PARENT SESSION
Poster Session #11: Elevated CO₂.
Monday, August 6, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

Stomatal conductance and stomatal limitation of photosynthesis in overstory sweetgum trees during four years CO₂ enrichment at the Duke Forest FACE experiment.

Herrick, Jeffrey¹, Maherali, Hafiz², Thomas, Richard¹, ^{1 2}

ABSTRACT- It is unclear whether stomatal function is altered by long-term atmospheric CO₂ enrichment in woody species. To address this issue, we characterized stomatal function and stomatal limitation of photosynthesis in sun and shade leaves of overstory sweetgum (Liquidambar styraciflua L.) trees during four years of growth under elevated CO₂. Our study was conducted at the Duke Forest FACE experiment which consists of three control plots and three plots fumigated with elevated CO₂ (ambient plus 200 ppm) since August 1996. We measured steady state stomatal conductance and C_i/C_a ratios during June and September of 1997, 1998, 1999 and 2000. We also calculated stomatal limitation of photosynthesis (lg) using the differential method from A-C_i curves measured at the same time. Averaged over the four growing seasons, CO₂ enrichment only slightly decreased (7%) stomatal conductance, but this was not statistically significant (P = 0.23). However, elevated CO₂ did decrease C_i/C_a ratios (P = 0.067). Stomatal limitation of photosynthesis was 21% lower in the shade and 40% lower in the sun leaves when grown under CO₂ enrichment (P = 0.03). Overall water use efficiency (WUE) was enhanced by 31% by elevated CO₂ (P = 0.056). Our data suggest that elevated CO₂ has had a minimal effect on stomatal function of sweetgum in Duke Forest. As a result, the 55% stimulation in sweetgum photosynthesis at elevated CO₂ is most likely responsible for reducing C_i/C_a and lg, as well as increasing WUE.

KEY WORDS: FACE, CO₂, stomatal conductance , WUE