Keeping mixtures of Norway spruce and birch in production forests: insights from survey data

ABSTRACT

Admixtures of birch in Norway spruce plantations are being promoted as a means to increase habitat and species diversity. The implications of this mixture were analysed with regional survey data from southern Sweden. Permanent sample plots from the Swedish National Forest Inventory (NFI), with Norway spruce and admixture of birch, were used to describe the temporal trends in the admixture, regarding species composition and competitive strength. Observations from thinned plots show a higher harvest removal in birch (35%) than for Norway spruce (19%). Observations without thinnings in the period before measurement showed that individual birch tree growth was lower compared to Norway spruce and it decreased even more with increasing stand age and competition. In addition, a complementary field survey, with multiple distributed sample plots in each stand, was used to detect within-stand variation of species composition and density. Although within-stand heterogeneity was larger in mixed stands in terms of species composition, it was not different from Norway spruce monocultures in terms of stand density. These two surveys show that the admixture of birch, for several reasons, decreases over stand age and although birch increases tree species diversity, it does not necessary imply a change in density.

KEYWORDS:

• Mixed forest
• national forest inventory
• Betula
• Picea
• thinning
• stand heterogeneity

Acknowledgements

Sveaskog and Södra forest owner association, has approved that information extracted from their forest properties can be published in anonymous summarized results, as managed in this manuscript. We are thankful for their data contribution. We are thankful to the Swedish National Forest Inventory, especially to Bertil Westerlund, who guided us with metadata and data delivery, and to all the forest technichians measuring the forest every summer.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability

Data from the national forest inventory (NFI) are open source and available from the SLU webpage: https://www.slu.se/en/Collaborative-Centres-and-Projects/the-swedish-national-forest-inventory/. Detailed information regarding exact coordinates of sample plots from both stand survey and NFI is not open source but may be retrieved after written agreements have been made with the original data supplier. The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request (excluding exact location of the plots).

Additional information

Funding

This study was partly funded by Formas.