PARENT SESSION
Oral Session #10: Elevated CO₂.
Presiding: D. Hileman
Monday, August 5. 8:00 AM to 11:30 AM. Grand Ballroom East, Radisson.

Photosynthesis versus nitrogen relationships at the Duke Forest FACE experiment indicate no evidence of photosynthetic acclimation after five years of CO₂ enrichment .

Springer, Clint^*,1, DeLucia, Evan², Thomas, Richard¹, ¹ West Virginia University, Morgantown, WV² University of Illinois, Urbana, IL

ABSTRACT- Photosynthetic acclimation to elevated CO₂ was assessed in the Duke Forest FACE experiment using the relationships between net photosynthesis (A) and foliar nitrogen (N). In this FACE experiment, treatment plots have been fumigated with elevated CO2 (ambient plus 200 ppm) since August 1996. Net photosynthesis was measured using growth and reciprocal CO₂ concentrations in the overstory using Pinus taeda (loblolly pine) and Liquidambar styraciflua (sweetgum) and in the understory using Acer rubrum, Carya glabra, Cercis canadensis, Liquidambar styraciflua and Cornus florida. Foliar N was 47% greater at the top of the canopy than at the bottom in both loblolly pine (p=0.005) and sweetgum (p=0.090) and A was stimulated by CO₂ at both leaf positions in loblolly pine (p=0.0029) and sweetgum (p=0.0068). A linear relationship between A and N was found through the forest canopy. Elevated CO₂ significantly increased the slope (65%; p=0.05) of the A-N relationship of sweetgum. Elevated CO₂ increased the slope of the A-N relationship of loblolly pine, however this increase was not statistically significant (p=0.30). There were no significant differences between the slopes of the A-N relationships for either species when A measured at the growth CO₂ concentration was compared to A measured at the reciprocal CO₂ concentration. Thus, we found no evidence of photosynthetic acclimation to CO₂ enrichment in the leaves of overstory loblolly pine or sweetgum trees after five years of treatment.

KEY WORDS: Elevated CO₂, Photosynthetic acclimation, Duke FACE, Nitrogen