State of island spruce forests in the western part of the Bolshezemelskaya tundra after 23 years

Modern climate warming, which began in the 1970s, has been observed throughout the Arctic including its Russian part [Доклад…, 2023; Druckenmiller et al., 2021]. It is accompanied by a large number of papers by Russian and foreign scientists on the forest boundary advancement to the north, and its upper boundary in the mountains – up the slopes [Шиятов и др., 2007, Harsch et al., 2009; Bolotov et.al., 2012; Grigor'ev et.al., 2013, 2019; Moiseev et.al., 2019; Shiyatov et al., 2020; Timofeev et.al., 2021; Dial et al., 2022; Hansson, 2022, etc.]. Climate change rate is high in the East European sector of the Arctic: over the last 35 years the average annual air temperature increase has reached +0.8°C/10 years [Malkova et.al., 2021], the length of the growing season has increased by an average of 2 weeks and the amount of heat accumulated during this period has increased by an average of 85°C [Lavrinenko et al., 2022]. The northern forest boundary (timberline) in East European Russia is formed by Picea obovata and runs at N 67°30ʹ-67°10ʹ. In the Bolshezemelskaya Tundra, spruce is found rather far north of the forest boundary and even north of N 68°. Spruce islands have been preserved here since the Holocene in refugia – sites with favorable microclimatic and soil conditions. Relict spruce islands are groups of closely spaced, thin-stemmed trees occupying upland landform elements on sandy outcrops of watersheds. Skirt-shaped growth trees are united by a common root system and appear to be clones formed by vegetative propagation [Lavrinenko, Lavrinenko, 2004]. In the framework of the international SPICE project, eight spruce islands were discovered and studied 8 spruce islands at latitude N 67°54'-67°56' (Fig. 1). Complete relevés were carried out within the boundaries of the 5 islands. Species abundance was estimated using the Brown-Blanquet scale [Becking, 1957]. The height of the tallest trunks was measured with a measuring tape and their diameter at the trunk base (in island E2 at a height of 50 cm) – with a caliper. In 2000, a spruce island was described at the northernmost site (N 68°17') near Cape Bolvansky Nos on the coast of the Pechora Bay of the Barents Sea (Fig. 1). The results of the spruce islands structure and cenoflora study have been published [Lavrinenko, Lavrinenko, 2003]. This data provided an opportunity to trace the changes of the islands 23 years later. All spruce islands in the Ortina Basin were resurveyed between 20 and 30 July 2023. The study included tree morphometric measurements, geobotanical relevés and comparative landscape photography. The surveys on the islet at Cape Bolvansky Nos were carried out in 2000, 2014 and 2020 and included plant community relevés and photography and height measurements of the 6 tallest living spruce tops; photos were taken during a short visit in 2017. Comparative analysis of the spruce islands composition and structure after almost a quarter of a century have shown: 1) In the Ortina River basin, in relict spruce islands on watersheds (E1, E4-E8), mean tree height has increased by 1.1-1.9 m and mean diameter – by 1.9-3.0 cm, i.e. mean height growth was 4.3-8.3 cm/year and radial growth was 0.41-0.65 mm/year. On a spruce island in the Ortina River valley (E2) with more favorable microclimatic conditions, these values were significantly higher – trees have grown on an average 2.8 m, diameter – 3.7 cm, i.e. height growth was 12.2 cm/year, radial growth – 0.8 mm/year (Table 1, Fig. 2а and б). In 2000 spruce island E3 was located on a sandy mound in the center of a sandy outcrop. By 2023 the mound has been almost completely destroyed by winds, the spruce looked like dying off and most likely it will disappear after some time (Fig. 9). 2) The shape of the tree crowns has changed. In 2000, spruce trees predominantly had "skirts" of well-developed lower branches. The upper part of the trees could have a cylindrical crown or the trunk could be partially devoid of branches with needles only at the top. By 2023, the crown of the most trees has become conical or narrow pyramidal with well-developed lower branches and green branches all over the trunk. On the E2 spruce island in the valley, the cone-shaped crowns of the trees have become lusher. 3) On all islands spruce has been spreading vegetatively by rooting lower branches and subsequently changing their growth from plagiotropic to orthotropic. This process has been especially active on the slopes of southern exposition. As a result, the area of the islands has slightly increased. Despite the abundance of both male strobiles and mixed-aged female cones, no undergrowth or freestanding young spruce trees were found in the surroundings. This indicates the absence of reproduction by seed for 23 years. The results prove the earlier suggestion that the northward advance of forests in watersheds is limited by the lack of quality seeds for sexual reproduction [Andreev, 1954; Norin, 1958; Surso, Barzut, 2010]. The earlier assumption that spruce islands could become a springboard for the spruce introduction into tundra communities under climate warming [Lavrinenko, Lavrinenko, 1999, 2004] is currently not confirmed. 4) Comparative photos taken from the same angles in 2000 and 23 years later are shown for all spruce islands (Fig. 3-8, 10). They display a significant tree state improvement. 5) At Cape Bolvansky Nos in the northernmost spruce islet (N 68°17'), both a surge (in 2014) and a decline in spruce vitality have been recorded over the past 20-year period. There was no increase in island area observed, in 2020 the condition of the spruce was depressed and close to 2000 (Fig. 11). 6) The dwarf shrub green-mossy spruce islands cenoflora was characterised by stability. Changes in the species composition were due to single, predominantly cryptogamous plants (Table 2). Key species, in addition to Picea obovata, are: Betula pubescens subsp. tortuosa, dwarf shrubs Empetrum hermaphroditum, Vaccinium vitis-idaea, Linnaea borealis, Arctous alpina, bryophytes Pleurozium schreberi, Hylocomium splendens and Ptilidium ciliare. Juniperus sibirica and Betula nana were often found in the shrub layer. The most active permanent herbaceous plant was Festuca ovina (Tables 1 and 2). 7) Landscape photos show the "greening" of surrounding tundra communities in watersheds and stream valleys in the Ortina River Basin due to climate warming. On watersheds, Betula pubescens subsp. tortuosa has actively introduced into tundra communities, and juveniles and young trees have gained straight trunks from the base of the tree (Fig. 13). In the river valley and its tributaries, the area and height of bushes of Juniperus sibirica, shrubby willows and especially Alnus fruticosa have increased (Fig. 8а and б, 14). 8) The current position of the island spruce sparse forests` northern boundary in the Ortina River valley recorded on the satellite image is at latitude N 67°53ʹ (Fig. 15) and has not changed over the last 20 years. The reason appears to be the lack of good quality seed for sexual reproduction. Monitoring studies could make it possible to trace the time when the boundaries of spruce sparse forests and spruce islands will close up in case of further possible climate warming. The distance between them is now quite small – 3-6 kilometers