Fungal communities as determinants of carbon dynamics in boreal forest soils

Boreal forests play an important role in the global carbon cycling, as their soils represent a substantial terrestrial sink for atmospheric CO2 globally. In this biome, fungi are pivotal components as drivers of decomposition and nutrient cycling. Rapid development of molecular techniques increases our knowledge of fungal species distribution in different environments. However, scaling from small-scale community composition to ecosystem level processes is challenging. High throughput sequencing, enzyme assays and stable isotope analyses were used to investigate how fungal diversity and community composition responded to ecosystem fertility and forest harvesting. Radiocarbon dating and a carbon sequestration model were combined to estimate long-term carbon dynamics in soil profiles. Using data collected within a large-scale national sampling program, drivers of organic matter accumulation across the entire latitudinal range of the Swedish boreal forest were explored. Multivariate statistics and structural equation modelling were used to yield correlative relationships between environmental parameters, fungal communities and soil carbon dynamics. The re-establishing ectomycorrhizal community during stand development after clear-cutting was dominated by Atheliaceae in younger stands and by Cortinarius and Russula in older stands. The latter genera correlated positively with nutrient-mobilising enzymes, indicating aggravated nutrient limitation. A risk that shorter rotation periods could lead to the loss of symbiosis-driven recycling of organic nutrient pools and constrain long-term forest productivity was identified. Fungal-driven oxidation constrained belowground organic matter accumulation and promoted ecosystem fertility in an old-growth forest. Oxidative decomposition was regulated by fertility-related shifts between fungal guilds with contrasting decomposing capacities. Long-term humus build-up was driven by differences in root decomposition rates, whereas leaf litter decomposition was found to be of minor regulatory importance. Within soil profiles, carbon and nitrogen dynamics were significantly related to ecosystem fertility, root decomposition and fungal community composition. Belowground fungal communities were important components in mediating effects of climate, soil fertility and forest management on accumulation of soil organic matter in Swedish boreal region. Overall, the results collected across different scales and ecosystem types underlined soil fungi as the principal drivers of carbon and nitrogen dynamics in boreal forest ecosystems. The thesis highlights a major potential to increase the predictive power of forest ecosystem models by including soil fungi as integrated components

Soil fertilisation with Cs-137-contaminated and uncontaminated wood ash as a countermeasure to reduce Cs-137 uptake by forest plants

The purpose of present study was to find out whether wood ash with a high pH value and neutralizing capacity reduces Cs-137 uptake by forest plants many years after the radionuclide fallout. The effects of one-time point fertilisation with Cs-137-contaminated and uncontaminated wood ash alone or in combination with KCl on Cs-137 transfer from soil to young leaves and green shoots of various dwarf shrubs and tree species were examined in a long-term fertilisation experiment (2012-2021) conducted in Bazar mixed forest, around 70 km from Chernobyl nuclear power plant. The results indicated minor effects of soil fertilisation, although there were differences between Cs-137 uptake by species and years. Soil amendment with Cs-137-contaminated wood ash generally did not affect Cs-137 uptake by young shoots and leaves of plants over the growing season in the first year and only slightly decreased Tag for Cs-137 in the following years. The effect of a single application of Cs-137-uncontaminated wood ash on reducing Cs-137 uptake by plants was generally negligible. Application of Cs-137-contaminated wood ash in combination with KCl reduced plant Cs-137 uptake by about 45%, however, such reduction was only significant in some years for bilberry berries, young leaves and green shoots of lingonberry and alder buckthorn. Thus application of wood ash to Cs-137-contaminated forest soil many years after radionuclide fallout generally does not reduce Cs-137 uptake by forest vegetation in a mixed forest ecosystem and this countermeasure should be applied with caution

Increase in dead wood, large living trees and tree diversity, yet decrease in understory vegetation cover: The effect of three decades of biodiversity-oriented forest policy in Swedish forests

In Sweden, the majority of forest area has been altered by industrial forestry over the decades. Almost 30 years ago, a shift towards biodiversity-oriented forest management practices occurred. Here we took advantage of long-term data collected by the Swedish National Forest Inventory to track developmental changes in forest structural components over this time. We assessed changes in structural components that play an important role in biodiversity (dead wood, large living trees, tree species composition, and understory vegetation) in four forest types with descending tiers of biodiversity protection: protected areas, woodland key habitats, low-productivity forests and production forests. Overall, we found a positive trend in the volumes of dead wood and large living trees, as well as in tree species diversity, while there was a general decline in understory vegetation coverage. Most observed changes were consistent with the intended outcomes of the current forest policy, adapted in the early 1990s. The implementation of retention forestry is likely driving some of the observed changes in forest structural components in the south. In contrast, we observed no changes in any of the focal structural components in the north, which could be attributed to the ongoing clear-cutting of forests previously managed less intensively. Dead wood and large living trees increased not only in managed, but also in unmanaged forests, likely reflecting historical management. The increased tree species diversity can be explained through current forest management practices that encourages maintenance of additional tree species. Decreasing understory vegetation coverage in both dense managed and unmanaged forests suggests that factors other than forestry contribute to the ongoing changes in understory vegetation in Swedish forests. Overall, the observed increase in structural components has not yet been reflected in documented improvements for red-listed forest species, which may be due to delays in species responses to small improvements, as well as a lack of detailed monitoring. Similarly, the increased availability of forest structural components might still be insufficient to meet the specific habitat requirements of red-listed species

A group of ectomycorrhizal fungi restricts organic matter accumulation in boreal forest

Boreal forest soils are important global carbon sinks, with significant storage in the organic topsoil. Decomposition of these stocks requires oxidative enzymes, uniquely produced by fungi. Across Swedish boreal forests, we found that local carbon storage in the organic topsoil was 33% lower in the presence of a group of closely related species of ectomycorrhizal fungi - Cortinarius acutus s.l.. This observation challenges the prevailing view that ectomycorrhizal fungi generally act to increase carbon storage in soils but supports the idea that certain ectomycorrhizal fungi can complement free-living decomposers, maintaining organic matter turnover, nutrient cycling and tree productivity under nutrient-poor conditions. The indication that a narrow group of fungi may exert a major influence on carbon cycling questions the prevailing dogma of functional redundancy among microbial decomposers. Cortinarius acutus s.l. responds negatively to stand-replacing disturbance, and associated population declines are likely to increase soil carbon sequestration while impeding long-term nutrient cycling

Soil microclimate changes affect soil fungal communities in a Mediterranean pine forest

Soil microclimate is a potentially important regulator of the composition of plant‐associated fungal communities in climates with significant drought periods. Here, we investigated the spatio‐temporal dynamics of soil fungal communities in a Mediterranean Pinus pinaster forest in relation to soil moisture and temperature. Fungal communities in 336 soil samples collected monthly over 1 year from 28 long‐term experimental plots were assessed by PacBio sequencing of ITS2 amplicons. Total fungal biomass was estimated by analysing ergosterol. Community changes were analysed in the context of functional traits. Soil fungal biomass was lowest during summer and late winter and highest during autumn, concurrent with a greater relative abundance of mycorrhizal species. Intra‐annual spatio‐temporal changes in community composition correlated significantly with soil moisture and temperature. Mycorrhizal fungi were less affected by summer drought than free‐living fungi. In particular, mycorrhizal species of the short‐distance exploration type increased in relative abundance under dry conditions, whereas species of the long‐distance exploration type were more abundant under wetter conditions. Our observations demonstrate a potential for compositional and functional shifts in fungal communities in response to changing climatic conditions. Free‐living fungi and mycorrhizal species with extensive mycelia may be negatively affected by increasing drought periods in Mediterranean forest ecosystems.info:eu-repo/semantics/acceptedVersio

Shifts in ectomycorrhizal fungal communities and exploration types relate to the environment and fine-root traits across interior Douglas-fir forests of western Canada

Large-scale studies that examine the responses of ectomycorrhizal fungi across biogeographic gradients are necessary to assess their role in mediating current and predicted future alterations in forest ecosystem processes. We assessed the extent of environmental filtering on interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) ectomycorrhizal fungal communities across regional gradients in precipitation, temperature, and soil fertility in interior Douglas-fir dominated forests of western Canada. We also examined relationships between fine-root traits and mycorrhizal fungal exploration types by combining root and fungal trait measurements with next-generation sequencing. Temperature, precipitation, and soil C:N ratio affected fungal community dissimilarities and exploration type abundance but had no effect on fungal richness and α‐diversity. Fungi with rhizomorphs (e.g., Piloderma sp.) and/or proteolytic abilities (e.g., Cortinarius sp.) dominated communities in warmer and less fertile environments. Ascomycetes (e.g., Cenococcum geophilum) and/or shorter distance explorers, which are potentially cost the plant less C, were favored in colder/drier climates where soils were richer in total nitrogen. Environmental filtering of ectomycorrhizal fungal communities is potentially related to co-evolutionary history between Douglas-fir populations and fungal symbionts, suggesting success of interior Douglas-fir as climate changes may be dependent on maintaining strong associations with local communities of mycorrhizal fungi. No evidence for a link between root and fungal resource foraging strategies was found at the regional scale. This lack of evidence further supports the need for a separate mycorrhizal symbiosis framework, independent from root trait frameworks, to aid in understanding below-ground plant uptake strategies across environments

Dissimilar responses of fungal and bacterial communities to soil transplantation simulating abrupt climate changes.

Both fungi and bacteria play essential roles in regulating soil carbon cycling. To predict future carbon stability, it is imperative to understand their responses to environmental changes, which is subject to large uncertainty. As current global warming is causing range shifts toward higher latitudes, we conducted three reciprocal soil transplantation experiments over large transects in 2005 to simulate abrupt climate changes. Six years after soil transplantation, fungal biomass of transplanted soils showed a general pattern of changes from donor sites to destination, which were more obvious in bare fallow soils than in maize cropped soils. Strikingly, fungal community compositions were clustered by sites, demonstrating that fungi of transplanted soils acclimatized to the destination environment. Several fungal taxa displayed sharp changes in relative abundance, including Podospora, Chaetomium, Mortierella and Phialemonium. In contrast, bacterial communities remained largely unchanged. Consistent with the important role of fungi in affecting soil carbon cycling, 8.1%-10.0% of fungal genes encoding carbon-decomposing enzymes were significantly (p < 0.01) increased as compared with those from bacteria (5.7%-8.4%). To explain these observations, we found that fungal occupancy across samples was mainly determined by annual average air temperature and rainfall, whereas bacterial occupancy was more closely related to soil conditions, which remained stable 6 years after soil transplantation. Together, these results demonstrate dissimilar response patterns and resource partitioning between fungi and bacteria, which may have considerable consequences for ecosystem-scale carbon cycling

Reindeer control over subarctic treeline alters soil fungal communities with potential consequences for soil carbon storage

The climate-driven encroachment of shrubs into the Arctic is accompanied by shifts in soil fungal communities that could contribute to a net release of carbon from tundra soils. At the same time, arctic grazers are known to prevent the establishment of deciduous shrubs and, under certain conditions, promote the dominance of evergreen shrubs. As these different vegetation types associate with contrasting fungal communities, the belowground consequences of climate change could vary among grazing regimes. Yet, at present, the impact of grazing on soil fungal communities and their links to soil carbon have remained speculative. Here we tested how soil fungal community composition, diversity and function depend on tree vicinity and long-term reindeer grazing regime and assessed how the fungal communities relate to organic soil carbon stocks in an alpine treeline ecotone in Northern Scandinavia. We determined soil carbon stocks and characterized soil fungal communities directly underneath and >3 m away from mountain birches (Betula pubescens ssp. czerepanovii) in two adjacent 55-year-old grazing regimes with or without summer grazing by reindeer (Rangifer tarandus). We show that the area exposed to year-round grazing dominated by evergreen dwarf shrubs had higher soil C:N ratio, higher fungal abundance and lower fungal diversity compared with the area with only winter grazing and higher abundance of mountain birch. Although soil carbon stocks did not differ between the grazing regimes, stocks were positively associated with root-associated ascomycetes, typical to the year-round grazing regime, and negatively associated with free-living saprotrophs, typical to the winter grazing regime. These findings suggest that when grazers promote dominance of evergreen dwarf shrubs, they induce shifts in soil fungal communities that increase soil carbon sequestration in the long term. Thus, to predict climate-driven changes in soil carbon, grazer-induced shifts in vegetation and soil fungal communities need to be accounted for