ABOUT MIGRATION OF PHYTOCENOSES IN THE HYPOARCTIC OF EASTERN SIBERIA AND ITS POSSIBLE CAUSE

ABOUT MIGRATION OF PHYTOCENOSES IN THE HYPOARCTIC OF EASTERN SIBERIA AND ITS POSSIBLE CAUSE

Mikhail Sysolin

student, Faculty of Geography, Moscow State University,

Russia, Moscow

ABSTRACT

An analysis of the dynamics of changes in the areas of three phytocenoses (woodlands, tundra and mountain tundra) in two areas of the Hypoarctic of Eastern Siberia over the past 30–40 years was carried out according to the study of the Google* Earth Pro archive. A consistent increase in the area of larch woodlands due to the unification of the tundra area has been established. The area of the mountain tundra has practically not changed over this time interval. Comparison of research results with observations of summer temperatures over the past 75 years (according to data from 12 Weather Stations in the Arctic and Hypoarctic zones of Russia)

Keywords: Hypoarctic, Eastern Siberia, phytocenoses, images, area, change, climate warming.

Introduction and research methods

Over the past one and a half to two decades, quite numerous works have appeared on the analysis of the dynamics of changes in the border between forest and tundra (forest tundra) in the northern hemisphere.

In North America and Russia in the Hypoarctic zone, the northern border of the forest has been moving northward at a speed of a few meters to tens of meters per year over the past 30-40 years [1, 2, 3, 12, 13].

The reasons for such a shift in the forest/tundra boundary (“greening”) are not precisely defined, but most researchers associate it with general (global?) warming [6, 7, 8, 11].

We set a goal to study the dynamics of changes in the areas of phytocenoses in the Hypoarctic of Eastern Siberia on a large and medium scale (in areas of a few thousand to tens of thousands of square kilometers) in the area of transition from forest to tundra/mountain tundra and compare these dynamics with climate change. In the basin of the river Olenek and its left tributary river Bur were selected two sites (Fig. 1) for the following reasons:

Figure 1. Large (white rectangle) and Small (red square) Olenyok Sites on Google* Earth Pro image

- due to the combination within the sites three types of vegetation (phytocenoses), clearly distinguished on Google* Earth Pro satellite images according to the photo image pattern and phototone (Fig. 2). When compared with small-scale Vegetation Maps [5, 10], these three types of vegetation can be identified as:

• shrub-moss with dwarf birch and dwarf birch moss-lichen tundra,

• plain larch (with spruce) woodlands,

• mountain tundra (with areas of rocky outcrops without vegetation).

Figure 2. Large (right) and Small (left) Olenyok Sites

- due to the change of areas with different types of vegetation (when using the "Historical images" GEP option) for the period from year1984, which is noticeable already during the first review of images from the Google* Earth Pro archive (hereinafter referred to as GEP) to year 2023;

- as an example of changes in vegetation areas on a different scale - within two sites, with an area of 1600 and 60,000 sq. km;

- as an example of a territory very slightly affected by human activity.

Interpretation (contouring of areas with the indicated three types of vegetation) of GEP images was initially performed for 1990, 2000, 2010 and 2020 years. Then, in order to clarify the rate of changes in the areas of tundra and woodlands, an image 2015 was additionally deciphered for the Small Olenyok site, and an image 2023 for the Large Olenyok site (Figs. 3, 4).

Figure 3. Migration of phytocenoses in the Large Olenyok site on images of the GEP archive: 1- tundra, 2- woodlands, 3- mountain tundra

Figure 4. Migration of phytocenoses at the Small Olenyok site on images of the GEP archive (see symbols in Figure 3)

Further, using the grids (2 x 2 km - for the Small Olenyok site and 15 x 15 km - for the Large Olenyok site), the areas occupied by various phytocenoses were calculated (as a percentage of the area of the entire site). The results are tabulated and shown in Figure 5.

Table 1.

Change in the area of phytocenoses of two sites for the period years 1990 - 2023, percent

Figure 5. Change in the area of woodlands: A - Large Olenyok site, B - Small Olenyok site

Migration of phytocenoses

Thus, it can be stated that:

• over a period of more than thirty years (1990 - 2023), a significant change in the areas of tundra and woodlands is recorded;

• the area of the mountain tundra has not practically changed over 35 years (1985-2020), its boundary (with flat tundra or woodlands) has not shifted vertically and is close to the horizontal 100m +/- 20m, i.е. in fact, it runs along the very foot of the low mountains (the Kyshtyk plateau and the Chekanovsky ridge).

• change in the ratio of the areas of tundra and woodlands - unidirectional: the area of woodlands grows in time due to a synchronous decrease in the area of the tundra.

• the noted conjugate change in the areas of tundra and woodlands can hardly be called “border shift” of these types of vegetation: there is a kind of “diffusion growth” of small isolated areas (“koloks”) of larch woodlands and their merging with each other almost over the entire area of the originally occupied tundra (Fig. 6). It can be noted only a noticeable migration (growth of a strip) of woodlands along the valley of the river Bur downstream.

Figure 6. "Greening" of the tundra, Small Olenyok site (on the left - photographs taken in year 1990, on the right - in year 2000)

• in some places, in a very limited area, one can notice the opposite process - the "reverse" replacement of woodlands with tundra (Fig. 7).

Figure 7. “Reverse” replacement of woodlands with tundra, Small Olenyok site (on the left - photographs taken in year 1990, on the right - in year 2000)

This process - the replacement of the tundra by woodlands - proceeds very unevenly, primarily in time. As can be seen from the trends (Table and Fig. 5), in the period from 1990 to 2000, there was a moderate increase in the area of woodlands in the Small Olenyok site - from 8% to 19%, and over the next decade and a half (2000 - 2015) a very slight increase was noted, from 19% to 25% - an increase in the area of woodlands. But the last five-year period (2015 - 2020) is characterized by a sharp increase (from 25% to 67%) in the area of woodlands. The process of replacing the tundra with woodlands is also uneven in the Large Olenyok site, but here the picture is different compared to the Small Olenyok area: the maximum growth in the area of woodlands - from 20% to 49% - falls on the first decade (1990 - 2000), and then the area of woodlands almost does not change. Such a fundamental difference in the trends in the growth of woodlands area in the two sites indicates the uneven nature of this process in space, along the territory of the Large Olenek site.

It should be noted that the interpretation of images of the GEP archive assumed the identification of only the three largest phytocenoses corresponding to the legend of the Vegetation Maps [5, 10]: a) tundra, b) larch woodlands, and c) mountain tundra. But in some places on the GEP images, one can notice the differentiation of the territories of these phytocenoses, i.e., fragmentary outlined more fractional structure. Such a differentiation of the areas of three first-order phytocenoses is somewhat more clearly visible on Landsat 7 images, although even on them it is possible to trace the boundaries of the areas of higher-order phytocenoses only presumably and far from everywhere. It is impossible to confidently identify such phytocenoses of a higher order due to the lack of detailed maps of the vegetation of this region. But for the preliminary identification of these higher-order phytocenoses one can use the legend of topographic maps at a scale of 1:200,000 for this territory (Figs. 8 and 9).

Figure 8. Block montages: Landsat-7 space images, flight 2000 [4] (left) and topographic maps 1:200 000 scale [14] (right) for the Large Olenyok site

Figure 9. Detailing of the vegetation map of the Large Olenyok site according to the interpretation of Landsat-7 images, flight 2000 and topographic maps of scale 1:200,000. The dotted line is the boundaries of high-order phytocenoses from Landsat-7 images.

Possible cause for the migration of phytocenoses

A separate issue is a possible reason for the shift of phytocenoses in the areas of Large and Small Olenyok sites. Most often in the literature one can find an indication of the general warming as a probable reason for the shift of the northern border of the forest and woodlands to the north - the so-called. "greening" [1, 2, 3, 6, 7, 8, 12, 13].

We carried out a test comparison of the intensity of the process of replacement of the tundra by woodlands with changes in air temperature. Data from 12 Meteorological Stations of the Russian Subarctic [9], from the Kola Peninsula in the west to Chukotka in the east, were analyzed for the period from 1945 (1947) to 2022 (Fig. 10).

Figure 10. Meteorological Stations of the Russian Subarctic and Large (white rectangle) and Small (red square) Olenyok sites

The most apparently important parameter for vegetation was calculated - the sum of average monthly temperatures for three summer months (Fig. 11).

Figure 11. Sum of average monthly summer temperatures (∑T^oC) over the past 75 years according to Weather Stations in the Russian Subarctic (graphs and trend  lines)

Trends in this parameter (Fig. 12) for all Weather Stations, with one exception (Pevek weather station), indicate a noticeable increase in temperature: usually smooth - from the beginning / middle of the 1990s and accelerated - from the beginning / middle of the 2010s.

Figure 12. Trends in: a) the sum of average monthly summer temperatures according to Weather Stations and b) the area of woodlands in the sites of Large (A) and Small (B) Olenyok

Since the trends in the area of woodlands for both sites also show an increase (albeit with different dynamics) of this parameter over the past 30–35 years, one can presumably consider the general increase in air temperature in the Russian Hypoarctic since the early/mid 1990s as the reason for the growth of woodlands due to the tundra (i.e., "greening").

But the heterogeneity of the "greening" process in space and time, which is very clearly manifested here, may indicate that the increase in air temperature is not the only reason for the "replacement" of the tundra by woodlands. Additionally, “greening” may also depend on the depth of permafrost thawing in summer, and on humidity, and even on the strength of the prevailing winds that disperse tree seeds [6, 7, 8].

Conclusion

The dynamics of changes in the area of the three main phytocenoses in the area of the border between woodlands and tundra for two sites in Eastern Siberia over the past 30–35 years indicates a steady “greening” - the replacement of the tundra by woodlands while maintaining the same area of mountain tundra, i.e. about the advancement of the northern border of woodlands to the north. This process of "greening" is characterized by a pronounced heterogeneity in space and time.

Such a "greening" of the tundra in the first approximation correlates with the general increase in summer temperatures over the past 75 years according to data from 12 Weather Stations in the Russian Hypoarctic. The heterogeneity of the “greening” process may indicate the role of additional factors, such as the depth of permafrost thawing, seasonal humidity, etc., which, in addition to general warming, affect the intensity of replacement of the tundra by woodlands.

*(At the request of Roskomnadzor, we inform you that a foreign person who owns Google information resources is a violator of the legislation of the Russian Federation - ed. note)

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