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Mixed forests increase the overall sustainability of forests
By establishing and managing mixed forests, we aim to better reconcile the objectives of timber production, carbon sequestration, forest resilience, and diversity. Updating forest management practices is essential to prepare for a changing climate and to ensure that forest use objectives, such as timber production and biodiversity, are met.
While growing pure spruce stands on fertile sites is beneficial for timber production, it does not support the overall sustainability of forests. Due to its sensitivity to changing climate conditions, spruce is expected to suffer from climate change. To prepare for these changes, increasing the proportion of Scots pine and silver birch, along with establishing mixed forests, can be beneficial. Mixed forests reduce the risks associated with spruce and enhance forest diversity.
When establishing both single-species and mixed forests, it is crucial to select the right tree species for the appropriate sites and to apply proper management at the right time. Mixed forests are established through regeneration and maintained through pre-commercial and commercial thinnings, with the goal of maintaining the species admixture throughout the entire rotation period. Implementing the right measures will also ensure the overall sustainability of forests in a changing climate.
The site illustrates how our main tree species, Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and silver birch (Betula pendula) thrive in single-species and mixed stands.
The stand is growing on a fertile site (OMT) characterized by fine sand mineral soil. In September 2022, a birch-dominated stand on this site was clearcut. The regeneration area was mounded by an excavator in May 2023 and seedlings were planted in June 2023.
The site contains a total of 10 plots plantedwith pine, spruce and silver birch as singlespecies stands and mixed stands of two tree species. There are plots where only every second mound has been planted. Natural birches are expected to regenerate on the empty mounds, supplementing the conifer seedlings. On two of the plots, Norway spruce seedlings propagated by somatic embryogenesis (SE) were planted instead of traditional seed-born seedlings.
Silviculture of mixed forests
In Finland, forest management has traditionally focused on even-aged forestry, favouring conifers in single-species stands. However, research indicates that mixed forests can offer a broader array of ecosystem services compared to single-species stands.
The forest management regimes of mixed forests are studied through permanent experiments (SEKAVA). These experiments are used to determine how to establish single-storey mixed forests on mineral soils, aiming to sustain the mixture throughout the rotation. The experiments also investigate the growth dynamics of various tree species and their impacts on soil characteristics and forest damage, providing new insights into the establishment and management of mixed forests.
A total of 18 experiments have been established in Southern and Central Finland, along with one site in the Punkaharju research forest. These experiments were set up between 2020 and 2023, with regeneration treatments varying according to site fertility. In addition to the treatments in Punkaharju, direct seeding of pine as well as spruce planting together with pine seeding have been applied. Each experiment includes 2–3 repetitions of treatments, while only one repetition was used in Punkaharju.
