Chugach Mountains, the Alaska Range, and the Aleutian Range. 

 The Brooks Range is the northern extension of the Rocky Moun- 

 tains. In the Intermontane Plateaus are the river systems of the 

 Yukon and Kuskokwim Rivers, the Yukon-Tanana Upland, and 

 other uplands. 



Mountains are significant to the distrihution of trees and shrubs. 

 The Coast Ranges separate the woody plants of the wet coastal 

 forests from those of the drier interior forests or taiga. Also, 

 the Brooks Range has acted as a major climatic barrier to many 

 interior species which reach their northern limits along its 

 southern slopes. The Arctic Coastal Plain lies beyond the northern 

 limit of trees. 



7. Geology (generalized). This map is from "Water 

 Resources of Alaska," an open file report published by the U.S. 

 Geological Survey (Feulner et al. 1971). It shows only three 

 broad categories of the basic geology: (1) areas where deposits of 

 alluvium, glacial debris, and wind-blown or eolian sand and silt 

 form a mantle over the subsurface rocks; (2) areas where the 

 bedrock is predominantly of sedimentary and metamorphic rocks; 

 and (3) areas of igneous rocks of both Quaternary and Tertiary 

 volcanics and of older intrusive rocks. This basic geology is im- 

 portant to the formation of soil types, the distribution of trees 

 and shrubs, and to the productivity of individual ecosystems. 



8. Extent of glaciation in Alaska. A "Map Showing 

 Extent of Glaciations in Alaska" was compiled by Coulter et al. 

 (1965) and published in color by the U.S. Geological Survey, 

 scale 1:2,500,000. The map in this Atlas is from the simplified 

 map in "Environmental Atlas of Alaska" (Johnson and Hartman 

 1971, plate 11), scale 1:10,000,000. 



Presently Alaska has more than 51,000 square miles of glaciers 

 and perennial or perpetual snow fields. It is then surprising to find 

 that large areas of Alaska were not glaciated during the Pleisto- 

 cene, even though glaciers occupied most of Canada and the 

 northern United States during parts of that epoch. In Alaska the 

 glaciers were limited primarily to the southern coastal areas and 

 to the main mountain ranges. The major portions of the Yukon 

 and Kuskokwim drainages were not glaciated. 



It is possible, then, that many species of trees and shrubs of 

 the interior forests or taiga of Alaska were able to survive either 

 on unglaciated areas in the interior or in areas west of present-day 

 Alaska that are now submerged beneath the Bering Sea. 



In southeastern Alaska, glaciation was nearly complete. How- 

 ever, the tops of some islands and mountain peaks, and also some 

 slopes on the seaward side of mountains, may have been refugia 

 for some plants. 



9. Permafrost. A "Permafrost Map for Alaska," scale 

 1:2,500,000, was published by Ferrians (1965), of the U.S. 

 Geological Survey. The map in this Atlas is from the modified and 

 simplified map in "Environmental Atlas of Alaska" (Johnson and 

 Hartman 1971, plate 10), scale 1:10,000,000. Another in color, 

 scale about 1:12,000.000, is on reverse side of "Major Ecosystems 

 of Alaska" (Joint Federal-State Land Use Planning Commission 

 for Alaska 1973). 



Permafrost, or permanently frozen ground, as defined by Midler 

 (1947) is "a thickness of soil or other surficial deposits or even 

 of bedrock, at a variable depth beneath the surface of the earth 

 in which a temperature below freezing has existed continuously for 

 a long time (from two to tens of thousands of years)." 



In Alaska permafrost, or permanently frozen ground, is found 

 to some degree in all but the southern coastal areas. The map 



distinguishes three different zones although, a- with climate, it 

 should In- recognized dial one zone grades gradually into the next. 



1. Generally underlain b) continuous permafrost. In thi- z 

 permafrost is found beneath marl) all '■urfa' ' large I 

 of water. 



2. Underlain by discontinuous permafrost. In much of the 

 interior basin and extensive areas on the south side of the Alaska 

 Range, permafrost is found under most poorly drained flat sur- 

 faces, on north slopes, and in moderately rolling topograph} 

 usually lacking on south-facing slopes and adjacent to rivers and 

 lakes. 



3. Underlain by isolated masses of permafrost. In this zone 

 permafrost is found in isolated patches, usually on north slopes 

 and in low-lying bog areas, but much of the substrate is perma- 

 frost free. 



4. Generally free from permafrost. Permafrost is lacking or 

 found only rarely in isolated patches in this zone, which includes 

 all of southeastern Alaska, much of south-central Alaska, the Alaska 

 Peninsula, and the Aleutian Islands. 



Permafrost has a strong influence on the distribution of I 

 and shrubs. It limits the rooting depth of plants, creates cold and 

 wet soil conditions, and provides conditions favorable for the 

 accumulation of peat deposits. In the interior basin of Alaska, 

 aspen and white spruce are usually found where permafrost is 

 lacking or at depths of more than 3 or 4 feet, whereas permafrost 

 sites are usually occupied by black spruce and tamarack. Paper 

 birch occurs commonly on both sites. 



10. Soils with agricultural potential. At present ther 



no generalized soil map of Alaska. The map of soils with agri- 

 cultural potential is simplified from that published in color in 

 "Alaska's Agricultural Potential" i Alaska Rural Development 

 Council 1974, after p. 22) at a scale of 1:6,000,000. It separates 

 upland from lowland agricultural soils and also distinguishes 

 some areas where soils are suitable for farming but where the 

 climate is likely to be marginal or restrictive for agricultural 

 crops. The larger colored map further subdivides agricultural areas 

 into those where 50% or more of the area is suitable for farming 

 and those in which 25—50% of the land is suitable for farming. 



Most of the land with agricultural potential lies in the Yukon 

 basin around Fort Yukon, in the Tanana Valley, and in the I 

 Inlet-Susitna Valley areas. Smaller areas occur along the lower 

 Yukon River, the Kuskokwim River, and in the Nushagak \ alley 

 and tributaries north of Dillingham. It should be pointed out that 

 these areas of good agricultural soils arc also sites of potential 

 forest growth and development. 



11. Climatic zones. The climatic /ones of Vlaska were 

 described and delineated b\ the National Weather Service Y\ 

 1959) and revised by Searbv (1968). of the Environmental 

 Sciences Services Administration. The map in this \tlas is modi- 

 fied from the simplified map in "Environmental Vtlas of Vlaska" 

 (Johnson and Hartman 1971, plate 26 I . Another in color, scale 

 about 1:12.000.000. is on reverse side of "Major Ecosystems of 

 Alaska" I Joint Federal-State land I se Planning Commission for 

 Alaska 1973). 



Four climatic /ones aie recognised for Uaska: Maritime. Tran- 

 sitional. Continental, and Arctic. 1 he Maritime Climate is cl 

 terized by high precipitation and a mean annual temperature 

 above freezing, and with a narrow range of variation b etw e en 



summer and \\ inter. 



In contrast, the Continental Zone lias low precipitation, 



