Impactful research: new paths for growing mixed forests

Improved vitality and resilience of mixed forests will help our forests adapt to climate change. The Natural Resources Institute Finland (Luke) has produced new data on the growth, establishment and cultivation of mixed forests. Extensive permanent experiments have been set up to help meet future data needs on mixed forests.

Forestry in Finland has traditionally focused on even-aged single-species forests, mostly pines and spruces. As the operating environment has changed, the role of forests has become more diverse. Mixed forests with broader variety of tree species offer good opportunities for multi-objective forest use and management in terms of timber production, climate resilience, recreation and biodiversity. 

Improved vitality and resilience of mixed forests contribute to maintaining wood production under changing conditions, thus enhancing forest carbon sinks and stocks. Luke has been studying the growth, establishment and silviculture of mixed forests and looking for best practices to support their growth.

In a changing climate, it is increasingly important to grow the tree species and seedlings of the most appropriate origin for the site. This will ensure the vitality of forests and the best possible resistance to damage in the future. It is also important to make greater use of the variety of tree species. 

“Mixed stands should be favoured on sites suitable for growing two or more tree species. For example, instead of spruce, silver birch can be grown on mesic heath forests and more fertile sites, and pine on coarse soils in mesic heath forests,” says senior researcher Saija Huuskonen from Luke.

More research was needed on how mixed forests should be established and at what rate pine, spruce and silver birch grow in mixed forests using current silvicultural methods. Luke's research projects have provided information on the differences between the regeneration results of spruce, pine and silver birch and the site factors that influence them.

Traditionally, the advice has been to remove all broadleaves during early cleaning and to leave broadleaves in pre-commercial thinning. New research has shown that when spruce regeneration has been successful and growth has been rapid, natural seed-born silver birch trees should be left as mixed stands already at the early cleaning stage.

“Otherwise, there is a risk that the birches will not catch up with the growth rate of the spruces. From the point of view of growing mixed forests, forest management should ensure the vitality and growth status of the broadleaf stand so that the tree species diversity can be maintained in the forest throughout the rotation period”, says Huuskonen.

When spruce regeneration has been successful and growth has been rapid, natural seed-born silver birch trees should be left as mixed stands already at the early cleaning stage.

Luke has made a significant investment in future forest research. The SEKAVA research project has established a total of 18 experiments, with 284 plots and more than 50,000 measurement points measuring the development of individual seedlings from regeneration onwards. Both single-species and two-species mixed forests have been established in the experiments. 

The tree-level measurements allow comparisons of the growth rates of spruce, pine and silver birch as they mature in pure stands and mixed stands. This series of permanent experiments will provide important new research data on the development of mixed forests and growth differences between tree species over the next decades. It will also enable long-term and extensive monitoring of the effects of mixed forests on, for example, ground vegetation, soil and biota.

The series of permanent experiments will provide important new research data on the development of mixed forests and growth differences between tree species over the next decades.