[Phenology, an indicator for environmental changes].

After a short introduction to phenology, the complex environmental factors influencing vegetation are explained and discussed. Three special phenological series are presented and analyzed (flowering time of summer lime trees at Liestal, 1951 to 1989, and of the cherry trees at Liestal, 1894 to 1990, and the time of appearance for leaves of chestnut trees at Geneva, 1808 to 1991). The temporal progress of the evolution of the vegetation between 1951 to 1990 in Switzerland shows that the "early", "normal" and "late" years succeed one another irregularly. For the processing of phenological data, the fact that growth and development of a plant are determined by numerous and complex environmental factors has also to be considered.