Stomatal conductance in three conifer species at different elevations during summer in Wyoming

Stomatal conductances to water vapor diffusion in Engelmann spruce (Piceaengelmannii Parry ex Engelm.), subalpine fir (Abieslasiocarpa (Hook.) Nutt.), and lodgepole pine (Pinuscontorta Engelm.) were compared to determine environmental influences on conductance at higher (3220 m) and lower (2860 m) elevations in the central Rocky Mountains. Measurements were taken on clear days, and soil water potentials remained at or greater than −0.1 MPa. Interspecific differences were small between spruce and fir at either site, but pine conductance was generally higher than spruce or fir at 2860 m. Daily maximum conductance in spruce and fir at 3220 m did not increase above 1.0 mm s−1 until daily minimum air temperature (early morning) increased to near 1 °C in early summer. Increases in maximum conductance above 2.0 mm s−1 occurred at both elevations when minimum air temperature rose above approximately 5 °C. At the lower elevation site, increases in maximum conductance during late July and mid-August appeared to depend strongly on soil temperature increasing above 7–8 °C. The persistence of cold soil temperatures in the highest elevations of the subalpine forest may serve to inhibit stomatal opening in spruce and fir in comparison to spruce, fir, and pine in lower elevation forests.