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The Unique Impact of Elevated CO2 on the Stomatal Conductances of Certain Pine Trees in China
Reference
Zhou, Y.M. and Han, S.J.  2005.  Photosynthetic response and stomatal behaviour of Pinus koraiensis during the fourth year of exposure to elevated CO2 concentration.  Photosynthetica 43: 445-449.

What was done
The authors grew four-year-old Pinus koraiensis saplings for four additional years out-of-doors (rooted in the ground) in open-top chambers that were maintained at atmospheric CO2 concentrations of approximately 350 and 700 ppm, at the conclusion of which time they measured needle stomatal numbers and conductances and light-saturated rates of net photosynthesis.

What was learned
The 350-ppm increase in the air's CO2 concentration did not elicit any change in stomatal numbers, but it led to a 20% increase in stomatal conductance (presumably due to an increase in stomatal apertures) and a 62% increase in the mean rate of light-saturated net photosynthesis.

What it means
Generally, stomatal conductance declines when plants are grown in CO2-enriched air, although this response is not as strong in woody species as it is in herbaceous species (Saxe et al., 1998).  In fact, some studies of trees have even reported no stomatal response to atmospheric CO2 enrichment (Roberntz and Stockfors, 1998; Bartak et al., 1999).  Consequently, Zhou and Han's results are rather unique, as they indicate a substantial increase in stomatal conductance in response to a doubling of the air's CO2 content.  Under conditions of adequate water availability, therefore, this response should enable P. koraiensis trees to display large growth enhancements in response to increases in the air's CO2 concentration, for as Zhou and Han note, "photosynthesis is dependent on stomata for its supply of CO2."  And with wider stomatal apertures and greater stomatal conductances in a CO2-enriched world of the future, P. koraiensis trees should experience large increases in net photosynthesis rates (as they did in this study) and, ultimately, commensurate increases in biomass production.

References
Bartak, M., Raschi, A. and Tognetti, R.  1999.  Photosynthetic characteristics of sun and shade leaves in the canopy of Arbutus unedo L. trees exposed to in situ long-term elevated CO2Photosynthetica 37: 1-16.

Roberntz, P. and Stockfors, J.  1998.  Effects of elevated CO2 concentration and nutrition on net photosynthesis, stomatal conductance and needle respiration of field-grown Norway spruce trees.  Tree Physiology 18: 233-241.

Saxe, H., Ellsworth, D.S. and Heath, J.  1998.  Tree and forest functioning in an enriched CO2 atmosphere.  New Phytologist 139: 395-436.

Reviewed 30 November 2005