A research led by the University of Jyväskylä (Finland) evaluated, for the first time, an ecological threshold for the amount of clearcutting in boreal forests. According to the results, maintaining populations of bird species specialized in old-growth forests requires either a forest rotation period of at least 90 years on average or approximately one-fifth of the forest area be excluded from clearcut management. The study provides a concrete target for the ecologically sustainable use of forests.

Clearcutting is a common forest management method worldwide, especially in boreal forests, but it is also a significant factor in the decline of forest biodiversity. Although the effects of individual harvests are relatively well understood, there has been little information to date on the long-term and cumulative effects of clearcutting at the landscape level.

Researchers analyzed national bird monitoring program data from South and Central Finland covering the years 2006 to 2020 and the European maps of forest disturbances, depicting the accumulation of clearcut areas.

- We noticed that the abundance of common forest species and species specialized in old-growth forests was lower in landscape where large areas of clearcutting had accumulated over 20 years, summarises Senior Researcher Rémi Duflot from the University of Jyväskylä.

Defining a "safe operating space" for forests

A key finding of the study was the definition of a so-called “safe operating space” for forest ecosystems. The researchers identified the ecological limits below which populations of bird species associated with old-growth forests can remain stable or recover from past declines.

- Based on our results, to maintain forest bird populations, the proportion of clearcutting should not exceed c. 10 percent of the forest landscape over a ten-year period. This corresponds to a rotation period of approximately 90 years in forestry or to the exclusion of approximately one-fifth of forest areas from clearcutting management, explains Duflot.

The researchers also emphasize that ecological thresholds may vary by region and among different groups of organisms. For example, lichens and mosses may be more demanding than forest birds. Therefore, the results should be supplemented by studies on other species groups before general thresholds are adopted.

A tool for more sustainable forest management

The results provide decision-makers, forest owners and forestry sector stakeholders with a concrete metric for planning ecologically sustainable forest management.

- Our study offers a clear and easily communicable goal that can be used to reconcile forest use with the protection of biodiversity. The method is also suitable for use around the world, says Professor in Applied Ecology Mikko Mönkkönen from the University of Jyväskylä.

The results were published in the Journal of Applied Ecology.