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The representation of 2-m air temperature and 10-m wind speed in the high-resolution (with a 2.5-km grid spacing) Copernicus Arctic Regional Reanalysis (CARRA) and the coarser resolution (ca. 31-km grid spacing) global European Center for Medium-range Weather Forecasts fifth-generation reanalysis (ERA5) for Svalbard, northern Norway, Sweden and Finland is evaluated against observations. The largest differences between the two reanalyses are found in regions with complex terrain and coastlines, and over the sea ice for temperature in winter. In most aspects, CARRA outperforms ERA5 in its agreement with the observations, but the value added by CARRA varies with region and season. Furthermore, the added value by CARRA is seen for both parameters but is more pronounced for temperature than wind speed. CARRA is in better agreement with observations in terms of general evaluation metrics like bias and standard deviation of the errors, is more similar to the observed spatial and temporal variability and better captures local extremes. A better representation of high-impact weather like polar lows, vessel icing and warm spells during winter is also demonstrated. Finally, it is shown that a substantial part of the difference between reanalyses and observations is due to representativeness issues, that is, sub-grid variability, which cannot be represented in gridded data. This representativeness error is larger in ERA5 than in CARRA, but the fraction of the total error is estimated to be similar in the two analyses for temperature but larger in ERA5 for wind speed.

Mammalian herbivores shape the structure and function of many nutrient-limited or low-productive terrestrial ecosystems through modification of plant communities and plant–soil feedbacks. In the tundra biome, mammalian herbivores may both accelerate and decelerate plant biomass growth, microbial activity and nutrient cycling, that is, ecosystem process rates. Selective foraging and associated declines of palatable species are known to be major drivers of plant–soil feedbacks. However, declines in dominant plants of low palatability often linked with high herbivore densities may also modify ecosystem process rates, yet have received little attention. We present data from an island experiment with a 10-year vole density manipulation, to test the hypothesis that herbivores accelerate process rates by decreasing the relative abundance of poorly palatable plants to palatable ones. We measured plant species abundances and community composition, nitrogen contents of green plant tissues and multiple soil and litter variables under high and low vole density. Corroborating our hypothesis, periodic high vole density increased ecosystem process rates in low-productive tundra. High vole density was associated with both increasing relative abundance of palatable forbs over unpalatable evergreen dwarf shrubs and higher plant N content both at species and at community level. Changes in plant community composition, in turn, explained variation in microbial activity in litter and soil inorganic nutrient availability. We propose a new conceptual model with two distinct vole–plant–soil feedback pathways. Voles may drive local plant–soil feedbacks that either increase or decrease ecosystem process rates, in turn promoting heterogeneity in vegetation and soils across tundra landscapes.

Identifying the scales of variation in forest structures and the underlying processes are fundamental for understanding forest dynamics. Here, we studied these scale-dependencies in forest structure in naturally dynamic boreal forests on two continents. We identified the spatial scales at which forest structures varied, and analyzed how the scales of variation and the underlying drivers differed among the regions and at particular scales. We studied three 2 km × 2 km landscapes in northeastern Finland and two in eastern Canada. We estimated canopy cover in contiguous 0.1-ha cells from aerial photographs and used scale-derivative analysis to identify characteristic scales of variation in the canopy cover data. We analyzed the patterns of variation at these scales using Bayesian scale space analysis. We identified structural variation at three spatial scales in each landscape. Among landscapes, the largest scale of variation showed the greatest variability (20.1–321.4 ha), related to topography, soil variability, and long-term disturbance history. Superimposed on this large-scale variation, forest structure varied at similar scales (1.3–2.8 ha) in all landscapes. This variation correlated with recent disturbances, soil variability, and topographic position. We also detected intense variation at the smallest scale analyzed (0.1 ha, grain of our data), partly driven by recent disturbances. The distinct scales of variation indicated hierarchical structure in the landscapes studied. Except for the large-scale variation, these scales were remarkably similar among the landscapes. This suggests that boreal forests may display characteristic scales of variation that occur somewhat independent of the tree species characteristics or the disturbance regime.

Aim: Elevational biodiversity patterns are understudied in high-latitude aquatic systems, even though these systems are important for detecting very early impacts of climatic changes on Earth. The aim of this study was to examine the elevational trends in species richness and local contribution to beta diversity (LCBD) of three biofilm microbial groups in freshwater ponds and to identify the key mechanisms underlying these patterns. Location: One hundred and forty-six ponds in subarctic Finland and Norway distributed across the tree line along an elevational gradient of 10-1,038 m a.s.l., spanning from forested landscape to barren boulder fields. Time period: July–August 2015. Major taxa studied: Diatoms, cyanobacteria and non-cyanobacteria. Methods: Generalized linear models were used to identify the most important pond variables explaining richness and LCBD. Structural equation models were used to explore the direct and indirect effects of multiscale drivers on richness and LCBD. Results: Diatom and cyanobacteria richness showed unimodal elevational patterns, whereas non-cyanobacteria richness decreased with increasing elevation. The LCBD–elevation relationship was U-shaped for all three microbial groups. Diatom and cyanobacteria richness and LCBD were best explained by local pond variables, especially by pH. Non-cyanobacteria richness and LCBD were related to pond variables, elevation as a proxy for climatic conditions, and normalized difference vegetation index as a proxy for terrestrial productivity. Main conclusions: Aquatic autotrophs were primarily controlled by environmental filtering, whereas heterotrophic bacteria were also affected by terrestrial productivity and elevation. All studied aspects of microbial diversity were directly or indirectly linked to elevation; therefore, climatic changes may greatly alter aquatic microbial assemblages.

ContextChanges in the structure of boreal old-growth forests are typically studied at a specific spatial scale. Consequently, little is known about forest development across different spatial scales.ObjectivesWe investigated how and at what spatial scales forest structure changed over several decades in three 4 km2 boreal old-growth forests landscapes in northeastern Finland and two in Quebec, Canada.MethodsWe used canopy cover values visually interpreted to 0.1-ha grid cells from aerial photographs taken at three time points between the years 1959 and 2011, and error distributions quantified for the interpretation. We identified the spatial scales at which canopy cover changed between the time points, and examined the credibility of changes at these scales using the error distributions in Bayesian inference.ResultsCanopy cover changed at three to four spatial scales, the number of scales depending on the studied landscape and time interval. At large scales (15.4–321.7 ha), canopy cover increased in Finland during all time intervals. In Quebec, the direction of the large-scale change varied between the studied time intervals, owing to the occurrence of an insect outbreak and a consequent recovery. However, parts of these landscapes also showed canopy cover increase. Superimposed on the large-scale developments, canopy cover changed variably at smaller scales (1.3–2.8-ha and 0.1-ha).ConclusionsOur findings support the idea that the structure of boreal old-growth forests changes at discernible spatial scales. Instead of being driven by gap dynamics, the old-growth forests in the studied regions are currently reacting to large-scale drivers by an increase in canopy cover.

This study revealed a significant reduction in tree growth across northern Fennoscandia following the 1600 AD eruption of Huaynaputina in Peru, the most powerful volcanic event in South America over the past two millennia. In the analysis, we utilized six tree-ring chronologies, which included the Finnish super-long chronology (5634 BC–2004 AD), the Kola Peninsula chronology (1445–2004 AD), and historical chronologies derived from old wooden churches in Finnish Lapland and Karelia, Russia. Using a superposed epoch analysis across these chronologies revealed a significant 24% (p < 0.01) decline in tree-ring growth in 1601 compared to the previous six years. The northernmost records, the Finnish super-long chronology (72%, p < 0.001) and the Sodankylä Old Church chronology (67%, p < 0.001), showed the most pronounced decreases. Statistical analysis confirmed significant (p < 0.05) similarities in tree-ring responses across all chronologies from 1601 to 1608. These findings underscore the reliability of using the 1600 Huaynaputina eruption as a chronological marker for dating historic wooden churches in northern Fennoscandia that were likely built between the late 17th and early 18th centuries. Additionally, analyzing church wood may provide insights into past climate patterns and environmental conditions linked to the eruption.

Basal ice formation in the terrestrial snow cover is a common phenomenon in northern circumpolar areas, one having significant impacts on ecosystems, vegetation, animals and human activities. There is limited knowledge on the spatial and temporal occurrence of basal ice formation because of the sparse observation network and challenges involved in detecting formation events. We present a unique dataset on the annual extent of ice formation events in northern Finland between 1948 and 2016 based on reindeer herders' descriptions of the cold season in their management reports. In extreme years, basal ice can form over wide geographical extents. In approximately half of the herding districts studied, it occurred more frequently in the period 1983-2016 than in the period 1948-1982. Furthermore, five out of seven of the most extensive basal ice formation events (90th percentile) occurred between 1991 and 2016. The most commonly reported processes related to ice formation were thaw or rain-on-snow events followed by freezing of the snow cover. Years with extensive basal ice formation were often characterized by above-average October-December air temperatures, air temperature variations around 0 °C and relatively high precipitation. However, basal ice did not occur during all warm and wet early winters, and formation events were generally weakly linked to the large-scale atmospheric teleconnections. Another risk factor for reindeer grazing associated with warm and rainy early winters is the growth of mycotoxin-producing molds below the snow. Approximately 24% of all reported mold formation events co-occurred with basal ice formation. The prevalence and frequency of basal ice formation events can be assessed based on our results. Our work contributes to understanding long-term fluctuations and changes in snow and ice conditions and the impacts of this variability in circumpolar areas.

We present an updated and validated seismic catalog for the northern Fennoscandian region, focusing on postglacial faults from the Merasjärvi fault system in the southwest to the Iešjávri fault system in the northeast. This work involved a comprehensive review of continuous waveforms derived from open datasets from 2007 to 2015 and processed using the Regressive ESTimator algorithm. The primary objective was to refine the delineation of seismicity along the above-mentioned postglacial faults and highlight their seismic potential. Our analysis revealed distinct waveform patterns originating primarily from two main sources: approximately 15% were associated with areas mapped as postglacial faults, and the remainder of the events outside these areas, 89%, were concentrated in areas with active mines. Compared to previously reported events in the Fennoscandian Earthquake Catalogue (FENCAT), we observed a 22% increase in seismic activity within postglacial fault zones. These results demonstrate that the Regressive ESTimator algorithm not only improves the detection of tectonic seismicity but also effectively identifies seismic signals resulting from mining activities in the study area. The Merasjärvi, Lainio–Suijavaara, Palojärvi, and Maze and Iešjávri fault systems appear to form a continuous deformation complex of approximately 300 km long, which we propose naming the Merasjärvi–Stuoragurra fault complex.

We studied climate trends and the occurrence of rare and extreme temperature and precipitation events in northern Fennoscandia in 1914–2013. Weather data were derived from nine observation stations located in Finland, Norway, Sweden and Russia. The results showed that spring and autumn temperatures and to a lesser extent summer temperatures increased significantly in the study region, the observed changes being the greatest for daily minimum temperatures. The number of frost days declined both in spring and autumn. Rarely cold winter, spring, summer and autumn seasons had a low occurrence and rarely warm spring and autumn seasons a high occurrence during the last 20-year interval (1994–2013), compared to the other 20-year intervals. That period was also characterized by a low number of days with extremely low temperature in all seasons (4–9% of all extremely cold days) and a high number of April and October days with extremely high temperature (36–42% of all extremely warm days). A tendency of exceptionally high daily precipitation sums to grow even higher towards the end of the study period was also observed. To summarize, the results indicate a shortening of the cold season in northern Fennoscandia. Furthermore, the results suggest significant declines in extremely cold climate events in all seasons and increases in extremely warm climate events particularly in spring and autumn seasons.

We show that the population ecology of the 9‐ to 10‐year cyclic, broadleaf‐defoliating winter moth (Operophtera brumata) and other early‐season geometrids cannot be fully understood on a local scale unless population behaviour is known on a European scale. Qualitative and quantitative data on O. brumata outbreaks were obtained from published sources and previously unpublished material provided by authors of this article. Data cover six decades from the 1950s to the first decade of twenty‐first century and most European countries, giving new information fundamental for the understanding of the population ecology of O. brumata. Analyses on epicentral, regional and continental scales show that in each decade, a wave of O. brumata outbreaks travelled across Europe. On average, the waves moved unidirectionally ESE–WNW, that is, toward the Scandes and the Atlantic. When one wave reached the Atlantic coast after 9–10 years, the next one started in East Europe to travel the same c. 3000 km distance. The average wave speed and wavelength was 330 km year⁻¹ and 3135 km, respectively, the high speed being incongruous with sedentary geometrid populations. A mapping of the wave of the 1990s revealed that this wave travelled in a straight E–W direction. It therefore passed the Scandes diagonally first in the north on its way westward. Within the frame of the Scandes, this caused the illusion that the wave moved N–S. In analogy, outbreaks described previously as moving S–N or occurring contemporaneously along the Scandes were probably the result of continental‐scale waves meeting the Scandes obliquely from the south or in parallel. In the steppe zone of eastern‐most and south‐east Europe, outbreaks of the winter moth did not participate in the waves. Here, broadleaved stands are small and widely separated. This makes the zone hostile to short‐distance dispersal between O. brumata subpopulations and prevents synchronization within meta‐populations. We hypothesize that hostile boundary models, involving reciprocal host–herbivore–enemy reactions at the transition between the steppe and the broadleaved forest zones, offer the best explanation to the origin of outbreak waves. These results have theoretical and practical implications and indicate that multidisciplinary, continentally coordinated studies are essential for an understanding of the spatio‐temporal behaviour of cyclic animal populations.