The upper forest boundary in the mountains is a reliable bioindicator of modern climate change, though its dynamics are influenced by multiple factors. Conjugate studies on the structure of tree stands and the soil properties across the elevation gradient were performed at the treeline on slopes of different exposures in the Altai Mountains. The oldest trees on the northern slope were established in the mid-15th century, but intensive expansion began in the 20th century, whereas tree establishment on the eastern slope began later and occurred over a shorter period. A significant positive correlation was revealed between summer and winter temperatures, winter precipitation, and the rate of Pinus sibirica establishment. The open forest boundary reaches its highest elevation on the eastern slope, where conditions are intermediate between the northern and southern slopes in terms of snow cover duration and land surface temperature.The differences in soil types reflect the contrasting conditions influenced by slope exposure. The earlier colonisation of northern slopes has led to the formation of typical forest soils (Skeletic Podzols) at higher elevations under open forest stands. These soils are more acidic, with slower litter decomposition and pronounced accumulation of carbon and nutrients in their organic surface horizons, while their mineral horizons are nutrient-poor. In contrast, soil at the eastern slope (Folic Leptosols) has a higher element content throughout the soil profile and displays minimal disparities and variation in the morphology and chemistry across the treeline ecotone. The mismatch transition between vegetation and soil belts suggests that soil properties respond to climatic changes more slowly than vegetation dynamics.

It has been established that with the improvement of climatic conditions in the high mountains of the Southern Urals, an upward shift of the upper forest boundary along the altitudinal gradient is observed. The process of forest regeneration in this area is highly prolonged over time. Cyclically uneven-aged and stepwise uneven-aged stands are forming, consisting of several morphologically indistinct forest generations.

At treeline, plant life forms and species change abruptly from low-stature plants in the tundra to trees in forests. Our study assesses how the vegetation shift affects the quality and elemental composition of the litter layer and consequently the microbial processing and nutrient release during decomposition. We sampled litter layers along elevation gradients across conifer- and broadleaf-dominated treelines in the Russian subarctic Khibiny Mountains and hemiboreal South Urals. Using microlysimeters at 5 and 15 degrees C, we measured carbon (C) mineralization and the release of inorganic nitrogen (N) and phosphorus (P), reflecting net N and P mineralization. Additionally, we quantified releases of dissolved organic C and N and analysed the stoichiometry and ecophysiology of microbial biomass. Our findings showed significant shifts in the chemical characteristics of the litter layer across both treeline ecotones. On average, C:N and C:P ratios decreased by 56 % and 65 %, while lignin contents increased by 110 % from tundra to forest. The consistent decrease in C:N:P ratios in the litter layer was paralleled by pronounced increases in net N and P mineralization from tundra to lower-elevation forest in both treeline ecotones. The negligible nutrient release from tundra litter was likely due to immobilization of mineralized N and P at molar C:N and C:P ratios exceeding 35 and 1100, respectively. In contrast to net nutrient mineralization, C mineralization and the release of dissolved organic C and N remained largely unchanged. Microbial biomass colonizing the litter layer showed average decreases of C:N and C:P ratios by 26 % and 74 % from tundra to forest, while potential activities of C-N-P-acquiring extracellular enzymes showed no consistent pattern. Mineralization of 13C-labelled glucose-6-phosphate decreased with decreasing C:N:P ratios from tundra to forest. As the 13C incorporation into microbial biomass remained unaffected, substrate-use efficiency (SUE) increased along the same trajectory. Overall, our results give evidence that the vegetation shift from tundra to forest is associated with an abrupt increase in net N and P mineralization in the litter layer, accelerating nutrient cycling and increasing N and P availability. In contrast, experimental warming by 10 degrees C was less important for net N and P mineralization than litter composition. This indicates that indirect effects of climatic warming through changes in plant community composition with treeline advances seem to be more important for soil N and P cycling than direct temperature effects.

Climate has changed sufficiently over the last 150 years and forced out upper treeline advance at the most studied sites around the world. The rate of advance has been extremely variable - from tens to hundreds meters in altitude. This is because the degree at which tree frontal populations respond to climate change depends on the complex interaction of biological and physical factors. The resulting stand pattern is the consequence of the interaction between dispersal and survival functions. A few publications have addressed the question of how this pattern is generated. In order to understand how the spatial structure of tree stands was formed at the upper limit of their distribution in the Ural Mountains, we assessed the distance and direction of dispersal of offspring from maternal individuals. We found that in frontal Larix sibirica Ledeb. populations, 'effective' dispersal of offspring ranges from 3 to 758 m (with a median of 20-33 m in open forest and 219 m in single-tree tundra in the Polar Urals and 107 m in open forest in the Northern Urals). We revealed that most of the offspring effectively dispersed not only in the direction of the prevailing winds, but also in the opposite direction up the slope, and the distance can reach 500-760 m. The data obtained can be used to develop an individual-based model which is capable of simulating in detail the dynamics of tree stands at the upper limit of their growth and reliably predicting the future position and pattern of treeline ecotone as growth conditions continue to improve in the face of observed climate change.

В статье представлены результаты оценки количественных и качественных показателей шишек и семян ели сибирской в экотоне верхней границы леса на примере горного массива Иремель (Южный Урал). Установлено, что в ельниках исследуемого экотона формируются некрупные шишки, заметно уступающие по длине, диаметру и весу шишкам из более благоприятных для роста древостоев равнинных условий. Из указанных параметров наибольшей изменчивостью характеризуется вес шишек, а наименьшей – их диаметр. Линейные размеры и вес шишек, содержание семян в шишках, средний вес 1000 семян и их всхожесть в пределах отдельных склонов закономерно уменьшаются с повышением высоты над уровнем моря. При прочих равных условиях эти показатели шишек и семян на юго-западном склоне выше, чем на северном. По посевным качествам большая часть семян относится к категории некондиционных. Низкая всхожесть семян ели на исследуемых объектах в основном обусловлена высокой долей пустых семян. В целом в экотоне верхней границы леса участки леса, различающиеся высотой над уровнем моря на 35–50 м и более, по условиям формирования шишек и семян принадлежат к качественно разнородным совокупностям. Между количеством семян, с одной стороны, и длиной, диаметром и весом шишек – с другой, наблюдаются достаточно устойчивые связи. Однако отдельные параметры шишек (длина, диаметр и вес) не могут в достаточной мере объяснить изменчивость количества семян в шишке. Полученные в результате исследования данные могут служить теоретической и экспериментальной базой для оценки семенной продуктивности и климатогенной динамики древостоев в высокогорьях Южного Урала. The article presents the results of assessing the quantitative and qualitative parameters of cones and seeds of Siberian spruce in the ecotone of the upper forest boundary using the example of the Iremel mountain range (Southern Urals). It has been established that in the spruce forests of the ecotone under study, small cones are formed, which are noticeably inferior in length, diameter and weight to cones from lowland conditions that are more favorable for the growth of tree stands. Of the indicated parameters, the weight of the cones is characterized by the greatest variability, and their diameter by the smallest. The linear dimensions and weight of cones, the number of disease-free seeds in cones, the average weight of 1000 seeds and their germination within individual slopes naturally decrease with increasing altitude. All other things being equal, these parameters of cones and seeds on the southwestern slope are higher than on the northern slope. According to the sowing qualities, most of the seeds belong to the category of substandard. The low germination of spruce seeds in the studied areas is mainly due to the high proportion of empty seeds. In general, in the ecotone of the upper forest boundary, forest areas that differ in height above sea level by 35–50 m or more, according to the conditions for the formation of cones and seeds, belong to qualitatively heterogeneous communities. Quite stable relationships are observed between the number of seeds, on the one hand, and the length, diameter and weight of the cones, on the other. However, individual parameters of cones (length, diameter and weight) cannot suffi ciently explain the variability in the number of seeds in a cone. The data obtained as a result of the study can serve as a theoretical and experimental basis for assessing seed productivity and climatogenic dynamics of forest stands in the highlands of the Southern Urals.

Treelines advance due to climate warming. The impacts of this vegetation shift on plant-soil nutrient cycling are still uncertain, yet highly relevant as nutrient availability stimulates tree growth. Here, we investigated nitrogen (N) and phosphorus (P) in plant and soil pools along two tundra-forest transects on Kola Peninsula, Russia, with a documented elevation shift of birch-dominated treeline by 70 m during the last 50 years. Results show that although total N and P stocks in the soil-plant system did not change with elevation, their distribution was significantly altered. With the transition from high-elevation tundra to low-elevation forest, P stocks in stones decreased, possibly reflecting enhanced weathering. In contrast, N and P stocks in plant biomass approximately tripled and available P and N in the soil increased fivefold toward the forest. This was paralleled by decreasing carbon (C)-to-nutrient ratios in foliage and litter, smaller C:N:P ratios in microbial biomass, and lower enzymatic activities related to N and P acquisition in forest soils. An incubation experiment further demonstrated manifold higher N and P net mineralization rates in litter and soil in forest compared to tundra, likely due to smaller C:N:P ratios in decomposing organic matter. Overall, our results show that forest expansion increases the mobilization of available nutrients through enhanced weathering and positive plant-soil feedback, with nutrient-rich forest litter releasing greater amounts of N and P upon decomposition. While the low N and P availability in tundra may retard treeline advances, its improvement toward the forest likely promotes tree growth and forest development.

Ground-based laser surveys of the upper parts (from the ridge crests to the border of closed forests) of six steppe slopes of the mountains of the Southern and Northern Kraka massifs (Southern Urals) on a total area of 20.82 hectares were carried out. As a result, 3584 trees were identified in all studied areas, their morphometric parameters (crown height and size) and exact geographical position were assessed with an accuracy of 3–10 cm. It was shown that the closure and density of tree stands are extremely low in the upper third of the surveyed altitudinal profiles, and a local increase in these indicators is observed only on some slopes. It is also widely noted that these characteristics of forest stands increase sharply at a distance of about 2/3rd of the length of the profiles from the ridge crests. Measurements of the depth of the soil layer showed that the soils in the upper parts of the slopes are thin (on average 7–12 cm), and their average depth reaches 29 cm only on some slopes in places where the density of forest stands increases. Soil depth on sections of profiles remote from the ridge crests 1/4th to 2/3rd of their length gradually increases and reaches average values of 20–30 cm. We associate both local and stable (in the lower part) increase in the density of forest stands in the forest–mountain steppe transition zone with the increase in the thickness of the soil layer and the volume of moisture retained by it.

In recent decades, the rapid climate warming in polar and alpine regions has been accompanied by an expansion of shrub vegetation. However, little is known about how changes in shrub distribution will change as the distribution of tree species and snow cover changes as temperatures rise. In this work, we analyzed the main environmental factors influencing the distribution and structure of Juniperus sibirica, the most common shrub species in the Southern Ural Mountains. Using mapping and digital elevation models, we demonstrated that J. sibirica forms a well-defined vegetation belt mainly between 1100 and 1400 m a.s.l. Within this zone, the abundance and cover of J. sibirica are influenced by factors such as rockiness, slope steepness, water regime and tree (Picea obovata) cover. An analysis of data spanning the past 9 years revealed an upward shift in the distribution of J. sibirica with a decrease in its area. The primary limiting factors for the distribution of J. sibirica were the removal of snow cover by strong winter winds and competition with trees. As a consequence of climatic changes, the tree line and forest limit have shifted upward, further restricting the distribution of J. sibirica to higher elevations where competition for light with trees is reduced and snow cover is sufficiently deep.

Мониторинг распространения древесно-кустарниковой растительности на верхней границе леса в горных регионах относится к наиболее простым и эффективным методам получения доказательств последствий климатических изменений для растительности. В условиях плато Путорана одним из наиболее крупных и распространенных кустарниковых видов является ольховник кустарниковый ( Duschekia fruticosa(Rupr.) Pouzar). В западной части плато Путорана в пределах экотона лес - горная тундра на склонах разной экспозиции был проведен анализ возрастной и морфологической структуры этого растения, произрастающего на разной высоте над уровнем моря (200-600 м) в древостоях разной сомкнутости. Установлено, что в ХХ в., преимущественно в его второй половине, происходила интенсивная экспансия ольховника кустарникового в горные тундры, редины и редколесья на склонах всех экспозиций массива Сухие горы. Выявлено влияние древостоев лиственницы Гмелина ( Larix gmelinii (Rupr.) Kuzen) на распространение и морфологическую структуру ценопопуляций ольховника кустарникового. Заселение и распространение ольховника кустарникового происходит сопряженно с древостоями лиственницы Гмелина. Установлены зависимости между высотой снега и суммой проекций крон кустарников ( R2 = 0.582). Выявлено, что при отсутствии снежного покрова ольховник кустарниковый не может существовать. Установлены значительные различия в распространении ольховника кустарникового в зависимости от экспозиции, на южных и восточных склонах оно выше. Наибольшее количество кустов произрастает преимущественно в нижней части экотона, где снежные массы накапливаются в большем количестве. Наиболее вероятным объяснением увеличения плотности и продвижения выше в горы ольховника кустарникового может быть общее изменение климатических условий в районе исследования. Monitoring the distribution of tree and shrub vegetation at the upper forest limit in mountainous regions is one of the simplest and most effective methods for obtaining evidence of the effects of climate change on vegetation. One of the largest and most widespread shrub species on the Putorana plateau is the shrub alder ( Duschekia fruticosa (Rupr.) Pouzar). The study presents an analysis of the age and morphological structure of shrub alder thickets, which grow at different altitudes (200-600 m above sea level) within the forest - tundra ecotone in forest stands of different density on slopes with different exposures of the Putorana plateau. It has been established that the intensive expansion of shrub alder into mountain tundras, sparse and open forests on the slopes of all exposures of the Sukhie Gory massif occurred in the 20th century, mainly in its second half. The influence of Gmelin larch ( Larix gmelinii (Rupr.) Kuzen) stands on the distribution and morphological structure of shrub alder cenopopulations was revealed. The colonization and spread of shrub alder occurs in conjunction with larch stands. We established relationships between snow depth and the sum of projections of shrub crowns ( R 2 = 0.582). Shrub alder cannot survive in the absence of snow cover. There are significant differences in the distribution of shrub alder depending on the slope exposure. The distribution of shrub alder is higher on the slopes of southern and eastern exposures. The largest number of shrubs grow mainly in the lower part of the ecotone, where snow masses accumulate in greater quantities. The most likely explanation for the increase in density and advance to the mountains of alder shrub may be a general change in climatic conditions in the study area.

Aims: We introduce ReSurveyEurope - a new data source of resurveyed vegetation plots in Europe, compiled by a collaborative network of vegetation scientists. We describe the scope of this initiative, provide an overview of currently available data, governance, data contribution rules, and accessibility. In addition, we outline further steps, including potential research questions. Results: ReSurveyEurope includes resurveyed vegetation plots from all habitats. Version 1.0 of ReSurveyEurope contains 283,135 observations (i.e., individual surveys of each plot) from 79,190 plots sampled in 449 independent resurvey projects. Of these, 62,139 (78%) are permanent plots, that is, marked in situ, or located with GPS, which allow for high spatial accuracy in resurvey. The remaining 17,051 (22%) plots are from studies in which plots from the initial survey could not be exactly relocated. Four data sets, which together account for 28,470 (36%) plots, provide only presence/absence information on plant species, while the remaining 50,720 (64%) plots contain abundance information (e.g., percentage cover or cover-abundance classes such as variants of the Braun-Blanquet scale). The oldest plots were sampled in 1911 in the Swiss Alps, while most plots were sampled between 1950 and 2020. Conclusions: ReSurveyEurope is a new resource to address a wide range of research questions on fine-scale changes in European vegetation. The initiative is devoted to an inclusive and transparent governance and data usage approach, based on slightly adapted rules of the well-established European Vegetation Archive (EVA). ReSurvey:Europe data are ready for use, and proposals for analyses of the data set can be submitted at any time to the coordinators. Still, further data contributions are highly welcome.

Many treeline and shrublines are not responding to climate warming as fast as expected. This lack of responsiveness could be explained by other non-thermal, climate drivers operating at the cold edge of distribution of trees and shrubs. To determine which are the main climate drivers of tree and shrub radial growth near the treeline we measured ring width and related it to climate variables (temperature, precipitation, snow depth) and vegetation greenness (NDVI, Normalized Different Vegetation Index). We compared two dwarf shrub (Vaccinium uliginosum, Dryas octopetala) and three tree species (Larix sibirica, Picea obovata, Pinus uncinata) sampled in three treeline sites: Polar or Northern Urals, Southern Urals, and Spanish Pyrenees. Dwarf shrubs presented lower first- order autocorrelation (AR1) than trees, excepting in the N. Urals site. In this site, V. uliginosum showed a negative growth trend, whereas this tendency was observed in P. obovata and P. uncinata trees from the S. Urals and Pyrenees sites, respectively. Shrub and tree growth indices correlated with NDVI at different months. Trees showed stronger and positive growth responses to warmer summer conditions and also negative responses to precipitation in the N. and S. Urals. The growth of D. octopetala in the Pyrenees was enhanced by prior-winter and current-spring precipitation showing a strong correlation with May snow depth (r = 0.66, p = 0.0006, period 1998-2020). Dwarf shrubs and trees coexisting near the treeline differently responded to regional climate variability. Our findings indicate a strong dependence of shrubs and trees on local (e.g., snow depth) and regional (e.g., growing-season air temperature) climate conditions, respectively.

The upper treeline ecotone is a global and typically climate-dependent phenomenon. Its elevation is usually coupled with the thermal limitations of tree growth. The air temperature rise connected with global warming is assumed as the main cause of treeline upslope shifts in the last century. It has been found that the treeline elevation also correlates with the distance from the coastline and the aridity or continentality of the climate or the mass elevation effect. However, previous and contemporary publications have not explained how the upper treeline position directly couples with climate parameters. Often, this has been restricted by a lack of climate measurements and spatial data. In our study, we obtained data from 339 regional weather stations for 1964–1974 and interpolated them to Altay and Western Sayan using regional DEMs and a specially developed regression model. Moreover, we semiautomatically identified the elevational position of the upper open forest boundary (OFB) (crown closure > 10%) on the slopes of 30 mountains in Altay and Western Sayan in 1960 and 2020. We took into account the slope aspect and edaphic constraints. The obtained data allowed us to undertake a regression analysis of the dependence of the OFB elevation on climatic parameters. As a result, we found that, in the 1960s, at OFB elevations rising from the outer to the inner parts of the study area to approximately 500–700 m, the summer air temperature and precipitation linearly decreased, but the summer sunshine duration increased. In the multiple regression analysis, including the climatic parameters as independent variables and the OFB elevation as a dependent variable, significant relations were found only for the combination of air temperature and sunshine duration. We assume that the OFB elevation is determined not only by the air temperature but also by the direct solar irradiation level, changing with latitude and cloudiness. We also found that the ratio between the OFB elevation on the northern and southern slopes varied with respect to latitude. The spatial analysis of OFB shifts in 1960–2020 revealed significant differences in its value in the central (80–90 m) and outer parts of the study area (110–130 m). We suppose that the OFB advance over the past 60 years has local specificity associated with the peculiarities of the climatic changes (summer temperature rise, precipitation decrease, and sunshine duration increase) in different parts of Altay and Western Sayan. Our results highlight the need to clearly determine climatic parameters when forecasting woody vegetation reactions to future climate changes.

Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1‐km 2 resolution for 0–5 and 5–15 cm soil depth. These maps were created by calculating the difference (i.e. offset) between in situ soil temperature measurements, based on time series from over 1200 1‐km 2 pixels (summarized from 8519 unique temperature sensors) across all the world's major terrestrial biomes, and coarse‐grained air temperature estimates from ERA5‐Land (an atmospheric reanalysis by the European Centre for Medium‐Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (−0.7 ± 2.3°C). The observed substantial and biome‐specific offsets emphasize that the projected impacts of climate and climate change on near‐surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil‐related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications.

Snow cover is one of the most important factors affecting the regeneration and growth of shrubs in cold arctic and alpine ecosystems. In many of these cold regions, climate change in the last century is manifested not only in a rapid rise of temperature, but also in an increase in winter precipitation. For instance, in the Ural Mountains, winter turned warmer and more humid during the past century, leading to higher snow accumulation. We investigated how the change trends in the cold season (November to March) climate conditions affected the recruitment of the shrub Juniperus sibirica Burgsd., the most widespread shrub conifer in mountains of this region where it is dominant in treeless areas. Specifically, we considered seven sites located in the Southern and Northern Urals that are subjected to lower and higher continentality, respectively. We assessed how juniper recruitment changed along altitudinal gradients going from the open forest to the alpine tundra and passing by the transition zone. We found that juniper shrubs recruited at higher elevations during the 20th century in most sites, with a rapid shrub encroachment into alpine tundra (shrubification) after the 1990s. This process was especially intensive in the last decades at the uppermost parts of convex slopes where the snowpack is shallow. We found positive associations between juniper recruitment and cold-season precipitation or temperature in the Northern and Southern Urals, respectively. Shrubification is following upward treeline shifts in the Southern Urals. Our findings indicate that juniper shrubs will tend to colonize sites with low snowpack depth if winter conditions keep warm and wet enough and the snowpack allows the effective protection of shrubs.

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.

Altitudinal forest limits are typically climatically dependent, such that increasing temperatures connected to global warming are causing upslope shifts in treeline ecotones worldwide. However, at the local and regional levels, the degree of such a response is dependent on differences in climate, topography and soil features. In recent decades, attempts have been undertaken to estimate tree stand dynamics with remote sensing methods, but their resolution is still too coarse for a precise assessment of stand structural changes, and requires ground-truthing, which is not possible without historical data collected on a single-tree level. We used aerial photos (1962) and satellite images (2021) in combination with historical inventory data to investigate changes in open forest positions at different spatial scales at the eastern macroslope of the Polar Urals over the past 60 years. Additionally, obtained remote sensing data were validated on a single-slope level using tree crown size estimations. Our investigations showed that since 1960 up to present day, the total crown coverage increased from 6.9 to 22.1% within the test polygon. A highly spatially variable upslope advance in an open forest boundary was identified from 1.7 up to 7.1 m in altitude per decade. We revealed that the rate of tree stand transformations was to a great extent depended on the stand density in the 1960s, soil substrate type, moisture regime, slope aspect and inclination. Our results highlighted the necessity to consider the abovementioned factors when trying to predict climate-induced tree distributional responses in subarctic mountain regions.

Climate change effects are strongest in forest ecosystems at the limit of their distributions. Despite the evidence that treelines have shifted upwards by hundreds of meters, knowledge of the associated changes in the stand biomass is limited. In this study, stand biomass and changes to it during the last centuries were estimated along 20 altitudinal transects reaching from the historical (located in the 1950s–1960s) closed forest line up to the current treelines on mountain slopes of three subarctic regions of Russia (Kola Peninsula, Polar Urals, and Putorana Plateau) along a 2200 km long longitudinal gradient. The estimates were based on allometric measurements of 139 trees of five species (Betula pubescens Ehrh. ssp. tortuosa, Pinus sylvestris L., Picea abies Ledeb. ssp. obovata, Larix sibirica Ledeb., and Larix gmelinii Rupr.), stand structure assessments, and the demographic patterns of 9300 trees. During the 20th century, the growth and establishment of trees at the forest–mountain tundra transition (340–500 m width) increased exponentially. Since 1910 forest expansion and densification led to an accumulation of 621–748 tons of aboveground stand biomass per km of treeline length. The accumulation was two times higher below than above the contemporary closed forest line. Data analysis of weather stations showed that the 20th century’s climate had changed in a similar manner in the three study regions, namely vegetation periods became longer (8–10 days) and warmer (0.6–0.9 °C) and more snow fell in the cold period (+10–30%). Our results indicate that regional patterns in stand biomass at the treeline ecotone are primarily related to tree species composition as determined by macroclimatic conditions (e.g., continentality, sunshine hours), snowpack depth, and growing season duration. However, the stand biomass accumulation was driven by increases of early summer temperatures and early winter precipitation during the last century.

While climatic research about treeline has a long history, the climatic conditions corresponding to the upper limit of closed alpine grasslands remain poorly understood. Here, we propose a climatic definition for this limit, the ‘grassline’, in analogy to the treeline, which is based on the growing season length and the soil temperature. Eighty-seven mountain summits across ten European mountain ranges, covering three biomes (boreal, temperate, Mediterranean), were inventoried as part of the GLORIA project. Vascular plant cover was estimated visually in 326 plots of 1 × 1 m. Soil temperatures were measured in situ for 2–7 years, from which the length of the growing season and mean temperature were derived. The climatic conditions corresponding to 40% plant cover were defined as the thresholds for alpine grassland. Closed vegetation was present in locations with a mean growing season soil temperature warmer than 4.9 °C, or a minimal growing season length of 85 days, with the growing season defined as encompassing days with daily mean ≥ 1 °C. Hence, the upper limit of closed grasslands was associated with a mean soil temperature close to that previously observed at the treeline, and in accordance with physiological thresholds to growth in vascular plants. In contrast to trees, whose canopy temperature is coupled with air temperature, small-stature alpine plants benefit from the soil warmed by solar radiation and consequently, they can grow at higher elevations. Since substrate stability is necessary for grasslands to occur at their climatic limit, the grassline rarely appears as a distinct linear feature.