Boreal Sweden Research Articles

Fungal trait‐environment relationships in wood‐inhabiting communities of boreal forest patches

Abstract Fungal traits can provide a mechanistic understanding of how wood‐inhabiting fungi interact with their environment and how that influences community assembly in deadwood. However, fungal trait exploration is relatively new and almost no studies measure fungal traits in their environment. In this study we tested species‐ and trait‐environment relationships in reproducing fungal communities inhabiting 571 Norway spruce (Picea abies) logs in 55 isolated forest patches (0.1–9.9 ha) of different naturalness types, located in Northern boreal Sweden. The studied patches were (1) semi‐natural set‐aside patches within highly managed landscapes, or (2) old‐growth natural patches located in an unmanaged landscape. We tested species and trait relationships to deadwood substrate and forest patch variables. We measured mean fruit body size and density for each of the 19 species within communities. Traits assembled in relation to log decay stage and forest patch naturalness, illustrating the important role of deterministic environmental filtering in shaping reproducing wood‐inhabiting fungal communities. Early decay stage communities had larger, less dense, annual fruiting bodies of half‐resupinate type and were more often white‐rot fungi. Species rich mid‐decay stage communities had mixed trait assemblages with more long lived perennial fruit bodies of intermediate size, and both brown‐ and white‐rot fungi equally represented. Finally, late decay stage communities had smaller, denser and perennial fruit bodies, more often of the brown‐rot type. The relationships between the studied traits and decay stages were similar in both set‐aside and natural patches. However, set‐aside semi‐natural patches in highly managed landscapes more frequently supported species with smaller, perennial and resupinate fruit bodies compared to natural patches in an unmanaged landscape. Synthesis. We found that log decay stage was the primary driver of fungal community assembly of species and traits in isolated forest patches. Our results suggest that decay stage filters four reproduction traits (fruit body density, size, lifespan and type) and one resource‐use trait (white or brown rot). Our results highlights, for the first time, that communities with diverse fungal reproductive traits are maintained foremost across all deadwood decay stages under different forest naturalness conditions. Read the free Plain Language Summary for this article on the Journal blog.

Will stumps from forest thinning and other standing deadwood provide habitat for saproxylic beetles if felling stumps on clearcuts are harvested?

ABSTRACT Increased level of stump harvest on clearcuts is expected due to growing demand for bioenergy. For saproxylic species, this will reduce habitat supply. Negative effects on populations may be buffered by alternative deadwood habitats, especially stumps after forest thinning. We investigated clearcuts and mature forest to see whether such stumps or other standing deadwood could host saproxylic beetles that occur in clear-felling stumps. The study was conducted in boreal Sweden, on clearcuts 3–14 years after harvest and in mature forests. Beetles were sampled from the bark of spruce, pine and birch, from stumps after clear-felling, stumps after thinning, snags within forests and high-stumps on clearcuts. Beta diversity differed significantly between deadwood types, but not tree species. The stumps after thinning had a species poor assemblage compared to other deadwood. Abundance of common species was higher in deadwood on clearcuts compared to forests, and higher in snags than in stumps. We conclude that stumps after thinning and other standing deadwood will not buffer habitat loss from increased stump harvest for species inhabiting stumps on clearcuts, because these communities require sun-exposed habitats. Within mature managed forests, improving the quality and diversity of deadwood will be important for increasing the diversity of saproxylic beetle communities.

A drained nutrient-poor peatland forest in boreal Sweden constitutes a net carbon sink after integrating terrestrial and aquatic fluxes.

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.

The Effects of Water Column Dissolved Oxygen Concentrations on Lake Methane Emissions—Results From a Whole‐Lake Oxygenation Experiment

AbstractLakes contribute 9%–19% of global methane (CH4) emissions to the atmosphere. Dissolved molecular oxygen (DO) in lakes can inhibit the production of CH4 and promote CH4 oxidation. DO is therefore often considered an important regulator of CH4 emissions from lakes. Presence or absence of DO in the water above the sediments can affect CH4 production and emissions by (a) influencing if methane production can be fueled by the most reactive organic matter in the top sediment layer or rely on deeper and less degradable organic matter, and (b) enabling CH4 accumulation in deep waters and potentially large emissions upon water column turnover. However, the relative importance of these two DO effects on CH4 fluxes is still unclear. We assessed CH4 fluxes from two connected lake basins in northern boreal Sweden where one was experimentally oxygenated. Results showed no clear difference in summer CH4 emissions attributable to water column DO concentrations. Large amounts of CH4 accumulated in the anoxic hypolimnion of the reference basin but little of this may have been emitted because of incomplete mixing, and effective methane oxidation of stored CH4 reaching oxic water layers. Accordingly, ≤24% of the stored CH4 was likely emitted in the experimental lake. Overall, our results suggest that hypolimnetic DO and water column CH4 storage might have a smaller impact on CH4 emissions in boreal forest lakes than previous estimates, yet potential fluxes associated with water column turnover events remain a significant uncertainty in lake CH4 emission estimates.

Coarse woody debris legacies and their dynamics in retained forest patches

Retaining trees in small patches at final harvest is a common forest conservation measure to maintain structural and biological diversity through the young forest phase. Long-term studies of coarse woody debris (CWD, breast height diameter ≥ 10 cm) changes in retention patches remain uncommon, especially in relation to different types of patches with varying tree species composition, ground moisture, size, shape, and exposure. In the present study we re-inventoried CWD three times (1–3, 5–7 and 18–20 years after harvesting) in 60 small tree retention patches (0.03–0.54 ha) of six commonly created retention patch types, in central boreal Sweden. Most retention patch types, despite being very small (<0.55 ha) and exposed, did not experience severe tree mortality and input of CWD over the two decades. Average total volumes of CWD did not change significantly over time, although volumes of downed CWD generally increased whilst standing CWD decreased. Volumes of different decay classes were dynamic over time, but all retention types supported a variety of decay stages after 18–20 years. The volume of CWD varied significantly between the six retention patch types, with buffer zones to mire having the lowest average volume of 9 m3 ha−1 and buffer zones to rocky outcrop the highest average volume of 32 m3 ha−1, when summarised over the whole study period. Very few patch-level environmental variables (except type) related to the amount of CWD in retention patches; only the retained living tree volume of patches during creation was clearly positively correlated with higher post-harvest CWD volumes. Patch characteristics such as size and shape index did not relate to the CWD volumes in retention patches, and patch exposure only related to higher CWD volumes within the first 1–3 years after harvest. Retention patches generally supported a variety of living trees, CWD volumes and qualities over time. Several retention patch types; such as free-standing coniferous and deciduous patches, rocky outcrop buffers, wet forest patches and buffers to water, supported average CWD volumes between 19–41 m3 ha−1 after 18–20 years, under remaining canopy cover. Our findings challenge the traditional management principles aimed at minimizing severe windthrow and CWD input (i.e., creating larger patches (>0.5–1 ha), choosing topographically sheltered areas, selecting specific tree species and ground conditions). However, our study was too small in scale to investigate multiple within-patch-type interacting environmental variables. Future larger-scale studies over extended time periods are needed to disentangle such interactions for the dynamics of CWD and associated biodiversity in retention patches.

Drainage Ditch Cleaning Has No Impact on the Carbon and Greenhouse Gas Balances in a Recent Forest Clear-Cut in Boreal Sweden

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 &lt;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.

Belowground resource utilization in monocultures and mixtures of Scots pine and Norway spruce

Mixed forests have been recommended to replace monocultures, often being more productive and resilient ecosystems. Those benefits of species mixing have been suggested to be attributed to lower competition (above- and belowground) due to potential separation of resource acquisition strategies, yet a mechanistic understanding of belowground processes has largely been missing. We applied an isotopic-labelling technique using 15N and 2H at ~5 cm soil depth to study acquisition of two important resources, nitrogen (N) and water, in a replicated field experiment including mature Scots pine and Norway spruce monocultures and their mixture in boreal Sweden. Based on the isotopic data, we modelled distance-dependent N and water uptake and described the horizontal reach of active rooting areas around trees. The active root area for both N and water uptake in monocultures of both species was approximately 6–10 m2. Substantially wider areas were observed inside the mixture for N acquisition (27 m2 for pine and 21 m2 for spruce). Water was mainly acquired from within a 12–15 m2 area in mixture. The mixture and the pine monocultures exhibited similar recovery of the added 15N (3.8–4.5%) and its uptake per unit of foliage mass. The recovery of 15N for the spruce monocultures was generally lower (2.1%) but no difference was noted in the uptake per unit of foliage mass between spruce monoculture and mixture. No differences were found for recovery of 2H between the stand types. Mixing pine and spruce did not improve the resource uptake in comparison with the estimated theoretical mixture (according to the normalized basal area in monocultures). However, the mixing revealed significant differences between tree species whereas pines inside the mixture recovered 11.3-fold more 15N than accompanying spruce. Root overlap of 3–11 trees on 1 m2 was observed in all stand types, far exceeding the degree of canopy overlap. We discuss this overlap in terms of competition between individual trees. Spruces have proportionally larger horizontal rooting area relative to their aboveground size than pines. Scots pine and Norway spruce mixing in this type of boreal forest does not significantly enhance resource acquisition but leads to compensatory partitioning of growth, suggesting fierce interspecific competition, rather than niche separation.

Forest floor fluxes drive differences in the carbon balance of contrasting boreal forest stands

The forest floor provides an important interface of soil-atmosphere CO2 exchanges but their controls and contributions to the ecosystem-scale carbon budget are uncertain due to measurement limitations. In this study, we deployed eddy covariance systems below- and above-canopy to measure the spatially integrated net forest floor CO2 exchange (NFFE) and the entire net ecosystem CO2 exchange (NEE) at two mature contrasting stands located in close vicinity in boreal Sweden. We first developed an improved cospectra model to correct below-canopy flux data. Our empirical below-canopy cospectra models revealed a greater contribution of large- and small-scale eddies in the trunk space compared to their distribution in the above-canopy turbulence cospectra. We found that applying the above-canopy cospectra model did not affect the below-canopy annual CO2 fluxes at the sparse pine forest but significantly underestimated fluxes at the dense mixed spruce-pine stand. At the mixed spruce-pine stand, forest floor respiration (Rff) was higher and photosynthesis (GPPff) was lower, leading to a 1.4 times stronger net CO2 source compared to the pine stand. We further found that drought enhanced Rff more than GPPff, leading to increased NFFE. Averaged across the six site-years, forest floor fluxes contributed 82% to ecosystem-scale respiration (Reco) and 12% to gross primary production (GPP). Since the annual GPP was similar between both stands, the considerable difference in their annual NEE was due to contrasting Reco, the latter being primarily driven by the variations in NFFE. This implies that NFFE acted as the driver for the differences in NEE between these two contrasting stands. This study therefore highlights the important role of forest floor CO2 fluxes in regulating the boreal forest carbon balance. It further calls for extended efforts in acquiring high spatiotemporal resolution data of forest floor fluxes to improve predictions of global change impacts on the forest carbon cycle.

Functional Diversity of Riparian Woody Vegetation Is Less Affected by River Regulation in the Mediterranean Than Boreal Region.

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.

Natural regeneration of lodgepole pine in boreal Sweden

The large-scale introduction of lodgepole pine (Pinus contorta ssp. latifolia, LP) into Swedish forests was initiated around 1970, and currently 520,000 hectares of the forest land are dominated by the species. Even though the tree has mostly serotine cones, adapted to open after forest fires, it has proved able to self-regenerate in Sweden. This study is the first to present, scientifically, the extent of self-regeneration covering the whole current range of LP-forests in Sweden. LP-saplings were recorded for three years, 2015–2017, in 8194 subplots distributed over 214 randomly selected LP-stands from latitude 59.6–66.9°N and altitude 88–710 m asl. Of all subplots, 3% contained LP-saplings, and regeneration was found in 53% of all stands. The probability of finding LP-saplings was significantly dependent on the plots’ distance from the edge of the LP-stand, and 78% of all saplings were found within and up to 15 m beyond the stand edge. Most, 63%, of the plots with LP were found on disturbed ground such as wheel tracks, roadsides and where there had been site preparation. The results show that LP can naturally spread under a range of conditions in the studied region. The regeneration is, however, concentrated in particular stands and should be possible to control with monitoring programmes and measures to eradicate self-dispersed trees in unwanted areas.

Impacts of tree species identity and species mixing on ecosystem carbon and nitrogen stocks in a boreal forest

Forest management practices, such as selection or mixing of particular tree species, may enhance forests’ carbon (C) sinks and resilience against climate change. While a majority of research on this subject has focused on aboveground production, far less is known about how these management decisions impact belowground C storage, as well as the C and nitrogen (N) stocks of the whole ecosystem. We used a well-replicated 60-year-old experiment in boreal Sweden comparing monocultures and a mixture of the two dominant coniferous species: Norway spruce (Picea abies (L.) Karst) and Scots pine (Pinus sylvestris (L.), set up at a site that was assessed as equally suitable for the growth of either species. Our aim was to evaluate the species identity and species mixing effects on ecosystem C and N stocks. We measured total standing volume, aboveground tree biomass, fine-root biomass, C and N pools in tree biomass and soil, litterfall inputs, and soil CO2 emissions. Our results show major differences in C allocation and growth patterns between spruce and pine. We found almost twice as high total standing volume and litterfall inputs in the pine stands than in the spruce stands. Higher proportion and amount of needle biomass resulted in larger amounts of N retained in the canopy and smaller accumulation of C and N in the humus in the spruce compared to pine stands. The C sinks in aboveground tree biomass and soil were larger in the pine compared to spruce stands at this site. In addition, a significantly higher soil CO2 efflux rate and fine-root biomass in the spruce compared to pine stands suggested greater tree internal allocation of C belowground to roots and ectomycorrhizal fungi in response to stronger N limitation. We found no significant mixing effect in the mixed stands, given the levels of the measured variables did not exceed levels of the most productive monoculture, with an exception of higher SOC stocks in the deeper (10–20 cm) mineral soil layer in the mixed stands. Our results do not support the idea of higher productivity and C sinks of forest mixtures compared to the best performing monoculture on the given site suggesting that these tree species are not complementary from a forest management perspective. However, in many cases the mixed stands performed equally well as the best monoculture, indicating that management for multi-species stands may not result in any loss in C uptake and storage.

Multi-layered Scots pine forests in boreal Sweden result from mass regeneration and size stratification

Understanding historic development of multi-layered Scots pine (Pinus sylvestris L.) stands and how they became multi-layered is essential for assessing the feasibility of using the selection system in these stands. To address this we measured trees (dbh ≥ 4 cm) and saplings (height > 0.5 m dbh < 4 cm) and used increment cores from 244 sample trees to reconstruct stand structure development, ingrowth and basal area increment in four multi-layered Scots pine stands in Sweden. Age distributions were quite homogeneous, three of the four stands had age distributions that were dominated by one or two 20 year age classes, suggesting that the irregular diameter distributions displayed in 2013 had developed from more homogeneous distributions. Analyses of the historical ingrowth of Scots pine into the tree layer suggested that the multi-layered structure was created by mass regeneration followed by size stratification caused by differences in growth rates within even-aged cohorts of regeneration. Large reductions of the basal area in the past resulted in abundant regeneration and ingrowth of Scots pine. When the over-story increased in basal area over time, there was a growth differentiation among the saplings and small trees, gradually creating a multi-layered stand structure as some of the trees grew into the larger size classes while others remained in the smaller size classes. When the stands reached a basal area of about 13 m2 ha−1 the ingrowth of saplings past 1.3 m height essentially stopped but the size stratification among the small trees continued, further enhancing the multi-layered structure. The results indicate that to receive regeneration pulses and sustain a multi-layered structure in Scots pine forests, the basal area needs to be significantly reduced. The growth consequences of this need to be studied.

Wildfire ignition from forestry machines in boreal Sweden

Several large Swedish wildfires during recent decades were caused by forestry machinery in operation, fires for which there is still no characterisation. We combined 18 years of data on dispatches, weather and fire danger and interviewed forestry workers to understand the spatial, temporal and weather distributions of these fires, and their underlying mechanisms. We estimate the average annual number of ignitions from forestry machinery in Sweden at 330–480 (2.0±0.4 ignitions per 1000ha clear-felling) of which 34.5 led to firefighter dispatches, constituting 2.2% of all forest fire dispatches and 40% of area burnt. Soil scarification causes the most ignitions and the main mechanism is likely high-inertia contact between discs and large stones, causing sparks igniting dry humus or moss, countering reports suggesting that such metal fragments cannot fulfil ignition requirements. We found a spatial relationship between forestry machine ignitions and abundance of large stones, represented by a Boulder Index generated from a nationwide dataset. Further, 75% of the dispatches occurred on days with relative humidity &amp;lt;45%, Duff Moisture Code (Canadian system) &amp;gt;26 and Fire Weather Index &amp;gt;12. 75% of the area burned when Fire Weather Index was &amp;gt;20. Results suggest machine-caused forest fires can be largely avoided by cancelling operations in stony terrain during high-risk weather.

Microbial community response to growing season and plant nutrient optimisation in a boreal Norway spruce forest

Interactions between Norway spruce trees and bacteria and fungi in nutrient limited boreal forests can be beneficial for tree growth and fitness. Tree-level effects of anthropogenic nutrient addition have been well studied, however understanding of the long-term effects on the associated microbiota is limited. Here, we report on the sensitivity of microbial community composition to the growing season and nutrient additions. High-throughput sequencing of the bacterial 16S rRNA gene and fungal ITS1 region was used to characterise changes in the microbial community after application of a complete mineral nutrient mixture for five and 25 years. The experiment was conducted using the Flakaliden forest research site in northern boreal Sweden and included naturally low nutrient control plots. Needle and fine root samples of Norway spruce were sampled in addition to bulk soil during one growing season to provide comprehensive insight into phyllosphere and belowground microbiota community changes. The phyllosphere microbiota was compositionally distinct from the belowground communities and phyllosphere diversity increased significantly over the growing season but was not influenced by the improved nutrient status of the trees. In both root and soil samples, alpha diversity of fungal, in particular ectomycorrhizal fungi (EMF), and bacterial communities increased after long-term nutrient optimisation, and with increasing years of treatment the composition of the fungal and bacterial communities changed toward a community with a higher relative abundance of nitrophilic EMF and bacterial species but did not cause complete loss of nitrophobic species from the ecosystem. From this, we conclude that 25 years of continuous nutrient addition to a boreal spruce stand increased phylotype richness and diversity of the microbiota in the soil, and at the root-soil interface, suggesting that long-term anthropogenic nutrient inputs can have positive effects on belowground biodiversity that may enhance ecosystem robustness. Future studies are needed to assess the impact of these changes to the microbiota on ecosystem carbon storage and nitrogen cycling in boreal forests.

Lake morphometry moderates the relationship between water color and fish biomass in small boreal lakes

AbstractLake morphometry may moderate the effects of water color on fish biomass in boreal lakes, but empirical evidence is scarce because there are a limited number of lakes for which both water color and bathymetry have been measured. We evaluated variations in catch‐per‐unit‐effort (CPUE), an indicator of fish biomass, across orthogonal gradients of light extinction and mean depth in 16 small Swedish lakes (mean depth 1.7–4.8 m, surface area 1–10 ha). Multiple regression coefficients indicated that the effect of light extinction on CPUE was negative, and that the relationship was more negative for deeper lakes than it was for shallower lakes. The pattern was strongest for lakes with mean depths between 2.1 m and 3.5 m. We estimated that 26% of small lakes in boreal Sweden fall within this mean depth range. These results contribute to the growing understanding of how variations in water color and lake morphometry influence patterns of fish biomass across the boreal landscape.

Reindeer husbandry in a mountain Sami village in boreal Sweden: the social and economic effect of introducing GPS collars and adaptive forest management

Reindeer husbandry in northern Sweden, Norway, and Finland is an extensive agroforestry system, and the management of reindeer occurs alongside other land uses such as forestry, hunting, and tourism. The economic, social, and environmental impact of using GPS collars to track the reindeer was evaluated in the Sami village of Njaarke in boreal Sweden. A second objective was to evaluate how adapted forest management influenced reindeer husbandry. The results show that the use of GPS tracking enables the improved monitoring of the reindeer but the financial costs were greater than the financial benefits. The increased income did not cover the costs of depreciation and maintenance. Whilst the labour inputs associated with reindeer husbandry were high, 30–40% of the gross value added by village activities were related to hunting, tourism, and the renting of fishing rights. The use of adaptive forest management was assumed to increase the number of reindeer that could be reared through to slaughter and it was calculated to increase the gross value added by reindeer husbandry by more than 30%.

Post-1980 shifts in the sensitivity of boreal tree growth to North Atlantic Ocean dynamics and seasonal climate

Abstract The mid-20th century changes in North Atlantic Ocean dynamics, e.g. slow-down of the Atlantic meridional overturning thermohaline circulation (AMOC), have been considered as early signs of tipping points in the Earth climate system. We hypothesized that these changes have significantly altered boreal forest growth dynamics in northeastern North America (NA) and northern Europe (NE), two areas geographically adjacent to the North Atlantic Ocean. To test our hypothesis, we investigated tree growth responses to seasonal large-scale oceanic and atmospheric indices (the AMOC, North Atlantic Oscillation (NAO), and Arctic Oscillation (AO)) and climate (temperature and precipitation) from 1950 onwards, both at the regional and local levels. We developed a network of 6876 black spruce (NA) and 14437 Norway spruce (NE) tree-ring width series, extracted from forest inventory databases. Analyses revealed post-1980 shifts from insignificant to significant tree growth responses to summer oceanic and atmospheric dynamics both in NA (negative responses to NAO and AO indices) and NE (positive response to NAO and AMOC indices). The strength and sign of these responses varied, however, through space with stronger responses in western and central boreal Quebec and in central and northern boreal Sweden, and across scales with stronger responses at the regional level than at the local level. Emerging post-1980 associations with North Atlantic Ocean dynamics synchronized with stronger tree growth responses to local seasonal climate, particularly to winter temperatures. Our results suggest that ongoing and future anomalies in oceanic and atmospheric dynamics may impact forest growth and carbon sequestration to a greater extent than previously thought. Cross-scale differences in responses to North Atlantic Ocean dynamics highlight complex interplays in the effects of local climate and ocean-atmosphere dynamics on tree growth processes and advocate for the use of different spatial scales in climate-growth research to better understand factors controlling tree growth.

Wolf and Bear Depredation on Livestock in Northern Sweden 1827–2014: Combining History, Ecology and Interviews

During the twenty-first century, large carnivores have increased in human dominated landscapes after being extinct or nearly extinct. This has resulted in increasing numbers of livestock killed by large carnivores. The intent of this paper is to give a land use-historical perspective on the recent livestock–carnivore conflict in boreal Sweden. More specifically we address: (1) depredation risks (livestock killed by carnivores) and (2) local knowledge of how to protect livestock from predation and whether it survived among pastoralists until the present. This study provides numeric information on carnivores, livestock and depredation, combined with oral information from summer farmers about livestock protection. We compare recent (since 1998) and historical (late nineteenth century) depredation rates in two Swedish counties. In Dalarna recent depredation rates are higher than historical rates while the opposite pattern is seen in Jämtland. Recent depredation rates in Dalarna are twice the recent rates in Jämtland, in contrast to the historical situation. Recent and historical depredation rates are of the same order. Summer farmers traditionally graze their livestock in forested areas where carnivores reside. Interviews show that traditional knowledge of how to protect livestock from carnivores was lost during the twentieth century, but recently new knowledge has developed leading to changes in summer farming practices. The carnivore–livestock situation today differs from the historical situation, not so much in levels of depredation, but mainly regarding the possibilities of farmers to face challenges associated with increasing carnivore populations.

Tree and stand structure of the non-native Pinus contorta in relation to native Pinus sylvestris and Picea abies in young managed forests in boreal Sweden

ABSTRACTManaged forest stands are typically younger and structurally less diverse than natural forests. Introduction of non-native tree species might increase the structural changes to managed forest stands, but detailed analyses of tree- and stand-structures of native and non-native managed forests are often lacking. Improved knowledge of non-native forest structure could help clarify their multiple values (e.g. habitat for native biodiversity, bioenergy opportunities). We studied the structural differences between the introduced, non-native Pinus contorta and the native Pinus sylvestris and Picea abies over young forest stand ages (13–34 years old) in managed forests in northern Sweden. We found that P. contorta stands had greater mean basal areas, tree heights, diameters at breast height, and surface area of living branches than the two native species in young stands. The surface area of dead attached branches was also greater in P. contorta than P. abies. Although this indicates greater habitat availability for branch-living organisms, it also contributes to the overall more shaded conditions in stands of P. contorta. Only one older 87 years old P. contorta stand was available, and future studies will tell how structural differences between P. contorta and native tree species develop over the full forestry cycle.

Ecological restoration in boreal forest modifies the structure of bird assemblages

Ecological restoration is increasingly recognised as a useful tool for biodiversity conservation in boreal forests. Most restoration methods in this environment aim to emulate natural disturbances, and thereby promote the development of key ecological structures. However, research about forest ecosystem restoration is still in its infancy and the responses of many boreal species groups remain to be described. We established a large-scale field experiment to evaluate the short-term effects of two restoration treatments – prescribed burning and gap cutting involving the creation of dead wood – on breeding bird assemblages in boreal Sweden. We censused breeding birds using territory mapping during two years in forest stands subjected to the two restoration treatments, in untreated controls and in old-growth references (nature reserves) (n=10 per treatment). Averaged over the two census years, we found 1145 territories of 36 bird species. Total bird species richness and abundance did not differ among treatments. However, prescribed burning led to clear changes in the structure of the bird assemblages. When dividing species according to four functional categorizations (migration, foraging, nesting and successional stage), we found that the abundance of long-distance migrants, ground breeders, strong cavity excavators and species preferring early-successional habitat was higher in burned stands than in untreated controls and gap-cut stands, as was the species richness of bark feeders and strong cavity excavators. In contrast, abundance of off-ground breeders and species richness of crown feeders were lower following prescribed burning than in the controls. The gap cutting treatment did not have any significant effects on the bird assemblages. Ecological restoration through prescribed burning can be a useful tool for the conservation of boreal forest birds, including the ecologically important strong cavity excavators (i.e. woodpeckers (Picidae)). Forest managers should therefore be encouraged to use prescribed burning as a restoration tool to quickly provide habitat for bird species adapted to natural disturbances in boreal forest.