The Transformation of Morphological Properties of Soils Due to the Low Arctic Tundra Shrubification
Climate warming in the Subarctic leads to the expansion of shrub ecosystems. The most common upland tundra shrubification is by alder in combination with dwarf birch and willows. However, the nature and rate of changes in the morphological properties of soils in the low arctic tundra during shrubification remain unknown. To study the impact of new shrub ecosystems on tundra soils, we studied alder shrubs in the south of the Western Siberian tundra. The key site is located between the rivers Taz and Pur. The village of Tazovsky, Yamalo-Nenets Autonomous Okrug (Russia) is located nearby. This area is called the Taz tundra, near its transition to the forest tundra. The coordinates of the center of alder shrubs are N67°22'17.4'', E78°42'11.7'' (Fig. 1). The formation of alder shrubs at the study site began no later than 1957. We studied 34 soil profiles. Of these, eight soil profiles were studied in the tundra, eight more in the tundra ecotone and alder shrubs. Eighteen soil profiles characterized the periphery or central zone of alder shrubs (Fig. 2, 3). In the field, we studied the vegetation, the depth of permafrost, photographed soil profiles, and took samples of soil horizons and a micromonolith. We quantified soil morphological parameters such as soil horizon boundary depth, soil horizon thickness, thixotropy index, gley patch percentage, root penetration depth, charcoal abundance, and horizon coloration in the CIE-L*a*b* system. A topographic survey was made within the key site. The age of shrubs and the relief form for each soil profile were determined. The obtained values were processed using the methods of basic statistics and the method of principal components. The studied alder shrubs in the Taz Tundra are located in the upper part of the slopes of stream valleys. The expansion of shrubs at the key site has been observed since the beginning of the second half of the 20th century. For the key site with alder, the succession stages of transformation of tundra into a shrub ecosystem are described. At the first stage, only a few young shoots of alder are observed in tundra (Fig. 10). Alder appears as a result of seed germination on devoid of vegetation areas of cryoturbated soils (patterned-ground). Alder actively colonizes the territory adjacent to the original places of germination, which leads to the expansion of the original range and the concentric structure of shrubs. The second stage is represented by tundra and alder ecotone. In this ecotone, the cover and height of alder increase, and so does the habitus of the original shrubs (dwarf birch and wild rosemary). The main transformations of soil properties are associated with an increase in the active layer thickness. The third stage is the peripheral part of the alder shrubs, where the height of the alder is maximum and reaches 4.5 m. The fourth stage is the central zone of the alder shrubs, where there appear reed-sedge meadows with fireweed. Meadows form in places where the alder bushes died out. The main changes at the third and fourth stages are associated with a radical improvement in the hydrothermal conditions of soil formation, which leads to deepening of the permafrost surface to 3-4.5 m, and a talik is formed. Soil fertility increases due to its enrichment with nitrogen by nitrogen-fixing alder. Alder leaves susceptible to decomposition fall on the soil surface, which is important for earthworms and has a priming effect for the decomposition of tundra litter. The depth of the root system increases. An increase in evapotranspiration leads to a decrease in soil moisture and disappearance of their thixotropic properties. The area of gley patches in soils decreases (Fig. 6). The thickness of peat and litter horizons decreases (Fig. 7). Initially, thixotropic horizons are structured according to the ooid type (Fig. 8). Root and animal tunnels, which are stable in seasonal cycles, are formed in the soil. The resulting pores and soil aggregates allow the developme