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Stefan Tretter


Bavarian State Institute of Forestry
Section Silviculture and Mountain Forests
Hans-Carl-von-Carlowitz-Pl. 1
D-85354 Freising

Phone: +49 (8161) / 71 - 4956
Fax:    +49 (8161) / 71 - 4971


Author(s): Editorial office – LWF
Editorial office: LWF, Germany
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Foreign tree species

Some euphorically endorse non-native tree species while others demonize them. Where in lies the truth? In central Europe many foreign tree species have already been established for a long time. Is that good or bad? Can all the tree species be lumped together? The goal of the research group "foreign tree species" is to research this topic and convey the results.


Chilenische Araukarie
Fig. 1: The Chilean pine (Araucaria araucana) in the Arboretum Burgholz nearby Wuppertal, Germany (Picture: M. Schölch).

Climatic changes from ice ages to warmer interglacial periods resulted in alterations to the ecosystems in central Europe. The southward migratory path for the plants collided with a natural barrier, the Alps, which are orientated from west to east. This resulted in a decrease in species diversity through extinction.

Means of transportation

Species can "hitch a ride" in many ways: Wind and water are the most common abiotic means of transportation (vectors) in nature. Even foreign continents can be reached. Additionally, other organisms can distribute plants. Humans are the most effective in this regard than any other species. New species have reached and will reach other islands and continents in the future by travelling with mariners. Trafficking through humans is labeled either as introduction (deliberate or non-deliberate) whereas the natural distribution is called migration. In Germany and Austria, about 1,000 known non-native plants (neophytes) have been identified.

Exotic tree species

Foreign/exotic tree species are those species of trees that did not occur in the central European species composition (Dendroflora) and were/are cultivated and distributed by humans. The existence of these non-native tree species should be quite thoroughly spatially and temporally confined.

Why were non-native tree species introduced in the fist place? The motives are diverse and the most common are:

  • Curiosity
  • Pleasure from their appearance or exotic nature
  • Expectations of usefulness
  • "Enrichment" of species composition
  • Diversification as a means for risk reduction due to climate change

For more than 2,000 years, foreign tree species have widened the spectrum of species in addition to natives. Particularly with the discovery of the new world in 1492 a plethora of new plant species was brought to Europe. This year serves as a separation between terms for old-established plants (archaeophyte) and the new arrivals (neophytes). These new tree species were typically planted first in parks and gardens. Species that are pertinent to forestry came relatively late to Europe. Douglas fir, for example, was introduced to Great Britain in 1892.

In the cultural landscape, we come across exotic species at every turn: apples, potatoes, lettuce, corn, tomatoes and wheat are just a few examples of cultivated species that are not native to Europe. They cannot survive here without human assistance. Is this also true for tree species?

Invasive species

"Invasive" actually means that species expand into new areas. Today this term suggests negative connotations: The connection is made with species that spread out intensively and result in undesirable situations for the native species or ecosystems. The German Federal Agency for Nature Conservation has produced a flyer listing invasive or potentially invasive tree species that include:

Arboretum Burgholz bei Wuppertal
Fig. 2: The art of combination: Mixing non-natives with native species in the Arboretum Burgholz nearby Wuppertal, Germany. In the foreground are the Noble firs and giant sequoias; in the background is a mixed beech stand in autumn (Picture: U. Nolden-Seemann).
Box elder Acer negundo
Tree of heaven Ailanthus altissima
Green or red ash Fraxinus pennsylvanica
Austrian or black pine Pinus nigra
White pine Pinus strobus
Carolina poplar Populus x canadensis
Black cherry Prunus serotina
Douglas fir Pseudotsuga menziesii
Red oak Quercus rubra
Black locust Robinia pseudoacacia
Staghorn sumac Rhus typhina

Arboretums (i.e. a collection of trees) have been established in which a small collective of trees or even a small forest stand of exotic species can be found. Their purpose is to realistically judge the long-term suitability and growth characteristics of trees. Furthermore there are also many botanical gardens. The arboretums are truly a treasure trove for scientific studies since they usually encompass over one hundred tree species, many of which are over one hundred years old and are spread out over a large area. It is worthwhile to conduct research at such locations because personal opinions can be traded in for knowledge.

Differences of opinion

Many people employed in forestry view modest and responsible cultivation of exotic species as a way to expand the tree species portfolio, to reduce risk from climate change and to increase timber yield. Advocates of nature conservation often have reservations.

As always, in such circumstances an adamant and diehard opinion is not helpful. Instead, a sound and lively knowledge will bring one further. It is a particular challenge to gain solid insights into foreign tree species due to the extensive time of observation required, the diversity of sites and origins as well as a changing environment.

The research group for foreign tree species – New colleagues are welcome!

The research group for foreign tree species has the following goals:

Früchte der Baumhasel
Fig. 3: We have the future in our hands; nuts of the Turkish hazel (Corylus colurna) (Picture: M. Schölch).
  • Reprocess existing knowledge
  • Analyzation of old cultivations
  • Creation of new knowledge
  • Illustrate advantages, disadvantages and risks
  • Objectively base decisions for the usage of non-native tree species on pros and cons
  • To stimulate communication between experts
  • Disseminate information to the general public
  • Provide information about the activities of the research group

Experts from Germany, Austria, Switzerland and Belgium were able to create two databases after round table discussions based on their respective experiences. Information about the cultivation from approximately 120 foreign tree species currently exists. Interested parties and dedicated colleagues are welcome to join!

The Results from many studies and observations of all cultivations show that only a few non-native tree species are economically attractive. Until now, only a dozen of the numerously planted tree species appealed to the forest industry. Due to climate change, a few more could be utilized and expand the species composition. This would result in an increase in the biological diversity. Further results show that non-native tree species perform better on disturbed sites than near to natural sites. Quite often they suffer under competition from native species, in particularly from European Beech.

The unknown stranger is unsettling. This is because the behavior of the new species with regard to the native population is relatively unknown and is viewed skeptically. As a result, experiences are particularly important. It seems that due to the quickly changing environment it is vital to gain experiences through well designed and scrutinized endeavors.

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