Soil fertilisation with 137Cs-contaminated and uncontaminated wood ash as a countermeasure to reduce 137Cs 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 137Cs uptake by forest plants many years after the radionuclide fallout. The effects of one-time point fertilisation with 137Cs-contaminated and uncontaminated wood ash alone or in combination with KCl on 137Cs 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 137Cs uptake by species and years. Soil amendment with 137Cs-contaminated wood ash generally did not affect 137Cs uptake by young shoots and leaves of plants over the growing season in the first year and only slightly decreased Tag for 137Cs in the following years. The effect of a single application of 137Cs-uncontaminated wood ash on reducing 137Cs uptake by plants was generally negligible. Application of 137Cs-contaminated wood ash in combination with KCl reduced plant 137Cs 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 137Cs-contaminated forest soil many years after radionuclide fallout generally does not reduce 137Cs 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 fertility in boreal forest relates to root-driven nitrogen retention and carbon sequestration in the mor layer.

Boreal forest soils retain significant amounts of carbon (C) and nitrogen (N) in purely organic layers, but the regulation of organic matter turnover and the relative importance of leaf litter and root-derived inputs are not well understood. We combined bomb 14 C dating of organic matter with stable isotope profiling for Bayesian parameterization of an organic matter sequestration model. C and N dynamics were assessed across annual depth layers (cohorts), together representing 256 yr of organic matter accumulation. Results were related to ecosystem fertility (soil inorganic N, pH and litter C : N). Root-derived C was estimated to decompose two to 10 times more slowly than leaf litter, but more rapidly in fertile plots. The amounts of C and N per cohort declined during the initial 20 yr of decomposition, but, in older material, the amount of N per cohort increased, indicating N retention driven by root-derived C. The dynamics of root-derived inputs were more important than leaf litter dynamics in regulating the variation in organic matter accumulation along a forest fertility gradient. N retention in the rooting zone combined with impeded mining for N in less fertile ecosystems provides evidence for a positive feedback between ecosystem fertility and organic matter turnover.

Below-ground organic matter accumulation along a boreal forest fertility gradient relates to guild interaction within fungal communities.

Plant-soil interactions link ecosystem fertility and organic matter accumulation below ground. Soil microorganisms play a central role as mediators of these interactions, but mechanistic understanding is still largely lacking. Correlative data from a coniferous forest ecosystem support the hypothesis that interactions between fungal guilds play a central role in regulating organic matter accumulation in relation to fertility. With increasing ecosystem fertility, the proportion of saprotrophic basidiomycetes increased in deeper organic layers, at the expense of ectomycorrhizal fungal species. Saprotrophs correlated positively with the activity of oxidative enzymes, which in turn favoured organic matter turnover and nitrogen recycling to plants. Combined, our findings are consistent with a fungus-mediated feedback loop, which results in a negative correlation between ecosystem fertility and below-ground carbon storage. These findings call for a shift in focus from plant litter traits to fungal traits in explaining organic matter dynamics and ecosystem fertility in boreal forests.

Shift in fungal communities and associated enzyme activities along an age gradient of managed Pinus sylvestris stands.

Forestry reshapes ecosystems with respect to tree age structure, soil properties and vegetation composition. These changes are likely to be paralleled by shifts in microbial community composition with potential feedbacks on ecosystem functioning. Here, we assessed fungal communities across a chronosequence of managed Pinus sylvestris stands and investigated correlations between taxonomic composition and extracellular enzyme activities. Not surprisingly, clear-cutting had a negative effect on ectomycorrhizal fungal abundance and diversity. In contrast, clear-cutting favoured proliferation of saprotrophic fungi correlated with enzymes involved in holocellulose decomposition. During stand development, the re-establishing ectomycorrhizal fungal community shifted in composition from dominance by Atheliaceae in younger stands to Cortinarius and Russula species in older stands. Late successional ectomycorrhizal taxa correlated with enzymes involved in mobilisation of nutrients from organic matter, indicating intensified nutrient limitation. Our results suggest that maintenance of functional diversity in the ectomycorrhizal fungal community may sustain long-term forest production by retaining a capacity for symbiosis-driven recycling of organic nutrient pools.

Changes in turnover rather than production regulate biomass of ectomycorrhizal fungal mycelium across a Pinus sylvestris chronosequence.

In boreal forest soils, ectomycorrhizal fungi are fundamentally important for carbon (C) dynamics and nutrient cycling. Although their extraradical mycelium (ERM) is pivotal for processes such as soil organic matter build-up and nitrogen cycling, very little is known about its dynamics and regulation. In this study, we quantified ERM production and turnover, and examined how these two processes together regulated standing ERM biomass in seven sites forming a chronosequence of 12- to 100-yr-old managed Pinus sylvestris forests. This was done by determining ERM biomass, using ergosterol as a proxy, in sequentially harvested in-growth mesh bags and by applying mathematical models. Although ERM production declined with increasing forest age from 1.2 to 0.5 kg ha-1  d-1 , the standing biomass increased from 50 to 112 kg ha-1 . This was explained by a drastic decline in mycelial turnover from seven times to one time per year with increasing forest age, corresponding to mean residence times from 25 d up to 1 yr. Our results demonstrate that ERM turnover is the main factor regulating biomass across differently aged forest stands. Explicit inclusion of ERM parameters in forest ecosystem C models may significantly improve their capacity to predict responses of mycorrhiza-mediated processes to management and environmental changes.