Austrian “Apfel strudel” and the Italian “Castagnaccio” are only two of the many culinary recipes that are traditional in Europe. But did you know that their main ingredients are the product of trees – Malus domestica and Castanea sativa – that are actually non-native to Europe? Many trees and crops have been introduced to Europe over the last centuries but when does a species become “invasive”? Could newly introduced tree species be an opportunity in this uncertain era of climatic changes? Scientists are trying to make light to many of these questions.

Last week I was hiking in Hungary, in the beautiful region of the lake Balaton. I did exactly the same hike five years ago. This time, however, I noticed something different growing under oaks, pines and maples. Many patches of forest were dominated by the Tree of Heaven (Ailanthus altissima; see cover figure). This species that was first introduced in Europe from Eastern Asia in the 18^th century, but that during the last decades has expanded more vigorously than before. With its long stem and weedy, palm-like habit, to my eyes Ailanthus looks a real “exotic” tree and I started wondering how this forest would look like if I would come back for a walk in a few decades. I also noticed that a large portion of the same forest was also dominated by black locust (Robinia pseudoacacia), which is also an introduced species but so widespread in Europe that it is considered almost naturalized. Differently from Ailanthus, whose presence rose sharply in the last years, black locust looks familiar to me, as I grew up seeing it in my neighborhood forests in Northern Italy. Although in some areas black locust is considered highly invasive, to me it did not really look very exotic.

Humans have introduced tree species into from one geographical area to another since millennia. This process has continued ever since, particularly since the time of the great explorations (1500s, 1600s). However, it is only since the advent of modern forestry in the late 19^th and 20^th century that non-European species started to play an important role in the continent’s economy and being present in substantial amount respect to native species. During the last decades, however, the situation started to change. Increases in temperature and drought conditions triggered mortality rates for some autochthonous species. Adapting management regimes to future climatic conditions became a hot topic among scientists and forest managers; introduced tree species started to be viewed as an opportunity to maintain ecosystem services from forests in which native tree species might experience reduced tree growth and vigor.

Introduced tree species in European forests: opportunities and challenge

All these aspects are covered in a very interesting publication: Introduced tree species in European forests: opportunities and challenges, published by the by European Forest Institute (EFI). With the coordination of the European Forest Institute, a pool of researchers from different institutions reviewed classic and state-of-the-art literature on the topic of introduced and invasive tree species in Europe. The result is a comprehensive piece of work (423 pages and 33 chapters!!) reporting the current knowledge on the subject. This book does not only provide a nice summary of the history of introduced tree species in the continent, but it also describe in a clear and impartial way the meaning of terms such as introduced (“A species in a given area whose presence there is due to intentional or accidental introduction as a result of human activity”), invasive (“A species that has overcome a series of barriers to be able to spread into novel areas in which it becomes dominant.”) or naturalized (“An introduced species that has established new selfperpetuating populations, undergoing widespread dispersal and becoming incorporated within the resident flora”) species, concepts are often confused and misused due to old prejudices.

What I found very interesting to read are the chapters dedicated to the case studies (Section 5 of the book, Case studies of introduced tree species) . For example, the case of black locust in Czech republic, the expansion of Ailanthus in Southern Switzerland, Eucalyptus in Portugal and the use of National Forest Inventory data to assess factors leading the spread of Acacia’s species in Northern Spain. In the publication is also presented the first harmonized European databases on invasive alien species. The European Information System on Invasive Alien Species (EASIN) was built on existing regional databases with the aim of facilitating the exploration of up-to-date information on invasive tree species and for supporting the implementation of the regulations at European level.

The book does not only report the most recent ecological findings on old and newly introduced species in Europe, but also includes economic and social aspects. The chapter by Hanewinkel and Knook entitled Economic aspects of introduced tree species – opportunities and risks, for instance, explains that the main drivers for introducing a new tree species in the past were often economic aspects. The two authors explain with an economic point of view there are not only risks but also opportunities from introduced species, as forests with high presence of invasive species may also be able to provide ecosystem services and mitigate the economic impact of climate change.  This, or course, does not mean that we should let grow any kind of non-native species on our forests. As also pointed out in a more ecological perspective in the chapter by Rigling and colleagues (Introduced or native tree species to maintain forest ecosystem services in a hotter and drier future?), the selection of new species to replace autochthonous is quite a complex process and requires careful investigations that should not only consider tree species identity but also different provenances. Assisted population migration, consisting of planting different provenances of the same species, for example, could be considered a good option for preparing forests of vulnerable regions to a warmer and dryer world (e.g., planting southern and more drought-resistant provenances of Pinus sylvestris in inner-Alpine valleys or in Central Europe).

READ MORE: Ecosystem services, mountain forests and climate change

So, what to do? As usual: it depends.

The two editors, F. Krumm and L. Vitkova, conclude that “the situation among introduced species is not black and white presenting us with a lot of grey areas”. Thus, as I have already pointed out on one of my previous posts: it depends. We could certainly expect that at such a large spatial scale (i.e., the European continent) the circumstances differ depending on the geographical regions, which are characterized by diverse histories and developments of introduced tree species. It is therefore not straightforward to determine whether an introduced species can have more positive opportunities than negative ecological and/or economical impacts. Clearly there are situations, in which introduced tree species become highly invasive and create damages to the environment. However, past-introduced species have also contributed significantly to the European economy and have well integrated not only to the native forest communities but also to the culture and history of many regions.

The first take-home message highlights the importance of local conditions and the need to address these issues at different scales than at European level. I believe that this is more directed to policy makers rather than to scientists. Second, it is important that we detach from the idea of “current conditions”. The distribution, presence, abundance and vigor of tree species in the continent have always varied over time (and I must add that trees have quite a different concept of time than human beings!). In addition, these changes have been subject to multiple factors rather than a single one. This add complexity to the issue. Environmental education and communication are also very important: invasive ecology should be part of school curricula, so the next generations will recognize the value of our forests in the surrounding environment. As climate is changing, accepting changes on species composition may be good.

In any case, I agree with the authors that more research is need on this regard: it is currently too risky to leave invasive species uncontrolled and adaptive management approaches carefully evaluated at local level are certainly needed. For doing this we need more international collaborations and enhanced communication across sectors (science, managers, stakeholders, policy makers).

So yes, aliens are coming. Or better, they have always been coming and they will continue to do so in the future. Together with threads they will also bring opportunities, and we better continue to keep an eye on them.

Source: Krumm, F. and Vítková, L. (eds) 2016. Introduced tree species in European forests: opportunities and challenges. European Forest Institute. 423 pp. The publication is freely available for download.

I leave you with this video made by colleagues of mine at WSL Birmensdorf. They tested the resistance of invasive Ailanthus to rockfall in Southern Switzerland. The video recently won an award by the Swiss National Science Foundation for best scientific video in the category Video-Loops. There is currently a research project that aims to assess the long-term effects of Ailanthhus altissima on the protection function of forests against rockfall in Switzerland and France. The project is called ALIEN, find more information here.

Main photo: Gaps in the forest dominated by Ailanthus altissima in the region of the lake Balaton (Hungary). Author of the photo: Marco Mina