Intraspecific variation in the water relations of Salix arctica, an arctic-alpine dwarf willow

The seasonal and diurnal water relations were investigated within arctic and alpine populations of the dwarf willow Salix arctica. Marked differences that were habitat dependent (e.g. xeric vs. mesic) occured both within and between the populations. The environmental variables that most affected plant water balance and the bulk tissue water relations were soil water potential ($\psi _{\text{soil}}$) and the leaf-to-air water vapor pressure gradient (Δw), however, low soil temperature (< 4.0° C) also had a marked effect in the wet to mesic habitats. The effects of declining$\psi _{\text{soil}}$and increased Δw were most pronounced in the plants growing in xeric habitats in both populations. Stomatal response to increased Δw was two-fold greater in alpine versus arctic plants and is hypothesized to have arisen in response to more frequent exposure to the higher evaporative conditions of alpine existance. Seasonal fluctuations in the osmotic potential closely followed changes in$\psi _{\text{soil}}$, suggesting that these were active rather that passive changes. Additionally, plants from xeric habiats had a lower bulk tissue elastic modulus (more elastic tissues) in both arctic and alpine populations. The osmotic and elastic properties enhanced turgor maintenance over a broad range of leaf water potentials and during periods when Δw was high. Turgor maintenance also correlated to continued transpiration despite fluctuations in soil and atmospherically induced water deficits. Arctic habitats have a shorter growing season, lower soil temperatures due to the presence of permafrost, but higher soil water potentials and lower leaf-to-air vapor pressure gradients than alpine habitats. The observed variation in patterns of stomatal conductance and in values of tissue water relations characteristics between arctic and alpine populations of S. arctica is hypothesized to have arisen in response to these different environmental regimes which represent different selective regimes that occur along the arctic-alpine environmental continuum inhabited by this wide ranging species.