Williams, D.G., and J.R. Ehleringer. 1996. Carbon isotope discrimination in three semi-arid woodland species along a monsoon gradient. Oecologia 106:455-460.

Leaf carbon isotope discrimination (D) was measured for three dominant, semi-arid woodland species along a summer monsoon gradient in the southwestern USA over a two-year period. We tested the hypothesis that decreased humidity levels during the growing season along this gradient resulted in lower leaf D 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 D values in Quercus gambelii were negatively correlated to w, a seasonally-weighted estimate of the evaporative humidity gradient, suggesting that stomatal conductance declined as transpiration potential increased. For two other trees co-occurring along this gradient, Pinus edulis and Juniperus osteosperma, D remained relatively constant despite large variation in w. 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 CO2 concentration) and presumably stomatal conductance decrease, and that of constancy where whole plant internal adjustments allow ci/ca to remain stable.