Temperature data used to determine a theoretical start to forest tree growth in spring

Summary The beginning of tree growth in the spring, denoted by the arrival of a phenophase, varies from year to year. The ambient air temperature is probably the most important factor for vegetation growth in spring. Starting from an initial concept of the beginning of the vegetation period as being seven days with a mean temperature of 5.0 °C or greater, we modify the threshold (going down to 2 °C) and the length of the reaction period (five to nine days). To estimate the effect of these changes on the results at several geographical sites, we calculated the modifications obtained for three weather stations in Switzerland: Zurich (590 m), Einsiedeln (914 m) and Davos (1600 m). To obtain a clear picture of the influence of these changes, we used a frequency of data obtained during the period 1931–1970 (maximum, 90%, 75%, 50%, 25%, 10%, minimum). In conclusion, the lower the threshold of temperature, the earlier the computed start date for growth. Similarly, the shorter the lapse time, the earlier the date produced for the phenophase. Both modifications have no significant influence on the picture of the frequential distribution of the results.With 5 Figures