Some results of a floristic inventory within the city of Zürich (1984-1998)

Floristic survey of the city of Zürich carried out from 1984 to 1998 covered an area of 122 km2. To assess possible floristic changes in the last 150 years, the results were compared with literature data from 1839 and herbaria. On the whole, the flora of Zürich includes nearly 2000 species. Of those, 1210 are either indigenous or introduced and subsequently naturalized. About 1/4 of the established species occur in more than a half of squares of 1 km2 grid, whereas 1/3 was found in less than 6 % of the squares. Average species number per square was 451, ranging from 294 to 607. Of the 1210 presently established species, 58 % are indigenous (native), 19 % archaeophytes, and 23 % neophytes. Within the developed parts of the city, neophytes form about a half of all species. The group of extinct species includes 60 % of indigenous species, 38 % of archaeophytes and 2 % of neophytes. At present, archaeophytes seem to be particularly threatened. Comparison with literature and herbaria revealed that 26 % of all species are at present as frequent as in 1839, 32 % increased their frequency or became newly naturalized, but 42 % decreased in frequency or became extinct. Extinct species amount to 188, newly introduced ones to 294. Compared to the newly established species, the extinct ones have higher indicator values for nutrient content and temperature, and lower values for continentality. These differences indicate that the environment has changed within the last 160 years towards higher soil nutrient content, higher temperatures, and milder winters.