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Northern peatlands provide a globally important carbon (C) store. Since the beginning of the 20th century, however, large areas of natural peatlands have been drained for biomass production across Fennoscandia. Today, drained peatland forests constitute a common feature of the managed boreal landscape, yet their ecosystem C balance and associated climate impact are not well understood, particularly within the nutrient‐poor boreal region. In this study, we estimated the net ecosystem carbon balance (NECB) from a nutrient‐poor drained peatland forest and an adjacent natural mire in northern Sweden by integrating terrestrial carbon dioxide (CO2) and methane (CH4) fluxes with aquatic losses of dissolved organic C (DOC) and inorganic C based on eddy covariance and stream discharge measurements, respectively, over two hydrological years. Since the forest included a dense spruce‐birch area and a sparse pine area, we were able to further evaluate the effect of contrasting forest structure on the NECB and component fluxes. We found that the drained peatland forest was a net C sink with a 2‐year mean NECB of −115 ± 5 g C m−2 year−1 while the adjacent mire was close to C neutral with 14.6 ± 1.7 g C m−2 year−1. The NECB of the drained peatland forest was dominated by the net CO2 exchange (net ecosystem exchange [NEE]), whereas NEE and DOC export fluxes contributed equally to the mire NECB. We further found that the C sink strength in the sparse pine forest area (−153 ± 8 g C m−2 year−1) was about 1.5 times as high as in the dense spruce‐birch forest area (−95 ± 8 g C m−2 year−1) due to enhanced C uptake by ground vegetation and lower DOC export. Our study suggests that historically drained peatland forests in nutrient‐poor boreal regions may provide a significant net ecosystem C sink and associated climate benefits. [ABSTRACT FROM AUTHOR]

Climate change is projected to increase the frequency and severity of droughts, possibly causing sudden and elevated tree mortality. Better understanding and predictions of boreal forest responses to climate change are needed to efficiently adapt forest management. We used tree‐ring width chronologies from the Swedish National Forest Inventory, sampled between 2010 and 2018, and a random forest machine‐learning algorithm to identify the tree, stand, and site variables that determine drought damage risk, and to predict their future spatial–temporal evolution. The dataset consisted of 16,455 cores of Norway spruce, Scots pine, and birch trees from all over Sweden. The risk of drought damage was calculated as the probability of growth anomaly occurrence caused by past drought events during 1960–2010. We used the block cross‐validation method to compute model predictions for drought damage risk under current climate and climate predicted for 2040–2070 under the RCP.2.6, RCP.4.5, and RCP.8.5 emission scenarios. We found local climatic variables to be the most important predictors, although stand competition also affects drought damage risk. Norway spruce is currently the most susceptible species to drought in southern Sweden. This species currently faces high vulnerability in 28% of the country and future increases in spring temperatures would greatly increase this area to almost half of the total area of Sweden. Warmer annual temperatures will also increase the current forested area where birch suffers from drought, especially in northern and central Sweden. In contrast, for Scots pine, drought damage coincided with cold winter and early‐spring temperatures. Consequently, the current area with high drought damage risk would decrease in a future warmer climate for Scots pine. We suggest active selection of tree species, promoting the right species mixtures and thinning to reduce tree competition as promising strategies for adapting boreal forests to future droughts. [ABSTRACT FROM AUTHOR]

Climate change predictions suggest that arctic and subarctic ecosystems will be particularly affected by rising temperatures and extreme weather events, including severe heat waves. Temperature is one of the most important environmental factors controlling and regulating microbial decomposition in soils; therefore, it is critical to understand its impact on soil microorganisms and their feedback to climate warming. We conducted a warming experiment in a subarctic birch forest in North Sweden to test the effects of summer heat waves on the thermal trait distributions that define the temperature dependences for microbial growth and respiration. We also determined the microbial temperature dependences 10 and 12 months after the heat wave simulation had ended to investigate the persistence of the thermal trait shifts. As a result of warming, the bacterial growth temperature dependence shifted to become warm‐adapted, with a similar trend for fungal growth. For respiration, there was no shift in the temperature dependence. The shifts in thermal traits were not accompanied by changes in α‐ or β‐diversity of the microbial community. Warming increased the fungal‐to‐bacterial growth ratio by 33% and decreased the microbial carbon use efficiency by 35%, and both these effects were caused by the reduction in moisture the warming treatments caused, while there was no evidence that substrate depletion had altered microbial processes. The warm‐shifted bacterial thermal traits were partially restored within one winter but only fully recovered to match ambient conditions after 1 year. To conclude, a summer heat wave in the Subarctic resulted in (i) shifts in microbial thermal trait distributions; (ii) lower microbial process rates caused by decreased moisture, not substrate depletion; and (iii) no detectable link between the microbial thermal trait shifts and community composition changes. [ABSTRACT FROM AUTHOR]

Boreal forests sequester and store vast carbon (C) pools that may be subject to significant feedback effects induced by climatic warming. The boreal landscape consists of a mosaic of forests and peatlands with wide variation in total C stocks, making it important to understand the factors controlling C pool sizes in different ecosystems. We therefore quantified the total C stocks in the organic layer, mineral soil, and tree biomass in 430 plots across a 68 km2 boreal catchment. The organic layer held the largest C pool, accounting for 39% of the total C storage; tree and mineral C pools accounted for 38% and 23%, respectively. The size of the soil C pool was positively related to modelled soil moisture conditions, especially in the organic soil layer (R2 = 0.50). Conversely, the tree C pool exhibited a unimodal relationship: storage was highest under intermediate wetness conditions. The magnitude and variation in the total soil C stocks observed in this work were comparable to those found at the national level in Sweden, suggesting that C accumulation in boreal landscapes is more sensitive to local variation resulting primarily from differences in soil moisture conditions than to regional differences in climate, nitrogen deposition, and parent material. [ABSTRACT FROM AUTHOR]

Longwave radiation is often the dominant source of energy for snowmelt in forests. Measurements at forest sites of varying density in Sweden and Finland show that downwelling longwave radiation is enhanced under forest canopies, even for sparse canopies and particularly for clear skies. Canopy density must be estimated accurately to predict this enhancement. Linear regression with above‐canopy longwave radiation and air temperature as predictors of sub‐canopy radiation gives good predictions of sub‐canopy longwave radiation with weightings for transmission through canopy gaps that are close to measured sky view fractions. Air temperature serves here as a proxy for effective canopy radiative temperature. Adding above‐canopy shortwave radiation as a predictor gives little improvement in the predictions, suggesting that daytime heating of trunks above the air temperature ("hot trees") has limited influence on longwave radiation under these continuous canopies. The influence of canopy temperatures falling below the above‐canopy air temperature ("cold trees") on calm, clear nights, however, is apparent. Decoupling of canopy and above‐canopy air temperatures in an energy balance model of the type used in large‐scale land surface modeling allows this cooling. Plain Language Summary: Longwave radiation is a component of the total energy budget between the atmosphere and the land surface. It is often the most important source of energy for snowmelt in forests. The amount of longwave radiation received by snow depends on the temperature and thermal emissivity of the forest canopy and sky. Forest canopies usually have higher emissivities than the sky and forests enhance longwave radiation, so accurate estimation of forest canopy density is critical for simulation of longwave radiation to the snow surface. Air temperature works well as a proxy for forest canopy temperature in boreal forests in Sweden and Finland. However, whether the air temperature used as a proxy is measured above or below a forest canopy can be important. Usually, above canopy air temperature works well as a proxy, but on calm, clear nights, forest canopy temperatures fall much lower than above canopy air temperatures. These periods of low canopy temperature require decoupling from above canopy air temperature by a model to simulate longwave radiation more accurately to snow. Key Points: Canopy density is the first‐order control on increases in longwave radiation to snow beneath forests away from canopy gaps and edgesSub‐canopy air temperature is an effective proxy for canopy temperature in calculating downward emission of longwave radiationAbove‐canopy air temperature is a less effective proxy, but a canopy model driven with above‐canopy temperature has lower errors [ABSTRACT FROM AUTHOR]

The ecosystem–atmosphere flux of biogenic volatile organic compounds (BVOCs) has important impacts on tropospheric oxidative capacity and the formation of secondary organic aerosols, influencing air quality and climate. Here we present within-canopy measurements of a set of dominant BVOCs in a managed spruce- and pine-dominated boreal forest located at the ICOS (Integrated Carbon Observation System) station Norunda in Sweden, collected using proton-transfer-reaction mass spectrometry (PTR-MS) during 2014–2016 and vertical emission profiles derived from these data. Ozone concentrations were simultaneously measured in conjunction with these PTR-MS measurements. The main BVOCs investigated with the PTR-MS were isoprene, monoterpenes, methanol, acetaldehyde, and acetone. The distribution of BVOC sources and sinks in the forest canopy was explored using Lagrangian dispersion matrix methods, in particular continuous near-field theory. The forest canopy was found to contribute ca. 86 % to the total monoterpene emission in summertime, whereas the below-canopy and canopy emissions were comparable (ca. 42 % and 58 %, respectively) during the fall period. This result indicates that boreal forest litter and other below-canopy emitters are a principal source of total forest monoterpene emissions during the fall months. During night, our results for methanol, acetone, and acetaldehyde seasonally present strong sinks in the forest canopy, especially in the fall, likely due to the nighttime formation of dew on vegetation surfaces. [ABSTRACT FROM AUTHOR]

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 FROM AUTHOR]

While even-aged forestry is the dominating forest management system in Sweden, there is an increasing interest in Continuous Cover Forestry. Consequently, the conversion of even-aged stands into uneven-aged ones using e.g. selection cutting can be expected to become more common in Sweden. However, there are no up-to-date studies available on harvester productivity during selection cutting under Nordic conditions. Studying harvest intensity during selection cutting is of interest because lighter harvest intensities lead to higher volume growth and better-preserved forest ambience than heavier intensities. The objective of this study was to examine the effect of harvest intensity on harvester productivity during selection cutting. The field study entailed harvesting either 14%, 28% or 48% of a mature stand's basal area. Harvester productivity was mainly explained by piece size (stem volume), while other factors, including harvest intensity, had only minor effects. This reality means that during selection cutting (thinning from above), piece size increases with decreasing harvest intensity, which in turn increases harvester productivity. Moreover, we observed a mild tendency that operators could select the stems' felling directions and order more freely when fewer trees are harvested. This amelioration increases productivity additionally during lighter harvesting intensities. [ABSTRACT FROM AUTHOR]

We report the results of two years' field performance of Scots pine (Pinus sylvestris) seedlings regenerated using artificially coated seeds. The coated seeds were used for regeneration on 12 clearcut sites, covering a 1000 km latitudinal gradient across Sweden. The coating was either combined with arginine-phosphate fertilizer (10 mg N per seed) or had no additions. Interactions with environmental variables associated with sites were also assessed. Coated seeds were deployed in May–June 2017 and surveyed in August–September of 2018 and 2019. After two years, the mean establishment rate of seedlings from coated seeds was 56 ± 4% across the 12 sites. The fertilizer addition did not affect survival, and the biomass response to fertilizer varied significantly between sites. Maximum precipitation and wind speed during the first six weeks after deployment were correlated with seedling survival, regardless of fertilization treatment. Establishment increased with increasing precipitation and decreased with increasing wind speed. This highlights the importance of initial weather conditions for the seeds' establishment. Our data suggest that Scots pine regeneration using coated seeds can be practiced in boreal forests, but also that the method is sensitive to the weather conditions at the time of deployment of the seeds. [ABSTRACT FROM AUTHOR]

Boreal forests are important global carbon (C) sinks and, therefore, considered as a key element in climate change mitigation policies. However, their actual C sink strength is uncertain and under debate, particularly for the actively managed forests in the boreal regions of Fennoscandia. In this study, we use an extensive set of biometric-and chamber-based C flux data collected in 50 forest stands (ranging from 5 to 211 years) over 3 years (2016–2018) with the aim to explore the variations of the annual net ecosystem production (NEP; i.e., the ecosystem C balance) across a 68 km2 managed boreal forest landscape in northern Sweden. Our results demonstrate that net primary production rather than heterotrophic respiration regulated the spatio-temporal variations of NEP across the heterogeneous mosaic of the managed boreal forest landscape. We further find divergent successional patterns of NEP in our managed forests relative to naturally regenerating boreal forests, including (i) a fast recovery of the C sink function within the first decade after harvest due to the rapid establishment of a productive understory layer and (ii) a sustained C sink in old stands (131–211 years). We estimate that the rotation period for optimum C sequestration extends to 138 years, which over multiple rotations results in a long-term C sequestration rate of 86.5 t C ha−1 per rotation. Our study highlights the potential of forest management to maximize C sequestration of boreal forest landscapes and associate climate change mitigation effects by developing strategies that optimize tree biomass production rather than heterotrophic soil C emissions. [ABSTRACT FROM AUTHOR]

Climate change currently manifests in upward and northward shifting treelines, which encompasses changes to the carbon (C) and nitrogen (N) composition of organic inputs to soils. Whether these changed inputs will increase or decrease microbial mineralisation of native soil organic matter remains unknown, making it difficult to estimate how treeline shifts will affect the C balance. Aiming to improve mechanistic understanding of C cycling in regions experiencing treeline shifts, we quantified priming effects in soils of high altitudes (Peruvian Andes) and high latitudes (subarctic Sweden), differentiating landcover types (boreal forest, tropical forest, tundra heath, Puna grassland) and soil horizons (organic, mineral). In a controlled laboratory incubation, soils were amended with substrates of different C:N, composed of an organic C source at a constant ratio of 30% substrate-C to microbial biomass C, combined with different levels of a nutrient solution neutral in pH. Substrate additions elicited both positive and negative priming effects in both ecosystems, independent from substrate C:N. Positive priming prevailed above the treeline in high altitudes and in mineral soils in high latitudes, where consequently climate change-induced treeline shifts and deeper rooting plants may enhance SOM-mineralisation and soil C emissions. However, such C loss may be compensated by negative priming, which dominated in the other soil types and was of larger magnitude than positive priming. In line with other studies, these results indicate a consistent mechanism linking decreased SOM-mineralisation (negative priming) to increased microbial substrate utilisation, suggesting preferential substrate use as a potential tool to support soil C storage. [ABSTRACT FROM AUTHOR]

Fire regimes and herbivore densities have in parts of the boreal been heavily impacted by human activities with consequences for vegetation, forage supply and ungulate use. In this study, we evaluate the effects of natural wildfires and prescribed burning on moose forage availability, use, and browsing in northern Sweden. We studied three wildfire locations, adjacent controls, and five prescribed burns 12 years after fire. To compare forage availability and browsing levels we combined related variables with Principal Component Analyses and used the first axis in a regression analysis. The wildfire locations had a different composition of forage compared with the controls with less forage in the field layer and more woody browse and deciduous saplings. Forage availability was best explained by the occurrence of fire and differed between wildfire and the unburned control areas. Browsing levels were higher in wildfire locations compared with unburned controls. Prescribed burns varied substantially and could resemble both unburnt control and wildfire in terms of forage availability and browsing. We conclude that prescribed burning as a restoration action potentially can improve forage availability, particularly of deciduous species, if they mimic the impact of wildfires in terms of larger area burned and high fire severity. [ABSTRACT FROM AUTHOR]

This paper argues that Sámi reindeer pastoralism in Sweden is highly stressed during the critical snow cover periods due to large-scale human interventions, especially forestry, and that these have over time significantly worsened the ecological conditions for natural grazing-based responses to changing snow conditions caused by climate change. Informed by a literature review, the paper conceptualises two, overlapping ecological dynamics that shape the availability of lichen as key forage resources within a Sámi pastoral landscape perspective: the grazing dynamics of reindeer during snow cover periods as determined by climatic stochasticity, and the more predictable vegetation dynamics of lichen habitat formation, growth and sustenance based on structured forestry practices. This could help articulate an intervention ecology that pursues sustainable ecological conditions for natural grazing-based Sámi reindeer pastoralism, along with other goals. As such alternatives are likely to face political resistance, the article discusses the implications of its findings within a science–politics interface. [ABSTRACT FROM AUTHOR]

Since the early nineteenth century, forest landscapes and socio-economic contexts have significantly changed in northernmost Sweden. These processes include agrarian colonisation, the Christianisation of the indigenous Sámi people, and the transfer of land tenure. We aim to analyse how Sámi religious practice manifested itself in a time of dramatic social transition in northern Sweden by focussing on the life and religious practice of a Sámi woman known as the shaman Guoksik-gummo, 'the Lady of the Siberian Jay.' We analyse a range of historical records and one specific sacred tree related to her to understand this period better and illuminate the changes in land-use religious practice and landscape transformation between the early nineteenth century and the early twenty-first century. We conclude that better documentation, more vital protection by law of Sámi cultural traces in northern landscapes, and better consultation with Sámi are needed in the future. [ABSTRACT FROM AUTHOR]

Across the boreal biome, clear‐cutting of old, previously non clear‐cut forests with high naturalness followed by tree planting or seeding is a major land use change. However, how much previously uncut forest has been converted to plantations remains unquantified. We combine Swedish national databases on clear‐cuts and forest inventories to show that at least 19% of all clear‐cuts since 2003 have occurred in old forests that were most likely not previously cut and planted or seeded. Old forests have been cut and lost at a steady rate of ∼1.4% per year for the same period, and at this rate they will disappear by the 2070s. There is further evidence that this type of unreported forest conversion is occurring across much of the world's boreal forest. Plain Language Summary: Natural ecosystems that have seen limited direct human impact are threatened by land use change. Land use in boreal regions is dominated by forestry but the rate at which older boreal forests with high naturalness are converted to planted and seeded forest for wood harvesting remains unquantified. We used a set of uniquely detailed databases and maps to estimate the share of old and previously uncut forests in annual harvests and at which rate these forests are being lost in Sweden. The results indicate that about a fifth of cut forest area since 2003 was cuts of previously uncut old forest. If the present trajectory continues, the remaining unprotected old and previously uncut forests will be converted to planted and seeded managed forests within a couple of decades. Given the slow development and distinct structural and ecological characteristics of these old forests, this loss may define the landscape for centuries. Key Points: Old previously uncut boreal forests are cut at a fast rateThe conversions of natural boreal forests to planted and seeded managed forests are not monitoredThe slow growth of boreal forests implies that the present natural to managed forest conversions may define the landscapes for centuries [ABSTRACT FROM AUTHOR]

Approximately 40% of earth's carbon (C) stored in land vegetation and soil is within the boreal region. This large C pool is subjected to substantial removals and transformations during periodic wildfire. Fire‐altered C, commonly known as pyrogenic carbon (PyC), plays a significant role in forest ecosystem functioning and composes a considerable fraction of C transport to limnic and oceanic sediments. While PyC stores are beginning to be quantified globally, knowledge is lacking regarding the drivers of their production and transport across ecosystems. This study used the chemo‐thermal oxidation at 375°C (CTO‐375) method to isolate a particularly refractory subset of PyC compounds, here called black carbon (BC), finding an average increase of 11.6 g BC m−2 at 1 year postfire in 50 separate wildfires occurring in Sweden during 2018. These increases could not be linked to proposed drivers, however BC storage in 50 additional nearby unburnt soils related strongly to soil mass while its proportion of the larger C pool related negatively to soil C:N. Fire approximately doubled BC stocks in the mineral layer but had no significant effect on BC in the organic layer where it was likely produced. Suppressed decomposition rates and low heating during fire in mineral subsoil relative to upper layers suggests potential removals of the doubled mineral layer BC are more likely transported out of the soil system than degraded in situ. Therefore, mineral soils are suggested to be an important storage pool for BC that can buffer short‐term (production in fire) and long‐term (cross‐ecosystem transport) BC cycling. Plain Language Summary: Northern forests store a substantial fraction of the earth's carbon. Monitoring these stores is crucial for understanding forest contribution to atmospheric CO2 levels as well as their health under a changing climate. Wildfire has a major impact on shifting forest carbon stores to the atmosphere via emissions due to burning. However, a fraction of burnt material is left in forests as pyrogenic carbon (PyC). PyC is known to have positive effects on forest health and is thought to have increased resistance to being emitted to the atmosphere. Little predictive knowledge exists regarding how much PyC is produced during wildfire and even less is known about how that PyC is transported into the larger environment. This study utilized a large area wildfire field sampling campaign to study PyC in northern forests. It found that the amount of PyC produced could not be predicted by forest or fire properties. However, it was discovered that a large fraction of wildfire produced PyC quickly moves downward in the soil profile where it will be protected from decomposition and future burning. This novel finding suggests that one of the first steps of PyC transport is also a protective one, conserving carbon and mitigating climate change. Key Points: Fifty separate summer 2018 boreal wildfires were sampled 1 year postfire for black carbon (BC) across the 57–67° latitudinal range within SwedenFire had an insignificant effect on total BC stocks in organic layers but approximately doubled stocks in mineral layersMineral soil may function to buffer short‐term (production) and long‐term (cross‐ecosystem transport) BC processes [ABSTRACT FROM AUTHOR]

Projected changes in precipitation regimes can greatly impact soil biota, which in turn alters key ecosystem functions. In moss-dominated ecosystems, the bryosphere (i.e., the ground moss layer including live and senesced moss) plays a key role in carbon and nutrient cycling, and it hosts high abundances of microfauna (i.e., nematodes and tardigrades) and mesofauna (i.e., mites and springtails). However, we know very little about how bryosphere fauna responds to precipitation, and whether this response changes across environmental gradients. Here, we used a mesocosm experiment to study the effect of volume and frequency of precipitation on the abundance and community composition of functional groups of bryosphere fauna. Hylocomium splendens bryospheres were sampled from a long-term post-fire boreal forest chronosequence in northern Sweden which varies greatly in environmental conditions. We found that reduced precipitation promoted the abundance of total microfauna and of total mesofauna, but impaired predaceous/omnivorous nematodes, and springtails. Generally, bryosphere fauna responded more strongly to precipitation volume than to precipitation frequency. For some faunal functional groups, the effects of precipitation frequency were stronger at reduced precipitation volumes. Context-dependency effects were found for microfauna only: microfauna was more sensitive to precipitation in late-successional forests (i.e., those with lower productivity and soil nutrient availability) than in earlier-successional forests. Our results also suggest that drought-induced changes in trophic interactions and food resources in the bryosphere may increase faunal abundance. Consequently, drier bryospheres that may result from climate change could promote carbon and nutrient turnover from fauna activity, especially in older, less productive forests. [ABSTRACT FROM AUTHOR]

Simple Summary: Native wild boar populations are expanding across Europe, causing concern due to their significant soil disturbances and considerable impact on ecosystems. However, little is known about how wild boar activities affect other organisms. This study investigated the effects of wild boars on the abundance of fungal sporocarps and their respective fungal guilds (i.e., symbiotic, saprophytic and pathogenic) in boreal forests in Sweden. We selected 11 forested sites in central Sweden: six with and five without the presence of wild boar. We determined the presence or absence of wild boar and rooting intensity at each site. Simultaneously, we investigated the abundance of fungal sporocarps and their fungal guilds. We found that the presence of wild boar and rooting intensity were associated with the abundance of fungal sporocarps. Wild boar rooting was positively correlated with saprotrophic fungi and negatively with symbiotic fungi. Pathogenic fungi were more abundant in plots with no rooting but in the presence of wild boar. We conclude that wild boar represents a recurrent disturbance agent and, based on sporocarp abundance, could eventually affect entire fungal populations. Native wild boar (Sus scrofa) populations are expanding across Europe. This is cause for concern in some areas where overabundant populations impact natural ecosystems and adjacent agronomic systems. To better manage the potential for impacts, managers require more information about how the species may affect other organisms. For example, information regarding the effect of wild boar on soil fungi for management application is lacking. Soil fungi play a fundamental role in ecosystems, driving essential ecological functions; acting as mycorrhizal symbionts, sustaining plant nutrition and providing defense; as saprotrophs, regulating the organic matter decomposition; or as plant pathogens, regulating plant fitness and survival. During autumn (Sep–Nov) 2018, we investigated the effects of wild boar (presence/absence and rooting intensity) on the abundance (number of individuals) of fungal sporocarps and their functional guilds (symbiotic, saprotrophic and pathogenic). We selected eleven forested sites (400–500 × 150–200 m) in central Sweden; six with and five without the presence of wild boar. Within each forest, we selected one transect (200 m long), and five plots (2 × 2 m each) for sites without wild boar, and ten plots for sites with boars (five within and five outside wild boar disturbances), to determine the relationship between the intensity of rooting and the abundance of sporocarps for three fungal guilds. We found that the presence of wild boar and rooting intensity were associated with the abundance of sporocarps. Interestingly, this relationship varied depending on the fungal guild analyzed, where wild boar rooting had a positive correlation with saprophytic sporocarps and a negative correlation with symbiotic sporocarps. Pathogenic fungi, in turn, were more abundant in undisturbed plots (no rooting) but located in areas with the presence of wild boar. Our results indicate that wild boar activities can potentially regulate the abundance of fungal sporocarps, with different impacts on fungal guilds. Therefore, wild boar can affect many essential ecosystem functions driven by soil fungi in boreal forests, such as positive effects on energy rotation and in creating mineral availability to plants, which could lead to increased diversity of plants in boreal forests. [ABSTRACT FROM AUTHOR]

Strong anthropogenic pressures on global forests necessitate that managed forests be evaluated as habitat for biodiversity. The complex pattern of habitat types created in forestry systems is ideal for analyses through the theoretical framework of alpha (local), gamma (total) and beta (compositional) diversity. Here I use saproxylic beetles, a species-rich threatened group, to compare four Norway spruce-dominated habitats representative of the boreal forest landscape of northern Europe: unmanaged semi-natural stands, nature reserves, unthinned middle-aged production stands and commercially thinned production stands. The beetles (in total 38 085 individuals of 312 species), including red-listed ones and three feeding guilds (wood consumers, fungivores and predators) were studied in 53 stands in central-southern Sweden, in two regions with differing amounts of conservation forest. Alpha diversity of saproxylic, but not red-listed, beetles was higher in the thinned stands than in the semi-natural stands, and did not differ for the other forest types. Beta diversity of saproxylic beetles was higher in unmanaged semi-natural stands than in the other forest types, but species composition did not differ noticeably. Furthermore, red-listed saproxylic beetles had higher gamma diversity in unmanaged semi-natural stands in the region with more conservation forest, but not in the one with less such forest. The local factors dead wood volume and dead wood diversity did not influence alpha diversity of beetles, but increasing canopy openness had a minor negative influence on saproxylic and red-listed beetles. While the local scale (alpha diversity) indicates the potential for managed forests to house many saproxylic beetle species associated with spruce forests in this boreal landscape, the larger scales (beta and gamma diversity) indicate the value of unmanaged forests for the conservation of the entire saproxylic beetle fauna. These results show the importance of analyses at multiple levels of diversity (alpha, beta, gamma) for identifying patterns relevant to conservation. [ABSTRACT FROM AUTHOR]

Context: Both climatic extremes and land-use change constitute severe threats to biodiversity, but their interactive effects remain poorly understood. In forest ecosystems, the effects of climatic extremes can be exacerbated at forest edges. Objectives: We explored the hypothesis that an extreme summer drought reduced the richness and coverage of old-growth forest species, particularly in forest patches with high edge exposure. Methods: Using a high-resolution spatially explicit precipitation dataset, we could detect variability in drought intensity during the summer drought of 2018. We selected 60 old-growth boreal forest patches in central Sweden that differed in their level of drought intensity and amount of edge exposure. The year after the drought, we surveyed red-listed and old-growth forest indicator species of vascular plants, lichens and bryophytes. We assessed if species richness, composition, and coverage were related to drought intensity, edge exposure, and their interaction. Results: Species richness was negatively related to drought intensity in forest patches with a high edge exposure, but not in patches with less edge exposure. Patterns differed among organism groups and were strongest for cyanolichens, epiphytes associated with high-pH bark, and species occurring on convex substrates such as trees and logs. Conclusions: Our results show that the effects of an extreme climatic event on forest species can vary strongly across a landscape. Edge exposed old-growth forest patches are more at risk under extreme climatic events than those in continuous forests. This suggest that maintaining buffer zones around forest patches with high conservation values should be an important conservation measure. [ABSTRACT FROM AUTHOR]

Ditch cleaning (DC) is increasingly applied to facilitate forest regeneration following clear-cutting in Fennoscandinavia. However, its impact on the ecosystem carbon and greenhouse gas (GHG) balances is poorly understood. We conducted chamber measurements to assess the initial DC effects on carbon dioxide (CO2) and methane (CH4) fluxes in a recent forest clear-cut on wet mineral soil in boreal Sweden. Measurements were conducted in two adjacent areas over two pre-treatments (2018/19) and two years (2020/21) after conducting DC in one area. We further assessed the spatial variation of fluxes at three distances (4, 20, 40 m) from ditches. We found that DC lowered the water table level by 12 ± 2 cm (mean ± standard error) and topsoil moisture by 0.12 ± 0.01 m3 m−3. DC had a limited initial effect on the net CO2 exchange and its component fluxes. CH4 emissions were low during the dry pre-treatment years but increased particularly in the control area during the wet years of 2020/21. Distance to ditch had no consistent effects on CO2 and CH4 fluxes. Model extrapolations suggest that annual carbon emissions decreased over the four years from 6.7 ± 1.4 to 1.6 ± 1.6 t-C ha−1 year−1, without treatment differences. Annual CH4 emissions contributed <2.5% to the carbon balance but constituted 39% of the GHG balance in the control area during 2021. Overall, our study suggests that DC modified the internal carbon cycling but without significant impact on the carbon and GHG balances. [ABSTRACT FROM AUTHOR]

Thin, hair‐like lichens (Alectoria, Bryoria, Usnea) form conspicuous epiphyte communities across the boreal biome. These poikilohydric organisms provide important ecosystem functions and are useful indicators of global change. We analyse how environmental drivers influence changes in occurrence and length of these lichens on Norway spruce (Picea abies) over 10 years in managed forests in Sweden using data from >6000 trees. Alectoria and Usnea showed strong declines in southern‐central regions, whereas Bryoria declined in northern regions. Overall, relative loss rates across the country ranged from 1.7% per year in Alectoria to 0.5% in Bryoria. These losses contrasted with increased length of Bryoria and Usnea in some regions. Occurrence trajectories (extinction, colonization, presence, absence) on remeasured trees correlated best with temperature, rain, nitrogen deposition, and stand age in multinomial logistic regression models. Our analysis strongly suggests that industrial forestry, in combination with nitrogen, is the main driver of lichen declines. Logging of forests with long continuity of tree cover, short rotation cycles, substrate limitation and low light in dense forests are harmful for lichens. Nitrogen deposition has decreased but is apparently still sufficiently high to prevent recovery. Warming correlated with occurrence trajectories of Alectoria and Bryoria, likely by altering hydration regimes and increasing respiration during autumn/winter. The large‐scale lichen decline on an important host has cascading effects on biodiversity and function of boreal forest canopies. Forest management must apply a broad spectrum of methods, including uneven‐aged continuous cover forestry and retention of large patches, to secure the ecosystem functions of these important canopy components under future climates. Our findings highlight interactions among drivers of lichen decline (forestry, nitrogen, climate), functional traits (dispersal, lichen colour, sensitivity to nitrogen, water storage), and population processes (extinction/colonization). [ABSTRACT FROM AUTHOR]

At the mid-twentieth century the pace of the transformation of the Swedish forest increased. New methods; large-scale clearcutting, mechanization of logging and planting of seedlings were developed. Chemicals were used to control insects and unwanted tree species. The aims of this study were to elucidate the timing, chain of events and the spatial extent of the large-scale spraying of phenoxy acids in Swedish forests and the drivers for this practice. More than 700 000 hectares of productive forest land was sprayed and the main driving force was a strong will to transform the forest into high-yield coniferous forest plantations. We conclude that; (1) the use of herbicides in forestry in Sweden was done on a very large scale in the period 1948–1984, (2) the ecosystem legacy of herbicide spraying must be investigated and (3) a homogenous cadre of like-minded professionals working across commercial companies, state agencies and universities is dangerous. [ABSTRACT FROM AUTHOR]

Wildfire is the main disturbance in most boreal forests. In the prolonged absence of wildfire, ecosystem retrogression occurs, which is characterized by reduced productivity, plant biomass and belowground process rates. Previous evidence suggests that phosphorus (P) decreases during retrogression, but the mechanisms involved remain poorly understood. Here we use 1-D 31P and 2-D, 1H-31P NMR to characterize changes in humus P composition across a 5000 year post-fire chronosequence in northern Sweden, to understand why P availability declines during long term fire absence. Against expectations, humus P composition varied only modestly with increasing time since fire. Using a method to back-calculate the in situ soil organic P speciation, we found that it was dominated by biologically active compounds such as RNA (41%), phospholipids (28%) and DNA (22%). The concentration of DNA and pyrophosphate was 19% and 29% lower, respectively, on infrequently burnt than recently burnt islands, and the concentration of DNA, phospholipids and nucleotides was positively correlated with net primary productivity (NPP). Given the lack of evidence for the accumulation of "recalcitrant" P or a geochemical P sink, reductions in P availability during retrogression may be associated with impaired P cycling through slower decomposition rates, and increasing humus depth separating surface humus from P-rich mineral soil. Our findings align with observed negative relationships between NPP and organic P concentration across other chronosequences. They also suggest that changing fire regimes in the boreal zone could indirectly affect the P cycle through changes in NPP and soil microflora rather than through changes in humus P composition. [ABSTRACT FROM AUTHOR]

While the carbon (C) content of temperate and boreal forest soils is relatively well studied, much less is known about the ratios of C, nitrogen (N), and phosphorus (P) of the soil organic matter, as well as the abiotic and biotic factors that shape them. Therefore, the aim of this study was to explore carbon, nitrogen, and organic phosphorus (OP) contents and element ratios in temperate and boreal forest soils and their relationships with climate, dominant tree species, and soil texture. For this purpose, we studied 309 forest soils located all over Sweden between 56 and 68 ∘ N. The soils are a representative subsample of Swedish forest soils with a stand age >60 years that were sampled for the Swedish Forest Soil Inventory. We found that the N stock of the organic layer increased by a factor of 7.5 from - 2.0 to 7.5 ∘C mean annual temperature (MAT), which is almost twice as much as the increase in the organic layer stock along the MAT gradient. The increase in the N stock went along with an increase in the N:P ratio of the organic layer by a factor of 2.1 from - 2.0 to 7.5 ∘C MAT (R2=0.36 , p<0.001). Forests dominated by pine had higher C:N ratios in the organic layer and mineral soil down to a depth of 65 cm than forests dominated by spruce. Further, also the C:P ratio was increased in the pine-dominated forests compared to forests dominated by other tree species in the organic layer, while the C:OP ratio in the mineral soil was not elevated in pine forests. C, N, and OP contents in the mineral soil were higher in fine-textured soils than in coarse-textured soils by a factor of 2.3, 3.5, and 4.6, respectively. Thus, the effect of texture was stronger on OP than on N and C likely because OP adsorbs very rigidly to mineral surfaces. Further, we found that the P and K concentrations of the organic layer were inversely related to the organic layer stock, while the N:P ratio was positively related to the organic layer stock. Taken together, the results show that the N:P ratio of the organic layer was most strongly related to MAT. Further, the C:N ratio was most strongly related to dominant tree species even in the mineral subsoil. In contrast, the C:P ratio was only affected by dominant tree species in the organic layer, but the C:OP ratio in the mineral soil was hardly affected by tree species due to the strong effect of soil texture on the OP concentration. [ABSTRACT FROM AUTHOR]

Fennoscandian boreal forests are managed intensively for wood production, which has had a negative impact on biodiversity. The harvesting pressure on the forests is likely to increase in the future because wood is seen as an important resource in a bio-based economy. Thus, there is an urgent need to better describe the trade-offs between wood production and biodiversity, and to understand how these trade-offs can be alleviated by adapting forest management. Encompassing a broad range of biodiversity aspects, we studied how forest management can increase biodiversity indicators while maintaining or increasing current harvest levels. We found that there is considerable leeway for forestry to pursue multiple objectives simultaneously in Fennoscandian forest landscapes. We show that it is possible to both increase harvests and structural elements of importance for biodiversity compared to present levels in a forest landscape that is representative of conditions in boreal forests in northern Sweden. Achieving this requires a variation in management strategies at the landscape level, and an adaptation of management practices to explicitly consider and implement multiple objectives in the planning process. [ABSTRACT FROM AUTHOR]

Forest management has a potential to alter below‐ground carbon storage. However, the underlying mechanisms, and the relative importance of carbon input and decomposition in regulation of soil carbon dynamics are poorly understood.We examined whether interactive effects of forest fertilization and thinning on carbon stocks in the topsoil of boreal forests were linked to changes in fungal community composition, biomass and enzyme activities, in a long‐term fertilization and thinning experiment distributed across 29 Pinus sylvestris forests along a 1,300 km latitudinal transect in Sweden.Nitrogen fertilization increased fungal biomass, particularly towards the north and mainly by promoting root‐associated Ascomycetes, but the response was moderated by thinning. Fungal biomass correlated positively with carbon stocks in the organic topsoil. However, ectomycorrhizal Cortinarius species were reduced in abundance by fertilization and correlated negatively with carbon stocks.Plausibly, increased soil carbon stocks after fertilization are linked to increased input of carbon in the form of root‐associated mycelium combined with the loss of ectomycorrhizal decomposers within the genus Cortinarius. These fungal responses to fertilization may mediate a natural climate solution by promoting carbon sequestration in the organic topsoil, but the effect of fertilization may also be undesired from a biodiversity perspective. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

While the carbon (C) content of temperate and boreal forest soils is relatively well studied, much less is known about the ratios of C, nitrogen (N), and phosphorus (P) of the soil organic matter, and the abiotic and biotic factors that shape them. Therefore, the aim of this study was to explore carbon, nitrogen, and organic phosphorus (OP) contents and element ratios in temperate and boreal forest soils and their relationships with climate, dominant tree species, and soil texture. For this purpose, we studied 309 forest soils with a stand age >60 years located all over Sweden between 56°N and 68°N. The soils are a representative subsample of Swedish forest soils with a stand age >60 years that were sampled for the Swedish Forest Soil Inventory. We found that the N stock of the organic layer increased by a factor of 7.5 from -2°C to 7.5°C mean annual temperature (MAT), it increased almost twice as much as the organic layer stock along the MAT gradient. The increase in the N stock went along with an increase in the N:P ratio of the organic layer by a factor of 2.1 from -2°C to 7.5°C MAT (R²=0.36, p<0.001). Forests dominated by pine had higher C:N ratios in the litter layer and mineral soil down to a depth of 65 cm than forests dominated by other tree species. Further, also the C:P ratio was increased in the pine-dominated forests compared to forests dominated by other tree species in the organic layer, but the C:OP ratio in the mineral soil was not elevated in pine forests. C, N and OP contents in the mineral soil were higher in fine-textured soils than in coarse-textured soils by a factor of 2.3, 3.5, and 4.6, respectively. Thus, the effect of texture was stronger on OP than on N and C, likely because OP adsorbs very rigidly to mineral surfaces. Further, we found, that the P and K concentrations of the organic layer were inversely related with the organic layer stock. The C and N concentrations of the mineral soil were best predicted by the combination of MAT, texture, and tree species, whereas the OP concentration was best predicted by the combination of MAT, texture and the P concentration of the parent material in the mineral soil. In the organic layer, the P concentration was best predicted by the organic layer stock. Taken together, the results show that the N:P ratio of the organic layer was most strongly related to MAT. Further, the C:N ratio was most strongly related to dominant tree species, even in the mineral subsoil. In contrast, the C:P ratio was only affected by dominant tree species in the organic layer, but the C:OP ratio in the mineral soil was hardly affected by tree species due to the strong effect of soil texture on the OP concentration. [ABSTRACT FROM AUTHOR]

Shielings are the historically known form of transhumance in Scandinavia, where livestock were moved from the farmstead to sites in the outlands for summer grazing. Pollen analysis has provided a valuable insight into the history of shielings. This paper presents a vegetation reconstruction and archaeological survey from the shieling Kårebolssätern in northern Värmland, western Sweden, a renovated shieling that is still operating today. The first evidence of human activities in the area near Kårebolssätern are Hordeum- and Cannabis-type pollen grains occurring from ca. 100 bc. Further signs of human impact are charcoal and sporadic occurrences of apophyte pollen from ca. ad 250 and pollen indicating opening of the canopy ca. ad 570, probably a result of modification of the forest for grazing. A decrease in land use is seen between ad 1000 and 1250, possibly in response to a shift in emphasis towards large scale commodity production in the outlands. Emphasis on bloomery iron production and pitfall hunting may have caused a shift from agrarian shieling activity. The clearest changes in the pollen assemblage indicating grazing and cultivation occur from the mid-thirteenth century, coinciding with wetter climate at the beginning of the Little Ice Age. The earliest occurrences of anthropochores in the record predate those of other shieling sites in Sweden. The pollen analysis reveals evidence of land use that predates the results of the archaeological survey. The study highlights how pollen analysis can reveal vegetation changes where early archaeological remains are obscure. [ABSTRACT FROM AUTHOR]

Greatly simplified ecosystems are often neglected for biodiversity studies. However, these simplified systems dominate in many regions of the world, and a lack of understanding of what shapes species occurrence in these systems can have consequences for biodiversity and ecosystem services at a massive scale. In Fennoscandia, ~90% of the boreal forest (~21Mha) is structurally simplified with little knowledge of how forest structural elements shape the occurrence and diversity of for example epiphytic lichens in these managed forests. One form of structural simplification is the reduction of the number and frequency of different tree species. As many lichen species have host tree preferences, it is particularly likely that this simplification has a huge effect on the lichen community in managed forests. In a 40–70 years old boreal forest in Sweden, we therefore related the occurrence and richness of all observed epiphytic lichens to the host tree species and beta and gamma lichen diversity at the forest stand level to the stand's tree species composition and stem diameter. Picea abies hosted the highest lichen richness followed by Pinus sylvestris, Quercus robur, Alnus glutinosa, Betula spp., and Populus tremula. However, P. tremula hosted twice as many uncommon species as any of the other tree species. Stand level beta and gamma diversity was twice as high on stands with four compared to one tree species, and was highest when either coniferous or deciduous trees made up 40–50% of the trees. The stem diameter was positively related to lichen richness at the tree and stand level, but negatively to beta diversity. For biodiversity, these findings imply that leaving a few trees of a different species during forest thinning is unlikely as effective as combining life-boat trees for endangered species with an even tree species mixture. [ABSTRACT FROM AUTHOR]

Aim: Global change effects on forest ecosystems are increasingly claimed to be context dependent, indicated by interactions between global and local environmental drivers. Most examples of such context dependencies originate from temperate systems, while limited research comes from the boreal biome. Here we set out to test if interactions between climate warming, nitrogen deposition, land‐use change resulting in increasing forest density, and soil pH drive long‐term changes in understorey vegetation in boreal forests. Location: Sweden. Time period: 1953–2012. Major taxa studied: Vascular plants. Methods: We used long‐term (50 years) National Forest Inventory data on forest understorey vegetation in Sweden to model the combined effects of climate warming, nitrogen deposition, increase in forest density (tree basal area), and soil pH. Results: Our results identify increasing temperature, nitrogen deposition and denser, shadier forest conditions as the main drivers of understorey vegetation changes during this time period. More importantly, we found that these effects varied with local conditions, that is, that the change towards a more nitrophilic understorey vegetation was more pronounced at low than high soil pH. Forest density was an important modulator of nitrogen deposition and temperature increase, with effects generally decreasing with density. Decreased cover of ericaceous dwarf shrubs was driven by both forest density and nitrogen deposition, with a stronger effect at low than at high pH. Main conclusions: Our results highlight that to understand forest ecosystems' response to global change, and to make adequate management decisions to mitigate the effects of global change, we need to understand how changes in local environmental factors (forest density and soil pH) interact with global‐scale drivers (nitrogen deposition and climate warming). Neglecting such interactions will lead to incorrect estimations of effects. In our case, we would for example, have underestimated the eutrophication effects on acid soils, which constitute a considerable part of the boreal biome. [ABSTRACT FROM AUTHOR]

The extreme 2018 hot drought that affected central and northern Europe led to the worst wildfire season in Sweden in over a century. The Ljusdal fire complex, the largest area burnt that year (8995 ha), offered a rare opportunity to quantify the combined impacts of wildfire and post‐fire management on Scandinavian boreal forests. We present chamber measurements of soil CO2 and CH4 fluxes, soil microclimate and nutrient content from five Pinus sylvestris sites for the first growing season after the fire. We analysed the effects of three factors on forest soils: burn severity, salvage‐logging and stand age. None of these caused significant differences in soil CH4 uptake. Soil respiration, however, declined significantly after a high‐severity fire (complete tree mortality) but not after a low‐severity fire (no tree mortality), despite substantial losses of the organic layer. Tree root respiration is thus key in determining post‐fire soil CO2 emissions and may benefit, along with heterotrophic respiration, from the nutrient pulse after a low‐severity fire. Salvage‐logging after a high‐severity fire had no significant effects on soil carbon fluxes, microclimate or nutrient content compared with leaving the dead trees standing, although differences are expected to emerge in the long term. In contrast, the impact of stand age was substantial: a young burnt stand experienced more extreme microclimate, lower soil nutrient supply and significantly lower soil respiration than a mature burnt stand, due to a thinner organic layer and the decade‐long effects of a previous clear‐cut and soil scarification. Disturbance history and burn severity are, therefore, important factors for predicting changes in the boreal forest carbon sink after wildfires. The presented short‐term effects and ongoing monitoring will provide essential information for sustainable management strategies in response to the increasing risk of wildfire. [ABSTRACT FROM AUTHOR]

Biochar soil amendment may provide the forestry sector with a formidable tool to simultaneously sequester carbon (C) in the soil and aboveground by enhancing plant productivity, yet several key uncertainties remain. Crucially, empirical evidence of long‐term effects of biochar management on vegetation and on greenhouse gas emissions in forest ecosystems is scarce. Using a large field experiment in a young managed boreal forest in northern Sweden, we investigated the effects of biochar (applied either on the soil surface or mixed in the soil 8–9 years prior to this study) on supply rates of soil nutrients, on survival and growth of planted Pinus sylvestris, on community composition of the understory vegetation, and on forest floor fluxes of N2O, CH4, and CO2. We found that biochar promoted P. sylvestris survival only when biochar was applied on the soil surface. Conversely, biochar enhanced P. sylvestris growth overall, resulting in a 19% increase in C stored in biomass. Biochar also altered the composition of the understory vegetation, especially when mixed into the soil, and promoted a more resource‐conservative community (i.e., with more ericaceous shrubs and less graminoids and forbs). Meanwhile, supply rates of the main soil nutrients were largely unaffected by biochar. Finally, we found that biochar did not alter overall N2O and CO2 emissions and CH4 uptake from the forest floor. Our findings show that biochar amendment increased the net C input to the system, since, besides directly increasing soil C stocks, biochar enhanced biomass growth without increasing soil C losses. Therefore, our study suggests that biochar could potentially be used for emissions abatement in intensively managed boreal forests. [ABSTRACT FROM AUTHOR]

Forest regeneration by tree planting on harvested sites in the boreal forests of northern Europe is frequently preceded by site preparation to increase survival and growth of the seedlings. We studied whether a small addition of arginine phosphate (AP treatment) at the time of planting would further enhance the seedlings' early performance. Following two growth seasons, we investigated survival and growth of Scots pine (Pinus sylvestris) seedlings on 11 locations between latitudes 61.1°N and 67.1°N in the boreal forest of northern Sweden. The planting positions of seedlings were on capped mounds and bare mineral soil following mechanical site preparation, and in non-prepared soil. We found that seedling survival following site preparation increased with AP treatment. On capped mounds, seedling survival was more variable and appeared more dependent on precipitation during the first month after planting than seedlings positioned in the mineral soil. The positive effect of AP treatment on seedling growth differed between sites and was more pronounced on sites with longer growing seasons. AP treatment had no significant effect on survival of seedlings planted in non-prepared soil, while the positive effect on growth was more pronounced at sites with higher fertility using this planting position. [ABSTRACT FROM AUTHOR]

• Eddy-covariance data 1–4 years after wildfire in managed European boreal forests. • Tracked impacts of fire, salvage-logging and replanting on CO 2 fluxes at 2 sites. • Both forest sites are net carbon sources for first 4 years after wildfire. • More carbon emitted during first 4 years post-fire than during fire itself. • May take over 30 or 40 years until forests reach carbon compensation point. Wildfire is one of the most important disturbances affecting boreal forests. Most previous research on boreal forest fires has occurred in North American forests which have different fire regimes, tree species and are less intensively managed than their Eurasian counterparts. Recent extreme fire years have highlighted the vulnerability of the Nordic boreal forest to climatic shifts that are increasing forest fire frequency and severity. The Ljusdal fire (2018) was one of the largest wildfires in recorded history in Sweden. We established eddy covariance flux towers to track the impacts of this fire on the carbon balance of two Pinus sylvestris sites subject to different fire severities and forest management strategies 1–4 years post-fire. The 'SLM' site was a mature stand that experienced low-severity fire (trees survived) followed by salvage-logging and reseeding, whilst the 'HY' site was 10 years old when it experienced high-severity fire (all trees killed) then was replanted with seedlings. During the study period, both sites were net carbon sources at the annual scale. It took up to 4 years after the fire until the first day of net CO 2 uptake was recorded at each site. We estimated that it will take 13 years (8, 21; mean ± 95 % confidence intervals) after the fire until the sites reach a neutral annual carbon balance. It will take up to 32 years (19, 53) at HY and 46 years (31, 70) at SLM to offset the carbon lost during and after the fire and salvage-logging. In addition, our measurements showed that more carbon was emitted in the first 4 years after the fire compared to the carbon lost from combustion during the fire. Quantifying carbon fluxes during the initial years after fire is therefore crucial for estimating the net impact of wildfire on the carbon budget of boreal forests. [ABSTRACT FROM AUTHOR]

• We investigated the thermal buffering of forest retention practices. • T max was lower in retention forests than in clear-cuts but higher than in old forests. • Basal area and canopy cover were identified as key factors impacting microclimates. • Preserving tree diversity and standing deadwood enhance stands' thermal buffering. • Lying deadwood related negatively to canopy cover, likely lowering thermal buffering. Retention forestry is increasingly adopted as an alternative to clearcutting practices and involves retaining structural and compositional complexity (e.g., living and dead trees) from preharvest to postharvest. Past studies have examined the role of retention forestry in supporting various ecosystem functions and biodiversity, whilst its microclimate buffering capacity has been largely neglected. We investigated the microclimates and the underlying mechanisms of retention forests relative to clearcuts and old forests in a boreal forest landscape in central Sweden. We found that both air temperature and vapour pressure deficit (VPD) differed significantly between the forest types. Old forests consistently exhibited the most buffered forest microclimates, followed by retention forests, while clearcuts displayed the lowest. Basal area and canopy cover were identified as the key determinants influencing air temperature and VPD across the forest types. Retention practices can also impact a stand's microclimates. Specifically, maintaining diverse tree species had the potential to lower the stand's maximum temperature, given its positive association with canopy cover. Large volumes of lying deadwood were found to be negatively correlated with both basal area and canopy cover, likely contributing to increased maximum temperatures. Furthermore, standing deadwood directly lowered the maximum temperature within forest stands. Finally, edge effects were observed in the retention forests, with south-facing edges experiencing significantly higher maximum temperature and VPD compared to north-facing edges and forest interiors. These south-facing edge effects were positively associated with the difference in lying deadwood volumes between forest edges and interiors. Our findings support the positive influence of retention practices on a stand's microclimate buffering, achieved through preserving diverse tree species, standing deadwood, and implementing measures to prevent severe wind-induced tree mortality, particularly in south-facing edges (e.g. creating south-facing buffer zones). Forest managers and policy makers can utilize these results to minimize the climate-change impacts on below-canopy biodiversity and functioning. [ABSTRACT FROM AUTHOR]

• We modelled reforestation of a boreal forest in Sweden for 200-years with LPJ-GUESS. • We studied the impact of forest management on carbon budgets under varied climates. • Forest management showed significant impact on carbon stock and fluxes. • The carbon compensation point was reached 12–16 years post clear-cut. • Net carbon sink of an unmanaged forest may decline by 11–68 % by 2100 under RCP 8.5. Boreal forests are key to global carbon (C) sequestration and storage. However, the potential impacts of climate change on these forests could be profound. Nearly 70 % of the European boreal forests are intensively managed, but our understanding of the combined effects of forest management and climate change on the forest's integral role as a C sink is still limited. In this study, we aim to fill this gap with simulations of the process-based dynamic global vegetation model LPJ-GUESS. We evaluated the effects of four forest management options under two different climate scenarios (RCP 4.5 and RCP 8.5), at a southern boreal forest stand in Sweden. These options were compared against a baseline without clear-cut or management interventions. We found that the projected increase in temperatures (+2 to +4 °C) during the latter part of the 21st century will reduce the net C sink strength, particularly in the unmanaged forest. The standing biomass C for reforestations was projected to be 57–67 % lower in 2100 than in the old forest in 2022. The study also revealed that the C sequestration potential of replanted pine forests may surpass that of 200-years old forests in the far future (2076–2100). The study did not detect statistically significant differences in overall net C exchange between the clear-cut with subsequent reforestation options and the baseline, even though specific reforestation strategies, such as pine plantations, enhanced the overall net C sink by 7–20 % relative to the baseline during 2022–2100. These findings underscore the profound influence of forest management on the net C budget, surpassing that of climate change scenarios alone. By adopting pertinent reforestation strategies, C uptake could be augmented, with concurrently improved forest productivity, resulting in favourable outcomes for the forest's critical role in C sequestration and storage amidst a changing climate. [Display omitted] [ABSTRACT FROM AUTHOR]

P-band synthetic aperture radar (SAR) is sensitive to above-ground biomass (AGB) but retrieval accuracy has been shown to deteriorate in topographic areas. In boreal forest, the signal penetrates through the canopy to interact with the ground producing variations in backscatter depending on ground topography, forest structure, and soil moisture. Tomographic processing of multiple SAR images Tomographic SAR (TomoSAR) provides information about the vertical backscatter distribution. This article evaluates the use of P-band TomoSAR data to improve AGB retrievals from backscattered intensity by suppressing the backscattered signal from the ground. This approach can be used even when the tomographic resolution is insufficient to resolve the vertical backscatter profile. The analysis is based on P-band data from two campaigns: BioSAR-1 (2007) in Remingstorp, southern Sweden, and BioSAR-2 (2008) in Krycklan (KR), northern Sweden. BioSAR airborne data were also processed to correspond as closely as possible to future BIOMASS TomoSAR acquisitions, with BioSAR-2-based results shown. A power law AGB model using volumetric HV polarized backscatter performs best in KR, with training residual root mean-squared error (RMSE) of 30%–36% (27–33 t/ha) for airborne data and 38%–39% for simulated BIOMASS data. Airborne TomoSAR data suggest that both vertical and horizontal tomographic resolution are of importance and that it is possible to greatly reduce AGB retrieval bias when compared with airborne P-band SAR backscatter intensity-based retrievals. A lack of significant ground slopes in Remningstorp reduces the benefit of using TomoSAR data which performs similar to retrievals based solely on P-band SAR backscatter intensity. [ABSTRACT FROM AUTHOR]

Stable nitrogen isotope ratios (δ15N) of plants and soil have been used to study changes in the N cycle during ecosystem succession and retrogression. However, little is known about how δ15N of soil mineral N and dissolved organic N (DON) change during retrogression, despite their potential to inform on processes contributing to N loss. Here, we examined the δ15N of NH4+ and DON together with δ15N of the dominant plant species and bulk soil across a 5,000‐year‐old retrogressive chronosequence of forested islands in northern Sweden. The δ15N of bulk soil N, NH4+ and DON did not change greatly during retrogression, suggesting that there are no major losses of N from the system. The δ15N of NH4+ and DON was significantly correlated with that of bulk soil N across islands, indicating that bulk soil N is an important determinant of the δ15N of dissolved soil N. The δ15N of DON was significantly higher than those of NH4+ and bulk soil N, probably because of the inclusion of microbial N to the DON fraction. Despite the lack of changes in δ15N of soil N as retrogression proceeded, the δ15N of most plant species increased. These results suggest that despite the relative importance of the three underlying mechanisms involved is unclear, the N resources of plants change in response to retrogression: they have an increasing reliance on DON, a decreasing dependence on N transferred from the mycorrhizal fungi and reduced reliance on N from surface soil layers as retrogression proceeds. [ABSTRACT FROM AUTHOR]

In arctic and boreal ecosystems, ground bryophytes play an important role in regulating carbon (C) exchange between vast belowground C stores and the atmosphere. Climate is changing particularly fast in these high‐latitude regions, but it is unclear how altered precipitation regimes will affect C dynamics in the bryosphere (i.e. the ground moss layer including senesced moss, litter and associated biota) and the closely associated upper humus layer, and how these effects will vary across contrasting environmental conditions. Here, we set up a greenhouse experiment in which mesocosms were assembled containing samples of the bryosphere, dominated by the feather moss Hylocomium splendens, and the upper humus layer, that were collected from across a boreal forest chronosequence in northern Sweden which varies strongly in nutrient availability, productivity and soil biota. We tested the effect of variation in precipitation volume and frequency on CO2 exchange and dissolved organic carbon (DOC) export, and on moss growth. As expected, reduced precipitation volume and frequency lowered net CO2 efflux, DOC export and moss growth. However, by regulating moisture, the lower bryosphere and humus layers often mediated how precipitation volume and frequency interacted to drive C dynamics. For example, less frequent precipitation reduced moss growth only when precipitation volume was low. When volume was high, high moisture content of the humus layer helped avoid moss desiccation. Variation in precipitation regime affected C cycling consistently in samples collected across the chronosequence, despite large environmental variation along the sequence. This suggests that the bryosphere exerts a strong buffering effect on environmental variation at the forest floor, which leads to similar responses of C cycling to external perturbations across highly contrasting ecosystems. As such, our study indicates that projected increases in droughts and ground evapotranspiration in high‐latitude regions resulting from climate change will consistently reduce C losses from moss‐dominated ecosystems. [ABSTRACT FROM AUTHOR]

In high‐latitude ecosystems bryophytes are important drivers of ecosystem functions. Alterations in abundance of mosses due to global change may thus strongly influence carbon (C) and nitrogen (N) cycling and hence cause feedback on climate. The effects of mosses on soil microbial activity are, however, still poorly understood. Our study aims at elucidating how and by which mechanisms bryophytes influence microbial decomposition processes of soil organic matter and thus soil nutrient availability.We present results from a field experiment in a subarctic birch forest in northern Sweden, where we partly removed the moss cover and replaced it with an artificial soil cover for simulating moss effects on soil temperature and moisture. We combined this with a fertilization experiment with 15N‐labelled N for analysing the effects of moss N sequestration on soil processes.Our results demonstrate the capacity of mosses to reduce soil N availability and retard N cycling. The comparison with artificial soil cover plots suggests that the effect of mosses on N cycling is linked to the thermal insulation capacity of mosses causing low average soil temperature in summer and strongly reduced soil temperature fluctuations, the latter also leading to a decreased frequency of freeze‐thaw events in autumn and spring. Our results also showed, however, that the negative temperature effect of mosses on soil microbial activity was in part compensated by stimulatory effects of the moss layer, possibly linked to leaching of labile substrates from the moss. Furthermore, our results revealed that bryophytes efficiently sequester added N from wet deposition and thus prevent effects of increased atmospheric N deposition on soil N availability and soil processes.Synthesis. Our study emphasizes the important role of mosses in carbon and nutrient cycling in high‐latitude ecosystems and the potential strong impacts of reductions in moss abundance on microbial decomposition processes and nutrient availability in subarctic and boreal forests. [ABSTRACT FROM AUTHOR]

It is well established that nutrient addition influences ecosystem features such as productivity, carbon storage, soil acidification and biodiversity. Less studied are long-term effects of sustained fertiliser application on forest soil characteristics and nutrient supplies, and especially direct and indirect mechanisms underlying changes. We investigated effects of 3 decades versus 1 decade of optimised fertiliser application on soil properties and nutrient supplies in a 30-year-old nutrient optimisation experiment in a Norway spruce plantation in northern Sweden. We tested for direct and indirect effects of fertiliser use through structural equation models and correlations among tree and soil variables. Results showed that soil characteristics, especially organic carbon and nutrient concentrations, were significantly affected by 10- and 30-year fertiliser application. Soil C/N was similar for the short-term versus controls, but decreased for the long-term versus short-term treatment. Although not explicitly measured, it was clear from our analyses and earlier studies at the site that litter accumulation played a key role in explaining these changes in soil properties, while foliar stoichiometry data suggest long-term effects of litter quality. Nutrient supply rates increased more after 30 than 10 years of fertiliser application. Summarized, we showed that the interplay of direct and indirect effects can yield nonlinear patterns over time, as exemplified by soil C/N. Furthermore, we conclude that lagged, indirect effects of fertilisation through altered litter quantity and quality dominate changes in soil characteristics in this forest. These soil characteristics have further relevance to nutrient availability, suggesting that nutrient optimisation can influence soil fertility also indirectly. [ABSTRACT FROM AUTHOR]

Residues from forest harvesting operations may be utilized as a renewable energy source. However, the sustainability of this practice has been questioned due to the losses of nutrients and exchangeable base cations, which may impair the forest's carbon sequestration capacity and lead to soil acidification. We report the 18 year response of biomass growth, soil carbon stock and soil chemistry to whole‐tree harvest at thinning and associated compensatory measures in a Pinus sylvestris forest in northern Sweden. The whole‐tree harvest at thinning was combined with nutrient additions to compensate for the nutrient loss caused by extracting the residues. Four main treatments, stem‐only thinning, whole‐tree thinning, whole‐tree thinning with one‐time nitrogen fertilization and whole‐tree thinning with repeated nitrogen fertilization every third year were applied, with plots split for wood‐ash treatment. Eighteen years after the treatments, whole‐tree thinning that had removed 3.0 ± 0.2 Mg C/ha in residues had no effect on forest growth, soil carbon and nitrogen stocks or soil chemistry. Both nitrogen fertilization regimes increased biomass growth, but neither one resulted in a significant increase in soil carbon stock. Wood‐ash addition increased soil pH and exchangeable base cations, but did not affect carbon stock in biomass or soil. Our long‐term data suggest that utilizing harvesting residues for biofuel feedstocks is appropriate in this type of forest. Hence, any nitrogen and wood‐ash additions appear unnecessary as compensatory measures for the removal of harvesting residues, but nitrogen can be applied to increase forest growth following thinning. [ABSTRACT FROM AUTHOR]

Herbivores can exert major controls over biogeochemical cycling. As invertebrates are highly sensitive to temperature shifts (ectothermal), the abundances of insects in high‐latitude systems, where climate warming is rapid, is expected to increase. In subarctic mountain birch forests, research has focussed on geometrid moth outbreaks, while the contribution of background insect herbivory (BIH) to elemental cycling is poorly constrained. In northern Sweden, we estimated BIH along 9 elevational gradients distributed across a gradient in regional elevation, temperature, and precipitation to allow evaluation of consistency in local versus regional variation. We converted foliar loss via BIH to fluxes of C, nitrogen (N), and phosphorus (P) from the birch canopy to the soil to compare with other relevant soil inputs of the same elements and assessed different abiotic and biotic drivers of the observed variability. We found that leaf area loss due to BIH was ~1.6% on average. This is comparable to estimates from tundra, but considerably lower than ecosystems at lower latitudes. The C, N, and P fluxes from canopy to soil associated with BIH were 1–2 orders of magnitude lower than the soil input from senesced litter and external nutrient sources such as biological N fixation, atmospheric deposition of N, and P weathering estimated from the literature. Despite the minor contribution to overall elemental cycling in subarctic birch forests, the higher quality and earlier timing of the input of herbivore deposits to soils compared to senesced litter may make this contribution disproportionally important for various ecosystem functions. BIH increased significantly with leaf N content as well as local elevation along each transect, yet showed no significant relationship with temperature or humidity, nor the commonly used temperature proxy, absolute elevation. The lack of consistency between the local and regional elevational trends calls for caution when using elevation gradients as climate proxies. [ABSTRACT FROM AUTHOR]

Gross primary production (GPP) is a key component of the forest carbon cycle. However, our knowledge of GPP at the stand scale remains uncertain, because estimates derived from eddy covariance (EC) rely on semi-empirical modelling and the assumptions of the EC technique are sometimes not fully met. We propose using the sap flux/isotope method as an alternative way to estimate canopy GPP, termed GPPiso/SF, at the stand scale and at daily resolution. It is based on canopy conductance inferred from sap flux and intrinsic water-use efficiency estimated from the stable carbon isotope composition of phloem contents. The GPPiso/SF estimate was further corrected for seasonal variations in photosynthetic capacity and mesophyll conductance. We compared our estimate of GPPiso/SF to the GPP derived from PRELES, a model parameterized with EC data. The comparisons were performed in a highly instrumented, boreal Scots pine forest in northern Sweden, including a nitrogen fertilized and a reference plot. The resulting annual and daily GPPiso/SF estimates agreed well with PRELES, in the fertilized plot and the reference plot. We discuss the GPPiso/SF method as an alternative which can be widely applied without terrain restrictions, where the assumptions of EC are not met. [ABSTRACT FROM AUTHOR]

River regulation may filter out riparian plants often resulting in reduced functional diversity, i.e., in the range of functions that organisms have in communities and ecosystems. There is, however, little empirical evidence about the magnitude of such reductions in different regions. We investigated the functional diversity patterns of riparian woody vegetation to streamflow regulation in boreal Sweden and Mediterranean Portugal using nine plant functional traits and field data from 109 sampling sites. We evaluated changes in mean plant functional traits as well as in indices of multidimensional functional traits, i.e., functional richness (FRic) and functional redundancy (FRed) within regions and between free-flowing and regulated river reaches. We found that regulation significantly reduced functional diversity in Sweden but not in Portugal. In Sweden, the increased magnitude of variations in water flow and water level in summer, the prolonged duration of extreme hydrological events, the increased frequency of high-water pulses, and the rate of change in water conditions were the likely main drivers of functional diversity change. Small riparian plant species with tiny leaves, poorly lignified stems, and shallow root systems were consistently associated with regulated sites in the boreal region. In Portugal, the similar functional diversity values for free-flowing and regulated rivers likely stem from the smaller streamflow alterations by regulation combined with the species legacy adaptations to the Mediterranean natural hydrological regimes. We conclude that streamflow regulation may reduce the functional diversity of riparian woody vegetation, but the magnitude of these effects will vary depending on the adaptations of the local flora and the patterns of streamflow disturbances. Our study provides insights into functional diversity patterns of riparian woody vegetation affected by regulation in contrasting biomes and encourages further studies of the functional diversity thresholds for maintaining ecosystems. [ABSTRACT FROM AUTHOR]

The boreal biome exchanges large amounts of carbon (C) and greenhouse gases (GHGs) with the atmosphere and thus significantly affects the global climate. A managed boreal landscape consists of various sinks and sources of carbon dioxide (CO2), methane (CH4), and dissolved organic and inorganic carbon (DOC and DIC) across forests, mires, lakes, and streams. Due to the spatial heterogeneity, large uncertainties exist regarding the net landscape carbon balance (NLCB). In this study, we compiled terrestrial and aquatic fluxes of CO2, CH4, DOC, DIC, and harvested C obtained from tall‐tower eddy covariance measurements, stream monitoring, and remote sensing of biomass stocks for an entire boreal catchment (~68 km2) in Sweden to estimate the NLCB across the land–water–atmosphere continuum. Our results showed that this managed boreal forest landscape was a net C sink (NLCB = 39 g C m−2 year−1) with the landscape–atmosphere CO2 exchange being the dominant component, followed by the C export via harvest and streams. Accounting for the global warming potential of CH4, the landscape was a GHG sink of 237 g CO2‐eq m−2 year−1, thus providing a climate‐cooling effect. The CH4 flux contribution to the annual GHG budget increased from 0.6% during spring to 3.2% during winter. The aquatic C loss was most significant during spring contributing 8% to the annual NLCB. We further found that abiotic controls (e.g., air temperature and incoming radiation) regulated the temporal variability of the NLCB whereas land cover types (e.g., mire vs. forest) and management practices (e.g., clear‐cutting) determined their spatial variability. Our study advocates the need for integrating terrestrial and aquatic fluxes at the landscape scale based on tall‐tower eddy covariance measurements combined with biomass stock and stream monitoring to develop a holistic understanding of the NLCB of managed boreal forest landscapes and to better evaluate their potential for mitigating climate change. [ABSTRACT FROM AUTHOR]

The riparian zone (RZ), or near-stream area, plays a fundamental role in the biogeochemistry of headwaters. Here, wet, carbon-rich soils can change groundwater chemistry before it enters the stream. In the boreal forest, the RZ plays an especially important role in the export of dissolved organic carbon (DOC) to streams. However, the RZ is not uniform, and spatial variability of riparian groundwater hydrology and chemistry can be large. Terrestrial topographic depressions create hydrological pathways towards focal points in the RZ, which we refer to as "discrete riparian inflow points" (DRIPs). Combining the chemical function of the RZ and the convergence of hydrological pathways, we hypothesize that DRIPs play a disproportionally large role in conveying DOC to small streams. Earlier work has demonstrated that runoff from DRIPs can make up the majority of riparian flow contributions to streams, but it is currently unknown how their groundwater chemistry differs from the rest of the RZ. Therefore, we ask the following question: are DOC concentrations in riparian groundwater linked to hydrological pathways in the boreal forest? To answer this question, we sampled riparian groundwater during six campaigns across three boreal headwater streams in Sweden. The groundwater wells were distributed into 10 DRIP and non-DRIP pairs (60 wells), following transects from the upland (20 m lateral distance from the stream bank) to the near-stream area (<5 m lateral distance from the stream bank). The variability in DOC, pH, and electrical conductivity (EC) was analyzed using linear mixed-effects models (LMMs). We explained the variability using three factors: distance from the stream, seasonality, and DRIP/non-DRIP. Our results showed that DRIPs provided DOC-rich water (34 mg L -1) with relatively low EC (36 µ S cm -1). The "non-DRIP" riparian water had 40 % lower DOC concentrations (20 mg L -1) and a 45 % higher EC (52 µ S cm -1) on average. Moreover, groundwater chemistry from DRIPs was spatially and temporally relatively homogeneous. In contrast, non-DRIP water transformed distinctly in the last 25 m towards the stream, and the chemical variability was also larger between seasons. We concluded that hydrological pathways and spatial variability in riparian groundwater DOC concentrations are linked, and that DRIPs can be seen as important control points in the boreal landscape. Characterizing DRIPs in headwater catchments can be useful for upscaling carbon inputs in boreal stream ecosystems and for delineating hydrologically adapted buffers for forest management practices. [ABSTRACT FROM AUTHOR]

Biological nitrogen (N) fixation is an important process supporting primary production in ecosystems, especially in those where N availability is limiting growth, such as peatlands and boreal forests. In many peatlands, peat mosses (genus Sphagnum) are the prime ecosystem engineers, and like feather mosses in boreal forests, they are associated with a diverse community of diazotrophs (N2-fixing microorganisms) that live in and on their tissue. The large variation in N2 fixation rates reported in literature remains, however, to be explained. To assess the potential roles of habitat (including nutrient concentration) and species traits (in particular litter decomposability and photosynthetic capacity) on the variability in N2 fixation rates, we compared rates associated with various Sphagnum moss species in a bog, the surrounding forest and a fen in Sweden. We found appreciable variation in N2 fixation rates among moss species and habitats, and showed that both species and habitat conditions strongly influenced N2 fixation. We here show that higher decomposition rates, as explained by lower levels of decomposition-inhibiting compounds, and higher phosphorous (P) levels, are related with higher diazotrophic activity. Combining our findings with those of other studies, we propose a conceptual model in which both species-specific traits of mosses (as related to the trade-off between rapid photosynthesis and resistance to decomposition) and P availability, explain N2 fixation rates. This is expected to result in a tight coupling between P and N cycling in peatlands. [ABSTRACT FROM AUTHOR]

Climate warming is likely to shift the range margins of species poleward, but fine‐scale temperature differences near the ground (microclimates) may modify these range shifts. For example, cold‐adapted species may survive in microrefugia when the climate gets warmer. However, it is still largely unknown to what extent cold microclimates govern the local persistence of populations at their warm range margin. We located 99 microrefugia, defined as sites with edge populations of 12 widespread boreal forest understory species (vascular plants, mosses, liverworts and lichens) in an area of ca. 24,000 km2 along the species' southern range margin in central Sweden. Within each population, a logger measured temperature eight times per day during one full year. Using univariate and multivariate analyses, we examined the differences of the populations' microclimates with the mean and range of microclimates in the landscape, and identified the typical climate, vegetation and topographic features of these habitats. Comparison sites were drawn from another logger data set (n = 110), and from high‐resolution microclimate maps. The microrefugia were mainly places characterized by lower summer and autumn maximum temperatures, late snow melt dates and high climate stability. Microrefugia also had higher forest basal area and lower solar radiation in spring and autumn than the landscape average. Although there were common trends across northern species in how microrefugia differed from the landscape average, there were also interspecific differences and some species contributed more than others to the overall results. Our findings provide biologically meaningful criteria to locate and spatially predict potential climate microrefugia in the boreal forest. This opens up the opportunity to protect valuable sites, and adapt forest management, for example, by keeping old‐growth forests at topographically shaded sites. These measures may help to mitigate the loss of genetic and species diversity caused by rear‐edge contractions in a warmer climate. [ABSTRACT FROM AUTHOR]