Introduction
The boreal forest, or as it is known in Russia, the taiga, constitutes
one of the largest biome in the world, covering some 12 M km2.
It is relatively depauperate in terms of species, being composed mainly
of spruces, firs, and conifers, with a smattering of of deciduous trees,
mostly along waterways. The boreal forest seems associated with the location
of the summertime arctic airmass - it begins generally where it reaches
its southern limit, and it extends to the southern most extension during
the winter. Thus, it lies between the summer and winter positions of the
arctic front (see Figure 3-2, Bolen, pg. 68).
Soils
Soils in this forest are called podsols, from the Russian
word for ash (the color of these soils) and their development podzolization.
This means that iron and aluminum are leached from the A horizon, and deposited
in the B horizon. Clays and other minerals migrate to lower layers, leaving
the upper one sandy in texture.
Because of the low temperatures, decomposition is fairly slow, and soil microorganism activity limited. The highly lignified needles of the dominant trees decompose slowly, creating a mat over the soil. Tannins and other acids cause the upper soil layers to become very acidic, and the permanent shade from the evergreen trees keeps evaporation to a minimum, and the soils are often wet. In some cases they are waterlogged nearly all year. This tends to limit nutrient cycling, compared to more southerly forests.
Major Plant and Animal Species
These forests are dominated by spruces, mainly black and white and
in the east, balsam firs. White birch can be found in more westerly areas,
and the fir drops out. In disturbed areas, aspen may be prominent. In Europe
and Asia, the spruces are replaced by two other species, Norway and Siberian.
In very wet areas, larches predominant. Larches are deciduous conifers,
and more abundant along the northern extremes.
The severe winters, and short growing season, favor evergreen species. These trees are also able to shed snow in the winter, which keeps them from breaking under the loads, and to begin photosynthesis early in the spring, when the weather becomes favorable.
Herbivores have to cope with highly lignified food, which is hard to digest. Since most of the trees bear cones, there are animals that have evolved adaptations to obtain seeds from the cones, and, conversely, the trees have adaptations to deter it, usually spines on the cones. Crossbills (which have crossed beaks) are highly efficient seed extractors. Snowshoe hares and lynx have unusually large feet, so they can walk on the snow. Moose are common large herbivores, and bear are abundant, along with wolves, where they haven't been exterminated. Caribou use the forest for shelter in the worst parts of the winter. Moose generally prefer deciduous browse and herbaceous plants, while caribou scavenge for lichens and can eat conifer needles. Thus, the two large herbivores have different food requirements - the former being an early successional species, and the latter a late successional species. Fire, which removes the lichen from the ground, can severely impact caribou, but favor moose. As human populations encroach on this remote forest area, they increase the frequency of fires, and caribou populations decline.
Insects can be a problem in this forest type, particularly the spruce budworm, which can devastate millions of hectares at a time. There have been 3 major outbreaks this century, each lasting 10 years or more. At times there can be so many moths that radar was used to follow their flight. Attacks have occurred in the 20's, 50's and 70's. Mortality can be 75% or higher! In response, DDT was sprayed in the early part of the century, and the resultant wildlife kills helped spur Rachel Carson to write her book, Silent Spring, which detailed the perils of this pesticide.
Salamandars are abundant in these forests, particularly in the southern Appalachians, and consitutute the largest animal biomass on a per hectare basis. Lemmings are the predominant rodent in the north, and have unusual 10-12 year population cycles. These have now been linked in a complicated fashion, to the vegetation. After several years of munching on plants, the plants produce toxins that deter herbivory. As the populations build up, predator populations also build up, and begin to exact a toll. When the plants are too noxious to eat, the lemmings (and hares) dye off. This sets in place a decline of the predator populations also. When the herbivore populations crash, they allow the plants to sprout freely, at which time they do not produce as much toxin in their tissues. This makes them more palatable, and the animals begin browsing again, starting the cycle over. It was only recently that the plant connection was discovered.
Associated Communities
Muskegs are low lying, water filled depressions, bogs really. They
harbor sphagnum moss, larches, heath shrubs, with a thick layer of peat.
Sphagnum moss may enhance the waterlogging - once established, it has the
ability to hold up to 4000% of its dry weight in water! It often limits
what species can establish once it gains a foothold. Some of the trees
can reproduce by layering, since the probability of seeds germinating are
low. Dr. Walker studied this in this Ph.D. thesis, and showed that some
trees were very old, and others were really all one genotype that had spread
by layering (this was white cedar, by the way).
Extensions of the boreal forest occur down the spines of mountains, at high elevations. In the east, this occurs at high elevation down to New Jersey, then West Virginia and again in the southern Appalachians. The mountains in most of Maryland and Virginia and Pennsylvania are too low to harbor this vegetation type. The trees are red spruce and balsam fir in the north, and fraser fir in the south. Fir tends to grow at the highest elevations. Yellow birch becomes prominent also, with a smattering of eastern hemlock. In the southern Appalachians, these forests start at about 4500', and in the north, where it is cooler, can be found at sea level (Maine and Canada). The boreal forest in the southern Appalachians is disjunct, and harbors several endemics. Due to its relatively small areal coverage, it is regarded as a highly endangered ecosystem. It is has been stressed recently by acidic deposition, warm temperatures, drought, and exotic insects (the balsam woolly adelgid has killed up to 95% of the fraser firs).
Ecosystem Productivity
As mentioned earlier, boreal forest occupies about 12 M km2
of land area. Net primary productivity (now referred to as NPP) ranges
from 400-2000 g m-2 yr-1, with a mean of 800 g m-2
yr-1. Worldwide, it contributes about 10 Gt yr-1,
making it a major carbon sink. As a percent of total worldwide NPP, boreal
forests contribute nearly 6% to that amount.
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Crandall, D.L. 1958. Ground vegetation patterns of the spruce-fir area of the Great Smoky Mountains National Park. Ecological Monographs 28:337-360.
Hairston, N.G. 1949. The local distribution and ecology of the plethodontid salamandars of the southern Appalachians. Ecological Monographs 19:48-73.
Krebs, C.J. , et al. 1995. Impact of food and predation on the snowshoe hare cycle. Science 268:112-115.
Larsen, J.A. 1980. The Boreal Ecosystem. Academic Press, NY. 500 pp.
Oosting, H.J. and W.D. Billings. 1951. A comparison of virgin spruce-fir forest in the northern and southern Appalachian system. Ecology 32:84-103.