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Elevated CO2 Stimulates Photosynthesis in Young Scots Pine Trees
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.

What was done
Three-year-old Scots pine (Pinus sylvestris L.) seedlings were rooted in the ground and grown in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 750 ppm for two 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 this investigation.

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

What it means
As the atmospheric CO2 concentration increases, Scots pine seedlings should respond by exhibiting increases in their photosynthetic rates, even if photosynthetic acclimation occurs.  The additional carbon obtained from the CO2-induced increases in photosynthesis will likely lead to increased biomass production, as elevated CO2 had no effect on daytime respiratory carbon losses.  Thus, it is likely that Scots pine forests and plantations will become increasingly stronger sinks for atmospheric carbon as the CO2 content of air continues to rise.