The consequences of forestry on fungal communities are seldom up to discussion in the debate, but new results indicate that logging have long-term impacts on important fungal groups. Disturbing the fungal balance may even lead to nutrient deficiency and reduced tree growth. This topic is one of many where researchers in the NEFOM network are involved.
Text: Mats Hannerz, Silvinormation
Molecular methods have led to a revolution in studies of fungi in soils, since old inventory methods, based on finding and identifying fruit bodies, only detect minor portions of the fungal species.
Mycologists can now use DNA sequencing to quantify nearly all the taxa present in soil, and this has enabled many interesting studies on the consequences of forest management methods. A number of Nordic published studies have investigated the impact on the fungal community from measures such as clear-cutting, ash fertilization and tree retention.
NEFOM network unites Nordic and Baltic mycologists
The mycology labs and research groups in the Nordic and Baltic countries help each other by sharing ideas and equipment. The cooperation has been going on for a long time, but since 2013, the network cooperation is formalised in the SNS-supported network NEFOM (North European Forest Mycologists).
A key activity is to establish and compile databases for identifying ectomycorrhizal fungi – and also all other fungi found in the environment. The largest is UNITE (Unified system for the DNA-based fungal species linked to the classification).
Several new results on the impacts of forestry were presented at a recent NEFOM conference held in Uppsala in February 2018. Almost 60 researchers from the Nordic and Baltic countries, as well as from the UK, Netherlands, USA, Canada and Switzerland, were assembled under the theme “Roles of fungi in sustainable forestry”.
Fungi important in boreal forests
Fungi have particularly important roles in boreal coniferous forests. The bacteria and earthworms that are important decomposers in richer soils are much less abundant in the acid soils of boreal ecosystems. Instead, fungi are the key decomposers of organic material and major agents in nutrient cycling.
Degradation of the top layer of the forest soil, the litter, is strongly dependent on saprotrophic fungi that release enzymes capable of efficiently decomposing not only simple substrates but also tough polymers such as lignin. Below, in the more decomposed humus layer, ectomycorrhizal fungi are responsible for much of the nutrient turnover.
Ectomycorrhizal fungi are essential in all forest ecosystems. They receive carbohydrates from the trees and pay them back by improving supplies of nutrients and water to their roots. But mycorrhizal fungi are not a homogenous group: some release more efficient degradative enzymes than others, which may have different functional roles.
Clear-cutting disturbs the fungal balance
When trees are cut, mycorrhizal fungi lose their symbiotic partners. Recent studies have shown that mycorrhizal species decrease after a clear-cut, and instead give room to free-living fungi with more efficient decomposition. This accelerates decomposition during the decade following clear-cuts resulting in a nutrient flush. Some mycorrhizal species recolonize the new forest stand, but others seem to be suppressed for longer times.
Fungi of the genus Cortinarius are particularly sensitive. Some of these fungi, despite being ectomycorrhizal, can produce strong enzymes which make them efficient scavengers of nutrients bound in organic matter. The nutrient peak after a clear-cut probably disfavour these fungi. In the long run, the balance between various groups of fungi will be disturbed.
Risks of nutrient deficiencies after clear-cutting
When the Cortinarius species decline, there is a risk of nutrient turnover slowing down as less efficient decomposers replace them. This may impair nutrient supplies to the trees.
Studies of chronosequences of managed pine stands in Sweden have shown that the fungal community is affected for a long time after clear-cutting. When all trees are cut, mycorrhizal fungi almost disappears. The fungi return slowly, but other species are dominating the young forest compared to the older. It is not until an age of 60 years or so before the community resemble that of the old forest.
The studies have for example shown that Cortinarius and Russula species are absent for several decades, and increase only slowly after that.
Single retention trees is not the final solution
What can be done to prevent the long decline of certain mycorrhizal fungi? Some of the studies have considered effects of retention trees. However, results have indicated that leaving single trees, like in a seed-tree stand, does not preserve the ectomycorrhizal composition. The fungi nearest the tree may still have a diversity resembling the old forest, but this is not enough to restore the fungal community in the whole stand.
It is probably more efficient to leave retention trees as forest patches. Uneven-aged forestry, avoiding clear-cuts, could also be effective.
Important to raise awareness of fungi
Karina Engelbrecht Clemmensen at SLU in Uppsala is coordinator of NEFOM and one of the organisers of the conference in Uppsala. She wants to raise the awareness of the important roles of fungi in the forest ecosystem. She finds, however, that the fungal community is usually neglected in the discussions about sustainable management.
– Two invited guests gave their perspectives on forest ecology and forestry research, and it was clear that fungi are overlooked in more general forestry research and management plans. We discussed means to raise the awareness of fungi. One idea was to conduct more cross-field research to ensure convergence between scientific research questions and forestry practices. We also need to tell our stories to the public and the forest sector. Research results must be made more easily available, in a context that engages the audience, she says.
The mycologists at SLU have made good efforts to produce popular news about their results, news that have been much exposed in media.
– The mission to disseminate new result applies to all NEFOM researchers. New results are constantly added, and we need to consider these when discussing sustainable forest management, says Karin Engelbrecht Clemmensen.
SNS network N2018-5 North European Forest Mycologist (NEFOM)
Contact: Karina Engelbrecht Clemmensen, firstname.lastname@example.org