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Photosynthetic Response of Sun and Shade Leaves to Elevated CO2
Reference
Herrick, J.D. and Thomas, R.B. 1999. Effects of CO2 enrichment on the photosynthetic light response of sun and shade leaves of canopy sweetgum trees (Liquidambar styraciflua) in a forest ecosystem. Tree Physiology 19: 779-786.

What was done
Sweetgum trees (approximately seven to eleven meters in height) growing in circular FACE plots (30-m diameter) receiving atmospheric CO2 concentrations of 350 and 560 ppm were studied in June and August to determine the photosynthetic response of sun and shade leaves to elevated CO2 during their first year of differential CO2 exposure. The soils in this natural forest ecosystem, 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
Elevated CO2 significantly increased photosynthetic rates in both sun and shade leaves, regardless of measurement date. However, the greatest CO2-induced photosynthetic stimulation occurred in August, when the mean maximum air temperature was 4C higher and monthly rainfall was 66% less than it was in June. In June, for example, elevated CO2 increased photosynthetic rates of sun and shade leaves by 92 and 54%, respectively, while in August, corresponding increases were 166 and 68%. Thus, the positive photosynthetic response of sweetgum leaves to elevated CO2 increased with temperature and water stress.

What it means
As the atmospheric CO2 concentration continues to increase, sweetgum trees should display photosynthetic increases even during conditions of low soil moisture or higher than normal air temperatures. This phenomenon will likely occur in all leaves, regardless of their access to sunlight. Thus, as photosynthesis increases with the CO2 content of the air, this species will likely utilize additional carbohydrates to increase its growth and biomass.


Reviewed 15 October 1999