CONTRIBUTIONS TO THE FLORA OF THE CITY OF ZURICH .1. INTRODUCTION - DESCRIPTION OF THE NEW FLORA - PTERIDOPHYTES AND GYMNOSPERMS

''Flora of the city of Zurich'', based on a square kilometer inventory of species, is presented. The data gathered within 122 square kilometers are compared with earlier records and herbarium specimens. Species characteristics and their distribution patterns will be treated in a series of publications. The present paper, first of the series, introduces the whole project and deals specifically with pteridophytes and gymnosperms. ''Flora of Zurich'' presently includes 1500 species. 200 to 300 of these are ephemerophytes or ergasiophytes, 150 - archaeophytes, 300 to 400 - neophytes, and 700 - idiochorophytes. The number of species corresponds on average to almost 400 per km(2). It was generally lower in areas of forests and of agricultural land use. On the other hand, more species were found in areas which consist of various urban, forest and agriculture elements. On the whole, 35 pteridophyte species were recorded. Nearly half of them did not change significantly their occurrence in the studied area over the last 150 years. On the other hand, 4 species increased their frequency, 5 are new arrivals, 6 are less frequent and 2 are now extinct (Blechnum spicant and Lycopodium inundatum). Of 10 gymnosperm species occurring within the study area, 5 are introduced forest plants. Four of the remaining species did not change conspicuously their frequency over the years, but one became rare and endangered (Juniperus communis).