Spring Frost Damage on Young Picea sitchensis 2. Predicted Dates of Budburst and Probability of Frost Damage

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Abstract
The probability of frost damage at the time of budburst on the lateral shoots of young trees of Picea sitchensis in Scotland, and at Masset on the Queen Charlotte Islands, was determined from (a) the dates of budburst, predicted from past temperature recosds using a thermal time / chilling model, and (b) the dates of last air frosts of −2.5°C. The mean date of budburst occurred between 6 and 13 May at a range of Scottish sites up to about 250 m altitude, but it occurred about 6 days later for each further 100 m increase in altitude up to 550 m. This resulted from an interplay between chilling and thermal time requirements for budburst. In different years, budburst dates ranged from mid-April to early June. From 1920 to 1960 there was a trend towards warmer springs and earlier dates of budburst, which has been reversed since 1960. Potentially damaging frosts have occurred near the date of budburst once every 3 years (on average) at Eskdalemuir and Braemar during the last 66 to 107 years, and once every 3 to 5 years in many upland plantation regions of Scotland. The probability of damaging frosts was greatest in the period 1930 to 1960 when budburst occurred relatively early, and has been relatively small since 1960, because budburst has occurred relatively late. Renewed warming of the climate in future may increase the probability of damaging frosts coinciding with the time of budburst. Masset, on the Queen Charlotte Islands, from where Britain has imported most P. sitchensis seed, has relatively cool springs so that budburst occurs relatively late (averaging 18 May). Also there are few damaging frosts at Masset after late April. Consequently, potentially damaging spring frosts have occurred there only once in 65 years.