Phenological observations are some of the most sensitive data in identifying how the biosphere responds to regional and local climate changes in mid-latitudes. A lot of recent phenological studies report on changes in phenological events, mainly in the springtime. In the most studies an advanced timing of spring events such as budding, leafing and flowering of plants between 2 and 4 days per decade was found. This investigation shows that the observed trends in plant phenology correspond well to recent changes in air temperature in the early spring. For the most spring events, which occur in the end of April or in the beginning of May, the average air temperature from February to April (T24) is a good indicator which explains the observed changes in the plant phenology.

For whole Europe a correlation coefficient between T24 and the average beginning of growing season (leaf unfolding index of 4 plant species: Betula pubescens, Prunus avium, Sorbus aucuparia and Ribes alpinum) of -0.83 (p<0.05) was found for the period 1969-1998. An increase in the mean air temperature between February and April of 1 °C corresponds to an advanced beginning of growing season by 6.7 days. In almost all natural regions of Europe a trend of an earlier onset of spring was observed. The strongest trends were noticed for regions in central Europe. For example, the average beginning of growing season in Germany starts on 20 April. In the last 30 years this date has advanced by 11 days (-3.7 d/10a, p<0.05). The found trend in the leafing of trees corresponds well with changes in air temperature in Germany (trend in T24: +0.41 °C per decade, p<0.05, 1961-2000). This seasonal warming led also to significant changes in the blossom of fruit trees (apple, cherry) by about -2 days per decade. An increase of T24 by 1 °C advances the beginning of flowering by nearly 5 days. The very early dates in the beginning of growing season and in the blossom of fruit trees were observed since the end of the 1980s. Since then positive air temperature anomalies prevail in the early spring. The observed temperature changes correspond well to changes in the circulation pattern over Europe. The increased frequency of positive phases of the North Atlantic Oscillation (NAO) index since 1989 led to milder temperatures in the late winter and in the early spring, because of the prevailing westerly winds during this time. This resulted in an advanced beginning of spring in Europe and in Germany in the 1990s and emphasis the strong interaction between atmosphere and biosphere.