Autumn stomatal closure in six conifer species of the Central Rocky Mountains.

Environmental and water relations parameters during fall were monitored for six conifer tree species common to the central Rocky Mountains growing naturally at the same location (Pinus contorta, Pinus ponderosa, Pinus flexilus, Pseudotsuga menziesii, Abies lasiocarpa, Picea engelmannii). Subsequent to what appeared to be the beginning of seasonal stomatal closure, leaf conductance to water vapor declined sharply following the onset of freezing air temperatures at night. A coincident rapid decline in morning xylem pressure potentials (ψ) also occurred which resulted in values that were considerably below afternoon ψ. Continuing decreases in maximum leaf conductance during the day were highly correlated with corresponding decreases in minimum nocturnal air temperatures of the preceding night. By mid-December, morning ψ returned to values very near afternoon ψ and were only slightly lower than before the onset of subfreezing nights. A preliminary model is proposed which interprets the qualitative interaction between air and soil temperatures, soil and plant water potentials, and leaf conductance during seasonal stomatal closure in fall.