Comparative drought physiology and biogeography of Quercus gambelii and Quercus turbinella

Seedlings of Quercus gambelii and Q. turbinella were subjected to two different levels of simulated drought and their responses compared. The simulated environments were designed to mimic summer drought conditions at their respective northern distributional limits in Utah, which are separated by ca. 4⚬lat. Both species lose positive turgor at ca. -3.0 MPa leaf water potential and the slopes of their turgor potential-leaf water potential relations are identical. Both species reach lethal drought stress at ca. -7.0 MPa leaf-water potential. However, Q. gambelii reached lethal leaf-water potential ca. 5 days earlier (on average) than Q. turbinella in a "low stress" environment. These observations imply that the differential drought response of the two species is due primarily to anatomical/morphological differences. Neither species receives enough precipitation (on average) at their respective northern limits to maintain positive turgor during the seasonal drought period. This study supports our previous conclusion that the northernmost populations of both species were established under a paleoclimate with more summer precipitation than the present (presumably the Hypsithermal Interval).