Boreal Forest Ecology

The term boreal forest or taiga is applied to coniferous forests of the Paleo-Arctic and Neo-Arctic Regions (the latter emphasized here); also included are Rocky Mountain Forest and the Temperate Rainforest of the Pacific Northwest. To the north, the boreal coniferous forest merges sinuously with both Arctic and alpine tundra. To the south, the boreal forest merges with the northern deciduous forest in some places and in other places with the plains and steppes of North America and Eurasia. Southward extensions of the boreal forest follow the three main north-south oriented ranges of North America.

The climate of the boreal forest region is characterized by short summers (50-100 frost-free days), long, cold, dark winters, great variations of temperature between winter lows and summer highs (at Fairbanks, Alaska (USA), record low, -52.2°C (-62°F); record high, 35.6°C (96°F)), and a mean annual precipitation of 380-500 mm. At Fairbanks, annual precipitation averages out at 272 mm, including an annual average of 1780 mm of snowfall. Snowfall totals vary widely from winter to winter— at Fairbanks, for example, 940 mm in 1969-1970; 3660 mm in 1971-1972. From the early 1900s to the present, average annual measurements at Fairbanks have changed as follows: temperature increase, 1.1°C; snowfall increase, 635 mm; and precipitation decrease, 38 mm.

Alaska's Kenai Peninsula is mostly free of permafrost, but permafrost occurs commonly throughout

Circumpolar Boreal Forest
A carpet of reindeer moss covers the boreal forest floor in Southern Labrador, Canada. Copyright Bryan and Cherry Alexander Photography

the circumpolar boreal forest region. However, its distribution is discontinuous, especially in the southern regions of the forest. Some south-facing slopes and the beds of the larger rivers lack permafrost. Generally, the farther north, the more continuous and closer to the surface is the permafrost. Larger trees are unable to develop supportive root systems in the thin, nutrient-poor soil overlying permafrost. Seeds that manage to germinate often culminate in a krummholz (from the German word meaning "twisted wood") community of small, straggly, misshapen trees that are characteristic of both the boreal and the alpine treelines. The boreal/alpine treelines are far from stable; for example, in Alaska the treeline is slowly moving farther into the tundra on the Seward Peninsula, and slowly creeping up some of the lower hillsides of the Alaska Range, both phenomena apparently expressions of global warming.

Both the Palearctic and Nearctic Regions share some species of plants and animals, but the general rule is that while the types of plants and animals are similar (often in common genera), the species are often different in the two regions. Although there is a certain apparent sameness of plant cover throughout the boreal forest region, ecologists have subdivided the forest into many dozens of ecozones or ecoregions, and within these they have further characterized numerous communities or habitats.

There is a pronounced north-south gradient in the numbers of plant and year-round resident animal species (fewer toward the north, especially if one starts counting in the High Arctic or on the North Slope of Alaska, far to the north of the boreal forest region). Spring thaw signals a huge influx of breeding migratory birds representing many species. These migrants exploit the summer food resources of the north, and then retire to warmer climes for the winter. The year-round residents also harvest the abundant foods of summer; their winter survival involves either hibernation or continued hunting (predators) or grazing (herbivores). A summer survey of a taiga community in the Yukon River valley of central Alaska revealed 37 species of mammals, 157 kinds of birds, and 14 species of fish. A very short list of some of the year-round animal residents of the boreal forest in North America would include the lynx (Felis lynx), shorttail weasel or ermine (Mustela erminea), beaver (Castor canadensis), red fox (Vulpes fulva), snowshoe hare (Lepus americanus), moose (Alces alces; called elk in Eurasia), white-winged crossbill (Loxia leucoptera), willow ptarmigan (Lagopus lagopus), and the great horned owl (Bubo virginianus). Caribou (Rangifer tarandus) are exceptional in that, in many instances, they winter in the boreal forest and summer on the Arctic or alpine tundra.

Food webs tend to be shorter and simpler (at least when compared to warmer biomes) in the boreal forest (e.g., lichens—caribou—wolf, or aquatic plants— moose—wolf). Plant and animal parasites and sym-bionts—all well represented in the forest—complicate food webs. The finer roots of white spruce (Picea glauca) are often enshrouded in mutualistic mycorrhizal fungi (numerous ascomycete and basidiomycete species) that promote water and nutrient absorption; the tree supplies sugars and amino acids to the fungi. These spruce trees (and several other conifer species) are parasitized (eventually fatally) by the spruce broom rust, Chrysomyxa arctostaphyli, which causes some of the spruce branches to form tangled masses called witches' brooms. Essential (as a telial host) for the completion of the broom rust life cycle is another plant, kinnikinnick (bearberry), Arctostaphylos uva-ursi. In preparation for the breeding season and for the next winter, northern flying squirrels (Glaucomys sobrinus) and red squirrels (Tamiasciurus hudsonicus) fashion nests inside the witches' brooms and later store harvested mycorrhizal fungi inside for winter food. As northern redbacked voles (Clethrionomys rutilus) graze the underground fruiting bodies of the mycorrhizal fungi, they spread the fungal spores about in their feces as they continue to forage.

Only a very few of the many kinds of insects and fungal pathogens that attack a wide variety of boreal forest trees are noted here. In cycles of 25-30 years, spruce budworm (Choristoneura fumiferana) populations explode in the forests of northern Maine (USA) and in Ontario and Québec (Canada), causing large-scale die-offs of balsam fir (Abies balsamea).

Many scientists are convinced that global warming is at least in part responsible for the outbreak of spruce bark beetles (Dendroctonus rufipennis) that has killed an estimated 38 million trees (white spruce; Sitka spruce, Picea sitchensis; Lutz spruce, Picea glauca X lutzii) in 1.6 million hectares of forest on the Kenai Peninsula and neighboring regions. These large tracts of dead trees are extremely susceptible to forest fires.

A new disease of forest trees, sudden oak death, caused by a funguslike pathogen, Phytophthora ramo-rum, has killed large stands of oak (Quercus spp.) and tanbark oak (Lithocarpus densiflorus) trees in California and Oregon, and is known to infect several other kinds of trees in the boreal forest. Infections have also been reported in the seedlings of Douglas fir (Pseudotsuga menziesii) and coast redwood (Sequoia sempervirens). Although this pathogen currently exists in a relatively restricted range (mainly western USA, the Netherlands, Germany, England, and Poland), the potential for the spread of this disease is still not clear.

Lightning-caused fires are natural and, for fire-adapted trees, essential events in forest ecosystems. Forest trees are frequent targets of lightning strikes; fires are a common result. The subsequent course of such a fire depends on multiple factors. If there is available combustible, sufficiently dry detritus, the fire may spread; but oftentimes the fire simply burns itself out in situ, especially if the lighting is accompanied by heavy rainfall. In the usual course of events, a forest may grow undisturbed by fire for 50-200 years (or some similar periodicity). During this time, combustible debris builds up on the forest floor. When hit by a dry strike (not accompanied by rain) of lightning, combustion proceeds and spreads until the available combustible materials are consumed or until rainfall dampens the fire. Such a burned tract will not again be available for another fire for a prolonged number of years because it will take many years to build up a layer of combustible forest detritus. But it is highly probable that lightning will set off another fire at some distant point in time, and the cycle of fire-induced succession will start all over again.

Fires in natural, unmanaged forests (not subject to human interventions) tend to move rapidly over the surface without producing extreme temperatures in the subsurface soils, but at the same time producing sufficient heat to provoke seed release from serotinous cones. Fire-susceptible seeds and seedlings are destroyed, mineral-rich ashes are deposited on the soil, and the way is cleared for the propagation of fire-adapted seedlings and windborne seeds of other plants. Generally, forest fires are good for moose and detrimental for caribou. Fire destroys reindeer lichens, but clears the way for new plant growth that makes for good moose forage (the lichens may eventually recover but only very slowly). This natural cycle of fires followed by new growth is the norm for the vast reaches of coniferous wilderness that encircle the Northern Hemisphere, forests that are mostly far removed from human interventions. Where there are fire prevention and control interventions, forest detritus builds up so much that when it burns, the temperatures generated are so high that both fire-adapted and fire-susceptible trees, seeds, and seedlings are often destroyed. Succession then must begin with the germination of windborne seeds and spores.

In central Alaska, bare ground scoured by fires or by river flooding and channel changes is first colonized by airborne seeds of various herbaceous and woody plants such as thinleaf alder (Alnus incana tenuifolia), balsam poplar (Populus balsamifera), and willows (Salix spp.). After about 15 years, the poplars overshadow the willows and the latter begin to wane and are replaced with horsetails (Equisetum spp.), highbush cranberry (Viburnum edule), prickly rose (Rosa acicularis), and seedlings of white spruce (Picea glauca). After about a century, the poplars largely disappear and white spruce becomes the dominant species. As white spruce trees progress towards dominance, a thick layer of moss and plant debris gradually accumulates on the forest floor, forming an insulating layer that keeps the soil temperatures cold, sometimes so cold that permafrost forms. Since such conditions are more favorable to black spruce (Picea mariana), this species eventually becomes dominant.

The large stands of mature red pine (Pinus resinosa) in the Boundary Water Canoe Area of Minnesota (USA) have been protected from forest fires for so long that many of the trees will die before being able to reproduce. The serotinous (closed) cones release their seeds usually only in response to forest fires. The fires also clear away the understory vegetation, making sunlight available to the new seedlings. It has been estimated that red pine stands can reproduce themselves if they are subjected to fire at least once every 300 years. Shorter intervals between fires would better serve other fire-adapted trees such as the jack pine (Pinus banksiana), a widespread species in Canada and in northern New England and the Lake States (USA) and the dominant tree species (or codominant with red pine) in the more southerly portion of the boreal coniferous forest, often growing in association with aspens (Populus spp.) and paper birch (Betula papyrifera). Some jack pines produce both serotinous (closed) cones that release seeds in response to high temperatures, usually caused by fires, and nonserotinous (open) cones that mature and release their seeds at ambient summer temperatures.

The boreal forest biome remained relatively unchanged for eons, but in recent years change-producing factors, mostly of human origin (but there may also be some factors related to interactions between cosmic rays and cloud cover), have begun to have impacts on this ancient ecosystem, including its human inhabitants, most especially nursing infants. Pollutants (pesticides, dioxins, PCBs (polychlorinated biphenyls), petroleum byproducts, radioactive wastes and emissions, especially from Chernobyl, above-ground nuclear device detonations and nuclear industry emissions, acid rain, black carbon aerosols, POPs (persistent organic pollutants), and many others), all products of human enterprises, are carried on winds and waters to the boreal regions where they become incorporated into the living tissues of fish, birds, and mammals that in part are the traditional foods of indigenous northern peoples (see Local and Transboundary Pollution). Pollution-generating industries are also located within the boreal forest biome. A satellite photograph shows large stands of trees killed by acid rain/sulfur dioxide from a nickel-copper smelter on Russia's Kola Peninsula; these pollutants also kill insects that prey on the willow-eating Melasoma lapponica, allowing populations of this leaf beetle to explode.

The global warming trend is impacting, among other things, glaciers, permafrost, tundra-taiga eco-tones, and the distributions of plants and animals and their diseases. Timber, petroleum and mineral exploitation, and hydroelectric projects have caused major alterations in the boreal forest ecosystem. Most of the world's supply of softwood timber (used to make paper) and of construction lumber comes from the boreal forest. Timber extraction operations invariably lead to habitat degradation and fragmentation, but restorative measures, implemented soon after harvest, can mitigate some of the damage. All of these impacts are uniformly both deleterious and capable of being mitigated to some extent if appropriate remedial measures are applied in a timely fashion. The most serious threat to the boreal forest wilderness is one of attitude, a mindset shared by many in government and industry, namely that the "frozen North" has value only in as much as it can be commercially exploited.

J. Richard Gorham

See also Coniferous Forests; Taiga; Treeline Dynamics

Further Reading

Arctic Monitoring and Assessment Programme, Arctic Pollution Issues: A State of the Arctic Environment Report, Oslo: AMAP, 1997 Bailey, Robert G, Ecoregions of North America (map), Washington, District of Columbia: Forest Service, US Department of Agriculture, 1997

-, Ecoregions of North America, Explanatory Note,

Washington, District of Columbia: Forest Service Miscellaneous Publication Number 1548, US Department of Agriculture, 1998 Berg, Edward, "Bark beetles and climate change on the Kenai Peninsula." 2000-2002, website http://chinook.kpc.alaska. edu/~ifeeb/cycles/cycles_index.html Chapin, F. Stuart III, Robert L. Jefferies, James F. Reynolds, Gaius R. Shaver, Josef Svoboda & Ellen W. Chu (editors), Arctic Ecosystems in a Changing Climate: An Ecophysiological Perspective, San Diego: Academic Press, 1992

Danks, H.V. & R.G. Foottit, "Insects of the boreal zone of Canada." Canadian Entomologist, 121(8) (1989): 625-690 Gawthrop, Daniel, Vanishing Halo: Saving the Boreal Forest, Vancouver, British Columbia: Greystone Books, 1999

Gunn, John M. & Rod Sein, "Effects of forestry roads on reproductive habitat and exploitation of lake trout (Salvelinus namaycush) in three experimental lakes." Canadian Journal of Fisheries and Aquatic Sciences, 57(Suppl. 2) (2000): 97-104

Hansen, Andrew J. & Jay J. Rotella, "Biophysical factors, land use, and species viability in and around nature reserves." Conservation Biology, 16(4) (2002): 1112-1122 Hanski, Ilkka & Peter Hammond, "Biodiversity in boreal forests." Trends in Ecology and Evolution, 10(1) (1995): 5-6 Humphries, Murray M., Donald W. Thomas & John R. Speakman, "Climate-mediated energetic constraints on the distribution of hibernating mammals." Nature, 418(6895) (2002): 313-316 Klein, David R., "Caribou in the changing North." Applied

Animal Behaviour Science, 29 (1991): 279-291 Lakehead University, Faculty of Forestry and the Forest Environment, Thunder Bay, Ontario, Canada, 2002, website Morrisset, Pierre & Serge Payette (editors), Tree-Line Ecology: Proceedings of the Northern Québec Tree-Line Conference (Collection Nordicana No. 47), Laval, Québec: Centre d'études nordiques, Université Laval, 1983 Packee, Edmond, C., "Silvicultural systems for Alaska's

Northern Forest." AgroBorealis, 32(1) (2000): 21-27 Stenseth, Nils Chr., Atle Mysterud, Geir Ottersen, James W. Hurrell, Kung-Sik Chan & Mauricio Lima, "Ecological effects of climate fluctuations." Science, 297(5585) (2002): 1292-1296

Viereck, Leslie A., "Forest succession and soil development adjacent to the Chena River in interior Alaska." Arctic and Alpine Research, 2(1) (1970): 1-26 Webster, Paul, "Biodiversity: bid to save Kamchatka's wildlife." Science, 297(5588) (2002): 1787-1788 Zvereva, E.L. & M.V. Kozlov, "Effects of air pollution on natural enemies of the leaf beetle Melasoma lapponica." Journal of Applied Ecology, 37(2) (2000): 298-309

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