Abstract
Potted 3–4-yr-old seedlings of white, red, and jack pine (Pinus strobus L., P. resinosa Ait., and P. banksiana Lamb.), white, black, and Norway spruce (Picea glauca (Moench) Voss, P. mariana (Mill.) B.S.P., and P. abies (L.) Karst.), and Eastern larch or tamarack (Larix laricina (Du Roi) K. Koch) were grown outdoors and subsequently subjected to freezing temperatures in a freezing chamber during the period from September 1968 to November 1969. Each freezing test consisted of five trees per species. Electrical impedance readings were taken on each tree. The frost hardiness was determined by visually assessing damage and measuring subsequent growth. Differences in frost hardiness between species were small. The minimum frost hardiness in the summer was between −3 and −5 C. The maximum frost hardiness in the winter was below −40 C, with the exception of Norway spruce, which had its maximum around −40 C. It is suggested that the processes of hardening and dehardening consist of two stages. Electrical impedance trends appear to correspond with frost hardiness trends, especially in the spruces and larch.
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