Chapter 11
Ecological Subregions of the United States

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Yukon Intermontane Plateaus Tayga - Meadow

Three Sections have been delineated in this Province: The area of these Sections, located in central Alaska, is about 55,000 mi2 (142,400 km2).

Section M131A--Nulato Hills

Geomorphology. The Nulato Hills generally consist of northeast-trending, even-crested ridges having rounded summits and gentle slopes. Valleys are narrow and flat-bottomed. Elevation ranges from sea level to 4,040 ft (1,230 m).

Lithology and Stratigraphy. The Section consists of Upper Cretaceous rocks with minor fragments of Koyukuk and Tozitna terranes.

Soil Taxa. The dominant soils are Aquepts that have a pergelic temperature regime. Cryaquepts occur on areas of impeded drainage underlain by permafrost. Cryumbrepts and Cryorthents are principal soils on high ridges and where surfaces drain well.

Potential Natural Vegetation. Most of the area supports alpine tundra vegetation of sedges and prostrate shrubs (e.g., willows). Barren areas are frequent at high elevations. Spruce-aspen-birch forests occur at lower elevations.

Fauna. Shrub thickets in the upland areas support breeding populations of bluethroats, yellow wagtails, and red-throated pipits. Surfbirds nest in the alpine tundra areas. River otters are found throughout the major river drainages. Wood frogs have been reported in this Section.

Climate. Average annual precipitation ranges to 12 to 16 in (300 to 400 mm). Average annual temperature ranges from 23 to 28 oF (-5 to -2 oC). The growing season is approximately from May 15 to September 10.

Surface Water Characteristics. Streams in the east flow to the Yukon River, whereas those of the west side flow towards Norton Sound. Following the prevailing topography, streams have parallel courses flowing either northeast or southwest. There are some thaw lakes in the valleys. Most of the Section is underlain by permafrost. Wetlands occupy up to 53 percent of the area.

Disturbance Regimes. Wildfires occur frequently.

Land Use. Subsistence and recreational hunting and fishing are the major human uses. Mining was formerly important.

Cultural Ecology. Residents include: Inupiaq Eskimos (north); Koyukon, Holikachuk, and Ingalik Athabaskans (central); and Yupik Eskimos (south).

Compiled by Alaska Region.

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Section M131B--Kuskokwim Mountains

Geomorphology. The Koskokwim Mountains are northeast-trending ridges having rounded-to-flat summits and broad, gentle slopes. Deep narrow valleys are prevalent. Elevation ranges from 1,310 to 4,430 (400 to 1,350 m).

Lithology and Stratigraphy. The Section consists of deep ocean floor and continental fragments of the Tozitna, Ruby, Innoko, and Nixon Fork terranes. These are tightly folded Paleozoic and earlier rocks, some of which have been metamorphosed.

Soil Taxa. The dominant soils are Aquepts that have pergelic temperature and aquic moisture regimes. Cryaquepts, which are underlain by permafrost, support extensive black spruce forests. The dominant soils of white spruce-birch forests are mainly Cryohumods, Cryorthods, and Cryochrepts.

Potential Natural Vegetation. Open black spruce forests are abundant. Alpine tundra vegetation of sedges and shrubs (willow and alder) cover most hills and ridges. White spruce-paper birch communities occur on hills bordering the Yukon and Kuskokwim Rivers.

Fauna. Horned larks, surfbirds, and white-tailed ptarmigan utilize the habitats in the alpine tundra in this Section. Sharp-shinned hawks and golden eagles are common avian predators in the Kuskokwim Mountains. Northern bog lemmings occur. The Section supports low to moderate populations of brown bears. The deep, oligotrophic lakes provide habitat for trout.

Climate. Average annual precipitation ranges from 12 to 22 in (300 to 560 mm). Average annual temperature ranges from 22 to 29 oF. (-6 to -2 oC). The growing season lasts approximately from May 15 to September 10. The average freeze-free period is 90-110 days.

Surface Water Characteristics. Major streams generally flow northeast to southwest along valleys, corresponding to faults. Streams are swift and meandering. A few oxbow and thaw lakes occur in the valleys, whereas some cirque lakes are located in the glaciated mountains. Permafrost underlies most lowland soils and high mountains, but is absent in soils covered by forests or by grasses and alders. Glaciers are not present. Wetlands occupy up to 53 percent of the area.

Disturbance Regimes. Wildfires are frequent events.

Land Use. Subsistence and recreational hunting and fishing are the main human uses. Mining was formerly important.

Cultural Ecology. Koyukon, Holikachuk, and Ingalik Athabaskans reside in this Section.

Compiled by Alaska Region.

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Section M131C--Nushagak-Lime Hills

Geomorphology. This Section comprises largely rounded, flat-topped ridges having broad, gentle slopes and broad, flat, or gently sloping valleys. Elevation ranges from 1,310 to 4,270 ft (400 to 1,300 m).

Lithology and Stratigraphy. The area is composed of continental fragments and near-shore oceanic rocks within the Dillinger and Nixon-Ford terranes. These are generally marine basin deposits of later Mesozoic age and geosynclinal deposits of earlier Paleozoic age. Most of the hills consist of tightly folded graywacke, argillite, conglomerate, and greenstone flows.

Soil Taxa. The dominant soils are Aquepts that have pergelic temperature and aquic moisture regimes.

Potential Natural Vegetation. Spruce-aspen-birch forests prevail at low elevations, whereas alpine tundra heath meadows and barrens dominate at high elevations.
Fauna. White-tailed ptarmigan utilize the habitats in the alpine tundra. Golden eagles are common avian predators in the Lime Hills. Northern bog lemmings occur. The Section supports low to moderate populations of brown bears.

Climate. Average annual precipitation ranges from 22 to 30 in (560 to 760 mm). Average annual temperature ranges from 27 to 32 oF (-3 to 0 oC). The growing season extends approximately from May 15 to September 10. Average freeze-free period is 90 to 110 days.

Surface Water Characteristics. The Section is drained by tributaries of the Kuskokwim River. Rivers that flow from glaciers in the Alaska Range (e.g., Stony and Swift) are braided muddy streams, whereas those that do not (e.g., Holitna) run clear and meander. There are some thaw lakes in valleys. Ponds are abundant in morainal areas. Most of the Section is underlain by permafrost; however, glaciers are not present. Wetlands occupy up to 55 percent of the area.

Disturbance Regimes. Wildfires are frequent.

Land Use. Subsistence and recreational hunting and fishing are the major human uses.

Cultural Ecology. Tanaina Athabaskans are the primary residents. Compiled by Alaska Region.

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