Frost hardening and dehardening in Abies procera and other conifers under differing temperature regimes and warm-spell treatments

Frequent bud frost damage in cultivation of Abies procera Rehder and pending climate changes are the background for this study of cold hardiness under varying acclimation regime (in closed-top chambers) and experimental warm spells during the cold season. LT50 values were established by freezing tests at different times of year. Damage and deaths were assessed on leader buds, subapical lateral buds, needles and cambium. Minor parallel experiments involved Abies nordmanniana, Picea abies and Picea sitchensis. Lower acclimation temperatures resulted in deeper frost hardiness during late autumn but less during spring, compared with ambient temperature controls. Elevated temperatures resulted in less deep frost resistance. Apical buds generally developed deeper frost hardiness than lateral buds but less deep than the cambium, varying with species, however. Frost damage in buds ranged from death over partially destroyed bud contents resulting in distorted shoots to buds seemingly remaining dormant. Responses to warm spells differed with duration, timing and species, ranging from dramatic decrease in frost hardiness with or without subsequent recovery to no reaction. Furthermore, the reactions did not show any clear relation to dormancy level. For A. procera, exposure to fluctuating temperatures appeared to be particularly problematic. This explains why this species develops best in coastal climates, and in sites sheltered from temperature extremes either by hedging, a winter snow cover, or topography. The Christmas tree production will suffer severely on sites with harsh temperatures due to losses of lateral and terminal buds, which destroy the crown symmetry. Clipping of greenery is less influenced by frost damages, although the development of normal branch whorls is often disturbed.