Frost hardiness of balsam poplar (Populus balsamifera L.) during the spring dehardening period

Changes in the frost hardiness of balsam poplar (Populus balsamifera L.) cuttings from four populations along a latitudinal transect from N. Wisconsin to Bearskin L, Ontario, were examined during the spring of 1987. Hardiness levels of dormant stem cuttings from the two extreme populations were examined after various incubation periods, under two different dehardening temperature regimes, with a standard freezing test (freezing temperatures: -S.-l 1 ,-19, and -27° C). Northern clones were less susceptible to frost injury than southern clones during the spring dehardening period, and this phenomenon was closely related to the tendency of northern clones to remain dormant longer than southern clones. High within-population variation was also noted in hardiness levels and bud break characteristics. Leaf tissue dehardened more rapidly than stem tissue, and the dehardening process occured more rapidly at the higher incubation temperature. A second study in which cuttings from the four provenances were subjected to a series of controlled freezing temperatures (-3,-6,-9,-12,-18, and -24° C) at parallel developmental stages revealed that provenance differences in frost injury were essentially a function of differential shoot phenology at the time of freezing. Cuttings were hardy to -18° C when leaf expansion first became visible, and could be subjected to -12° C without injury when the newly expanding shoot became visible, indicating that an attenuated form of hardiness may exist even when the shoots are actively growing