Environmental control of leaf water conductance in conifers

Diurnal measurements of leaf conductance, xylem water potential, temperature, humidity, and radiation were taken on six sites throughout Oregon. Xylem water potential was hypothesized to influence leaf conductance in two ways. First, predawn xylem water potential controlled the early-morning maximum leaf conductance of Pseudotsugamenziesii (Mirb.) Franco. Second, irrespective of predawn water potential, a threshold diurnal water potential was found. Whenever water potential of Pseudotsugamenziesii dropped below −20 bars (−20 × 105 Pa) during the day, a significant decrease in leaf conductance resulted. If the water potential threshold was not reached during the day, leaf conductance stayed at the morning maximum or decreased slightly in response to decreasing humidity