Spring Frost Damage on Young Picea sitchensis 1. Occurrence of Damaging Frosts in Scotland Compared with Western North America

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Abstract
Air frosts (in Stevenson screens) of −2.5°C or below in Scotland were judged to be potentially damaging to the newly emerging shoots on young trees of Picea sitchensis at the time of budburst. This was based on a knowledge of the killing tissue temperature and the relationship between air and grass minima in May. Dates of last spring air frosts of −2.5°C and −4.5°C were obtained for 42 meteorological stations in northern Britain with runs of 19 to 116 years. Frequency distributions of last frost dates were approximately normal. About 80% of the variation between stations could be attribuoted to altitude, distance from the sea and latitude. The variance in last dates of −2.5°C frosts decreased from mild to cold sites. Multiple regressions were used to produce maps of last frost dates in 20×20 km grid squares in Scotland. Dates of last 28° F (−2.2°C) and 24°F (−4.4°C) frosts were obtained for 20 sites in western North America, spanning the natural range of P. sitchensis . Mean dates increased, and variances decreased, from south to north. Most Scottish upland plantation sites (e.g. Glentress and Eskdalemuir) experience −2.5°C air frosts until later in the year than all coastal stations in western North America south of Cordova, Alaska. On average, Eskdalemuir (242 m altitude) experiences −2.5°C air frosts about 4 weeks later in the spring than Masset (3 m) on the coast of the Queen Charlotte Islands, which has been the source of much of the P. sitchensis planted in Scotland. Masset has a spring frost climate somewhat milder than Durham.

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