Cold-hardiness of silver sagebrush seedlings

Abstract

Silver sagebrush (Artemisia cana Pursh), a common shrub on Northern Mixed Prairie in Canada, is an excellent species to consider for ecological restoration. On the Canadian Prairies, freezing temperature can occur during April and early May, months when most silver sagebrush seedlings emerge. Decreasing temperatures in autumn or exposure to freezing temperature through winter may also be lethal to seedlings of this long-lived shrub. The purpose of this study was to characterize freezing tolerance in silver sagebrush seedlings because low temperatures may reduce establishment. Seedlings were grown from 1 week to 1 full growing season, exposed to freezing temperatures under controlled conditions, 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 degree C, respectively. Changes in mortality with temperature variations were more gradual in younger than older seedlings, and mean LT95 was 2.8 degree 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 degree C, while mortality of acclimated seedlings was nearly nil in most cases. Only 6.9% (SE = 5.5) of seedlings grown under field conditions died in November after exposure to -39 degree C. Freezing tolerance of field-grown seedlings remained high over winter; seedling mortality after exposure to -39 and -45 degree C averaged 5.6% (SE = 4.1) in March. No seedlings survived temperatures lower than -15 degree C in April, and predicted LT50 and LT95 averaged -15.6 and -19.3 degree C, respectively. Increased mortality after freezing in April indicates seedlings de-acclimated as temperatures rose and day length increased in spring. Since the potential of developing freezing tolerance is greater in older than younger seedlings, silver sagebrush seedlings that germinate early in growing season may survive the winter better than those germinating later. Under normal circumstances, temperatures on the Canadian Prairies should not threaten survival of silver sagebrush seedlings during their first winter.