Arctic Plants Under Environmental Stress: A Review

Abstract

Arctic plants inhabit extremely cold environments and are exposed to a range of abiotic stress factors. Arctic species exhibit remarkable adaptability to multiple environmental challenges, including a short growing season, low summer temperatures, continuous 24-h daylight during the polar day, limited nitrogen availability in soils, water scarcity, and strong winds. This review examines the key features of growth, development, and reproduction in Arctic plants, as well as their physiological and genomic adaptations to extreme climatic conditions. While Arctic plants show remarkable physiological tolerance, community-level resistance varies regionally and remains an open question.

1. Introduction

The Arctic covers approximately 10% of the Earth’s surface at high latitudes and represents one of the planet’s most extreme environments [1]. Arctic vegetation was formed under conditions of repeated glaciations, geological landscape transformations, and permafrost development [2]. Since the Last Glacial Maximum (LGM), when massive ice sheets covered most of North America and Eurasia, many plant species that survived in refugia have gradually colonized the ice-free areas they now occupy [3]. Low temperatures, a short growing season, and limited nitrogen availability have created one of the world’s least productive and most species-poor ecosystems [4].

Arctic plants grow beyond the northern tree line. Their resistance and adaptability allow them to endure diverse environmental stresses, including recurrent frosts, low summer temperatures, continuous daylight during the polar day, water scarcity, and strong winds [5]. Environmental stressors such as cold, drought [6] and intense light [7] exert strong selective pressure on the regulation of cellular and physiological processes [8], particularly affecting nutrient and energy uptake, photosynthesis, and reproduction [9].

Three main groups of primary producers dominate in Arctic plant communities: vascular plants, bryophytes, and lichens [10]. Lichens and mosses are generally more tolerant of extreme climatic conditions [11,12]. Compared with their temperate relatives, Arctic plants display numerous distinctive morphological and physiological traits [5]. They exhibit long life cycles; the development of shoots and flower primordia over multiple growing seasons [13,14,15,16,17]; well-developed root systems [18], high resistance to cold and drought [19,20,21]; low chlorophyll content [22,23]; and 24-h photosynthetic activity under polar-day conditions [24,25]. The purpose of this review is to analyze available data on the main features of Arctic plant growth, development, and reproduction, their morphological, physiological and genomic adaptations to extreme cold climates, and the responses of plant communities to ongoing climate warming in high-latitude regions.

2. Growth, Photosynthesis and Resistance

2.1. Arctic Dwarfs

The vascular plants of the Arctic flora have developed a complex set of morphological adaptations to cold climates. Most species in tundra communities are hemicryptophytes and chamaephytes, characterized by dwarf growth forms that develop close to the soil surface [26]. Plant height typically ranges from 1 to 20 cm, and many species display rosette or semi-rosette growth forms [27]. Most grasses and sedges form tussocks, in which dead leaves create a protective layer against strong winds and icing [28]. Cushion-shaped (pillow) growth forms are common among both herbaceous and shrub species, providing favorable microclimatic conditions, and are typical of mountain tundra in the Low Arctic and of polar deserts in the High Arctic (Figure 1).

There are very few therophytes; only seven annual species have been recorded in the polar deserts [26]. The limited number of annual and biennial species in the Arctic is mainly due to the short growing season, which prevents completion of the full life cycle. The causes of dwarfism in Arctic plants remain a subject of scientific debate [29,30,31,32]. Growth limitation in polar latitudes extends beyond low temperatures and includes the extremely short growing season, strong winds, and low light intensity. The molecular mechanisms underlying specific Arctic plant adaptations—such as dwarfism and slow growth—are still not fully understood [5]. Morphological adaptations to cold include prostrate growth, shortening of vegetative shoots, and a reduction in the size and number of metameres [30,31] (Figure 2). Dwarfism is ecologically associated with warmer air temperatures near the soil surface and protection from wind [27,28]. As a result, Arctic species can survive not only through winter but also during the cold, short summer, when temperatures are only a few degrees above the physiological limit for growth. In the high-latitude polar deserts, the “miniaturization” of life becomes especially evident.

Decreased growth intensity in Arctic plants was identified by B. A. Yurtsev [29] as one of the primary forms of resistance to the extreme conditions of the Arctic. Yu.V. Gamaley [32] proposed a hypothesis suggesting that, depending on the balance between light-dependent synthesis of assimilates and their distribution, plants develop either a “leaf-type” or a “stem-type” habitus. Arctic dwarf species represent the “leaf-type” habitus, in which growth remains incomplete due to suppression of photosynthate outflow [32], thereby realizing their growth potential to a lesser extent than their photosynthetic capacity [33]. All Arctic plants show increased growth under experimentally elevated temperatures. Phytotron experiments have demonstrated significant differences between the temperature optima for photosynthesis (+10–15 °C) and for growth (above +20 °C) [33,34]. According to Yu.V. Gamaley [32], adaptive evolution resulted in the paradox of “temperature scissors”: the optimum temperature for photosynthesis in cold climates occurs at levels (10–15 °C) where full realization of growth is impossible, since the growth optimum is much higher (above +20 °C). As a result of this evolutionary adaptation, a remarkable diversity of life forms has emerged, reflecting various strategies for stabilizing functional balance.

All Arctic plants are characterized by microphyllous leaves—very small foliage. A reduction in leaf surface area and leaf thickening, tissue compaction (resulting from a decrease in intercellular space volume), weak cell vacuolization, an increased number of stomata per unit area, a dense venation network, and folding and thickening of the cuticle are indicators of suppressed leaf cell growth [35]. Some authors [30] suggest that the reduction in leaf area is caused by a decrease in the number of cell divisions. This represents an adaptation to the short growing season: when cell division ends earlier, chloroplasts mature more rapidly. Structural compensation for small leaf size involves an increase in the active components of the cell, including the volume of the chondriome, the surface area of chloroplasts, and the extent of the endoplasmic reticulum [35,36].

2.2. Photosynthesis and Photosynthetic Pigments

The Arctic is predominantly inhabited by C₃ species utilizing the Calvin cycle. Plants growing in various Arctic regions are considered ideal “Calvin species”, as carbohydrates constitute the primary products of photosynthesis [24]. These plants photosynthesize under continuous daylight during the polar day, and 24-h assimilation is one of their key ecophysiological characteristics. This phenomenon was first described in 1928 by Müller (cited in [26]) for Salix glauca in western Greenland, then by S. P. Kostychev in 1930 on the Barents Sea coast, and by H. G. Wager in 1941 for Oxyria digyna and Beckwithia glacialis in eastern Greenland. Subsequent studies confirmed 24-h assimilation in vascular plants across various Arctic regions [24,25,37,38,39,40]. Most Arctic vascular plants begin to grow in late spring after snowmelt, when air temperature and photosynthetic activity increase [41]. The rate of photosynthesis remains relatively high from mid-June to mid-August. Arctic plants are adapted to photosynthesis across a wide range of light intensities [42]. Because of the low solar angle and weak light intensity, photosynthesis in most species reaches saturation at moderate or even low illumination levels [40]. Evergreen shrubs can photosynthesize even beneath the snow in early spring, when light becomes sufficient for carbon assimilation [38]. The light compensation point of Arctic plants varies considerably—from 0.65 to 2.5 klux—which is comparable to values found in desert plants.

Arctic plants photosynthesize across a wide temperature range. Some species are photopsychrophiles—photosynthetic organisms obligatorily adapted to low temperatures (0–15 °C) but unable to survive at higher temperatures (≥20 °C) [25]. Photopsychrotolerant species, by contrast, are more resistant and can survive within a broad thermal range (5–40 °C). Photopsychrophily and photopsychrotolerance are likely the result of multiple cellular adaptations enabling survival in the extreme polar environment [25]. The optimal temperatures for photosynthesis in High Arctic plants are close those in the Subarctic (

Table 1. Optimal temperatures for photosynthesis of plants in the Arctic and Subarctic.

Plants	Region	T °C	Authors
Vascular plants	Wrangel Island (High Arctic)	+10–20	[24]
Vascular plants	Kola Peninsula (Subarctic)	+15–20	[24]
Mosses	Khibiny Mountains (Subarctic)	+10–24	[44]

). The lower temperature limit for photosynthesis is usually determined by the freezing point of the protoplast and the onset of intracellular ice formation. In the Khibiny Mountains of the Kola Peninsula, this threshold ranges from –5 °C to +1 °C [43].

The ability of photosynthesis to occur at 0 °C and subzero temperatures is enabled by enzyme systems capable of functioning in extreme environments. Enzymes in Arctic mosses rapidly increase their activity with rising temperature, particularly within the range of 0–10 °C [44]. Genomic studies of Arctic grasses have shown that transcripts associated with photosynthesis and abiotic stress responses are significantly enriched among cold-responsive genes in all studied species [19]. Some plant species from both the Low and High Arctic exhibit high rates of CO₂ assimilation not only at low but also at elevated temperatures. Such results have been obtained in Wrangel Island, the Kola Peninsula, and Canada [24,43]. The observed optimum for photosynthesis in many Far Northern species is +15–20 °C, with an upper limit of +30–37 °C and occasionally up to +40–44 °C [43]. The optimum potential intensity of photosynthesis is most often between +20–25 °C and +20–30 °C. CO₂ uptake by plants on Wrangel Island occurs at temperatures up to +40–45 °C, and in the Taimyr region and the Khibiny Mountains up to +50 °C [24]. Overall, the temperature range for photosynthesis in Arctic species is exceptionally wide, from −3–6 °C to +45–50 °C. The high upper temperature limit of photosynthesis is determined by genetically fixed (hereditary) traits of the species. Several authors have noted a weak correlation between the optimum temperature for photosynthesis and the ambient environmental temperature [45,46]. Many plant species are adapted to the thermal conditions of their origin environments but also possess the capacity to acclimate to short-term temperature fluctuations in their habitats [47]. However, the mechanisms underlying these responses remain poorly understood. Numerous studies have shown that photosynthetic reactions vary geographically, suggesting a genetic adaptation of species to the climates of their origin [46,48].

Chlorophyll content decreases under extreme conditions and with increasing latitude toward the North. Most Arctic plants contain low levels of chlorophyll—typically 1–2 mg/g of fresh weight—while species with higher concentrations are rare [49]. In vascular plants from Wrangel Island, the total chlorophyll (a + b) content ranges from 0.5–2.9 mg/g of fresh weight t [43]. Similar results were obtained in the Arctic tundra of the Taimyr Peninsula, where chlorophyll content was 40–60% lower than in comparable species from the vicinity of St. Petersburg [49]. Among Arctic plants of different life forms, chlorophyll concentrations can vary by as much as forty times [39]. Researchers have suggested that chlorophyll quantity is species-specific and influenced by the microclimatic conditions of the habitat. The lability of this parameter is probably an adaptive trait that ensures a broad photosynthetic range, promoting species dominance in various plant communities and enhancing survival under environmental stress [39]. N. Shmakova and E. Markovskaya [22] studied pigments in eleven Saxifraga species from Svalbard and found that chlorophyll content correlates with growth form, increasing from cushion plants to semi-rosette forms and then to rosette plants. Plants with primitive cushion-like growth forms exhibit the lowest pigment content. A low chlorophyll content indicates a potentially reduced rate of photosynthesis and overall biological productivity [40] but cushion plants compensate for this through a larger volume of above-ground photosynthetic biomass [23]. Ninety-eight species of vascular plants from twenty families (Caryophyllaceae, Brassicaceae, Saxifragaceae, Rosaceae, Poaceae, Cyperaceae, Juncaceae et al.) were studied on West Svalbard Island [23]. Chlorophyll content varied widely among species: approximately 70% had low chlorophyll levels (up to 1.0 mg/g of fresh weight), while about 5% exhibited maximum values (above 2.0 mg/g of fresh weight). The authors identified two groups of Arctic angiosperms. The first group includes species with chlorophyll content below 1 mg/g of fresh weight (Caryophyllaceae, Brassicaceae and Saxifragaceae), which dominate in Arctic plant communities. The second group (above 1 mg/g of fresh weight) consists mainly of species from the families Juncaceae, Cyperaceae, Rosaceae and Poaceae, which prefer specialized habitats, with only a few being dominant within plant communities [23].

3. Genomic Adaptations to Abiotic Stressors

The ability of plants to survive under subzero temperatures is linked to frost tolerance [19]. For Arctic species, maintaining the integrity of cell membranes is crucial to prevent osmotic stress during prolonged freezing [50]. Frost resistance develops through a range of physiological modifications mediated by distinct molecular pathways [51]. These include an increase in intracellular sugar concentration, changes in membrane lipid composition, and the synthesis of antifreeze proteins [52]. Although the molecular mechanisms underlying genomic adaptations of Arctic plants to abiotic stress are still not fully understood, active research in this field has expanded in recent years [1,8,19,20,21,41,53,54,55,56,57].

Arctic plant genomes possess a notable feature—a high frequency of gene duplications [58,59]. Duplicated genes can enhance expression levels and improve tolerance to environmental stress [60]. Additional gene copies may also accumulate mutations and develop new functions or expression patterns, increasing genomic plasticity [8]. Key genes involved in cold and shade responses independently maintain duplicated copies across different evolutionary lines [61]. These extra copies have evolved new interactions with other genes, contributing to plant survival in cold climate [8]. Many Arctic plant genes are inhibited under abiotic stress [62]. However, most stress-response genes display similar expression changes across species, suggesting that regulatory mechanisms are likely ancestral and shared among diverse plant taxa [8]. Genes involved in signal transduction, stress responses, redox homeostasis, photosensitivity, and meiosis play key roles in Arctic plant adaptation to stress [1]. Molecular studies of Eriophorum vaginatum populations in Arctic Alaska revealed correlations between allele frequency, temperature, and precipitation along a latitudinal gradient [41]. In the genome of E. vaginatum, transcription factors were identified that belong to gene families associated with stress response [63]. These genes are critical important for plant adaptation to extreme temperature fluctuations in the Low Arctic [41]. Recent evolutionary studies indicate that diverse plant species often use the same genes, gene families, and regulatory networks for adaptation [57,64].

One of the first genomic models identifying adaptations of Arctic plant species to extreme environmental factors was the genome study of Draba nivalis (Brassicaceae), which demonstrated that its stress adaptations are polyvariate [1]. The genome of D. nivalis contains expanded gene families associated with drought and cold stress tolerance, including genes involved in maintaining of redox homeostasis, meiosis, and signaling pathways. It remains unclear whether Arctic plants exhibit cold responses distinct from those of their closely related temperate species [21]. Genetic studies of Arctic Brassicaceae species have shown that the cold responses of Cardamine bellidifolia, Cochlearia groenlandica and Draba nivalis are highly species-specific, with most cold-induced genes unique to each taxon. It is interesting that the number of genes shared by all three Arctic species and the temperate Arabidopsis thaliana exceeds the number of genes shared exclusively among the Arctic taxa. This suggests that the cold responses of Arctic Brassicaceae evolved independently, although certain components have likely been conserved throughout the family’s evolutionary history [20]. Thus, C. bellidifolia, C. groenlandica, and D. nivalis utilize different genes within similar stress-response pathways, indicating that each represents an independent line of adaptation to the Arctic climate [21]. These results demonstrate that Arctic plants exhibit polyvariate molecular strategies for responding to low temperatures rather than a single unified pathway [21]. A molecular study of five Arctic cereal species revealed that 10–30% of highly conserved genes respond to cold, consistent with the cold-adaptation genetic programs observed in Arabidopsis thaliana [19,53]. At the same time, nearly half of all cold-responsive genes in these cereals were species-specific according to differential expression analyses [19]. These studies indicate that the evolution of cold acclimation occurred at least partially independently across lineages. Although species follow distinct evolutionary trajectories, their cold-response mechanisms are largely built upon a general genetic foundation [19]. Overall, genomic adaptations of Arctic plants involve multiple genetic pathways shaped by positive selection for cold and low-light tolerance, often realized through different genes and functional routes in different species. Available genomic data suggest polyvariate, possibly independent trajectories of cold adaptation, although conserved stress-response modules may persist across taxa.

Very little is currently known about the genomes of Arctic plants. In the Arctic, cold temperatures and short growing seasons favor genomes capable of rapid replication [65], while also elevated rates of polyploidy [66]. Recently, one of the first genomic sequencing results was published for thirteen dominant Arctic plant species from Svalbard [67]. The same researchers analyzed draft genomes of the genera Oxyria and Cochlearia using long-read sequencing data [67]. It was found that Oxyria digyna and Cochlearia groenlandica possess relatively small genome sizes and low chromosome numbers. The creation of complete chromosome-reference genomes of Arctic plants will enable the study of the genomics of adaptation to extreme conditions of polar latitudes [57]. This allows for the exploration of potential convergent similarities in repetitive elements, including centromere types [68] and telomeric repeats. Several unique multiple telomeric repeats have recently been identified in Arctic plants. It is possible that the evolution of repetitive regions had occurred in the extreme climate of the Arctic [67].

K. Elphistone [57] conducted molecular–genetic studies of plant species from various families in Arctic Canada. She produced a genome assembly for Oxyria digyna and subsequently compared the genomes of four diploid Arctic species—Oxyria digyna, Dryas octopetala, Draba nivalis, and Cochlearia groenlandica—with annotated genomes of several non-Arctic species from Polygonaceae (five species), Brassicaceae (five species), and Rosaceae (six species). The results demonstrated that many gene functions were enriched in the expanded and/or contracted gene families of all four Arctic species. These functions were primarily associated with responses to abiotic factors such as temperature, light, water, and, also, to biotic interactions [57]. It is well known that Arctic plants increase cellular sugar content to mitigate stress caused by rapid freezing and thawing. This process is supported by new evidence showing the expansion of gene families involved in polysaccharide binding and starch synthesis across all four studied species [57]. However, genomic responses of Arctic species to temperature stress appear to be more complex. Only a few expanded or contracted gene families related to temperature response were identified, except in O. digyna. Functions associated with temperature response and heat-shock protein binding were expanded in D. nivalis and C. groenlandica but contracted in D. octopetala [57]. In the polar deserts of the Canadian Arctic, GO terms related to “water response” were significantly enriched by genes belonging to expanded gene families in O. digyna and D. octopetala. Genomic adaptations to light were driven by gene-family expansions in three species—O. digyna, C. groenlandica, and D. nivalis. Enriched GO terms included “blue light signaling pathway”, “cellular response to blue light”, “response to red or far-red light”, “photosynthesis light harvesting” and several related categories.

4. Reproductive Development

The reproductive strategies of Arctic plants are shaped by the timing of snowmelt, the short growing season, and low temperatures [15,69,70]. Adaptation to the brief vegetation period at high latitudes is reflected in the extension of the complete generative cycle across several seasons [17]. The initiation and differentiation of floral primordias typically occur in summer and autumn of the year preceding flowering. Such observations have been reported for plants in Greenland [13], Alaska [14], the Canadian Arctic Archipelago [15,71], Chukotka [72], Taimyr [73] and the Kola Peninsula [16,17,74]. Low temperatures often inhibit flower differentiation in some Arctic species, and their reproductive buds may take several seasons to fully develop [74]. T. Sørensen [13] examined 169 plant species in northern Greenland and found that, during winter, floral primordia of 92% of species were at various developmental stages. Similar data were obtained by H. Hodgson [14] in Alaska, where all high-latitude grass species possessed inflorescence primordia during winter, while more southern ecotypes did not. The primary function of this strategy is to allocate resources in advance for the subsequent growing season [75,76].

A study of reproductive buds in plants from the polar semi-desert of King Christian Island (Canada) revealed that most species (27 out of 35) formed reproductive buds one year prior to flowering, with floral primordias becoming visible in July [15]. The flowers of Potentilla hyparctica and Papaver radicatum were nearly fully developed by autumn, while the development of reproductive buds in Puccinellia vaginata continued for several years. It was also found that some floral buds did not develop fully by the flowering phase—up to 50% of reproductive buds in Papaver radicatum and Potentilla hyparctica were eliminated before plants emerged from winter dormancy [15]. Similar data were obtained in tundra communities of the Khibiny and Lovozersky Mountains on the Kola Peninsula [16,17,74,77]. Studies of floral organogenesis in dwarf shrubs Arctous alpina, Phyllodoce caerulea, and Cassiope tetragona showed that floral initiation and differentiation began in June, one year before flowering. The androecium and gynoecium formed in the middle of the growing season; floral organogenesis was completed by September. Polyvariance of reproductive development of plants was revealed, which was due to reflecting differences in the duration of the primary intra-bud phase (from one to several years) and in the development period of monopodial monocarpic shoots—from initial bud formation to flowering (ranging from 2 to over 10 years) [16,17,77]. Depending on climatic conditions, floral meristems are not formed on vegetative shoots for several years [77]. Terminal inflorescences of A. alpina typically develop in the 3rd–4th year of shoot growth, although flowering may occur as late as the 5th or even 10–11th year [16]. In P. caerulea, terminal inflorescences form in the 3rd–4th, and occasionally the 8th year of growth [17].

The beginning of flowering in Arctic plants correlates with the timing of snowmelt and changes in total solar radiation intake [78]. Flowering is regulated by temperature, photoperiod, or a combination of both (Figure 3).

Several researchers have suggested that for early—flowering Arctic plants, the main abiotic factors influencing reproduction are photoperiod and the time of snowmelt, whereas for late-flowering species the key driver is cumulative heat during spring and summer [79,80]. Temperature is generally considered the dominant abiotic factor affecting flowering in the Arctic [81,82].

Cassiope tetragona is a model species for studying how temperature variation affects vegetative and reproductive development in the High Arctic [83,84]. C. tetragona is a polycarpic shrub with monopodial growth and intercalary inflorescences [74,77]. This species blooms for many consecutive years: 10–18 years in the mountain tundra of the Subarctic [74] and up to 26–35 years in the polar deserts of the Arctic archipelagos [83]. Floral primordias of C. tetragona are formed one year prior to flowering [13,74]. In the Kola Subarctic, the initiation of inflorescence primordias begins in the 5th–6th year of vegetation; floral meristems form in June, and flower organogenesis develops in July. According to N. P. Deeva [73], C. tetragona exhibits the most differentiated reproductive buds among tundra plants of Taimyr, with flowers and microspores fully formed by early winter. J. F. Johnstone [84] observed in the Canadian Arctic that flower production in C. tetragona was positively correlated with vegetative growth during the previous year but negatively correlated with growth in the current year. This pattern was interpreted as a resource-allocation strategy, in which developing flowers receive priority in internal resource distribution immediately after initiation [84]. The strongest correlations in flower number were recorded with May–June temperatures of the preceding year. Experimental studies involving winter icing and altered snow depth in Arctic habitats have demonstrated that winter climate changes negatively affect the flowering success of dwarf shrubs such as C. tetragona [85,86,87]. On Svalbard, experimental winter icing significantly reduced flower numbers in the following growing season, likely due to winter damage [88]. In another experiment on Svalbard, increased snow depth and delayed snowmelt resulted in later onset of the first phenological phases in early-flowering species such as Cardamine, Cassiope, Dryas, Papaver, Salix, and Saxifraga [85]. This delay has also been documented in alpine and subarctic ecosystems [79].

Several studies have demonstrated that flowering in Arctic tundra ecosystems is highly sensitive to changes in summer temperatures [71,80,87,89,90]. Higher temperatures and a longer growing season positively influence flower abundance [80,87,89], whereas low temperatures at the onset of the growing season can cause freezing and floral mortality, thereby reducing population reproductive success [90]. Flower production appears to be the most temperature-sensitive parameter of all reproductive traits [71]. Some Arctic plant species exhibit accelerated growth and flowering under climate warming, though these responses are species-specific [91,92,93].

Numerous studies have examined reproductive responses to increased ambient temperatures in the Arctic [94,95,96,97,98]. In the High Arctic of Greenland, long-term monitoring of six plant species revealed that flowering duration decreased with rising summer temperatures [99]. Climate warming in future will provide the shortening of flowering seasons in Arctic tundra ecosystems [100], although longer growing seasons and greater biomass accumulation may create additional resources for flower formation [80,101]. Experimental studies in the Low Arctic of Alaska showed that prolonged warming led to earlier flowering and increased flowering duration and density in both evergreen and deciduous shrubs [98]. For example, the prolonged blooming of the early-flowering shrub Arctous alpina is important for pollinators, serving as an early nectar source for bumblebees emerging from hibernation [102]. In Alaska, only evergreen and deciduous shrubs responded to long-term warming by increasing flower density, whereas this effect was not observed in herbaceous species [98]. Similar findings were reported from Arctic Canada, where Frei and Henry [103] observed increased flowering density under prolonged warming, although the responses were species-specific. At the same time, long-term warming was found to reduce the number of flowering species [98]. Other studies also report a decline in community species richness associated with warming [104,105]. Extensive evidence suggests that Arctic warming promotes greater shrub cover and height, which in turn can reduce plant species diversity [104,106,107]. Increased shrub height also causes shading, which may suppress reproductive productivity [108].

Climate change exhibits contrasting trends between the Low and High Arctic, leading to different effects on plant reproduction. In the High Arctic, mean annual temperatures remain below zero, but increasing temperatures and earlier snowmelt have been observed, resulting in damage to reproductive buds and flowers in certain species [80]. In Svalbard, for example, Cassiope tetragona and Dryas octopetala have shown declining flowering productivity [87]. Similar patterns have been recorded in northern Greenland for Papaver and Cassiope, where flower production demonstrates a strong downward trend. For pollinators, a reduction in flower abundance may increase competition for pollen, nectar, and seed resources [80]. In contrast, long-term monitoring of nine vascular plant species in the Low Arctic region of Greenland revealed that total flower density nearly doubled over the past 13 years, opposing the decline observed in the High Arctic [80]. In some Low Arctic species prolonged warming enhances flowering and seed set, while in the High Arctic it can reduce reproductive output due to frost damage and resource limitations.

To better understand tundra plant phenology under natural, non-experimental conditions, in ten-year study in Utqiaġvik (Alaska) recorded seasonal flower counts for 23 species across four plant communities [106]. The results showed no significant shift in the timing of reproductive cycles when temperatures rose above a threshold of 10 °C [106]. It is possible that Low Arctic tundra plants experience heat stress like that of temperate species but at lower temperature thresholds [109]. Some ecologists suggest that Arctic reproductive phenology is relatively resilient to climate warming and exhibits only moderate changes during periods of extreme heat [106,109]. However, such temperature spikes maynegatively affect plant growth and seed viability.

Molecular studies on the genetic regulation of reproductive development in Arctic plants are only beginning [57]. In a genomic analysis of four Arctic species (Oxyria digyna, Draba nivalis, Cochlearia groenlandica, and Dryas octopetala) from Arctic Canada, numerous GO terms were identified in expanded and contracted gene families related to “sexual reproduction”, “meiosis”, “pollen recognition”, “regulation of reproductive processes” and “anther wall tapetum development” [57]. Genes regulating reproduction were expanded in O. digyna, whereas no such expansion was found in D. nivalis and C. groenlandica, both predominantly self-pollinating species. Expansion of pollen-recognition genes was detected in O. digyna and D. octopetala, but not in D. nivalis and C. groenlandica.

5. Pollination and Seed Reproduction of Arctic Plants

5.1. Pollination

Pollination plays a key role in genetic exchange and differentiation between plant populations [110]. For a long time, it was believed that wind pollination (anemophily) and self-pollination dominate in the Arctic tundra [27]. Some ecologists have proposed that wind pollination promotes more frequent and widespread genetic exchange over long distances than insect pollination, potentially reducing the loss of genetic diversity during range shifts [111]. Insect-mediated cross-pollination was once considered incidental in the Arctic [112]. It was also assumed that sexual reproduction plays a minor role, with vegetative propagation being the main method of reproduction [113], and that Arctic plants exhibit low pollen productivity [114]. The modern conception is another: Arctic plants show high plasticity in their pollination and reproductive strategies [115,116,117]. Recent studies demonstrate that pollinators are essential for successful seed formation in many Arctic and Subarctic alpine species, most of which employ mixed reproductive systems [117,118,119]. Experimental warming has produced mixed results: some studies report enhanced sexual reproduction under higher temperatures [94,96], while others found no significant effect [120,121]. Extreme temperature fluctuations may inhibit successful sexual reproduction, though the responses appear to be species-specific [122].

An international research team analyzed DNA from pollinators visiting several Dryas species (D. drummondii, D. integrifolia, D. octopetala) across the Atlantic Arctic (Canada, Greenland, Norway, Finland) and identified 1360 species of arthropods [123]. Most pollinators belonged to the orders Diptera and Hymenoptera, reflecting the high β-diversity of Arctic pollinator communities and the widespread occurrence of entomophilous cross-pollination. A distinctive feature of Arctic pollination systems is the dominance of nectar- and pollen-feeding flies, which play an increasingly important role as pollinators [119,124,125]. Diptera are the main pollinators in Arctic ecosystems [126], while butterflies visit only a limited subset of flowering species [127].

The lability of pollination mechanisms is one of the most important adaptive features of tundra dicotyledonous plants [128,129]. Under specific environmental conditions, the same species may employ different pollination strategies—crosspollination by wind or insects, various forms of self-pollination, and even transitions from entomophily to anemophily [130]. For example, Empetrum hermaphroditum is entomophilous in continental Arctic regions but becomes anemophilous in oceanic climates. In such cases, the entomophilous corolla is retained, but nectar and smell is lost, while anther filaments elongate to facilitate wind pollination. Willows (Salix sp.) in the Arctic exhibit both wind and insect pollination. E. Warming [131] described willows in Greenland and Novaya Zemlya as wind-pollinated, whereas modern research in northwestern Greenland has shown that Salix polaris is actively pollinated by flies of the families Muscidae and Empididae [132]. Yu. I. Chernov [133] also documented active pollination of Subarctic and Arctic willows by bumblebees in the Yugorsky Peninsula, Vaigach Island, and near Dikson, while E. Tikhmenev [128] observed willow pollination by bumblebees and flies in Chukotka and on Wrangel Island. Ecologists suggest that in both High and Low Arctic regions, where pollen availability is limited, natural selection favors those floral traits that either attract insect pollinators or facilitate reproduction through self-pollination [117].

The flowers of Saxifraga oppositifolia on Wrangel Island produce large amounts of nectar even at very low temperatures and are pollinated exclusively by insects such as bumblebees and various Diptera species [134]. Bumblebees were observed pollinating S. oppositifolia continuously under the 24-h daylight of the polar day—even during strong winds and snowfall, when air temperatures dropped below 0 °C [134]. The isolated bumblebee populations of Bombus glacialis inhabiting Novaya Zemlya, Wrangel, and Kolguev Islands are considered relict and currently threatened by climate change [135]. The 21 bumblebee species (genus Bombus) are found in the Arctic zones of the United States and Canada, six of which occur at high latitudes. They belong mainly to two subgenera: Alpinobombus, specialized in tundra pollination, and the more generalist Pyrobombus [125].

Ecological interactions between flowering plants and pollinators are among the most important in Arctic terrestrial ecosystems [136]. Arthropods constitute most of the terrestrial biodiversity in the Arctic and are extremely sensitive to abiotic changes [137]. Warming may disrupt plant–pollinator relationships [124], and the temporal dynamics of these networks remain poorly studied [78]. Understanding phenological responses of plants to climate change and possible asynchrony with insect life cycles is therefore essential [138]. One of the global environmental issues is the worldwide decline of insect populations, including in the Arctic [139]. Pollinator diversity is relatively low in high-altitude regions of the High Arctic [140], yet data on pollinator abundance and efficiency remain contradictory. Studies in Svalbard found that pollinator communities there are more effective than in Subarctic Sweden [99]. Conversely, research in Greenland indicated that reduced pollinator abundance (Chironomids and Muscids) was linked to shorter flowering periods in mountain tundra communities, signaling disruptions in plant–pollinator interactions [99,141]. With rising temperatures, floral visitors may face reduced resource availability later in the season due to earlier and shorter blooming periods [99]. Many Arctic insects have multi-year life cycles, and their phenological responses depend heavily on overwintering strategies [127,142].

Long-term monitoring in Zackenberg (Northeast Greenland, 1996–2016) produced surprising results: despite a 1.5 °C temperature increase, the plant–pollinator network structure remained stable [132]. Functional stability in pollen transport was also observed, as the total amount of pollen transferred and deposited on Dryas flowers showed no significant change. This suggests that High Arctic pollination networks are resistant over long timescales [132]. Such resistance is likely due to several key plant genera—particularly Dryas and Salix arctica—which are long-lived and long-blooming,, increasing their tolerance to environmental variability and current climate change [143]. The main pollinators in these systems are flies (Muscidae and Empididae), which transport large quantities of pollen, remain active for extended periods, and pollinate numerous plant taxa [144]. Earlier research at Zackenberg (1996–1997) similarly demonstrated high dynamic stability of the pollination network despite differences in temperature, growing season length, and insect abundance [78]. Current evidence suggests that phenological changes in the High Arctic mainly occur at the beginning and end of the growing season and have limited influence on insect activity [99].

Some data suggests that historical climate fluctuations had only minor effects on plant–pollinator networks, indicating relative long-term stability [145]. However, the effects of modern climate change are likely to be much stronger. Experimental studies on the impact of warming on pollination and pollinators in the Arctic have been conducted since 1992, yet the amount of available data remains limited [98]. Over the past three decades, only a few works have directly addressed this key ecological problem [146]. Current data on pollen production of Arctic plants are contradictory: some studies report about limited pollen quantities [119,147], whereas others find no such evidence [148]. Research on how climate change affects nectar production is particularly rare [149]. Several experiments have explored how snowmelt timing influences pollinator activity [102,150,151,152]. Artificial heating experiments in Alaska showed that species such as Vaccinium uliginosum increased nectar production under both short-term and long-term warming [98]. However, other studies report the opposite effect—decreased nectar volume under warming [153]. In the same Alaskan experiments, in heated chambers recorded less insect visits compared to control plots [98], likely due to the “barrier effect” of chambers that physically impede insect access [154,155]. Another explanation is that the chambers heat plants but not insects, disrupting phenological synchrony [156]. Moreover, warming promotes shrub growth and shading, leading to lower species diversity among flowering plants [105,106,107]. A prevailing hypothesis is that the reduction of insects in the High-Arctic regions results from a short flowering season and limited floral resources [99]. Most flower visits in the High Arctic are made by generalist insects [132,152]. Such generalist-dominated networks act as buffers against rapid environmental change and compensate for low pollinator diversity [152]. In the Alaskan tundra, only a few plant species attract insects from a single order: Arctous alpina is mainly pollinated by bumblebees, while Dryas octopetala, Stellaria spp., and Potentilla hyparctica are predominantly visited by flies [98]. Highly specialized pollinators are rare in the Arctic [157]. As latitude increases, the diversity of Hymenoptera declines, making Diptera (flies) especially important pollinators [158]. Flies visit flowers for multiple reasons—nectar, pollen, warmth, caloric intake, and shelter [159]. Experiments in Alaska confirm that bumblebees and flies are the key pollinators of tundra plants: bumblebees transport large pollen loads, while flies visit flowers more frequently [98]. Similarly, V. Koch [148] found that eight tundra plant species in Lapland were pollinated by insects and were not pollen limited.

5.2. Seed Reproduction

Even by the late XX and early XXI centuries, information about sexual reproduction and seed productivity of Arctic plants remained very limited, since it was long believed that seed reproduction played a minor role in subpolar ecosystems [112]. However, in recent decades, researchers have increasingly recognized the ecological importance of the formation of seed banks in the High Arctic soils [96,160,161]. Many Arctic plant species exhibit good seed productivity, and their seeds often ripen before winter [72,112,115,162,163]. Some species reproduce apomictically, forming viable seeds without pollination. Most Arctic seeds lack a distinct period of winter dormancy, but their germination is restricted by low soil temperatures [164]. The optimal temperature range for germination is 20–25 °C [165], and germination tends to occur simultaneously once suitable conditions are reached [164]. In certain species, exposure to freezing is required before germination. According to B. A. Tikhomirov [162], 58 out of 106 species in the Taimyr flora has produced mature seeds. Seed ripening is successful in the Low Arctic. V. A. Gavrilyuk [72] reported that in southeastern Chukotka, most plant species (around 300) produced mature fruits and seeds before winter, with particularly stable fruiting observed in the families Empetraceae and Cyperaceae. In contrast, fruiting in the High Arctic is irregular and strongly influenced by environmental conditions of the current and preceding autumn seasons. Some plants complete seed ripening beneath the snow cover [162].

Studies in the polar semi-desert of King Christian Island (Canada) showed that 27 plant species under the study flowered annually, but seeds most of them don t achieved full maturation [15]. Field germination was slow, increasing during the second summer, and typically occurred after snowmelt, in meltwater, or following rare summer rains [15]. The ability of tundra plants to reproduce by seeds, bulbs, and spores is very important for colonization and primary succession in Arctic ecosystems. Without continual seed input and seed bank formation, most Arctic landscapes would remain barren [112]. Data on the effects of experimental soil warming on the reproductive efforts of Arctic plants (defined as the ratio of the weight of generative structures to total plant biomass)are inconsistent [95,96,166]. Short-term monitoring of tundra species in the Subarctic revealed strong interspecific and interannual variability in reproductive efforts [95,166]. In contrast, a 12-year experiment in Arctic Canada showed a significant long-term increase in reproductive efforts under continuous warming [96]. These differences appear to reflect distinct reproductive strategies between the Low and High Arctic.

In polar deserts, where competition for resources is minimal, plants tend to allocate more energy to seeds production. Conversely, in the Low Arctic, where interspecific competition is stronger, plants invest more in vegetative growth and biomass to drive out competitors [95]. The limitation of sexual reproduction by low temperatures in the High Arctic is less as the growing season becomes warmer [96]. Under warming, the number of seeds, their weight, germination rate, frequency, and seed set all tend to increase [167,168]. With ongoing glacier retreat, changes in environmental conditions are expected to enhance the colonization potential of Arctic ecosystems. Sexual reproduction—particularly in the High Arctic—plays a crucial role as a source of seeds for the colonization of barren polar deserts [96,163]. Increased flowering and sexual reproduction under warming can also boost seed set frequency by elevating pollen availability and attracting more pollinators [117].

6. The Arctic Plants and Warming of the Climate

6.1. Short History of Arctic Climate Changes and Vegetation Responses

Modern Arctic ecosystems are strongly limited by temperature [169]. Throughout glacial and interglacial cycles, Arctic climates have undergone repeated long-term fluctuations. Since the Last Glacial Maximum (LGM), many plant species that survived in ice-free refugia gradually recolonized new deglaciated areas as the ice retreated [57,111]. The demographic history of Arctic species—their range contractions and expansions following climate shifts—has shaped their adaptive potential, which is closely linked to genetic diversity [170].

To study the origins and migration routes of Arcto-alpine flora, researchers have used metagenomic analyses of ancient plant DNA [171]. These studies revealed consistent, large-scale plant DNA responses to climatic oscillations over the past 50,000 years. According to molecular data, overall floristic diversity peaked around 26.5 thousand years ago, at the onset of the Last Glacial Maximum, when herbaceous plants were the dominant life form [171]. Around 19,000 years ago, floristic diversity declined due to the loss of several grass taxa. Subsequent warming between approximately 14.6 and 12.9 thousand years ago triggered a major vegetation transition: woody plants such as Salix and Betula began to spread, while grass diversity continued to decline [171]. After the Younger Dryas, increasing summer insolation and atmospheric CO₂ levels reached near-Holocene values. During the early Holocene, continued warming promoted higher floristic diversity and the expansion of herbaceous plant communities. During interglacial periods, the distribution range of Arctic flora approached its present-day extent [171]. Complex genetic responses of plants to climatic shifts and isolation in refugia have been documented across glacial and interglacial periods [172]. Fossil records indicate that many Arctic plants survived glaciations on exposed moraine soils near glacier margins and recolonized new habitats as temperatures rose and soils developed during the Holocene [171,173,174,175].

6.2. Modern Warming in the Arctic

Average annual temperature trends in the Arctic are more than twice exceed the global mean, according to several long-term analyses [176]. Other studies estimate that the Arctic is warming up to four times faster than the global average—a process known as Arctic amplification [177]. Since the beginning of the XX century, the mean temperature in the Arctic has increased by approximately 2.3 °C, which is significantly higher than in most other regions of the planet [178]. The reflective ice and snow cover of the Arctic normally act as a mirror, returning much of the solar radiation back into space. As ice melts, darker ocean and land surfaces are exposed, absorbing more solar energy and accelerating further warming [179]. Glaciers and ice caps are retreating, permafrost is melting, and sea levels are rising [173]. Climate models project that by the end of the XXI century, Arctic temperatures will rise by 3–5 °C in spring and by 7–13 °C in autumn [180]. Permafrost degradation is accompanied by a reduction of thickness (Figure 4) and duration in snow cover [176,181]. Permafrost represents a unique complex of ecosystems, stabilizes landscape hydrology, and stores enormous amounts of organic carbon [140]. It is estimated to contain about 1700 gigatons (Gt) of carbon [182]. Melting of just three meters of permafrost could release up to 624 million tons of CO₂ by 2100, potentially triggering large-scale feedback effects on both regional and global climates [183,184,185].

Recent research associated Arctic warming with the weakening and destabilization of polar vortices, which allows cold air masses to move southward and warm air to penetrate high latitudes [179]. Deposition of black carbon on ice surfaces further reduces albedo, accelerating melting [186]. Ecologically, Arctic warming is expected to lengthen the growing season, enhance plant phenology, and increase reproductive success [187]. Some studies predict that continued warming will favor more efficient sexual reproduction in tundra plants [95]. However, the strongly synchronized life cycles of Arctic species with spring and early summer conditions mean that earlier snowmelt and flowering could disrupt “plant—pollinators” networks in the ecosystems [140]. While an overall increase in ecosystem productivity—particularly in the Low Arctic—remains one of the most pronounced trends, recent studies show that plant community responses are more complex, region-specific, and species-dependent [188,189,190].

Since the 1970s, ecologists have expressed growing concern over the anthropogenic impacts on Arctic ecosystems, particularly those associated with mineral exploration, extraction, and the use of heavy machinery, as tundra landscapes require long periods for recovery [190,191]. In several Arctic regions, vegetation cover and species diversity have declined, while areas of exposed soil have expanded [192], partly due to permafrost melting.

Conversely, some reports indicate localized increases in vegetation cover—for example, in abandoned quarries and vehicle ruts [193]. On the Kola Peninsula, population growth of certain orchid species has been observed in habitats contaminated by organic fuel residues [194]. In the Low Arctic tundra, the proportion of grasses is rising, while the dominance of evergreen shrubs is declining [192].

6.3. International Tundra Experiment

The International Tundra Experiment (ITEX) was conducted across all Arctic countries from the late XX to the early XXI century. The primary goal of this large-scale initiative was to investigate the effects of climate warming on the growth, development, and phenology of Arctic plants. Monitoring stations were established in Arctic, Subarctic, and Alpine ecosystems of Alaska, Canada, Eurasia, and Japan [89,97,195]. The experimental design employed open-top chambers (OTCs), in diverse plant communities and also the control plots. Across all monitoring sites, six key phenological phases were recorded: leaf unfolding, beginning of flowering, end of flowering, fruiting, seed dispersal, and leaf senescence—with observation periods ranging from 1 to 20 years. A wide range of vascular plant species was studied, including Cassiope tetragona, Salix arctica, Oxyria digyna, Empetrum nigrum, Loiseleuria procumbens, Carex sp., and others (Figure 5) [83,84,85,89,196]. ITEX remains one of the most comprehensive circumpolar research initiatives examining the biome-level impacts of climate warming on tundra vegetation [97]. The largest number of species was monitored at Latnjajaure (Sweden), where flowering phenology of 144 species was tracked over a 10-year period. The longest continuous records were obtained from Zackenberg, Greenland (1996–2018), and Utqiaġvik, Alaska (1994–2019) [89].

Analysis of ITEX data has demonstrated that, the timing of leaf emergence and beginning of flowering in Arctic ecosystems has shifted during the past three decades, both under long term experimental warming and in field observations [97]. Within the circumpolar ITEX network, experimental warming using open-top chambers (OTCs) has been maintained at several tundra stations for up to 30 years [146]. In heated plots, researchers observed earlier flowering, increased plant height, greater leaf area, and higher seed mass and viability [57,95,96,106].

The results revealed notable shifts in DNA methylation and gene expression in plants from Low Arctic sites (Alaska and Sweden), where long-term warming in OTC chambers was applied. Genome-wide methylation levels were consistently higher in Dryas individuals from heated plots, though this pattern disappeared in their offspring. In both Sweden and Alaska, genes that were differentially methylated and expressed were significantly enriched for functions related to oxidative stress and defense against fungi and other organisms. By contrast, no significant differences were detected in High Arctic populations from Svalbard and Nunavut (Canada) [197]. These data are consistent with previous results showing that High Arctic plant communities exhibit weaker growth responses to warming [106]. Satellite-derived greenness indices also confirm more pronounced vegetation changes in the Low Arctic than in the High Arctic [198]. Ecologists suggest that High Arctic plant species are less temperature-sensitive because they are already near their physiological upper limits and lack sufficient plasticity for stronger responses [57,199].

T. Callaghan [200], reviewing publications on the effects of climate warming in high latitudes, notes that most authors report a trend of Arctic greening, particularly in Arctic Russia, Scandinavia, and Alaska. Arctic greening—the increase in vegetation productivity and biomass—is generally associated with rising temperatures, but it may also result from declining herbivore populations, especially lemmings and reindeer [201]. However, other studies have described minor or absent vegetation changes in parts of the High Arctic [202,203]. For instance, no significant shifts in plant growth, floristic composition, or vegetation structure were recorded over 70 years on Svalbard [202], 49 years in West Greenland [204], and 40 years near Dikson on the Taimyr Peninsula [204]. This remarkable stability may be explained by the physiological plasticity of Arctic species, including their historical adaptation to warmer periods, or by the inability of some High Arctic plants to respond to further warming. For example, the growth of Phleum alpinum in western Greenland, located near the northern edge of its distribution, is constrained by morphological limits of growth rather than temperature [200]. In recent years, researchers have also reported about the browning of the Arctic vegetation—a decline in vegetation productivity due to physical damage, reduced biomass, and extreme climate events, such as severe frost without snow cover, freezing rain, and storms [188]. Browning may also result from biotic stressors, including fungal infections and insect outbreaks [200]. According to L. Berner [205], satellite data from 1985–2016 indicate that 37.3% of the High Arctic has experienced greening, 4.7% browning, and 58% remained unchanged. Despite rapid climate warming, the resistance and stability of Arctic ecosystems continue to surprise ecologists, and the mechanisms behind this resistance remain poorly understood [200].

However, several studies indicate that plant phenology in the High Arctic may be even more sensitive to temperature fluctuations than in the Low Arctic [89,95,206]. Experimental research and long-term monitoring in polar tundra and desert regions have shown that in the High Arctic plants often respond to warming with accelerated phenological development, increased growth, and enhanced reproduction [95,207]. Many ecologists have observed significant increases in plant biomass in open-top chamber (OTC) warming experiments after several years [95,208,209]. A meta-analysis of short-term ITEX data demonstrated that experimental soil warming of 2–3 °C in tundra ecosystems led to greater height and cover of deciduous shrubs and grasses, while moss and lichen cover, as well as overall vascular plant diversity, declined [209]. Regional warming has altered Arctic plant community composition, trophic interactions, and surface energy balance [210]. The sensitivity of plants to temperature depends on thermal conditions of biotops (higher sensitivity in colder habitats) and on species-specific phenological features [206]. Late-flowering species exhibit greater temperature sensitivity [82]. In Arctic ecosystems, snow depth and melting rate determine the length of the growing season and the period available for resource assimilation [211]. Premature exposure to low spring temperatures following early snowmelt can shift flowering phenology and reduce flower abundance [56,212]. Early flowering species such as Dryas octopetala, Saxifraga oppositifolia, and Cassiope tetragona response more strongly to snowmelt timing, as earlier exposure allows them to exploit the full growing season. In contrast, late-flowering species may depend more on temperature or day length to synchronize with stable mid-summer conditions [56].

6.4. Consequences of Climate Warming in the Arctic

In 2001, the Arctic Council’s Conservation of Arctic Flora and Fauna (CAFF) published its first circumpolar biodiversity report, Arctic Flora and Fauna: Status and Conservation [140]. The report emphasized that reliable data on the status and dynamics of Arctic flora were almost entirely lacking. By the early XXI century the Arctic began to undergo rapid transformations driven by new environmental stressors and challenges [140]. One of the major problesm is the replacement of Arctic and Hypo-Arctic species by boreal taxa. In several regions, typical tundra plant communities—composed of grasses, sedges, mosses, and lichens—are being displaced by evergreen shrubs and other species characteristic of more southern biomes. Tree penetration into tundra has also intensified. According to model projections under high-emission scenarios up to 2100, the tree line may shift up to 500 km northward, potentially leading to the loss of more than half (≈51%) of tundra habitats. As a result, such new ecosystems cannot be considered as the “Arctic” [140].

R. Barrett and R. Hollister [207] noted that while many studies have examined the long-term impacts of warming on Arctic plant community composition [106,211,213], far fewer have focused on the detailed responses of individual species—particularly their growth, reproduction, and phenology under sustained warming [96]. Data from long-term (17–19 years) experimental warming plots in Alaska indicate that Arctic plants typically respond to elevated temperatures with earlier reproductive phenology, taller inflorescences, and longer leaves, though reproductive effort varies among species and years [207]. Other long-term field studies similarly report increases in inflorescence height [214], total plant height [215], reproductive biomass [96,216], leaf length [215], and photosynthetic productivity [217,218], as well as earlier beginning of flowering [187]. Arctic plants continue to exhibit physiological and phenological responses to experimental warming even after two to three decades of continuous exposure [106]. Nonetheless, the absence of clear, consistent trends across many sites and species indicates that tundra vegetation is remarkably resistant to climate warming—at least over decadal timescales [106].

The Arctic Climate Impact Assessment (ACIA) project recommended expanding long-term biodiversity monitoring across the Arctic, leading to the establishment of the Circumpolar Biodiversity Monitoring Program (CBMP) in 1992. This program monitoring species diversity and composition, phenology, spatial structure, demographics, productivity, and other key ecological processes [189]. Changes in plant phenology—including the timing and duration of vegetative and reproductive phases—are among the most evident biological consequences of Arctic climate warming [206]. This aspect is particularly important for the tundra biome, where phenological responses to climate change remain among the least studied [97]. Using remote sensing methods [219] and in situ control-site monitoring [220], researchers have identified a gradual trend toward earlier green-up in several Arctic regions [189]. Studies of reproductive phenology at monitoring plots show that flowering dates of some species have advanced, while others remain unchanged [220]. Satellite-based analyses have also detected an increase in the length of the growing season in two of the five Arctic subzones [219]. However, despite these changes, vascular plant biodiversity has not increased over three decades of monitoring across tundra sites, but the moss-lichen layer is shrinking [106]. In certain regions—particularly Alaska, northern Iceland, and the western Russian Arctic—processes of naturalization and invasion by adventive species have been recorded [189]. Long-term experimental studies indicate that grasses and shrubs respond positively to warming, suggesting potential future expansion of their populations. Shrub abundance tends to increase mainly in warmer tundra regions with mesic or moist soils, while no similar trend has been observed in colder or drier areas [106].

Plants growing in Earth’s cold environments fix a substantial proportion of the total biospheric CO₂ and therefore make an important contribution to mitigating global climate change [221]. New habitats for Arctic plants are emerging on recently exposed glacial moraines. In response to rising temperatures, shrubs and trees are expanding their ranges northward [222]. Because shrubs are generally darker than the surrounding tundra vegetation, they lower surface albedo and increase local heat absorption, thereby amplifying environmental warming [223]. In the High Arctic, predictions of vegetation response to climate change rely primarily on remote sensing data [224] and experimental warming studies [209]. Satellite observations based on the Normalized Difference Vegetation Index (NDVI) suggest that Arctic vegetation productivity is increasing, largely due to the expansion of shrub cover, which correlates positively with rising high-latitude temperatures [225]. In many Arctic regions, shrubs are spreading rapidly, colonizing open habitats, and the shrub line is moving upward [107]. Increased shrub height change microclimatic conditions, influencing hydrology, near-surface soil temperature, and carbon fluxes [225,226]. They are very few uniform pan-Arctic vegetation trends, but this does not mean that Arctic vegetation does not change, this occurs at local and regional levels, which depends on ecosystem and climatic conditions [189]. Studies in Greenland [227] revealed that species richness is not directly correlated with temperature or precipitation during the growing season. Instead, topography and soil moisture content are the main factors which influence on biodiversity. The weak impact of climate on species diversity in High Arctic Greenland perhaps is relates to the scarcity of tall shrubs in this region. Researchers therefore conclude that future impacts of climate change on Arctic vegetation will depend primarily on changes in soil moisture rather than direct temperature increases [227]. Many ecologists emphasize the need to maintain multiple long-term monitoring sites across diverse Arctic habitats and to intensify the study of local ecological factors—not just temperature—to better understand the mechanisms of Arctic vegetation change [220].

7. Future Perspectives

It is likely that the distribution of Arctic plants will become increasingly restricted in the future. Some species—especially those forming small endemic populations—may even disappear [173]. However, it is important to remember that during the early Holocene, Arctic temperatures were about 2 °C warmer than today, and summer temperatures were even higher, particularly in Greenland. Yet, according to palaeobotanical evidence, no Arctic plant species went extinct during the Quaternary period, suggesting that these species possess remarkable resistance to climate change [173]. At the same time, the ranges of Arctic species are shrinking, which may lead to a loss of intraspecific genetic diversity [228] and reduce the adaptive potential of plants in response to future environmental shifts [229].

Ecologists emphasize that the recent drastic reduction of sea-ice extent and regional warming are largely anthropogenic phenomena, rather than ordinary climatic fluctuations [179]. These transformations are closely connected with biological, ecological, and biogeochemical processes that form species interactions and ecosystem networks. Data indicate that Arctic sea-ice volume has declined by approximately 75% over the past 40 years [230], and terrestrial ice volume has decreased by about 60% [231]. Projections suggest that the Arctic Ocean could become ice-free in summer within the next 15 years, which would have profound global climatic consequences [232]. In the Low Arctic, a “greening effect” is underway—tundra habitats are being gradually replaced by boreal plant communities, shrubs, and trees [233]. These vegetation changes are also altering trophic structures: for example, reindeer populations are declining due to the loss of lichen cover, while their migration routes are shifting, threatening the stability and biodiversity of Arctic ecosystems [234].

One of the central ecological questions remains: “Can Arctic plants survive in a rapidly warming climate?” [173]. Some researchers are optimistic. According to H. Birks [173], many Arctic species may persist due to their efficient reproductive strategies. While most seeds, fruits, and spores fall close to parent plants, a portion is dispersed over long distances. Species tolerant of higher temperatures may survive in localized refugial habitats [235]. Additionally, many Arctic plants are polyploid, and their large genomes may enhance adaptive plasticity under changing environmental conditions [236]. On the other hand, genetic variability is decreasing due to shrinking ranges and species loss. Therefore, ecologists propose the creation of genetic sample archives for Arctic plant species—to preserve their evolutionary potential, enable restoration efforts, and facilitate prediction of species responses to climate change [140] The Arctic thus presents both unprecedented challenges and unique opportunities for understanding how biological communities and ecosystems respond to climate change [56].