Clear-cut forestry has long-term effects on the community composition of ectomycorrhizal fungi in boreal forest

Rotation forestry based on clear-cutting is a common practice in boreal forests. Clear-cutting has detrimental short-term effects on ectomycorrhizal fungal communities, and the communities that re-establish after clear-cutting differ in species composition from old forests, but the long-term time trajectories of ectomycorrhizal fungal biomass, species richness and community composition in secondary forest remains uncertain. We collected soil samples from almost 1600 locations distributed systematically across Swedish coniferous forests, in conjunction with the Swedish National Forest and Forest Soil Inventories, and analysed ectomycorrhizal fungal communities by sequencing of amplified ITS2 markers.

We found that the relative abundance of ectomycorrhizal species in the fungal community increases to similar levels as before clear-cutting within two decades. In the following decades, species richness increases to a somewhat higher level than in old stands, peaking about 40 years after harvesting. Clear-cutting has strong and long-lasting effects on the composition of ectomycorrhizal fungal communities, with harvesting effects remaining for up to 100 years. Many species that attain high abundance in old forests (mainly certain Cortinarius and Russula species) are adapted to the acidic, unfertile soil conditions and have a well-developed capacity to mobilise nutrients from recalcitrant organic matter. These species are negatively affected by rotation forestry, which raises pH and increases nutrient availability.

This means that rotation forestry based on clear-cutting seems to be sustainable with regards to the abundance and species richness of ectomycorrhizal fungi, which return to pre-harvest levels well within the time limits of a rotation period and even reach somewhat higher levels than in old forests. However, rotation forestry progressively changes the ectomycorrhizal community at the landscape level. Many of the species that are characteristic of the predominantly nutrient poor and acidic boreal forests decrease in abundance. This declining community is also likely to contain many rare species, which risk extinction in large areas if transformation of the forest landscape proceeds.