328 



TIMBER RESOURCES FOR .AMERICA'S FUTURE 



to Alaska are not accompanied by such weed 

 species as gray birch or red maple. Seen from 

 the air the forest and other vegetation cover 

 appears as a complex mosaic of types. In general, 

 the forest occupies the valleys, often appearing as 

 belts which follow the meanders of the streams, 

 and the lower slopes and low benchlands. 

 Throughout most of the region timberline is com- 

 paratively low, between 2,000 and 3,000 feet 

 elevation. 



The complexity of the vegetation pattern is, in 

 large measure, the result of fire. Only when the 

 influence of past fires is appreciated can one begin 

 to account for the seemingly haphazard distribu- 

 tion of vegetation types. The sharp boundaries 

 between stands of quaking aspen or Alaskan white 

 birch and white spruce are then recognized as the 

 edges of burns. Isolated stands of a few acres of 

 white spruce, the upland stringers, and even the 

 scattered trees of white spruce may also be rec- 

 ognized as remnants or relics of former extensive 

 stands that have been destroyed by fire. Some 

 areas, now treeless, on close examination prove to 

 have formerly supported full forest stands which 

 were destroyed by repeated burning. 



Another influence that contributes to the diver- 

 sity of vegetation cover is the occurrence, in some- 

 what complicated pattern, of permanently frozen 

 ground. This phenomenon frequently results in 

 poor soil drainage with the attendant evils of poor 

 soil aeration, restricted root space, and low soil 

 temperatures. Within the Alaskan Interior, either 

 greatly impeded drainage (whether associated with 

 permanently frozen ground or not) or very exces- 

 sive drainage lead to outstandingly poor sites for 

 tree growth. 



Sharp boundaries between vegetation types are 

 most frequently caused by fire whereas those 

 caused by topography and associated influence are 

 apt to be diffuse. 



No reliable information is available as to the 

 relative areas of individual softwoods (white 

 spruce, black spruce) or of hardwood species (white 

 birch, Cottonwood, and aspen). It has been 

 roughly estimated that about 55 percent of the 

 commercial forest land has coniferous cover, 17 

 percent has a cover of broadleaved trees and the 

 balance — 28 percent — is a mixture of broadleaved 

 species and conifers, having a ratio of 60 percent 

 softwoods and 40 percent hardwoods. The 40 

 million acres, when broken down in this way, are 

 composed of 29 million acres of softwoods and 1 1 

 million acres of hardwoods. 



Following are the recognized forest types of 

 Interior Alaska. Their characteristics are some- 

 what at variance with similar types farther east. 

 The white spruce and the white birch types and 

 their mixtures comprise the bulk of the commercial 

 forest on the better sites. Aspen and tacamahac 

 poplar (balsam poplar) also form merchantable 

 stands. Black spruce usually becomes of mer- 



chantable size only when it invades better drained 

 areas on uplands. 



1 . White spruce is the climax forest community 

 on upland areas of the Interior. The essentially 

 pure stands are broken by areas of white birch or 

 aspen or types transitional betw^een these and pure 

 spruce. A stand may be even aged or many aged, 

 depending on whether it started as a seedling 

 stand or by gradual entry into a paper birch or 

 aspen overstory. Average maximum heights at 

 maturity (about 160 years) are 85-100 feet, and 

 average maximum diameters 24-28 inches with 

 individuals of much larger size. 



2. The white birch type follows fire, but later 

 white spruce enters the stand to form a mixed 

 type. Fires perpetuate the birch and reduce the 

 spruce representation. At 100 3'ears or so birch 

 declines as decay increases and the stand moves 

 gradually toward the climax. In the essentially 

 pure stands, birch at maturity seldom exceeds 80 

 feet in height or 18-20 inches in diameter. 



3. Quaking aspen also follows fire and in the 

 absence of fire or cutting is gradually replaced by 

 white spruce. Fire maintains aspen because that 

 species reproduces fi'om both root suckers and 

 seed. After 50 or 60 years deca}^ opens up the 

 stand. Average maximum heights and diameters 

 are 60 feet and 10 inches, respectively. 



4. White spruce-white birch and white spruce- 

 aspen are transition types. With absence of fire, 

 spruce gradually invades the white birch type or 

 the aspen type to form a mixture, with spruce 

 dominant after the birch or aspen reach maturity. 

 When mixtures are about even, mature heights 

 are: spruce 65-75 feet; birch 60-70 feet. Aver- 

 age maximum diameters: spruce 13 inches; birch 

 15 inches. 



5. The tacamahac poplar type often maintains 

 itself, especially if the streams along which it 

 occurs are subject to periodic overflow. Heights 

 of 70 feet and diameters of 36 inches are common. 

 White spruce sometimes enters the stand and 

 gains dominance, \^^lere this happens it will 

 eventually replace the poplar. Fires are uncom- 

 mon in poplar stands. The species has a thick 

 bark which makes it more fire resistant than 

 other Alaskan forest trees and reduces damage 

 from most of the few fires which do occur. Fol- 

 lowing destructive fires it regenerates much as 

 aspen does. 



6. The black spruce type also maintains itself, 

 as it commonly occurs where drainage is poor and 

 the permafrost table is close to the surface. On 

 such sites it seldom becomes of merchantable 

 size. Competition from other species is light on 

 the poorly drained habitat and black spruce is 

 considered a physiographic climax on these sites. 

 Stand densities are high; even at 100 years there 

 may be 2,000-3,000 trees per acre 1 inch in diam- 

 eter and larger. Average maximum heights in 

 mature stands seldom exceed 45 feet and diameters 



