Adaptation of resource acquisition and investment in resource acquiring tissues of Betula nana in response to climatic constraints

Rapid climate change in the Arctic is altering biological communities and their subsequent effects on ecosystem functioning. For example, warming‐induced shrub expansion accelerates biogeochemical cycles in part by increasing high‐quality litter inputs. Likewise, warming may enable higher densities of wolf spiders, which are dominant invertebrate predators whose activities indirectly alter plant litter decomposition rates. Although shrubs and wolf spiders are responding to climate change simultaneously, it is unclear how more shrub litter and more spiders together will influence elemental cycling in Arctic ecosystems. To test how warming could influence these processes, we used a fully factorial mesocosm experiment to quantify effects of wolf spiders on litter decomposition of an expanding species of dwarf deciduous shrub Betula nana under ambient and warmed conditions. We found higher densities of wolf spiders were consistently associated with more litter mass loss, and more C and N release regardless of warming treatment, indicating biotic interactions may be a stronger driver of short‐term B. nana litter decomposition than warming when wolf spiders are present. Our findings suggest the combined effects of warming‐induced shifts in plant and arthropod communities may further accelerate C and N cycling, which could cause positive feedbacks on Arctic shrub expansion.

The Arctic tundra biome is undergoing rapid shrub expansion ('shrubification') in response to anthropogenic climate change. During the previous ~2.6 million years, glacial cycles caused substantial shifts in Arctic vegetation, leading to changes in species' distributions, abundance and connectivity, which have left lasting impacts on the genetic structure of modern populations. Examining how shrubs responded to past climate change through genetic data reveals the demographic dynamics that shaped their current diversity and distribution and sheds light on the resilience of Arctic shrubs. Here we test scenarios of Quaternary demographic history of dwarf birch species (Betula nana L. and Betula Glandulosa Michx.) using Single Nucleotide Polymorphism (SNP) markers obtained from RAD sequencing and approximate Bayesian computation. We compare the timings of modelled population events with ice sheet reconstructions and other paleoenvironmental information to untangle the impacts of alternating cold and warm periods on dwarf birch. Our best supported model suggested that the species diverged in the Mid-Pleistocene Transition as glaciations intensified. We found support for a complex history of inter- and intraspecific divergences and gene flow, and secondary contact occurred during both ice sheet expansion and retreat. Our spatiotemporal analysis suggests that the modern genetic structure of dwarf birch results from transitions in climate between glacials and interglacials, with ice sheets acting alternatively as a barrier or an enabler of population mixing. Tundra shrubs may have had more nuanced responses to past climatic changes than phylogeographic analyses have often suggested, with implications for future eco-evolutionary responses to anthropogenic climate change.

Abstract The results of a ground-penetrating radar survey and multiproxy studies of the sediment cores collected from two lakes in the Valdai Highlands (East European Plain) provide new insights into the late glacial and Holocene environmental history of the region situated in the marginal zone of the last Scandinavian ice sheet. The cores were analyzed for organic carbon and nitrogen content, as well as for pollen and diatoms. The chronology of the cores is based on radiocarbon dates and pollen-based stratigraphy. The studied records document that vast dead ice masses and associated ice-dammed lakes existed in the Valdai Highlands area until ∼14 cal ka BP. Open tundra-steppe communities dominated the study area during the Oldest Dryas, Bølling, and Older Dryas (between ca. 17 and 14 cal ka BP), but dwarf birch ( Betula nana ), shrub alder ( Alnus fruticosa ), and willow ( Salix ) were also common. Scots pine forest ( Pinus sylvestris ) became common for a short interval during the Bølling warming (ca. 14.9 and 14.4 cal ka BP). The appearance of spruce ( Picea ) forest in the landscape occurred in the beginning of the Allerød warming (∼14 cal ka BP), but the open steppe-like plant communities remained common until the onset of the Holocene. The modern lake systems emerged at ∼10 cal ka BP, marked by an onset of organic-type sedimentation and the appearance of modern-type forests. The Mid-Holocene (∼8–4 cal ka BP) was the warmest time, as documented by the maximal distribution of temperate and broadleaved taxa in the region. The onset of agricultural land use and simultaneous trend of increasing lake trophic state and increasing paludification in the area is recorded at ∼2.5 cal ka BP.

The rhizosphere contains diverse groups of bacteria and fungi living near plant roots and mycorrhizal hyphae whose composition and function are key drivers of ecosystem and biogeochemical processes. Despite rich literature on rhizosphere communities, no studies have examined the drivers of rhizosphere communities across plants or soil types in the tundra. We collected 513 root samples from 141 individual plants representing six plant species and three mycorrhizal association types across four glacial histories in Northern Alaska. Glacial drifts ranged from 11,000 to 4.5 million years since deglaciation representing a gradient in glacial history and mineralogical weathering. We found that glacial history, a strong proxy for soil mineralogy, explained the most variation in rhizosphere bacterial communities (13.3%) while interactions between glacial history and host plants explained the most variation in fungal rhizosphere communities (11.6%). We found strong correlations between ectomycorrhizal and rhizosphere communities across spatial scales and sites for the shrub Betula nana (30.7% to 54.7% correlated), and that ectomycorrhizal composition was most similar among root fragments of the same plant, followed by plants at the same site, and plants at different sites. This work serves to advance the ecological understanding of rhizosphere and ectomycorrhizal communities in response to shrubification.

BackgroundIntrogressive hybridization is common in natural birch woodlands in Iceland, where two birch (Betula) species (diploid dwarf birch B. nana and tetraploid tree birch B. pubescens) coexist and hybridize readily. Our previous morphological, cytogenetic and palynological studies show that triploid hybrids are likely to have mediated gene flow between the two species. Our previous molecular study based on chloroplast haplotyping confirms the hybrid introgression and provides information about the genetic origin of Betula species in Iceland. The question remains, however, as to what extent nuclear gene flow is involved in this hybrid introgression process. The objective of the present study was therefore to use nuclear markers to probe birch introgressive hybridization.ResultsAFLP (Amplified Fragment Length Polymorphism) analysis was performed on genomic DNA isolated from 169 individual Betula plants (67 diploid B. nana, 82 tetraploid B. pubescens and 20 triploid hybrids), from birch woodlands in Iceland in comparison to those from northern Scandinavia. The generated 115 polymorphic markers were subjected to analysis of molecular variance across ploidy groups, locations, and major chloroplast haplotypes. A new R package, Linarius, was developed for use with this mixed ploidy dataset. All markers were considered nuclear as no allele specific to any chloroplast haplotypes was detected. The results were to a certain extent congruent with those from our previous chloroplast study. No ploidy- or species-specific alleles were detected. Almost all alleles were shared among all three ploidy groups, indicating gene flow via hybridization. The difference, however, was that the nuclear markers clearly differentiated between diploid B. nana and tetraploid B. pubescens, whereas the chloroplast haplotype variation between species was non-significant. The triploid hybrid group was scattered within both ploidy clusters, in line with its role as a bridge to introgression. This nuclear separation between the two species is comparable to that from our previous analysis based on species- specific morphological characters, implying that the whole genomes may be selected for species adaptability in their different habitats. Furthermore, the present AFLP study depicted a clear east–west geographical separation among Icelandic Betula populations, based on both genetic distance analysis and anamorphosis modelling. This geographical separation is prominent in B. nana while B. pubescens is more genetically homogeneous.ConclusionThe present study shows that despite extensive gene flow, Betula species maintain their species integrity and ploidy stability. This in turn allows the long-term survival of the species in their local habitats.

ABSTRACTPersistent warming and higher frequency of heat waves in the Arctic are causing alterations in Arctic vegetation and plant functionality, potentially redefining the role of the Arctic ecosystem. Vegetation influences atmospheric composition through exchanges of CO2 and volatile organic compounds (VOCs), both processes exhibiting a strong response to temperature variations. However, our quantitative understanding of how increased temperatures interact with extreme weather events, namely heat waves and drought, to affect Arctic plant processes remains limited. Here, we measure phenology, photosynthesis, leaf fluorescence and VOC emissions from three widely distributed Arctic shrubs, Betula nana, Empetrum hermaphroditum and Salix spp., in response to future climate. We use state‐of‐the‐art climate chambers to test the effects of warmer growth temperatures on Arctic shrub responses to heat waves and drought. Our results show that increased growth temperatures advance leaf unfolding by 24 days in B. nana and 17 days in E. hermaphroditum, and increase VOC emissions across species. For B. nana, photosynthesis decreased by 42% during the heat wave and by 72% during drought. In contrast, Salix spp. and E. hermaphroditum experienced decreased photosynthesis only during drought, by 62% and 71%, respectively. The VOC emissions during the heat wave shifted toward a less diverse compound profile: acetaldehyde emissions increased for both control and warmed plants in all species, and isoprene emissions increased in Salix spp. Additionally, plants grown at higher temperatures exhibited a twofold increase in emissions compared to control plants during the heat wave, suggesting a higher temperature sensitivity of emissions. Our study indicates that warming and increasingly frequent extreme weather events will significantly impact Arctic plant phenology, photosynthesis and the diversity and rates of VOCs emitted into the atmosphere, contributing to modifying the regional climate.

Hybrid plants were verified in Betula nana populations of the Urals by flow cytometry and were especially numerous in the northern part of the region. ITS1 nucleotide sequences were obtained by Sanger sequencing. Based on their analysis, low differentiation was observed for B. nana and B. pubescens and associated with current and ancestral hybridization processes. Intragenomic polymorphism of ITS1 sequences was assessed using the Illumina technology and detected both B. nana and B. pubescens rDNAs in the genomes of first-generation interspecific hybrids.

The continuous palynological record of sediments from the El'gygytgyn crater lake document the changes in vegetation during marine isotope stages 12-17, which span the interval between 424,000 and 712,000 years ago. An important chronological reference point for this interval is the Brunhes-Matuyama boundary (781,000 years ago), established by paleomagnetic analysis of the lake sediments. The regional vegetation of the warm interglacial stages was characterized by the widespread presence of larch forests, which included thickets of dwarf birch, alder, and dwarf pine. In the cold stages, a mosaic of herb tundra dominated, giving way to forest-tundra communities in the valleys of the Anadyr Plateau that surrounds the lake. In the interval between 424,000 and 712,000 years ago, the warmest climate occurred during MIS15, whereas the most severe cooling occurred during MIS12.

ABSTRACT Human disturbance in the Arctic is increasing. Abrupt changes in vegetation may be expected, especially when spots without vegetation are made available; additionally, climate change alters competition between species. We studied whether 34- to 35-year-old seismic operations had left imprints on local vegetation and whether changes could be related to different soil characteristics. The study took place in Jameson Land in central east Greenland where winter seismic operations in search of oil took place from 1985 to 1989. This area is dominated by continuous dwarf shrub heath with Cassiope tetragona, Betula nana, and Vaccinium uliginosum as dominant species. Using point frame analyses, we registered vascular plants and other surface types in frames along 10-m transects in vehicle tracks (hereafter “damages”) and in undisturbed vegetation parallel to the track (hereafter “references”) at eleven study sites. We also measured temperature and pH and took soil samples for analysis. Damaged and reference vegetation types were compared with Sørensen similarity indices and detrended correspondence analyses. Although most vascular plant species were equally present in damaged vegetation and in references the detrended correspondence analyses showed that at ten out of eleven study sites the damages and references still differed from each other. Graminoids and the herb Polygonum viviparum had the highest occurrence in damages. Shrubs and the graminoid Kobresia myosuroides had the highest occurrence in references. Cassiope tetragona was negatively impacted where vehicles had compacted the snow. Moss, organic crust or biocrust, soil, and sand occurred more often in damages than in references, whereas lichens and litter had the highest occurrence in references. The richness of vascular plant species varied between the eleven study sites, but between damages and references the difference was only up to four species. Temperature was the soil parameter with the most significant differences between damages and references. Total recovery of the damaged vegetation will most likely not occur within several decades. The environmental regulations were important to avoid more serious impacts.

The identification of materials collected in Svalbard led to the discovery of the species Stictis radiata (L.) Pers. This species is a facultatively lichenized fungus and an epiphyte whose main range lies outside the Arctic. We collected two specimens of this species in the Colesdalen, Svalbard. This place is one of ‘Arctic hotspot complexes’, as Arve Elvebakk called them, where the only place in the archipelago with growing Betula nana is located. Colesdalen also had suitable conditions of moisture and “sheltered” from the winds for the formation of sufficiently large woody remains of vegetation, which became a suitable substrate for Stictis radiata, whose main range is confined to the forest zone. The closest known locations of the species are in the Pechenga region of the Murmansk Region (Russia) and Scandinavia. This article discusses the ecological preferences and distributional features of the species.

Plant communities formed by shrub willows are widespread in the North of the Koryak Okrug (the mainland of the Kamchatka Territory) in valleys and floodplains of rivers and streams, on gentle mountain slopes, hollows and depressions, along the mire edges. The main dominants are Salix pulchra, S. alaxensis, and S. krylovii. Communities formed by some other species of shrubby willows (Salix lanata, S. saxatilis, S. glauca) are quite rare. The detailed geobotanical characteristics of shrub willow communities of the Koryak Okrug are presented. The dominant-determinant classification based on 54 relevés was elaborated following the main principles and classification methods of the Russian geobotanical school approach. As the result 21 associations, included into 13 association groups and 5 formations are revealed. The peculiarities of their species composition, communities’ structure, ecology and geographical distribution are considered. Communities formed by Salix pulchra are characterized by the higher community diversity in the study area; they occupied a wide range of habitats — from floodplains and tussock tundra to drained slopes of ridges. The low community diversity was typical for phytocoenoses formed by Salix lanata, which are quite rare in the North of the Koryak Okrug, but widespread in the Arctic regions of Eurasia. Shrub willow communities formed by Salix pulchra, S. alaxensis or S. krylovii are represented by Oligoherbosa, Calamagrostidosa, Varioherbosa, Caricosa appendiculatae and Vacciniosa uliginosi association groups; they occupy depressions, hollows, foots of ridges, the slopes of hills, and flat watersheds. Sphagnosa and Paludiherbosa willow communities are found along the mire edges and in the lake hollows. The predominance of herbaceous mesophytes is character for Varioherbosa association group; among these constant are Calamagrostis purpurea s. l. prevailed and Cirsium kamtschaticum, Chamerion angustifolium, Equisetum arvense, Galium boreale, Geranium erianthum, Rubus arcticus, Saussurea pseudotilesii, Thalictrum minus, Trientalis europaea, Veratrum oxysepalum, Viola epipsiloides. The moss layer is formed by Aulacomnium palustre, Brachythecium salebrosum, Bryum pseudotriquetrum, Climacium dendroides, Dicranum majus, Hylocomium splendens, Sanionia uncinata, Sciuro-hypnum reflexum, S. starkei, etc. Alluvial gley soils and podburs are character for the sites of Varioherbosa and Calamagrostidosa association groups. The predominance of mesohygrophytes and hygrophytes in the grass layer, and green and Sphagnum mosses in the ground layer are character for Paludiherbosa willow communities. The grass layer is usually predominated by sedges Carex appendiculata and C. lyngbyei subsp. cryptocarpa with common hygrophilous herbs (Comarum palustre, Equisetum fluviatile, Galium trifidum, Caltha palustris). Mosses Calliergon cordifolium, Plagiomnium ellipticum, Philonotis fontana, Pseudobryum cinclidioides, Rhizomnium punctatum, Straminergon stramineum occur in wet depressions. Gley, peat-gley and peat soils are character for Sphagnosa and Paludiherbosa willow communities. Arctoboreal dwarf-shrubs Empetrum nigrum s. l., Ledum palustre s. l., Vaccinium uliginosum s. l., V. vitis-idaea s. l., dwarf birches Betula exilis and B. middendorffii and mesophilous mosses are main species in the communities of Vacciniosa uliginosi association group. Caricoso lugentis-eriophorosa vaginatae association group formed by Salix pulchra is transitional to zonal tundra communities in which tussock-forming plants cotton grass Eriophorum vaginatum and sedge Carex lugens subsp. soczavaeana in the grass layer predominate. The community site differed by peat-gley and cryosols underlained by permafrost. The shrub willow community’s classification units were compared with the syntaxa identified due to the floristic (Brown-Blanquet) classification approach. Shrub willow communities occupy vast areas in river valleys and intermountain depressions; they are extremely important and valuable reindeer pastures during the summer and early autumn grazing seasons.

The high-resolution pollen record retrieved from dated sediments of the Khanka Depression made it possible for the first time to reconstruct evolution of vegetation in the south of the Russian Far East that occurred during one of the coldest phase of MIS 2 – the Yonger Dryas. The results of palynological analysis showed that strong global cooling occurred after the first, slight warming between 18200 and 15500 cal BP, which followed after one of the coldest and driest Gydan Stage of the Sartan Glaciation, again led to spread of boreal flora plants. The spruce and small-leaved forests, sparse larch and dwarf birch, alder and elfin pine forests, and also Sphagnum mires dominated in the ecosystems of the region. This plants were typical of the south of the Russian Far East during the drier and colder Gydan Stage of the Sartan Glaciation. Their ranges under the cold climate, which were significantly different from the modern ones, due to increasing cooling, again began to shift southward.

New palaeoecological records from two glacial lakes (the Mały Staw – 1183 m a.s.l. and the Wielki Staw – 1225 m a.s.l.) from the Polish Western Sudetes were obtained with the aim of better understanding the long-term vegetation development, the relationship between postglacial migration patterns, climate changes and human interference in mountainous areas, as well as to verify the local survival of some cold-adapted species during the Holocene maximum warming. Vegetation changes were reconstructed using pollen, spores and macrofossils. Several major stages of plant cover evolution over the last 12 000 years were identified. The end of the Late Vistulian (~12 100–11 700 cal BP) was documented for the first time in lake sediments from the region. During this period, the local vegetation was characterized by cold alpine meadows and patches of communities with shrubs (Betula nana, Alnus viridis, Salix, Juniperus, Ephedra) and trees growing at some distance from the lakes. In the Early Holocene, the expansion of boreal forests, consisting of Betula, Pinus sylvestris, as well as continental Larix and Pinus cembra, reached an altitude of ~1180 m a.s.l. An important discovery was the presence of Larix macrofossils in both studied profiles, which together with pollen evidence, confirmed its local persistence from the Early Holocene to the Middle Ages. It was also demonstrtaed that Betula nana, Selaginella selaginoides, Huperzia selago most probably persisted in the area from the Younger Dryas to at least the Middle Ages or even to the modern times, surviving through the Holocene climatic optimum. The increase in grassland representatives from ~4100 cal BP and the appearance of the cultivated plants (Triticum type pollen) from ~3300 cal BP, was due to the long-distance transport of pollen reflecting the development of agriculture and settlement outside the Karkonosze Mountains. It was not until the 10th century AD that the environment underwent a stronger anthropogenic transformation. Growing economic activities (e.g. metallurgy, mining of non-ferrous metal ores, glass production, forest industry) that developed, especially from the 12th century onwards required the supply of wood raw material. The development of agriculture in the region promoted the expansion of meadows and pastures and the greatest taxonomic diversity of herbaceous plants was recorded between the 13th and 15th centuries.

In this study, we describe plant communities along the mesotopographic gradient in the low‐elevation subcontinental mountains of NE Finland (Utsjoki region). We sampled vascular plants, bryophytes and lichens along 18 mesotopographic ridge‐snowbed transects comprising a total of 180 plots. We used non‐metric multidimensional scaling (NMDS) ordination with envfit to explore the differentiation of plant communities in relation to mesotopography, elevation, rock cover, cover of bare ground, snowbed size and snowmelt time. The classification of communities was performed using DIANA clustering. Plant communities were differentiated along the mesotopographic gradient, snowmelt time, elevation and rock cover. The DIANA analysis distinguished seven clusters corresponding to the following communities: Betula nana–Lichenes heath, Empetrum–Myrtillus–Stereocaulon heath, Empetrum–Pleurozium–Lichenes heath, graminoid‐rich snow‐protected heath, Oreojuncus trifidus–Avenella flexuosa snow‐protected heath, Polytrichastrum sexangulare–liverwort snowbed, and Salix herbacea–Kiaeria starkei snowbed. Because of the strong impact of snowmelt time on plant community structure and distribution of communities, it is likely that climate change‐induced changes in snow conditions are affecting tundra vegetation and especially snowbeds are threatened. Snowbed communities in the Utsjoki region roughly align with previously described vegetation associations of mountain areas in NW Europe. The assignment of the graminoid‐rich snow‐protected heath community remains uncertain.

The tiny fishing settlement of Tareya (73.253389° N, 90.596806° W) on the right bank of the river Pyasina (Fig. 1, this and others see in text) in its middle reaches (Western Taymyr) is well known in the circumpolar scientific community due to the long-term Biogeocenological field station of the Komarov Botanical Institute of the Academy of Sciences of the USSR, which operated in 1965-1977. A huge amount of complex researches has been done by numerous scientists, and the results were published in a lot of proceedings, reports at the Arctic conferences, and papers published in various journals, which formed the basis of several monographs as well as the large article in the multi-volume international edition «Ecosystems of the world» [Chernov, Matveyeva, 1997]. It was the reason why just this site was considered as point number one for doing work within the project “Back to the Future” (hereinafter BTF). The idea of visiting the sites of long-term work carried out in circumpolar Arctic within UNESCO “International Biological Program” arose in connection with the popular concept of global warming. The BTF task suggested to assess the current state of arctic ecosystems in details studied half a century ago. In several sites in the North American Arctic this was achieved on the eve of the International Polar Year (2008) [Callaghan et al., 2011a]. The Taymyr trip, took place in July–August 2010. Only the first author worked at the station from its beginning in 1965 and last time was there 40 years ago (1970). The period of field works in 2010 (July 21 – August 8), was not promising for detailed researches due both to the extremely short (18 days) stay and unfavorable weather. Botanists managed to re-inventory the flora of vascular plants and assess their activity in landscape, to make relevés at two permanent experimental stands and selectively some communities as well walk around the territory with vegetation map [Matveyeva, 1978]. The results on the flora were published [Matveyeva et al., 2014]. This paper presents the results of assessing the state of plant cover. We were well aware that opportunities for such a short time of repeated study in assessing the state of ecosystems and making not just expert conclusions about any changes, but to evaluate these quantitatively and to explain their reasons, were minor. In our case, different not only at moments far apart in time, but also at the same time in the past were the methodology doing relevés, including the size of sample plots, the totality of species records and quantitative assessment of their presence in communities, as well as professionalism by researchers, including their field work experience. We kept all this in mind when assessing the results, trying to distinct objectivity, subjectivity and expertise when interpreting these. In the past, detailed studies were carried out at six permanent sites [see: Matveyeva, 1968, 1969; Matveyeva et al., 1973], the most important of which were zonal communities on watersheds – frost-boils and hummock stands. DRyad–sedge–moss frost-boils stand (Matveyeva, 1968: Fig. 1, 3-5, Table 1; Matveyeva et al., 1973: Fig. 4, Table 1. Site N 2) is located on terrace above the floodplain close to high river bank of approx. 10 m high. In the checklist of Taymyr communities, according to the dominant classification [Matveyeva, 1985] it is classified as the ass. Hylocomium splendens var. alaskanum+Aulacomnium turgidum+Tomentypnum nitens–Carex ensifolia+Dryas punctata; according to the Zürich-Montpellier (hereinafter Z-M) school floristic one (Braun-Blanquet (hereinafter B-B) approach) – to the ass. Carici arctisibiricae–Hylocomietum alaskani Matveyeva 1994. In the past, the relevés were carried out on two sample plots located in close proximity to each other, 10 × 10 m (in 1966) and 15 × 15 m (in 1969), with lists of species (vascular plants, mosses, liverworts, lichens) according to 3 nanorelief elements (soil patch, rim, trough), with measurements of their size, and the horizontal structure schemes on both. We did not find those sample plots in 2010, so the relevé was performed on a new plot of 10 × 10 m. Only vascular plants were guaranteed to be identified totally with assessment of their abundance/cover on the B-B scale while that of bryophytes and lichens was estimated only for the most common and large-sized species, relatively easy identified in the field. Due to nanorelief of cryogenic genesis community horizontal structure is of 3-item regularly cyclic type [see. Matveyeva, 1988, 1998], with module repeating in space: soil patch (up to ~0.8 m diam.) at different stages of overgrowth on the medallion (up to 1.3 m diam.) +rim along its periphery (up to ~0.5 m wide)+trough (~0.3 m wide) between medallions (Fig. 2). This type of horizontal structure was preserved in 2010, although some values of element sizes were close, but not identical (Appendix 1, Table П1). However, the fact that after more than 40 years the number of modules per 100 m2 (32 and 31) and the ratio of their elements (patches 30%, rims 50%, troughs 20%) are the same, is rather evidence in favor of the horizontal structure stability, with variances due to measurement error of items widely varying in shape. Visually, the share of bare soil decreased slightly (no more than 2-3%), that caused a minor increase of total community plant cover, ~90% in 1966, 1969. up to ~92% in 2010. The dominating species in the ground layer were Hylocomium splendens var. alaskanum, Aulacomnium turgidum, Tomentypnum nitens, in the sparse upper one – Carex bigelowii ssp. arctisibirica and Dryas punctata. There were 197 species (60 vascular plants, 49 mosses, 27 liverworts, 61 lichens) on two sample plots, being different (135 and 180) on each one, due to some nuances of methodology (the lower number in 1966 is the work of a beginning graduate student, while later is the professional job by specialists in bryophytes; the lichen number was close because lichenologists were working on both plots). This community is the richest in species known in circumpolar Arctic [Matveyeva, 2009]. In 2010, on 10 × 10 m plot the composition of vascular plant species was identified with assessment of their abundance/coverage in points on the B-B scale for the entire area; that of bryophytes and lichens was estimated only for the most common and large-sized species. The most abundant ( 1%) species in the sparse low dwarf shrub-herbaceous layer were the same as before – sedge Carex bigelowii subsp. arctisibirica and dryad Dryas punctata. 11 species (all previously with low abundance/occurrence or single specimen) were not found, including two (underlined) in the past were recorded only on one of the two plots – Androsace chamaejasme, Cardamine bellidifolia, Koeleria asiatica, Orthilia obtusata, Papaver pulvinatum, Pedicularis capitata, P. hirsuta, Petasites frigidus, Nardosmia gmelinii, Ranunculus nivalis, Saxifraga oppositifolia, Vaccinium vitis-idaea subsp. minus) and found, also single specimen, 6 (Carex misandra, Eriophorum brachyantherum, Hedysarum arcticum, Polygonum bistorta, Ranunculus affinis, Saxifraga foliolosa). Such small variances gave practically the same species richness of vascular plants – 55/57 and 56. The abundance of species and their pattern at nanorelief elements remained unchanged except the cover increase of the most active species in the landscape – sedge Carex bigelowii subsp. arctisibirica. For entire community with rims occupying half of its area, this gives an increase of ~10% in layer density, i. e. the sedge abundance over the whole area remained the same (2 points). As cryptogams composition was not completely assessed, we cannot comment their richness, however all co-dominants in ground layer (mosses Hylocomium splendens var. alaskanum, Aulacomnium turgidum, Tomentypnum nitens and liverwort Ptilidium ciliare), as well species with previously significant ( +) cover kept their abundance. The obtained results provide the basis for a partly objective, partly expert conclusion that there are no significant changes in the composition of species and in their distribution within this stand. DRyad–sedge–moss hummock stand [Matveyeva, 1968: Fig. 6, 8, Table 1; Matveyeva et al., 1973: Fig. 3, Table 1. Site N 1] is located on the first terrace above the floodplain in the upper part of stream valley gentle slope at 1.5 km from the riverbank. In the checklist of Taymyr communities, according to the dominant classification [Matveyeva, 1985] it is classified as the ass. Hylocomium splendens var. alaskanum+Aulacomnium turgidum+Tomentypnum nitens–Carex ensifolia+Dryas punctata. This community with closed cover is the same in dominants as the above frost-boils one: Hylocomium splendens var. alaskanum, Aulacomnium turgidum, Tomentypnum nitens, and Ptilidium ciliare in the ground layer, and Carex bigelowii ssp. arctisibirica and Dryas punctata in the sparse upper one. Despite the common dominants and significant number of species with similar abundance, communities with closed cover are poorer in species due to the lack of species obligate to bare or partly overgrown soil. The positioning of such communities in the classification of the Z-M school (B-B approach) was not proposed. In the future, it is possible either to describe new association or to identify a subassociation. There is nanorelief of cryogenic genesis, caused by frost ground cracking and its consequences – hummocks 0.10-0.12 m high and 0.15-0.30 m diam. which sometimes, merging together, form chains or almost locked rims, and troughs 0.15-0.20 m wide, with no patches of bare soil (Fig. 3). The type of horizontal structure is irregular mosaic (Matveyeva, 1988). In 2010, what awaited us in this community was not just a surprise, but rather a shock. A transformation took place that we [Matveyeva et al., 2011; Matveyeva, Zanokha, 2013] formulated as “polygonization” of loamy watersheds – the previously leveled surface (with described nanorelief) turned into a system of mounds (7-10 m diam.) and trenches (2-5 m wide) with significant (0.5-1.0 m) excess in height (Fig. 4). In terms of the area size and the pattern of heterogeneity with rows of mounds and trenches, these are most similar to the massifs of bajdzharakhs (the Yakutian name for mounds that appears a result of the fossil ice wedge melting). Such serious changes occurred without disturbances in the plant cover, as well as in the absence of erosion, with the previous nanorelief and the same irregular mosaic type of horizontal structure both on the surface of mounds and their almost vertical slopes, and in trenches. Since there were no signs of this until 1994 (evidence from colleagues who worked here after 1970), and the system already existed in 2003 (Google Earth Quick Birds, 8.11.2003), the transformation has occurred in less than 9 years. We were not able to find the old sample plots in 2010, and only a wooden stick and small (10 × 20 and 50 × 50 cm) metal frames (used for horizontal structure study) near it convinced us that this was the same permanent stand. More than 40 years later, the horizontal structure on both new microrelief elements looked the same: the familiar combination of hummocks and troughs, but visually the surface became smoother due to the decrease in the height of the elements relative to each other. The link of species with nanorelief elements did not change, with the same dominants on hummocks (mosses Hylocomium splendens var. alaskanum and Aulacomnium turgidum, sedge Carex bigelowii ssp. arctisibirica and dwarf shrub Dryas punctata) and in troughs (Тоmentypnum nitens and Ptilidium ciliare and the same vascular plants but with lower abundance). In general, the variances in species composition between the sample plots in 1966 and 1969 were similar to those recorded in the frost boils stand, but noticeably more dissimilar (69 and 141), and not only in cryptogams but in vascular plants (Appendix 1, Table П3). In 2010, full information was obtained only about vascular plants: 43 species (32 and 33 on 2 mounds) with the same dominants both on mounds and in trenches that were previously on the flat stand surface. The abundance of sedge Carex bigelowii ssp. arctisibirica has increased up to 3 points versus 2 and that of cotton grass Eriophorum angustifolium to 2 versus 1 and +, with the same abundance of dwarf shrubs Dryas punctata and Cassiope tetragona. We found no changes in species composition or abundance in dry trenches compare to the formerly flat surface of the community and the current mound one. The second object is 3-element rim-polygonal mire. RIM-POLYGONAL MIRE [Matveyeva et al., 1973: Fig. 4, Table 3. Site N 4] in 1969 was located in: flat-concave lake depression on a river terrace above the floodplain in about 1 km from the riverbank. There are from hundreds to thousands of modules polygon center+rim+trench – wet polygon 15-20 m diam. with 1) concave center and 2) rim along it periphery 1.0-1.5 m wide, rising (0.15-0.20 m) above central part and 3) water trenches between polygons in a polygonal system (Fig. 6). Quite arbitrarily, without assigning their vegetation to any units of any classification, lists of species were made for three microrelief elements. Altogether there are 110 species (vascular plants 24, mosses 47, liverworts 24, lichens 15) were identified, with respectively 34 (10, 24, 0, 0) on polygon centers, 80 (16, 28, 21, 15) on rims, and 34 (8, 23, 3, 0) in trenches. Co-dominants in continuous moss layer are Cinclidium latifolium, Sarmentypnum sarmentosum, Scorpidium revolvens, Meesia triquetra on polygon centers and in trenches, and Aulacomnium turgidum, Hylocomium splendens var. alaskanum, Tomentypnum nitens on rims; these in the sparse above moss layer are Carex aquatilis subsp. stans and C. chordorrhiza on polygon centers and Carex aquatilis subsp. stans in trenches, and Betula nana, Dryas punctata and Salix pulchra on rims. The classification of such complex object is debatable in all respects, beginning from the relevé methodology (choice of sample plots, their size, number) as well as defining the object status. It is most logical to consider the plant cover of each of the 3 elements as communities, trying to classify them independently, however this is not too obvious: there are 18 numbers in the scheme legend, that demonstrates both the obvious cover complexity (3 types of communities) and the mosaic nature of each type – 7 units on polygon centers, 8 on rims, 3 in trenches. In the Z-M school system (B-B approach), vegetation on polygon centers and in trenches is classified as mires of the class Scheuczerio–Caricetea nigrae (Nordh. 1936) R. Tx. 1937; while that on rims as communities close to zonal ones of the class Carici arcrtisibiricae–Hylocomietea alaskani Matveyeva Lavrinenko 2023 (ass. Carici arcrtisibiricae–Hylocomietum alaskani Matveyeva 1994). In 2010, we not only failed to make relevé on previous sample plot, but could not determine its exact location in wet depression. This was because the general picture of microrelief in the area, where site in question was situated, was so different from described above, that an attempt to obtain a photo of a “classical” rim-polygonal mire for a lecture course for students (which was so easy to do before) turned in vain: there were only isolated hummocks due to partial going down (subsidence) of most rims (Fig. 7), In another massif (south of Lake Bolshoye), which vegetation on map [Matveyeva, 1978] is shown as a 3-item rim-polygonal mire, all rims went downwards, and the polygon surface became flat (Fig. 8, а). As a result, the previously clearly heterogeneous plant cover visually (from a human height) became looked homogeneous. Although heterogeneity remained (Fig. 8, б): in 2010, obviously hygrophilic grasses (Carex aquatilis subsp. stans, Eriophorum medium, Hierochloë pauciflora) and mosses (Sarmentypnum sarmentosum, Cinclidium latifolium, Scorpidium revolvens, Meesia triquetra, etc.) and just as obviously mesophilic shrub/dwarf shrub (Betula nana and Dryas punctata) and mosses (Aulacomnium turgidum, Hylocomium splendens var. alaskanum, Tomentypnum nitens, etc.) cohabit at the same surface level with high soil moisture. Anyone who has seen this would not be able to find an adequate explanation for this phenomenon without knowing the past of such areas. Our expert conclusion is that, despite significant transformations in microrelief, the heterogeneity of plant cover as well as species composition are the same as before, with slight change in the abundance of some dominants. Another type of polygonal complexes is developed in the upper reaches of numerous brook valleys. BOG-TUNDRA POLYGONAL COMPLEX [Matveyeva et al., 1973: Fig. 5, Table 3. Site N 3] in 1969 was located on a river terrace above the floodplain in 1 km from the riverbank in a depression in the upper reaches of a short valley directly close to settlement. The structure of sample plot (50 × 60 m) is a complex of drained polygons of diverse shape and size (15-30 m diam.) and trenches (0.5-6.0 m wide and 0.2-0.3 m deep), filled with water (Fig. 9). The area ratio polygons/trenches is 80/20%. The name of the complex reflects the heterogeneity of its vegetation. Plant cover on polygons is close to that of low watersheds with dominance of willows Salix reptans, S. pulchra and dwarf birch Betula nana in the shrub layer, sedge Carex bigelowii subsp. arctisibirica and cotton grass Eriophorum angustifolium and dwarf shrubs Dryas punctata, Cassiope tetragona, Vaccinium vitis-idaea subsp. minus in dwarf shrub–herbaceous, and Aulacomnium turgidum, Hylocomium splendens var. alaskanum, Tomentypnum nitens in moss one; and mire in trenches with the same shrubs as on the polygons, sedge Carex aquatilis subsp. stans and cotton grass Eriophorum angustifolium and hygrophilic mosses Sarmentypnum sarmentosum, Cinclidium latifolium, Scorpidium revolvens, Meesia triquetra, Polyrichum jensenii. There were 85 species (35 vascular plants, 41mosses, liverworts were not detected, 9 lichens), respectively – 59 (30, 20, 9) on polygons and 35 (12, 23, 0) in trenches. The classification of this object is no less problematic in all respects, as of rim-polygonal mire vegetation. Most logical is to consider the vegetation on each of two elements as communities and try to classify them separately, which is quite difficult. There are 19 numbers in the map legend – two community types with 13 inside units on polygons and 6 ones in trenches. Such complexes so far have not been described in literature. In 2010, visually everything looked as before, however this conclusion is subjective being based upon only on two routes through a vast complex system, including a stationary site with wooden sticks. TUNDRA AND NIVAL-MEADOW COMMUNITIES ON THE SOUTHERN SLOPE OF THE RIVER BANK [Matveyeva et al., 1973: Fig. 7, Table 4, Site N 5]. The steep slope, is cut by hollows (with 3-5 m snow beds) formed due to the ice wedge melting (Fig. 10). Ridges, in winter with little snow, melting completely in June, are in summer the warmest biotopes with the maximum (up to 1.5 m) depth of frozen ground seasonal thawing. The great biotope diversity determines the heterogeneity of the plant cover, with elements small (2-3 m2) in size that form ecological series, contrasting in soil moisture and heating. There are 13 community types on sample plot (70 × 70 m). The most contrasting in comparison with stands in zonal habitats were in 1969 and remained (visually) in 2010 are herb communities on ridges (Fig. 11) with grasses (Festuca brachyphylla, Koeleria asiatica, Trisetum sibiricum subsp. litorale) and forbs (Astragalus alpinus, Cerastium maximum, Myosotis alpestris subsp. asiatica, Oxytropis adamsiana, Pachypleurum alpinum, Pedicularis verticillata, Polemonium boreale) from 0.10-0.15 to 0.30-0.35 m high and thin (up to 0.01 m), and sparse moss layer of Hypnum revolutum, Sanionia uncinata, Thuidium abietinum. Later such community types became the object of close attention [Zanokha, 1993] in different areas of Taymyr (but not in Tareya), and was classified as the ass. Pediculari verticicillatae–Astragaletum arctici Zanokha 1993, but with no placing in any higher unit. The plant cover of such herb communities, in terms of life form set and horizontal and vertical cover structure is closest to boreal meadows of the class Molinio-Arrhenatheretea Tüxen 1937, however composed of not boreal, but of arctic and arctic-alpine species, that stops these from being placed in this class. As well, conditional is the positioning [Matveyeva, Lavrinenko, 2021] of such communities in the class. Mulgedio-Aconitetea Hadač et Klika in Klika et Hadač 1944. In 2010, the lists of vascular plant species were compiled for such herb communities along the whole riverbank of the field station area, and no differences were recorded in their activity [Matveyeva et al., 2014]. It is worth to notice that the methodology for getting data in the past is not described, and it differs from that adopted in the Z-M school. This will not allow objectively assessing possible changes in the future that should be kept in mind by those who will manage to visit this area. VEGETATION UNDER MAN IMPACT [Matveyeva et al., 1973; Fig. 8. Site n 6]. In 1965, when six BIN researchers arrived to Tareya, life in tiny fishing settlement was in full swing. The basis of this was a vast man-made cave in the permanently frozen ground of the high river bank. It was used to store fish that was caught by teams of fishermen from Norilsk State Industrial Enterprise, scattered across the vast Western Taymyr territory. Fishermen were flown to “points” on AN-2 planes, from where the catch was regularly taken, brought to Tareya, frozen and stored until the autumn fishing season, when ships with barges arrived along the river from the Norilsk city. There were three small houses (at the edge of the floodplain) for living and a house where the radio operator lived and worked. In addition, there was a large plank house owned by the Arctic and Antarctic Research Institute (AARI), permission for its use became the basis for organization of a long-term BIN field station (Appendix 2, Fig. П1). In the first summer (1965), the pioneer group lived in a plank house (future laboratory). The following summer, scientific field station began to function, which gathered from 18 (1966) to 30-40 (1967-1969) people from various scientific institutes, who lived in numerous tents located on a gentle slope between the laboratory and the radio operator' house. After 1977, the living buildings continued to be used by fishermen, as well as geologists. Fishing intensity gradually decreased becoming private. In a spring (the year is unknown) high flood, three small houses were carried away by water; the laboratory house was burned down in 1998. Before 1965, the plant cover was quite changed, since for a long time the base of the geological expedition of the AARI was located here. Its initial state is dryad-sedge-moss hummock tundra, common on gentle slopes with the dominance of mosses Aulacomnium turgidum, Hylocomium splendens var. alaskanum, Tomentypnum nitens, sedge Carex bigelowii subsp. arctisibirica, dwarf shrub Dryas punctata. During the field station functioning, the load (trampling) on plant cover in summer (late June–early September) was quite strong. In 1968, the vegetation of the territory was verbally described, and a map-scheme was made, with 12 items in legend [Matveyeva et al., 1973]. In 1968, the most of area between houses, where the original vegetation was damaged almost completely, was occupied by suppressed and sparse grass cover. In 2010, it looked like the original dryad-sedge-moss tundra, with no obvious signs of disturbance and with no high abundance of apophytic grasses (Alopecurus alpinus and Poa alpigena). However walking along, it at the end of July–beginning of August was possible only in rubber boots, i. e., the soil moisture was significantly higher than before, when we lived in tents and walked in light sports shoes. Vegetation map. The conclusion that in 2010 communities have kept their belonging to the same earlier classified community types is made according to their look when walking around the territory with vegetation map that would not have to be changed (Fig. 13). Some of the objectivity of this opinion is supported by the fact that it was done by the researcher who made this map, as well as by few relevés, where the community structure and species composition remained the same. Flora of vascular plants. There were 212 species on the territory that was studied in 2010 [Polozova, Tikhomirov, 1971]. After 40 years, we did not find 29 species (all rare in the landscape) and discovered 10 new ones (all in the floodplain of the Pyasina River, rare, many in a single specimen). We refer a reader to the publication [Matveyeva et al., 2014], the main conclusions of which are as follows: 1) the main reason for the incomplete identification of the flora is the short duration of the research in 2010; 2) there is no firm conviction that newly found species were absent 40 years ago; 3) assuming that the last are still present, the systematic and geographical structure flora remains unchanged. It is possible to assess changes in species activity within landscape only for a total of 184 species – in 162 (88.5%) it remained unchanged, in 5, with the same activity, abundance slightly increased or decreased; activity decreased by 1 point in 22 (mean and low active) species. Small changes in the landscape pattern of species with low activity may be considered both objective and subjective (short duration of observations in 2010 and uncertainty in estimations in the 1971 annotated list). No information was obtained on the cryptogam flora (mosses, liverworts, lichens), earlier detailed studied. Our partly expert opinion is that their composition and presence in communities have not undergone noticeable changes. However, for an objective assessment it is necessary to conduct studies similar to those that were done at high professional level [Pijn, Trass, 1971; Blagodatskikh, 1973; Zhukova, 1973]. THE DISCUSSION OF THE RESULTS. The most general conclusion based on the results of various observations in the course of repeated (after 40 years) visit to the area of long-term field station functioning can be formulated as follows: stability in the plant cover with significant transformations in the landscape, micro- and nanorelief, and as a consequence in changes in surface/inside soil water flow. From the diverse cryometamorphic processes, we focus the most significant and noticeable one, that might considerably change the plant cover on the above-floodplain terrace, where previously there were 2 systems, both in depressed landscape sites: 1) rim-polygonal mires (in lake depressions, bottoms of drained lakes of thermokarst origin) and 2) bog-tundra complexes (concave surfaces of watersheds, dissected by trenches as a result of backward erosion). The third one, with flat mounds of different height and size and trenches of various width and depth, appeared in zonal sites. (Fig. 15). This happened on a large area, lot of watersheds is transformed completely with some (most wide and flat) being so far rather uniform. The beginning or early stages of this process in the form of future polygonal system were recorded already in 1968 by geocryologist [Danilov et al., 1971]. In 2010, already in the field on many interfluves between brook valleys, especially on the widest ones, with a horizontal surface in their middle part, we observed the beginning of polygonization so far with no upcoming mound exceeding the trenches in height ( 1-2 cm), which is clearly visible on satellite images (Fig. 16). Potentially, the presence of trench system on watersheds may strengthen the hydrological cycle through higher inside soil flow (that will eliminate trench wetting), however as drainage system it will reduce the moisture amount on watersheds, that may lead to larger frozen soil seasonal thawing, and greater thermokarst in zonal sites. What will be a result of such large transformation is a subject of professional interest for geocryologists. We can only state the landscape instability, which was not recorded 40 years ago in Tareya. The second phenomenon of significant change is the coming down of rims in rim-polygonal mires, in the place of which only isolated hummocks remained, or the surface of the polygons has become flat, the most important consequence of which was a radical change in hydrological regime. In classic rim-polygonal mire systems, the water on the isolated concave polygon centers surrounded by rims is standing water, while in trenches between polygons it is running, and there is a general waterway, which gathers water from connected trenches. This is the source of brooks through which the general (surface and inside) water flow is running away the wetland (Fig. 17). Without rims, the previously standing water on polygons, being no longer isolated, has become running, that increased the total flow (a kind of drainage). On the downed rims, the plant cover is so far (visually) the same. Although the fact that the mire, heavily watered throughout the growing season in 1967-1969 (and according to satellite image in 2003), in 2010 has lost part of water, affected the activity of the most important grasses – the abundance of Hierochloë pauciflora and Carex chordorrhiza previously dominated on the most watered polygons became less, while that of Carex aquatilis subsp. stans (previously also rather abundant) increased. This expert conclusion is based on difficulty in finding the first two species, which previously were common in these biotopes. At first sight, our judgment about stability in plant cover along with great landscape transformation, looks at least contradictory. In our defense, we propose thesis that stability does not mean the absence of any changes. The latter includes changes in the activity (abundance, occurrence) of some vascular plant species, dominants in communities in zonal sites (Carex bigelowii subsp. arctisibirica) and in mires (Carex chordorrhiza, C. aquatilis subsp. stans, Hierochloë pauciflora). However, for the majority (88.5%) of species it remained unchanged; for few ones the abundance slightly increased or decreased, which did not cause noticeable changes in the structure of communities and their diversity. To explain the slight increase in the cover density on ground patches in frost-boils stands and that of main dominant, the long-rhizome sedge Carex bigelowii subsp. arctisibirica, is hardly makes sense to attach the argument, most common in the last decade, about global warming. A series of questions arises – what do we know about vegetation before we worked in this area 40 years ago? how much do we know about the life cycles of Arctic species populations, about the species individual growth? as well, without single-vector climate trend, changes in vegetation do not occur? or we ignore natural succession? Our conclusion about the stability of syntaxonomic diversity, with small changes in the communitiy structure and with minor variation in vascular plant species set in local flora and their activity in landscape, in general coincides with the opinion of colleagues, who worked within the BTF project in Canada and Greenland, and repeated studies over shorter periods in Alaska and the European North, differing in minor details. This is inspiring and at the same time amazing, because only on Taymyr (besides Tareya, in the Dickson area) this stability takes place against the background of spectacular landscape transformation – polygonization of watersheds and modification of rim-polygonal mires. The formation of the third polygonal system on watersheds, in addition to the widespread polygonal mires and bog-tundra polygonal complexes in depressions, may continue, which gradually lead to radical transformation of the Arctic landscape on the plains. However, to predict exactly, what consequences will follow, is difficult. The existence of new formed trenches proposes their greater moisture, in comparison with mounds and the former flat surface, but the fact that these are not isolated, but form system, suggests a drainage effect. We are not ready to predict to what extent the intra-soil moisture runoff increasing will balance or exceed the current greater moisture in trenches, this is a matter for soil scientists. However, there is no doubt that the dynamics of vegetation in zonal sites depends on this, and significant changes in the plant cover may be expected over vast areas. The data obtained by us and other researchers in different Arctic regions indicate the stability of the plant cover in the course of the period that coincides with the ascending wave of climate warming in high latitudes, which is the second in the 20th century [Vize. 1937; Rosenbaum, Shpolyanskaya, 2000; Malinin, Vainovsky, 2018], even in situations of mobile landscape.

The main objective of the study was to evaluate the population characteristics of Betula nana under different anthropogenic influences. The study was conducted in the vicinity of the exploited Šepeta peatland (northeastern Lithuania). The population status of B. nana was determined by comparing the ramet density and morphology (height, branching, and leaf size), the age structure, the number of generative ramets, and their flowering characteristics in four study areas at different distances from the exploited peatlands and in different habitats. Around 20 environmental factors were included in the analysis, covering water levels, peat, and vegetation characteristics. Shading, drainage and increased amounts of nitrogen in the habitats are the main factors contributing to the differences and structure of B. nana cenopopulations. Although taller ramets with larger leaves are observed under the changed conditions as an adaptation to shading, the negative anthropogenic effects in the most affected habitats are reflected in a reduction in the number of flowering ramets, lower vegetative regeneration, and an increase in the number of dead twigs on mature ramets.

Climate change can have positive and negative effects on the carbon pools and budgets in soil and plant fractions, but net effects are unclear and expected to vary widely within the arctic. We report responses after nine years (2012−2021) of increased snow depth (snow fences) and summer warming (open top chambers) and the combination on soil and plant carbon pools within a tundra ecosystem in West Greenland. Data included characteristics of depth-specific soil samples, including the rhizosphere soil, as well as vegetation responses of NDVI-derived traits, plant species cover and aboveground biomass, litter and roots. Furthermore, natural vegetation growth through the study period was quantified based on time-integrated NDVI Landsat 8 satellite imagery.Our results showed that summer warming resulted in a significant and positive vegetation response driven by the deciduous low shrub Betula nana (no other vascular plant species), while snow addition alone resulted in a significant negative response for Betula. A significant positive effect of summer warming was also observed for moss biomass, possibly driven increasing shade by Betula. The aboveground effects cascaded to belowground traits. The rhizosphere soil characteristics differed from those of the bulk soil regardless of treatment. Only the rhizosphere fraction showed responses to treatment, as soil organic C stock increased in near-surface and top 20 cm with summer warming. We observed no belowground effects from snow addition. The study highlights the plant species response to treatment followed by impacts on belowground C pools, mainly driven by dead fine roots via Betula nana. We conclude that the summer warming treatment and snow addition treatment separately showed opposing effects on ecosystem C pools, with lack of interactive effects between main factors in the combination treatment. Furthermore, changes in soil C are more clearly observed in the rhizosphere soil fraction, which should receive more attention in the future.

Can dwarf birch (Betula nana) growth rings be used as indicators of permafrost degradation?

Abstract In the Arctic tundra, warming is anticipated to stimulate nutrient release and potentially alleviate plant nutrient limitations. Typically simulated by fertilization experiments that saturate plant nutrient demand, future increases in soil fertility are thought to favor ectomycorrhizal (EcM) over ericaceous shrubs and have often been identified as a key driver of Arctic shrub expansion. However, the projected increases in fertility will likely vary in their alleviation of nutrient limitations. The resulting responses of shrubs and their mycorrhizae across the gradient of nutrient limitation may be nonlinear and could contradict the current predictions of tundra vegetation shifts. We compared the functional responses of two dominant shrubs, EcM dwarf birch (Betula nana) and ericaceous Labrador tea (Rhododendron tomentosum), across a long‐term nitrogen and phosphorus fertilization gradient experiment in Arctic Alaska. Using linear mixed‐effects modeling, we tested the responses of shrub cover, height, and root enzyme activities to soil fertility. We found that B. nana cover and height linearly increased with soil fertility. In contrast, R. tomentosum cover initially increased, but decreased after surpassing the intermediate levels of increased soil fertility. Its height did not change. Enzyme activity did not respond to soil fertility on EcM‐colonized B. nana roots, but sharply declined on R. tomentosum roots. Overall, the nonlinear responses of shrubs to our fertility gradient demonstrate the importance of experiments grounded in replicated regression design. Our results indicate that under moderate increases in soil fertility, Arctic shrub expansion may not only include deciduous EcM shrubs but also ericaceous shrubs. Regardless of shifts aboveground, changes in root enzyme activity belowground point to some EcM shrub species playing a more influential role in tundra soils; as EcM roots remained steady in their liberation of soil organic nutrients with heightened soil fertility, degradative root enzyme activity on the dominant ericaceous shrub dropped—in some instances with even the slightest increase in fertility.