The Impact of Temperature on the Northern Distribution Limits of the Introduced Species Fallopia japonica and Impatiens glandulifera in North-West Europe

This paper tests the predictions of sensitivity ana- lysis performed on two matrix population growth models, for annual and perennial vegetation (Woodward, 1988). Climatic correlations have been established for two intro- duced invasive plant species Fallopia japonica (Japanese knotweed) and Impatiens glandulifera (Himalayan balsam) in north-west Europe. These two species represent exam- ples of perennial and annual vegetation respectively. In accordance with the model predictions, the northern distri- bution limit of F. japonica was found to be controlled by two climatic variables-the length of the growing season, measured in day-degrees, and the minimum temperature- while for I. glandulifera only the length of the growing season was critical. Experimental verification of the results is crucial for understanding the mechanisms operating to limit plant distributions. By use of the predictions established by the climatic cor- relations for I. glandulifera and F. japonica, maps have been produced forecasting the potential northward spread of both species on the basis of a 1.5?C and a 4.5?C rise in global mean surface air-temperature. These temperatures represent the minimum and maximum increases predicted to occur by general circulation models for a doubling of CO2 concentration in the atmosphere. For a global tempera- ture increase of 1.5?C the maps demonstrate a maximum northward spread of F. japonica from a latitude of 63? to 68?N and for I. glandulifera from 64? up to 69?N. The greater temperature increase of 4.5?C indicates a consider- able potential for expansion in the range of F. japonica in Norway and Sweden but a distribution which was restricted along the eastern coast of Sweden and southern Finland. In contrast, I. glandulifera could colonize the whole of the Fennoscandinavian area. The predictions are made assum- ing no migrational lag and no soil 'incompatibility'. Geo- graphical differences in the patterns of spread between the species were explained with reference to the different factors affecting perennial and annual vegetation.