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Effects of Elevated CO2 on Needle Characteristics of Young Scots Pine Trees
Lin, J., Jach, M.E. and Ceulemans, R.  2001.  Stomatal density and needle anatomy of Scots pine (Pinus sylvestris) are affected by elevated CO2New Phytologist 150: 665-674.

What was done
Seven-year-old Scots pine (Pinus sylvestris L.) seedlings were rooted in the ground and grown in open-top chambers receiving atmospheric CO2 concentrations of 350 and 750 ppm for four years to determine the long-term effects of elevated CO2 on various needle characteristics in this important European timber species.  In addition, in order to make the experimental results more representative of the natural world, no additional nutrients or irrigation waters were applied to the soils during this investigation.

What was learned
After the fourth year of atmospheric CO2 enrichment, a detailed analysis indicated that elevated CO2 reduced needle stomatal density by an average of 7.4%.  In contrast, elevated CO2 increased needle thickness, mesophyll tissue area, and total cross-sectional area by averages by 6.4, 5.7, and 10.4%, respectively.  In addition, atmospheric CO2 enrichment increased the average relative area occupied by phloem cells by 4.4%.

What it means
As the atmospheric CO2 concentration increases, Scots pine seedlings will likely respond by reducing their stomatal density, which may allow them to conserve water and better cope with periods of drought and water stress.  Other CO2-induced changes in needle structural and anatomical characteristics will likely allow Scots pine to increase and maintain enhanced rates of photosynthesis in CO2-enriched environments.  Increases in mesophyll tissue area, for example, which is the photosynthetic tissue in plants, will undoubtedly increase photosynthetic rates in Scots pine needles.  Moreover, CO2-induced increases in phloem cell area will allow for greater transport of photosynthetic sugars from needles to actively growing sinks, which may keep photosynthetic acclimation from being triggered by photosynthetic sugar accumulation in source needles.  In summary, as the air's CO2 content rises, anatomical and structural changes associated with needles of Scots pine will likely allow this forest species to exhibit ever-increasing rates of photosynthesis and biomass production.