Mountain Birch Research Articles

Natural colonization as a means to upscale restoration of subarctic woodlands in Iceland

The global climate and biodiversity crisis has led to extensive restoration initiatives, calling for cost‐effective strategies harnessing the potential of natural processes. Natural colonization of target species is a key process in the scaling up of woodland restoration, and effective planning and implementation of restoration strategies requires a mechanistic understanding of colonization processes. In this study, we investigated patterns and processes of natural colonization of mountain birch (Betula pubescens ssp. tortuosa), the only native woodland‐forming tree species in Iceland, into adjacent treeless lands under diverse conditions, and its implications for low‐intensity restoration strategies. Accordingly, we assessed spatial patterns of seedling densities on transects extending from 10 birch woodlands in Iceland and analyzed the impact of local environments on colonization dynamics. Seedling densities generally decreased rapidly with distance from the seed source to around 40 m. In many study areas, however, birch seedlings were registered at high enough densities to potentially form woodlands, up to 140 m from a standing seed source. We identified a range of biotic and abiotic determinants of colonization, with the largest number affecting the early‐establishment phase of birch colonization, including safe site availability, wind speed and dominant direction, and grazing. Our results demonstrate a good potential for natural colonization as a restoration strategy for birch woodlands in subarctic environments, and they can furthermore be used to identify areas where birch colonization is likely to occur naturally and determine appropriate interventions that can facilitate birch colonization in areas where it is less likely.

Spiders (Araneae) from three human-influenced habitats in the Golsfjellet mountain area, central southern Norway

Three sites at 900 m a.s.l. in the sub-alpine mountain birch forest region and under different levels of human-influence were investigated. Site 1: An open birch forest with some scattered spruces, a rich bush and field layer and a rich moss cover; very little grazed. Site 2: Tree and bush layer removed, a rich field layer invaded by open area species and some ’culture indicators’; there was a rich moss cover. Site 3: Grazed; tree layer, bush layer and moss cover absent. Pitfall trapping (18 May – 18 Aug. 2000) in site 1 - 3 resulted in 48, 48 and 22 spider species (in total 64 species). In site 1 and 2 Pardosa riparia (C.L.Koch, 1833) and Alopecosa pulverulenta (Clerck, 1757) were dominating. P. sphagnicola (Dahl, 1908) was also fairly common in site 1, P. palustris and P. hyperborea (Thorell, 1872) in site 2. Site 3 was almost completely dominated by Pardosa palustris (73.2 %) and Erigone atra Blackwall, 1841 (18.2 %); present (absent elsewhere) were also some specimens of Oedothorax retusus (Westring, 1851) (4.6 %).

Enhanced herbaceous encroachment due to niche overlap in alpine tundra of Northeast China

The directional ecological filtering hypothesis posits that species in high-altitude environments typically exhibit broad environmental tolerance, influencing the colonization success of non-native species through niche overlap with natives. This study examines this hypothesis on Changbai Mountain, focusing on the encroachment of the herb Deyeuxia angustifolia from mountain birch forests into alpine tundra, where it competes with the endemic Rhododendron aureum. We established a permanent plot containing 10,000 continuous quadrats (2 × 2 m) in the summer of 2014, covering from treeline to the upper limit of alpine tundra. Each quadrat was divided into 40 × 40 cm resolution cells, totaling 250,000 cells. In 2014 we photographed and manually interpreted tundra plant species for all cells, assessing topographic and edaphic niche characteristics (the range of the factors) for both species. Findings reveal: (1) Over 50 % niche overlap between D. angustifolia and R. aureum, suggesting intense competition; (2) Broader niche width for R. aureum compared to D. angustifolia; and (3) The expansion of D. angustifolia is facilitated by available nitrogen, while R. aureum is uniquely adapted to low phosphorus levels at higher altitudes. These results demonstrate that ecological and environmental filtering jointly shape distribution patterns of R. aureum and D. angustifolia, supporting the idea that environmental filtering is critical in the colonization of invading plant species. Our research sheds light on the pivotal roles played by microenvironmental factors, notably topography and soil, in determining the composition and distribution of plant communities, and contributes valuable insights for understanding and forecasting future shifts in plant composition of alpine tundra.

Effects of industrial pollution and ambient air temperature on larval performance and population dynamics of Eriocrania leafminers (Lepidoptera)

Pollution is an integral part of global environmental change, yet the combined and interactive effects of pollution and climate on terrestrial ecosystems remain inadequately understood. This study aims to explore whether pollution alters the impacts of ambient air temperature on the population dynamics of herbivorous insects. Between 1995 and 2005, we studied populations of two closely related moths, Eriocrania semipurpurella and E. sangii, at eight sites located 1 to 64 km from a large copper‑nickel smelter in Monchegorsk, Russia. We found that pollution and temperature influence the performance of Eriocrania larvae mining in the leaves of mountain birch, Betula pubescens var. pumila, through multiple pathways. This is evident from the unconsistent changes observed in larval and frass weight, mine area, and leaf size. We found increases in both leaf quality and larval weight with decreasing pollution levels at both spatial and temporal scales and attributed these to the impact of sulphur dioxide, rather than trace elements (nickel and copper). The quality of birch leaves increased with spring (May) temperatures, enabling Eriocrania larvae to achieve greater weight while consuming less biomass. During the larval growth period (early June to early July), Eriocrania larvae increased their consumption with rising temperatures, presumably to compensate for increased metabolic expenses. Contrary to our expectations, the per capita rate of population change did not correlate with larval weight and did not vary along the pollution gradient. Nevertheless, we detected interactive effects of pollution and climate on the rate of population change. This rate decreased with rising winter temperatures in slightly polluted and unpolluted sites but remained unchanged in heavily polluted sites. We conclude that pollution disrupts mechanisms regulating the natural population dynamics of Eriocrania moths.

High seed losses in mountain birch (Betula pubescens ssp. tortuosa) and developmental, ecological, and environmental correlates

Abstract Plants typically experience great losses from their reproductive potential represented by ovule production to the post-dispersal crop of viable seed. We examined seed density and viability in a founder population of mountain birch (Betula pubescens ssp. tortuosa), aiming to quantify losses at different stages and examine potential selection forces on the reproduction success of the founder generation of an isolated population. At the time of the study (2017–2020), the population had recently reached reproductive maturity, following its colonization around 1990 through long-distance dispersal onto an early successional outwash plain in southeast Iceland. Seed densities were high, but 89% of apparently intact seeds did not contain an embryo, despite being visually indistinguishable from filled seeds. Externally evident losses amounted to about 45% of the total seed crop and were mostly due to predation by the gall midge Semudobia betulae. When all losses were accounted for, 2.7% of the seed crop remained viable and germinated. Pollen limitation may partially explain the high incidence of empty seeds. Excessive flower production is compatible with the predator satiation hypothesis but cannot explain pre-dispersal losses. Another adaptation to predation, masting, appears poorly developed in Iceland. Our results suggest the presence of constraints on the reproduction potential of the new island population, that are more limiting than in neighbouring populations, and we discuss their developmental, ecological, and environmental correlates.

Quantifying the roles of climate, herbivory, topography, and vegetation on tree establishment in the treeline ecotone

AbstractThe transition zone between the upper forest boundary and the treeless alpine vegetation (i.e., the treeline ecotone) is expected to shift upward in elevation with global warming. Multiple factors in addition to climate are influencing tree establishment, but it is uncertain to what degree. In this study, we took advantage of a unique dataset with field measurements of mountain birch trees acquired from 32 boreal–alpine treeline ecotone sites situated along a 1100‐km latitudinal gradient in Norway to investigate the role of climate, herbivory, topography, and vegetation on the establishment success of trees. To represent tree establishment, we recorded occurrence, survival, and height growth of short juvenile birch trees (stem height ≤1 m) as response variables at each site in 2008, 2012, and 2018. We used already available climate and herbivory data, and airborne laser scanning (ALS) and unmanned aerial vehicle (UAV) surveyed imagery together with field measurements to compute topography and vegetation explanatory variables per site on microsite scale (approx. 1 m2). We observed a clear decrease in short juvenile birch tree establishment due to livestock herbivory, evidenced by lower occurrence, reduced survival rate, and diminished height growth in treeline ecotones with higher livestock densities. Winter precipitation and the two land cover classes, tree‐covered area and lee side vegetation, were also important for tree establishment. However, the direction and magnitude of the relationships depended on the stage of establishment. While winter precipitation can lead to higher tree occurrence, long‐term survival is not necessarily ensured, reflecting diverse requirements for optimal conditions of tree establishment. Thus, our results show that occurrence, survival, and height growth of short juvenile birch trees differ in their requirements to climate, topography, and vegetation, but tree establishment is consistently low with livestock herbivory. Overall, our results suggest that treeline ecotones will shift toward higher elevations in areas with little or no livestock herbivory and close to already established trees and in lee sides. Regarding management and conservation issues, our results suggest that free‐roaming livestock can reduce tree establishment and hinder treeline ecotone shifts into threatened alpine ecosystems, at least for birch‐dominated treelines.

Ozone stress response of leaf BVOC emission and photosynthesis in mountain birch (Betula pubescens spp. czerepanovii) depends on leaf age.

Oxidative stress from ozone (O3) causes plants to alter their emission of biogenic volatile organic compounds (BVOC) and their photosynthetic rate. Stress reactions from O3 on birch trees can result in prohibited plant growth and lead to increased BVOC emission rates as well as changes in their compound blend to emit more monoterpenes (MT) and sesquiterpenes (SQT). BVOCs take part in atmospheric reactions such as enhancing the production of secondary organic aerosols (SOA). As the compound blend and emission rate change with O3 stress, this can influence the atmospheric conditions by affecting the production of SOA. Studying the stress responses of plants provides important information on how these reactions might change, which is vital to making better predictions of the future climate. In this study, measurements were taken to find out how the leaves of mature mountain birch trees (Betula pubescens ssp. czerepanovii) respond to different levels of elevated O3 exposure insitu depending on leaf age. We found that leaves from both early and late summers responded with induced SQT emission after exposure to 120 ppb O3. Early leaves were, however, more sensitive to increased O3 concentrations, with enhanced emission of green leaf volatiles (GLV) and tendencies of both induced leaf senescence as well as poor recovery in the photosynthetic rate between exposures. Late leaves had more stable photosynthetic rates throughout the experiment and responded less to exposure at different O3 levels.

Solar and Climatic Factors Affecting Tree-Ring Growth of Mountain Birch (Betula pubescens) beyond the Northern Timberline on Kola Peninsula, Northwestern Russia

A 105-year chronology (AD 1917–2021) was developed from mountain birch (Betula pubescens Ehrh.) from beyond the coniferous treeline on the Kola Peninsula in Northwestern Russia (68.86 N, 34.69 E). A total of 22 trees were cored, including the oldest living mountain birch of 105 years old. The highest correlations occurred for the May temperature (r = 0.39, p < 0.01) and July sunshine duration (r = −0.39, p < 0.05). The increase in radial growth in May seemed to be caused by snowmelt giving rise to soil temperature, which can lead to a resumption in radial growth after winter dormancy. The negative correlation with the July sunshine duration seemed to be connected to changes in the spectral composition of solar radiation in the red to far-red ratio in the end of the polar day in July. The application of wavelet coherency revealed a significant (>95%) connection between the radial growth of B. pubescens, and solar activity in frequency bands encompassed the main solar cycles: 5.5 years (the second harmonic of the Schwabe cycle), 11 years (the Schwabe cycle) and 22 years (the Hale cycle). The results show that the northernmost birch trees in Europe are suited for tree-ring research. This allows us to expand the area of dendrochronological research further beyond the conifer treeline above the Polar Circle.

Subalpine ”pinification”: Early signs of a pending and distinct biogeographic shift in the Swedish Scandes: Review and updates

This study focuses, by in situ records and long-term observations, on recent (post-Litte Ice Age), arboreal change in a mountain birch dominated (Betula pubescens ssp. czerepanovii) valley in the Swedish Scandes. During the early Holocene thermal optimum and up to the onset of the mid-Holocene Neoglaciation, Scots pine (Pinus sylvestris) dominated the tree cover of this valley and formed the treeline ecotone adjacent to the early alpine tundra. Subsequently, and consistent with progressive cooling until the late 19th century, prevailing pine stands demised and opened for landscape-level expansion of birch and spruce (Picea abies). A short and distinct break in that process took place by the Medieval Climate warming phase, about 1000-800 years before present. Subsequently, during pre-industrial time, temperatures reached their lowest levels of the entire post-glacial period. This was the so-called Little Ice Age, which ended the long-term Holocene cooling. That cold-climate epoque was broken by the late-19th and early 20th century. Thereafter and up to the present-day, temperatures in the study region (summer and winter) have increased by slightly less than 2 °C. As a consequence, treelines of all species have advanced by a maximum of more than 200 altitudinal meters. Pine displays the most persistent expansion, particularly over the past few decades. “Falangist” occurrences appear in the pure birch forest, tens of kilometers and hundreds of meters, respectively, beyond and above the outposts by the early 20th century. Occasionally, scattered young pine trees now grow close to the birch forest limit and somewhat above. In particular, at sites where pine stands demised during the Little Ice Age, c. AD. 1300-1850, prolific regeneration and insignificant winter mortality are recorded over the past 15 years. In comparison, birch and spruce provide no analogous signs of recent expansion. It may be hypothesized that unabated climate warming and corresponding arboreal progression will profoundly transform the plant cover of this valley, and others alike, into the same pine-dominated state that prevailed during the early Holocene. In other words, a new biogeographic zonation pattern may be on the rise, with pine back as the dominant subalpine species. This option is focused by continued monitoring.

Genetic variation and origin of mountain birch on a recently colonized glacial outwash plain by Vatnajökull glacier, southeast Iceland

Genetic variation and origin of mountain birch on a recently colonized glacial outwash plain by Vatnajökull glacier, southeast Iceland

Herbivore–shrub interactions influence ecosystem respiration and biogenic volatile organic compound composition in the subarctic

Abstract. Arctic ecosystems are warming nearly 4 times faster than the global average, which is resulting in plant community shifts and subsequent changes in biogeochemical processes such as gaseous fluxes. Additionally, herbivores shape plant communities and thereby may alter the magnitude and composition of ecosystem respiration and biogenic volatile organic compound (BVOC) emissions. Here we determine the effect of large mammalian herbivores on ecosystem respiration and BVOC emissions in two southern and two northern sites in Swedish Scandes, encompassing mountain birch (LOMB) and shrub heath (LORI) communities in the south and low-herb meadow (RIGA) and shrub heath (RIRI) communities in the north. Herbivory significantly altered BVOC composition between sites and decreased ecosystem respiration at RIGA. The difference in graminoid cover was found to have a large effect on ecosystem respiration between sites as RIGA, with the highest cover, had 35 % higher emissions than the next highest-emitting site (LOMB). Additionally, LOMB had the highest emissions of terpenes, with the northern sites having significantly lower emissions. Differences between sites were primarily due to differences in exclosure effects and soil temperature and the prevalence of different shrub growth forms. Our results suggest that herbivory has a significant effect on trace gas fluxes in a productive meadow community and that differences between communities may be driven by differences in shrub composition.

Decreased Soil Microbial Nitrogen Under Vegetation ‘Shrubification’ in the Subarctic Forest–Tundra Ecotone: The Potential Role of Increasing Nutrient Competition Between Plants and Soil Microorganisms

The consequences of warming-induced ‘shrubification’ on Arctic soil carbon storage are receiving increased attention, as the majority of ecosystem carbon in these systems is stored in soils. Soil carbon cycles in these ecosystems are usually tightly coupled with nitrogen availability. Soil microbial responses to ‘shrubification’ may depend on the traits of the shrub species that increase in response to warming. Increase in deciduous shrubs such as Betula nana likely promotes a loss of soil carbon, whereas the opposite may be true if evergreen shrubs such as Empetrum hermaphroditum increase. We analyzed soil organic matter stocks and 13C NMR fractions, microbial CO2 respiration, biomass, extracellular enzyme activities (EEAs), and their association with shrub density in northern Sweden after 20 years of experimental warming using open top chambers (OTCs). Our study sites were located in a tundra heath that stores high soil carbon quantities and where the OTCs had increased deciduous shrubs, and in a mountain birch forest that stores lower soil carbon quantities and where the OTCs had increased evergreen shrubs. We predicted that organic matter stocks should be lower and respiration and EEAs higher inside the OTCs than untreated plots in the tundra, whereas no effect should be detected in the forest. Soil organic matter stocks and 13C NMR fractions remained unaffected at both sites. When expressed as per gram microbial biomass, respiration and EEAs for carbohydrate and chitin degradation were higher inside the OTCs, and contrasting our prediction, this effect was stronger in the forest. Unexpectedly, the OTCs also led to a substantially lower microbial biomass carbon and nitrogen irrespective of habitat. The decline in the microbial biomass counteracted increased activities resulting in no effect of the OTCs on respiration and a lower phenol oxidase activity per gram soil. Microbial biomass nitrogen correlated negatively with evergreen shrub density at both sites, indicating that ‘shrubification’ may have intensified nutrient competition between plants and soil microorganisms. Nutrient limitation could also underlie increased respiration per gram microbial biomass through limiting C assimilation into biomass. We hypothesize that increasing nutrient immobilization into long-lived evergreen shrubs could over time induce microbial nutrient limitation that contributes to a stability of accumulated soil organic matter stocks under climate warming.

Correction to: Population structure and genetic variation of fragmented mountain birch forests in Iceland

Correction to: Population structure and genetic variation of fragmented mountain birch forests in Iceland

Twig selection on mountain birch Betula pubescens by winter‐feeding willow grouse Lagopus lagopus in a subarctic forest

In a subarctic forest at Kvaløya, northern Norway, willow grouse Lagopus lagopus fed at snow level by clipping bits of twigs from mountain birch Betula pubescens during winter. Birch has two types of twigs ending in a terminal bud: long twigs with a smooth bark, and short twigs with rings of thicker bark. The grouse selected ringed twigs above smooth twigs despite a surplus of smooth twigs in the forest. Ringed twigs had more bark cm−1 of twig length and a higher relative bark/wood ratio than smooth twigs. Smooth twigs had growth nodes that increased in diameter inwards from the tip. Because of the non‐linear relation between the area and the circumference of a circle, the bark/wood ratio decreased for each node. Although being able to clip much thicker twigs, 90% of smooth twigs clipped by grouse were ≤ 2 mm in diameter. It is concluded that willow grouse fed optimally on birch in winter by selecting twigs to minimize fibrous wood intake.

Trees out‐forage understorey shrubs for nitrogen patches in a subarctic mountain birch forest

Nitrogen (N), acquired by roots and mycorrhizal fungi and supplied to plant foliage, is a growth‐limiting nutrient at the subarctic treeline. Due to this limitation, interspecific competition and acquisition of N is an important control on plant community composition and distribution. The ability of trees and shrubs to access N shapes community dynamics at this ecotone undergoing species range shifts and changes in primary productivity driven by climate change. Using 15N soil labelling we investigate the fate of soil inorganic N, and spatial distances over which trees and understorey shrubs access soil N, in a treeline forest. 15N was injected into soil rooting zones in discrete 1 m2 patches and foliar samples were collected from trees between 1 and 50 m away, and understorey shrubs between 0.5 and 11 m away from labelled soil. The 15N label was found in mountain birch trees up to 5 m, and in understorey shrubs up to 2 m, away from labelled soil. We estimate that 1.27% of pulse‐derived N was found in foliage of birch trees, compared to 1.16% in the understorey. However, mountain birch trees contributed only 31% of ecosystem leaf area index (LAI), thus there was a disproportionate allocation of added label to the birch canopy compared with its contribution to ecosystem LAI. The difference in root and mycorrhizal exploration distances and community N partitioning between mountain birch trees and understorey shrubs may confer competitive advantage to trees with respect to nitrogen and nutrient patches, which may alter plant community structures within these forests. This is particularly important considering predicted climate‐driven tree and tall shrub expansion in subarctic regions, with likely consequences for ecosystem N and carbon (C) cycling, as well as for community composition and biodiversity.

Recent and Past Arboreal Change: Observational and Retrospective Studies Within a Subalpine, Birch-Dominated (Betula Pubescens Ssp. Czerepanovii) Mountain Valley in the Southern Swedish Scandes-Responses to Climate Change and Land Use

Multi-scale observations and reconstructions of treeline and associated plant cover changes are reviewed in this study. The main focus is on the dynamics of the treeline ecotone within a mountain valley in the southern Swedish Scandes. This area is well-researched for more than 100 years, with a background of historical studies covering the entire Holocene epoch. Trees were present on ice-free nunataks already during the Late-Glacial, 17 000-13 000 cal. a BP. Mountain birch (Betula pubescens ssp. czerepanovii), Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) then grew 400-700 m higher than their current treelines, indicative of summer temperatures 3 °C higher than present. In addition, the tree flora contained Siberian larch (Larix sibirica) and broad-leaf thermophilic and deciduous species, Quercus robur, Corylus avellana, Ulmus glabra and Alnus glutinosa. Eventually, the climate turned cooler, particularly enhanced around 5000 cal. a BP. This course of change initiated the Neoglacial period, when thermophilic species were extirpated. Concomitantly, mountain birch forest and spruce benefitted from this course of change. Pine, on the other hand was disfavored and regressed in the treeline elevation and general abundance on the landscape. These trends continued until the Mediaeval period AD 1000-1300, when temperature raised above present-day standards and the pine treeline locally shifted upslope by at least 100 m. This favourable epoch was discontinued quite abruptly by resurgence of Neoglacial cooling; the “Little Ice Age”, which prevailed until the late 19th century. Widespread glacier expansion, high-elevation arboreal decline and treeline retraction then occurred. With a gentle onset in the late 19th century or somewhat earlier, a modern and still prevailing warming period was accentuated by the late 1930s. Subsequently, slight cooling prevailed for 3-4 decades, whereupon warming was resumed with short interruptions until the present day. One distinct short-term cooling-conditioned setback occurred during the 1980s. Transient permafrost formed close to the treeline, and locally some treeline recession took place. Thereafter, warming has re-occurred and glaciers and permafrost have receded. Treeline rise and densification of the treeline ecotone are once again active processes in the mountainscape. Currently, most conspicuous is a process of pine reclamation of local ground lost during the Little Ice Age. Hereabouts, seed regeneration, still virtually without winter injuries, is taking place at an unprecedented rate and abundance. Mountain birch is gaining ground in snow rich depressions in the birch belt and is currently regressing in exposed and snow-poor sections of the local topography. Current biotic changes within the treeline ecotone are well within the ranges of inferred natural variabilty during the postglacial period.

Population structure and genetic variation of fragmented mountain birch forests in Iceland.

Betula pubescens Ehrh. (mountain birch) is the only forest-forming tree in Iceland. Since human settlement (874 AD), the continuous 25,000 to 30,000 km2 forest has shrunk to 1.200 km2 of fragmented patches, making it a good object to study population genetic consequences of habitat fragmentation and disturbance. Further, genetic studies have also shown that hybridization between the tetraploid (2n = 56) B. pubescens and the diploid (2n = 28) Betula nana L. (dwarf birch) occurs among Iceland's natural populations. This study assessed the genetic variation within and among 11 birch forests remaining across Iceland. Genotype-by-sequencing methodology provided a total of 24,585 single nucleotide polymorphisms (SNP´s), with a minor allele frequency >5% for genetic analyses. The analysis showed similar diversity within forests, suggesting that fragmentation and hybridization have had a limited effect on the genetic variation within sites. A clear genetic divergence is found among forests from the different regions of Iceland that may reflect historical isolation; the differentiation between forests increased with geographic distances reflecting isolation by distance. Information on the distribution of genetic variation of birch in Iceland is essential for its conservation and to establish genotype-phenotype associations to predict responses to new environmental conditions imposed by climate change and novel biotic/abiotic stressors.

Establishment of mountain birch (Betula pubescens ssp. tortuosa) on a glacial outwash plain: Spatial patterns and decadal processes.

Most of the Earth's surface has now been modified by humans. In many countries, natural and semi‐natural ecosystems mostly occur as islands, isolated by land converted for agriculture and a variety of other land‐uses. In this fragmented state, long‐distance dispersal may be the only option for species to adapt their ranges in response to changing climate. The order of arrival of species may leave a lasting imprint on community assembly. Although mostly studied at and above the species level, such priority effects also apply at the intraspecific level. We suggest that this may be particularly important in subarctic and arctic ecosystems. Mountain birch (Betula pubescens ssp. tortuosa) is characterized by great intraspecific variation. We explored spatio‐temporal patterns of the first two mountain birch generations on a homogeneous, early successional glacial outwash plain in SE Iceland that was the recipient of spatially extensive long‐distance dispersal ca. 30 years ago. We evaluated the decadal progress of the young population by remeasuring in 2018, tree density and growth form, plant size, and reproductive effort on 30 transects (150 m2) established in 2008 at four sites on the plain and two adjacent sites ca. 10 km away. All measured variables showed positive increases, but contrary to our predictions of converging dynamics among sites, they had significantly diverged. Thus, two of the sites (only 500 m apart) could not be distinguished in 2008, but by 2018, one of them had much faster growth rates than the other, a higher growth form index reflecting more upright tree stature, greater reproductive effort, and much greater second‐generation seedling recruitment. We discuss two hypotheses that may explain the diverging dynamics, site‐scale environmental heterogeneity, and legacies of intraspecific priority effects.

Microclimate relationships of intraspecific trait variation in sub‐Arctic plants

Within‐species trait variation is a substantial part of plant functional diversity. However, this intraspecific trait variation (ITV) is rarely investigated in relation to a key characteristic of the Arctic and alpine ecosystems: fine‐scale microclimatic heterogeneity. Here, we quantified the influence of microclimate (soil moisture, snow and local temperatures) on plant functional traits, specifically on ITV. We focused on six widespread northern latitude vascular plant species, and measured four traits: plant height, leaf area, leaf dry matter content (LDMC) and specific leaf area (SLA). We related ITV to field and remotely sensed microclimate data from 150 study plots within six study grids. The grids were located within a 76‐m altitudinal belt in three environments: the tundra, tundra–forest ecotone and mountain birch forest in Kilpisjärvi, northwestern Finland. We compared the range of trait values between this local trait dataset (n = 5493) and global trait databases (n = 10 383). We found that information in the local dataset covers a relatively large portion of the trait ranges in global databases. The proportion varies among traits and species; the largest portion was 74% for variation in leaf area of Vaccinium uliginosum, and the lowest was 19% for LDMC of Betula nana. We found that ITV in height was mostly related to local temperatures, whereas SLA and LDMC were more related to soil moisture and snow conditions. However, species showed contrasting relationships with the microclimate drivers. We conclude that microclimate profoundly shapes ITV in northern latitude plants and that even a very compact geographic area can contain a large amount of ITV. The influence of the microclimatic conditions varies among functional traits and species, which indicates that plastic response or adaptive potential of the species to climate change may also vary across species, but that necessary variation may often be present within local plant populations.

Long-term spatiotemporal dynamics in a mountain birch (Betula pubescens ssp. czerepanovii) forest in south-east Norway.

Mountain birch forest covers large areas in Eurasia, and their ecological resilience provides important ecosystem services to human societies. This study describes long-term stand dynamics based on permanent plots in the upper mountain birch belt in SE Norway. We also present forest line changes over a period of 70 years. Inventories were conducted in 1931, 1953, and 2007. Overall, there were small changes from 1931 up to 1953 followed by a marked increase in biomass and dominant height of mountain birch throughout the period from 1953 to 2007. In addition, the biomass of spruce (Picea abies) and the number of plots with spruce present doubled. The high mortality rate of larger birch stems and large recruitment by sprouting since the 1960s reveal recurrent rejuvenation events after the earlier outbreak of the autumnal moth (Epirrita autumnata). Our results demonstrate both a high stem turnover in mountain birch and a great ability to recover after disturbances. This trend is interpreted as regrowth after a moth attack, but also long-term and time-lagged responses due to slightly improved growth conditions. An advance of the mountain birch forest line by 0.71 m year-1 from 1937 to 2007 was documented, resulting in a total reduction of the alpine area by 12%. Most of the changes in the forest line seem to have taken place after 1960. Regarding silviculture methods in mountain birch, a dimension cutting of larger birch trees with a cutting interval of c. 60 years seems to be a sustainable alternative for mimicking natural processes.