Reading 4.1 Management principles for sustainable use of non-native trees

Cedrus libani A. RICH.- the Lebanon cedar is an example of a NNT involving no reports on invasiveness, and there is no need to take urgent measures. Its timber is durable and can be processed easily. While Cedrus libani is not a tree relevant to forestry in the Alpine region, some experimental plots maintained by forest research institutions exist. Afforestation outside the species’ natural range has been carried out in Italy and France, and several cultivation and provenance trials were implemented in Switzerland, Germany, France, and Italy.

The impacts of climate change are expected to be particularly severe in the Alpine region. A shifting precipitation regime will likely lead to more intense and frequent drought events during summers. Droughts tend to lead to reductions in growth and CO2 uptake in subsequent years, and successive drought events likely affect tree quality and increase mortality, leading to changes in species composition and forest structure. Another challenge in the Alpine region is the likely increase in mean temperature, which will cause increased evaporation and thus further changes to precipitation patterns, potentially resulting in even more severe drought episodes impacting forest growth and resilience. Although forest fires are currently not a pressing problem in the Alpine space, fire intensity and frequency are also likely to increase. On a large scale, forest tree species and communities will experience increased stress, and cultivation will no longer be possible in lowland areas, thereby strongly affecting some habitats and possibly leading to impacts on biological diversity. The ideal climate zones for many tree species in Europe will shift northwards and upwards. For the Alpine regions, this implies that climate change will further alter the distribution of many species, with an upward shift in elevation for various plant communities and earlier onset of spring (according to phenological observations) for most tree species. So-called ecologically effective population densities serve as guidelines for determining the minimum densities of individuals necessary to maintain critical interactions and ensure resilience against ecosystem degradation and extreme events. Non-native trees (NNT) are sometimes viewed as part of a solution for adapting forests to future climate conditions.

Forests in the Alpine region are an important part of the landscape and provide various ecosystem services and functions that are crucial for nature-based solutions for a sustainable society. Among the most important of these are:

  1. productive functions (timber and non-timber products)
  2. protective functions in places where human populations and ecosystems are permanently threatened (soil erosion, landslides, snow avalanches, biodiversity conservation)
  3. social functions such as recreation, aesthetic functions, and many others.

Of particular importance in the Alpine region are the protective functions of forests, which have a stabilizing effect on the natural environment (water purification and circulation, precipitation, air circulation, temperature, macro- and microclimate). In addition, the interdependence of all three forest function groups in the Alpine region is profound, and well-planned forest management is therefore required. Sustainable forest management concepts need to consider the compatibility between forest functions, ecosystem properties, and different stakeholders. Add to this interconnectedness of functions the diverse requirements of different stakeholders, the increasing effects of climate change, and the presence of existing and future non-native tree species (NNT), and successful forest management can become a very challenging task. Owing to the high complexity and heterogeneity of overall forest management, we will focus only on the principles for managing NNT. Forest management principles may vary between countries in the Alpine region, but management principles for sustainable use of NNT should generally be the same. And regardless of whether the species are native or non-native, management interventions can increase their economic, ecological, and social value. The most important guidelines for forest management should be those formulated by the Convention on Biological Diversity (Second Ministerial Conference on the Protection of Forests in Europe, MCPFE (16–17 June 1993, Helsinki/Finland)): “Native species and local provenances should be preferred where appropriate. The use of species, provenances, varieties, or ecotypes outside their natural range should be discouraged where their introduction would endanger important/ valuable indigenous ecosystems, flora, and fauna.” NNT may still be used even when they are considered invasive, if their negative impacts will likely not occur in the area of usage, and if their populations can be controlled with little effort. If possible, native species should always be preferred, while NNT can be considered if they are clearly superior to native species for achieving forest management objectives, or if native species are no longer able to fulfill crucial forest functions or ecosystem services. But there is a risk if no negative impacts are identified in the short term in the area where they are used, they will most likely occur in the future. Therefore, the use of NNT can also be highly controversial, as they can cause significant environmental changes as part of large-scale plantations or when they spread beyond their intended area of introduction into natural ecosystems. Some NNT species have existed in our forests for centuries, and others will arrive in the near future. The reality in the EU is that around 4% or 8.5 million ha of European forests are currently composed of NNT species, with large regional differences. Most of these NNT were intentionally introduced into forests after being initially used in parks for aesthetic purposes, mainly from the 19th century onwards. The criteria for selecting tree species were experience, ease of cultivation, productivity (economic attractiveness), and in some cases the potential to improve site conditions. In general, our NNT management in the Alpine region should be based on EU, national, and sub-national legislation.

Juglans nigra L. – the Black walnut grows faster than the native Juglans regia and is very resistant to diseases and pests. In very good stands and with appropriate crown size, diameters (DBH) of 60 cm can be easily achieved in 60 to 70 years. For cost reasons and because of the comparatively good growth forms, relatively wide planting clusters are preferred.

For NNT management, we must therefore first know which NNT are present within a given area. Overall, Brus et al. (2019) determined at least 145 NNT occurring in European forests (excluding trials and arboreta), with almost half of them originating in North America. With such a large number of NNT, management principles should generally be based on our experience and knowledge along with the risks and benefits accompanying the presence of NNT under existing legislation. The following three legal instruments are relevant to the introduction, use, and management of NNT at the EU level: (1) Council Directive 1999/105/ EC of 22 December 1999 on the marketing of forest reproductive material, (2) Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and flora (the “Habitats Directive”) and (3) Regulation (EU) No 1143/2014 on the prevention and management of the introduction and spread of invasive alien species (IAS), which includes invasive NNT. Regarding the national and subnational levels, Pötzelsberger et al. (2020b) reported 335 relevant legal instruments regulating the use of NNT in place in the investigated 116 geopolitical legal units across the EU in June/July 2019.

Only some NNT species are well known and studied enough for us to be aware that they can represent both benefits and risks in a changing environment. Because rising CO2 concentrations and further climate warming are expected to affect site suitability, productivity, species composition, and biodiversity. The mismatch between climate change and tree adaptation will have serious implications for forest growth and composition – and thus important consequences for management and conservation. NNT can be part of the solution for adapting to such changes in forest ecosystems, but at the same time they pose risks to biodiversity and ecosystem functions. We therefore need a suitable risk assessment tool especially for NNT that should be site-specific and topical. Over the last centuries, many NNT have only been tested and planted on a small scale. Documentation of these trials and plantations is patchy and not centrally available.

Our forest ecosystems are changing drastically due to increasing global warming, and some forest sites and forest types are more heavily invaded by NNT than others. For the Alpine region, some NNT – especially those with an invasive character and the ability to adapt to changing forest site conditions – are expected to spread spontaneously through our semi-natural forests, including Robinia pseudoacacia, Ailanthus altissima, Acer negundo, Quercus rubra, and others. Such expansion at the expense of indigenous growth space is already a major problem from the point of view of biodiversity and ecosystem functioning, and it is expected to become even more serious. For a long time, we have been confronted with the phenomenon of invasions by various NNT, and we still need to study three different aspects: invasiveness of species, invasibility of ecosystems, and impacts of invasive species. Invasive NNT affect the environment at different levels including competition, hybridization, disease/pest transmission, and chemical, physical, or structural impacts. Regarding such invasions, our efforts should focus on the restriction and eradication measures included in the respective management plan. Such measures include mechanical, chemical, and biological methods for eradicating and restricting the spread of NNT.

Whenever we decide to promote certain (tested) NNT in our forests, proper selection of provenance material is essential, as some important NNT originate from large distribution ranges in which numerous races, ecotypes, and climes have evolved. When using NNT seeds and planting material from our own seed stands, it is important to be aware that the loss of genetic diversity and even inbreeding depression can be a disadvantage: unlike many native species, introduced trees often do not have large populations in their area of introduction. Another important issue is the response of NNT to future climatic conditions, and this is the area in which provenance testing can be useful. For example, a provenance that works relatively well in one location may not be suitable for another potential area of use with similar conditions in the future due to climate change. Some authors believe that adaptation to extreme events may even be more important than adaptation to changing average values of environmental variables. A promising example in the Alpine region could be the Douglas fir (Pseudotsuga menziesii) as a replacement for the Norway spruce, which is disappearing from spruce, beech, and silver fir forests due to climate change and past management mistakes. The Douglas-fir can be also combined with native tree species in mixed forests. Invasiveness is not an issue in such mixed Douglas fir stands; in fact, the early growth of the Douglas fir needs to be supported due to its relatively high light requirements. However, in our rapidly changing ecosystems, further studies and monitoring are required. In general, the integration of NNT into mixed forest stands with native tree species is easiest and safest when the ecological characteristics of the introduced NNT are similar to the native species and the NNT do not exhibit strong dominance behavior.

25 Recommendations for the sustainable use of NNT in forests & urban areas of the Alpine Space

  1. Promote and prefer native tree species whenever possible and reasonable.
  2. Evaluate potential benefits and risks of each NNT in the Alpine Space with respect to ecosystem services and disservices provided or jeopardized by that species.
  3. Do not introduce and promote NNT until a pre-entry risk assessment has been performed (Ennos et al. 2019).
  4. Promote and facilitate discussions and information sharing between forest owners, public authorities, regional agencies, NGOs, environmentalists, and the timber industry.
  5. Support the study, collection, breeding, propagation, and local production of forest and horticultural plant reproductive material for diverse NNT and native species.
  6. Use of Site-Specific Risk Assessment (Bindewald et al. 2021) which can be defined as an assessment of risks posed by NNT, distinguishing between locations, habitats or ecosystem types and of which the results can be clearly communicated to policymakers, practitioners, and public users.
  7. Use only those NNT in the area of interest, for which knowledge and/or experience have shown any risks can be easily mitigated or nearly eliminated by effective management measures.
  8. Promote tree diversity in planting sites, including the breeding of native tree species.
  9. Explore new methods to improve the rapid and species-specific response for identifying and potentially eradicating high-risk NNT.
  10. Prevent and manage the further spread of invasive NNT that have already escaped and cannot be eradicated more, in regard to the ecosystem services at risk.
  11. Develop species-specific sustainable management plans to ensure a coordinated multilateral approach across sectors and the general public.
  12. Improve data availability on NNT occurrence and their growth characteristics, through continuous monitoring, for example by explicit inclusion in national or regional forest inventories or other monitoring programs.
  13. NNT that currently pose no risk of becoming invasive can be integrated in mixed-species stands under close observation of their reproduction to prevent any possible invasions caused by increasing propagule pressure.
  14. Consider multiple criteria for the selection of suitable NNT for planting in urban areas, wherein the selection should be limited to those species that are climate-adapted, non-invasive, supporting urban cooling, and aesthetically attractive, and do pose no threat to human health.
  15. Regularly re-evaluate recommendations for site-specific suitability of NNT, integrating assessed changes of their risks and benefits according to the latest research on the predicted range of future climatic conditions.
  16. Improve wildlife management so that climate-adapted native tree species and NNT are not browsed to an extent that they fail to establish and/or successfully grow up.
  17. Ensure close cross-sector cooperation and exchange of expertise in the fields of forestry, horticulture, and urban forestry.
  18. Enable cross-border trade and utilization of forest reproductive materials, in full accordance to OECD and plant health regulations.
  19. Develop and maintain networks in the Alpine Space to provide and exchange information on the distribution, management, and use cases of NNT to ensure free and user-friendly access to comprehensive information on them.
  20. Invest in translational research activities to investigate the population genetics to identify suitable tree species and their long-term management requirements, the potential effects of NNT on forest-dwelling species at different spatial scales and mixing ratios, and the adaptation potential of native trees and NNT in the Alpine Space.
  21. Coordinate the establishment of training initiatives to ensure that stakeholders are equipped with the appropriate skills and knowledge to conduct a risk assessment and manage NNT.
  22. When communicating with the public: always refer to species- and site-specific information to avoid generalizations and clearly distinguish between NNT that currently pose no risks of becoming invasive or whose risks can be kept low, and NNT that are expected to always pose high risks that cannot be controlled through specific management actions.
  23.  Translate guidance and training material, as well as policy documents, and all relevant project results to local languages.
  24. Support appropriate education/guidance initiatives at all levels, in Schools, Universities, and stakeholders’ organizations, including environmental societies and networks.
  25. Facilitate the spread of information through established knowledge platforms (e.g. in the Alpine Space region.

Further resources:

PODCAST – The Strategy on non native trees in the Alpine Space
After 22 months of working on the “ALPTREES Strategy on the use and management of non-native tree species in the Alpine Space”, the ALPTREES team reflects on the work, the progress, the successes and the challenges we all faced in our various work packages to meet the core goal of the project: To provide policymakers, stakeholders, political decisionmakers and the general public with a guideline on how to deal with nonnative tree species throughout the Alpine Space in Slovenia, Italy, France, Austria, and Germany.Available at:

The Usage Value of Non Native Trees – Alptrees Timber Report
Although NNT products are already widely used in Europe, they have not received attention in the New EU Forestry Strategy agenda. Accordingly, the ALPTREES project has the merit of promoting the added value of NNTs for Europe´s forests and related value chains. The authors also open the debate on the existing and potential markets of NNT assortments and create more confidence in their current and future use. The document can be seen as a first attempt to raise awareness among customers and decision-makers about the use of NNT wood. In a compact format, the report summarizes the practical scientific findings on the most important NNT species for the timber market. The report also provides a preliminary outlook on the economic value of NNTs wood products along the timber value chains of some Alpine countries. Furthermore, it illustrates in a simple and intuitive way some valuable experiences made by companies active in the Alpine Space. Available at: alptrees_timber_report_final.pdf

The Usage Value of Non Native Trees – Alptrees Timber Report
Although NNT products are already widely used in Europe, they have not received attention in the New EU Forestry Strategy agenda. Accordingly, the ALPTREES project has the merit of promoting the added value of NNTs for Europe´s forests and related value chains. The authors also open the debate on the existing and potential markets of NNT assortments and create more confidence in their current and future use. The document can be seen as a first attempt to raise awareness among customers and decision-makers about the use of NNT wood. In a compact format, the report summarizes the practical scientific findings on the most important NNT species for the timber market. The report also provides a preliminary outlook on the economic value of NNTs wood products along the timber value chains of some Alpine countries. Furthermore, it illustrates in a simple and intuitive way some valuable experiences made by companies active in the Alpine Space. Available at: alptrees_timber_report_final.pdf

Scientific References

Bindewald, A. , Brundu, G., Schueler, S., Starfinger, U., Bauhus, J., Lapin, K. (2021) Site-specific risk assessment enables trade-off analysis of non-native tree species in European forests. Ecology and Evolution, doi: 10.1002/ece3.8407.

Bolte, A., C. Ammer, M. Löf, P. Madsen, G. J. Nabuurs, P. Schall, P. Spathelf, and J. Rock. 2009. Adaptive forest management in central Europe: Climate change impacts, strategies and integrative concept. Scandinavian Journal of Forest Research 24(6): 473–482.

Brang, P., A. Breznikar, M. Hanewinkel, R. Jandl, and B. Maier. 2013. In: Managing alpine forests in a changing climate. Cerbu, G.(Ed.), Management Strategies to Adapt Alpine Space Forests to Climate Change Risks, pp. [PB1] 369–383. InTech. Paris. DOI: 10.5772/56933; ISBN: 978-953-51-1194-8. eBook (PDF) ISBN: 978-953-51-4240-9

Brundu, G., A. Pauchard, P. Pyšek, J. Pergl, A. Bindewald, A. Brunori, S. Canavan, T. Campagnaro, L. Celesti-Grapow, and M. De. 2020. Global guidelines for the sustainable use of non-native trees to prevent tree invasions and mitigate their negative impacts. NeoBiota 61: 65–116.

Brus, R., E. Pötzelsberger, K. Lapin, G. Brundu, C. Orazio, L. Straigyte, and H. Hasenauer. 2019. Extent, distribution and origin of non-native forest tree species in Europe. Scandinavian Journal of Forest Research: 34: 533–544.

Campagnaro T, Brundu G, Sitzia T (2018) Five major invasive alien tree species in European Union forest habitat types of the Alpine and Continental biogeographical regions. Journal for Nature Conservervation 43:227-238.

Castro-Díez, P., Á. Alonso, A. Saldaña-López, and E. Granda. 2021. Effects of widespread non-native trees on regulating ecosystem services. Science of The Total Environment 778: 146141.

Castro‐Díez, P., A. S. Vaz, J. S. Silva, M. Van Loo, Á. Alonso, C. Aponte, Á. Bayón, P. J. Bellingham, M. C. Chiuffo, and N. DiManno. 2019. Global effects of non‐native tree species on multiple ecosystem services. Biological Reviews 94: 1477–1501.

Chakraborty, D., J. Gaviria, D. Bednárová, A. Bolte, and C. Bouissou. 2019. Interreg SUSTREE, Policy Brief; Implementing assisted migration.

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Pötzelsberger, E., H. Spiecker, C. Neophytou, F. Mohren, A. Gazda, and H. Hasenauer. 2020b. Growing non-native trees in European forests brings benefits and opportunities but also has its risks and limits. Current Forestry Reports: 6: 339–353.

Richardson, D. M., and M. Rejmánek. 2011. Trees and shrubs as invasive alien species–a global review. Diversity and Distributions 17(5): 788–809.

Rigling, A., A. Gessler, L. Feichtinger, V. Queloz, and T. Wohlgemuth. 2016. 4.3 Introduced or native tree species to maintain forest ecosystem services in a hotter and drier future? In: Krumm F, Vítková L (eds.) Introduced tree species in European forests: Opportunities and challenges. pp. 236–246.

Vaz, A. S., P. Castro-Díez, O. Godoy, Á. Alonso, M. Vilà, A. Saldaña, H. Marchante, Á. Bayón, J. S. Silva, and J. R. Vicente. 2018. An indicator-based approach to analyze the effects of non-native tree species on multiple cultural ecosystem services. Ecological Indicators 85: 48–56.

Wohlgemuth, T., B. Moser, E. Pötzelsberger, A. Rigling, and M. M. Gossner. 2021. Über die Invasivität der Douglasie und ihre Auswirkungen auf Boden und Biodiversität. Schweizerische Zeitschrift für Forstwesen 172(2): 118–127.