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NORTHERN CONIFEROUS FOREST BIOME

 

The northern coniferous forest biome occupies a vast area below the tundra, extending completely across Canada and into interior Alaska. The biome is also referred to as the boreal forest or taiga

Compared to the arctic tundra, the climate of the boreal forest is characterized by a longer and warmer growing season. Precipitation averages 20 inches per year, but ranges from 40 inches in the eastern regions to 10 inches in interior Alaska. Available soil moisture is high as a result of cool temperature and low evapotranspiration rates. Mineral soils are generally thin and poorly drained. Large expanses of land are covered with thick deposits of peat and organic soils, ranging in depth from several feet to nearly a hundred. These soils have very high moisture holding capacities and are often completely saturated.

 

 

A broad belt of forest-tundra transition lies along the northern edges of the boreal forest. Here, forest trees are interspersed with tundra plants, the former occupying more stable soils not subject to movement or heaving due to cyclic freezes and thaws.

The diversity of tree species in the boreal forest is quite low, with black spruce (Picea mariana), larch or tamarack (Larix laricina), and white spruce (P. glauca) the most common species. The former two species generally occupy wet sites with poorly drained mineral or organic soils, while white spruce is the climatic climax species on sites that are drier and higher in nutrient content. Balsam fir (Abies balsamea) is a dominant tree species in the eastern half of the biome.

This species is very tolerant of understory conditions (more so than the spruces which are also relatively tolerant) and competes well with tolerant hardwoods. Balsam fir is, however, susceptible to injury by insects, fungi, fire, and ice. Like the above species, it is also shallow-rooted and subject to wind throw. 

 

Red spruce replaces white spruce as a dominant south of the St. Lawrence Valley and into the northern Appalachians. Still further south in the high Appalachians, Fraser fir (Abies fraseri) replaces balsam fir and forms a high altitude association with red spruce as far south as the Carolinas and Tennessee. In the northwestern part of the biome, balsam fir is replaced by subalpine fir (A. lasiocarpa), in association with white spruce. 

 

Much of the northern coniferous forest biome is covered with lakes, which eventually fill with mineral and organic matter to form bogs. A succession of plant communities is associated with the change in substrate and microenvironment. Thus, floating and submerged aquatic plants occupy the shallow waters near the shore, slowly accumulating wind-blown soil and decaying organic matter. This initial stage is replaced by invading sedges and grasses and in turn by shrubs, including Labrador tea (Ledum groenlandicum), leather leaf (Chamaedaphne calyculata), sweet gale (Myrica gale), alder (Alnus spp.), and various species of blueberry (Vaccinium spp.). Tamarack, with a high tolerance for wet, cold, highly acid and highly organic substrates, is the first tree species to invade bogs. It is gradually replaced by black spruce, the edaphic climax on wet, boggy sites in the biome. 
Surrounding the Great Lakes in both the U.S. and Canada is the white pine-red pine association, which is included in both the northern coniferous forest and temperate deciduous forest biomes (northern hardwoods association) by various authors. The original extensive stands of eastern white pine (Pinus strobus) that once covered this area have largely been destroyed by improper logging practices and white pine blister rust; most of these stands have been replaced by northern hardwooods or jack pine (P. banksiana). Eastern white pine can be considered an edaphic climax species or a sub-climax species of long duration (300-500 years). It reaches its maximum development on sandy loam soils. Moist sites in the Lake States often supported pure stands of white pine, while drier sites were more often occupied by mixed stands of red pine (P. resinosa) and jack pine, with jack pine occupying the most extreme of dry sandy sites. On heavier (i.e., clayey) soils, none of the pines were able to compete favorably with the northern hardwoods.

 

 

 

Jack pine, with its serotinous cone habit, is a prime example of a "fire species," commonly occurring in pure stands on recently burned areas. On all but the driest of these sites, jack pine is replaced by white and red pine. Quaking aspen (Populus tremuloides), balsam poplar (Populus balsamifera), and paper birch (Betula papyrifera), are all short-lived, intolerant pioneer species which quickly invade the richer or moister sites that have been denuded by fire or logging. All are good invaders with their large crops of lightweight, wind-dispersed seeds; the aspens and poplars are further enhanced in this regard by their prolific sprouting ability. In the northernmost part of the biome, black spruce (also with serotinous cones) replaces jack pine as the fire-related invader species. 

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