Microclimate, freezing tolerance, and cold acclimation along an elevation gradient for seedlings of the Great Basin Desert shrub, Artemisia tridentata Michael E. Loik % * & Sean P. Redar w % Department of Environmental Studies, University of California, Santa Cruz, CA, 95064, U.S.A. wUnited States Forest Service, Fawnskin, CA, 92333, U.S.A. Vegetation, microclimate, seedling frequency, freezing tolerance, and cold acclimation were compared for seedlings of Artemisia tridentata collected from 1775, 2175, and 2575 m elevation in the eastern Sierra Nevada, California. Data were used to test the hypothesis that ecotypic differences in stress physiology are important for seedling survival along gradients from desert to montane ecosystems. The vegetation canopy cover and A. tridentata seedling frequency were greatest at 2575 m, compared to 1775 and 2175 m. Snow cover ameliorated temperatures near the soil surface for part of the winter and depth varied across elevations. Freezing tolerance was compared for seedlings maintained in growth chambers at day/night air temperatures of 251C/151C. The temperature at which electrolyte leakage and Photosystem II function (F V /F M ) from leaves were half-maximum was approximately 13?51C for leaves of seedlings from all three elevations. Shifting day/night air temperatures from 251C/151C to 151C/51C initiated about 1?51 of acclimation by plants from all three altitudes, with seedlings from the highest elevation exhibiting the greatest acclimation change. Measurements of ambient air and canopy temperatures at the three elevations indicated that wintertime average low temperatures were consistent with the measured degree of freezing tolerance. At small spatial scales used in this study, pollen and seed dispersal between study sites may have precluded resolution of ecotypic differences. Patterns of freezing tolerance and cold acclimation may depend on a combination of mesoclimate and microclimate temperatures, canopy cover, snow depth, and snow melt patterns. # 2003 Elsevier Science Ltd. Keywords: air temperature; electrolyte leakage; frost; sagebrush; stress Introduction Cold is considered one of the most important factors limiting the productivity of plants (Levitt, 1980; Larcher, 1995). Many species are geographically limited by sub- zero temperatures that affect key physiological processes (Levitt, 1980; Nobel, 1988; Loik & Nobel, 1993a). Episodic sub-zero air temperatures (i.e. ‘cold snaps’) are *Corresponding author. Fax: +1 831 459-4015. E-mail: mloik@cats.ucsc.edu 0140-1963/03/040769 + 14 $30.00/0 # 2003 Elsevier Science Ltd. Journal of Arid Environments (2003) 54: 769–782 doi:10.1006/jare.2002.1106, available online at http://www.sciencedirect.com