Ten-year exposure to elevated CO2 increases stomatal number of Pinuskoraiensis and P.sylvestriformis needles

Stomatal number and stomatal conductance are important structural and functional parameters for the assessment of carbon assimilation and water use under elevated CO2. We studied stomatal density, number of stomatal rows and stomatal conductance of Pinussylvestriformis and P.koraiensis needles exposed to elevated CO2 (500 μmol mol−1 CO2) in open-top chambers for 10 years (1999–2009). Elevated CO2 increased stomatal density on P. sylvestriformis by 10.8 % (13.5 % on abaxial surface and 8.0 % on adaxial surface) and the number of stomatal rows on P. koraiensis by 7.9 % (5.0 % in 1-year-old needles and 10.7 % in current-year needles). Increased stomatal density for P. sylvestriformis and number of stomatal rows for P. koraiensis indicate that elevated CO2 increases stomatal number in both tree species. Needle age significantly influenced stomatal density and number of stomatal rows in P.koraiensis but not in P.sylvestriformis. For both species, elevated CO2 did not significantly affect stomatal conductance but increased water use efficiency. The increase in stomatal number is not accompanied by significant changes in stomatal conductance at elevated CO2 for both tree species suggesting that there may be no direct relationship between stomatal conductance and stomatal numbers.