Long-term trends in spring phenology in a boreal forest in central Finland

We studied the onset of spring phenology and the long-term trends of 31 species at the Oulainen-Ohineva site (64°13'N, 24°53'E) in central Finland. The species studied represented a wide range including deciduous trees, shrubs, grasses, 18 species of migratory birds and six insect species. The duration of the species-specific time series varied from 4 years up to 54 years during the 1952-2005 observation period. An advancing trend (p < 0.05) in the timing of spring phenology was only found for five species (with over 15 years of observations), namely, Formica rufa group, Rana temporaria, Grus grus, Ficedula hypoleuca and Hirundinidae. In local species with more than 38 years of observations, the temperature trend turned out to be the best variable to describe the advancement of the phenological event. For birds in six cases out of ten the best factor for explaining the advancement, as well as the temperature, was the NAO index.