Document: DAV-3-34-59

CO₂ responses in fifteen species across four free-air CO₂ enrichment (FACE) experiments: Is leaf C_i/C_a maintained?

ELLSWORTH, D.S.* ^1,4, G.W.KOCH ², P.B.REICH ³ and G.G.KATUL ⁴

Brookhaven National Laboratory, Upton, NY 11973 USA ¹
Northern Arizona University, Flagstaff, AZ 86011 USA ²
University of Minnesota, Saint Paul, MN 55108 USA ³
Duke University, Durham NC 27708 USA ⁴

Abstract:
It has been hypothesized that the ratio of leaf intercellular [CO₂] (C_i) to air [CO₂] is maintained in a relatively narrow range for a wide variety of species grown across a range in environments. Since this C_i/C_a ratio defines the 'set-point' for leaf metabolism, it is important to understand whether the stomatal and metabolic responses of leaves to elevated atmospheric CO₂ alter this ratio in a consistent manner. If C_i/C_a ratio is maintained despite a large increase in C_e, then this would substantially simplify ecophysiological models and formulations for predicting leaf C_i and hence CO₂ assimilation rates. We conducted comprehensive gas exchange studies to estimate C_i and C_i/C_ain ambient and elevated CO₂ for 15 herbaceous and woody species among 4 free-air CO₂ enrichment (FACE) sites in North America. Across the range of environments and species with 5-fold variation in maximum photosynthesis, C_i/C_a ratio was remarkably constant in enriched CO₂, deviating by 5% or less from C_i/C_a of ambient-grown leaves. We also analyzed leaves for stable carbon isotope content (13C/12C), and using a simple source-mixing model we confirmed the C_i/C_a change in elevated CO₂ estimated from gas exchange analysis. Despite maintenance of nearly constant C_i/C_a ratios, different functional groups maintained it by different types of CO₂ responses. Grass and herbaceous species generally showed significant stomatal responses to long-term CO₂ while woody species mostly did not. Strong stomatal responses to CO₂ in grasses and herbs suggest that leaf warming will occur in elevated CO₂, though water relations may be improved. Lack of stomatal responses to CO₂ in some tree species indicates a larger proportional photosynthetic enhancement by CO₂ than in grasses and herbs. The constancy of C_i/C_a ratios in elevated vs. ambient atmospheric CO₂ can help simplify practical formulations for C_i with the aim of predicting photosynthesis from stomatal conductance.

Keywords: elevated CO2, photosynthesis, FACE, forest ecophysiology

This abstract is being presented at: 10:30 AM in session:
Poster Session #18: Elevated CO₂.