Carbon isotope discrimination in three semi-arid woodland species along a monsoon gradient.

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 c i /c a (ratio of internal to ambient CO2 concentration) and presumably stomatal conductance decrease, and that of constancy where whole plant internal adjustments allow c i /c a to remain stable.