In 1989-90, the species' tolerance to cold storage increased in the order Douglas fir < Japanese larch < Sitka spruce (Queen Charlotte Islands). This order was evident in 1990-91 too. Within Sitka spruce, the more northerly the seed origin the greater its tolerance to cold storage. Scots pine was only slightly more tolerant of cold storage than Sitka spruce of Oregon origin. Undercutting and regular wrenching had a minor effect on the ability of Japanese larch to withstand cold storage but did not have a consistent, significant effect on the tolerance of Sitka spruce or Douglas fir.
Plants were more tolerant of storage in the colder winter of 1990-91 than in the preceding milder winter. On the basis of the leakage patterns found in the two contrasting years, safe lifting dates are given for Sitka spruce (Queen Charlotte Island) and Japanese larch. Safe lifting dates for Douglas fir, Scots pine, Oregon Sitka spruce and Alaskan Sitka spruce are suggested; further research is needed to verify these dates.
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