Relationships among cold hardiness, root growth potential and bud dormancy in three conifers.

Greenhouse-cultured, container-grown ponderosa pine (Pinus ponderosa var. scopulorum Engelm.), interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) and Engelmann spruce (Picea engelmannii (Parry) Engelm.) were cold acclimated and deacclimated in growth chambers over 19 weeks. Stem cold hardiness, total new root length at 14 days and days to bud break were measured weekly. Relationships among cold hardiness, root growth potential (RGP) and bud dormancy suggest that cold hardiness, which can be measured quickly, could provide a useful basis for estimating the two other parameters. During cold acclimation, there was a lag period in which stem cold hardiness remained at -15 degrees C and RGP was at a minimum, in all three species. Douglas-fir and Engelmann spruce buds remained fully dormant during this lag period. Ponderosa pine buds had no chilling requirement for the loss of dormancy, and reached quiescence during the lag period. Immediately following the lag period, as stem cold hardiness progressed to -22 degrees C, RGP increased to a high plateau in all three species, and Douglas-fir and Engelmann spruce buds approached quiescence. Cold deacclimation and bud development began immediately on exposure to warm, long days, but RGP remained high until stem cold hardiness returned to approximately -15 degrees C. At bud break, cold hardiness and RGP were at the minimum.