Effects of light exposure in freezing temperatures on winter damage to foliage of Norway spruce container seedlings in mid and late winter: Pilot experiments in an open field

Norway spruce (Picea abies (L.) Karst.) is widely planted for reforestation in the boreal zone. It is sensitive to frost and high irradiance during the growing season, and also to winter damage, which cause growth losses in reforestation. This study made a pilot attempt to examine the needle damage and seedling vigour on hardened Norway spruce seedlings under freezing temperatures using natural and artificial light exposure from a day to weeks in an open field in mid and late winter in central Finland. The treatments induced needle browning and decreased seedling vigour, which reduced shoot and root growth during the following growing season. Visibly damaged, mottled needles of one-year terminal shoots had practically no healthy-looking cells. The new buds, however, were healthy and were able to grow during the following season. Our results suggests that, above the snow cover, the freezing temperatures and wind, rather than the intensity of light radiation, induced the observed needle damage found immediately after the treatments, and the subsequent growth reduction in the following growing season. The preliminary methods used outdoors could not distinguish all the different environmental conditions and their mechanisms of effects on seedlings, which demonstrates the need for further method development in controlling experimental conditions of air temperature, radiation intensity, and air current in future research of seedling winter damage outdoors.