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

Stomatal response to CO₂ in conifers of semi-arid regions.

DeMarco, Jennie^*,1, Edgar, Colin¹, Koch, George¹, ¹ Northern Arizona University, Flagstaff, AZ

ABSTRACT- Coupled biosphere-atmosphere models of global change increasingly incorporate key physiological responses of vegetation, including reduced stomatal conductance (g) in elevated CO₂ . The g decrease with CO₂ doubling averages about 25%, there being considerable variation among species, and relatively little information for conifers. Studies in FACE experiments indicate short-term g responses qualitatively reflect long-term response, but the response magnitude may acclimate. We have examined the g response to a short-term (< 1 hr) increase in CO₂ partial pressure (from 29 to 56 Pa) in piñon pine (Pinus edulis) and one-seeded juniper (Juniperus monosperma), which co-occur throughout high elevation semi-arid woodlands of the southwestern U.S. The conductance decrease was small for both species, averaging about 8% for piñon and 12% for juniper. The response of juniper was more variable; g decreased by 22% during the dry season, but by only 6% during the summer monsoon season. Including data from both seasons, the g response of piñon was significantly and negatively correlated with the CO₂ internal (Ci) at ambient CO₂ . In contrast, there was no significant correlation between Ci and the degree of conductance response in juniper. We analyze these responses in light of published xylem vulnerability to cavitation in piñon and juniper and observed changes in xylem water potential associated with the responses of g and transpiration to elevated CO₂ .

KEY WORDS: elevated CO2, stomatal conductance, water relations, transpiration