Iturup, Kunashir and Lesser Kuril Ridge are located on south Kuril Arc, stretching from South Kamchatka to Hokkaido Island. Iturup Island (44.45°- 45.55°N, 146.80°- 148.88°E)
the largest island of Kuril Island Arc, is about 200 km long and from 5.5 to 46 km wide. Ekaterina Strait (6.3 km wide, up to 520 m depths) divides Iturup from Kunashir, Friz Strait ( 17.82 km wide, up to 890 m depth) - from Urup (Figure 1).
Kunashir Island (43°40'- 44° 30' N, 145°23'- 146°30' E) is divided by Izmena Strait (15.5 km wide) from northeastern Hokkaido, the island is about 123 km long, 7 to 35 km wide, and 1490 km in area. The Pacific Ocean borders the islands on the east and the Sea of Okhotsk on the west. Lesser Kurils are located parallel Great Kuril Arc and divided from Kunashir Island by South Kuril Strait (48 km wide, depth up to 200 m). The islands are separated one by another by narrow shallow straits (1.9-5.6 km wide, up to 54 m depth), from Nemuro Peninsula - by strait with 0.7 km wide, depth up to 89 m. Shikotan island is largest and has 182 km2 in area , 29 km long and 9 km wide. The islands located on south of the arc are small, less than 60km2 in area, maximal length is 11.4 km, width - 9.2 km (Zeleniy Island).
Iturup and Kunashir Islands exhibit mountain relief (up to 500-1800 m) resulting from some volcanic groups, separated by low isthmuses (from 3-4 m to 60 m high). Shikotan has dissected low mountain relief (maximal elevation 412 m). Other islands located to the south from Shikotan are small flat island with elevation up to 40 m.
The islands have oceanic climate with small annual temperature amplitude, warm winter and cool summer. During the winter the north-west air masses from Asia produce severe cold and snow. During the summer moist cool Pacific air masses move south or east across the Kuril Islands toward the Asian low-pressure area, bringing with them extensive rainfall, fog and typhoons in August-September. Annual mean temperature is about 4.3°C on Iturup Island, 4.7°C on Kunashir, and 5.0-5.2°C on Lesser Kurils, mean monthly temperature differs from -7.2°C in February on Iturup Island, -6.7°C on Kunashir Island, -5.3...-5.9°C on Lesser Kurils and about 16°C in August on Itirup Island, 16.6°C on Kunashir, and 16.1-16.3°C on Lesser Kurils. Annual sum of mean daily temperatures (above 10°C) is 1451°C on Iturup, 1593°C on Kunashir, and 1562°C on Lesser Kurils. Annual rainfall is about 1040 mm on Iturup, 1255 mm on Kunashir, 1020-1240 mm on Lesser Kurils. The islands have large amount of days with strong winds (101-106 days with wind > 15 m/c) and lower snow cover (35 cm on Shikotan up to 68 cm on Iturup). Snow cover on small flat islands is unstable due to strong winds. N and NW winds prevail at October-March shifting to S at April-September. Amount of fog days are about 83, cloudy days - from 211 on Iturup up to 162 on Zeleniy Island [38, 39].
The Pacific Ocean with the Oyashio cold current borders the island on the east and the Sea of Okhotsk on the west. The warm Soya current penetrates up to Kurilsky Bay on Okhotsk side of the Iturup Island. Ocean currents are particularly important in influencing the regional climate of Southern Kuril Islands. The warm Soya current is most important, as it has a warming effect on the southern part of the Sea of Okhotsk. The Oyashio cold current brings cold water from north to south in the Kuril region. The warm Kuroshio Current produces synoptic eddies , which influence the southern part of the island.
Marine currents, mountain relief, and hot springs define a wide range of microclimatic conditions and a high diversity of ecotypes . Vegetation period within the Sea of Okhotsk side is warmer than Pacific coast. Phenological data show that plants in Okhotsk side develop early than in Pacific side, the same phases of plant development begin on 8-10 days earlier . This is explained by two causes: influence of warm Soya current and barrier role of mountain relief, protected from fog and cold winters forming by cold water of the Oyashio current.
Vegetation of north part of Iturup Island (northern from Vetrovoi Isthmus) belongs to Urup floristic district, central and southern part of Iturup Island and Kunashir are divided to South Kuril floristic district and Lesser Kurils are separated in Lesser Kuril floristic district . South Kuril floristic district has the most diversity flora with high contents of heat-loving plants. Lesser Kuril floristic district differs by absence of some species, typical for Great Kuril Arc, such as Pinus pumila, Quercus and other trees. Coniferous and birch are dominate, larch forest is met on SE . Grows of juniper are widespread on mountain slopes. Tree vegetation is absence on small islands on south Lesser Kuril Arc.
The moist climate, wide latitudinal range, and mountain relief combine to produce some botanical formations on the islands [2, 50]. Boreal coniferous forests of the Kunashiri-Iturup Formation with the dominant Abies sachalienensis are extensively distributed in northern Kunashir and south Iturup. Cool-temperate broadleaf and mixed coniferous-broadleaf forests of the Nemuro-Kunashiri Formation occupy the southern part of the Kunashir Island. Broad-leaved taxa include Quercus crispula, Q. dentata, Acer pictum and A. ukurunduense in association with Kalopanax septemlobum, Phellodendron sachaliense, Cerasus sachalinensis, C. maximoviczii, Ulmus laciniata and U. propinqua. Conifers are represented by Abies sachalinensis, Picea microsperma, P. glehnii, and Taxus cuspidata. Such thermophylous species as Magnolia obovata, Betula maximowicziana, Alnus japonica, Fraxinus manshurica, Syringa amurensis, Actinidia arguta, Botrocarium controversum, and some others are found only in this southern part of the Kuril Islands. The boundary between these two vegetation formations is located in the central part of the island.
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