Leaf carbonisotope discrimination (?) 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 ? 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 ? values in Quercus gambelii were negatively correlated with ?, a seasonally-weighted estimate of the evaporative humidity gradient, suggesting that stomatal conductance declined [...]
