Long-term Effects of Clear-cutting Forestry on Ectomycorrhizal Fungi in Boreal Forest Revealed
- Gauri Khanna
- 4 hours ago
- 3 min read
Too long to read? Go for the highlights below.
Clear-cutting boreal forests does not reduce the total number of ectomycorrhizal fungal species, but permanently alters which species are present, with effects lasting at least a century.
Species richness peaks surprisingly quickly, around 30 to 40 years after harvest, yet community composition continues to diverge from old-growth forests well beyond a typical rotation period.
The findings suggest that rotation forestry, as currently practised across Sweden, is progressively eroding the distinct fungal communities of ancient boreal forests at a landscape scale.
The Hidden Cost of a Timber Rotation
Walk through a Swedish boreal forest and the trees tell one story; the soil tells quite another. Beneath the moss and root mats lives a community of ectomycorrhizal fungi, organisms that wrap themselves around tree roots and exchange soil nutrients for the sugars that trees produce through photosynthesis. These fungi are obligate biotrophs, meaning they cannot survive without living host trees and perish rapidly after clear-cutting. What happens to their communities over the following century is the subject of a major new study published in New Phytologist by researchers at the Swedish University of Agricultural Sciences.
The team, led by Björn Lindahl, analysed DNA extracted from soil samples taken across more than 1,500 sites distributed systematically throughout Swedish boreal forests. By treating forests of different ages as snapshots of a single long timeline, a technique known as space-for-time substitution, they tracked how fungal communities changed from freshly harvested clear-cuts to forests over 200 years old.
Species Richness Recovers, but Community Identity Does Not
The headline finding is reassuring in one respect and sobering in another. Species richness, the raw count of how many fungal species are present at a given site, recovers well within a standard rotation period of 60 to 100 years. In fact, it peaks around 30 to 40 years after harvest, likely because soil pH rises after clear-cutting and creates temporarily favourable conditions for a wider range of fungi. In this narrow sense, rotation forestry appears ecologically sustainable.
The more troubling conclusion concerns which species are present, rather than how many. Community composition in secondary forests remained measurably different from that of old continuity forests even in stands approaching 100 years of age. A statistical index the researchers developed to track community maturity suggested that full convergence with old-growth communities was not reached until stands exceeded 100 years, well beyond the point at which most Swedish forests are harvested again.
The species patterns are consistent across such a large dataset that they become hard to dismiss. Pioneer fungi, notably Thelephora terrestris and Laccaria laccata, colonise recently harvested sites rapidly but fade as stands mature. Several species in the Atheliaceae family, including Piloderma and Tylospora, proliferate in managed secondary forests and persist for decades. Meanwhile, species from the genera Cortinarius, Russula, and Hebeloma, which are adapted to the acidic, nutrient-poor soils of ancient forests and are capable of extracting nitrogen from particularly tough organic matter, decline steadily across the managed landscape. In Fennoscandia, approximately 15% of the roughly 2,000 known ectomycorrhizal fungi are already nationally red-listed, with clear-cutting cited as a primary driver.
What This Means for Forest Management
The practical implications are significant. Because 91% of Swedish forest land is managed for timber production and harvesting is almost exclusively by clear-cutting, these community shifts are not localised curiosities but landscape-scale trends. The research suggests that with each successive rotation, the fungi best suited to nutrient-poor old-growth conditions become rarer across the country as a whole.
This matters beyond taxonomy. The species in decline play disproportionate roles in nutrient cycling, particularly in how organic nitrogen is made available to trees in boreal soils. Losing them could have consequences for decomposition rates and carbon storage that are not yet fully understood.
The researchers point to continuous cover forestry, where enough trees are retained at harvest to prevent the sharp rise in soil pH and fertility that follows clear-cutting, as a partial remedy. The key condition is that retention must be sufficient to maintain the acidic, low-nutrient soil conditions that old-growth fungal specialists require. Without that, even partial harvesting could accelerate the broader pressures on fungal biodiversity already documented across European forests.