Physiological quality of lodgepole pine and interior spruce seedlings: effects of lift date and duration of freezer storage

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Abstract
Owing to frozen ground, nurseries in interior Oregon, Washington, and British Columbia are often unable to lift bare-root seedlings during midwinter when they are in peak physiological condition. Therefore, seedlings are normally lifted in the fall and planted either immediately or after overwinter storage, or lifted in the spring and planted either immediately or after brief storage. The objective of this study was to ascertain which of the above strategies produced planting stock with the highest physiological quality. The species evaluated were lodgepole pine (Pinuscontorta Dougl.) and interior spruce (Piceaglaucaengelmannii complex). Seedlings were grown in a Weyerhaeuser Canada nursery near Armstrong, B.C., and lifted on nine dates spanning early October 1982 to late March 1983. Seedling quality was evaluated at each lift date and following 2 and 6 months in freezer (−1 °C) storage. Evaluations consisted of measurements of dormancy release index, root growth potential (RGP), stress resistance (SR), and frost hardiness (FH). The main effects of lift date and storage duration were generally highly significant on all seedling-quality attributes. Lift date × storage duration interactions were also highly significant in most cases. Dormancy weakened throughout winter in the nursery, with accumulated chilling. Storage slowed the release of dormancy for most lift dates. RGP was very high in fall-lifted seedlings, but declined rapidly after storage. RGP again increased in late winter, but was not appreciably affected by storage except for March-lifted seedlings. SR was relatively high in fall-lifted seedlings and very low in spring-lifted seedlings. SR was also reduced by storage, especially after 6 months. However, SR did not fall as rapidly in stored seedlings as it did in seedlings left in the nursery bed. FH was −30 °C in early October; during midwinter FH was beyond the limit of measurement (−40 °C). Seedlings of both species appeared to deharden in storage, but at a relatively slow rate. The best operational strategy seems to be fall lifting, beginning November 1, with overwinter freezer storage. Spring-lifted seedlings tended to exhibit low RGP, low SR, low FH, and poor storability.

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