Thermal Time, Chill Days and Prediction of Budburst in Picea sitchensis

where thermal time was day degrees > 5 IC accumulated from 1 February, and chill days were the number of days <5 C counted from 1 November, both based on mean daily air temperature ((max. + min.)/2). This model may be used to estimate the date of budburst on young P. sitchensis of most provenances growing in upland Britain. (3) The following features or assumptions of the model were examined with reference to the literature and/or by experimentation: the small effect of provenance; linearity in the relationship between bud growth rate and temperature; the large effect of chilling on thermal time to budhurst; the omission of daylength and soil temperature as variables; the choice of starting dates for effective chilling and thermal time; and the use of simple fixed base temperatures. (4) The model was applied to mean daily temperatures at Eskdalemuir for the period 1912-82. The predicted dates of budburst ranged from 23 April in 1961 to 30 May in 1923, with a mean date of 12 May.