It is important to include introduced tree species into forest ecosystems similarly as the native and non-invasive ones. Due to the changes in periglacial and postglacial periods, such species can fill specific niches (e.g. Douglas-fir (Pseudotsuga menziesii) and black walnut (Juglans nigra)). Although some of the species are really invasive, they did not escape from forests but from urban greenery and parks (black locust (Robinia pseudoacacia) and tree of heaven (Ailnathus altissima)). The silvicultural treatments aiming at including non-invasive tree species are a major challenge for modern forestry. The improvement of the timber assortment supply may be considered as the major opportunity.

 

There is a hope for using introduced tree species in the future. Not only because of the climate change but also due to replacing non-native conifers (Norway spruce (Picea abies) in commertial forests) with those more suited to the particular site (Douglas-fir). Some declining natives (alders (Alnus spp.) and ashes (Fraxinus spp.)) will need partial substitution by other species that have the ability to produce valuable timber (walnuts and cherries (Prunus spp.)).

 

Yes, this is proven by a lot of research. For example, Douglas-fir influences the habitats a lot less in comparison to Norway spruce and Scots pine (Pinus sylvestris). Some exotic species can represent danger but not in general. Nonetheless, the substitutuion is not complete, it is only partial on convenient places. Protected areas should be managed without non-native species.

 

This is a question, what is for forestry success in natural regeneration, comparable to natives, for somebody can be successfull regeneration a sign of invasiveness. But considering the silvicultural treatments a a whole, Douglas-fir should be not considered as invasive species. But ideology is very dangerous.

 

Not in general, however, black locust is an example of this as it regenerates in many places from reclaimed land and from parks. Another case is the tree of heaven since it escapes from urban areas.