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Peatlands are carbon dioxide (CO2) sinks that, in parallel, release methane (CH4). The peatland carbon (C) balance depends on the interplay of decomposer and CH4-cycling microbes, vegetation, and environmental conditions. These interactions are susceptible to the changes that occur along a successional gradient from vascular plant-dominated systems to Sphagnum moss-dominated systems. Changes similar to this succession are predicted to occur from climate change. Here, we investigated how microbial and plant communities are interlinked with each other and with ecosystem C cycling along a successional gradient on a boreal land uplift coast. The gradient ranged from shoreline to meadows and fens, and further to bogs. Potential microbial activity (aerobic CO2 production; CH4 production and oxidation) and biomass were greatest in the early successional meadows, although their communities of aerobic decomposers (fungi, actinobacteria), methanogens, and methanotrophs did not differ from the older fens. Instead, the functional microbial communities shifted at the fen–bog transition concurrent with a sudden decrease in C fluxes. The successional patterns of decomposer versus CH4-cycling communities diverged at the bog stage, indicating strong but distinct microbial responses to Sphagnum dominance and acidity. We highlight young meadows as dynamic sites with the greatest microbial potential for C release. These hot spots of C turnover with dense sedge cover may represent a sensitive bottleneck in succession, which is necessary for eventual long-term peat accumulation. The distinctive microbes in bogs could serve as indicators of the C sink function in restoration measures that aim to stabilize the C in the peat.

Aim The diversity and composition of natural communities are rapidly changing due to anthropogenic disturbances. Magnitude of this compositional reorganization varies across the globe, but reasons behind the variation remain largely unknown. Disturbances induce temporal turnover by stimulating species colonizations, causing local extinctions, altering dominance structure, or all of these. We test which of these processes drive temporal community changes, and whether they are constrained by natural environmental gradients. Moreover, we assess to what degree identity shifts translate to changes in dominance structure. Location Finland. Time period Observations 1985–2006, disturbance history > 140 years. Major taxa studied Vascular plants. Methods We investigated temporal turnover of boreal forest understorey in response to disturbance, here forest management, along a soil fertility gradient. We disentangle the roles of species gains, losses and abundance changes in driving temporal turnover in response to and after disturbance by comparing turnover rates in different forest age categories along a fertility gradient. We quantify temporal turnover using richness‐based complement of Jaccard’s similarity index and proportional‐abundance based dissimilarity index. We also test whether disturbance history or fertility influence the relationship between identity shifts and dominance structure. Results We found that the impact of disturbance on temporal turnover depends on soil fertility. The greatest turnover occurred in the most fertile forests immediately after disturbance. There, species gains and losses strongly altered dominance structure leading to high turnover, whereas undisturbed old forests and nutrient‐poor habitats were characterized by stable dominant species even when the majority of species shifted their identity. Main conclusions Our results suggest that human impacts on temporal biodiversity change vary along environmental gradients. In boreal forests, the fertile habitats have a higher probability than nutrient‐poor sites of changing their composition in response to anthropogenic disturbances. Resource availability and disturbance history may thus influence consequences of temporal turnover for ecosystem functioning.

Publisher Copyright: © 2022 During the last decade it has been increasingly acknowledged that carbon (C) contained in root exudates can accelerate decomposition of soil organic matter (SOM), a phenomenon known as rhizosphere priming effect (RPE). However, the controlling factors and the role of different soil microorganisms in RPE are not yet well understood. There are some indications that the response of the soil microbial decomposers to labile C input in the rhizosphere depends on microbial demand of nutrients for growth and maintenance, especially that of C and nitrogen (N). To test this hypothesis, we assessed SOM decomposition induced by 13C-glucose additions during one week in forest soils with different C:N ratios (11.5–22.2). We estimated SOM respiration, the potential activity (concentration) of a range of extracellular enzymes, and incorporation of 13C and deuterium (D) in microbial phospholipid fatty acids (PLFAs). Glucose additions induced positive priming (a 12–52% increase in SOM respiration) in all soil types, but there was no linear relationship between priming and the soil C:N ratio. Instead, priming of SOM respiration was positively linked to the C:N imbalance, where a higher C:N imbalance implies stronger microbial N limitation. The total oxidative enzyme activity and the ratio between the activities of C and N acquiring enzymes were lower in soil with higher C:N ratios, but these findings could not be quantitatively linked to the observed priming rates. It appears as if glucose addition resulted in priming by stimulating the activity rather than the concentration of oxidative enzymes. Microbial incorporation of D and 13C into in PLFAs demonstrated that glucose additions stimulated both fungal and bacterial growth. The increased growth was mainly supported by glucose assimilation in fungi, while the increase in bacterial growth partly was a result of increased availability of C or N released from SOM. Taken together, the findings suggest that the soil C:N ratio is a poor predictor of priming and that priming is more dependent on the C:N imbalance, which reflects both microbial nutrient demand and nutrient provision.

Despite the importance of vegetation uptake of atmospheric gaseous elemental mercury (Hg(0)) within the global Hg cycle, little knowledge exists on the physiological, climatic and geographic factors controlling stomatal uptake of atmospheric Hg(0) by tree foliage. We investigate controls on foliar stomatal Hg(0) uptake by combining Hg measurements of 3,569 foliage samples across Europe with data on tree species traits and environmental conditions. To account for foliar Hg accumulation over time, we normalized foliar Hg concentration over the foliar life period from the simulated start of the growing season to sample harvest. The most relevant parameter impacting daily foliar stomatal Hg uptake was tree functional group (deciduous versus coniferous trees). On average, we measured 3.2 times higher daily foliar stomatal Hg uptake rates in deciduous leaves than in coniferous needles of the same age. Across tree species, for foliage of beech and fir, and at two out of three forest plots with more than 20 samples, we found a significant (p

Climate change is a pervasive threat to biodiversity. While range shifts are a known consequence of climate warming contributing to regional community change, less is known about how species' positions shift within their climatic niches. Furthermore, whether the relative importance of different climatic variables prompting such shifts varies with changing climate remains unclear. Here we analysed four decades of data for 1,478 species of birds, mammals, butterflies, moths, plants and phytoplankton along a 1,200 km high latitudinal gradient. The relative importance of climatic drivers varied non-uniformly with progressing climate change. While species turnover among decades was limited, the relative position of species within their climatic niche shifted substantially. A greater proportion of species responded to climatic change at higher latitudes, where changes were stronger. These diverging climate imprints restructure a full biome, making it difficult to generalize biodiversity responses and raising concerns about ecosystem integrity in the face of accelerating climate change.The authors analyse four decades of distribution data for various taxonomic groups to understand the shift of species within their climatic niches and the changing influences of different climate factors. The diverse and diverging climate imprints raise concerns about future ecosystem integrity.

There is mounting evidence demonstrating that fluxes and chemical composition of precipitation is substantially changed after passing through tree canopies, particularly in the case of atmospheric nitrogen (N) compounds, with important implications on forest N cycling. However, the processes underpinning those changes – beyond the leaf retention and/or leaching of N compounds - have been less investigated. In a previous study we provided isotopic evidence that biological nitrification in tree canopies was responsible for significant changes in the amount of NO3- from rainfall to throughfall across two UK forests at high nitrogen (N) deposition. This finding strongly suggested that forest canopies are not just passive filters for precipitation water and dissolved nutrients, and that the microbial life hidden within them can be responsible for transforming atmospheric N before it reaches the soil. We extended the isotopic approach at the European scale, and combined it to next-generation sequence analyses with the aim of elucidating canopy nitrification and identify phyllosphere microbes responsible for it. Specifically, in this study we: 1) estimated the relative contribution of NO3- derived from biological canopy nitrification vs. atmospheric deposition by using δ18O and δ17O of NO3- in rainfall and throughfall water; 2) quantified the functional genes related to nitrification, and finally 3) characterized the microbial communities harboured in tree canopies (i.e., phyllosphere) and in the underlying soils for two dominant tree species in Europe (Fagus sylvatica L. and Pinus sylvestris L.) using metabarcoding techniques. We considered twelve sites included in the European ICP Forests monitoring network, chosen along climate and N deposition gradients, spanning from Fennoscandia to the Mediterranean. We will show that presence of nitrifying microbes (as assessed through qPCR) and their activity (as derived from δ18O and δ17O) were detected in the tree canopies across most of the sites, and that canopy nitrification was significantly correlated with atmospheric N deposition. Finally, we will discuss differences in microbial community structure and composition across phyllosphere (and between the two tree species considered), water and soil samples in the investigated forests. Our study demonstrates the potential of integrating stable isotopes with microbial analyses to advance our understanding on canopy-atmosphere interactions and their contribution to N cycling.

doi: 10.1111/avsc.12525 Questions Forest classifications are tools used in research, monitoring, and management. In Finland, the Cajanderian forest site type classification is based on the composition of understorey vegetation with the assumption that it reflects in a predictable way the site's productive value. In Russia, the Sukachevian forest classification is similarly based on understorey vegetation but also accounts for tree species, soil wetness, and paludification. Here we ask whether Cajander's and Sukachev's forest types are effectively the same in terms of species composition, site productivity, and biodiversity. Location Boreal forests on mineral soils in Finland and the Russian part of Fennoscandia. Methods We use vegetation and soil survey data to compare the Cajanderian and the Sukachevian systems in terms of the understorey community composition (that is supposed to define them), soil fertility and tree productivity (that they are expected to indicate), and biodiversity (that is of interest for conservation purposes). We create and employ class prediction models to divide Russian and Finnish sites into Cajander's and Sukachev's types, respectively, based on vegetation composition. We perform cross-comparisons between the two systems by non-metric multidimensional scaling ordination and statistical tests. Results Within both systems, the site types formed similar, meaningful gradients in terms of the studied variables. Certain site types from the two systems were largely overlapping in community composition and arranged similarly along the fertility gradient and may thus be considered comparable. Conclusions The Cajanderian and the Sukachevian systems were both developed in the European boreal zone but differ in terms of the exact rules by which site types are determined. Our results show that analogous types between the systems can be identified. These findings aid in endeavours of technology and information transfer between Finnish and Russian forests for the purposes of basic or applied ecological research and forest management.

Biological fixation of atmospheric nitrogen (N2) by bryophyte-associated cyanobacteria is an important source of plant-available N in the boreal biome. Information on the factors that drive biological N2 fixation (BNF) rates is needed in order to understand the N dynamics of forests under a changing climate. We assessed the potential of several cryptogam species (the feather mosses Hylocomium splendens and Pleurozium schreberi, a group of Dicranum bryophytes, two liverworts, and Cladina lichens) to serve as associates of cyanobacteria or other N2-fixing bacteria (diazotrophs) using acetylene reduction assay (ARA). We tested the hypotheses that the legacy of chronic atmospheric N deposition reduces BNF in the three bryophyte species, sampled from 12 coniferous forests located at latitudes 60–68° N in Finland. In addition, we tested the effect of moisture and temperature on BNF. All species studied showed a BNF signal in the north, with the highest rates in feather mosses. In moss samples taken along the north–south gradient with an increasing N bulk deposition from 0.8 to 4.4 kg ha−1 year−1, we found a clear decrease in BNF in both feather mosses and Dicranum group. BNF turned off at N deposition of 3–4 kg ha−1 year−1. Inorganic N (NH4-N + NO3-N) best predicted the BNF rate among regression models with different forms of N deposition as explanatory variables. However, in southern spruce stands, tree canopies modified the N in throughfall so that dissolved organic N (DON) leached from canopies compensated for inorganic N retained therein. Here, both DON and inorganic N negatively affected BNF in H. splendens. In laboratory experiments, BNF increased with increasing temperature and moisture. Our results suggest that even relatively low N deposition suppresses BNF in bryophyte-associated diazotrophs. Further, BNF could increase in northern low-deposition areas, especially if climate warming leads to moister conditions, as predicted.

Pollen is known to affect forest throughfall biochemistry, but underlying mechanisms are not fully understood. We used generalized additive mixed modelling to study the relationship between long-term series of measured throughfall fluxes in spring (April–June) at forest plots and corresponding airborne pollen concentrations (Seasonal Pollen Integral, SPIn) from nearby aerobiological monitoring stations. The forest plots were part of the intensive long term monitoring (Level II) network of the UNECE International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) with dominant tree genera Fagus, Quercus, Pinus and Picea, and were distributed all across Europe. We also conducted a 7-day laboratory dissolution experiment with bud scales and flower stalks of European beech (Fagus sylvatica L.), pollen of beech, common oak (Quercus robur L.), silver birch (Betula pendula L.), Scots pine (Pinus sylvestris L.), Corsican pine (Pinus nigra Arnold ssp. laricio (Poiret) Maire), Norway spruce (Picea abies (L.) Karst.) and sterilized pollen of silver birch in a nitrate (NO3--N) solution (11.3 mg N L-1). Throughfall fluxes of potassium (K+), ammonium (NH4+-N), dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) showed a positive relationship with SPIn whereas NO3--N fluxes showed a negative relationship with SPIn. In years with massive seed production of beech and oak SPIn and throughfall fluxes of K+ and DOC were higher, but fluxes of NO3--N were lower. The experiment broadly confirmed the findings based on field data. Within two hours, pollen released large quantities of K+, phosphate, DOC and DON, and lesser amounts of sulphate, sodium and calcium. After 24-48 hours, NO3--N started to disappear, predominantly in the treatments with broadleaved pollen, while concentrations of nitrite and NH4+-N increased. At the end of the experiment, the inorganic nitrogen (DIN) was reduced, presumably because it was lost as gaseous nitric oxide (NO). There was no difference for sterilized pollen, indicating that the involvement of microbial activity was limited in above N transformations. Our results show that pollen dispersal might be an overlooked factor in forest nutrient cycling and might induce complex canopy N transformations, although the net-impact on N throughfall fluxes is rather low.

Changing environmental conditions may substantially interact with site quality and forest stand characteristics, and impact forest growth and carbon sequestration. Understanding the impact of the various drivers of forest growth is therefore critical to predict how forest ecosystems can respond to climate change. We conducted a continental-scale analysis of recent (1995–2010) forest volume increment data (ΔVol, m3 ha−1 yr−1), obtained from ca. 100,000 coniferous and broadleaved trees in 442 even-aged, single-species stands across 23 European countries. We used multivariate statistical approaches, such as mixed effects models and structural equation modelling to investigate how European forest growth respond to changes in 11 predictors, including stand characteristics, climate conditions, air and site quality, as well as their interactions. We found that, despite the large environmental gradients encompassed by the forests examined, stand density and age were key drivers of forest growth. We further detected a positive, in some cases non-linear effect of N deposition, most pronounced for beech forests, with a tipping point at ca. 30 kg N ha−1 yr−1. With the exception of a consistent temperature signal on Norway spruce, climate-related predictors and ground-level ozone showed much less generalized relationships with ΔVol. Our results show that, together with the driving forces exerted by stand density and age, N deposition is at least as important as climate to modulate forest growth at continental scale in Europe, with a potential negative effect at sites with high N deposition.

Boreal forests are rich in non-timber forest products from plants: wild berries and herbs used commercially or by households as food, medicine, decoration, or raw material. Approximately two hundred wild plant species have been documented for their nutritional uses in Finland, and many of these species occur in forests. However, the provisioning of edible plants by managed forests has received little attention, despite the fact that forest vegetation is altered by forest management practices. In this study, we use nation-wide forest vegetation and tree stand data consisting of a total of 1,778 sample plots to quantify the richness and abundance of edible wild plants in Finnish forests. Responses of edible species richness, abundance, and composition to stand characteristics such as site type, tree species composition, stand density, and management history are analyzed with regression models, NMDS ordination, and diagnostic species analysis, for forests on mineral soils and on peatlands separately. A total of 68 edible wild plant species occur in our dataset, with their occurrence and abundance varying between species and between sites. Our results indicate that habitat characteristics, namely site fertility and stand density, are the strongest determinants of overall edible plant provisioning. The richness of edible species as well as their total abundance were lower in less fertile site types and in denser stands. Recent timber harvesting and plantation as opposed to natural regeneration had a negative effect on edible species abundance in mineral soil forests. Several edible plant species confined to the richest site types accounted for the increase in species richness, while different forest management practices had generally none or a negligible number of diagnostic species. We conclude that a large and diverse set of edible wild plants occurs in Finnish forests, and the effects of stand characteristics and management on overall edible plant richness and abundance may be muddled by opposite species-level responses. Edible plant provisioning should be further analyzed at the level of smaller species groups or individual species to reveal the opportunities to support it in managed forests.

Forest ecosystem productivity is strongly linked to site nutrient availability, which is influenced by litter inputs and their decomposition rates. Fine roots and mycelia are key contributors in belowground soil carbon (C) accumulation, but studies have seldom reported how belowground litter C input is related to site types in boreal forests. In this study, three mature and one young Pinus sylvestris forests along a site type gradient in southern Finland were chosen for measurements of fine root biomass, fine root longevity, below- and aboveground growth phenology and annual litter input from tree and understorey vegetation. Site types were distinguished by understorey vegetation composition, which indicated the site fertility. Fine root biomass per tree stand basal area decreased significantly from nutrient-poor to nutrient-rich sites, the nutrient-poor sites with longer fine root longevity resulted in an equal belowground litter input with the nutrient-rich site. Above- and belowground annual litter inputs were 131–236 and 70–91 g m−2 year−1, respectively. Aboveground litter increased with site fertility, resulting into belowground litter having a decreasing trend from 37% to 23% of total litter inputs with increasing site fertility. Ectomycorrhizal mycelia and understory production contributed 8–13% and 18–41% of belowground production, respectively. Contribution of understorey vegetation to the belowground litter C input was lower than that of trees at xeric and sub-xeric sites but equaled to that of trees at the mesic site. Our study showed distinct dimensions of below- and aboveground litter inputs influenced by site types. Moreover, we emphasize that the belowground C inputs from ectomycorrhizal mycelia and the understorey in addition to those of trees should always be considered in C balances and C reporting in boreal conifers.

We analysed the driving forces of temporal change in the cover of boreal forest plant species in Finland over 21 years (1985–2006). Our study focused on the effects of forest cuttings of different intensity, i.e. regeneration cuttings (RC) and intermediate cuttings (IC), on 11 common understory species (including vascular plants, mosses and lichens) and their recovery after cuttings. General linear models (LM) were applied in modelling the change in the cover of plant species. The plant species displayed contrasting responses to RC and IC in accordance with their light demand, which also accounted for their recovery rate after cutting disturbance. After RC of old forest, shade and semi-shade tolerant species (herb Oxalis acetosella, moss Hylocomium splendens, dwarf shrub Vaccinium myrtillus) decreased strongly in cover (>70%) and recovered relatively slowly, but increased (40–100%) after IC. Species adapted to semi-light conditions (mosses Dicranum polysetum and Pleurozium schreberi, dwarf shrub Vaccinium vitis-idaea) decreased rather strongly (20–60%) after RC, but recovered relatively fast, reaching their old-forest cover level at a stand age of ca. 30 years. These species also increased after IC. In contrast, light-demanding species (dwarf shrubs Empetrum nigrum and Calluna vulgaris, grass Deschampsia flexuosa) increased after RC and decreased after IC. Cladina lichens decreased across the country. In general, early successional light-demanding species increased and late successional shade-tolerant species decreased after RC. IC favoured all but the most light-demanding species. Species-specific responses to cuttings showed considerable variation within plant functional groups (e.g. dwarf shrubs, herbs, mosses). Cuttings determined the principal change in vegetation, while only slight signals of the effect of climate warming were found in few species. The legacy of forest management deserves special attention when e.g. tracking the effect of global change on vegetation.

Mosses take up nitrogen (N) mainly from precipitation through their surfaces, which makes them competent bioindicators of N deposition. We found positive relationships between the total N concentration (mossN%) of common terrestrial moss species (feather mosses Pleurozium schreberi and Hylocomium splendens, and a group of Dicranum species) and different forms of N deposition in 11-16 coniferous forests with low N deposition load in Finland. The mosses were collected either inside (Dicranum group) or both inside and outside (feather mosses) the forests. Deposition was monitored in situ as bulk deposition (BD) and stand throughfall (TF) and detected for ammonium (NH4+-N), nitrate (NO3--N), dissolved organic N (DON), and total N (N-tot, kg ha(-1)yr(-1)). N-tot deposition was lower in TF than BD indicating that tree canopies absorbed N from deposition in N limited boreal stands. However, mossN % was higher inside than outside the forests. In regression equations, inorganic N in BD predicted best the mossN% in openings, while DON in TF explained most variation of mossN% in forests. An asymptotic form of mossN% vs. TF N-tot curves in forests and free NH4+-N accumulation in tissues in the southern plots suggested mosses were near the N saturation state already at the N-tot deposition level of 3-5 kg ha(-1) yr(-1). N leachate from ground litterfall apparently also contributed the N supply of mosses. Our study yielded new information on the sensitivity of boreal mosses to low N deposition and their response to different N forms in canopy TF entering moss layer. The equations predicting the N-tot deposition with mossN% showed a good fit both in forest sites and openings, especially in case of P. schreberi. However, the open site mossN% is a preferable predictor of N deposition in monitoring studies to minimize the effect of tree canopies and N leachate from litterfall on the estimates. (C) 2020 Elsevier Ltd. All rights reserved.

The quantitative assessment of forest management impacts on ecosystem services calls for ecological models to be linked to forest planning systems. In these systems, such models are expected to produce valid and accurate predictions to quantify the effects of alternative forest management strategies on ecosystem services. As an example, we present an evaluation of the model for the percent cover of ground lichens in forests on mineral soils in Finland. First, the performance of the lichen cover model fitted to the data of forest vegetation surveyed in 1985–1986 was comprehensively validated with the data resurveyed on the same plots in 1995. The differences between the observed and predicted values in lichen cover were calculated and analysed. Second, stand-level analyses were conducted to validate the predicted development of lichen cover in accordance with the rotation length development and management of forest stands. Third, the impacts of forest management scenarios on the average lichen cover at regional level were calculated to validate the results over larger areas. Lichen cover was predicted by site fertility, growing stock, forest harvests, soil preparation and geographic location. Our evaluation suggested that the model logically predicted the 10-year change in lichen cover at the stand level. A negative bias was found in predictions especially on xeric sites in northern Finland, although this was not related to stand management, but probably to global greening and reindeer grazing. The model based on the cross-sectional vegetation survey data showed its ability to describe the long-term changes in lichen cover along with the development and management of stands in different site fertility classes. On xeric and barren sites, the analyses showed decreasing trends in the average lichen cover at the regional level during the 30-year simulation period. However, the development of the average lichen cover varied only slightly among the alternative forest management scenarios. This study provides a modelling approach to evaluate the impacts of forest management on ecosystem services, here the cover of ground lichens, as an indicator of forest biodiversity and quality of reindeer pastures.

Unidad de excelencia María de Maeztu CEX2019-000940-M Nutrient availability influences virtually every aspect of an ecosystem, and is a critical modifier of ecosystem responses to global change. Although this crucial role of nutrient availability in regulating ecosystem structure and functioning has been widely acknowledged, nutrients are still often neglected in observational and experimental synthesis studies due to difficulties in comparing the nutrient status across sites. In the current study, we explain different nutrient-related concepts and discuss the potential of soil-, plant- and remote sensing-based metrics to compare the nutrient status across space. Based on our review and additional analyses on a dataset of European, managed temperate and boreal forests (ICP [International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests] Forests dataset), we conclude that the use of plant- and remote sensing-based metrics that rely on tissue stoichiometry is limited due to their strong dependence on species identity. The potential use of other plant-based metrics such as Ellenberg indicator values and plant-functional traits is also discussed. We conclude from our analyses and review that soil-based metrics have the highest potential for successful intersite comparison of the nutrient status. As an example, we used and adjusted a soil-based metric, previously developed for conifer forests across Sweden, against the same ICP Forests data. We suggest that this adjusted and further adaptable metric, which included the organic carbon concentration in the upper 20 cm of the soil (including the organic fermentation-humus [FH] layer), the C:N ratio and of the FH layer, can be used as a complementary tool along with other indicators of nutrient availability, to compare the background nutrient status across temperate and boreal forests dominated by spruce, pine or beech. Future collection and provision of harmonized soil data from observational and experimental sites is crucial for further testing and adjusting the metric.

Biological fixation of atmospheric nitrogen (N

Explaining the large-scale diversity of soil organisms that drive biogeochemical processes—and their responses to environmental change—is critical. However, identifying consistent drivers of belowground diversity and abundance for some soil organisms at large spatial scales remains problematic. Here we investigate a major guild, the ectomycorrhizal fungi, across European forests at a spatial scale and resolution that is—to our knowledge—unprecedented, to explore key biotic and abiotic predictors of ectomycorrhizal diversity and to identify dominant responses and thresholds for change across complex environmental gradients. We show the effect of 38 host, environment, climate and geographical variables on ectomycorrhizal diversity, and define thresholds of community change for key variables. We quantify host specificity and reveal plasticity in functional traits involved in soil foraging across gradients. We conclude that environmental and host factors explain most of the variation in ectomycorrhizal diversity, that the environmental thresholds used as major ecosystem assessment tools need adjustment and that the importance of belowground specificity and plasticity has previously been underappreciated.

The tree root–mycorhizosphere plays a key role in resource uptake, but also in the adaptation of forests to changing environments. The adaptive foraging mechanisms of ectomycorrhizal (EcM) and fine roots of Picea abies, Pinus sylvestris and Betula pendula were evaluated along a gradient from temperate to subarctic boreal forest (38 sites between latitudes 48°N and 69°N) in Europe. Variables describing tree resource uptake structures and processes (absorptive fine root biomass and morphology, nitrogen (N) concentration in absorptive roots, extramatrical mycelium (EMM) biomass, community structure of root-associated EcM fungi, soil and rhizosphere bacteria) were used to analyse relationships between root system functional traits and climate, soil and stand characteristics. Absorptive fine root biomass per stand basal area increased significantly from temperate to boreal forests, coinciding with longer and thinner root tips with higher tissue density, smaller EMM biomass per root length and a shift in soil microbial community structure. The soil carbon (C) : N ratio was found to explain most of the variability in absorptive fine root and EMM biomass, root tissue density, N concentration and rhizosphere bacterial community structure. We suggest a concept of absorptive fine root foraging strategies involving both qualitative and quantitative changes in the root–mycorrhiza–bacteria continuum along climate and soil C : N gradients.

Nitrogen (N) is typically the growth-limiting factor in boreal forest ecosystems. Therefore, knowledge on forest N stocks and fluxes is crucial in order to predict and evaluate the effects of different anthropogenic factors (e.g. climate change, air pollutant deposition, forest management practices) on the condition, development and sustainability of boreal forests. In this study, we evaluated the amount and distribution of N and biomass in different compartments of forest ecosystem, including not only tree stand and soil, but also such rarely reported N stocks as litter layer, ground vegetation and fine and small roots. We also calculated the theoretical export of N in three forest harvest regimes of different intensity (stem-only harvest, whole-tree harvest, whole-tree harvest and stump uplifting) and assessed the time required for N deposition to compensate the N losses occurring in them. The study included seven Scots pine and eight Norway spruce dominated stands belonging to the UN-ECE ICP Forests Level II programme in Finland. The average effective temperature sum and stand age of the sites ranged 658–1351 d.d. and 55–200 yrs, respectively. Among the study sites, the total biomass (needles, living and dead branches, stems, bark, stumps, coarse roots, fine and small roots, understory, litter, humus and mineral soil layers) ranged from 178 Mg ha −1 to 541 Mg ha −1 , the respective range for N stock being 1890–7530 kg ha −1 . The two largest pools of N in forest ecosystem were mineral soil (depth 0–40 cm; mean = 70%) and humus layer (mean = 16%). The largest living biomass N stock was in stems in pine stands (88 kg ha −1 ) and in needles in spruce stands (134 kg ha −1 ). Nstored in tree biomass accounted for 7–19% of the total ecosystem N stock. The proportion of N stored in potential logging residues or biofuel (needles, living and dead branches, stumps and coarse roots) was 67 ± 4% and 53 ± 5% of the tree N stock in northern spruce stands and in southern pine stands, respectively. The understory vegetation N stock was the largest in northern spruce stands, and the lowest in southern spruce stands. Our results supported the hypothesis that in boreal coniferous forests, inputs of N by deposition accumulating during the following rotation period will be able to replenish the export of N caused by conventional stem-only-harvest in final cutting, but the sustainability of the site productivity will be challenged when more intense whole tree harvest regimes are practiced, especially in Norway spruce stands.

Significance In peatlands, the external sources of nitrogen are mainly atmospheric, but the atmospheric nitrogen deposition alone cannot explain the long-term annual nitrogen accumulation rates to these ecosystems. Because of methodological problems, methane-induced fixation of atmospheric dinitrogen gas has been previously overlooked as an additional nitrogen input mechanism. We found that the activity of methane-oxidizing bacteria provides not only carbon but also nitrogen to peat mosses and, thus, contributes to carbon and nitrogen accumulation in peatlands, which store approximately one-third of the global soil carbon pool. Our results imply that nitrogen fixation in wetlands may be strongly underestimated when methods inhibiting methane oxidizers are used.

Boreal mires are globally important carbon stores. In nutrient poor mires, i.e. bogs, vegetation is highly structured between hydrologically different microforms: hummocks, lawns and hollows. We studied whether the vegetation structure in four boreal bogs is reflected in the actinobacterial decomposer communities. Vegetation was determined by projection cover of plant species. Actinobacteria were detected from peat profiles at depths of 0–20, 20–40 and 40–60 cm by molecular methods. The data was analysed by a hierarchical set of ordinations. The inter-bog variation in plant communities was insignificant, while the intra-bog differences (microforms) at each bog were highly significant. This variation was reflected in actinobacteria communities in the two upper peat layers. The deepest peat layer (40–60 cm) showed some inter-bog differences, possibly due to the different history of the bogs. Our results show that the actinobacteria communities reflect the variation in bog vegetation, which in turn is tied to hydrological conditions. The possible alterations in the water level caused by changing climate are likely to cause predictable changes in vegetation and microbial communities in bog ecosystems.

Model validation experiments are fundamental to ensure that the peat growth models correspond with the diversity in nature. We evaluated the Holocene Peatland Model (HPM) simulation against the field observations from a chronosequence of peatlands and peat core data. The ongoing primary peatland formation on the isostatically rising coast of Finland offered us an exceptional opportunity to study the peatland succession along a spatial continuum and to compare it with the past succession revealed by vertical peat sequences. The current vegetation assemblages, from the seashore to a 3000 year old bog, formed a continuum from minerotrophic to ombrotrophic plant communities. A similar sequence of plant communities was found in the palaeovegetation. The distribution of plant functional types was related to peat thickness and water-table depth (WTD) supporting the assumptions in HPM, though there were some differences between the field data and HPM. Palaeobotanical evidence from the oldest site showed a rapid fen–bog transition, indicated by a coincidental decrease in minerotrophic plant functional types and an increase in ombrotrophic plant functional types. The long-term mean rate of carbon (C) accumulation varied from 2 to 34 g C/m2 per yr, being highest in the intermediate age cohorts. Mean nitrogen (N) accumulation varied from 0.1 to 3.9 g N/m2 per yr being highest in the youngest sites. WTD was the deepest in the oldest sites and its variation there was temporally the least but spatially the highest. Evaluation of the HPM simulations against the field observations indicated that HPM reasonably well simulates peatland development, except for very young peatlands.

Fine root acclimation to different environmental conditions is crucial for growth and sustainability of forest trees. Relatively small changes in fine root standing biomass (FRB), morphology or mycorrhizal symbiosis may result in a large change in forest carbon, nutrient and water cycles. We elucidated the changes in fine root traits and associated ectomycorrhizal (EcM) fungi in 12 Norway spruce stands across a climatic and N deposition gradient from subarctic-boreal to temperate regions in Europe (68°N–48°N). We analysed the standing FRB and the ectomycorrhizal root tip biomass (EcMB, g m−2) simultaneously with measurements of the EcM root morphological traits (e.g. mean root length, root tissue density (RTD), N% in EcM roots) and frequency of dominating EcM fungi in different stands in relation to climate, soil and site characteristics. Latitude and N deposition explained the greatest proportion of variation in fine root traits. EcMB per stand basal area (BA) increased exponentially with latitude: by about 12.7 kg m−2 with an increase of 10° latitude from southern Germany to Estonia and southern Finland and by about 44.7 kg m−2 with next latitudinal 10° from southern to northern Finland. Boreal Norway spruce forests had 4.5 to 11 times more EcM root tips per stand BA, and the tips were 2.1 times longer, with 1.5 times higher RTD and about 1/3 lower N concentration. There was 19% higher proportion of root tips colonized by long-distance exploration type forming EcM fungi in the southern forests indicating importance of EcM symbiont foraging strategy in fine root nutrient acquisition. In the boreal zone, we predict ca. 50% decrease in EcMB per stand BA with an increase of 2 °C annual mean temperature. Different fine root foraging strategies in boreal and temperate forests highlight the importance of complex studies on respective regulatory mechanisms in changing climate.

In this study we hypothesized that a change in the composition of vegetation communities is closely linked to a concurrent change in the structure of microbial communities through the decisive role of plant residues in regulating the quality of carbon sources for soil microbes. The study was carried out in a boreal forest along an 80-m-long successional transect (willow, alder, birch, spruce) located on the land-uplift coast of the Gulf of Bothnia, western Finland. The field and ground layer vegetation were recorded by species percentage cover on 2 m2 sample plots located at 10 m intervals. Organic layer samples were taken from around each vegetation sample plot for the determination of phospholipid fatty acid (PLFA) profiles and chemical composition of the organic layer, as characterized by the concentrations of neutral and acidic sugar units in non-cellulosic polysaccharides, cellulose, the acid-soluble lignin (ASL), and acid-insoluble residue (AIR). The organic layer along the transect showed a decrease in pH (from 5.2 to 4.4), moisture content and concentrations of ASL and the hemicellulosic sugars arabinose, rhamnose, xylose, galactose, and glucuronic acid, while the AIR concentration increased. These organic matter fractions and pH also showed a strong positive or negative correlation with the vegetation and the PLFA ordination. The percentage cover of grasses significantly correlated with the PLFA data, as well as with that of herbs and field layer species. However, PLFA ordination showed a higher correlation with the organic layer properties than with the coverage of plant species groups. In conclusion, our data showed a strong directional relationship between the organic matter characteristics and structure of the vegetation and microbial communities along the study transect. This study is, to our knowledge, the first investigation to apply acid methanolysis in the characterization of non-cellulosic polysaccharides in the organic layer.

Change history: In the HTML version of this Article, author 'Filipa Cox' had no affiliation in the author list, although she was correctly associated with affiliation 3 in the PDF. In addition, the blue circles for 'oak' were missing from Extended Data Fig. 1. These errors have been corrected online.

We studied methanogen activity—measured by in vitro methane production potential and by detection of the messenger RNA (mRNA) of a functional gene—in two boreal fens under high and deep water table (WT) level conditions resulting from a rainy growing season and a dry growing season. The depth of the highest CH4-producing layers differed between the years. In the wet year, the highest CH4 production rate was around 20 cm below the mean WT. In the dry year, the highest rates were measured close to the peat surface, well above the mean WT. The distribution of activity in the peat profiles of the two fens appeared to be site specific. Under deep-WT conditions, CH4 production potential was generally lower than that under high-WT conditions. Detection of the mRNA of the methanogen-specific mcrA gene indicated in situ methanogenesis in both water-saturated peat (below the WT) and unsaturated peat (above the WT). Analyses of DNA-derived and mRNA-derived methanogen community structures showed greater similarity between those two in water-saturated peat than in unsaturated peat. This suggested that favorable conditions promoted the activity of most members in methanogen communities, but unfavorable conditions showed differences between distinct community members in adaptation to adverse conditions.

Variations in ectomycorrhizal (EcM) short root tips of Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) in 16 stands throughout Finland were studied, and their relationships with latitude, organic layer C:N ratio, temperature sum and foliage biomass were determined. There were no significant differences in EcM root tip frequency (number per milligram of fine roots) or root tip mass between tree species or between northern and southern sites. The EcM root tip number per unit area of the organic layer plus the 0-30 cm mineral soil layer varied between 0.8 and 2.4 million per m(2) for Norway spruce and between 0.7 and 2.9 million per m(2) for Scots pine, and it was higher in the northern Scots pine stands than in the southern Scots pine stands. Over 80% of the EcM root tips of both species were in the organic layer and the upper 0-10 cm mineral soil layer. We related EcM root tips to foliage mass because these two components are the most important functional units in boreal tree physiology. Both species, especially the Scots pine trees, had more EcM root tips in relation to foliage mass in northern Finland than in southern Finland. Scots pine trees had more EcM root tips in relation to foliage mass than Norway spruce in the same climatic region. The EcM root tip:foliage biomass ratio of Norway spruce was positively related to the C:N ratio in the organic layer, whereas that of Scots pine was negatively related to the temperature sum. The number of EcM root tips per milligram of fine root biomass was constant, implying that trees of both species increase nutrient uptake by increasing fine root production and hence their total number of EcM tips and the area of soil occupied by mycelia. Both tree species responded to nitrogen (N) deficiency by maintaining more EcM tips per foliage unit, and this may be related to a higher proportion of N uptake in an organic form.

International audience; Key messageAtmospheric deposition of nitrogen compounds and soil and foliar variables related to N deposition resulted important factors accounting for the variability of defoliation in European forest plots.ContextNitrogen (N) deposition has increased in the northern hemisphere because of anthropogenic-related emission of N compounds. Increased N availability may have an adverse impact on forest sustainability.AimsThis study aims to test the importance of throughfall N (Nthr) deposition in explaining the variability of the frequency of trees with defoliation >25 % (F25), an indicator of forest condition.MethodsA pan-European data set (71 plots) with enhanced quality control was considered. The additive effect of Nthr-related predictors (identified conceptually and by rank correlation) in explaining F25 was investigated by partial least square regression in comparison with a reference model based on site, stand, management and climate data. Reported damage to foliage, Nthr deposition, foliar N ratios and mineral top-soil variables were added stepwise to the reference model.ResultsN-related variables improved defoliation models. Higher Nthr deposition led to higher F25 for beech and Norway spruce, while the effect was opposite for Scots pine. Higher foliar N ratios led to higher F25 for all species.ConclusionNthr deposition, damage to foliage, foliar N/P, N/Ca, N/Mg, N/K, top-soil pH, C/N and exchangeable base cation resulted important factors (although with possible diverse effect) in explaining the variability of F25 among plots.

Forest harvesting, especially when intensified harvesting method as whole-tree harvesting with stump lifting (WTHs) are used, may increase mercury (Hg) and methylmercury (MeHg) leaching to recipient water courses. The effect can be enhanced if the underlying bedrock and overburden soil contain Hg. The impact of stem-only harvesting (SOH) and WTHs on the concentrations of Hg and MeHg as well as several other variables in the ditch water was studied using a paired catchment approach in eight drained peatland-dominated catchments in Finland (2008–2012). Four of the catchments were on felsic bedrock and four on black schist bedrock containing heavy metals. Although both Hg and MeHg concentrations increased after harvesting in all treated sites according to the randomized intervention analyses (RIAs), there was only a weak indication of a harvest-induced mobilization of Hg and MeHg into the ditches. Furthermore, no clear differences between WTHs and SOH were found, although MeHg showed a nearly significant difference (p = 0.06) between the harvesting regimes. However, there was a clear bedrock effect, since the MeHg concentrations in the ditch water were higher at catchments on black schist than at those on felsic bedrock. The pH, suspended solid matter (SSM), dissolved organic carbon (DOC), and iron (Fe) concentrations increased after harvest while the sulfate (SO4-S) concentration decreased. The highest abundances of sulfate-reducing bacteria (SRB) were found on the sites with high MeHg concentrations. The biggest changes in ditch water concentrations occurred first 2 years after harvesting.

Vegetation and water table are important regulators of methane emission in peatlands. Microform variation encompasses these factors in small-scale topographic gradients of dry hummocks, intermediate lawns and wet hollows. We examined methane production and oxidization among microforms in four boreal bogs that showed more variation of vegetation within a bog with microform than between the bogs. Potential methane production was low and differed among bogs but not consistently with microform. Methane oxidation followed water table position with microform, showing higher rates closer to surface in lawns and hollows than in hummocks. Methanogen community, analysed by mcrA terminal restriction fragment length polymorphism and dominated by Methanoregulaceae or 'Methanoflorentaceae', varied strongly with bog. The extent of microform-related variation of methanogens depended on the bog. Methanotrophs identified as Methylocystis spp. in pmoA denaturing gradient gel electrophoresis similarly showed effect of bog, and microform patterns were stronger within individual bogs. Our results suggest that methane-cycling microbes in boreal Sphagnum bogs with seemingly uniform environmental conditions may show strong site-dependent variation. The bog-intrinsic factor may be related to carbon availability but contrary to expectations appears to be unrelated to current surface vegetation, calling attention to the origin of carbon substrates for microbes in bogs.

Peat accumulating mires are important sources of the greenhouse gas methane. Methane emissions and methanogenic Archaea communities have been shown to differ between fens and bogs, implying that mire succession includes an ecological succession in methanogen communities. We investigated methane production and the methanogen communities along a chronosequence of mires (ca. 100-2,500 years), which consisted of five sites (1-5) located on the land-uplift coast of the Gulf of Bothnia. Methane production was measured in a laboratory incubation experiment. Methanogen communities were determined by amplification of a methyl coenzyme M-reductase (mcr) gene marker and analyzed by terminal-restriction fragment length polymorphism. The terminal-restriction fragment length polymorphism fingerprinting resulted in 15 terminal restriction fragments. The ordination configuration of the terminal restriction fragments data, using nonmetric multidimensional scaling, showed a clear gradient in the methanogen community structure along the mire chronosequence. In addition, fingerprint patterns of samples from the water table level and 40 cm below differed from one another in the bog site (site 5). Methane production was negligible in the three youngest fen sites (sites 1-3) and showed the highest rates in the oligotrophic fen site (site 4). Successful PCR amplification using mcr gene primers revealed the presence of a methanogen community in all five sites along the study transect.

The aim of this study was to identify the most significant site, stand and climate factors affecting needle (LFneedle) and total (LFtotal) above-ground litterfall production and to develop multiple linear regression (MLR) models that can be used to reliably predict litterfall production in the boreal zone using readily available variables. Unlike most other litterfall production studies, we use climate data for the actual sites and annual litterfall values. A data set including 34 Scots pine stands located throughout Finland was compiled. The data for some stands covered a period of more than 30 years. The age of the stands ranged from 35 to >200 years and all were growing on upland, mineral soil sites. Stand mean annual LFneedle ranged from 22 g m−2 (northern Finland) to 157 g m−2 (southern Finland); corresponding values for LFtotal were 32 and 230 g m−2. Annual LFneedle production accounted between 49 and 75% of stand LFtotal production and explained 88% of the variation in LFtotal over all stands. There was considerable annual variation in litterfall production also within the same stand. The coefficient of variation in LFneedle in each stand ranged from 4 to 58% (mean = 19%) and from 3 to 39% (mean = 22%) for LFtotal. Both LFneedle and LFtotal were highly significantly (p

Since 70 % of global forests are managed and forests impact the global carbon cycle and the energy exchange with the overlying atmosphere, forest management has the potential to mitigate climate change. Yet, none of the land-surface models used in Earth system models, and therefore none of today's predictions of future climate, accounts for the interactions between climate and forest management. We addressed this gap in modelling capability by developing and parametrising a version of the ORCHIDEE land-surface model to simulate the biogeochemical and biophysical effects of forest management. The most significant changes between the new branch called ORCHIDEE-CAN (SVN r2290) and the trunk version of ORCHIDEE (SVN r2243) are the allometric-based allocation of carbon to leaf, root, wood, fruit and reserve pools; the transmittance, absorbance and reflectance of radiation within the canopy; and the vertical discretisation of the energy budget calculations. In addition, conceptual changes were introduced towards a better process representation for the interaction of radiation with snow, the hydraulic architecture of plants, the representation of forest management and a numerical solution for the photosynthesis formalism of Farquhar, von Caemmerer and Berry. For consistency reasons, these changes were extensively linked throughout the code. Parametrisation was revisited after introducing 12 new parameter sets that represent specific tree species or genera rather than a group of often distantly related or even unrelated species, as is the case in widely used plant functional types. Performance of the new model was compared against the trunk and validated against independent spatially explicit data for basal area, tree height, canopy structure, gross primary production (GPP), albedo and evapotranspiration over Europe. For all tested variables, ORCHIDEE-CAN outperformed the trunk regarding its ability to reproduce large-scale spatial patterns as well as their inter-annual variability over Europe. Depending on the data stream, ORCHIDEE-CAN had a 67 to 92 % chance to reproduce the spatial and temporal variability of the validation data.

Key message Exceedance of critical limits in soil solution samples was more frequent in intensively monitored forest plots across Europe with critical loads for acidity and eutrophication exceeded compared to other plots from the same network. Elevated inorganic nitrogen concentrations in soil solution tended to be related to less favourable nutritional status. Context Forests have been exposed to elevated atmospheric deposition of acidifying and eutrophying sulphur and nitrogen compounds for decades. Critical loads have been identified, below which damage due to acidification and eutrophication are not expected to occur. Aims We explored the relationship between the exceedance of critical loads and inorganic nitrogen concentration, the base cation to aluminium ratio in soil solutions, as well as the nutritional status of trees. Methods We used recent data describing deposition, elemental concentrations in soil solution and foliage, as well as the level of damage to foliage recorded at forest plots of the ICP Forests intensive monitoring network across Europe. Results Critical loads for inorganic nitrogen deposition were exceeded on about a third to half of the forest plots. Elevated inorganic nitrogen concentrations in soil solution occurred more frequently among these plots. Indications of nutrient imbalances, such as low magnesium concentration in foliage or discolouration of needles and leaves, were seldom but appeared more frequently on plots where the critical limits for soil solution were exceeded. Conclusion The findings support the hypothesis that elevated nitrogen and sulphur deposition can lead to imbalances in tree nutrition.

We investigated the changes in basal respiration (BASAL), microbial biomass as substrate-induced respiration (SIR) and their ratio ( q CO 2 ) in the organic layer along a primary successional transect (alder/rowan, birch, birch/spruce, spruce I and spruce II) typical of the land-uplift coast in western Finland. PLFA analysis was used to detect concurrent successional changes in microbial community structure. Along the transect the soil C/N ratio increased (from 16 to 37) and pH (H 2 O) decreased (from 5.11 to 3.98) substantially. Concurrently, BASAL and SIR remained mainly stable although, during the most favorable temperature and moisture conditions in the field, they tended to increase along the transect from the alder/rowan site to spruce I, and decreased again in spruce II. q CO 2 showed no consistent trend along the transect. The soil of the birch site had the highest total PLFA and bacterial PLFA concentrations, while the soils of both spruce sites had the lowest. The ordination configuration of the PLFA data in non-metric multidimensional scaling was clearly related to the C/N ratio and pH, and separated the forest sites relatively well. It was possible, on the basis of the similarities in the variation pattern along the transect, to divide the PLFAs into six groups.

We monitored net and gross N transformations in the organic layer along a primary successional transect (alder/rowan, birch, birch/spruce, spruce I and spruce II) typical for the land-uplift coast in Western Finland. The relationships between N transformations, vegetation succession and organic matter quality (i.e. concentration of dissolved forms of N, C/N ratio, moisture and acidity) were then evaluated. Net N mineralisation rates in the organic layer were estimated in 5-week incubation experiments in situ using intact soil cores, and in the laboratory on sieved, fresh organic layer samples. Microbial biomass N (fumigation–extraction) and gross N mineralisation (15N-isotope dilution method), were determined once in the laboratory. The C/N ratio increased and pH and net N mineralisation decreased in the organic layer along the succession transect. The alder/rowan site was the only site to show net nitrification. Microbial biomass N tended to increase along the transect from the alder/rowan site to spruce I, and decreased again in spruce II. Concurrently, gross N mineralisation showed a tentative increasing trend along the transect, although the differences between the sites were non-significant. The higher net N mineralisation rates in the alder/rowan site compared to the spruce sites were thus due to lower microbial immobilisation rather than to greater gross N mineralisation. Possible methodological reasons for the lack of response of gross N mineralisation rate to decreasing soil organic matter quality are discussed.

The state of health of 30 Norway spruce (Picea abies L. Karst.) stands located on the uplifting coast of western Finland was examined in 1992. Relationships between crown condition, elemental concentrations of the needles, organic layer properties and soil type were studied using sitewise data. The site scores along the first axis of the Principal Component Analysis for certain needle and organic layer variables were used as the site fertility index. The spruces in the study area showed a higher level of defoliation and this occurred in younger stands than elsewhere in southern Finland. Defoliation and discoloration correlated positively with stand age, and negatively with the site fertility index. Low N and Cu concentrations of the needles were found in older stands where spruces were highly defoliated and discolored. Boron concentrations of the needles were highest in these stands. Total N of organic layer and extractable S concentrations were negatively correlated with stand age, defoliation and discol...