E., Gaindh, D., Ryals, R., Finzi, A.C., Jackson, R.B., Stemmler, E.A. and Schlesinger, W.H. 2008. Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment. Global Change Biology 14: 2910-2922.

Maier, C.A., Johnsen, K.H., Butnor, J., Kress, L.W. and Anderson, P.H. 2002. Branch growth and gas exchange in 13-year-old loblolly pine (Pinus taeda) trees in response to elevated carbon dioxide concentration and fertilization. Tree Physiology 22: 1093-1106.

Pritchard, S.G., Strand, A.E., McCormack, M.L., Davis, M.A. and Oren, R. 2008. Mycorrhizal and rhizomorph dynamics in a loblolly pine forest during 5 years of free-air-CO2-enrichment. Global Change Biology 14: 1-13.

Turnbull, M.H., Tissue, D.T., Griffin, K.L., Rogers, G.N.D. and Whitehead, D. 1998. Photosynthetic acclimation to long-term exposure to elevated CO2 concentration in Pinus radiata D. Don. is related to age of needles. Plant, Cell and Environment 21: 1019-1028.

Last updated 27 October 2010