Diurnal and Seasonal Changes in Leaf Conductance, Xylem Water Potential, and Abscisic Acid of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] in Five Habitat Types

Sapling Douglas-fir [ Pseudotsuga menziesii (Mirb.) Franco] trees from five habitat types in the Coast and Cascade Mountain Ranges of Oregon were measured five times from summer 1977 through spring 1978 for diurnal patterns of leaf conductance (gL), xylem water potential (Ψ x ), and abscisic acid (ABA). Vapor pressure deficit (VPD) and solar radiation were measured as well. During summer drought, g L and Ψ x were low except at the wet coastal site. During fall, winter, and spring, g L was high, suggesting photosynthetic potential at these times, especially for the wet coastal site. Winter Ψ x was generally low, especially on the sites with high Ψ x in summer. At several sites during winter and spring, Ψ x rose during the day and fell in late afternoon, contrary to usual behavior in other seasons. This inverse pattern may indicate a freezing adaptation of Doug L as-fir to a relatively mild winter climate. ABA showed large diurnal fluctuations in summer and fall at some sites while remaining relatively constant in winter and spring but generally showed no close relationship with the other variables. In multiple regression analyses, g L was related to VPD and ABA in early summer, to VPD in late summer, to Ψ x in fall, to radiation in winter, and to Ψ x and VPD in spring. Stomatal responses seem linked to stresses of the season. Forest Sci. 28:627-638.