SPATIAL AND PHYSIOLOGICAL OVERLAP OF 3 COOCCURRING ALPINE WILLOWS

The distribution, seasonal and diurnal patterns of stomatal conductance, and bulk tissue water relations characteristics were examined in three co-occurring species of alpine Salix spp. in the C Canadian Rocky Mountains. The species differ in their distribution along a topographic-soil moisture gradient. Correlated to the differences in spatial distribution were differences in water relations among the species. Salix reticulata inhabited the most xeric end of the gradient and exhibited the lowest osmotic potentials near full-tissue hydration and turgor loss, had relatively inelastic tissues, was able to adjust osmotically, had low rates of stomatal conductance, and a relatively high tolerance to an increased leaf-to-air vapour pressure gradient (ΔW). S. barrattiana grew at the wettest end of the gradient and demonstrated comparatively high osmotic potentials near full-tissue hydration and turgor loss, highly elastic tissues, no ability to adjust osmotically, had high rates of stomatal conductance, and a very marked stomatal sensitivity to increased ΔW. S. arctica inhabited intermediate sites along the gradient and its tissues demonstrated intermediate osmotic and elastic properties, ability to adjust osmotically, rates of stomatal conductance, and stomatal sensitivity to increased ΔW. A measure of spatial and physiological overlap was calculated using Pianka's (1974) symmetrical index. -from Author