Climatic forcing on the dispersal of exotic species
Changes in vegetation composition and distribution are often paralleled to a changing climate. Especially at the boundaries of the distribution area of a given vegetation type, shifts due to changing environmental conditions as e.g. climatic change are expected to be detected first. In a recent study conspicuous changes in vegetation composition have been reported in climatically sensitive areas with particular boundary position as e.g. the Insubrian area in southern Switzerland. More than a dozen exotic species recently started to colonise - and in some places even to dominate - forest understorey vegetation. All the exotic species share equal characteristics (evergreen broad-leaved or laurophyllous plant functional type) and a synchronous pattern of dispersal in time and space. This stresses the fact that external factors may determine the process. Climatic data from local meteorological stations are evaluated with special regard to limiting climatic parameters for laurophyllous species. With this background, climatic forcing must be considered as one of the major contributing factors for this increasing tendency of exotic species dispersal and their capability to establish in forests. The present case study for southern Switzerland gives insights on the rapid reorganisation of a forest community with shifting ranges of indigenous species and pre-adapted non-indigenous species invading possible vacant niches.