Seasonal variation in root hardiness of container-grown Scots pine, Norway spruce, and lodgepole pine seedlings

Roots of containerized seedlings of Scots pine (PinussylvestrisL.), Norway spruce (Piceaabies (L.) Karst.), and lodgepole pine (Pinuscontorta Dougl.) grown for one season were experimentally frozen during autumn, winter, and spring. After freezing, root growth capacity and seedling survival were determined by growing the seedlings for 3 weeks in a growth chamber. Root vitality was also tested with the triphenyl tetrazolium chloride method. The seedlings showed a progressive increase in root hardiness towards midwinter and a dehardening as spring approached. Scots pine and lodgepole pine roots were more sensitive to low temperatures than Norway spruce roots. Northern provenances of Scots pine and Norway spruce had the hardiest roots during the autumn. During winter and spring, provenance differences became less clear. Northern Scots pine and lodgepole pine seedlings sown in July tended to be more sensitive to low root temperatures than older seedlings sown in May. No age effects were found in southern Scots pine and Norway spruce seedlings. Although the root systems survived temperatures as low as −15 °C to −25 °C during late fall and midwinter, the capacity to grow roots tended to be stunted at considerably higher temperatures. This indicates the sensitivity of roots and emphasizes the importance of protection against low root temperatures.