Regional phytophenological trends in Switzerland

The Swiss phenological network started in 1951 comprises 160 stations where 69 phenophases from 26 plant species are observed and registered. These stations are distributed in the different regions of Switzerland and at different altitudes. A first trend analysis including the data 1951-1998 had demonstrated an earlier occurrence of the spring phenophases (11.6 days) and a light delay of the autumn phases (1.7 days). The proportion of significant trends showed important regional and altitudinal variations. A recent analysis including the data 1951-2002 was performed in order to better understand these differences. The trends were calculated in days / 50 years. The phenological seasons (spring, summer, autumn) and the types of phenophases (leaf unfolding, full flowering, leaf colouring and leaf fall) were compared in 6 regions and for the lowland and the altitudes above 1000 m. The strongest trends reach -28 days (full flowering in the south of the Alps), -27 days (leaf unfolding in Engadin) and -19 days (Swiss Plateau). With the exception of the autumn phases in the south of the Alps, the occurrence of all phenophases is registered earlier in all the regions. As it is well known, the spring and summer phases are much influenced by the air temperature; the trends that were recorded can then be related to the climate warming that occurred in the last decades. In the north of the Alps, the stations located above 1000 m present a stronger reaction to that climate change than in the lowland. The reaction is very different in the south of the Alps. The various local topographic and climatic situations make from Switzerland an interesting region for phenological investigations.