Spatial and Physiological Overlap of Three Co-Occurring Alpine Willows

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Abstract
The distribution, seasonal and dirunal patterns of stomatal conductance, and bulk tissue water relations characteristics were examined in three co-occurring species of alpine willows [Salix spp.] in the central 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 L. inhabited the most xeric end of the gradient and exhibited to 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 (.DELTA.w). Conversely, S. barrattiana Hook, grew at the wettest end of the gradient and demonstrated comparatively high osmotic potentials near full-tissue hydration and tugor loss, highly elastic tissues, no ability to adjust osmotically, had high rates of stomatal conductance, and a very marked stomatal sensitivity to increased .DELTA.w, S. arctica Pall. 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 .DELTA.w. A measure of spatial and physiological overlap was calculated using Pianka''s (1974) symmetrical index. Spatial distribution (habitat), maximum stomatal conductance to water vapour (gmax), stomatal response to increased .DELTA.w, and four bulk tissue water relations characteristics were used in the calculation of the index. The index values were compared using Mantel''s test (1967). The analysis revealed that the null hypothesis, i.e. no association exists between spatial and physiological overlap, must be rejected suggesting close correspondence between water relations characterisitcs and spatial distribution along the topographic-soil moisture gradient. The types of ecological and physiological information gathered here, when applied in conjunction with the calculation of Pianka''s index, and its statistical analysis, demonstrated that this approach may be useful for examining co-occurrence in multispecies assemblages of plants from an ecophysiological perspective, especially where a limiting resource, such as water, can be identified as a possible factor contributing to differential species distributions.

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