Trees

All of the broad-leaved trees of Saskatchewan are flowering plants (angiosperms) and dicots (i.e., where seedlings have two seed leaves or cotyledons). They are deciduous, shedding their leaves at the end of summer. Broad-leaved trees are also called hardwoods because of the high density of their wood (compared to softwoods such as conifers). The eight species which regularly exceed 10 m in height at maturity, or large trees, are the subject of this entry. Eleven other native woody dicots can be small trees: speckled alder, water birch, mountain ash, two or three willows, saskatoon, chokecherry, pincherry, wild plum, mountain maple, and nannyberry. The diversity of broad-leaved trees mirrors their nationally and globally higher diversity as compared to coniferous trees.

Five of these trees (Manitoba maple, green ash, American elm, bur oak, eastern cottonwood) belong to the western elements of the eastern deciduous temperate forest of North America, and commonly occur together. The other three species (trembling aspen, balsam poplar, white birch) are associated with both the boreal forest and the eastern deciduous forest but are usually thought of as boreal species.

Manitoba maple (Acer negundo: family Aceraceae) is most prevalent along the river valleys and coulees of south-central and southeastern Saskatchewan, with an extension westward along the South Saskatchewan River. It is associated with green ash and with American elm, trembling aspen, white birch, and balsam poplar in the north-central and northeastern part of its range. The mature tree is usually 10-15 m in height, with diameters up to 50 cm; it has ridged, light grey bark, and the opposite leaves are normally divided into several parts. The winged seeds spin like helicopters when released from the tree.

The distribution of green ash (Fraxinus pensylvanica: family Oleaceae) is almost identical to that of Manitoba maple, excluding the South Saskatchewan. Green ash is a denizen of alluvial floodplains, lake shores, and coulees. It may be confused with Manitoba maple, but its opposite leaves are divided into five or seven leaflets. The 3 cm- long winged seed spins when released from the tree. Mature trees may be over 20 m tall, with diameters over 50 cm; the trunk is straight and tapers a little, with ridged brownish-grey bark. It is an important landscaping tree.

American or white elm (Ulmus americana: family Ulmaceae) is often planted for shade. The most extensive natural stands, with the largest trees, are found on the rich alluvial soils of the floodplains of the Carrot and Saskatchewan rivers near the Manitoba border. The species also occurs in the Dirt Hills and in the wooded coulees of the Missouri Couteau, but is much smaller in these prairie environments. The tree reaches heights of 25 m and diameters of over 50 cm, with entire, sharply toothed, opposite leaves up to 10 cm long and dark, ridged and furrowed bark. The species has been ravaged by beetle-borne Dutch elm disease, and most natural populations are in serious decline.

Bur oak (Quercus macrocarpa: family Fagaceae) is found in the southeast of the province, mainly along the valleys of the Qu'Appelle and Souris Rivers, with a small population along the Assiniboine River. The alternate leaves are deeply lobed, and the bark is rough and deeply furrowed. This tree is relatively small in Saskatchewan, reaching heights of 15 m and diameters of 30 cm.

The eastern cottonwood (Populus deltoides: family Salicaceae) is a large poplar found along the flood plains of the North and South Saskatchewan rivers, and sporadically along the Qu'Appelle and Souris river valleys. It also inhabits moist, sandy depressions in the Great Sand Hills. The tree reproduces by seed and by suckering from roots and branches carried downstream by water. The leaves, triangular and with rounded teeth, have a length of 5-10 cm. The cottony seeds are borne in capsules called catkins. Cottonwood can be a large tree over 25 m in height and 100 cm in diameter, with corky, deeply furrowed bark. These trees are used in shelterbelts and for landscaping (often as hybrids). The alluvial ecosystems dominated by cottonwood are considered endangered because of changes produced by hydroelectric dams that prevent the spring floods which have for ages created the alluvial seed beds required for the perpetuation of this species.

Trembling aspen (P. tremuloides) is perhaps the best known of our native broad-leaved trees. It is found from coulees in the south to the groves of the Aspen Parkland to the boreal forest. It makes up the bulk of pure hardwood stands in the Boreal Plain and is a major component of mixedwood forests (associated with white spruce or jack pine). It becomes less common northward across the Boreal Shield, and is rare in the Taiga Shield. This is a slender, graceful tree having smooth bark (made whitish-green by a powdery bloom) and a rounded crown. The leaves, with fine teeth, are nearly circular with a short tip. The leaf stalk is flattened, which enables the leaves to tremble in the slightest breeze, giving the tree its name. The downy seed is found in capsules on the long catkins, but the tree usually reproduces vegetatively by suckering. The trembling aspen, which may attain heights of over 30 m and diameters of over 40 cm, is a source of commercial pulp and wood products.

Balsam poplar (P. balsamifera) is a tree of moist depressions, lake shores, and river valleys. It ranges from the Qu'Appelle and Saskatchewan valleys to the boreal forest to the Northwest Territories (NWT) border. This species has a bullet-shaped crown occupying the upper half of the trunk. The bark is dark and furrowed on the bottom of the trunk, but smoother and olive-green higher up the bole. The cottony seeds are in capsules on long catkins. The finely toothed leaves are egg-shaped, with a gradually tapering, pointed tip. The resinous buds give off an exquisite scent on moist spring evenings. The name of the tree derives from this scent, which resembles the perfume of the balm of Gilead (P. candicans), itself deriving its name from a resemblance to that of the Old World balm of Gilead (Commiphora gileadensis). Balsam poplar can be a very large tree, with heights of over 30 m and diameters of over 100 cm.

White birch (Betula papyrifera: family Betulaceae) is the arboreal emblem of the province of Saskatchewan. This tree is found from Moose Mountain and the Cypress Hills north through the boreal forest to the NWT border. Birch seldom forms pure stands of any extent, being almost always associated with other broad-leaved or coniferous trees. It flourishes in dry sandy sites and on rock outcrops as well as in moist, rich riparian areas such as river valleys and lake shores. The leaves are egg-shaped, with large, pointed teeth and a length of up to eight cm. The winged seeds are dropped from the catkins in fall and winter, and it is common to see the scales and seeds scattered over snow surfaces beneath birch trees. The outer papery bark peels away from the trunk over time.

The shapely coniferous trees of Saskatchewan's boreal forest and Cypress Hills are all naked-seeded plants (gymnosperms) in the family Pinaceae. Their seeds are borne on scales arranged into cones (hence the name coniferous). They all have needle-like leaves and fragrant, resinous wood of considerable commercial value in some species. Mature trees have short branches, straight trunks, and conical to columnar crowns. All are evergreen (their needles live for two or more years), with the exception of tamarack, our only deciduous conifer. All of our species are shade-intolerant, and most of the canopy of an old stand must be removed in order for these species to be successfully regenerated by fire or logging (balsam fir is the exception to this rule). The low diversity of this group (six species) is typical of the boreal forest, an unproductive biome with harsh winters and short, dry summers. The native conifers are widespread across the rest of the Canadian boreal forest.

The native pines are tough trees adapted to growth on dry sands or rocky sites. Lodgepole pine (Pinus contorta) and jack pine (P. banksiana) are sister species thought to have arisen from an ancestral pine whose population was split by the last continental ice sheet. Jack pine is found from the prairie-forest ecotone to the NWT border, while lodgepole pine is restricted to the Cypress Hills. Both species have 2-4 cm-long needles, grouped in pairs. Both attain heights of 20-25 m and diameters of over 40 cm. The cones are serotinous, i.e., their scales stay firmly closed until heated in a forest fire or by sun-warmed ground. The cone scales of lodgepole pine are armoured with prickles. These pines grow in pure stands, but associations with trembling aspen, white birch or black spruce (for jack pine only) are fairly common. Jack pine is an important lumber species.

White spruce (P. glauca) and black spruce (P. mariana) are trees of moist to wet sites. White spruce is the signature tree of the southern boreal mixedwood zone (the Mid-boreal Upland). It is also a conspicuous tree of moist, nutrient-rich riparian zones bordering waters across the Boreal Plain. Its prominence in the forest declines on the Shield, and it is virtually absent north of Lake Athabasca. Black spruce is the characteristic boreal conifer of poorly drained fens and bogs (peatlands) with cold and nutrient-poor organic (peaty) soils. On the Shield it also occurs on rocky upland sites. White spruce has longer needles (2 cm) than black spruce (1.25 cm), and its cones are up to 5 cm long and narrowly cylindrical, while black spruce cones are egg-shaped and up to 2.5 cm long. Black spruce cones are serotinous. The presence of minute white hairs on the young twigs of black spruce conclusively differentiate it from white spruce. White spruce is the largest provincial tree, reaching heights of over 30 m and diameters well over 50 cm. Black spruce is a smaller tree, with a narrower crown and down-sweeping branches; it has a height of over 20 m and a diameter of over 30 cm. White spruce is the most valuable lumber tree in Saskatchewan.

Tamarack or larch (Larix laricina) is a deciduous conifer found in swamps with groundwater flow or in very acid black spruce sphagnum peat bogs with little groundwater influence. The species occurs throughout the provincial boreal forest. Tamarack is the most shade-intolerant of the native conifers and always grows in open stands. Its soft, light-green needles (up to 2.5 cm long) appear in clusters of 10-20 on dwarf twigs. In autumn, it lights up the wetlands with a gorgeous golden-yellow display as its needles turn colour and fall. Larch cones are about 1.5 cm long and open to shed seed in the fall. The tree may attain a height of over 25 m and diameters of over 40 cm.

Balsam fir (Abies balsamea) seedlings are able to grow in the shade of other trees and, in the absence of fire, may become the canopy dominants. Because fir sheds its cone scales and seeds each fall, it has no canopy seed bank and if fire sweeps through a stand the species is likely to be extirpated, with no seed bank available for reestablishment. Balsam fir is of sporadic occurrence in Saskatchewan (largely restricted to the southern half of the boreal forest), most often appearing on moist, rich sites in riparian areas in the lee of lakes or rivers, where the trees have the greatest chance of escaping periodic forest fires. Fir needles are flat; the bark is smooth and grey, with resin-filled blisters that pop when pressed, exuding the aromatic resin characteristic of this fragrant tree. The crown is narrow, pyramidal, and spire-like at its apex. The tree may attain a height of over 25 m and a diameter of over 40 cm.

Robert A. Wright

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Further Reading

Farrar, J.L. 1995. Trees in Canada. Ottawa: Natural Resources Canada, Canadian Forest Service; Hosie, R.C. 1969. Native Trees of Canada, 7th edition. Ottawa: Canadian Forest Service, Department of the Environment; Johnson, D., L. Kershaw, A. MacKinnon and J. Pojar. 1995. Plants of the Western Boreal Forest and Aspen Parkland. Edmonton: Lone Pine Publishing.