The significance of tree retention on fungal communities and below-ground stocks 12 years after harvest of boreal forest

Forest harvesting has a major impact on fungal diversity, but the subsequent successional dynamics of soil fungi and their links to organic matter dynamics below-ground remain uncertain. Release of saprotrophic fungi from ectomycorrhizal fungal competition after clear-cutting could stimulate decomposition, leading to accelerated loss of organic matter (a Gadgil effect). On the other hand, clear-cutting could select against ectomycorrhizal fungi with decomposer capacity, hindering mobilization of organic nutrient stocks. If so, tree retention strategies may buffer such effects on fungal communities and organic matter turnover. We evaluated fungal communities in a low-productive, old-growth pine forest in northern Sweden that was harvested with different levels of tree retention. Previously, the experiment was investigated 3 years after harvesting. Here, we returned 12 years after harvesting, using long-read sequencing to assess taxonomic and phylogenetic diversity of fungal communities. We also estimated carbon and nitrogen stocks in the organic soil layer, as well as fungal biomass by PLFA analyses. Community assembly transitioned from largely stochastic processes 3 years post-harvest toward more deterministic environmental filtering over the following decade, with higher harvesting levels driving increasingly distinct ectomycorrhizal fungal communities. Specifically, certain Cortinarius species were particularly sensitive to tree harvesting, while for example Piloderma cinicola increased in relative abundance at lower retention levels. Carbon and nitrogen stocks in the organic layer declined at higher harvesting levels, consistent with increased organic matter turnover, although we found no evidence for increased biomass of saprotrophic fungi 12 years after harvest. In this nutrient-limited system, an initial post-harvest phase with rapid decomposition, consistent with a Gadgil effect, may have led to the loss of organic stocks, with the released nitrogen rapidly taken up by ground vegetation and remaining trees. Tree retention strategies lessened such harvesting impacts on both fungal communities and below-ground carbon and nutrient dynamics during the first decade after harvest. By maintaining ectomycorrhizal fungal communities, moderate retention levels hindered losses of organic matter stocks and sustained carbon sequestration capacity compared to clear-cuts. Read the free Plain Language Summary for this article on the journal's blog.