Dmitriy S. Balakin

Global warming, which is mainly linked to CO₂ increase, has led to a growing interest in assessing carbon conservation in forest biomass. Despite evidence that treelines have advanced by hundreds of meters, knowledge of associated stand biomass changes is insufficient for comprehensive estimation of their role in carbon sequestration. Traditionally, the biomass assessment is based on data collected by field measurements. While this approach provides accurate data for local sites, it cannot be extrapolated properly to larger areas. A more appropriate approach would be to combine field measurements with remote sensing methods. We used data obtained by tree morphometry and annual ring measurements, model-based biomass estimation, processing of laser scanning results, and satellite imagery to model and calculate changes in stand above-ground biomass (AGB) since 1900 at treeline ecotone in Altai and Western Sayan. We developed simulations to predict AGB changes over the coming four decades in these regions. Our findings revealed that the upslope shift of the treeline ecotone by 58–86 m of altitude over the past century was accompanied by an exponential increase in AGB of stands within the 200–400 m forest–tundra transition zone. This resulted in an AGB increment of 120–139 tons per 100 m of treeline. We expect that stand AGB at the treeline ecotone will become 2.3–3.3 times bigger by 2060. All exposures must be considered when estimating stand AGB within treeline ecotones because there are significant differences in treeline elevation, tree-dominant proportions, and stand structure on different slopes.

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.

Aim. Analysis of the current distribution of mountain tundra in the South Urals, determination and cataloging of areas occupied by mountain tundra communities to assess the possible threat of their extinction in the 21^st century.Methodology. A visual search for potential locations of alpine tundra communities in the Southern Urals was conducted on satellite images from open mapping services (Yandex, Google) in maximum resolution. The principal peaks and ridges were subjected to detailed survey (Bolshoy Taganay, Urenga, Zyuratkul, Bolshaya Suka, Uvan, Zigalga, Yagodny, Mashak, Kumardak, Nary, Kuyantau, Iremel, Nurgush), and the locations of alpine tundra were identified. In a geographic information system, the obtained data were combined with a digital elevation model in order to calculate the area of alpine tundra. The species composition and the principal types of mountain tundra communities were identified through the utilisation of the route reconnaissance survey.Results. The present study demonstrates that alpine tundra communities are pervasive in the Southern Urals mountains, spanning latitudes between 53.7 and 55.4°N across a minimum of 37 peaks (comprising 12 mountain ranges and massifs). The largest alpine tundras in terms of area are located on the Zigalga, Nurgush ridges and the Kuyantau and Iremel massifs. These areas represent 80% of the total area of mountain tundra in the Southern Urals (at least 546 ha). The areas comprising alpine tundra, which collectively encompass less than 20 ha, are situated on the ridges of Bolshoy Taganay, Zyuratkul, Urenga, Nurgush, Yagodny, Bolshaya Suka, Nary, Mashak, Kumardak, and the individual peaks of Zigalga. The total area of alpine tundra on these ridges is estimated to be at least 140 hectares, which represents approximately 20% of the total area of all alpine tundra in the Southern Urals. A comprehensive catalogue and map of the contemporary distribution of alpine tundra in the Southern Urals have been compiled. The locations where mountain tundra has undergone a significant decline over recent decades and is likely to continue this trend in future decades have been identified.Research implications. The findings of this study can be utilized to develop models of climatogenic transformation in high-mountain ecosystems of the Southern Urals, and subsequently used to monitor their condition under a range of climate change scenarios in the future.

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.

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 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.

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.

Against the general background of an increase in global temperature and spatiotemporal rearrangements in the distribution of precipitations in the 20th century, there are significant changes in the structure and properties of terrestrial ecosystems at all levels of biosphere organization, while their carbon-depositing role is widely discussed in connection with an increase in the concentration of carbon dioxide (CO2 ) in the last century. In the massif of the Kraka mountains (South Urals), extrazonal mountain (petrophytic) steppes are located on the southern, highly insolated slopes of the ridges, where peculiar harsh conditions are formed that restrict the growth of woody plants. Due to climate change, forests are expanding to mountain steppes and their area is increasing. The aim of our research is to study the contemporary structure of tree stands and the features of their phytomass accumulation in the transition zone between the forest and mountain steppes on the slopes of the Kraka mountains (53°15′ – 53°50′N, 57° 36′ – 58°12′E), as well as to assess their changes in the last decades (See Fig. 1). In the massifs of the North and South Kraka mountains on the forest-mountain steppe ecotone, 8 altitudinal transects were installed, with division into levels depending on the crown density: the upper (first) – at the upper border of the sparse tree stands (groups of trees with crown density 0.05–0.1), middle (second) – at the upper border of open forests (crown density 0.2–0.3), lower (third) – at the upper border of closed forests (crown density 0.4–0.5). At the lower and middle altitudinal levels, 3 plots with a size of 20 × 20 m were installed. At the upper level, polygons were set up in the form of a rectangle with a size of 1-3 hectares. On Bashart Mount, a polygon profile was installed, without division into levels due to the mosaic distribution of trees along the entire profile. For each tree, the following parameters were determined: height, trunk diameter at the base and at a height of 1.3 m, crown projection diameter in two directions, and vital state. To determine the age of trees with a diameter of more than 3 cm, a bored wood sample (core) was taken. The age was determined using standard dendrochronological methods. The data on the phytomass of 16 model pine trees and 12 larch trees were obtained with subdivision into the following fractions: phytomass of the trunk, branches, needles and generative organs. The trunk phytomass in the bark was determined by direct weighing in the field with an accuracy of 50 g, for which the trunk was cut into meter sections. The percentage of dry matter, both in the wood and in the bark, is determined by the discs cut from the ends of the sawn sections. Wood and bark from the discs were weighed with an accuracy of 0.1 g at the site, and then they were sent to the laboratory for further drying and determination of their weight in an absolutely dry state. When determining the phytomass of the crown and its structural parts, all branches from the trunk were chopped off. The total weight of the crown was determined by direct weighing at the site in the field, after which the crown was divided into parts covered with needles and without them, and they were weighed separately. A sample of up to 20–30% of the total weight was taken. The needles in the sample were separated from the branches and weighed. To determine the absolutely dry matter, a sample weighing 30-60 g was taken from each crown fraction for subsequent drying to an absolutely dry state in laboratory conditions. The assessment of the scale of changes in the area of treeless territories on the slopes of the Kraka mountain ranges was carried out using the functionality of the SAS.Planet 160707 program. Using the “tools” from the “Tags” tab, on the basis of the images in the layers of modern (2015) satellite images of sub-meter resolution and maps (~ 1986) of the GosGisCentre (M 1: 25000), all non-forested areas were identified where the forest stands below 35-40%. For each treeless territory, its area was estimated in 1986 and 2016. Comparative characteristics of the habitat within the ecotone between closed forests and the mountain steppe and the patterns of changes in the morphometric parameters of tree stands at different altitude levels on the slopes of the South Kraka massif showed that the trunk diameter, height and age of trees increase from the lower to the upper level. At the upper level, single, old-growth, strongly intermittent trees are most often found. Down the slope, the number of trees at the middle level increases 15 times, and 40 times at the lower level. At the same time, the average age and diameter of trees decrease, while the height increases. In the North Kraka, the average values of the diameter, height and age, on the contrary, decrease from the lower to the upper level. Here the number and the average age of trees are significantly lower than in the South Kraka, especially at the upper levels (See Table 2). On the basis of the set of model trees of Scots pine and Sukachev larch, we studied the dependences of the aboveground phytomass and fractional structure (trunk, crown, needles) between different parameters of trees. We developed regression equations for the total aboveground phytomass of trees, as well as of the trunk, crown and needles from the diameter at the base and the height of the trunk (See Fig. 2 and 3; Table 4). On the basis of the equations, the reserves of aboveground phytomass were calculated for all altitudinal levels of the studied transects, and separately for the South and North Kraka mountains. The revealed features of the accumulation of phytomass of various fractions of trees and their ratio depend on the altitudinal position in the ecotone, the age and tree species. We found out that the total aboveground...