How can I reproduce native trees

Olaf Schmidt
Non-native tree species between nature conservation and forestry - LWF currently 123

Don't be afraid of the stranger. It is important to analyze and evaluate the advantages and disadvantages objectively and without prejudice

Forestry in Bavaria, as in the whole of Germany, will not be able to avoid including non-native tree species in its tree species portfolio in view of the ongoing climate change. However, nature conservation representatives, among others, have concerns that these “new” tree species could also pose a threat to the forest ecosystem. The forest chances and the nature conservation risks have to be analyzed impartially and the right conclusions to be drawn. For the benefit of forestry and nature conservation.

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Fig. 1: The sweet chestnut has been proving its "forest suitability" for many centuries in Rhineland-Palatinate. (Photo: E. Segatz)

In the course of climate change, the interest of forest owners and foresters is increasing in alternative, i. H. Increasing the use of rare native and non-native tree species for climate-tolerant forest conversion. Alternatives are being sought for the climate-prone spruce in particular. In the already warmest areas of Bavaria, on the Untermain and in the Schweinfurt dry area, some native tree species seem to be reaching the limits of their possibilities due to climate change.

In order to preserve the forest with all its functions, it is necessary to increase the cultivation of climate-tolerant alternative tree species. Overall, a mixture of forests, not only according to tree species, but also according to grading, structural richness, unequal age and genetic diversity, is becoming more and more important. When growing non-native tree species, however, risks for forest owners such as pest susceptibility, climatic vulnerabilities, wood quality and for society (invasion potential) and consequences for the forest ecosystem, such as the decline in insects, cannot always be ruled out.

Conservation vs. forestry

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Fig. 2: Different perspectives when considering the same facts of non-native tree species (graphic: LWF)

In 2013, the Federal Agency for Nature Conservation (BfN) presented a nationwide nature conservation assessment of non-native vascular plants living in Germany for the first time (Nehring et al. 2013a / b). According to these results, some tree species, including the forest-grown tree species Douglas fir, red oak, strobe, robinia and hybrid poplar, are listed as invasive species that are supposed to endanger native species. They are kept on a "black list", for which a special nationwide management is planned to control and combat the spread. In an open letter from German forest scientists to the BfN on June 4, 2014, the general statements that non-native tree species affect native nature were questioned. The background to this discussion is still the different ways of looking at non-native tree species from the point of view of nature conservation or from the point of view of forestry.

While foresters and forest owners are happy about the growth potential, competitiveness and rejuvenation of some non-native tree species, nature conservation representatives see the danger here that native tree species are displaced or pushed back. While foresters appreciate it when tree species rejuvenate naturally, nature conservation representatives also see the danger of invasiveness of this tree species in a good nature rejuvenation option. H. penetration into other forest stands and natural forest communities (Figure 2).

Furthermore, for example, the lack of mycorrhizal partners and the lack of adaptation of native insects to non-native tree species, which can lead to a loss of biodiversity in insects and a shortage of food for insectivorous bird species, amphibians, reptiles and small mammals, are presented. Indeed, this argument of food poverty due to a lack of insect species cannot be denied. Some studies that have been carried out in recent years show clear effects here (Kolb 1996; Kolbe 1995; Gossner 2004). Our native insects are not on introduced tree species, especially tree species that are not native to us (e.g. Pseudotsuga, Tsuga, Cedrus, Liriodendron), customized.

The forestry sees this insect poverty as an advantage, since it also goes hand in hand with greater vitality, while the conservationists see a loss of diversity in it. With a general decline in insects, pollinators and natural predators (predators) can also be affected. In addition, it can be assumed that in forests, as is already the case in open land, the insect biomass could decrease and with it the food basis for many higher species.


There are two different approaches to defining the term "invasive species". From the perspective of natural science, all species are "invasive" that are not native to an area and that reproduce and spread there (Kowarik 2010).

According to the second definition, invasive species are nothing more than problematic alien species. The Federal Nature Conservation Act (BNatSchG) also pursues this approach, in which Section 7 defines the term "invasive species": "a species whose occurrence outside its natural range represents a significant risk potential for the naturally occurring ecosystems, biotopes or species there." This definition is significant because it not only assumes a possible endangerment of a new species, but is based on a "considerable hazard potential".

Invasive species pose a threat to biodiversity worldwide for isolated habitats such as islands, mountain peaks, and drainless lakes. The threat to biological diversity in Central Europe does not come from invasive animal and plant species, but here eutrophication, climate change, sealing of the landscape, impoverishment of the fields and the use of insecticides play the much more important role that endanger and decrease our biodiversity to lead. Not a single native species has become extinct in Central Europe as a result of introduced or introduced species. Essl & Rabitsch (2013) also explain this in their book “Biodiversity and Climate Change”: “Compared to other regions of the world, however, the ecological effects in Central Europe are less dramatic. So far, no case is known in which native species have become extinct solely through invasive alien species. "

Kowarik (2010, p. 395) himself also writes: »However, the problem status of a species is not a biological property. The same species can… lead to problems… elsewhere, ecosystem services of the species may even be desired… Here, a blanket negative species assessment obstructs possible leeway. «

Effects on species

What effects non-native tree species have or could have on fauna is discussed below using herbivorous, flower-visiting and wood-dwelling insects, as well as examples from the bird world.

Phytophagous insects

Particularly noteworthy here is the work »Diversity and structure of arboreal arthropod communities of foreign and native tree species«, which Gossner presented in 2004. Here he makes comparisons between English oak and red oak and between spruce and Douglas fir.

Red List species are found on the Douglas fir to a similar extent as on the spruce, but in a significantly lower number of individuals. From the point of view of species protection, a decline in endangered insect species can therefore be expected with a strong expansion of Douglas fir cultivation, especially in pure stands.

When comparing pedunculate oak to red oak, Gossner came to the following conclusions:
  • Crown zenoses of the red oak are to be classified as few individuals and species compared to the pedunculate oak.
  • The differences are more pronounced in the pure stand.
  • The red oak has a significantly lower number of individuals and species of endangered insect species. Nevertheless, the red oak is not an "ecological desert".
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Fig. 3: The Little Schiller Butterfly is dependent on native poplar species and the common willow as a food plant, but also uses the non-native hybrid poplar. (Photo: J. Hlasek)

Investigations of weevils, bark beetles, glossy bark beetles and rump-winged beetles in stands with and without non-native tree species revealed noticeable differences in three years of catching (Kolbe 1995). The beech population was the richest in species and individuals, but the mixed forest with non-native tree species also performed similarly well in terms of the number of species. The numbers of individuals in the groups of weevils and bark beetles in the mixed exotic forest were significantly lower than in beech or spruce forests.

One of the most persistent misjudgments of the impact of neophytic woody plants on our native biodiversity was the opinion that the hybrid poplar (P. x canadensis) a "food trap" for endangered species of butterflies such as the lesser schiller butterfly (Apatura ilia) would represent. As early as 1987 Hafner had shown that the caterpillars of the lesser schiller moth also use the leaves of the Canadian hybrid poplar (Barsig 2004). Nevertheless, this allegation of the alleged bio-pot trap has persisted in nature conservation circles until recently.

In more recent studies by the Bavarian State Institute for Viticulture and Horticulture (LWG), native and south-eastern European tree species were compared in pairs (hornbeam and hop beech, ash and manna ash, winter and silver linden). A total of 804 window traps, 416 yellow panels and 390 knock samples could be evaluated. This means that over 90,000 insects and spiders were caught on the 30 trees over the entire recording period.

There is no significant difference in the biodiversity of insects and spiders between native and south-eastern European tree species. Only 200 of the 90,000 insects caught in the study have so far been identified by species, that is, the groups of cicadas and wild bees. In fact, no significant differences in the number of species between alien and indigenous tree species were found in the two groups. The species examined are European tree species that even come from the same genus (Tilia, Fraxinus) or closely related genera (Carpinus, Ostrya). From the outset, no major differences between the phytophagous species are to be expected.

This certainly also applies to our hazelnut and tree hazel, for example. It would look different if one were to compare Douglas fir with spruce, pine, fir or oak with tulip tree, or Paulownia with beech. Likewise, the assessment of the sweet chestnut has changed due to recent research. Since the genus Castanea is closely related to the genus Quercus, it could be assumed that the species spectrum of phytophagous and xylobiont insect species of both genera is very similar. This was impressively demonstrated in the extensive studies carried out as part of the INTERREG project “The Sweet Chestnut on the Upper Rhine”.

The studies show that old chestnut stands that are rich in structure and deadwood can become just as important for biodiversity as old oak stands (Segatz 2015).

Flower visiting insects

The non-native tree species Robinia and horse chestnut, for example, are very popular with flower-visiting insects such as honeybees, hover flies and others. Beekeepers therefore like to encourage these tree species. For years, however, a major misjudgment was also the view that the non-native silver linden tree (Tilia tomentosa) would produce nectar poisonous for bees and bumblebees. Madel came to this conclusion in 1977.

In the years and decades that followed, this led to a veritable "crusade" against the silver linden tree in our cities. In Munich, as in many other cities, silver linden trees were felled with this argument. It was not until the studies by Baal and Surholt (1994) that the deaths of bees and bumblebees under silver linden trees were investigated more precisely and came to the conclusion that it was not the nectar that was poisonous, but that the bees' death was caused by a lack of food! The dead bumblebees and bees under silver linden trees simply starved to death! The scientists then called for more silver linden trees to be planted in cities in order to increase the supply of flowers for bees and bumblebees.

This case of the silver linden tree shows very clearly that one has to be very careful with the blanket rejection or the euphoric approval of the cultivation of non-native tree species. Only more detailed investigations provide us with the decisive arguments for the pros and cons, in order to be able to assess opportunities and risks and to be able to differentiate them without dogmatics (Schmidt 2006).

Xylobionte insects

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Fig. 4: The red oak is not only interesting because of its wood and growth, its autumn red color is also appealing. (Photo: K. Schreiber, LWF)

In forests, xylobionte insects play a major role in biodiversity. When examining the deadwood beetle fauna in Cologne, it was found that xylobionte beetles use both native and foreign tree species for their development. The only important thing here is the difference between conifers and deciduous trees. Tree species of the genera were of the greatest importance there Populus, Tilia, Ulmus and Acer. But also the non-native species Aesculus, Sophora, Robinia and especially Catalpa were important sites of xylobiont beetles (Stumpf 1994).

For Berlin, Möller (1998) calls for a process of weighing up the maintenance and safety measures for neophytic woody plants. The deadwood beetle fauna of the late blooming bird cherry was surprisingly diverse (Prunus serotina). 27 xylobionte and mycetophagous insect species were found on a fragile specimen. Möller's assessment of xylobionte on the red oak is similarly positive.

Bird life

In winter, Douglas fir crowns lack insect stages and spiders. Because of this lack of food, pure Douglas fir stands are rarely populated with insectivorous bird species such as golden grouse, pine and crested tit (Gossner & Utschick 2004). In 1996 in the Weinheim exotic forest, Kolb found poorer reproductive successes in stocks with non-native tree species than in near-natural forests in breeding biological studies on the great tit. The reasons are the lower food supply due to a lack of insects.


In summary, one can draw the conclusion for the forest that the cultivation of non-native tree species does have ecological effects - for example on the native fauna. Foreign tree species should therefore only be planted in our forests after a prior intensive risk assessment and by no means in pure stands, but only in an intensive mixture with native tree species in order to keep these consequences as low as possible. Here, too, the mixed forest is the first choice (see article by Tretter et al. And Falk et al.).

If, as forecast, the climate in our latitudes becomes warmer and drier, and extreme events increase at the same time, completely different tree species could gain importance in the not too distant future. For example, the sweet chestnut (Castanea sativa)which are already showing good growth rates in some areas of Bavaria, for example on the Untermain, and also help to create interesting forest images when mixed with oak, beech and pine.

As in many questions in the forestry sector, there is only one differentiated approach that does not include an “either or” strategy, but rather an “as well as” strategy. In forests in particular, trees play a decisive role in comparison to the other organisms of the ecosystem due to their longevity and their dominant structure. The choice of tree species is therefore particularly crucial and the manager has to take the various aspects into account as comprehensively as possible.

In conclusion, one can only advise foresters and forest owners not to sound the horn of fears of "foreign" species, but to observe objectively and conscientiously and to identify advantages and disadvantages without bias.


As global warming progresses, non-native tree species will increasingly play a greater role in silviculture and forest construction. This also increases tensions between forestry and nature conservation. First of all, the article describes in general the tension between forestry and nature conservation. In the following, concrete examples are used to show that no generalizations, but only factual, unbiased analyzes help. At the end it is critically noted that the cultivation of non-native tree species has to be done carefully and prudently and that the mixed forest option should be the first choice for foresters and forest owners.
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