The Alaska Vegetation Classification - Alaska Geobotany Center ...

I.B. 1.d. Closed Paper Birch ForestDescrlption-These are communities withpaper birch dominating the overstory andwith at least 60 percent tree cover (fig. 26).Large trees reach 30 to 45 centimeters (12 to18 in) in d.b.h. and 18 to 25 meters (60 to 80ft) in height. Densities can range from 15,500trees per hectare (6,300 treeslacre) for youngstands (20 to 25 years old) to 370 trees perhectare (150 treeslacre) for old stands (120years). Annual aboveground tree biomassincrement is about 343 to 572 grams persquare meter (3,060 to 5,100 Iblacre). Leaffall is heavy, on the order of 250 grams persquare meter per year (2,230 Iblacre). Birchregeneration is limited mostly to sternsuckers sprouting from the bases of oldtrees. White spruce and black spruceseedlings and saplings may be present butusually are not abundant.Figure 26Closed broadleaf forest of paperbirch with a scattered shrub layer of Wburnumedule and Rosa acicularis and an herbaceouslayer of Equisetum awense and E. sylvaticumin the uplands of interior AlaskaIn most stands. a discontinuous tall-shrublayer several meters tall is made up of Alnus crispa or A. sinuafa. Rosa acicularisand Viburnum edule commonly form a shrub layer 1 to 2 meters (3 to 6 ft) high,though in some stands, particularly those with abundant alder, these shrubs maybe lacking or nearly so. Dwarf shrubs may be absent or may be represented only byVaccinium vifis-idaea and Linnaea borealis. The herb layer usually is dominated byCalamagrostis canadensis. The importance of Calamagrostis generally increases withage of the stand. Merfensia paniculafa and €quisefum spp. also may be common.Mosses and lichens are rare, probably because of the heavy leaf litter.Distributlon and slte characteristics-Paper birch forests are common on severalupland sites in interior and south-central Alaska. Soils generally are moderatelydrained to well-drained silts. In some areas, the soil is stony and shallow overbedrock. Permafrost usually is absent, but some birch forests develop on soils withpermafrost and a shallow active layer. The organic horizon over the mineral soil isthin and consists mostly of decaying birch leaves. Mosses and lichens usually areabsent from the forest floor.Successional status-Paper birch communities generally result from fires or otherdisturbances and usually will be replaced by open or closed white spruce, blackspruce, or black spruce-white spruce communities after passing through severaltypes of spruce-birch mixtures. In south-central and interior Alaska, open mixturesof white spruce and birch with grassy openings may be climax on some sites(Neiland and Viereck 1977).White spruce and paper birch may become established at the same time; however,the birch grows faster than the spruce. When the birch become overmature and die,the spruce is already present. It is more difficult for spruce to invade after a birchforest is well established, because the heavy leaf fall prevents the survival of spruceseedlings (Gregory 1966). Eventually, however, a few spruce become established asthe aging birch weaken and die.