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Seasonal Photosynthetic Response of Pine to Elevated CO2
Hymus, G.J., Ellsworth, D.S., Baker, N.R. and Long, S.P.  1999.  Does free-air carbon dioxide enrichment affect photochemical energy use by evergreen trees in different seasons?  A chlorophyll fluorescence study of mature loblolly pine.  Plant Physiology 120: 1183-1191.

What was done
Fifteen-year-old loblolly pine trees growing in circular FACE plots (30-m diameter) receiving atmospheric CO2 concentrations of 350 and 560 ppm were studied to determine the seasonal effects of elevated CO2 on various photosynthetic parameters.  The soils within these face plots, located in North Carolina, USA, are characteristically low in available nitrogen and phosphorus.  Hence, any CO2-induced influences on photosynthesis may involve additional interactions resulting from reduced soil fertility.

What was learned
During the warmer months of the year, elevated CO2 significantly increased the proportion of light energy used in the generation of ATP and NADPH (products of the light-dependent reactions of photosynthesis), which are subsequently used in the fixation of CO2 (light-independent reactions of photosynthesis).  Such CO2-induced increases in photochemical productivity resulted in net photosynthetic rates that were up to 65% greater than rates observed under ambient CO2 concentrations.  In contrast, elevated CO2 caused significant reductions in the utilization of light energy during the cooler months of the year.  However, cool-season rates of net photosynthesis were still greater in trees exposed to elevated CO2, suggesting that the light-independent reactions were still more-limiting to the overall photosynthetic process than those reactions directly involved with the utilization of light energy.

What it doesn't mean
The authors closed with a statement suggesting that elevated CO2 "may add a further stress to overwintering evergreen vegetation in temperate regions."  However, we do not view these results as imposing any additional stress on such species.  Although elevated CO2 reduced photosynthetic light utilization in cooler months, CO2-enriched trees still exhibited net photosynethic rates that were higher than those displayed by trees grown in ambient CO2.  Thus, in totality, elevated CO2 provided a net benefit to photosynthesis in this species; albeit it could have been even greater in the absence of cool-season light utilization reductions.

What it means
As the atmospheric CO2 concentration continues to increase, loblolly pine trees should continue to display seasonal changes in photosynthesis; with higher rates being exhibited in the warmer, rather than cooler, months of the year.  However, even in the cooler months, photosynthetic rates will still likely be greater than those that would have been displayed at lower atmospheric CO2 concentrations.  Thus, the rising CO2 content of the air should stimulate annual carbon gain, carbohydrate synthesis, and biomass production in this perennial evergreen species.

Reviewed 15 September 1999