title: Carbon isotope discrimination in three semi-arid woodland species along a monsoon gradient.
abstract: Leaf carbonisotope discrimination (Δ) was measured for three dominant, semi-arid woodland species along a summer monsoon gradient inthe southwestern United States over a 2-year period. We tested the hypothesis that decreased humidity levels during the growing season along this gradient resulted in lower leaf Δ values. Sites of similar elevation along the transect were selected and the range in monsoon contribution to overall annual precipitation varied from 18 to 58%, while total annual precipitation differed by a maximum of only 25% across this gradient. Leaf Δ values in Quercus gambelii were negatively correlated with ϖ, a seasonally-weighted estimate of the evaporative humidity gradient, suggesting that stomatal conductance declined as transpiration potential increased. For two other trees that co-occurred along this gradient, Pinus edulis and Juniperus osteosperma, Δ remained relatively constant despite large variation in ϖ. These woodland species represent the full spectrum of responses of carbon isotope discrimination to increases in evaporative potential; that of decline where ci/ca(ratio of internal to ambient CO2concentration) and presumably stomatal conductance decrease, and that of constancy where whole plant internal adjustments allow ci/cato remain stable.
authors.affiliation: Department of Biology, University of Utah, 84112, Salt Lake City, UT, USA. dgw@ag.arizona.edu.
authors.forename: David G
authors.initials: DG
authors.lastname: Williams
authors.affiliation: Department of Biology, University of Utah, 84112, Salt Lake City, UT, USA.
authors.forename: James R
authors.initials: JR
authors.lastname: Ehleringer
keywords: Carbon isotope discrimination; Humidity; Monsoon; Pinyon-juniper ecosystem