Spanda et al. (1998) monitored 15-year-old Norway spruce trees in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm for a period of four additional years, in order to study the long-term effects of elevated CO2 on photosynthetic acclimation; and when they measured the trees photosynthetic rates at reciprocal growth CO2 concentrations, they found that the CO2-enriched shoots displayed a reduction of 18% compared to shoots grown in ambient air, indicative of the presence of CO2-induced photosynthetic acclimation. Likewise, when analyzing photosynthetic pigments, they found the total amounts of chlorophylls and carotenoids in needles produced in CO2-enriched air to be 17 and 14% less, respectively, than the amounts found in needles produced in ambient air. Nevertheless, when measured at growth CO2 concentrations, current-year CO2-enriched shoots still displayed rates of net photosynthesis that were 78% greater than those exhibited by shoots of trees grown in ambient air.

Contemporaneously, Egli et al. (1998) rooted several saplings of different Norway spruce genotypes directly into calcareous or acidic soils in open-top chambers and exposed them to atmospheric CO2 concentrations of 370 or 570 ppm and low or high soil nitrogen contents, in order to determine the effects of elevated CO2 and soil quality on the saplings' photosynthesis and growth rates. In doing so, they found that the elevated CO2 stimulated light-saturated rates of net photosynthesis under all conditions, with all genotypes exhibiting stimulations as great as 35%. Elevated CO2 also led to a down regulation of photosynthesis. However, rates of leaf photosynthesis still remained higher for the trees grown in the CO2-enriched air, despite the occurrence of this phenomenon; and this increase in photosynthesis ultimately contributed to greater instantaneous water-use efficiencies of the trees grown in elevated CO2, which were also promoted by CO2-induced decreases in needle stomatal conductance. And all these things operating together consistently led to increased aboveground biomass production, regardless of genotype, soil type and nitrogen content.

Shortly thereafter, Wiemken and Ineichen (2000) completed an experiment where they grew Norway spruce seedlings for three years in growth chambers maintained at atmospheric CO2 concentrations of 280, 420 and 560 ppm. In addition, the seedlings received either low, medium or high levels of nitrogen fertilization. This work revealed that nitrogen fertilization did not affect the concentrations of any sugars within mature needles of the seedlings; but atmospheric CO2 enrichment significantly enhanced needle glucose contents in a season-dependent manner.

In the highly productive growing phases characteristic of spring and early summer, for example, glucose contents in mature needles of the CO2-enriched trees were not significantly different from those observed in needles of the trees exposed to atmospheric CO2 concentrations of 280 and 420 ppm. In late summer, fall and winter, however, glucose concentrations in needles on the CO2-enriched trees were 40 to 50% higher than those of needles on trees subjected to ambient and sub-ambient CO2 concentrations (420 and 280 ppm, respectively). And these seasonal fluctuations in needle glucose concentrations suggested to the two researchers that glucose levels may be mediating a seasonal photosynthetic down regulation in spruce needles, as had previously been noted by others.

In light of these latter observations, it would appear that as the air's CO2 content continues to rise, Norway spruce trees will likely increase their photosynthetic rates, which should result in greater needle concentrations of soluble carbohydrates, including glucose. During favorable growing conditions associated with spring and early summer, the additional glucose produced by trees growing in CO2-enriched air will likely be mobilized and sent to active sinks to support their growth and development. As growing conditions become less favorable in late summer, however, glucose may not be mobilized from needles as rapidly as it was during the spring and early summer, which may lead to a seasonal photosynthetic down regulation in this species.

In spite of this intimation of a temporary seasonal down regulation of photosynthesis in Norway spruce seedlings, it seems logical to expect that they will still exhibit greater biomass production at higher atmospheric CO2 concentrations, as demonstrated by the positive findings of Spanda et al. (1998) and Egli et al. (1998), as well as the work of Tjoelker et al. (1998), who studied Picea mariana (black spruce), together with quaking aspen, paper birch, tamarack and jack pine in controlled-environment chambers for three months at atmospheric CO2 concentrations of either 370 or 580 ppm and day/night temperatures ranging from 18/12 to 30/24°C, and who found that the 57% increase in the CO2 content of the air significantly stimulated net photosynthesis in all of the studied species by an average of 28%, regardless of temperature, over the entire three-month study period. And here again, elevated CO2 decreased leaf nitrogen levels in all species, causing differing degrees of photosynthetic down regulation; but the mobilization of nitrogen from the trees' leaves/needles, coupled with the sustained enhancement of photosynthetic rates, led to increased photosynthetic nitrogen-use efficiencies in all plants grown in elevated CO2. In addition, the elevated CO2 decreased stomatal conductance by 10 to 25% in all species, leading to 40 to 80% increases in their instantaneous water-use efficiencies. And as in essentially all such studies conducted to date, these benefits of atmospheric CO2 enrichment more than compensated for the incomplete acclimation that sometimes occurs in such circumstances.

References
Egli, P., Maurer, S., Gunthardt-Goerg, M.S. and Korner, C. 1998. Effects of elevated CO2 and soil quality on leaf gas exchange and aboveground growth in beech-spruce model ecosystems. New Phytologist 140: 185-196.

Spunda, V., Kalina, J., Cajanek, M., Pavlickova, H. and Marek, M.V. 1998. Long-term exposure of Norway spruce to elevated CO2 concentration induces changes in photosystem II mimicking an adaptation to increased irradiance. Journal of Plant Physiology 152: 413-419.

Tjoelker, M.G., Oleksyn, J. and Reich, P.B. 1998. Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and temperature. Tree Physiology 18: 715-726.

Wiemken, V. and Ineichen, K. 2000. Seasonal fluctuations of the levels of soluble carbohydrates in spruce needles exposed to elevated CO2 and nitrogen fertilization and glucose as a potential mediator of acclimation to elevated CO2. Journal of Plant Physiology 156: 746-750.