COLD-HARDINESS OF SILVER SAGEBRUSH SEEDLINGS

Silver sagebrush (Artemisia cana Pursh), a common shrub on Northern Mi xed Prairie in Canada, is an excellent species to consider for ecologi cal restoration. On the Canadian Prairies, freezing temperatures can o ccur during April and early May, months when most silver sagebrush see dlings emerge. Decreasing temperatures in autumn or exposure to freezi ng temperatures through winter may also be lethal to seedlings of this long-lived shrub. The purpose of this study was to characterize freez ing tolerance in silver sagebrush seedlings because low temperatures m ay reduce establishment. Seedlings were grown from 1 week to 1 full gr owing season, exposed to freezing temperatures under controlled condit ions, and lethal temperatures for 50 and 95% mortality (LT50 and LT95) were determined. Averaged across 1- to 6-week-old seedlings, LT50 and LT95 were -7.7 and -11.1 degrees C, respectively. Changes in mortalit y with temperature variations were more gradual in younger than older seedlings, and mean LT95 was 2.8 degrees C lower in 1- and 2-week than 4- and 6-week-old seedlings. Within age groups, death after freezing was greater in non-acclimated than acclimated seedlings. Virtually no non-acclimated seedlings survived -14 degrees C, while mortality of ac climated seedlings was nearly nil in most cases. Only 6.9% (SE = 5.5) of seedlings grown under field conditions died in November after expos ure to -39 degrees C. Freezing tolerance of field-grown seedlings rema ined high over winter; seedling mortality after exposure to -39 and -4 5 degrees C averaged 5.6% (SE = 4.1) in March. No seedlings survived t emperatures lower than -15 degrees C in April, and predicted LT50 and LT95 averaged -15.6 and -19.3 degrees C, respectively. Increased morta lity after freezing in April indicates seedlings de-acclimated as temp eratures rose and day length increased in spring. Since the potential of developing freezing tolerance is greater in older than younger seed lings, silver sagebrush seedlings that germinate early in growing seas on may survive the winter better than those germinating later. Under n ormal circumstances, temperatures on the Canadian Prairies should not threaten survival of silver sagebrush seedlings during their first win ter.