Phytophenological trends in the Swiss Alps, 19512002
Defila, Claudio; Clot, Bernard
This work aims to study the long term phenological trends in Switzerland and is focused upon the differences between the lowland (lower than 600 m asl) and the alpine regions (higher than 1000 m asl) for the 1951-2002 period. Trends for 651 time series (n >= 30 years) from 75 observation stations and 19 phenophases were calculated. Compared to the results obtained with the dataset 1951-1998, the proportion of the significant trends like the intensity of the trends calculated in days have been reinforced. In every season, a general earlier occurrence of the phenophases has been observed, except by the leaf fall, where a slight delay is more likely to occur. The proportion of significant trends is clearly higher in the alpine regions (42 %) than in the lowland (33 %). On the contrary, the shift towards earlier occurrences is stronger in the lowland. The spring phenophases occur 20 days in advance in the lowland and 15 days in the alpine regions, in summer 19 and 16 days, and in autumn 7 and 4 days earlier in the lowland and the alpine regions respectively. The proportion of significant trends in the regions located in the south of the Alps is remarkable: 64 % in the lowland and 50 % in the alpine regions. In this area, the small number of data only allowed the study of the leaf unfolding and the full flowering. Concerning the full flowering, an advance of 32 days in the lowland and 20 days in the alpine regions was observed. The leaf unfolding occurred 12 days earlier in the lowland and 1 day later in the alpine regions than 50 years before. The trends concerning the full flowering present a stronger trend in the south than in the north of the Alps. On the contrary, the trends concerning the leaf unfolding are stronger in the north. In Switzerland, the climate warming was stronger in the last decades than the global one. The increase in temperature reaches 1.5° for 100 years in winter and 0.4 to 0.8 in summer for the 1864 to 2001 period. As it was several times mentioned, the spring and early summer phenophases are above all influenced by the temperature of the air. Then the trend towards earlier occurrence of the phenological phases is clearly related to the climate warming. In autumn, apart from the temperature, other parameters such as dry periods during the vegetation period play also an important role. Short time weather events such as strong winds, frost or snow fall are the main drivers of the leaf fall. This study gives prominence that phenology is a good indicator of the influences of a climate warming on the development of the vegetation. Strong regional and altitudinal differences are observed in Switzerland.