Cold acclimation ofPinus sylvestrisandPinus contortaprovenances as measured by freezing tolerance of detached needles

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Abstract
Differences in cold acclimation were evaluated for detached needles of six Pinus contorta Dougl. and six Pinus sylvestris (L.) provenances originating from latitudes 55° to 68° N in western Canada and northern Sweden. The needles were collected in a Swedish field trial and freeze tested at various temperatures in the range of ‐ 6°C to ‐ 29°C on four occasions from August to October. Temperatures causing 50% discolored (dead) needle tissue were considered a measure of the level of cold acclimation. Provenance variation in needle injury was evident during the whole test period. Variation within species may largely be explained by the latitude of provenance origin, the more northern, the better the frost tolerance. The cold acclimation of lodgepole pine was comparable to that of Scots pine of approximately eight to five latitudes more northern origin from August to October.