Seasonal Changes in the Frost Hardiness of Provenances of <i>Picea sitchensis</i> in Scotland

Abstract

Changes in the natural level of frost hardiness of shoots of four provenances of Picea sitchensis were monitored over two growing seasons by detaching shoots from 7 to 10-year-old trees growing in a nursery in Scotland, and subjecting them to freezing temperatures under conditions which simulated night frosts. Six seasonal phases of frost hardiness were identified (Fig. 3). During each autumn, killing temperatures (the level of hardiness) decreased from −5°C to below −20°C, beginning several weeks after shoot elongation ceased. Alaskan provenances hardened in September, apparently in response to shortening day lengths alone, whereas an Oregon provenance did not harden until November, after repeated frosts. Queen Charlotte Islands provenances were intermediate. From November to March all provenances were hardy to below −20°C, which is adequate to prevent direct freezing injury at most plantation sites. In March-April, several weeks before bud-burst, old shoots dehardened to killing temperatures of about −10°C in response to warm temperatures, and southerly provenances did so before northerly ones. During bud-burst the newly-emerging shoots were hardy to only −3°C to −5°C until they were about 3.5 cm long. All provenances burst bud at the same time and were equally frost susceptible at this time. During May-July the elongating shoots fluctuated in hardiness between −5°C and −10°C apparently in response to fluctuating ambient temperatures. In August 1980 there was a period of late summer dehardening to killing temperatures of about −3°C. Seasonal changes in hardiness are discussed in relation to changes in shoot growth and environmental factors. The main opportunities for selecting frost hardy genotypes seem to be in the rate of autumn hardening, the time of pre-bud burst dehardening, and the time of bud-burst.