PLANT INVASIONS AND THE ROLE OF RIPARIAN HABITATS - A COMPARISON OF 4 SPECIES ALIEN TO CENTRAL-EUROPE

We compared the rate of invasion of four plant species which are alien to central Europe and the highest-growing representatives of different life-forms: Impatiens glandulifera (annual), Heracleum mantegazzianum (monocarpic perennial), Reynoutria japonica and R. sachalinensis (polycarpic perennials). The spread of these species in the Czech Republic was reconstructed on the basis of floristic data. Cumulative numbers of localities reported from the time of introduction to the present were used for comparison. Exponential regression models were found to best fit the increase in the cumulative number of localities over time and the slope b was considered a convenient measure of the invasion rate. The ranking of species according to the decreasing rate of invasion was: I. glandulifera, Reynoutria japonica, Heracleum mantegazzianum, R. sachalinensis. The rate of invasion in riparian habitats, if these were treated separately, decreased in the following order: I. glandulifera, R. sachalinensis, R. japonica, H. mantegazzianum. The lag and exponential phases of spread were distinguished and the timing of the beginning of invasion was estimated at 1936 in I. glandulifera, 1938 in R. japonica, 1943 in H. mantegazzianum and 1952 in R. sachalinensis. H. mantegazzianum and I. glandulifera began to spread exponentially after having reached only a few localities in the area studied and their invasion rates during the exponential phase were higher than those of both Reynoutria species, whose invasion proceeded at a more even rate. Habitat preferences differed between species in both lag and exponential phases of spread. Different patterns of affinity to riparian habitats were found among the species studied. The role of river corridors in encouraging plant invasions is discussed on the regional scale with respect to the autecology of the species and frequency of suitable habitats.