Forestry's impact on mycorrhizal fungi: Species richness returns, but composition changes

Forestry's impact on mycorrhizal fungi: Species richness returns, but composition changes

Forests managed under current Swedish forestry practices experience a shift in the species composition of mycorrhizal fungi, yet they maintain the same number of species as forests that have never been clear-cut. This concludes a major new study from the Swedish University of Agricultural Sciences (SLU).

Mycorrhizal fungi live in symbiosis with tree roots; the fungi provide the trees with nutrients from the soil and, in return, receive energy in the form of sugars from the trees' photosynthesis.

This symbiosis makes mycorrhizal fungi sensitive to logging, as their energy source disappears when the trees are removed. While many studies show that these fungi largely vanish immediately after clear-cutting, knowledge regarding how they recolonize the next forest generation − and how they are affected by forestry in the long term − has been limited. Researchers at SLU sought to fill this knowledge gap by examining more than 1,500 forests of various ages across Sweden, focusing on the most common fungal species in the soil.

− We chose to DNA-identify the presence of fungal mycelium. This provides a more complete picture of the entire fungal community, especially the most common species, compared to identifying fruiting bodies. The DNA sampling has spanned twelve years as part of the Swedish National Forest Soil Inventory, says Björn Lindahl at the Department of Soil and Environment, who led the study alongside Johan Stendahl and Anders Dahlberg.

Altered composition, stable species count

The results show that when a forest begins to regrow after harvesting and regeneration, the number of mycorrhizal fungal species increases steadily until the forest is approximately 40 years old. After that, the increase levels off. The number of species in managed forests is at the same level as, or even slightly higher than, in older forests that have not been clear-cut.

− One explanation could be that the soil becomes less acidic and nutrient availability increases after logging, which allows many fungal species to thrive there, says Björn Lindahl.

While species richness in individual forests does not appear to be disadvantaged by modern forestry (where forests are harvested in cycles of 60–100 years), the study does not provide an answer as to how the total number of species across Sweden has been, or is being, affected.

If, however, one takes a closer look at the species composition in forests of different ages, one can see that managed forests differ significantly from old-growth forests that have not been clear-felled. Certain common mycorrhizal fungi, such as Russula decolorans and Russula paludosa, and several Cortinarius species, are specialized for nutrient-poor soil conditions and appear to be inhibited by competition from other species when forestry alters the conditions. A clear impact of forestry on the fungal community can be observed in stands up to 100 years old.

− This may be one reason why red-listed species, which are usually too rare to be found in a survey like this, are disadvantaged, says Anders Dahlberg at the Department of Forest Mycology and Plant Pathology.

Different ways to evaluate biodiversity

This study shows that forestry does not appear to affect the local number of mycorrhizal fungal species in the long term, but that the composition changes at the expense of species adapted to living in older coniferous forests. Whether forestry should be considered positive or negative for mycorrhizal fungi depends on how one views biodiversity and natural value.

− If you see the number of species as the important factor, the local impact of forestry is limited to a few decades after felling. If, however, you place value on the mycorrhizal species that are historically and naturally characteristic of Swedish coniferous forests, then forestry is not sustainable, but rather leads to a long-term change across the forest landscape, says Björn Lindahl.

The study clearly illustrates that biodiversity can be valued in different ways, depending on whether the focus is on species quantity or species composition.

The species that are disadvantaged by forestry also appear to have certain specific traits, such as an especially good capacity to release hard-to-access nutrients in the soil. This means that the change in the fungal communities also entails changes in the soil's biological processes. At present, the researchers cannot say whether this affects the long-term productivity of the forests.

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