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Subarctic ecosystems are subjected to increasing nitrogen (N) enrichment and disturbances that induce particularly strong effects on plant communities when occurring in combination. There is little experimental evidence on the longevity of these effects. We applied N-fertilization (40 kg urea-N ha−1 year−1 for 4 years) and disturbance (removal of vegetation and organic soil layer on one occasion) in two plant communities in a subarctic forest-tundra ecotone in northern Finland. Within the first four years, N-fertilization and disturbance increased the share of deciduous dwarf shrubs and graminoids at the expense of evergreen dwarf shrubs. Individual treatments intensified the other's effect resulting in the strongest increase in graminoids under combined N-fertilization and disturbance. The re-analysis of the plant communities 15 years after cessation of N-fertilization showed an even higher share of graminoids. 18 years after disturbance, the total vascular plant abundance was still substantially lower and the share of graminoids higher. At the same point, the plant community composition was the same under disturbance as under combined N-fertilization and disturbance, indicating that multiple perturbations no longer reinforced the other's effect. Yet, complex interactions between N-fertilization and disturbance were still detected in the soil. We found higher organic N under disturbance and lower microbial N under combined N-fertilization and disturbance, which suggests a lower bioavailability of N sources for soil microorganisms. Our findings support that the effects of enhanced nutrients and disturbance on subarctic vegetation persist over decadal timescales. However, they also highlight the complexity of plant–soil interactions that drive subarctic ecosystem responses to multiple perturbations across varying timescales. [ABSTRACT FROM AUTHOR]

The spatial and temporal dynamics of the Siberian larch (Larix sibirica Ledeb.) at the upper limit of its growth on the south-eastern macroslope of the Rai-Iz massif (Polar Urals, Russia) during the second half of the 20th to the beginning of the 21st century were analyzed. Current climate changes were accompanied by increased stand density on previously wooded parts of the mountain slopes and the appearance of new forest generations in lightly wooded or unforested parts of the studied area. Our original method for the automated recognition of boundaries among the key phytocoenohoras (closed forest, open forest, light forest, and tundra with single trees) is universally applicable and improves objectivity in selecting boundaries for these phytocoenohora types. With regard to the total area of the study site, the area of closed forest, open forest, and light forest, respectively, increased from 2.9% to 6.8%, from 9.6% to 13.1%, and from 7.5% to 15.6%, while the area of tundra lots with single trees decreased from 79.9% to 64.5%. Phytocoenohora type replacement in the course of the study period was characterized by a transition from forms with lower density to higher-density forms. Changes in the opposite direction were not discovered. Natural wind protection barriers for young larch tree generations included hummocks and groups of grown trees. The process of gradual tundra and forest tundra forestation then began on the leeward side of the barrier close to seed-producing trees. [ABSTRACT FROM AUTHOR]

Investigating the distribution of internal physiological indicators and the cold resistance of woody plants in the alpine treeline ecotone is of great ecological importance to explain the mechanism of alpine treeline formation. Less research has been conducted on the cold resistance mechanisms of alpine treeline woody plants than on commercial crops. In this paper, five different tree species in the alpine treeline ecotone of the Sygera Mountains were used as the research objects and the leaves, branches, and roots of 19 woody plants were collected in the non-growing season (November) of 2019. Their non-structural carbohydrate content (soluble sugar and starch), malondialdehyde, hydrogen peroxide (H2O2), proline, superoxide dismutase, and peroxidase levels were measured. The contents of C, N, P, and K elements were analyzed, along with the distribution characteristics of physiological indices and organs of various woody plants and their relationship to plant nutrients. Results showed that the MDA (5.46 ± 1.95 μg·g−1) and H2O2 (4.11 ± 0.76 mmol·g−1) of tree root organs and the MDA (3.03 ± 2.05 μg·g−1) and H2O2 (4.25 ± 1.03 mmol·g−1) of shrub leaf organs were higher than those of other organs, indicating that under the stress of low temperatures, the root organ of arbor species and the leaf organ of shrub species experienced the most damage. Osmotic substances, particularly soluble sugars, play a crucial role in the response of the woody plants in Sygera Mountains to low-temperature stress. Plant nutrients could enhance plant stress resistance by further activating the activity of the antioxidant system and increasing the synthesis of osmotic substances. This study hypothesized that the stress on the root organs of the arbor species in the treeline ecotone may not be repaired in time, which may be a key mechanism for the formation of the alpine treeline in the Sygera Mountains. [ABSTRACT FROM AUTHOR]

The spatial and temporal dynamics of the Siberian larch (Larix sibirica Ledeb.) at the upper limit of its growth on the south-eastern macroslope of the Rai-Iz massif (Polar Urals, Russia) during the second half of the 20th to the beginning of the 21st century were analyzed. Current climate changes were accompanied by increased stand density on previously wooded parts of the mountain slopes and the appearance of new forest generations in lightly wooded or unforested parts of the studied area. Our original method for the automated recognition of boundaries among the key phytocoenohoras (closed forest, open forest, light forest, and tundra with single trees) is universally applicable and improves objectivity in selecting boundaries for these phytocoenohora types. With regard to the total area of the study site, the area of closed forest, open forest, and light forest, respectively, increased from 2.9% to 6.8%, from 9.6% to 13.1%, and from 7.5% to 15.6%, while the area of tundra lots with single trees decreased from 79.9% to 64.5%. Phytocoenohora type replacement in the course of the study period was characterized by a transition from forms with lower density to higher-density forms. Changes in the opposite direction were not discovered. Natural wind protection barriers for young larch tree generations included hummocks and groups of grown trees. The process of gradual tundra and forest tundra forestation then began on the leeward side of the barrier close to seed-producing trees.

Free-living saprotrophic fungi and symbiotic mycorrhizal fungi affect organic matter dynamics differently because of contrasting ecological adaptations. We investigated how mass-loss, C:N-ratio and stable isotope dynamics of leaf litter and humus substrates depended on presence of living tree roots and associated fungal communities in a forest-to-tundra ecotone over three years. Litter mass-loss was stimulated by tree roots, contrary to a Gadgil effect. Increases in the litter nitrogen pool and δ15N suggested import of nitrogen from deeper soil by the dominating saprotrophic fungi. Over time, humus first lost, then gained, mass, and corresponding shifts in δ15N and δ13C suggested fluctuating pools of fine roots and fungal mycelium. Ectomycorrhizal tree roots consistently reduced longer-term humus mass-gain, counteracting positive effects of ericoid roots and associated fungi. Across all substrates, mass dynamics correlated with the balance between ectomycorrhizal and litter-saprotrophic fungi, both linked to mass-loss, and ericaceous shrubs and associated fungi, linked to mass-gain. [ABSTRACT FROM AUTHOR]

Past anthropogenic disturbances lowered the altitudinal distribution of the Mediterranean Fagus sylvatica forests below 2,000m a.s.l. Accordingly, our current understanding of the southern distribution range of F. sylvatica forests is restricted to managed stands below this elevation, neglecting relic forests growing above. This study has shed light on the structure and species assemblage of an unmanaged relict subalpine F. sylvatica stand growing within the core of its southernmost glacial refugia and at its highest species range elevation limit (2,140m a.s.l.) in southern Apennines (Italy). Here, tree biometric attributes and understory species abundances were assessed in eight permanent plots systematically positioned from 1,650 to 2,130m a.s.l. In the subalpine belt, F. sylvatica had formed a dense clonal stem population that was layered downward on the steepest slopes. The density and spatial aggregation of the stems were increased, while their stature and crown size were decreased. Above 2,000m, changes in tree growth patterns, from upright single-stemmed to procumbent multi-stemmed, and canopy layer architecture, with crowns packed and closer to the floor, were allowed for the persistence of understory herbaceous species of biogeographic interest. Clonal layering represents an adaptive regeneration strategy for the subalpine belt environmental constraints not previously recognized in managed Mediterranean F. sylvatica forests. The clonal structure and unique species assemblage of this relic forest highlight the value of its inclusion in the priority areas networks, representing a long-term management strategy of emblematic glacial and microclimatic refugia. [ABSTRACT FROM AUTHOR]

Past anthropogenic disturbances lowered the altitudinal distribution of the Mediterranean Fagus sylvatica forests below 2,000 m a.s.l. Accordingly, our current understanding of the southern distribution range of F. sylvatica forests is restricted to managed stands below this elevation, neglecting relic forests growing above. This study has shed light on the structure and species assemblage of an unmanaged relict subalpine F. sylvatica stand growing within the core of its southernmost glacial refugia and at its highest species range elevation limit (2,140 m a.s.l.) in southern Apennines (Italy). Here, tree biometric attributes and understory species abundances were assessed in eight permanent plots systematically positioned from 1,650 to 2,130 m a.s.l. In the subalpine belt, F. sylvatica had formed a dense clonal stem population that was layered downward on the steepest slopes. The density and spatial aggregation of the stems were increased, while their stature and crown size were decreased. Above 2,000 m, changes in tree growth patterns, from upright single-stemmed to procumbent multi-stemmed, and canopy layer architecture, with crowns packed and closer to the floor, were allowed for the persistence of understory herbaceous species of biogeographic interest. Clonal layering represents an adaptive regeneration strategy for the subalpine belt environmental constraints not previously recognized in managed Mediterranean F. sylvatica forests. The clonal structure and unique species assemblage of this relic forest highlight the value of its inclusion in the priority areas networks, representing a long-term management strategy of emblematic glacial and microclimatic refugia.

At high elevation or latitude, the margin of the life-form tree is set by low temperature, with trees defined as upright woody species taller than 2–3 m. Globally, the temperature limit of the life-form tree occurs whenever the growing season mean soil temperature declines to 6.7 ± 0.8 °C. Disturbance and human land use, however, can cause trees to be absent from the climatic treeline. After addressing definitions and concepts related to treeline ecophysiology and examining treeline structure and dynamics, the focus will be on future treeline developments with respect to climate, competition and land use change. Finally, changes in economic structure and land use within the treeline ecotone are outlined with respect to net ecosystem production and year-round evapotranspiration. [ABSTRACT FROM AUTHOR]

Historical black-and-white (B&W) aerial images have been recognized as an important source of information for assessing vegetation dynamics. However, the use of these images is limited by the lack of multispectral information, as well as by their varying quality. It is therefore important to study and develop methods that are capable of automatic and accurate classification of these B&W images while reducing the need for tedious manual work. The goal of this study was to assess changes over 30 years in woody vegetation cover along alpine treeline ecotones using B&W aerial images from two time points. A convolutional neural networks model was firstly set up based on three structure classes calculated from Airborne Laser Scanning data using the B&W aerial images from 2010. Then, the model was improved by active addition of training samples of those that were wrongly predicted from historical B&W aerial images from 1980. A comparison with visual image interpretation revealed generally high agreement for the class "dense forest" and lower agreement for the class "group of trees". The study illustrates that vegetation changes at the treeline ecotone can be detected in order to assess areawide long-term vegetation dynamics at a fine spatial resolution. [ABSTRACT FROM AUTHOR]

The alpine treeline ecotone is defined as a forest-grassland or forest-tundra transition boundary either between subalpine forest and treeless grassland, or between subalpine forest and treeless tundra. The alpine treeline ecotone serves irreplaceable ecological functions and provides various ecosystem services. There are three lines associated with the alpine treeline ecotone, the tree species line (i.e., the highest elevational limit of individual tree establishment and growth), the treeline (i.e., the transition line between tree islands and isolated individual trees) and the timber line (i.e., the upper boundary of the closed subalpine forest). The alpine treeline ecotone is the belt region between the tree species line and the timber line of the closed forest. The treeline is very sensitive to climate change and is often used as an indicator for the response of vegetation to global warming. However, there is currently no comprehensive review in the field of alpine treeline advance under global warming. Therefore, this review summarizes the literature and discusses the theoretical bases and challenges in the study of alpine treeline dynamics from the following four aspects: (1) Ecological functions and issues of treeline dynamics; (2) Methodology for monitoring treeline dynamics; (3) Treeline shifts in different climate zones; (4) Driving factors for treeline upward shifting. [ABSTRACT FROM AUTHOR]

The biodiversity of tundra areas in northern high latitudes is threatened by invasion of forests under global warming. However, poorly understood nonlinear responses of the treeline ecotone mean the timing and extent of tundra losses are unclear, but policymakers need such information to optimize conservation efforts. Our individual-based model LAVESI, developed for the Siberian tundra-taiga ecotone, can help improve our understanding. Consequently, we simulated treeline migration trajectories until the end of the millennium, causing a loss of tundra area when advancing north. Our simulations reveal that the treeline follows climate warming with a severe, century-long time lag, which is overcompensated by infilling of stands in the long run even when temperatures cool again. Our simulations reveal that only under ambitious mitigation strategies (relative concentration pathway 2.6) will ∼30% of original tundra areas remain in the north but separated into two disjunct refugia.

This paper presents results of analyzing the second half of the 20th–early 21st century changes in lateral spatial structure of Larix sibirica Ledeb. population in the upper treeline ecotone located on the Rai-Iz massif (Polar Urals, Russia). The GIS layers characterizing distribution of Siberian larch trees and undergrowth together with their crowns was produced for a 7.32 square kilometer area based on aerial images recognition. Using statistical models, we assessed probabilities for assigning trees to age intervals of 1–10, 11–40, and 40+ years based on the average radius of tree crown projection. These maps and layer showing locations of trees that grew in the upper part of the ecotone, and died during the Little Ice Age, allow for assessing specifics of forest cover proliferation at different parts of upper treeline ecotone, and comparing current location of the trees with one from the past. The proposed method for probability-based recognition of Siberian larch tree generations in the upper treeline ecotone using average crown radius can be used to reconstruct time and spatial forest dynamics at the upper growth boundaries for time spans up to 100 years and more. [ABSTRACT FROM AUTHOR]

The elevational and latitudinal diversity patterns of microbial taxa have attracted great attention in the past decade. Recently, the distribution of functional attributes has been in the spotlight. Here, we report a study profiling soil microbial communities along an elevation gradient (500-2200 m) on Changbai Mountain. Using a comprehensive functional gene microarray (GeoChip 5.0), we found that microbial functional gene richness exhibited a dramatic increase at the treeline ecotone, but the bacterial taxonomic and phylogenetic diversity based on 16S rRNA gene sequencing did not exhibit such a similar trend. However, the β-diversity (compositional dissimilarity among sites) pattern for both bacterial taxa and functional genes was similar, showing significant elevational distance-decay patterns which presented increased dissimilarity with elevation. The bacterial taxonomic diversity/structure was strongly influenced by soil pH, while the functional gene diversity/structure was significantly correlated with soil dissolved organic carbon (DOC). This finding highlights that soil DOC may be a good predictor in determining the elevational distribution of microbial functional genes. The finding of significant shifts in functional gene diversity at the treeline ecotone could also provide valuable information for predicting the responses of microbial functions to climate change.

Birch (Betula tortuosa) is one of the treeline forming species within the Siberian Mountains. We analysed the area dynamics of birch stands and the upslope climb of birch treeline based on the Landsat time series scenes and on-ground data. We found that since the warming onset (1970th) birch area increased by 10%, birch stands and treeline boundary were moving upslope with a rate of 1.4 m/yr and 4.0 m/yr. Birch upslope shift correlated with air temperatures at the beginning (May-June) and the end (August-October) of the growth period. Meanwhile, no correlation was found between birch upslope migration and precipitation. Winds negatively influenced both birch area growth and birch upslope climb during spring, fall, and wintertime. In the windy habitats, birch, together with larch and Siberian pine, formed clusters (hedges) which mitigated the influence of adverse winds. These clusters are the adaptive pattern for trees' upslope climb within windward slopes. The other adaptation to the harsh alpine ecotone habitat is non-leaf (bark) photosynthesis which supports tree survival. Thereby, Betula tortuosa upslope climb depends on the wind impact and warming in spring and fall that extended growth period. With ongoing warming and observed wind speed decrease on the background of sufficient precipitation, it is expected to further birch advance into alpine tundra in the Siberian Mountains. [ABSTRACT FROM AUTHOR]

Abstract Background In the northern hemisphere, the north face of the mountains has a high diversity of species which is attributed to the moist and shady conditions at the north face. Other environmental variables may also influence the species diversity on the northern faces of the mountain and therefore needs to be studied in detail. The northern slopes represent three different sub-slopes—north, north east, and north west. During the current investigation of Pir-Panjal and Dauladhar ranges in Bhadarwah valley, fine-scale studies on the relationship between vegetation and four variables (soil pH, moisture content, electrical conductivity, and steepness) were conducted. The study determined the role of these variables on the vegetation of three different sub-slopes. The sampling was done at the confluence of two communities (forest and grassland) at three sites on the northern sub-slopes of the lesser stratum in western Himalayan. Results The result revealed that rich herbaceous diversity prevails on the lesser Himalayan stratum (Bhadarwah valley). As many as 65.8% species differ from site to site, whereas species commonality among the sites is minimum. The role of environmental variables on the species composition at different sub-slopes of the north mountain face is deduced through canonical correspondence analysis (CCA). Conclusions The availability of solar light increases air and soil temperature on the north east–facing slope. All the environmental variables (soil pH, moisture, electrical conductivity, and mountain steepness) are insignificant at pure north face for the species composition. Therefore, it can be concluded that some other environmental variables may influence the species composition which are needed to be further investigated.

Both biotic and abiotic components, characterizing the mountain treeline ecotone, respond differently to climate variations. This study aims at reconstructing climate-driven changes by analyzing soil evolution in the late Holocene and by assessing the climatic trends for the last centuries and years in a key high-altitude climatic treeline (2515 m a.s.l.) on the SW slope of the Becca di Viou mountain (Aosta Valley Region, Italy). This approach is based on soil science and dendrochronological techniques, together with daily air/soil temperature monitoring of four recent growing seasons. Direct measurements show that the ongoing soil temperatures during the growing season, at the treeline and above, are higher than the predicted reference values for the Alpine treeline. Thus, they do not represent a limiting factor for tree establishment and growth, including at the highest altitudes of the potential treeline (2625 m a.s.l.). Dendrochronological evidences show a marked sensitivity of tree-ring growth to early-summer temperatures. During the recent 10-year period 2006–2015, trees at around 2300 m a.s.l. have grown at a rate that is approximately 1.9 times higher than during the 10-year period 1810–1819, one of the coolest periods of the Little Ice Age. On the other hand, soils show only an incipient response to the ongoing climate warming, likely because of its resilience regarding the changeable environmental conditions and the different factors influencing the soil development. The rising air temperature, and the consequent treeline upward shift, could be the cause of a shift from Regosol to soil with more marked Umbric characteristics, but only for soil profiles located on the N facing slopes. Overall, the results of this integrated approach permitted a quantification of the different responses in abiotic and biotic components through time, emphasizing the influence of local station conditions in responding to the past and ongoing climate change. [ABSTRACT FROM AUTHOR]

We used pollen, plant macrofossil, and charcoal records to investigate local long-term timberline shifts and changes in vegetation composition in relation to fire activity at the modern upper forest limit (ca. 2,000 m a.s.l.) in the Mont Bégo area, Maritime Alps of France and Italy. The area is an important place for Alpine archaeology because it has thousands rock-art carvings whose age cannot be directly assessed. Our new record confirms the occurrence of distinct land use phases (7,450–7,150, 6,200–4,900, and 4,250–3,700 cal bp), as suggested by earlier studies of rock art typology. Moreover, the vegetation reconstruction from macrofossils, with co-dominance of Pinus and Betula, suggests that early Holocene conditions were moister than in drier inner Alpine valleys, where Larix decidua played a more important role, both in the past as well as in modern timberline forests. After 8,000 cal bp, the timberline shifted upwards and mixed Abies alba and Pinus cembra stands established around the study site. These fire sensitive trees were finally replaced during the Bronze Age (around 4,000 cal bp) by L. decidua, which still dominates the subalpine woodlands in the area today. Our study supports the notion that while the range of A. alba has been reduced at the colder end of its natural distribution, that of L. decidua has been widened by land use changes and fire disturbances to create high alpine wood pastures. [ABSTRACT FROM AUTHOR]

Tundra ecosystems are global belowground sinks for atmospheric CO2. Ongoing warming‐induced encroachment by shrubs and trees risks turning this sink into a CO2 source, resulting in a positive feedback on climate warming. To advance mechanistic understanding of how shifts in mycorrhizal types affect long‐term carbon (C) and nitrogen (N) stocks, we studied small‐scale soil depth profiles of fungal communities and C–N dynamics across a subarctic‐alpine forest‐heath vegetation gradient. Belowground organic stocks decreased abruptly at the transition from heath to forest, linked to the presence of certain tree‐associated ectomycorrhizal fungi that contribute to decomposition when mining N from organic matter. In contrast, ericoid mycorrhizal plants and fungi were associated with organic matter accumulation and slow decomposition. If climatic controls on arctic‐alpine forest lines are relaxed, increased decomposition will likely outbalance increased plant productivity, decreasing the overall C sink capacity of displaced tundra. [ABSTRACT FROM AUTHOR]

At high elevation or latitude, the margin of the life-form tree is set by low temperature, with trees defined as upright woody species taller than 2–3 m. Globally, the temperature limit of the life-form tree occurs whenever the growing season mean soil temperature declines to 6.7 ± 0.8 °C. Disturbance and human land use, however, can cause trees to be absent from the climatic treeline. After addressing definitions and concepts related to treeline ecophysiology and examining treeline structure and dynamics, the focus will be on future treeline developments with respect to climate, competition and land use change. Finally, changes in economic structure and land use within the treeline ecotone are outlined with respect to net ecosystem production and year-round evapotranspiration.

Historical black-and-white (B&W) aerial images have been recognized as an important source of information for assessing vegetation dynamics. However, the use of these images is limited by the lack of multispectral information, as well as by their varying quality. It is therefore important to study and develop methods that are capable of automatic and accurate classification of these B&W images while reducing the need for tedious manual work. The goal of this study was to assess changes over 30 years in woody vegetation cover along alpine treeline ecotones using B&W aerial images from two time points. A convolutional neural networks model was firstly set up based on three structure classes calculated from Airborne Laser Scanning data using the B&W aerial images from 2010. Then, the model was improved by active addition of training samples of those that were wrongly predicted from historical B&W aerial images from 1980. A comparison with visual image interpretation revealed generally high agreement for the class “dense forest” and lower agreement for the class “group of trees”. The study illustrates that vegetation changes at the treeline ecotone can be detected in order to assess areawide long-term vegetation dynamics at a fine spatial resolution.

This paper presents results of analyzing the second half of the 20th–early 21st century changes in lateral spatial structure of Larix sibirica Ledeb. population in the upper treeline ecotone located on the Rai-Iz massif (Polar Urals, Russia). The GIS layers characterizing distribution of Siberian larch trees and undergrowth together with their crowns was produced for a 7.32 square kilometer area based on aerial images recognition. Using statistical models, we assessed probabilities for assigning trees to age intervals of 1–10, 11–40, and 40+ years based on the average radius of tree crown projection. These maps and layer showing locations of trees that grew in the upper part of the ecotone, and died during the Little Ice Age, allow for assessing specifics of forest cover proliferation at different parts of upper treeline ecotone, and comparing current location of the trees with one from the past. The proposed method for probability-based recognition of Siberian larch tree generations in the upper treeline ecotone using average crown radius can be used to reconstruct time and spatial forest dynamics at the upper growth boundaries for time spans up to 100 years and more.

Aims: Pinus uncinata is the major treeline‐forming species in the Pyrenees. Yet, the role of its reproduction and dispersal as drivers of treeline dynamics remains unknown. Here we quantify seed production, dispersal and germination changes along the elevation gradient to assess whether they may constrain the foreseen treeline advance in the Pyrenees. Location: Central Pyrenees, Catalonia, NE Spain. Methods: We established four plots along an elevation gradient from the closed subalpine forest to the krummholz zone at five study sites. In each plot, we collected cones from five to six trees, measured their length, and triggered their opening in the laboratory to count the number of empty seeds and the number and weight of full seeds. We used the collected seeds in a germination experiment under controlled conditions in growth chambers. Additionally, we installed seed traps along the forest–alpine grassland transition to measure seed rain for three consecutive years in three of the study sites. Results: The number of full seeds per cone decreased along the elevation gradient and was correlated with cone length. However, the proportion of full seeds per cone and their weight did not differ between elevation positions. Seed rain decreased drastically with elevation and no seeds arrived into the alpine grassland traps consistently across study years. Although germination success did not significantly differ between elevation provenances (i.e., elevation position of origin), we found significant differences in germination dynamics between study sites and between elevation provenances within sites. Conclusions: Our results indicate that whereas the viability of Pinus uncinata seeds is not limited by elevation, seed production and dispersal are constraining the ongoing rates of treeline advance in the Pyrenees. [ABSTRACT FROM AUTHOR]

Climate warming impacts on alpine treeline dynamics. However, we still lack robust assessments of the long-term impacts of climate on tree recruitment at the treeline, particularly in remote areas such as the subarctic regions of Russia subjected to different climate influences. We expected that the treelines in two regions may have different features and dynamics patterns. We analyzed climate variables and assessed treeline dynamics by quantifying recruitment using the tree rings of ca. 7000 trees of four species (Betula pubescens Ehrh. ssp. tortuosa, Pinus sylvestris L., Picea abies Ledeb. ssp. obovata, Larix gmelinii Rupr.) along 14 altitudinal transects (series of study plots). We compared the Khibiny Massif (Kola Peninsula) and the western Putorana Plateau, subjected to oceanic and continental influences, respectively. In both regions, summers became warmer, and winters became snowier during the past century. At the low part of the treeline ecotone, tree recruitment has slowly increased since the mid-18th century at the Putorana Plateau and the mid-19th century at the Khibiny but accelerated in the early 20th century at both regions and reached a maximum peak in the second half of the past century. Treeline encroachment intensified in the 1930s at the Khibiny and the 1950s at the Putorana Plateau. Trees encroached in the tundra leading to upward treeline shifts in the late 20th century. The slope exposure affected the rates of treeline shift with higher upward advances on southern-oriented slopes. Tree recruitment and early-winter precipitation were positively correlated. The differences in species composition, treeline altitude and influences of slope orientation on treeline dynamics can be explained primarily by differences in the degree of continentality. The abundance of saplings in both regions allows the future encroachment of trees into tundra and further treeline upward shifts to be forecast.

Himalayan region represents the highest and most diverse treeline over the world. As one of the most conspicuous boundaries between montane forests and alpine vegetation, the alpine timberline attracted the interest of researchers for many decades. However, timberline in the Himalayas is understudied compared with European counterparts due to remoteness. Here we review the distribution pattern of timberline and its climatic condition, the carbon and nutrient supply mechanism for treeline formation, and treeline shift and treeline tree recruitment under climate change scenarios. Growth limitation, rather than carbon source limitation is the physiological cause of timberline under the low temperature condition. Nutrient limitation and water stress are not the direct cause of timberline formation. However, more clear local limitation factors are need to integrate in order to enable us to predict the potential impacts and changes caused by human activity and related global change in this sensitive region. [ABSTRACT FROM AUTHOR]

Questions: Shrub expansion in arctic, alpine and subalpine areas has been occurring recently due to warming, land abandonment and increasing nitrogen deposition. However, shrub expansion is strongly affected by largely unknown site‐specific regeneration patterns. We evaluated the importance of seed production, suitable microsite availability and seedling survival for seedling establishment of the subalpine conifer shrub Pinus mugo. Location: Hrubý Jeseník Mts., Czech Republic (50° N, 17° E). Methods: We collected data on: (a) seed quantity and quality; (b) seedling microsite preferences; and (c) seedling survival during initial ontogenetic stages. From 2006 to 2016 we assessed cone production annually and tested seed viability every second year. Microsite preferences were analyzed for 650 seedlings at six sites. The following microsite characteristics were recorded for 1‐m2 around each seedling: percentage of bare surface; surrounding plant species cover; surface shape; and organic horizon thickness. Additionally, distance and direction to the nearest adult pine were recorded. Finally, we tested survival of 200 newly established seedlings in relation to microsite type. Results: Seed source and viability were not critical factors for seedling establishment. Far more important was the presence of suitable bare‐surface microhabitats with thin or no organic layer near mature pine stands. Vegetation communities dominated by Avenella flexuosa and Nardus stricta facilitated seedling establishment; communities dominated by Vaccinium myrtillus showed strong competitive effects on seedlings. Vegetation's effect on seedling establishment was closely related to disturbance regime, which influences vegetation type. Seedling mortality was highest in the first year of life irrespective of microsite vegetation type. Highest mortality was associated with open mineral‐soil microsites, while microsites with dense herbaceous vegetation showed the lowest. Conclusions: Potential Pinus mugo expansion will occur in close proximity to existing mature stands, and its intensity will be strongly modified by site‐specific density of suitable microsites and/or dominant species of host communities. [ABSTRACT FROM AUTHOR]

Understanding the processes that control alpine treelines, the elevational limits of tree growth forms, has been a central question in ecology and is growing in importance with concern over climate change. Cool summer air temperatures are currently thought to be the ultimate limiter of upright tree growth at alpine treelines globally. However, winter damage has long been recognized as a shaping force near alpine treelines. Low‐growing krummholz growth forms provide an opportunity to test hypotheses about the controls of upright growth in environments above current treelines.To distinguish between effects of growing season temperature, winter damage and their interaction on preventing upright growth in krummholz, we conducted a field experiment on krummholz growth forms of Pinus albicaulis over the summer and winter of 2015–2016 at 10 mountain top sites in the Tobacco Root Mountains, Montana, USA. We experimentally manipulated four factors using a fully crossed design: shoot position (natural low position in the krummholz mat vs. propped up above the krummholz mat), summer warming (warming chamber vs. ambient), winter exposure (shelter cage vs. exposed), and elevation position (local high vs. low krummholz limits). We also conducted an observational study of the climatic conditions associated with recent natural emergent stem establishment from krummholz.Experimentally propped shoots that were exposed in winter experienced the highest mortality (10%–50%), while propped shoots in shelter cages and shoots located within the krummholz mat, whether caged or not, had low mortality (0%–10%). Summer warming had little influence on shoot mortality. Surviving mat shoots had marginally higher growth rates than surviving propped shoots during the early growing season after treatments were established. Natural emergent stem establishment was associated with warmer than average summer temperatures, but also warmer winter temperatures, lower winter wind speeds, and lower snowpack.Synthesis. Our results suggest winter damage plays a more important role than does growing season temperature in maintaining the krummholz growth form. While warming may increase opportunities for emergent shoot establishment above krummholz mats, establishment of upright trees in the krummholz zone will also require climatic change that reduces wind and snow transport which cause winter damage. [ABSTRACT FROM AUTHOR]

Developing sustainable adaptation pathways under climate change conditions in mountain regions requires accurate predictions of treeline shifts and future distribution ranges of treeline species. Here, we model for the first time the potential distribution of Betula utilis, a principal Himalayan treeline species, to provide a basis for the analysis of future range shifts. Our target species Betula utilis is widespread at alpine treelines in the Himalayan mountains, the distribution range extends across the Himalayan mountain range. Our objective is to model the potential distribution of B. utilis in relation to current climate conditions. We generated a dataset of 590 occurrence records and used 24 variables for ecological niche modelling. We calibrated Generalized Linear Models using the Akaike Information Criterion (AIC) and evaluated model performance using threshold-independent (AUC, Area Under the Curve) and threshold-dependent (TSS, True Skill Statistics) characteristics as well as visual assessments of projected distribution maps. We found two temperature-related (Mean Temperature of the Wettest Quarter, Temperature Annual Range) and three precipitation-related variables (Precipitation of the Coldest Quarter, Average Precipitation of March, April and May and Precipitation Seasonality) to be useful for predicting the potential distribution of B. utilis. All models had high predictive power (AUC ≥ 0.98 and TSS ≥ 0.89). The projected suitable area in the Himalayan mountains varies considerably, with most extensive distribution in the western and central Himalayan region. A substantial difference between potential and real distribution in the eastern Himalaya points to decreasing competitiveness of B. utilis under more oceanic conditions in the eastern part of the mountain system. A comparison between the vegetation map of Schweinfurth (1957) and our current predictions suggests that B. utilis does not reach the upper elevational limit in vast areas of its potential distribution range due to anthropogenically caused treeline depressions. This study underlines the significance of accuracies of current environmental niche models for species distribution modelling under climate change scenarios. Analysing and understanding the environmental factors driving the current distribution of B. utilis is crucial for the prediction of future range shifts of B. utilis and other treeline species, and for deriving appropriate climate change adaptation strategies.

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The rapid warming in Himalaya has begun to impact ice and snow and high elevation ecosystems, of which alpine treelines and meadows are particularly affected. The main objective of this study is to find out the impact of early snowmelt on herb communities of treeline ecotone (3200-3300 m elevations) located in Tungnath, Uttarakhand Himalaya. The focus is on species composition, species diversity and phenology. For this, we have followed two research approaches: first, comparing the herb communities of sites and microsites differing in the timing of snowmelt and the amount of snow cover, and second, comparing the phenology of common herb species with the past studies undertaken at the same and similar sites. We sampled several microsites which differed in the timing of snowmelt and were accordingly divisible into early snowmelt and late snowmelt microsites, the difference between them being of 3-5 days. Of the 86 species recorded across the sites, 84% were hemicryptophytic perennial forbs, and about 70% of them were native. Both herb species richness and species diversity were significantly higher in early snowmelt microsites than in late snowmelt microsites, both in high snow cover and low snow cover habitats. The total plant density ranged between 82-626 individuals m-2 in early snowmelt microsites and 69-288 individuals m-2 in late snowmelt microsites. It seems that the early snowmelt in a warming climate would promote species diversity and plant density. Comparison to the past studies indicates that because of climatic warming species have advanced and lengthened their vegetative and flowering phenophases. We could not assess whether migration of species from lower elevations contributed to high species richness of early snowmelt microsites. However, it cannot be ruled out, given that continent-wide species enrichments near mountain summits have been recorded elsewhere. [ABSTRACT FROM AUTHOR]

In this study on treeline phenology in Tungnath, Garhwal Himalaya (altitude, 2955-3334 m asl), we (i) collected data on micro-climatic conditions, timing of major phenophases, leaf and shoot growth phenology and leaf nitrogen resorption in four major tree species (Abies spectabilis, Betula utilis, Quercus semecarpifolia, Rhododendron arboreum) and a krummholz species (R. campanulatum); and (ii) compared the treeline species phenology with mid-altitude forest tree species phenology of this region. Meteorological data reported for this site revealed that the mean monthly atmospheric temperature during growing period (July-September) has increased at the rate of 0.11 °C yr-1 in the last two decades. Leaf bud-break occurred in all the species in May, however, the proportion of marked tree population in bud-break phase at the time of first observation in May varied considerably across the species from 10% in Q. semecarpifolia to 50% in R. arboreum. Leaf initiation and leafing in the treeline species was delayed by about two months, compared to the mid-altitude tree species. Also, the leaf expansion within one month of leafing was conspicuously slower in treeline species than mid-altitude species (43% vs. 83% in Quercus spp. and 49% vs. 76% in R. arboreum). The treeline species were characterized by short growing period (2-4 months), lower period of steady-state in peak leaf mass and rapid leaf mass loss, low nitrogen concentration in leaves (1.7% vs. 2.5%), slow shoot growth, lower shoot length (5.6 cm vs. 9.5 cm) and shoot growth period, and higher leaf density in shoots (1.2 vs. 0.8 leaves cm-1 shoot length) than the mid-altitude forest trees. It is expected that with the increasing rate of warming in Tungnath the phenological behaviour of treeline species would change markedly in future leading to changes in ecosystem properties. [ABSTRACT FROM AUTHOR]

Sensitivity of alpine treeline ecotone to climate change and upward shift of plants due to warming have been reported from many parts of the globe. However, such research in the climate sensitive western Himalaya is still in infancy due to paucity of past climatic and vegetation distribution data to detect the change. In this paper we determined the expansion of Rhododendron campanulatum krummholz using seedling population and growth characteristics along an altitudinal transect rummimg from the upper forest limit to treeline ecotone and beyond it in Tungnath (i.e., 3242 m asl), Uttarakhand. In the studied transect (altitudinal gradient of 3511-3665 m asl) in 2017, a total of 17 trees and 36 seedlings of R. campanulatum were found growing. Almost half of the trees were devoid of any seedlings around their canopies. Height and circumference at collar height of both the adults and seedlings were positively correlated (P < 0.05). Taking the mean distance of seedlings from the mother tree (2.8 m; range = 0.50-4.72 m) and mean age of the seedlings (2-9 yrs; mean = 3.9 yr; reckoned by the number of internodes) the rate of expansion of R. campanulatum population was computed about 1.4 m yr-1 Occurrence of mature individuals at the mountain top without any seedlings indicates that as no space left for upward movement of plants, the interspaces between the krummholz are being filled by the regenerating individuals. This may leave little grazing grounds for the migratory livestock and change ecosystem properties. It may be further pointed out that due to rise in atmospheric temperature (@ 0.11 °C yr-1 in the past two decades) and continued biotic stress of grazing and tree lopping, R. campanulatum, a non-palatable species of wider niche width might preponderate at the expense of herbs and other treeline species in future and may bring out compositional changes in treeline vegetation and carbon storage. [ABSTRACT FROM AUTHOR]

Quantitative and qualitative characteristics of cones and seeds and the emergence and survival of seedlings in the Siberian larch (Larix sibirica Ledeb.) have been studied in different parts of the ecotone at the upper boundary of arboreal vegetation (treeline ecotone) on Serebryanskii Kamen' Mountain (Northern Urals) and on the hills (with a landmark 312 m above sea level) surrounding Chernaya Mountain (Polar Urals) from 2005 to 2011. We have found a decrease in the parameters of cones, number of seeds in the cones, their viability in laboratory with an increase in altitude, and differences in the number of seedlings between parts of the treeline ecotone. It is shown that the formation of Siberian larch generations on the Northern Urals occurs only after moist years. It is noted that, in the Polar Urals, Siberian larch produces a big seed crop every 2-3 years. It is proven that the seed production and mortality of seedlings in Siberian larch are influenced not only by air temperature and soil moisture, but also by wind load, snow depth, and soil temperature rapidly changing along the slope. [ABSTRACT FROM AUTHOR]