FREEZING TEMPERATURES AND EXPOSURE TIMES DURING BUD BREAK AND SHOOT ELONGATION INFLUENCE SURVIVAL AND GROWTH OF CONTAINERIZED BLACK SPRUCE (PICEA-MARIANA) SEEDLINGS

Spring frosts frequently cause significant damage to conifer seedlings during bud flushing and shoot elongation in forestry nurseries. To in sure adequate protection, levels of frost sensitivity must be known du ring these stages of development. Eight- or 9-month-old containerized black spruce seedlings (Picea mariana (Mill.) BSP) were submitted to f reezing temperatures of 0 degrees to -10 degrees C for 1-6 h at the fo llowing stages: (1) nonswollen terminal buds, (2) swollen terminal bud s, (3) terminal bud scales bursting, needle tips emerging, and (4) sho ots elongating, 1-5 cm. After freezing, seedlings were grown for 130 d ays in a greenhouse. Seedling survival was estimated; dead seedlings d iscarded; and damage to buds, needles, and roots as well as diameter a nd shoot increment were measured on the remaining seedlings. Frost sen sitivity increased as buds flushed and new shoots elongated. Decreased seedling and bud survival was noted with increasing time of freezing exposure and decreasing temperature in stages 2, 3, and 4. Damage to n eedles and roots increases, while diameter decreases, with decreasing temperatures at all stages; however, shoot increment was influenced by decreasing temperatures only at stages 2 and 3.