Invasions and collapses of tree species after the Ice Age.
Pollen and macrofossils are used to reconstruct mechanisms and processes of past invasions and collapses of tree species. They are also providing bottom lines for future invasions and collapses. The Swiss forest communities were always shifting to adjust to climate and land use changes. Some invasions occurred surprisingly synchronous in space. Ecosystem adjustment processes started rapidly after climate change or disturbance, but usually lasted several centuries, given that they involved several tree generations. The available climate and vegetation history evidence suggests that the today's rather homogenous sweet chestnut, oak, beech and spruce forests are particularly prone to be invaded, because the communities were created by millennial-long land use and also because the realized climatic niche of the dominant species is strongly anthropogenically widened. On the other hand, the realized climatic niche of silver fir is strongly narrowed by human impact. This implies that silver fir is far more resilient to climate change than its today's range would suggest. Thus new plant species may have considerable difficulties to enter quasi-natural mixed silver fir forest communities. A similar finding, although less pronounced, was obtained for linden, elm, ash and maple species. Evergreen broadleaved species from the Mediterranean region, which are native to Switzerland since millennia but were not able to expand due to climatic reasons (e.g. Quercus ilex), will presumably spread spontaneously within a few decades under changing climate. Such future invasions can be considered as a natural process from an ecological point of view, if the invading species involved originate from the neighboring subtropics.