The dynamic land-cover of the Altai Mountains: Perspectives based on past and current environmental and biodiversity changes

Abstract

We present climate-dependent changes in the high-mountain forest ecotone, old-growth forests, alpine phytocenoses, and deglaciated forelands in the Aktru glacial basin (Altai Republic, Russia). A number of independent sources (variations in upper treeline altitude, dendrochronological data, analysis of lacustrine sediments and botanical and geographical studies linked with the dynamics of glacial-dammed lakes in the Chuya and Kurai intermountain depressions) suggest Holocene temperatures reached about 4 °C higher than today. Unlike the European Alps, glaciers in the continental Altai Mountains disappeared before forming again. Also, the upper altitudinal limit of mountain forests during the Holocene was greater than in the European Alps. The high variability of mountain ecosystems in southern Siberia suggests their potential instability in a currently changing climate. However, periglacial successions associated with the strong continental climate and glacier retreat represent an area of increasing biodiversity and plant cover. The historical and current sensitivity of the continental mountains to climate variations which exceeds that of the European Alps requires greater understanding, environmental protection, and increased social responsibility for the consequences of anthropogenic contributions to climate change: the isolated Altai areas contribute little to climate changes, but are greatly affected by them.

Supplementary Information

The online version of this article contains supplementary material available (10.1007/s13280-021-01605-y).

Introduction

Mountain systems are an important focal area for research as their landscapes and biodiversity are particularly vulnerable to environmental and land use changes. They are the “water towers” of many areas, storing water as glacial ice, providing a refuge for a specific biodiversity and often offer different, and more extensive, land uses than areas in the surrounding lowlands (Messerli et al. 2004; Steinbauer et al. 2018).

Currently, mountain environments are changing due to climate warming and changes in land use. Based on the Representative Concentration Pathway 8.5 scenario (IPCC 2014), mountain glaciers are expected to lose 37–57% of their mass by 2100 (Hock et al. 2019). This accelerating ice shrinkage will lead to terrestrial ecosystem shifts conditioned by ecological succession, soil development (Eichel et al. 2016), and other kinds of paraglacial adjustment (Hedding et al. 2020). Such ecosystem changes will lead to, or interact with ecosystem changes caused by changing land use, for example, reduced grazing by cattle in the European Alps (Mandych et al. 2012). As such changing ecosystem dynamics and loss of biodiversity are among the most pressing scientific problems at the present time, we need to document and understand the changes and design and implement appropriate environmental management to protect mountain environments and their ecosystems. However, to understand current changes in the mountains, we need a long-term perspective because mountains and their environments are dynamic: glaciers advance and retreat according to changes in temperature and precipitation; ecosystems change following climatic changes and they respond to—or stimulate—changes in land use; biodiversity changes according to climate, conservation, and exploitation; mountain surfaces change according to glacier movement and outburst floods; slope processes, earthquakes, and volcanism.

Many of these processes are researched in the world’s more accessible and populated mountains (https://www.mountainresearchinitiative.org/). However, in Russia, and particularly in southern Siberia, there is relatively poor knowledge of the current dynamics of mountain ecosystems despite an anomalous increase in temperatures in the continental regions of northern Asia (Volkova et al. 2021). These mountains, the Altai-Sayan system, are isolated and surrounded by the arid regions of the steppes and semi-deserts. Consequently, they host glaciers and a specific biodiversity and indigenous land use totally anomalous for the region. Furthermore, direct anthropogenic influences on the mountains and their ecosystems, such as land use, are much less than in many other mountain areas such as the European Alps allowing a clearer signal of climate change impacts to be determined. As these mountains are isolated and generally inaccessible, we aim to provide a holistic picture of the changes taking place there. However, our objectives are not to review a vast literature of global mountains and their environmental and biodiversity changes, but rather to increase global awareness of a remote and vulnerable mountain environment and its exotic and often endemic species. We discuss the general context of the area in time and space, but focus specifically on a particularly well-known valley, the Aktru Valley, where the first observations of glacier fronts were made in 1911 by professor of Tomsk Imperial University V.V. Sapozhnikov (Sapozhnikov 1949) and have been pursued since 1936 by professor M.V. Tronov (Okishev and Narozhniy 2001). We also wish to draw attention to areas of increasing biodiversity that are somewhat neglected among the vast global literature on biodiversity loss (Spehn et al. 2011). This paper consequently contributes to a range of studies dedicated to Siberian Environmental Change (Callaghan et al., 2021).

General characteristics of the Russian Altai Mountains

Geographical features

The Altai Mountains as a part of the Altai-Sayan highland, belong to the periphery of the magnificent mountain ranges of continental Asia. Development of the Altai Mountains began in the late Cretaceous—Early Paleogene as a result of neotectonic activation and transformation of the surface of the peneplain formed in the era of platform development of the area. In general, the mountain glacier basin is complicated by monotonous and highly dislocated sericite-chlorite shales with an admixture of quartzite and other rocks of the Devonian age. Metamorphic shales are easily weathered and, when destroyed, contribute to the accumulation of debris material at the foot of the slopes of the valleys, kars and tongues of glaciers The Altai Mountains contain the headwaters of the Katun and Biya rivers (Figs. 1, 2). The confluence of these rivers forms the biggest river in Western Siberia—the 3650 km long River Ob, which is of great importance for nature and humans downstream, all the way to the Arctic Ocean.

Fig. 1.

The dynamic land-cover of the Altai Mountains: perspectives based on past and current environmental and biodiversity changes. Schematic map of the Aktru catchment basin within the southwestern Altai Mountains

Fig. 2.

Cross-sectional relief of the Aktru Valley: A-lower part of the valley (A to B on Fig. 1); B-upper part of the valley (C to D on Fig. 1). 1. Example alpine plants on stones; 2. Alpine meadows of low plants near melting snowfields; 3. Bush-dominated communities (Betula rotundifolia); 4. Mountain meadow; 5. Sparse mountain forest; 6. Aktru river bed; 7. Mountain forest; 8. Talus; 9. Mountain tundra dominated by Dryas oxyodonta; 10.Sparse vegetation on stony substrates of the valley floor

The landscapes of the Altai Mountains resemble those of the European Alps but with sharper contrasts due to a transient humid and continental climate. The Altai Mountains are recognized by WWF as an area with high biological diversity (WWF 2012). The western macro slope of the Altai Mountains, which receives a significant part of the moisture brought by the Atlantic cyclone, is much more similar to the European Alps than the other, more arid part of this mountain system. Lush alpine meadows and mountain forests develop here. However, at a distance of less than 200 km to the east in the Chuya Valley, forests are replaced by dry steppes and deserts, which separate the area from the alpine belt of thorny cushion plants common at 2200–2400 m above sea level (https://www.vtourisme.com/altaj/priroda/zapovedniki/1219-geopark-altaj).

The vegetation of the alpine belt here is represented by mountain steppes, tundras, communities dominated by plants from the genus Kobresia, and cryophytic cushion plants. This most southeastern part of the Russian Altai bears more resemblance to the mountains of the Mongolian Altai located to the south. In fact, the southeastern part of the Russian Altai is a part of the vast region of inland arid mountainous areas of Asia. Here is the northern part of the range of typical species of the Central Asian flora and fauna with many endemic species (Pyak et al. 2008). The traditional type of economy of the Indigenous Peoples of the Altai Republic is associated with distant pasture farming of sheep, horses and cattle. However, today, the Altai Republic is experiencing a boom in tourism, mainly national rather than international.

Glaciers

The very existence of glaciers deep in Siberia was considered impossible for a long time: low temperatures alone do not contribute to glaciation because cold air is dry (Kalesnik 1947). However, the Altai and Sayan mountains receive significant precipitation (more than 1000 mm annually) due to a cyclone regime and high elevations (over 3000–4000 m a.s.l.). Furthermore, the local redistribution of precipitation associated with the relief facilitates glacier development. However, the relatively small size of the individual Altai glaciers contributes to their instability in the face of a progressive increase in global average temperatures (Narozhniy and Zemtsov 2011).

The glaciers of the southeastern part of the Russian Altai are characterized by high temporal dynamics (Narozhniy and Zemtsov 2011). They completely disappeared and the forest regenerated about 7000 years ago where there are now glaciers (Agatova et al. 2012a, b). For comparison, mountain glaciers in central Europe did not disappear, although 10,500 years ago their size was smaller than at the end of the twentieth century (Solomina et al. 2015). During total deglaciation in the Altai Mountains, restructuring of the mountain ecosystems took place and paleoreconstructions of such past natural environments make it possible to predict future changes in ecosystems under conditions of modern climatic change.

Climate

Long-term observations at high-mountain meteorological stations in the Altai Mountains show that there was a sharp increase in the average summer (June to August) air temperature by 1 °C from 1997—to present (Galakhov et al. 2019). At the same time, over the period 1986–2014, the summer air temperature increase in the highlands of Central Altai Mountains was higher than in the neighboring mountains (Paromov et al. 2018; Zhang et al. 2018). This observation confirms that climate change in the alpine belt, as in the Arctic region (Second Roshydromet Assessment Report 2014), is proceeding faster than in other regions of the Earth. In addition, according to observations at nine stations in Russia and eight stations in China, over the past 50 years, the degree of climate aridity is gradually increasing in the northern Altai Republic, while in the south (in China) the climate is becoming more humid (Zhang et al. 2018).

Ground instability

The Aktru Valley is located in a seismically active belt. The last major earthquake, with a magnitude of up to 7.5 and intensity at the epicenter of 9 points, was on September 27th 2003. The earthquake left a linear system of seismic ruptures more than 70 km long on the northern slopes of the North-Chuiskiy and South-Chuiskiy ranges. Seismic processes are a trigger of such slope processes as rockfalls, landslides, avalanches, mudflows, and slower soil creep. They all leave noticeable traces in the forest belt. Floods and mudflows might be formed during overflow and breakthrough of cirque and moraine lakes, as well as breakthrough of intraglacial ice dams. The results of dendrochronological analysis allowed Agatova et al. (2014) to state that a recurrence period of strong earthquakes in the southeastern Altai Mountains over the past 3000 years was ~ 400 years.

Characteristics of the Aktru Valley

We focus on the Aktru river valley (Fig. 1) in the eastern part of the North-Chuiskiy range. This valley is a left tributary of the Chuya river which, in turn, flows into the Katun river. In the Aktru Valley, constant climate and glacial-hydrological research activities have been pursued since the International Geophysical year in 1956–57 and a research station has been operated by Tomsk State University since 1978 (Okishev and Narozhniy 2001). Within the past 9 years, the Aktru research station has been part of INTERACT—the International Network for Terrestrial Research and Monitoring in the Arctic—and it is described on INTERACT’s web site (https://eu-interact.org/field-sites/aktru-research-station/).

Climate

The local distribution of precipitation in the Aktru catchment basin plays a huge role in the formation of glaciers. However, the average annual precipitation of 590 mm annually for the Aktru station does not show the actual rate of precipitation in the glacier accumulation zone, estimated at 1000 mm in the Aktru basin and up to 1300 mm for the glacial area (Tronov 1972). According to meteorological observations at different altitudes, the total water input to the Aktru catchment basin was 968 mm in 1968–1980 (Galakhov et al. 1987). This explains the fact that the extreme continental climate in the Altai Mountains is relatively favorable for the existence of a large current glaciation that feeds high-flow rivers (Tronov 1972).

In the Altai-Sayan region, including the Aktru catchment basin, there are many proxies that allow the paleoreconstruction of the climate to cover a large time range within the past 2367 yrs. These proxies include dendrochronology (Timoshok et al. 2009, 2016) and analysis of the remains of dead trees preserved in moraines and on the upper forest line (Agatova et al. 2012b; Nazarov et al. 2016).

Glaciers and moraines

Precipitation and mountain relief features, in particular the system of kars or cirques, provide conditions for the stable existence of multiple small glaciers in the Altai Mountains (Tronov 1954) and the Aktru catchment basin is representative for studying glaciation processes in Central Asia (Tronov 1968). According to our field monitoring data on the Altai benchmark glacier “Leviy Aktru” (5.53 km² in area), in 2020 the glacier lost more than 107 cm in height. The year 2019 was also anomalously warm, but in 2020 the rate of glacier mass loss increased by more than two times (World Glacier Monitoring Service (WGMS) database: https://wgms.ch/data_exploration/).

The Maliy Aktru glacier (Figs. 1, 3-1, 3-2) front was located at a height of 2310 m a.s.l. in 2019 (Table 1). The forelands of the Maliy Aktru comprise vast talus slopes, a moraine complex, and a glacial-fluvial and proluvial complex. The moraine complex consists of two lateral moraines, two terminal moraines, several fragments of a recessional moraine, and a ground moraine. The glacier is rapidly retreating (Galakhov et al. 2015).

Fig. 3.

Glaciers of the Aktru Valley and moraine complexes on which successions develop: 1—Aktru landscape view from the book of V.V. Sapoznikov 1949 “Along the Russian and the Mongolian Altai”; 2—photograph taken in 2019 that shows how far the front of the Maliy Aktru glacier has receded during this time; 3—pioneer plants on the moraine of the Maliy Aktru glacier; 4—The upper part of the Aktru Valley (the Leviy Aktru glacier is in the center) and the bottom moraine, on which the succession develops. In the lower right part of the photograph, a strip of forest is visible, which occupies the highest position in the bottom of the valley; a part of the left lateral moraine of the Maliy Aktru glacier can be also seen; 5—Ice-cap glacier Vodopadniy and Vodopadniy plateau (under the glacier (on the right), bordered by a strip of persistent snow on the slopes); 6—Vegetation at the first stage of vegetation succession near the Vodopadniy glacier front.

Photo 2 by A. Erofeev; photo 3-6 by I. Volkov

Table 1.

Morphological characteristics of glaciers in the Aktru catchment basin, on the moraines of which periglacial successions were studied (mapping was carried out using Pléiades space images* (dated 08.26.2019) and 2019 aerial photography from UAVs). The “open part” of the glacier is that area not covered with debris)

Glacier	Morphological type	Aspect	Area, km2		Altitude, m a.s.l (± 1 m)
			Total	Open part	Minimum	Maximum
Maliy Aktru	Valley	N	2.82	2.80	2310	3405
Leviy Aktru	Valley	SE	5.35	5.34	2613	4044
Vodopadniy	Ice-cap	N	0.8	0.8	3034	3550

*The Pléiades stereo-pair and DEM were provided by the Pléiades Glacier Observatory initiative of the French Space Agency (CNES) as part of the European Space Agency program “Observing glaciers from space”

The periglacial complex of the formerly single-branched Bolshoy Aktru glacier (Fig. 3-4) includes a terminal moraine with three arched ramparts, as well as lateral, central and ground moraines. The absence of lateral moraines at the junction of the Leviy and Praviy Aktru glaciers can be explained by their rapid retreat. The processes of disintegration of the Bolshoi Aktru glacier began in the early 1960s (Okishev 1982). Before that, there was a single ablation field, with an area of about 1 km² at 2300–2500 m, which gradually degraded by 0.54 km² from the mid-nineteenth century until 1936—the time of the first instrumental survey of the glacier conducted by M.V. Tronov (Narozhniy 2001). The past glacier front positions in the Aktru catchment basin are morphologically well expressed in the relief and are confirmed by numerous radiocarbon, dendrochronological and lichenometric datings (Agatova et al. 2012b).

The Vodopadniy glacier (Fig. 3-5, 3-6) is a small ice cap glacier with an area of 0.8 km². Its relief is composed of several recessional moraines, ridges, bottom moraines and talus slopes. A moraine complex opening in a fan-like manner to the north in the form of five recessional moraine ridges mark the historical position of the glacier front.

Land use

Anthropogenic pressure in the Aktru Valley is mainly due to recreation activities and is most noticeable along popular tourist routes. Timber logging was carried out on a small scale on the left slope of the valley at the end of the twentieth century. Livestock grazing is insignificant and fires cause much more harm to forest ecosystems.

Overview of ecosystems of the Aktru Valley

The specific nature of the ecosystems of the Aktru Valley is associated with a wider distribution of poorly populated moraines of various ages than in the neighboring valleys of the northern slope of the North Chuya Ridge. The most significant factor in the spatial differentiation of the valley ecosystems is the altitude zoning. The lower part of the Aktru Valley, approximately up to 2200–2400 m above sea level, is located within the forest belt. However, the lowest part of the bottom of the glacial trough valley, covered with moraine and alluvial deposits of the meandering Aktru river, is mainly occupied by rather sparse shrubby thickets and unclosed herbaceous vegetation (Figs. 4-4 3-3, respectively). Thus, there are two treelines: an upper treeline constrained by climate and a lower one constrained by geomorphological processes and features but still within the potential forest belt defined by climate.

Fig. 4.

Plant communities of the Aktru Valley. 1—Myricaria bracteata—typical plant of the rocky banks of the Aktru river, 2—forest belt with the dominance of siberian larch (Larix sibirica) in the lower part of the Aktru Valley, 3—dryad tundra (Dryas oxyodonta), 4—vegetation communities dominated by birch shrubs (Betula rotundifolia) and lichens, 5—carpets of alpine plants under the snowfield in the Aktru Valley, and 6—communities of sad sedge (Carex tristis).

Photo by I. Volkov

Forest belt

In the upper part of the forest belt of the Aktru Valley, Siberian larch (Fig. 4-2) and Siberian larch-Siberian pine (Pinus sibirica) forests with well-developed shrub and herbaceous canopy levels are common. The Siberian pine forests are the oldest in the region (Altai Republic) reaching 500 years old (Timoshok et al. 2001). The diversity of vascular plants in the forest belt of the Aktru catchment basin reaches 166 species, with an average density of 36 species per 100 m² (Timoshok et al. 2010). In the area surrounding the Aktru station and along the nearby tourist trails, 20 species of synanthropic plants were found, which are associated with a high recreational load (Timoshok et al. 2001) (Fig. 4).

The forest belt of the Altai Mountains is characterized by a typical mountain-taiga complex of small mammals, including 31 species such as chipmunk and squirrel. There are also 6 species of reptiles, but there are practically no amphibians (Voznichuk, 2014). Of the terrestrial invertebrates, ground beetles dominate, among which the proportion of endemic species is quite high (Dudko 1998). In the forest belt, Nucifraga caryocatactes associated with Pinus sibirica is common. For moraines and rocky slopes, a common component of the fauna is the small rodent Ochotona alpina, which is recognized by its ability to emit a sharp whistle. The Aktru Valley and its surroundings are the habitat of several herds of Siberian mountain goats (Capra sibirica), which are not too afraid of people.(Fig. 5). Probably, a large recreational load at Aktru scares off the most dangerous predators for goats—the endangered snow leopard (Uncia uncia) and wolves Canis lupus, although it will not save the goats from attack by large birds such as the golden eagle (Aquila chrysaetos).

Fig. 5.

Group of Siberian mountain goat males climbing the Uchitel Pass (Aktru Valley).

Photo by I. Volkov

Upper treeline

The Aktru catchment basin forest-tundra ecotones occur at 2200–2500 m a.s.l. (Timoshok et al. 2016). At the upper (and lower) limits of the ecological distribution of trees in the forest ecotone of Aktru, Siberian pine (Pinus sibirica) usually prevails over Siberian larch (Larix sibirica) (Fig. 6). In the more arid regions of the southeastern Altai Republic, where the forest is found on the slopes of northern exposure, Siberian larch predominates and in the most humid regions of the western part of the Altai Republic, Siberian pine, Siberian spruce (Picea obovata) and Siberian fir (Abies sibirica) form the upper boundary of the forest. Currently at Aktru, the climatic border of the forest with Siberian pine and Siberian larch is confined to 2335–2350 m a.s.l.; the stunted growth form of Siberian pine rises to 2475 m a.s.l. and Siberian larch up to 2420 m a.s.l. (Timoshok et al. 2016). Seedlings of Siberian pine are found in Dryas turf in sheltered habitats at an altitude of 2800 m a.s.l., i.e., at the species upper limit (Fig. 7).

Fig. 6.

Siberian pine (Pinus sibirica) and fir (Larix sibirica) trees above the upper border of the forest on the left slope of the Aktru Valley.

Photo by I. Volkov

Fig. 7.

Young Siberian pine (Pinus sibirica) growing in the dryad's turf (Dryas oxyodonta) on a ridge in the left side of Aktru Valley.

Photo by I. Volkov

Alpine belt

The alpine zone is understood in a broad sense: its lower boundary in the Aktru Valley is the upper border of the forest, and the upper boundary is the snow line and glaciers (nival-glacial belt). The altitude of the Aktru alpine zone is approximately 2300—3300 m, except for those places where the tongues of glaciers descend. Therefore, not all of the upper part of the Aktru Valley is included in the alpine zone. The vegetation of the Alpine belt is very diverse. A significant part of the upper part of the Aktru Valley is occupied by complexes of plant communities that superficially resemble the sparse vegetation of the lower part of the valley. Dryad tundra (Fig. 4-3) on moraine complexes is mixed with willows and rarely occurring low trees. On the protruding sides of the valley, there are communities dominated by dwarf birch (Fig. 4-4), which alternate with Dryad tundra and colorful carpets of small alpine plants. The most widespread alpine plant communities are distributed near the Vodopadniy glacier where they are fed by glacial melt water and late-melting snowfields (Fig. 4-5). Alpine meadows dominated by sedges and grasses, which bloom in spring with white Callianthemum sajanense flowers, grow on the northeast side of the valley. At the base of the Vodopadniy plateau, there are dense turf communities of Carex tristis (Fig. 4-6). The upper vegetation belt is formed by cryophyte plants. In terms of endemism, the Altai alpine flora surpasses most of the mountain systems of Central Asia: 6.16% of alpine plants are local endemics (Revushkin 1988). In the Aktru alpine zone, the alpine jackdaw (Pyrrhocorax graculus) and partridge (Perdix perdix) are found.; on the moraine deposits of the valleys you can find Oenanthe isabellina nesting in open spaces and in abandoned rodent burrows.

Dynamics of alpine and forest vegetation

Treeline

The Aktru catchment basin forest-tundra ecotones currently occur at 2200–2500 m a.s.l. and show the biggest response to global and regional climatic changes (Timoshok et al. 2016). Dendrochronological and radiocarbon analyses have made it possible to reconstruct the thermal regime of the Aktru catchment basin (as in other regions of the southeastern Altai Mountains: Nazarov et al. 2016). As Siberian larch wood growth is dependent on the temperatures in June and July as measured at the Aktru hydrometeorological station (where the average temperature in June and July at the modern treeline (2365 m a.s.l.) is 8.1 °C), it has been possible to estimate the thermal potential of the treeline as expressed in the vertical migration of the June–July isotherm over the last 10 380 years (Table 2).

Table 2.

Changes in positions of the upper treeline and directions of changes: in brackets—at the southeastern part of Russian Altai Mountains, whereas other figures specifically related to the Aktru Valley. Data from: Nazarov et al. 2012, 2016; Agatova et al. 2012a, b; Solomina 2014. (“<" denotes less than, “>” denotes more than, “$\wedge$” denotes increase and “$\vee$” denotes decrease in height)

Time period	Location and changes of the treeline limit positions
10 380 ± 200 BP	(< 2700 m a.s.l.)
7200 BP	(> 2500 m a.s.l.)
6500 BP	(> 2500 m a.s.l.)
5900–3200 BP	$\vee$ 2360 m a.s.l
3100–2600 BP	< 2260 m a.s.l
2600–1200 BP	$\wedge$ 2250–2350 m a.s.l
800–1050 AD	2360 m a.s.l
1100–1400 AD	Average 2340 reaching 2440 m a.s.l
1400–1650 AD	$\vee$ 2370–2350 m a.s.l
1650–1800 AD	$\vee$ 2340–2290 m a.s.l
1800–1850 AD	$\vee$ 2290 m a.s.l
1850–current	$\wedge$ 2260–2365 m a.s.l

The modern invasion of Siberian pine and Siberian larch into the forest-tundra ecotones began in the 1860s due to a warmer climate. At present, the most common tree species in the ecotone is Siberian pine (60–80% of all trees), while the presence of Siberian larch is much lower (20%). Here, the age structures of tree populations, where there is only one adult generation of Siberian pine and Siberian larch with trees 50–130 years old, are completely different from the age structures in the old forest (Timoshok et al., 2016).

Trees of the second generation of Siberian pine are young (4–50 years) and numerous. The second generation of Siberian larch is represented by individuals 36–49 years old. The invasion of Siberian pine into the lower and upper parts of the ecotone occurred simultaneously and took place in 1860–1960. The invasion of Siberian larch into the lower part of the ecotone took place in 1870–1960, and into the upper part 20 years later (from 1900 onwards). The rapid invasion of Siberian pine began in the 1920s and continues to this day. The invasion of Siberian larch was insignificant in the twentieth century and ceased in the 1980s (Timoshok et al. 2016).

Since the beginning of increased temperatures in the 1860s, the treeline has risen by 60–210 m. From 1960 to the mid-1970s, it rose by 40–120 m (up to 2390 m a.s.l.) and another 20–90 m during the past 40 years (Siberian pine up to 2480 and Siberian larch up to 2420 m a.s.l.). Monitoring data show that the invasion of Siberian pine into the ecotone continues (Timoshok et al. 2016). A study in approximately the same area at an altitude 2150–2300 m a.s.l. (Cazzolla Gatti et al. 2019) showed that trees recruited before 1954 do not grow above 2150 m and that trees recruited before 1999 do not grow above 2200 m a.s.l. The maximum altitude of 2300 m a.s.l. was reached between 1994 and 2002, while after 2002 the youngest observed trees (recruited around 2006) grow at a lower altitude of 2250 m a.s.l. An increase in air temperature over the past 52 years has contributed to a rapid rise of the treeline on the left slope near the Aktru station by about 150 m (Cazzolla Gatti et al. 2019). This is much higher than the current rate of treeline rise in the subarctic mountains—50 m of rise in 40–50 years (Rees et al. 2020) and in the European Alps, where from 2000 to 2008, the forest border rose by 10 m (Leonelli et al. 2011). It is also somewhat higher than the rate of treeline altitudinal increase by Abies spectabilis in the Himalayan Mountains (Gaire et al. 2014).

Tree growth

General decreases in radial growth occurred at the beginning and the end of the sixteenth century, in the middle of the first half of the seventeenth century, at the turn of the seventeenth and eighteenth centuries, in the middle of the first half of the nineteenth century, and at the beginning and in the middle of the second half of the twentieth century (Bocharov 2011). The slow-down in radial growth in the middle of the first half of the nineteenth century, and at the beginning and in the middle of the second half of the twentieth century correspond to small depressions of the climatic treeline according to Nazarov et al. (2016).

According to Timoshok et al. (2016), the most noticeable and prolonged periods of increase in tree growth took place in 1544–1575, 1704–1776, 1828–1902 and during the last decade of the twentieth century. The main growth periods coincide with the establishment of separate generations of Siberian pine at the treeline determined by an inventory of forest stands, accordingly: 1550–1613 (1st generation), 1654–1732 (2nd generation) and 1835–1910 (3rd generation). The generations of Siberian pine and Siberian larch are separated by 40–50 years due to the different ways these species react to temperature. This is visible in the modern generation of Siberian pine, which consists of trees aged between 1 and 3 years, while the same generation of Siberian larch consists of trees aged between 40 and 50 years (Timoshok et al. 2016).

Holocene climate dynamics

Based on the radiocarbon and dendrochronological analysis of a data set for dozens of wood samples, it was concluded that the climatic conditions at Aktru were warmer and more humid in the first half of the Holocene. Several periods were characterized by a relatively large temperature increase (Rusanov 2004; Nazarov et al. 2012). According to Rusanov (2004), in the Atlantic period of the Holocene, the mean annual temperature in the Northern Altai Mountains increased by 4 °C, with a decrease of 300 mm in the mean annual precipitation. This is equivalent to the current temperature difference for the growing season (from May to September) averaged for the vertical temperature gradient on the southeastern slope from the Aktru station (2150 m a.s.l.) to the Uchitel Pass (3050 m a.s.l.) (Fig. 1) (Sevastyanov 1998). This increase temperature would have allowed the treeline to rise almost to the pass.

There is additional evidence of a sharp temperature jump in the western part of the Chuya and Kurai arid basins, located at a distance of less than 50 km east of the Aktru Valley. During the periods of glacier advance no later than ~ 13,000–15,000 years BP, there was a glacier-dammed lake (Rudoy 2002; Herget et al. 2020), which excluded the existence of modern complexes of xerophytic plants (currently localized at altitudes of 1750–1900 m a.s.l.). After the ice-dammed lake drained in a glacial outburst mega-flood, the modern complexes of xerophytic plants must have in migrated from the Mongolian Altai Mountains (although some individual species possibly arrived by long-distance dispersal). This would have involved overcoming the passes of the Saylyugem Ridge with a height of about 2600 m a.s.l. In order for the temperatures at the passes to approach the mean annual temperatures of modern habitats of xerophytic plants in the Chuya depression, an estimated increase in temperatures of the growing season by 3–3.6 °C would have been necessary. With a theoretical increase in average annual temperatures by about 4 degrees, the temperature of the Saylyugem passes would have approached the current temperature of the bottom of the Chuya depression (Volkov 2003) with its current xerophytic plant complexes.

A comprehensive study of bottom sediments of lakes in the Altai foothills (Rusanov 2004), also indicate a warming of the climate in the Atlantic period of the Holocene about 7,350–5,030 years ago, which coincides with the data of Agatova et al. (2012b) on glacier degradation in the Aktru catchment basin.

These independent studies unambiguously indicate a change in the natural environment in the Southeastern Altai Mountains at the beginning of the Holocene, the scale of which was clearly larger than that in the European Alps. These studies show a much greater dynamism of the environment of the inland mountains, which can lead to faster and more catastrophic changes in their ecosystems compared to the mountain systems on the periphery of the continents.

Periglacial primary successions

Expanding glacial forefields

The Aktru Valley, as a part of a mountain-glacial basin, is unique for the study of periglacial successions because the moraines cover almost the entire alpine belt—from the upper part of the forest belt, where the succession on the Maliy Aktru glacier forefield currently ends (2206 m a.s.l.), up to the altitudes where the succession begins on the Vodopadniy glacier forefield (3050 m a.s.l.) (Fig. 1). This is important because we can observe the specifics of the development of successions at different altitudes. Besides that, detailed dating of deglaciated moraines forms the basis for chronology of the vegetation successions. Such studies are important because the expanding glacial forefields are rare examples in a global context of where biodiversity is increasing.

The areas on which the primary successions occur vary greatly, primarily due to the different morphology of the glacial beds. The tongue of the Maliy Aktru glacier retreated by 1120 m since 1911. Currently, the area covered with successions is 0.36 km². The average velocity of forefields creation is about 0.33 ha per year.

The tongue of the Bolshoy Aktru glacier, after splitting into the Leviy and Praviy Aktru glaciers, in the 1950s retreated by 820 and 660 m, respectively. The area covered with successions is 0.32 km² with an annual velocity of forefields creation and primary successions of 0.45 hectares per year.

Initial studies of succession in the Altai-Sayan Mountains

Climatic changes, which caused a rapid deglaciation process in the twentieth century, led to the massive development of primary successions in the expanding areas freed from glaciers. Revyakina (1996) was the first to systematically study the restoration of vegetation after the retreat of the glaciers of the Altai-Sayan Mountains describing three stages and observing the rate of colonization of moraines by plants. On the Gebler glacier (Katunsky Ridge, Altai Republic), 20 vascular plants were recorded in the deglaciated area from 1972 to 1983 (Revyakina 1996). In the Aktru catchment basin, in 1973 Revyakina (1996) found the first plants (a tree and a shrub) only 10 m from the front of the Maliy Aktru glacier indicating slow glacial retreat and/or rapid succession. Comparative studies of the dynamics of vegetation of alpine and periglacial phytocenoses showed a higher rate of change in vegetation near glaciers (Appendix S1).

Microbial and fungal colonization

In this nitrogen (N) poor environment, microbes are usually the first colonizers and keystone players (Bradley et al. 2016). The first study (Cazzolla Gatti et al. 2018), which provides data on microorganisms on the Maliy Aktru moraine, showed that at a distance of 50 m from the front of the glacier, the number of colony-forming units (CFU) is small, but they are present even at low nutrient levels. Between 50 and 250 m the number of CFU increases slightly, but at a distance of 300–600 m the number of microorganisms is 100–500% higher than at the initial stages. Nevertheless, the species diversity of microorganisms remains almost stable throughout the entire distance of succession. In contrast, the diversity of fungi fluctuates significantly. Ectomycorrhizal fungi partially replace saprotrophs at a distance of more than 350 m from the glacier front. This stage of colonization of the glacial forefield by microorganisms can be considered the zero stage of succession.

Plant successions

The distance between the glacier and the first plants has increased from 10 m in 1973 (Revyakina 1996) to 20 m in 2016 (Cazzolla Gatti et al. 2018) (Fig. 3-3). This, together with the existence of trees and shrubs in the pioneer stage of succession in the recent past but now absent, confirms that the rate of retreat of the glacier is progressively increasing faster than woody plants can colonize the emerging habitats. This is in stark contrast to one of the medieval advances of the Maliy Aktru glacier, when ice buried (thereby preserving) the trees at its front, while the mature forest continued to grow to the East of the glacier tongue and died much later (Nazarov et al. 2016).

Stages of plant succession on Maliy Aktru glacier moraines

Current plant successions in the Aktru catchment basin show three stages (Revyakina 1996) and a number of sub-stages on the bottom moraine of the Maliy Aktru glacier (Timoshok et al. 2020; Table 3, Fig. 8A).

Table 3.

Characteristics of changes in plant and lichen biodiversity during the succession on the Maliy Aktru glacier.

(Modified from Timoshok et al. 2020)

Stage No	Sub-stage No	Distance from the front of the glacier, m	How many years ago the moraine was exposed	Number of vascular plant species	Number of moss species	Number of lichen species	Vegetation cover, %
1	1.1	50–150	5–20	37	5	0	1
	1.2	150–230	20–50	49	5	3	5
2	2.1	230–340	40–60	83	13	5	15–20
	2.2	470–600	60–100	111	29	5	50
3		600–810	Over 100	84	24	6	80

Fig. 8.

Plant successions at Aktru. A Scheme showing the general change in the composition of species and life forms of plants at different stages of the succession on the bottom moraine of the Maliy Aktru glacier: 1—Dracocephalum imberbe, 2—Crepis karelinii, 3—Saxifraga oppositifolia, 4—Trisetum mongolicum, 5—Myricaria bracteata, 6—mosses, 7—Salix saposhnikovii, 8—Mesostemma martjanovii, 9—Pinus sibirica, 10—Larix sibirica, 11—Salix berberifolia, 12—Betula rotundifolia, 13—Hedysarum neglectum. B Scheme showing the general change in the composition of species and of life forms of plants at different stages of succession on the bottom moraine, Leviy Aktru glacier (beginning of succession is on the left): 1—Crepis karelinii, 2—Dracocephalum imberbe, 3—Saxifraga oppositifolia, 4—Trisetum mongolicum, 5—moss, 6—Salix saposhnikovii, 7—Chamaenerion latifolium, 8—Draba cana, 9—Poa glauca, 10—Salix berberifolia, 11—Juniperus sibirica, 12—Dryas oxydonta. C Scheme showing the general change in the composition of species and life forms of plants at different stages of succession on the bottom moraine of the Vodopadniy glacier: 1—Cerastium pusillum, 2—Festuca brachyphylla, 3—Crepis karelinii, 4—Saxifraga cernua, 5—Papaver pseudocanescens, 6—Trifolium eximium, 7—Paracolpodium altaicum, 8—Sagina saginoides, 9—Hierochloё alpina, 10—moss I, II, III—stages of succession

At the first stage of plant succession, individual grasses occur. Initial simple groupings gradually become more complex and grow in size, occupying mainly micro-depressions where fine grained particles accumulate and fragmented soils are formed. In the course of the succession, the dominant role gradually passes from grasses to shrubs, dwarf shrubs and then trees at the final stage; the ground coverage increases and the vertical structure of the plant community becomes more complex, including layers of 1) mosses, 2) herbs and dwarf shrubs, 3) shrubs and 4) trees (Timoshok et al. 2020).

The termination of the Maliy Aktru glacier in the upper part of the current forest belt (the climax vegetation) creates the largest diversity in ecological conditions at different stages of deglaciation. This leads to high biomorphological gradients and increasing complexity in the structure of the phytocenoses, varying from the extremely sparse communities of alpine extremophile plants to communities close to the climax (Volkov et al. 2019).

Stages of plant succession on Leviy Aktru glacier moraines

The primary succession on the moraines of the Leviy Aktru glacier (Fig. 8A) occurs at higher altitudes than the succession on the Maliy Aktru glacier, and includes three stages and four sub-stages (Timoshok et al. 2020; Table 4).

Table 4.

Characteristics of changes in the biodiversity of plants and lichens during the succession on the Leviy Aktru glacier.

(Modified from Timoshok et al. 2020)

Stage No	Sub-stage No	Distance from the front of the glacier, m	How many years ago the moraine was exposed	Number of vascular plant species	Number of moss species	Number of lichen species	Vegetation cover %
1	1.1	50–200	5–20	37	5	0	1
	1.2	250–500	20–50	49	8	5	1–5 (Up to 15)
2	2.1	500–750	70	68	13	5	15
	2.2			72	18	5	15–20
3		750–960	100	90	19	5	Up to 60

In the alpine belt, succession is more ecologically homogeneous than in the forest belt (Fig. 8B). This is because harsh climatic conditions determine the predominance of extremophile plants, which inhabit both the ice-free territories and dominate the phytocenoses at all stages of succession and the dominance of trees at the last stage of succession is absent. Therefore, while pioneer vegetation on the Maliy Aktru and Leviy Aktru glaciers is similar, there are significant differences at the second stage of succession during which differences in species composition increase and shrub growth forms differ from prostrate (< 20 cm height) on the Leviy Aktru moraine to predominantly erect on the Maliy Aktru moraine (Timoshok et al. 2020).

The third stage of succession on the Maliy Aktru moraine has no analogy with the succession at the Leviy Aktru glacier forefield. While individual trees appear at the second stage of succession on the bottom moraine of the Leviy Aktru glacier, this does not lead to the formation of forest communities.

Stages of plant succession on Volopadniy glacier moraines

On the young moraines of the Vodopadniy glacier, three stages of vegetation formation were distinguished (Timoshok et al. 2010): (1) pioneer stage on moraines 2–6 years old (20 species of vascular herbaceous plants and 12 species of mosses) 2–6) grass-moss-herb stage (37 species of vascular plants, 15 species of mosses and two species of lichens) and (3) herb-moss stage (59 species of flowering plants, 24 species of mosses, and 16 species of lichens). However, the differentiation of vegetation on the Vodopadniy plateau is exerted not by the glacier, but by the relief of the plateau (Volkov and Minchinskaya 2011).Therefore, the second and third stages of succession, accordingly, are the vegetation on convex rocky areas and in depressions along streams flowing from the glacier (Fig. 8C).

The roles of mosses and lichens in plant successions

The predominance of vascular plants over mosses at the first pioneer stages of the succession at the Maliy Aktru and Leviy Aktru glaciers does not adhere to the classical pattern of succession, where simpler organisms and communities prepare the ground for the more highly organized organisms and complex communities or coexist at the first stage of succession, such as at Glacier Bay in Alaska (Worley 1973). The successions at Aktru therefore, appear to promote rapid increase of biodiversity. However, Maliy and Leviy Aktru glaciers show significant differences in the composition of the dominant species of mosses and lichens throughout succession. Mosses play an important role in the succession at Maliy Aktru, whereas lichens are important on the Leviy Aktru moraines (Timoshok et al. 2020). A study of the differences in exposure on the lateral moraine of the Leviy Aktru glacier (Volkov 2006) reveals that mosses are very sensitive to strong solar radiation and their development on the southern slopes is hindered. As the bottom moraine of the Maliy Aktru glacier has a northern orientation and is largely shielded from morning and midday solar radiation, mosses can flourish there. In contrast, the bottom moraine of the Leviy Aktru glacier is almost entirely exposed to the midday sun, which suppresses mosses and favors lichens, which are more resistant to desiccation.

Soil development

Substrate weathering and soil-forming processes during the succession on the moraine of the Maliy Aktru glacier are slow (Davydov and Timoshok 2010). During the first stage of vegetation formation, there is a redistribution and accumulation of fine earth and then, during the stage with a predominance of dwarf shrubs and mosses, leaching of carbonates begins with the formation of a thin organogenic horizon on the surface. After 60 years, these processes lead to the formation of an embryonic humus horizon. On 60–150-year-old moraines, an under-developed soil profile is formed. During the formation of the forest community, the processes of leaching and the formation of the soil profile are completed—podburs (Entic Podzolsin) drained habitats and cryozems (Cryosolsin) poorly drained habitats.

The authors conclude that it takes 200–300 years for the formation of soils (as well as vegetation) corresponding to the climax. It is interesting to compare the slow, but continuous process of soil formation at Aktru with the succession at Werenskiold Glacier, SW Spitsbergen (Kabala and Kubicz 2012), during the first stages of which the accumulation of organic carbon and nitrogen proceeds at a high rate, comparable to the rate in the low Arctic and European Alps; however, in the fourth to fifth decades after deglaciation, this process reaches a quasi-stationary state, associated with low annual precipitation, and this is probably the decisive factor controlling the succession of plants and weathering of minerals (the importance of soils as a decisive factor of weathering on moraines of the glaciers of the European Alps is discussed in the work of Egli et al., 2006). The moisture conditions on the moraine of the Maliy Aktru glacier are therefore, quite favorable for the development of succession, and its rate is limited by the temperature regime.

Invertebrate colonization

Studies of primary succession by invertebrates in the Altai glacier forefields are currently in their infancy and their role in the dynamics of subglacial ecosystems is underestimated. Thus, when studying successions on the moraines of the Maliy Aktru glacier, vegetation elements and microbial populations were recorded, but invertebrates were ignored (Cazzolla Gatti et al 2018). However, our preliminary studies conducted near the Maliy Aktru glacier in 2013–2019 showed that the group composition of invertebrates in the terrestrial fauna of the subglacial moraine is largely similar to that in Southern Norway. The territories released from the ice are inhabited firstly by settlers with high migration opportunities: coleoptera insects of the families Carabidae, Staphylinidae, and Collembola. Like most insects, the Carabidae and Staphylinidae are active in the subglacial belt only for a relatively short period of time, from mid-June to early September. The rest of the year they spend in diapause, hiding in the crevices of the substrate, under rocks or in the moss pads closest to the glaciers. However, it remains unclear which species of invertebrates are pioneers, what allows them to successfully occupy the territory, how quickly the number of species changes, and how plants and animals interact during succession (Appendix S2).

Conclusions

While numerous studies of mountain environments and their ecosystem processes exist for populated and accessible mountains such as those of the European Alps and Scandinavia, isolated and largely inaccessible mountain systems such as the Altai-Sayan Mountains of Southern Siberia are less well represented in the literature. However, the mountains of the Southeastern Altai have experienced a greater change in their natural environment in the Holocene than, for example, in the European Alps. This greater dynamism of the environment of the inner continental mountains can lead to faster and more catastrophic changes in their glacial landscapes and ecosystems compared to the mountain systems on the periphery of the continents. The biogeographical isolation of these mountains, and particularly their location surrounded by steppes and semi-deserts, has led to high levels of endemism and suggests great biodiversity and ecosystem sensitivity to the current period of climate warming, if this persists. Over the short-term, however, the existence of large areas of moraine undergoing primary succession together with areas of land currently being released from glacial retreat facilitate an increase in biodiversity and ecosystem development that are largely under-represented in climate change impact studies. We present the changes that have taken place. Based on these studies, future changes in local ecosystems can be predicted.

The historical and current sensitivity of the continental mountains to climate variations which exceeds that of the European Alps requires greater understanding, environmental protection, and increased social responsibility for the consequences of anthropogenic contributions to climate change: the isolated Altai Mountains areas contribute little to climate changes, but are greatly affected by them.

Supplementary Information

Below is the link to the electronic supplementary material.

Biographies

Igor V. Volkov

is PhD in Botany, Associate Professor of Department of General Biology and Methods of Teaching Biology at Tomsk State Pedagogical University. His research interests include plant ecology, adaptations to extreme environments, geobotany, landscape ecology, nature wise use, and conservation.

Valeriy A. Zemtsov

is DSc in Geography, Professor, Head of the Hydrology Department of the Geology and Geography Faculty at Tomsk State University. His research interests include hydrology of lands, geo-ecology, and hydrological implications of climate change.

Alexander A. Erofeev

is PhD in Geography, an Associate professor and Head of the Glacioclimatology Laboratory of the Geology and Geography Faculty at Tomsk State University. His research interests include glacier morphology, glaciers dynamic and monitoring, and modeling.

Adrei S. Babenko

is DSc in Biology, Professor, Head of Department of Agricultural Biology, Institute of Biology, Ecology, Soil Science, Agriculture and Forestry at Tomsk State University. His research interests include biodiversity of insects, soil zoology, and environmental management.

Anastasia I. Volkova

is Junior Researcher of PaleoData Lab at Institute of Archaeology & Ethnography, Russian Academy of Sciences. Her research interests are landscape dynamics, paleobotany, and palynology.

Terry V. Callaghan CMG

is DSc, Professor of Arctic Ecology at Sheffield University, UK, and Professor of Botany at Tomsk State University, Russia. He is a founder of INTERACT—the international network of terrestrial research stations for research and monitoring in the Arctic and its scientific adviser. He is a co-founder and scientific director of the Siberian Environmental Change Network. His main interests include Arctic Ecology, Climate change, Traditional Indigenous Knowledge, Adaptation, and Science Diplomacy.