Variations of the climatological growing season (1951-2000) in Germany compared with other countries

We analyse how variations in the climatological growing season, defined by single-value thresholds of daily minimum and mean air temperature, mirror recent changes in plant phenological phases. In Germany (1951-2000, 41 climate stations), the dates of last spring frost T_min < 0°C (<-3°C and <-5°C) advance by 0.24 (0.23 and 0.32) days per year on average, and the first day when the daily mean temperature constantly exceeds 5°C (7°C and 10°C) advances by 0.13 (0.21 and 0.09) days per year. The respective autumn dates are delayed up to 0.25 days per year; in total, the climatological growing season is lengthened by 0.11 to 0.49 days per year, depending on the criterion analysed. However, a certain station-to-station variability is displayed. Mean trends of phenological phases correspond well to those measures of the climatological growing season having similar seasonal occurrence. Thus, for example, the extension of the growing seasons of trees (up to 0.22 days per year) averages the lengthening of the period where the daily mean temperature constantly exceeds 5°C (0.23 days per year). However, the greatest changes result for the lengthening of the frost-free period (0.49 days per year) due to the observed stronger increase in daily minimum rather than maximum temperatures. Consequently, trees may not take advantage of the frost-free season as before, but may profit from a reduced risk of damage by late spring frosts. A parallel evaluation of trends in the climatological growing season in other European countries (Austria, Switzerland, Estonia) supports this finding of a stronger lengthening of the frost-free period than that of the growing season based on daily mean air temperature. The mean lengthening of the frost-free period of 0.50 days per year in Austria and Switzerland (1951-99, 18 stations <950 m a.s.l) and 0.34 days per year in Estonia (1951-2000, two stations) is quite similar to that in Germany. The lengthening of the growing season (T_mean ≥ 7°C) in Austria and Switzerland (0.39 days per year) also approximates the respective result for Germany (0.36 days per year). However, in several cases, the trends increase with altitude (up to 500-900 m a.s.l.), but at high-elevation climate stations (>950 m a.s.l.) weaker trends are again generally observed.