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Needle hardiness of introduced yellow pine, Pinus banksiana Lamb., lodgepole pine, P. contorta Dougl. and native white spruce, Picea glauca (Moench) Voss, were assessed by the effective prefreezing temperature method. Yellow pine needles were less hardy than lodgepole pine or white spruce needles in Alaska on each date measured. Although hardiness decreased in springtime in all species, decreases in hardiness in yellow pine began before temperatures were above -20°C, apparently in response to day length, while decreases in hardiness in lodgepole pine and white spruce began only when mean temperatures were above 0°C. Hardiness was increased by decreasing the water content of yellow pine and spruce needles. However, only the latter increased its field hardiness by decreased water contents, and only to a small degree. Large decreases in phospholipid occurred during the dehardening period, indicating the presence of major membrane-associated changes. However, changes in hardiness did not closely parallel those in phospholipid; hardiness decreased before phospholipid did in spruce and after phospholipid did in lodgepole pine. In yellow pine, changes in hardiness were more closely related to changes in phospholipid content. Decreases in phospholipid appeared to be correlated with the day length in all species. [ABSTRACT FROM AUTHOR]