Pinophyta (conifers)

From Wikipedia, the free encyclopedia

Pinophyta Temporal range: Carboniferous – Present PreЄ Є O S D C P T J K Pg N
Conifer forests, though comprising few species, cover vast areas, as in this forest in the Cascade Range of western North America.
Scientific classification
Kingdom:	Plantae
Clade:	Spermatophytes
Division:	Pinophyta
Class:	Pinopsida
Orders and families
Cordaitales † Pinales Pinaceae Araucariaceae Podocarpaceae Sciadopityaceae Cupressaceae Cephalotaxaceae Taxaceae Vojnovskyales † Voltziales †
Synonyms
Coniferophyta Coniferae

The Pinophyta, also known as Coniferophyta or Coniferae, or commonly as conifers, are a division of vascular land plants containing a single extant class, Pinopsida. They are gymnosperms, cone-bearing seed plants. All extant conifers are perennial woody plants with secondary growth. The great majority are trees, though a few are shrubs. Examples include cedars, Douglas firs, cypresses, firs, junipers, kauri, larches, pines, hemlocks, redwoods, spruces, and yews. As of 1998, the division Pinophyta was estimated to contain eight families, 68 genera, and 629 living species.

Although the total number of species is relatively small, conifers are ecologically important. They are the dominant plants over large areas of land, most notably the taiga of the Northern Hemisphere, but also in similar cool climates in mountains further south. Boreal conifers have many wintertime adaptations. The narrow conical shape of northern conifers, and their downward-drooping limbs, help them shed snow. Many of them seasonally alter their biochemistry to make them more resistant to freezing. While tropical rainforests have more biodiversity and turnover, the immense conifer forests of the world represent the largest terrestrial carbon sink. Conifers are of great economic value for softwood lumber and paper production.

Evolution

The narrow conical shape of northern conifers, and their downward-drooping limbs, help them shed snow.

 

The earliest conifers in the fossil record date to the late Carboniferous (Pennsylvanian) period (about 300 million years ago), possibly arising from Cordaites, a genus of seed-bearing Gondwanan plants with cone-like fertile structures. Pinophytes, Cycadophytes, and Ginkgophytes all developed at this time. An important adaptation of these gymnosperms was allowing plants to live without being so dependent on water. Other adaptations are pollen (so fertilisation can occur without water) and the seed, which allows the embryo to be transported and developed elsewhere.

Conifers appear to be one of the taxa that benefited from the Permian–Triassic extinction event, and were the dominant land plants of the Mesozoic era. They were overtaken by the flowering plants, which first appeared in the Cretaceous, and became dominant in the Cenozoic era. Conifers were the main food of herbivorous dinosaurs, and their resins and poisons would have given protection against herbivores. Reproductive features of modern conifers had evolved by the end of the Mesozoic era.

Taxonomy and naming

Conifer is a Latin word, a compound of conus (cone) and ferre (to bear), meaning "the one that bears (a) cone(s)". 

The division name Pinophyta conforms to the rules of the International Code of Nomenclature for algae, fungi, and plants (ICN), which state (Article 16.1) that the names of higher taxa in plants (above the rank of family) are either formed from the name of an included family (usually the most common and/or representative), in this case Pinaceae (the pine family), or are descriptive. A descriptive name in widespread use for the conifers (at whatever rank is chosen) is Coniferae (Art 16 Ex 2). 

According to the ICN, it is possible to use a name formed by replacing the termination -aceae in the name of an included family, in this case preferably Pinaceae, by the appropriate termination, in the case of this division ‑ophyta. Alternatively, "descriptive botanical names" may also be used at any rank above family. Both are allowed. 

This means that if conifers are considered a division, they may be called Pinophyta or Coniferae. As a class they may be called Pinopsida or Coniferae. As an order they may be called Pinales or Coniferae or Coniferales. 

Conifers are the largest and economically most important component group of the gymnosperms, but nevertheless they comprise only one of the four groups. The division Pinophyta consists of just one class, Pinopsida, which includes both living and fossil taxa. Subdivision of the living conifers into two or more orders has been proposed from time to time. The most commonly seen in the past was a split into two orders, Taxales (Taxaceae only) and Pinales (the rest), but recent research into DNA sequences suggests that this interpretation leaves the Pinales without Taxales as paraphyletic, and the latter order is no longer considered distinct. A more accurate subdivision would be to split the class into three orders, Pinales containing only Pinaceae, Araucariales containing Araucariaceae and Podocarpaceae, and Cupressales containing the remaining families (including Taxaceae), but there has not been any significant support for such a split, with the majority of opinion preferring retention of all the families within a single order Pinales, despite their antiquity and diverse morphology.

Phylogeny of the Pinophyta based on cladistic analysis of molecular data.

 

The conifers are now accepted as comprising seven families, with a total of 65–70 genera and 600–630 species (696 accepted names). The seven most distinct families are linked in the box above right and phylogenetic diagram left. In other interpretations, the Cephalotaxaceae may be better included within the Taxaceae, and some authors additionally recognize Phyllocladaceae as distinct from Podocarpaceae (in which it is included here). The family Taxodiaceae is here included in family Cupressaceae, but was widely recognized in the past and can still be found in many field guides. A new classification and linear sequence based on molecular data can be found in an article by Christenhusz et al.

The conifers are an ancient group, with a fossil record extending back about 300 million years to the Paleozoic in the late Carboniferous period; even many of the modern genera are recognizable from fossils 60–120 million years old. Other classes and orders, now long extinct, also occur as fossils, particularly from the late Paleozoic and Mesozoic eras. Fossil conifers included many diverse forms, the most dramatically distinct from modern conifers being some herbaceous conifers with no woody stems. Major fossil orders of conifers or conifer-like plants include the Cordaitales, Vojnovskyales, Voltziales and perhaps also the Czekanowskiales (possibly more closely related to the Ginkgophyta).

Morphology

All living conifers are woody plants, and most are trees, the majority having monopodial growth form (a single, straight trunk with side branches) with strong apical dominance. Many conifers have distinctly scented resin, secreted to protect the tree against insect infestation and fungal infection of wounds. Fossilized resin hardens into amber. The size of mature conifers varies from less than one metre, to over 100 metres. The world's tallest, thickest, largest, and oldest living trees are all conifers. The tallest is a Coast Redwood (Sequoia sempervirens), with a height of 115.55 metres (although one Victorian mountain ash, Eucalyptus regnans, allegedly grew to a height of 140 metres, although the exact dimensions were not confirmed). The thickest, or tree with the greatest trunk diameter, is a Montezuma Cypress (Taxodium mucronatum), 11.42 metres in diameter. The largest tree by three-dimensional volume is a Giant Sequoia (Sequoiadendron giganteum), with a volume 1486.9 cubic metres. The smallest is the pygmy pine (Lepidothamnus laxifolius) of New Zealand, which is seldom taller than 30 cm when mature. The oldest is a Great Basin Bristlecone Pine (Pinus longaeva), 4,700 years old.

Foliage

Pinaceae: needle-like leaves and vegetative buds of Coast Douglas fir (Pseudotsuga menziesii var. menziesii)

 

Araucariaceae: Awl-like leaves of Cook Pine (Araucaria columnaris)

 

In Abies grandis (grand fir), and many other species with spirally arranged leaves, leaf bases are twisted to flatten their arrangement and maximize light capture.

 

Cupressaceae: scale leaves of Lawson's Cypress (Chamaecyparis lawsoniana); scale in mm

 

Since most conifers are evergreens, the leaves of many conifers are long, thin and have a needle-like appearance, but others, including most of the Cupressaceae and some of the Podocarpaceae, have flat, triangular scale-like leaves. Some, notably Agathis in Araucariaceae and Nageia in Podocarpaceae, have broad, flat strap-shaped leaves. Others such as Araucaria columnaris have leaves that are awl-shaped. In the majority of conifers, the leaves are arranged spirally, exceptions being most of Cupressaceae and one genus in Podocarpaceae, where they are arranged in decussate opposite pairs or whorls of 3 (−4). In many species with spirally arranged leaves, such as Abies grandis (pictured), the leaf bases are twisted to present the leaves in a very flat plane for maximum light capture. Leaf size varies from 2 mm in many scale-leaved species, up to 400 mm long in the needles of some pines (e.g. Apache Pine, Pinus engelmannii). The stomata are in lines or patches on the leaves, and can be closed when it is very dry or cold. The leaves are often dark green in colour, which may help absorb a maximum of energy from weak sunshine at high latitudes or under forest canopy shade. Conifers from hotter areas with high sunlight levels (e.g. Turkish Pine Pinus brutia) often have yellower-green leaves, while others (e.g. blue spruce, Picea pungens) have a very strong glaucous wax bloom to reflect ultraviolet light. In the great majority of genera the leaves are evergreen, usually remaining on the plant for several (2–40) years before falling, but five genera (Larix, Pseudolarix, Glyptostrobus, Metasequoia and Taxodium) are deciduous, shedding the leaves in autumn and leafless through the winter. The seedlings of many conifers, including most of the Cupressaceae, and Pinus in Pinaceae, have a distinct juvenile foliage period where the leaves are different, often markedly so, from the typical adult leaves.

Tree ring structure

The internal structure of conifer

 

Tree rings are records of the influence of environmental conditions, their anatomical characteristics record growth rate changes produced by these changing conditions. The microscopic structure of conifer wood consists of two types of cells: parenchyma, which have an oval or polyhedral shape with approximately identical dimensions in three directions, and strongly elongated tracheids. Tracheids make up more than 90% of timber volume. The tracheids of earlywood formed at the beginning of a growing season have large radial sizes and smaller, thinner cell walls. Then, the first tracheids of the transition zone are formed, where the radial size of cells and thickness of their cell walls changes considerably. Finally, the latewood tracheids are formed, with small radial sizes and greater cell wall thickness. This is the basic pattern of the internal cel structure of conifer tree rings.

Reproduction

Most conifers are monoecious, but some are subdioecious or dioecious; all are wind-pollinated. Conifer seeds develop inside a protective cone called a strobilus. The cones take from four months to three years to reach maturity, and vary in size from 2 mm to 600 mm long.

In Pinaceae, Araucariaceae, Sciadopityaceae and most Cupressaceae, the cones are woody, and when mature the scales usually spread open allowing the seeds to fall out and be dispersed by the wind. In some (e.g. firs and cedars), the cones disintegrate to release the seeds, and in others (e.g. the pines that produce pine nuts) the nut-like seeds are dispersed by birds (mainly nutcrackers, and jays), which break up the specially adapted softer cones. Ripe cones may remain on the plant for a varied amount of time before falling to the ground; in some fire-adapted pines, the seeds may be stored in closed cones for up to 60–80 years, being released only when a fire kills the parent tree.

In the families Podocarpaceae, Cephalotaxaceae, Taxaceae, and one Cupressaceae genus (Juniperus), the scales are soft, fleshy, sweet and brightly colored, and are eaten by fruit-eating birds, which then pass the seeds in their droppings. These fleshy scales are (except in Juniperus) known as arils. In some of these conifers (e.g. most Podocarpaceae), the cone consists of several fused scales, while in others (e.g. Taxaceae), the cone is reduced to just one seed scale or (e.g. Cephalotaxaceae) the several scales of a cone develop into individual arils, giving the appearance of a cluster of berries.

The male cones have structures called microsporangia that produce yellowish pollen through meiosis. Pollen is released and carried by the wind to female cones. Pollen grains from living pinophyte species produce pollen tubes, much like those of angiosperms. The gymnosperm male gametophytes (pollen grains) are carried by wind to a female cone and are drawn into a tiny opening on the ovule called the micropyle. It is within the ovule that pollen-germination occurs. From here, a pollen tube seeks out the female gametophyte, which contains archegonia each with an egg, and if successful, fertilization occurs. The resulting zygote develops into an embryo, which along with the female gametophyte (nutritional material for the growing embryo) and its surrounding integument, becomes a seed. Eventually the seed may fall to the ground and, if conditions permit, grow into a new plant.

In forestry, the terminology of flowering plants has commonly though inaccurately been applied to cone-bearing trees as well. The male cone and unfertilized female cone are called male flower and female flower, respectively. After fertilization, the female cone is termed fruit, which undergoes ripening (maturation). 

It was found recently that the pollen of conifers transfers the mitochondrial organelles to the embryo, a sort of meiotic drive that perhaps explains why Pinus and other conifers are so productive, and perhaps also has bearing on (observed?) sex-ratio bias.

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  Pinaceae: unopened female cones of subalpine fir (Abies lasiocarpa)
  
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  Taxaceae: the fleshy aril that surrounds each seed in the European Yew (Taxus baccata) is a highly modified seed cone scale
  
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  Pinaceae: pollen cone of a Japanese Larch (Larix kaempferi)
  

Life cycle

Conifers are heterosporous, generating two different types of spores: male microspores and female megaspores. These spores develop on separate male and female sporophylls on separate male and female cones. In the male cones, microspores are produced from microsporocytes by meiosis. The microspores develop into pollen grains, which are male gametophytes. Large amounts of pollen are released and carried by the wind. Some pollen grains will land on a female cone for pollination. The generative cell in the pollen grain divides into two haploid sperm cells by mitosis leading to the development of the pollen tube. At fertilization, one of the sperm cells unites its haploid nucleus with the haploid nucleus of an egg cell. The female cone develops two ovules, each of which contains haploid megaspores. A megasporocyte is divided by meiosis in each ovule. Each winged pollen grain is a four celled male gametophyte. Three of the four cells break down leaving only a single surviving cell which will develop into a female multicellular gametophyte. The female gametophytes grow to produce two or more archegonia, each of which contains an egg. Upon fertilization, the diploid egg will give rise to the embryo, and a seed is produced. The female cone then opens, releasing the seeds which grow to a young seedling.

1. To fertilize the ovum, the male cone releases pollen that is carried on the wind to the female cone. This is pollination. (Male and female cones usually occur on the same plant.)
2. The pollen fertilizes the female gamete (located in the female cone). Fertilization in some species does not occur until 15 months after pollination.
3. A fertilized female gamete (called a zygote) develops into an embryo.
4. A seed develops which contains the embryo. The seed also contains the integument cells surrounding the embryo. This is an evolutionary characteristic of the Spermatophyta.
5. Mature seed drops out of cone onto the ground.
6. Seed germinates and seedling grows into a mature plant.
7. When the plant is mature, it produces cones and the cycle continues.

Female reproductive cycles

Conifer reproduction is synchronous with seasonal changes in temperate zones. Reproductive development slows to a halt during each winter season, and then resumes each spring. The male strobilus development is completed in a single year. Conifers are classified by three reproductive cycles, namely; 1-, 2-, or 3- . The cycles refers to the completion of female strobilus development from initiation to seed maturation. All three types or reproductive cycles have a long gap in between pollination and fertilization.

One year reproductive cycle:The genera includes Abies, Picea, Cedrus, Pseudotsuga, Tsuga, Keteleeria (Pinaceae) and Cupressus, Thuja, Cryptomeria, Cunninghamia and Sequoia (Cupressaceae). Female strobili are initiated in late summer or fall in a year, then they overwinter. Female strobili emerge followed by pollination in the following spring . Fertilization takes place in summer of the following year, only 3–4 months after pollination. Cones mature and seeds are then shed by the end of that same year. Pollination and fertilization occurs in a single growing season.

Two-year reproductive cycle:The genera includes Widdringtonia, Sequoiadendron (Cupressaceae) and most species of Pinus. Female strobilus initials are formed in late summer or fall then overwinter. It emerges and receives pollen in the first year spring and become conelets. The conelet goes through another winter rest and in the spring of the 2nd year. The Archegonia form in the conelet and fertilization of the archegonia occurs by early summer of the 2nd year, so the pollination-fertilization interval exceeds a year. After fertilization, the conelet is considered an immature cone. Maturation occurs by autumn of the 2nd year, at which time seeds are shed. In summary, the 1-year and the 2-year cycles differ mainly in the duration of the pollination- fertilization interval.

Three-year reproductive cycle: Three of the conifer species are pine species (Pinus pinea, Pinus leiophylla, Pinus torreyana) which have pollination and fertilization events separated by a 2-year interval. Female strobili initiated during late summer or autumn in a year, then overwinter until the following spring. Female strobili emerge then pollination occurs in spring of the 2nd year then the pollinated strobili become conelets in same year (i.e. the second year). The female gametophytes in the conelet develop so slowly that the megaspore does not go through free-nuclear divisions until autumn of the 3rd year. The conelet then overwinters again in the free-nuclear female gametophyte stage. Fertilization takes place by early summer of the 4th year and seeds mature in the cones by autumn of the 4th year.

Tree development

The growth and form of a forest tree are the result of activity in the primary and secondary meristems, influenced by the distribution of photosynthate from its needles and the hormonal gradients controlled by the apical meristems (Fraser et al. 1964). External factors also influence growth and form. 

Fraser recorded the development of a single white spruce tree from 1926 to 1961. Apical growth of the stem was slow from 1926 through 1936 when the tree was competing with herbs and shrubs and probably shaded by larger trees. Lateral branches began to show reduced growth and some were no longer in evidence on the 36-year-old tree. Apical growth totalling about 340 m, 370 m, 420 m, 450 m, 500 m, 600 m, and 600 m was made by the tree in the years 1955 through 1961, respectively. The total number of needles of all ages present on the 36-year-old tree in 1961 was 5.25 million weighing 14.25 kg. In 1961, needles as old as 13 years remained on the tree.The ash weight of needles increased progressively with age from about 4% in first-year needles in 1961 to about 8% in needles 10 years old. In discussing the data obtained from the one 11 m tall white spruce, Fraser et al. (1964) speculated that if the photosynthate used in making apical growth in 1961 was manufactured the previous year, then the 4 million needles that were produced up to 1960 manufactured food for about 600,000 mm of apical growth or 730 g dry weight, over 12 million mm³ of wood for the 1961 annual ring, plus 1 million new needles, in addition to new tissue in branches, bark, and roots in 1960. Added to this would be the photosynthate to produce energy to sustain respiration over this period, an amount estimated to be about 10% of the total annual photosynthate production of a young healthy tree. On this basis, one needle produced food for about 0.19 mg dry weight of apical growth, 3 mm³ wood, one-quarter of a new needle, plus an unknown amount of branch wood, bark and roots. 

The order of priority of photosynthate distribution is probably: first to apical growth and new needle formation, then to buds for the next year's growth, with the cambium in the older parts of the branches receiving sustenance last. In the white spruce studied by Fraser et al. (1964), the needles constituted 17.5% of the over-day weight. Undoubtedly, the proportions change with time.

Seed dispersal mechanism

Wind and animals dispersals are two major mechanisms involved in the dispersal of conifer seeds. Wind bore seed dispersal involves two processes, namely; local neighborhood dispersal (LND) and long- distance dispersal (LDD). Long-distance dispersal distances ranges from 11.9–33.7 kilometres (7.4–20.9 mi) from the source. The bird family, Corvidae is the primary distributor of the conifer seeds. These birds are known to cache 32,000 pine seeds and transport the seeds as far as 12–22 kilometres (7.5–13.7 mi) from the source. The birds store the seeds in the soil at depths of 2–3 centimetres (0.79–1.18 in) under conditions which favor germination.

Invasive species

A Monterey Pine forest in Sydney, Australia.

A number of conifers originally introduced for forestry have become invasive species in parts of New Zealand, including radiata pine (Pinus radiata), lodgepole pine (P. contorta), Douglas fir (Pseudotsuga mensiezii) and European larch (Larix decidua).

In parts of South Africa, maritime pine (Pinus pinaster), patula pine (P. patula) and radiata pine have been declared invasive species. These wilding conifers are a serious environmental issue causing problems for pastoral farming and for conservation.

Radiata pine was introduced to Australia in the 1870s. It is "the dominant tree species in the Australian plantation estate" – so much so that many Australians are concerned by the resulting loss of native wildlife habitat. The species is widely regarded as an environmental weed across southeastern and southwestern Australia  and the removal of individual plants beyond plantations is encouraged.

Predators

At least 20 species of roundheaded borers of the family Cerambycidae feed on the wood of spruce, fir, and hemlock (Rose and Lindquist 1985). Borers rarely bore tunnels in living trees, although when populations are high, adult beetles feed on tender twig bark, and may damage young living trees. One of the most common and widely distributed borer species in North America is the whitespotted sawyer (Monochamus scutellatus). Adults are found in summer on newly fallen or recently felled trees chewing tiny slits in the bark in which they lay eggs. The eggs hatch in about 2 weeks, and the tiny larvae tunnel to the wood and score its surface with their feeding channels. With the onset of cooler weather, they bore into the wood making oval entrance holes and tunnel deeply. Feeding continues the following summer, when larvae occasionally return to the surface of the wood and extend the feeding channels generally in a U-shaped configuration. During this time, small piles of frass extruded by the larvae accumulate under logs. Early in the spring of the second year following egg-laying, the larvae, about 30 mm long, pupate in the tunnel enlargement just below the wood surface. The resulting adults chew their way out in early summer, leaving round exit holes, so completing the usual 2-year life cycle. 

Globosa, a cultivar of Pinus sylvestris, a northern European species, in the North American Red Butte Garden

Cultivation

Conifers – notably Abies (fir), Cedrus, Chamaecyparis lawsoniana (Lawson's cypress), Cupressus (cypress), juniper, Picea (spruce), Pinus (pine), Taxus (yew), Thuja (cedar) – have been the subject of selection for ornamental purposes (for more information see the silviculture page). Plants with unusual growth habits, sizes, and colours are propagated and planted in parks and gardens throughout the world.

Conditions for growth

Conifers can absorb nitrogen in either the ammonium (NH₄⁺) or nitrate (NO₃⁻) form, but the forms are not physiologically equivalent. Form of nitrogen affected both the total amount and relative composition of the soluble nitrogen in white spruce tissues (Durzan and Steward 1967). Ammonium nitrogen was shown to foster arginine and amides and lead to a large increase of free guanidine compounds, whereas in leaves nourished by nitrate as the sole source of nitrogen guanidine compounds were less prominent. Durzan and Steward noted that their results, drawn from determinations made in late summer, did not rule out the occurrence of different interim responses at other times of year. Ammonium nitrogen produced significantly heavier (dry weight) seedlings with higher nitrogen content after 5 weeks (McFee and Stone 1968) than did the same amount of nitrate nitrogen. Swan (1960) found the same effect in 105-day-old white spruce. 

The general short-term effect of nitrogen fertilization on coniferous seedlings is to stimulate shoot growth more so than root growth (Armson and Carman 1961). Over a longer period, root growth is also stimulated. Many nursery managers were long reluctant to apply nitrogenous fertilizers late in the growing season, for fear of increased danger of frost damage to succulent tissues. A presentation at the North American Forest Tree Nursery Soils Workshop at Syracuse in 1980 provided strong contrary evidence: Bob Eastman, President of the Western Maine Forest Nursery Co. stated that for 15 years he has been successful in avoiding winter “burn” to Norway spruce and white spruce in his nursery operation by fertilizing with 50–80 lb/ac (56–90 kg/ha) nitrogen in September, whereas previously winter burn had been experienced annually, often severely. Eastman also stated that the overwintering storage capacity of stock thus treated was much improved (Eastman 1980).

The concentrations of nutrient in plant tissues depend on many factors, including growing conditions. Interpretation of concentrations determined by analysis is easy only when a nutrient occurs in excessively low or occasionally excessively high concentration. Values are influenced by environmental factors and interactions among the 16 nutrient elements known to be essential to plants, 13 of which are obtained from the soil, including nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur, all used in relatively large amounts (Buckman and Brady 1969). Nutrient concentrations in conifers also vary with season, age and kind of tissue sampled, and analytical technique. The ranges of concentrations occurring in well-grown plants provide a useful guide by which to assess the adequacy of particular nutrients, and the ratios among the major nutrients are helpful guides to nutritional imbalances.

Economic importance

The softwood derived from conifers is of great economic value, providing about 45% of the world’s annual lumber production. Other uses of the timber include the production of paper and plastic from chemically treated wood pulp. Some conifers also provide foods such as pine nuts and Juniper berries, the latter used to flavor gin.

Boreal forest of Canada

From Wikipedia, the free encyclopedia

Boreal Forests occur in the more southern parts of the Taiga ecoregion that spreads across the northern parts of the world.

 

Canada's Boreal forest comprises about one third of the circumpolar boreal forest that rings the Northern Hemisphere, mostly north of the 50th parallel. Other countries with boreal forest, include Russia, which contains the majority, the United States in its northern most state of Alaska, and the Scandinavian or Northern European countries (e.g. Sweden, Finland, Norway and small regions of Scotland). In Europe, the entire boreal forest is referred to as taiga, not just the northern fringe where it thins out near the tree line. The boreal region in Canada covers almost 60% of the country's land area. The Canadian boreal region spans the landscape from the most easterly part of the province of Newfoundland and Labrador to the border between the far northern Yukon and Alaska. The area is dominated by coniferous forests, particularly spruce, interspersed with vast wetlands, mostly bogs and fens. The boreal region of Canada includes eight ecozones. While the biodiversity of regions varies, each ecozone has a characteristic native flora and fauna.

The boreal forest zone consists of closed-crown conifer forests with a conspicuous deciduous element (Ritchie 1987). The proportions of the dominant conifers (white and black spruces, jack pine (Pinus banksiana Lamb.), tamarack, and balsam fir) vary greatly in response to interactions among climate, topography, soil, fire, pests, and perhaps other factors.

The boreal region contains about 13% of Canada's population. With its sheer vastness and forest cover, the boreal makes an important contribution to the rural and aboriginal economies of Canada, primarily through resource industries, recreation, hunting, fishing and eco-tourism. Hundreds of cities and towns within its territory derive at least 20% of their economic activity from the forest, mainly from industries like forest products, mining, oil and gas and tourism. The boreal forest also plays an iconic role in Canada's history, economic and social development and the arts.

Overview

Location and size

The Canadian boreal forest is an intermediate tract of land over 1,000 kilometres in width (north to south) separating the arctic tundra region from the various landscapes of southern Canada. It extends in length from the Yukon-Alaska border right across the country to Newfoundland and Labrador. The taiga growth (as defined in North America) along the northern flank of the boreal forest creates a transition to the tundra region at the northern tree line. On the southern flank, the mountainous terrain in British Columbia that continues into the foothills of the Rockies in central Alberta makes it difficult or impossible to identify a transition zone between the northern alpine boreal forest and the montane and temperate rain forests further south. However, across the Prairie Provinces, a band of aspen parkland clearly marks the change from boreal forest to grassland. In Central Canada, a transition from northern boreal forest to the deciduous woodlands of Southern Ontario can be found in the southeastern boreal shield region of Central Ontario and western Quebec. It consists mainly of mixed coniferous and broad-leaf woodlands. 

Canada's boreal forest is considered to be the largest intact forest on earth, with around three million square kilometres still undisturbed by roads, cities and industrial development. Its high level of intactness has made the forest a particular focus of environmentalists and conservation scientists who view the untouched regions of the forest as an opportunity for large-scale conservation that would otherwise be impractical in other parts of the world.

General forest ecology

The Canadian boreal forest in its current form began to emerge with the end of the last Ice Age. With the retreat of the Wisconsin Ice Sheet 10,000 years ago, spruce and northern pine migrated northward and were followed thousands of years later by fir and birch. About 5,000 years ago, the Canadian boreal began to resemble what it is today in terms of species composition and biodiversity. This type of coniferous forest vegetation is spread across the Northern Hemisphere. These forests contain three structural types: forest tundra in the north, open lichen woodland further south, and closed forest in more southern areas. White spruce, black spruce and tamarack are most prevalent in the four northern eco-zones of the Taiga and Hudson Plains, while spruce, balsam fir, jack pine, white birch and trembling aspen are most common in the lower boreal regions. Large populations of trembling aspen and willow are found in the southernmost parts of the Boreal Plains.

One dominant characteristic of the boreal is that much of it consists of large, even-aged stands, a uniformity that owes to a cycle of natural disturbances like forest fires, or outbreaks of pine beetle or spruce budworm that kill large tracts of forest with cyclical regularity. For example, the many stands of white spruce, black spruce, and balsam fir are vulnerable to the cyclical outbreaks of a species of the spruce budworm, the Choristoneura fumiferana. Since the melting of the great ice sheet, the boreal forest has been through many cycles of natural death through fire, insect outbreaks and disease, followed by regeneration. Prior to European colonization of Canada and the application of modern firefighting equipment and techniques, the natural burn/regeneration cycle was less than 75 to 100 years, and it still is in many areas.

Terms like old growth and ancient forest have a different connotation in the boreal context than they do when used to describe mature coastal rain forests with longer-lived species and different natural disturbance cycles. However, the effects of forest fires and insect outbreaks differ from the effects of logging, so they should not be treated as equivalent in their ecological consequences. Logging, for example, requires road networks with their negative impacts, and it removes nutrients from the site, which may deplete nutrients for the next cycle of forest growth. Fire, on the other hand, recycles nutrients on location (except for some nitrogen), it removes accumulated organic matter and it stimulates reproduction of fire-dependent species.

Ecosystems

Typical upland taiga in Quebec

 

Canada's boreal region can be divided into seven ecozones. These seven can be divided into two main groups. The northern regions of the boreal forest consists of four eco-zones – Taiga Cordillera, Taiga Plains, Taiga Shield and Hudson Plains – that are the most thinly treed areas where the growing season and average tree size progressively shrinks until the edge of the Arctic tundra is reached. The southern tier of the boreal meanwhile consists of three other ecozones that form the largely uninterrupted or continuous forest in stretching as far south as Lake Superior in Ontario (as the Central Canadian Shield forests ecoregion) and the Manitoba-North Dakota border. These three southern zones are the Boreal Shield, at 1,630,000 square kilometres the largest of the eight zones, the Boreal Plains and Boreal Cordillera. A typical ecoregion of this southern tier would be the Eastern Canadian Shield taiga that covers northern Quebec and most of Labrador. Within the boreal region, there are about 1,890,000 square kilometres that are between 80% to 100% forested and another 650,000 square kilometres with 60% to 80% forest cover.

Forest species

The Calypso orchid grows in the shade of boreal forest.

 

Most trees native to the Canadian boreal are conifers, with needle leaves and cones. These include: black spruce, white spruce, balsam fir, larch (tamarack), lodgepole pine, and jack pine. A few are broad-leaved species: trembling and large-toothed aspen, cottonwood and white birch, and balsam poplar. There are large areas of black spruce, a species which is tolerant of shallow soil, permafrost and waterlogged substrates, although as a consequence they have relatively low biological productivity. Owing to the short growing season, generally infertile soils, generally shallow soils, and frequent waterlogging, most of these forest types are slow-growing species, which generally tend to predominate in stressed habitats. Similarly, many of the understory shrubs are in the Ericaceae, a family known to tolerate acid, infertile and flooded habitats: examples include Labrador tea, sheep-laurel and blueberry. Since nutrient levels are so low, overall, the productivity of forest trees is highly dependent on the rate at which mineral elements such as nitrogen and phosphorus are recycled by litterfall and decomposition. After logging, the loss of nutrients may convert forested areas into shrub barrens dominated by shrubs such as sheep-laurel. Many of the plant species are fire-dependent, since fire removes neighbouring plants, and recycles nutrients locked in organic matter. 

Although there are rather few species of trees in the boreal forest, there is a considerable diversity of other kinds of plants. An accurate summary is difficult, since most compendia on plants are organized by political, rather than ecological boundaries; one exception addresses the flora of the Hudson Bay Lowland, but much of this area is not forested. One portion of the boreal forest can be used to illustrate plant diversity; consider the Flora of the Yukon. In this western part of the boreal forest, there are, for example 127 species of grass (Poaceae), 118 species of Asteraceae, 115 species of sedge (Cyperaceae), 93 species of crucifer (Brassicaceae), 52 species of Rosaceae, 37 species of Saxifragaceae and 36 members of the snapdragon family (Scrophulariaceae). Overall, the flora has 1112 species—there are even 15 species of orchids. 

A Sphagnum bog with spruce trees on a forested ridge in Quebec

Inland water and wetlands

Canada's boreal landscape contains more lakes and rivers than any comparably sized landmass on earth. It has been estimated that the boreal region contains over 1.5 million lakes with a minimum surface area of 40,000 square metres as well as some of Canada's largest lakes. Soft water lakes predominate in central and eastern Canada and hard water lakes predominate in Western Canada. Most large boreal lakes have cold water species of fish like trout and whitefish, while in warmer waters, species may include northern pike, walleye and smallmouth bass.

The boreal forest also has vast areas of wetland, particularly bogs and fens. Two wetland areas, the Hudson Bay Lowland and the Mackenzie River basin, are among the ten largest wetlands in the world. The boreal forest wetlands provide wildlife habitat (particularly for migratory birds), they maintain water flow in rivers, and they store significant amounts of carbon that otherwise would be released to the atmosphere.

Deforestation

Sheep laurel grows in clearings and shallow soils. It can form extensive shrub barrens after logging.

 

In contemporary times, the boreal forest has suffered little deforestation, defined as the permanent conversion of forest area to non-forest due to activities associated with agriculture, urban or recreational development, oil and gas development, and flooding for hydroelectric projects. In Alberta, the province with the largest oil and gas industry, more trees are cut for agriculture or oil and gas exploration than for timber. In eastern Canada, over 9,000 square kilometres of peatlands and forest have been flooded over the past four decades for hydroelectric projects. As of 2005, Canada as a whole has 91% of the forest cover that existed at the dawn of European settlement. More deforestation has occurred outside the boreal region, in more southerly areas of the country. The forest sector annually harvests approximately ½ of 1% of the region. However, this is not considered deforestation by some, given that provincial laws are meant to ensure that areas harvested by the forest sector are replanted or regenerated naturally. However, the resulting road network from logging has effects that persist long beyond the period of harvest; indeed, one can make the case that road construction is one of the most harmful and persistent effects of logging.

The Canada warbler nests on the ground in boreal forests.

Wildlife

There may be as many as five billion landbirds, including resident and migratory species. The Canadian boreal region contains the largest area of wetlands of any ecosystem of the world, serving as breeding ground for over 12 million waterbirds and millions of land birds, the latter including species as diverse as vultures, hawks, grouse, owls, hummingbirds, kingfishers, woodpeckers and passerines (or perching birds, often referred to as songbirds). It is estimated that the avian population of the boreal represents 60% of the landbirds in all of Canada and almost 30% of all landbirds in the United States and Canada combined.

Many of the wildlife species, are, like the forests, dependent upon natural disturbance from fire and insect outbreaks. For example, at least three species of warbler (Cape May warbler, bay-breasted warbler and Tennessee warbler), have distributions and abundance related to spruce budworm outbreaks. The black-backed woodpecker shows a preference for burnt over forests, where it forages for insects burrowing in the dead trees that remain standing. Fireweed, as the name suggests, is a plant that similar thrives in recently burned areas. Blueberries and huckleberries are also stimulated by fires, probably benefiting from the removal of shade, and the nutrients released in ashes. The resulting berries are an important food source for boreal forest animals. 

Few species of boreal wildlife are classified under government conservation regimes as being at risk of extinction. However, the decline of some major species of wildlife is a concern. Boreal woodland caribou, whose lichen-rich, mature forest habitat spans the boreal forest from the Northwest Territories to Labrador, is designated as a threatened by the Committee on the Status of Endangered Wildlife in Canada. The Newfoundland population of marten is threatened by habitat loss, accidental trapping and prey availability.

Boreal life cycles

Natural regeneration

The particular mixture of tree species depends upon factors including soil moisture, soil depth, and organic content. Upland forests can be closely mixed with forested peatlands. The resulting conifer forests are produced by and dependent upon recurring disturbance from storms, fires, floods and insect outbreaks. Owing to the accumulated peat in the soil, and the predominance of coniferous trees, lightning-caused fire has always been a natural part of this forest. It is one of many ecosystems that depend upon such recurring natural disturbance. For example, fire dependent species like lodgepole and jack pine have resin sealed cones. In a fire, the resin melts and the cones to open, allowing seeds to scatter so that a new pine forest begins. It has been estimated that prior to European settlement, this renewal process occurred on average every 75 to 100 years, creating even-aged stands of forest. Fire continues to cause natural forest disturbance, but fire suppression and clear-cutting has interrupted these natural cycles, leading to significant changes in species composition.

Boreal vegetation never attains stability because of interactions among fire, vegetation, soil–water relationships, frost action, and permafrost (Churchill and Hanson 1958, Spurr and Barnes 1980). Wildfires produce a vegetation mosaic supporting an ever-changing diversity of plant and animal populations (Viereck 1973). In the absence of fire, the accumulation of sphagnum peat on level upland sites would eventually oust coniferous vegetation and produce muskeg.

Fire effects

Fireweed is a native wildflower that grows after forest fires.

 

Despite today's sophisticated and expensive fire-spotting and fire-fighting techniques, forest fires in Canada still burn, on average, about 28,000 square kilometres of boreal and other forest area annually. That average annual burn area is equivalent to more than three times the current annual industrial timber harvest. It can be many more times that in intense fire years. However, although logging also removes trees, fire is not the same as logging, since fire has been a part of coniferous forests for millennia. Fire not only stimulates regeneration of many plant species, it recycles phosphorus and removes accumulated organic matter. Fire is increasingly used as a management tool to maintain forest health in some parts of North America. Different parts of the boreal have different burn cycles. The drier western region, which receives lower average rainfall, had higher natural fire frequencies. Hence, more area is burned annually on average in the west than in central and eastern Canada. When natural burn cycles are interrupted by fire suppression, natural renewal is obstructed and species composition is changed. In addition, fire suppression causes fuel loads to increase so that fires, when they do occur, become more intense. One can argue that fire suppression actually creates a positive feed back loop, where ever more expensive fire suppression generates the conditions for ever larger fires. The negative effects of fire suppression are still under study, and not fully measured, but they need to be considered when making decisions about the future health of boreal forests.

Economic activities

Region-wide planning

Because parts of the boreal forest region are found in nearly every province and territory in Canada, there has not been much in the way of coordinated planning to develop the region. Prime Minister Diefenbaker talked of his "northern vision" but little was done to see it come to pass. A proposal was authored by Richard Rohmer in 1967 called Mid-Canada Development Corridor: A Concept and was discussed by officials and politicians but was never implemented. In 2014, John van Nostrand attempted to revive the concept.

In the absence of a nationwide plan, private industry and the provinces have pursued development in particular products or certain regions. These include the Athabasca Oil Sands in Alberta, the Ring of Fire (Ontario), and Quebec's Plan nord.

Land ownership

Forest land in Canada is largely Crown land. Over 90% of the boreal forest is provincial Crown land; another 5% is federally controlled and includes national parks, First Nations reserves and national defence installations.

Industrial activity

A skidder is used to clear forest and move logs.

 

About 1,400 communities within the Boreal region rely on resource industries for at least part of the livelihood and stability. Many of these communities were carved out of the forest to support a sawmill, pulp and paper mill, mine or railway maintenance facility. Boreal forestry activities support almost 400,000 direct and indirect jobs across Canada. Forestry, pulp and paper, mining, and oil and gas exploration and development are the largest industries along with tourism, trapping, recreation, light manufacturing and the services to support industry and communities. The forest products sector is one of Canada's largest export industries, representing approximately 3% of GDP, with about half of the annual wood harvest coming from the boreal forest.

Roughly one quarter of the boreal forest is managed for industrial forestry. The remaining three-quarters is either in parks, conservation areas, model forests or is considered non-timber-productive, generally defined as unsuitable for managed forestry or inaccessible. As recently as 2003, it was estimated that the annual harvest in the boreal was about 7,500 square kilometres per year, equivalent to about 0.2% of the total Canadian boreal forest. The sharp downturn in the market for lumber because of the collapse of the housing market in the United States that began in 2006, coupled with import tariff and tax barriers, have knocked the bottom out of Canada's forest industry. In Ontario, Canada's most populous province, where most forestry activity is in the boreal, government statistics suggest that the harvest declined 18% from 2005 to 2006. Given the high number of mill closings from 2005 onward, mostly in Ontario and Quebec, it is a trend that most likely persisted through 2007 and 2008. Most of Canada's conventional onshore oil and gas production, including the rapidly expanding oil sands production in Alberta, is located in the boreal region as is Canada's largest uranium producing zone in northern Saskatchewan and Quebec's largest hydroelectric generating facilities in the La Grande watershed.

Aboriginal participation

About eighty percent of Canada's Aboriginal population resides in forested areas – including one million in over five hundred First Nations and Métis settlements in boreal zones. Of that amount, over 17,000 work in the forest products industry, mostly in silviculture and woodlands operations in the boreal and other forest regions.

Sustainable development

Since the early 1990s, a strong impetus has been created to focus on conserving Canada's boreal legacy and sustainably managing economic activity within the entire region. The Canadian boreal is largely intact and available for multiple uses like timber harvest, recreation and hunting. Forestry companies have come to adopt the management practices known as eco-system based management, which takes into consideration criteria and indicators for sustainability – social, economic and environmental. A number of key principles have come to underpin Canadian forestry practices as mandated by forestry legislation, including the obligation for forestry companies operating on public lands to fully regenerate all areas harvested for timber and to consult the public on the preparation of forest management/harvest plans submitted to the relevant provincial authorities.

Certification for sustainable forest management

As a result of growing public concern with sustainable development and conserving the integrity of the boreal forests, conservation initiatives are progressing on various fronts. The area in national and provincial parks and protected conservation areas is approximately 10% of the total boreal area. Most large forest products companies have certified their boreal forestry operations to one of three third-party, independently audited standards for sustainable forest management:

• The Forest Stewardship Council's FSC Boreal Standard;
• The Canadian standard CAN/CSA Z809;
• The Sustainable Forestry Initiative.

Sustainable Forest Management refers to managing a forest ecosystem in a manner that maintains and enhances its long-term health.

Protection

In July 2008 the Ontario government announced plans to protect 225,000 square kilometres of the Northern Boreal lands. In February 2010 the Canadian government established protection for 5,300 square miles (14,000 km²) of boreal forest by creating a new reserve of 4,100 square miles (11,000 km²) in the Mealy Mountains area of eastern Canada and a waterway provincial park of 1,200 square miles (3,100 km²) that follows alongside the Eagle River from headwaters to sea. A report issued in 2011 by the Pew Environment Group described the Canadian boreal forest as the largest natural storage of freshwater in the world.

Boreal in culture and popular imagination

The boreal forest is deeply ingrained in the Canadian identity and the images foreigners have of Canada. The history of the early European fur traders, their adventures, discoveries, aboriginal alliances and misfortunes is an essential part of the popular colonial history of Canada. The canoe, the beaver pelt, the coureur des bois, the voyageurs, the Hudson's Bay Company and the North-West Mounted Police, the construction of Canada's transcontinental railways – all are symbols of Canadian history familiar to school children that are inextricably linked to the boreal forest.

The forest – and boreal species such as the caribou and loon – are or have been featured on Canadian currency. Another iconic and enduring image of the boreal was created by 20th-century landscape painters, most notably from the Group of Seven, who saw the uniqueness of Canada in its boreal vastness. The Group of Seven artists largely portrayed the boreal as natural, pure and unspoiled by human presence or activity and hence only partly a reflection of reality.