Annual and spatial variability of the beginning of growing season in Europe in relation to air temperature changes

To investigate the annual and spatial variability in the beginning of growing season across Europe, phenological data of the International Phenological Gardens for the period 1969-1998 were used. The beginning of growing season (BGS) was defined as an average leaf unfolding index of 4 tree species (Betula pubescens, Prunus avium, Sorbus aucuparia and Ribes alpinum). The study shows significant changes in the mean air temperatures from February to April and in the average BGS in Europe since 1989. In the last decade the mean temperature in early spring increased by 0.8degreesC. As a result the average BGS advanced by 8 d. Between 1989 and 1998, 8 out of 10 years tend towards an earlier onset of spring. The earliest date was observed in 1990. The relationships between air temperature and the beginning of growing season across Europe were investigated by canonical correlation analysis (CCA). The spatial variability of both fields can be described by 3 pairs of CCA patterns. The first pattern, which explains most of the variance, shows a uniform structure with above (below) normal temperatures in whole Europe and consequently an advanced (delayed) beginning of growing season. The other 2 patterns show regional differences in the anomaly fields. Whereas the second CCA pattern has a meridional structure, the third pattern shows a zonal distribution. In all cases the anomalies of the regional air temperature and of the beginning of growing season correspond very well. The correlation coefficients between the anomaly fields range between 0.90 and 0.66, For all patterns appropriate examples in the observed data were found.