1. Two contrasting disturbance regimes-wildfire and clear-cutting-are common in boreal forests and create fundamentally different conditions for succession of wood-inhabiting fungi. We investigated (i) how species richness and community composition change after these two disturbances and (ii) which stand-level characteristics drive diversity trajectories. 2. In two chronosequences-managed clear-cut (1-109 years since clear-cut; n = 18) and unmanaged fire (4-375 years since fire; n = 18)-we combined fruiting-body surveys with DNA metabarcoding to quantify species richness of wood-inhabiting fungi, including total number of species, Agaricomycete and red-listed species. To identify drivers, we measured deadwood attributes and forest structural complexity using terrestrial laser scanning. 3. Species richness, including red-listed species, was highest in unmanaged fire stands. Unmanaged fire stands had similar to 55 more total species than managed clear-cut stands at comparable time since disturbance (<= 109 years), and similar to 156 more species in stands >109 years. Communities differed markedly between chronosequence types. Managed clear-cut stands harboured a subset of species found in unmanaged fire stands, and nearly all red-listed and indicator species were exclusive to unmanaged fire stands. Total and Agaricomycete species richness increased with time in both chronosequences without saturating. Red-listed species richness remained low and did not increase with time in managed clear-cut stands, but was higher and increased with time in unmanaged fire stands. Conditional random forest models identified spruce deadwood percentage, deadwood volume, and forest structural complexity as dominant diversity drivers, with deadwood quality replacing forest structure as the best predictor for red-listed species. Species richness rose steadily with deadwood volume, levelling at similar to 50 m(3) ha(-1) for total species and >100 m(3) ha(-1) for red-listed species. 4. Synthesis. Clear-cutting altered fungal recovery trajectories differently from fire. While fires leave standing and fallen dead trees that host fungal communities for centuries, clear-cutting removes these legacies and simplifies forest structure, resulting in a lack of recovery of red-listed species. These contrasting disturbance pathways shape boreal fungal communities through their effects on deadwood and structural continuity. Retaining high deadwood volumes and structural complexity can help maintain fungal diversity in managed forests; however, maintaining old-growth stands is essential for conserving highly diverse communities and red-listed species.

There is a public debate on how boreal forests can deliver climate change mitigation benefits. While most debates regarding Fennoscandian forests have centered on the contrasting effects of actively managed and old-growth unmanaged forests on carbon uptake and storage, the impact of surface albedo has often been overlooked. According to the new EU forest strategy for 2030, with aim of improving quantity and quality of forests by promoting primary old-growth forests and avoiding clear-cutting, among others, we examined how albedo across a wide age range of boreal Pinus-dominated forests develops over time after wildfire (defined as unmanaged) and clear-cutting (defined as managed). We find that albedo decreases over time after disturbance, but mainly in managed forests. Annual mean albedo in young (<30 years) managed forests (0.36 +/- 0.04) is markedly larger than in young unmanaged forests (0.18 +/- 0.04). This difference is particularly prominent during winter, when snow-covered ground is present. The mean albedo over the entire unmanaged forest-age gradient (0.17 +/- 0.05) is significantly lower (p < 0.05) than that of the managed forest-age gradient (0.23 +/- 0.10). Considering the typically higher frequency of clear-cuts compared to wildfires in Fennoscandian forests, these albedo differences would be even larger over long time scales. Our findings reveal the importance of considering the climatic cooling potential of albedo when making decisions on how to optimize future forest management in northern boreal forests to mitigate climate change.

Warming temperatures can promote arctic tree seedling establishment and growth, but these effects depend on the environmental context and the seedlings' capacity to acquire limiting resources. Here, we tested how temperature, soil properties, and the presence of a neighbouring shrub influence seedling growth rates. We further explored how the environment‐induced shifts in growth rates are related to traits associated with resource acquisition strategy. Seedlings of mountain birch Betula pubescens subsp. czerepanovii were grown in a growth chamber experiment with a fully factorial design manipulating temperature (ambient, warm), soil origin (low elevation, high elevation), and the presence or absence of a common neighbour, crowberry Empetrum nigrum subsp. hermaphroditum . Growth rate, carbon dioxide (CO 2 ) assimilation rates, ectomycorrhizal fungal colonisation, and above‐ and belowground functional traits that are linked to resource acquisition strategies, were measured for the mountain birch seedlings. Warming significantly increased seedling growth, and growth rates were positively associated with several above‐ and belowground traits associated with resource acquisition. Surprisingly, growth rates were not correlated with leaf traits such as specific leaf area and leaf dry matter content. Soil origin and the presence of Empetrum did not affect the growth rate responses to warming, although seedlings grown in high‐elevation soils generally grew faster and had higher CO 2 assimilation rates. These seedlings also expressed traits, commonly associated with conservative growth strategy. Our results highlight that both above‐ and belowground plant traits adjust differently to warming as part of their growth response. While seedling functional traits can be highly plastic in response to changes in temperature and soil conditions, they are not always coupled with changes in seedling growth rates. This is likely because birch seedlings adopt different growth‐promoting strategies under different environmental conditions.

Inter-continental study systems are crucial for testing ecological hypotheses, such as the widely cited Enemy Release Hypothesis (ERH), which seeks to explain the superior performance of plant species when they are introduced to new regions. Pinus contorta (lodgepole pine), native to North America, has been extensively introduced to Europe and the Southern Hemisphere, making it an ideal tree species for studying invasion hypotheses from a biogeographical perspective. We compared foliar fungal communities, especially pathogens, of P. contorta across two native–introduced region pairs (NIRPs): a northern NIRP (from Canada to Sweden) and a southern NIRP (from the USA to Patagonia), while also examining the differences between source plantations and invasion fronts within Patagonia. P. contorta underwent significant fungal community shifts and experienced pathogen release during its large-scale introduction from North America to Sweden and Patagonia. The fungal richness and relative abundance changes were more pronounced for the southern NIRP pair, where no closely related tree species to P. contorta are present in Patagonia. In Sweden, the presence of the phylogenetically related P. sylvestris and its associated local fungal community appears to play a role in influencing the foliar fungal communities associated with introduced P. contorta. In Patagonia, the incomplete co-invasion of fungal taxa from the USA emerges as a principal driver of the observed variability in fungal community composition and pathogen release following the introduction of P. contorta. In Patagonia, fungal community composition differences between source plantations and invasion fronts provided insufficient evidence that pathogen release occurs at this local scale. Integrating both biogeographical and phylogenetic perspectives, our study suggests that priority effects of local fungi appear to be a dominant community assembly process when introduction is done in a phylogenetically similar community; whereas, co-invasion of fungal communities is the dominant process in phylogenetically distant communities.

Climate change has increased the size and frequency of wildfires across the boreal biome. Severe wildfires in boreal forests have been found to trigger shifts from evergreen to deciduous canopies, which has cascading effects on carbon and nitrogen cycling. Ecosystem productivity and carbon uptake in boreal forests are strongly linked with nitrogen, and Earth system models increasingly depend on our understanding of the nitrogen balance to predict post-fire carbon uptake. To investigate the post-fire boreal nitrogen balance, we combined a mass balance approach and literature synthesis to estimate rates of nitrogen accumulation and nitrogen inputs across a network of 18 boreal wildfire chronosequences that varied in both wildfire regime and post-fire canopy type, comprising 527 forest stands. We found that deciduous- or mixed-dominance boreal forests establishing after severe, stand-replacing fires had the highest nitrogen accumulation rates (15.7 ± 3.8 kg ha-1 year-1), while evergreen-dominated forests establishing after surface- or mixed-severity fires had the lowest nitrogen accumulation rates (1.4 ± 1.1 kg ha-1 year-1). Annual known inputs from nitrogen deposition and biological nitrogen fixation combined, estimated from published data, largely failed to explain the rate of nitrogen accumulation, particularly in deciduous or mixed-dominance forests establishing after stand-replacing fires, suggesting that the origins of most nitrogen in these forest types remain poorly understood. As the frequency of severe wildfires increases across the boreal biome and shifts toward deciduous canopies become more common, our study reveals a large knowledge gap in the resulting nitrogen balance that needs to be resolved in order to improve predictions of forest carbon uptake.

There has been much recent interest in understanding how abiotic factors such as light, nutrients, and soil moisture affect the composition and biomass of lichen communities. Meanwhile, whether and how ground layer vegetation such as bryophytes and shrubs also influence lichen communities have received much less attention, particularly regarding how these effects vary across environmental gradients. In this study, we used a long‐term (19‐year) biodiversity manipulation experiment to assess the importance of feather moss and ericaceous dwarf shrub removals on the composition and diversity (assessed via metabarcoding) and biomass (assessed via PLFA markers) of terricolous lichen communities along a 5000‐year boreal forest post‐fire chronosequence in northern Sweden. Overall, our results showed that shrub removals had a greater impact than moss removals on the biomass and composition of lichen communities. Shrub removals increased lichen alpha‐diversity while decreasing lichen beta‐diversity. This is mainly because, although the number of lichen species increased in the absence of shrubs, lichen communities were strongly dominated by Cladonia spp. However, the effects of shrub removals were context‐dependent, with greater effects observed in older ecosystems. Our results highlight that shrubs had a greater impact than moss in shaping terricolous lichen communities in boreal forests, with increasing effects from young ecosystems to older ones. We conclude that the foreseen expansion of vascular plants such as ericaceous shrubs into high latitude regions will probably have negative consequences on lichen cover, but that these effects will be dependent on the environmental context.

In Boreal forests, wildfires are common disturbance agents, important for sustaining forest regeneration and ecosystem processes. However, climate warming has intensified fire activity and severity in recent years, and warmer conditions following fire can alter environmental factors affecting tree seedling survival and growth. Management interventions can potentially counterbalance the effects of fire severity and warming. Using a field experiment, we investigated the main and interactive effects of fire severity, experimental warming, understory shrub presence and salvage logging on the growth of different conifer (Picea abies and Pinus sylvestris) provenances, to provide insight about how tree seedlings of various species respond to multiple environmental drivers following fire. Our study shows that low fire severity without salvage logging resulted in poorer seedling above-ground growth, whereas experimental warming and ericaceous shrubs removal increased seedling biomass regardless of fire severity or logging treatment. Our results suggest that enhanced light availability and reduced resource competition from the understory supports the growth of conifer seedlings following fire. Post-fire warming further stimulated the growth of conifer seedlings (particularly in high-severity burn areas) potentially due to increased early colonization by fast-growing deciduous trees. Moreover, in warmer conditions, northern provenances of P. sylvestris planted in southern locations also showed greater height growth compared to local populations, highlighting the potential for assisted migration under climate change. Synthesis and applications. This study demonstrates that artificial regeneration through planting can support forest recovery following wildfire, especially under high fire severity in sites where natural regeneration is poor. Salvage logging can enhance the growth of P. sylvestris seedlings and indirectly benefit P. abies on low-severity sites by facilitation by deciduous species that create favourable microhabitats. However, the broader ecological impacts of salvage logging (such as on biodiversity loss and soil and habitat structure) must be considered in management planning. Post-fire vegetation management, including control of competitive ericaceous shrubs, may further improve seedling establishment. Selecting climate-adapted seedling provenances can boost reforestation success under future warming. These findings support an adaptive management framework that integrates fire severity, logging intensity, understory control and assisted migration to foster resilient forest landscapes in a changing climate. I boreala skogar & auml;r skogsbr & auml;nder viktiga f & ouml;r att uppr & auml;tth & aring;lla skogens f & ouml;rnyelse och ekosystemfunktioner. Klimatf & ouml;r & auml;ndringar har under senare & aring;r lett till oftare f & ouml;rekommande br & auml;nder och br & auml;nder som resulterar i h & ouml;gre brandh & aring;rdhet vilket kan p & aring;verka skogarnas & aring;terh & auml;mtning och f & ouml;ryngring. Skogssk & ouml;tsel & aring;tg & auml;rder kan potentiellt motverka negativa effekter av b & aring;de h & ouml;gre brandh & aring;rdhet och ett varmare klimat. Ett f & auml;ltexperiment genomf & ouml;rdes f & ouml;r att unders & ouml;ka hur brandh & aring;rdhet, experimentell uppv & auml;rmning, n & auml;rvaro av undervegetation samt avverkning av brandskadad skog p & aring;verkar tillv & auml;xten hos tr & auml;dplantor av Picea abies (gran) och Pinus sylvestris (tall) fr & aring;n olika provenienser. Syftet var att & ouml;ka f & ouml;rst & aring;elsen f & ouml;r hur tr & auml;dplantor av olika ursprung svarar p & aring; flera samverkande milj & ouml;faktorer efter brand. Resultaten visade att l & aring;g brandh & aring;rdhet i fr & aring;nvaro av avverkning hade en negativ effekt p & aring; tr & auml;dplantornas tillv & auml;xt. Samtidigt bidrog b & aring;de experimentell uppv & auml;rmning och borttagning av risartade v & auml;xter till & ouml;kad biomassa hos tr & auml;dplantorna, oberoende av brandh & aring;rdhet och avverkning. Vi fann att & ouml;kad ljustillg & aring;ng och minskad resurskonkurrens fr & aring;n undervegetationen gynnade barrtr & auml;dens tillv & auml;xt efter brand. Uppv & auml;rmning efter brand stimulerade tillv & auml;xten ytterligare, s & auml;rskilt i omr & aring;den med h & ouml;g brandh & aring;rdhet. Detta berodde sannolikt p & aring; att snabbv & auml;xande l & ouml;vtr & auml;d etablerade sig snabbt och skapade gynsamma f & ouml;rh & aring;llanden f & ouml;r barrtr & auml;dsplantorna. Nordliga provenienser av P. sylvestris, uppvisade h & ouml;gre tillv & auml;xt & auml;n lokala populationer n & auml;r de planterades i sydligare och experimentellt varmare f & ouml;rs & ouml;ksytor. Detta tyder p & aring; en potential f & ouml;r klimatanpassad migration av genetiskt material. Till & auml;mpning och syntes. Denna studie visar att f & ouml;ryngring genom plantering kan fr & auml;mja skogens & aring;terh & auml;mtning efter en skogsbrand, s & auml;rskilt i omr & aring;den med h & ouml;g brandh & aring;rdhet d & auml;r den naturliga f & ouml;ryngringen & auml;r begr & auml;nsad. Avverkning av brandskadad skog kan f & ouml;rb & auml;ttra tillv & auml;xten hos P. sylvestris och indirekt gynna P. abies i omr & aring;den med l & aring;g brandh & aring;rdhet genom att fr & auml;mja l & ouml;vtr & auml;d som i sin tur skapar gynnsamma mikrohabitat f & ouml;r barrtr & auml;dsplantor. De bredare ekologiska konsekvenserna av avverkning efter brand, s & aring;som f & ouml;rlust av biologisk m & aring;ngfald och d & ouml;d ved samt f & ouml;r & auml;ndringar i markstruktur, b & ouml;r dock beaktas i planeringen av skogssk & ouml;tsel & aring;tg & auml;rder efter brand. Att kontrollera konkurrerande risv & auml;xter, kan ocks & aring; hj & auml;lpa plantorna att etablera sig b & auml;ttre. Att v & auml;lja klimatanpassade provenienser kan & ouml;ka framg & aring;ngen f & ouml;r & aring;terbeskogning under

The understory vegetation of boreal forests plays a crucial role in maintaining biodiversity by creating habitats, supplying food resources, and regulating microclimate and soil conditions. This essential layer is frequently affected by disturbances such as forest fires and clear‐cutting, which significantly alter understory communities and the ecosystem resource availability and heterogeneity. This study aimed to understand how these disturbances influence the spatial and temporal dynamics of key ecosystem resources, and subsequently the patterns of understory diversity. We analyzed and compared understory vegetation diversity in a rotational management chronosequence and an unmanaged fire chronosequence of Scots pine Pinus sylvestris forests across northern Sweden. We assessed the relationship of above‐ and belowground resource availability and heterogeneity with alpha and beta diversity using generalized additive models and multivariate analyses. We found that belowground resource availability (especially inorganic nitrogen) and aboveground resource heterogeneity (especially variation in forest structural complexity) were most strongly positively correlated with alpha and beta diversity, varying across successional stages. In early stages (0–60 years), high availability of belowground resources and aboveground heterogeneity was associated with high alpha and beta diversity. In mid‐stages (100–200 years), reduced belowground resource availability and aboveground heterogeneity was linked to lower diversity. In late stages (> 250 years, which only exists in the unmanaged fire chronosequence), increased aboveground heterogeneity associated with tree mortality was linked to a resurgence in alpha and beta diversity. These results highlight the necessity of maintaining a mosaic of stands with different disturbance regimes and successional stages, particularly early post‐fire stands and late successional stands, which are currently much rarer on the landscape, to support biodiversity at the landscape level.

Ecosystem productivity and carbon uptake in the circumpolar boreal forest are contingent on available nitrogen, which ultimately originates from inputs via deposition and biological nitrogen fixation. Nitrogen deposition rates in boreal forests are relatively small compared to other biomes, and most biological nitrogen fixation research has focused on moss-diazotroph associations. However, the relative contributions of these two primary nitrogen inputs to ecosystem nitrogen stocks have not been widely investigated. In this study, we combined a mass balance approach and literature synthesis to estimate rates of nitrogen accumulation and nitrogen inputs across a network of 18 wildfire chronosequences spanning the boreal biome. We found that nitrogen accumulation rates were strongly linked with fire regime (stand-replacing versus surface fires) and canopy dominance (deciduous versus evergreen canopies). Furthermore, a considerable amount of accumulating nitrogen in these boreal forests was unexplained by the known inputs estimated from the literature synthesis, particularly in forests with stand-replacing fire regimes and more deciduous tree cover that together had the highest nitrogen accumulation rates. This unexplained fraction of nitrogen inputs in some forests may originate from poorly quantified niches of biological nitrogen fixation. Exploring this research frontier will help improve predictions of boreal forest nitrogen cycling and carbon uptake in changing climate and wildfire regimes.

Boreal forests are frequently subjected to disturbances, including wildfire and clear-cutting. While these disturbances can cause soil carbon (C) losses, the long-term accumulation dynamics of soil C stocks during subsequent stand development is controlled by biological processes related to the balance of net primary production (NPP) and outputs via heterotrophic respiration and leaching, many of which remain poorly understood. We review the biological processes suggested to influence soil C accumulation in boreal forests. Our review indicates that median C accumulation rates following wildfire and clear-cutting are similar (0.15 and 0.20 Mg ha-1 year-1, respectively), however, variation between studies is extremely high. Further, while many individual studies show linear increases in soil C stocks through time after disturbance, there are indications that C stock recovery is fastest early to mid-succession (e.g. 15-80 years) and then slows as forests mature (e.g. >100 years). We indicate that the rapid build-up of soil C in younger stands appears not only driven by higher plant production, but also by a high rate of mycorrhizal hyphal production, and mycorrhizal suppression of saprotrophs. As stands mature, the balance between reductions in plant and mycorrhizal production, increasing plant litter recalcitrance, and ectomycorrhizal decomposers and saprotrophs have been highlighted as key controls on soil C accumulation rates. While some of these controls appear well understood (e.g. temporal patterns in NPP, changes in aboveground litter quality), many others remain research frontiers. Notably, very little data exists describing and comparing successional patterns of root production, mycorrhizal functional traits, mycorrhizal-saprotroph interactions, or C outputs via heterotrophic respiration and dissolved organic C following different disturbances. We argue that these less frequently described controls require attention, as they will be key not only for understanding ecosystem C balances, but also for representing these dynamics more accurately in soil organic C and Earth system models.

It is often speculated that non-native invasive species undergo rapid changes in their phenotypic properties (i.e., traits) that provide adaptive advantage in their new environment. However, few studies have directly compared traits of invasive non-native species with their native counterparts to reveal whether such phenotypic changes occur, and which stages of initial introduction and subsequent invasion contribute to these shifts. We studied trait variation of an invasive tree, Pinus contorta, which is native to northwestern North America and invasive in the Patagonia region of South America (i.e., Argentina and Chile). Commercial plantations of P. contorta were introduced extensively in Patagonia from the 1970s onward, from an unknown seed origin within the Pacific Northwest, USA, where three sub-species are found, including subsp. contorta, latifolia, and murrayana. We employed a home-versus-away study approach, where we compared mean growth, defense, and reproduction trait values, and mean within-stand trait variation (Coefficient of Variation, CV) of Patagonia plantations, with the three native sub-species. We further compared mean traits, and trait CVs between invasive P. contorta and the Patagonia plantations from which they escaped. Patagonia plantations shared the most similar mean trait values with subsp. latifolia and murrayana, suggesting possible source populations. However, both mean trait values and trait CVs of Patagonia plantations differed from all three native sub-species, indicating potential founder effects, population bottlenecks, and/or plastic responses to their new environment that occurred during or after introduction. We also found evidence for selective change during invasion; however, these differences did not suggest growth traits were prioritized over defense traits, which was inconsistent with hypotheses that invaders exhibit an evolutionary trade-off between defense traits and growth traits. Our study highlights that processes occurring both at first introduction and establishment, as well as the subsequent invasion phase can influence the phenotype of successful invaders.

IntroductionSurvival and growth of tree seedlings are key processes of regeneration in forest ecosystems. However, little is known about how climate warming modulates seedling performance either directly or in interaction with understory vegetation and post-fire successional stages.MethodsWe measured survival (over 3 years) and growth of seedlings of three tree species (Betula pubescens, Pinus sylvestris, and Picea abies) in a full-factorial field experiment with passive warming and removal of two plant functional groups (feather moss and/or ericaceous shrubs) along a post-fire chronosequence in an unmanaged boreal forest.ResultsWarming had no effect on seedling survival over time or on relative biomass growth. Meanwhile, moss removal greatly increased seedling survival overall, while shrub removal canceled this effect for B. pubescens seedlings. In addition, B. pubescens and P. sylvestris survival benefitted most from moss removal in old forests (>260 years since last fire disturbance). In contrast to survival, seedling growth was promoted by shrub removal for two out of three species, i.e., P. sylvestris and P. abies, meaning that seedling survival and growth are governed by different understory functional groups affecting seedling performance through different mechanism and modes of action.DiscussionOur findings highlight that understory vegetation and to a lesser extent post-fire successional stage are important drivers of seedling performance while the direct effect of climate warming is not. This suggests that tree regeneration in future forests may be more responsive to changes in understory vegetation or fire regime, e.g., indirectly caused by warming, than to direct or interactive effects of rising temperatures.

Changes in fire regime of boreal forests in response to climate warming are expected to impact postfire recovery. However, quantitative data on how managed forests sustain and recover from recent fire disturbance are limited.Two years after a large wildfire in managed even-aged boreal forests in Sweden, we investigated how recovery of aboveground and belowground communities, that is, understory vegetation and soil microbial and faunal communities, responded to variation in the severity of soil (i.e., consumption of soil organic matter) and canopy fires (i.e., tree mortality).While fire overall enhanced diversity of understory vegetation through colonization of fire adapted plant species, it reduced the abundance and diversity of soil biota. We observed contrasting effects of tree- and soil-related fire severity on survival and recovery of understory vegetation and soil biological communities. Severe fires that killed overstory Pinus sylvestris promoted a successional stage dominated by the mosses Ceratodon purpureus and Polytrichum juniperinum, but reduced regeneration of tree seedlings and disfavored the ericaceous dwarf-shrub Vaccinium vitis-idaea and the grass Deschampsia flexuosa. Moreover, high tree mortality from fire reduced fungal biomass and changed fungal community composition, in particular that of ectomycorrhizal fungi, and reduced the fungivorous soil Oribatida. In contrast, soil-related fire severity had little impact on vegetation composition, fungal communities, and soil animals. Bacterial communities responded to both tree- and soil-related fire severity.Synthesis: Our results 2 years postfire suggest that a change in fire regime from a historically low-severity ground fire regime, with fires that mainly burns into the soil organic layer, to a stand-replacing fire regime with a high degree of tree mortality, as may be expected with climate change, is likely to impact the short-term recovery of stand structure and above- and belowground species composition of even-aged P. sylvestris boreal forests.

Abstract It is well established that application of biochar to soils can promote soil fertility, which ultimately may enhance plant growth. While many mechanisms have been proposed to explain this, one specific mechanism, the “microbial refugia hypothesis,” suggests that biochar may provide physical protection for soil microbe from soil microfauna that otherwise exert top‐down control on microbial biomass and activity. We tested the microbial refugia hypothesis by incubating two boreal soils with and without biochar derived from a wood mixture of boreal tree species (Picea abies and Pinus sylvestris), and with and without soil nematodes. We measured phospholipid fatty acids (PLFA) as a relative measure of microbial biomass, and several variables indicative of microbial activity, including extractable nutrient concentrations (NH4+, NO3−, and PO4−), heterotrophic N2‐fixation, and soil respiration. Contrary to our expectations, we found that biochar by itself did not stimulate microbial biomass or activity. Furthermore, we found that nematode addition to soil stimulated rather than depressed the biomass of several bacterial PLFA groups. Finally, interactive effects between the nematode treatment and biochar never worked in a way that supported the microbial refugia hypothesis. Our findings suggest that a typical boreal biochar applied to boreal soils may not have the same stimulatory effect on microbial biomass and activity that has been shown in some other ecosystems, and that enhanced plant growth in response to biochar addition sometimes observed in boreal environments is likely due to other mechanisms, such as direct nutrient supply from biochar or amelioration of soil pH.

Feather mosses and associated cyanobacteria serve as an important source of nitrogen (N) in boreal forests; however, few studies have effectively traced the N2 fixation in-situ for more than one year. The purpose of this study was to assess the long-term fate of N2 fixed in feather moss carpets in boreal forests of northern Sweden. We conducted a seven-year 15N2(g) labelling and tracing experiment in a boreal forest with a moss bottom layer dominated by Pleurozium schreberi Brid (Mitt). Mesocosms of forest floor with and without mosses present were exposed to a 10% 15N2(g) atmosphere for five days after which samples of moss, shrub, pine seedling and O horizon were taken at varied intervals and analysed for δ15N. Plots were split and burned in year seven to assess the effect of fire on liberating moss N for uptake by Scots pine (Pinus sylvestris L.) seedlings. Feather mosses treated with 15N2(g) were highly enriched in 15N one month after labelling which subsequently declined after seven years to 13% of the original 15N enrichment in green tissue. No measurable 15N enrichment was observed in shrubs or the O horizon in the presence of labelled mosses until greater than five years after the labelling experiment. Fire can liberate moss N, but we did not observe a direct effect of fire on N uptake by pine seedlings. This long-term study indicates that N2 fixed by cyanobacteria in feather moss carpets is conserved in moss tissue for extended periods and slowly transferred to the forest soil O horizon as moss tissue decomposes suggesting that moss N2 fixation is most important as an ecosystem N source at long time scales.

Summary Mycorrhizal fungi associated with boreal trees and ericaceous shrubs are central actors in organic matter (OM) accumulation through their belowground carbon allocation, their potential capacity to mine organic matter for nitrogen (N) and their ability to suppress saprotrophs. Yet, interactions between co‐occurring ectomycorrhizal fungi (EMF), ericoid mycorrhizal fungi (ERI), and saprotrophs are poorly understood. We used a long‐term (19 yr) plant functional group manipulation experiment with removals of tree roots, ericaceous shrubs and mosses and analysed the responses of different fungal guilds (assessed by metabarcoding) and their interactions in relation to OM quality (assessed by mid‐infrared spectroscopy and nuclear magnetic resonance) and decomposition (litter mesh‐bags) across a 5000‐yr post‐fire boreal forest chronosequence. We found that the removal of ericaceous shrubs and associated ERI changed the composition of EMF communities, with larger effects occurring at earlier stages of the chronosequence. Removal of shrubs was associated with enhanced N availability, litter decomposition and enrichment of the recalcitrant OM fraction. We conclude that increasing abundance of slow‐growing ericaceous shrubs and the associated fungi contributes to increasing nutrient limitation, impaired decomposition and progressive OM accumulation in boreal forests, particularly towards later successional stages. These results are indicative of the contrasting roles of EMF and ERI in regulating belowground OM storage.

Wildfires are natural and important disturbances of boreal forest ecosystems, and they are expected to increase in parts of the boreal zone through climate warming. There is a broad understanding of the immediate effects of fire on soil nitrogen (N) transformation rates, but less is known about these effects several years after fire. In July 2014, a large wildfire in the boreal forest zone of Central Sweden took place. Four years after the wildfire, we measured processes linked to the soil N cycle using the N-15 pool dilution method (for gross N mineralization, consumption and nitrification) and the buried bags method (for net N mineralization), in soils from stands of different fire severity that had or had not been subjected to salvage logging. Gross N mineralization and consumption rates per unit carbon (C) increased by 81 % and 85 % respectively, in response to high fire severity, and nitrification rates per unit C basis decreased by 69 % in response to high fire severity, while net N mineralization was unresponsive. There was no difference in the effect of salvage logging across stands of differing fire severity on N transformation rates, although concentrations of resin adsorbed nitrate (NO3-) were overall 50 % lower in logged compared to unlogged stands. We also found that irrespective of burn severity, N immobilization rates exceeded N nitrification rates, and immobilization was therefore the dominant pathway of gross N consumption. Gross N consumption rates were higher in burned than unburned stands, despite there being a higher active microbial biomass in unburned soil, which suggests an even higher immobilization of N over time as the microbial biomass recovers following fire. Our study shows that soil N transformation rates were more affected by changes in fire severity than by salvage logging, and that four years after the fire many aspects of the N cycle did not differ between burned and unburned stands, suggesting substantial resilience of the N cycle to fire and salvage logging. However, we note that long term impact and many additional ecosystem properties or processes should be evaluated before concluding that salvage logging has no ecosystem impact. Furthermore, shortened fire regimes following climate warming accompanied with shorter intervals between salvage logging practices, could still impact the capability for the N cycle to recover after an intense fire. While wildfire in the boreal region results in a shift from nutrient conserving to nutrient demanding plant species, our results suggest this shift is dependent on a relatively short-lived pulse of higher N cycling processes that would have likely dissipated within a few years after the fire.

In 2018, an extreme drought affected large parts of Europe and led to the worst fire season in over a century in Sweden. We investigated the impacts of the Ljusdal fire, the largest fire complex that year, on soil CO2 and CH4 fluxes, nutrient concentrations and microclimate in a Scots pine forest. The measurements were conducted during the first growing season after the fire. In three separate analyses, we compared stands that differed in terms of burn severity (unburnt, low and high burn severity), salvage-logging (logged or unlogged) and stand age (young: 12 years old or mature: ~100 years old at the time of the fire). A mature stand affected by a high severity burn (100% tree mortality) had significantly lower soil respiration compared to a stand affected by a low severity burn (nearly 100% tree survival), but there was no difference in soil respiration between the low burn severity and unburn stands. These results indicate that autotrophic respiration plays a key role in determining post-fire soil respiration. After a high severity burn, salvage logging had no significant effects on forest soils compared to a stand where the dead trees had been left standing, although differences between these two stands are likely to become significant in the future. Stand age had a clear impact on most of the soil properties tested. Despite mean soil temperature being 5 °C warmer at a young site compared to a mature site after a high severity burn, soil respiration was lower at the young site. The young site had been clear-cut and undergone soil scarification and replanting 12 years before the fire, which is likely to have contributed to the lower nutrient availability and thinner soil organic layer there compared to the mature site. Short return intervals between disturbances such as harvesting and wildfire that remove part of the soil organic layer can thus have significant and long-term impacts on nutrient cycling and carbon exchange in the boreal forest. The boreal forest is thus vulnerable to becoming a carbon source, especially in regions where climate change is increasing the frequency of high severity wildfire and commercial timber production is expanding.