Impact of warm spells during late fall and winter on frost hardiness of short-day treated Norway spruce seedlings

Warm spells during the late fall and winter are expected to become more frequent in boreal areas due to climate change. The aims of our study were to investigate whether warm spells in the late fall and winter cause deac-climation of Norway spruce (Picea abies (L.) Karst.) seedlings, whether the seedlings are capable of reacclimating in response to a subsequent cold period, and whether a short-day (SD) treatment applied in a nursery affects seedlings' responses to warm and cold spells during the non-growing season (hereafter off-season). During the preceding growing season, the seedlings were treated either with a SD treatment (SD1: June 29 - July 20 or SD2: July 20 - August 10) or were grown in a natural photoperiod. The seedlings were artificially exposed to a 7-day warm spell, followed by a 7-day cold spell on four occasions (late fall: October and November; winter: January and March). The frost hardiness of the needles and the probability of bud damage were determined before and after the warm spell and after the cold spell. The dormancy status of the seedlings was also determined. Warm spells reduced the frost hardiness in needles and increased the probability of bud damage in January and March, but not in October and November. Needles were able to reacclimate from November to January, and buds also in early March. Our results indicate that in boreal areas, warm spells followed by low temperatures may predispose Norway spruce seedlings to freezing damage later in the winter, but not in the late fall. The seedlings that received SD2 treatment retained an ability to reacclimate after a warm spell in March, unlike the seedlings in the other photoperiod treatments, indicating that the responses of seedlings to the fluctuating temperature condi-tions in winter are affected by the growth conditions during the previous growing season. Therefore, the use and timing of SD treatment may affect survival of fall-planted seedlings and seedlings that overwinter outdoors in nurseries, especially during snowless winters.

Automated summary

The frequency of warm spells in boreal areas during late fall and winter is expected to increase due to climate change. This study aimed to investigate the effects of warm spells in late fall and winter on the deacclimation of Norway spruce seedlings, their ability to reacclimate during subsequent cold periods, and the impact of short-day treatment in a nursery on their responses to temperature fluctuations during the non-growing season. The results showed that warm spells reduced frost hardiness in needles and increased the probability of bud damage in January and March, but not in October and November. Needles were able to reacclimate from November to January, and buds in early March. Seedlings that received the SD2 treatment retained the ability to reacclimate after a warm spell in March, unlike those in other photoperiod treatments. The timing and use of SD treatment may therefore affect the survival of fall-planted seedlings and seedlings overwintering outdoors in nurseries, particularly in snowless winters.