More broadleaved tree species for the Nordic countries – a call out for a common effort

Will the future Nordic forests contain more species than today? With climate change, the conditions for forestry will also change, which may be background for a new composition of tree species. EUFORGEN points to the need for increased knowledge building on species that today have only minor economic importance. Such species can make a greater contribution to the Nordic forests, both economically and in relation to biodiversity.

Text and photos by Gunnar Friis Proschowsky (The Danish Nature Agency). The top photo shows a field-trial in Denmark with the noble hardwood Silver birch (Betula pendula) that is part of a Skogforsk breeding programme and contains genetic resources from Sweden, Finland and Denmark.

Different regeneration strategies can be used in the context of climate change: a natural regeneration strategy, an assisted migration strategy or a combined strategy.

Assisted migration and noble hardwoods

Assisted migration means intentional moving of forest reproductive material (FRM) to new locations. In light of climate change, this may be desirable in order to ensure properties such as drought resistance and adaptation to a longer growth period for species that are already a valuable part of forestry in the locality.

Both the tree species that today are commercially important as well as the species not presently part of forestry production can be part of assisted migration.

Species that supplement the commercially important tree species – providing diverse forests with benefits for biodiversity and possibilities of niche production – are often referred to as noble hardwoods. Noble hardwoods include species such as alder, maple, rowan, hornbeam, ash, birch, walnut, wild cherry and small-leaved lime. For Nordic forestry, beech may also be added to the list. It is at present cultivated to a significant extent in Denmark, but only to a lesser extent in Sweden and Norway.

Tree plant surrounded by soil.
Common ash is a noble hardwood. Nordic cooperation and exchange of resistant genetic material is an important part of mitigating ash-dieback.

These under-utilised species have only to a lesser extent been included in provenance trials and breeding and therefore knowledge about the options for choosing material for assisted migration is limited.

Knowledge about appropriate forest reproductive material

Using appropriate forest reproductive material is essential to forest and nature management. Appropriate material is suitable for the actual purpose of forestry, whether it is production, outdoor life, nature or biodiversity – or a combination of several purposes. In practice, this means that forest reproductive material must be able to both grow and survive under the given environmental conditions and at the same time meet requirements for e.g. good shape and good growth. Facing climate change it becomes an increasing challenge to predict the performance of FRM and to secure a supply of it. The work with breeding has for many decades built up knowledge about this, using field-trials and through comparisons of provenances from different places within the species’ distribution area. Furthermore, testing of selected individual trees is an important element in breeding. On this basis, there is good knowledge in the Nordic countries about the recommended planting-areas for the commercially important tree species and provenances. For the most important forest tree species, breeding improvements have also been accomplished, especially in relation to quality, production and health. In the Nordic countries, experimental and breeding co-operation between neighboring countries creates a basis for assessing which material is most suitable within a larger geographical area. Furthermore, it is possible to assess whether the material that has already been improved for e.g. production or quality can be used in other countries.

A field with long rows of tree plants.
Planting of a field trial of beech in Hørup, Denmark, October 2022.

Intensive breeding is carried out in Sweden, Finland and Norway for Norway spruce and Scots pine whereas in Iceland and Denmark a broader range of species have been in focus in less intensive programs. The database NOLTFOX (http://noltfox.metla.fi/) contains information of Nordic long-term field trials, including many trials with spruce and pine and only a few with noble hardwoods. Noble hardwood trials are often located in Denmark.

The Danish breeding of nobel hardwood species is mainly built on national genetic resources but in 2017 provenances from Central and Southern Europe were compared with Danish seed sources of beech, hornbeam, wild cherry, wild service tree, chestnut and walnut. This project is expected to provide knowledge about opportunities and limitations for assisted migrations. In order to be able to use such results in the other Nordic countries, field trials will be needed in the countries concerned. A new EU project “optFORESTS” may provide a background for this. Here, the species and provenances are tested in several different countries, which is expected to give additional knowledge on how relocations of forest reproductive material are appropriately conducted.

FACTS