THE DYNAMICS OF PLANT INVASIONS - A CASE-STUDY OF 3 EXOTIC GOLDENROD SPECIES (SOLIDAGO L.) IN EUROPE

The patterns of spread of the three exotic species Solidago altissima L., S. gigantea Ait. and S. graminifolia (L.) Salisb. after their intr oduction to Europe were investigated, based on herbarium specimens and literature records. The spread was analysed by mapping the localities for each decade since 1850. Cumulative numbers of localities as well as numbers of occupied grid squares showed a continuous increase since 1850 for all three species. The slopes, however, were significantly d ifferent among the species. Solidago gigantea had the highest coloniza tion rate, followed by S. altissima, and finally S. graminifolia. The latter showed only a slight increase in abundance over time. The incre ase in diameter of the range in central Europe was logistic for S. alt issima and S. gigantea with a rapid increase between 1850 and 1880. Th e spread of the species in area and time over Europe showed no clear f ront; new localities at large distances were simultaneously colonized. A large part of the actual range of S. altissima and S. gigantea was already achieved about 1950. It is assumed that the spread of the spec ies followed the hierarchical diffusion model with Several independent foci from which the species began to spread. The data suggest that S. altissima and S. gigantea will successfully spread further, leading t o an increase in abundance and area, while S. graminifolia seems to sp read slowly.