Vegetation of the Central and Southern Mackenzie River Basin: Encyclopedia Arctica 6: Plant Sciences (Regional)

Author Stefansson, Vilhjalmur, 1879-1962

Vegetation of the Central and Southern Mackenzie River Basin

(EA-Plant Sciences. Hugh M. Raup)



Scroll Table to show more columns

Topography and Soils 1
Types of Vegetation 3
Forests 3
Semi-Open Prairies 8
Deltas and River Flood Plains 9
Margins of Ponds and Lakes 10
Sand Dunes 15
Subalpine Scrub 16
Alpine Plant Communities 17
Bibliography 19

EA-Plant Sciences (Hugh M. Raup)

The area of northwestern Canada covered in this article embraces roughly the southern two-thirds of the drainage basin of the Mackenzie River and its tributaries. Eastward it includes the basins of the Great Slave and Athabaska lakes; southward it covers the valleys of the Clearwater and Athabaska rivers to about latitude 55° N.; in the southwest and west it includes the drainage basin of the Peace River southward to latitude 55° N. and westward to the sum– mits of the Rocky Mountains, and also the drainage of the Liard River westward approximately to Watson Lake. Still farther northward its western boundary is the central axis of the Mackenzie Mountains as far north as the Canol Road. Northward the area extends approximately to Fort Norman on the Mackenzie River.
In terms of broad vegetational belts, the whole area, with the exception of the higher slopes of the mountains, is in the coniferous forest belt of subarctic Canada. Topographically, it is in the northern Great Plains of the continent and on the eastern slopes of the cordillera. Its surface east of the mountains can be subdivided into three physiographic regions. Eastward from the base of the mountains is a broad plain, known as the Alberta Plateau, with altitudes reaching 3,500 feet. The major western part of this plateau, composed principally of more or less flat-lying Cretaceous sediments, is

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deeply dissected by the principal tributaries of the Mackenzie: the Atha– baska, Peace, and Liard rivers. The plateau itself, away from the major streams, is rather poorly drained. Its surface rises gradually from west to east. In the central Mackenzie Ba ^ s ^ in, its eastern portions consist of isolated remnants known as the Buffalo Head Hills, the Birch Mountains, the Caribou Mountain plateau, the Eagle Mountains, and the Horn Mountain plateau. Between the eastern margins of the Cretaceous sediments and the valleys of the Slave and Mackenzie rivers is a lower surface underlain principally by Paleozoic sediments. It extends northward into the valley of the upper Mackenzie.
The immediate valleys of the Mackenzie and Slave rivers, as well as those of the lower reaches of the Peace, Athabaska, and Liard rivers, con– stitute a second physiographic region known as the Mackenzie Lowland. It is characterized by broad flood-plain deposits.
Eastward from a line which follows the valley of the lower Athabaska River, the Slave River, and the northern arm of Great Slave Lake, is the rocky surface of the Canadian Shield, or Laurentian Plateau. This surface is underlain by granitic rocks and metamorphosed sediments of pre-Cambrian age. Its soils are thin and unevenly distributed, and its drainage is im– mature. It is mottled with countless lakes and ponds, and contributes the third physiographic region.
Most of the surfaces in the area under description have been highly mod– ified by glacial erosion and deposit. The principal valleys were deepened by glacial scouring, and the intervening uplands were smoothed, grooved, and rounded. The surface underlain by Paleozoic and younger sediments usually have a deep mantle of glacial deposits such as tills and outwash gravels.

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The central portion of the area, involving the basins of Athabaska and Great Slave lakes and extending westward into the upper valleys of the Peace and Athabaska rivers, contained vast lakes in postglacial times. The beds of these lakes are marked by accumulations of lacustrine silts, and their margins by delta deposits and shore forms.
Upland White Spruce . The most widespread forest type in the central and southern parts of the Mackenzie Basin is composed principally of white spruce ( Picea glauca ). This forest has been much modified by fire practically through– out its extent. Although it occasionally exists as pure stands of spruce, it is usually composed of a great variety of mixtures involving trembling aspen, balsam poplar, Bankslan or jack pine or lodgepele pine. A common constituent of the forest is the white birch, but the distribution of this tree is irregu– lar. Intense fires that have not only killed the spruce but have burned the accumulation of raw humus down to the mineral spoils give rise to stands of one or the other pine. Fires that do not burn the humous layers are followed by mixtures of spruce, aspen, and pine. In either case the aspens and pines usually persist for only one generation and are replaced by white spruce. The existing complex mixtures of all these trees are due to the great frequency and wide extent of burning.
The upland forests of white spruce cover most of the Alberta Plateau east of the mountains. Eastward from the lower Athabaska, Peace, and Liard rivers, the pine which follows fire is Pinus banksiana . Throughout the cordilleran region, the foothills, and the western portions of the Cretaceous plateau,

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the pine so doing is P. contorta var. latifolia , the lodgepole pine.
The upland white spruce forests have a relatively small flora of shrubs and herbs. The forest floor is usually covered by a mat of woodland mosses and lichens, 3 to 6 inches, in thickness. Undergrowth is scanty. On favor– able sites — that is, on so ^ i ^ ls that are well drained but not too light — the individual trees are of good form, 60 to 80 feet high and 2 to 3 feet in diameter at [: ]breast height. However, the stands are usually open and not heavily stocked.
Flood-Plain White Spruce . Another spruce forest of wide extent in the central Mackenzie Basin occupies the Lackenzie Lowland region. Here the trees are of somewhat better form and the stands are more fully stocked. Individual trees commonly reach a height of 80 to 100 feet. The undergrowth is heavier, composed of willows, alders, and viburnums. The forest floor has a scanty cover of mosses and the soils are chiefly alluvial silts. Nar– row valley floors throughout the upland parts of the region support represent– tatives of the flood-plain spruce forest, but the type is most highly devel– oped in the lowlands along the great rivers.
Parklike White Spruce . The third type of white spruce forest occupies deposits of glacial outwash, stony tills, and sand plains in the Canadian Shield. It also appears on similar soils on [: ] the Alberta Plateau. It has been called the parklike spruce forest because of its open, parklike appear– ance. The trees are widely spaced, and the intervening ground is covered by thin mats of reindeer lichens ( Cladonia spp.), low heaths, and crowberries. The individual trees are of comparatively small stature, with branches extend– ing to the ground. Wherever it has been observed, this forest is composed of the Alberta spruce ( Picea glauca var. albertiana ).

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White Spruce and Balsam Fir . Spruce-fir forests on the interior plains are limited to the southeastern portions of the area under discussion. In general appearance they resemble the upland spruce forests, except that they contain the eastern balsam fir ( Abies balsamea ). Their most northerly ex– tent, so far as is known, is in the delta of the Athabaska River. Westward, the balsam fir becomes occasional in the Athabaska Valley between the mouth of the Clearwater and Lesser Slave Lake. Its western limits are probably in the Lesser Slave Lake region.
Pine Forests . These have already been mentioned as being widespread in areas of spruce th ^ a ^ t have been burned. It is necessary, however, to consider certain types of pine woods as more or less stable in that they are able to regenerate themselves lastingly by natural seeding. Both Banksian and lodge– pole pines may behave in this way on very light, sandy or gravelly soils. Such soils are widespread in the region. They occur as stabilized dune sand on modern or ancient lake shores, and as glacial outwash, plains. The pines on these sites usually are widely space, the ground being covered with a thin mat of moss, reindeer lichen, and trailing heaths — much as in the parklike spruce forests described above. The pines are usually of poor form and low stature, although trees 2 feet in diameter and 50 feet high are not uncommon. Forests of Banksian pine are also widely distributed over the rocky terrain of the Laurentian Plateau in the eastern part of our area. Here the trees occupy local gravel and sand deposits or crevasses in the rocks and are widely scattered and of poor growth.
It is probable that the pine forests on these dry sites will eventually support spruce. It is not unusual, however, to find spruce in an uneven-aged condition, which indicates that they are able to persist through at least two or three generations of pine.

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Muskeg with Black Spruce and Larch . Muskeg (boggy) forests are widespread throughout the area. They are regularly composed of black spruce ( Picea mariana ), with or without an admixture of America larch (i.e., tamarack, Larix laricina ). Muskegs, that is, bogs and swamps, occupy vast expenses of un– drained or very poorly drained land. They owe their abundance to several physiographic factors. In the Canadian Shield they develop in shallow de– pressions scoured out of the bedrock, or in areas that have been dammed by glacial debris or barrier beaches. Throughout the regions of deeper soils, they exist in depressions that are formed over heavy clay or behind morainic dams. In the silts of the Mackenzie Lowland, they commonly form in abandoned channels of deltas and local flood plains. Through the entire area their drainage is commonly prevented by permafrost.
The spruce forest of the muskegs is usually of comparatively low stature and small trees. Their lateral spacing is extremely variable, from open parklike stands to dense thickets. They usually have an undercover of shrubs composed of Labrador tea ( Ledum groenlandicum ), shrubby cinquefoil ( Potentilla fruticosa ), scrub rich ( Betula glandulosa ), and a few willows. The ground is a hummocky mass of sphagnum and woodland mosses. Permafrost is usually high in muskegs throughout the summer, being commonly found a foot or two below the surface. Floristically the muske y ^ g ^ s are remarkably uniform throughout the region.
Aspen Parkland . In the agricultural region of the upper Peace River, and on the Alberta Plateau a few miles west of the Slave and upper Mackenzie rivers, are representatives of what has been called the aspen parkland. This type is widespread in central Alberta, central Saskatchewan, and southern Manitoba — around the northern border of the grasslands on the Great Plains.

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The aspen parkland is composed of “bluffs” of aspen and balsam poplar scat– tared among natural prairie openings.
The question is commonly raised as to whether the aspen is a permanent type in such situations, and indeed there is considerable evidence that it can persist for a long time, at least in the region of central Alberta and Saskatchewan. On the other hand there is some evidence that the aspen bluffs of the Peace and Slave river regions are greatly accentuated, if not perpetu– [: ]ated, by fire. Prairies are occasionally found that are margined directly by spruce forests, and old aspen timbre commonly has young spruce in it. The parkland is nearly all developed on relatively heavy soils.
Montane and Foothill Forest . The forest of the eastern foothills and lower slopes of the Rocky Mountains may be described in two phases. On steeper, well-drained slopes is a heavy timber composed of Alberta spruce, alpine fir ( Abies lasiocarpa ), lodgepole pine, and black spruce. In the southwestern part of the area, Engelmann spruce ( Picea engelmannii ) is added. In timber that has not been disturbed by fire for a long time the trees are of excellent growth, form, and size, individuals 100 feet high and 3 feet in diameter at breast height being not uncommon. The undergrowth is scanty and the ground is covered by a heavy mat of woodland mosses.
Another type of foothill forest is composed of a mixture of white and black spruce and lodgepole pine. Widespread burning has allowed the entry of much trembling aspen into this type. Its principal occurrence is on heavy soils derived from lacustrine clays or heavy clay tills. Such soils are of wide occurrence in the eastern foothills and in valleys leading into the moun– tains. The lacustrine clays originated in glacial lakes and usually are varved. In general appearance this forest resembles that of the muckegs; thus there is

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a considerable undergrowth of willow, Labrador tea, and scrub birch, and on the ground a wet mat of sphagnum and other bog mosses. The mat of humus, however, is only a few inches thick and directly overlies the clay. Permafrost is found within two or three feet of the surface during most of the summer. This type of forest is usually found on gently but evenly sloping terrain, the surface commonly showing an incipient drainage pattern. There are iso– lated occurrences of such forest far east of the mountains. It has been found on the upper slopes of the Caribou Mountain plateau, 100 miles or so went of the Slave River.
Semi-Open Prairies
Natural prairies are of common occurrence throughout the region of the Peace River east of the mountains, and on the Alberta Plateau of the central Mackenzie Basin. Suggestions of them are to be found also within the area of the Canadian Shield on soils derived from the weathering of dolomitic rocks. The grasslands are usually on heavy soils composed of lacustrine clays or fine silts. Most of them are fairly well drained, though in some places they may be swampy in spring.
Throughout the Peace and Slave River regions the prairies can be classed with the “mid-grass” types of southern Manitoba and central Saskatchewan. Their principal species are Agropyron trachycaulum vars., Stipa comata , and Koeleria cristata [: ]. In the central part of our area, notably between the Peace and Liard rivers, the prairies grade into other types wherein bluegrass ( Poa pratensis ) and bluejoint ( Calamagrostis canadensis ) occur. Still farther north, in prairies on the uplands bordering the upper Mackenzie River valley, C. neglecta , P. pratensis , and Juncus vaseyi become the predominant species.

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No evidence has come to light to indicate that the prairies were pre– ceded by any woody vegetation. It is more probable that they developed from some form of grass or sedge tundra as the climate ameliorated in post– glacial times.
A notable variation in the prairie vegetation is to be found on the so-called “salt plains” immediately west of the Slave River. These plains are watered by springs and spring-fed streams from gypsiferous Silurian limestones that outcrop in this area. The streams and settling basins are margined by such halophytic species as Salicornia europaea , Triglochin mari–timum , Herdeum jubatum , and Puccinella nuttalliana .
The agricultural development that has taken place in the last fifty years in the upper Peace River country is based upon the naturally semi-open prairie land of that region. In spites of its northern latitude it has proved to be a successful venture. The more northern prairies have been used in a few places as a source of wild bay. In early times these northern prairies supported herds of bison, as did those of the northern Great Plains.
Deltas and River Flood Plains
Flood plains and deltas throughout the region develop a forest of white spruce. The early stages of this development, however, consist of several types of vegetation which succeed one another as the deposits are formed. Sand bars usually have an initial vegetation of Juncus spp. and Equisetum spp. with a few other perennial flowering plants. These communities are open, with the individual plants often widely spaced. In species composition they are extremely variable from place to place. The first woody vegetation is of sand-bar willow ( Salix interior ), but at higher levels this gives way to alders ( Alnus tenuifolia ) and several other willows (such as Salix lutem,

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S. mackenzicana , and S. lasidandra ). This community is followed by nearly pure stands of balsam poplar, which in turn is followed by spruce.
On delta plains where fine-textured muds are deposited in shallow water, the initial vegetation is ^ u ^ sually of bulrushes ( Scirpus validus ) and sedges (such as Carex rostrata , C. aquatilis , or C. atherodes ). As the plain is built up and partially drained, broad meadows of Calamagrostis canadensis are formed. Natural embankments along stream channels carry a shrub community of willows, mostly Salix planifolia . On higher, better-drained parts of the deltas, S. plani–folia gives way to S. bebbiana , and this in turn is invaded by balsam poplar and white spruce.
There are many intermediate types of deposit and vegetation that occur in sloughs and abandoned channels in the valleys of the main rivers, and on local river deposits. Taken together, they constitute a distinctive phase of the vegetation of the region. Their widest expanse is in the lower valleys of the principal rivers. The deltas are highly developed where the Athabaska and Peace rivers enter the basin of Lake Athabaska, and where the Slave River enters Great Slave Lake.
Margins of Ponds and Lakes
Lake and pond shore vegetation can be divided into two major categories: that on the shores of ponds and small lakes, and that on the shores of large bodies of water such as Lake Athabaska and Great Slave Lake. Although there are many points of similarity between these two categories, the differences are so great that it becomes necessary to consider them separately.
Ponds and small lakes are extremely numerous and varied, particularly on the Canadian Shield. Some idea of their abundance can be gathered from the

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fact that an area of 16 square miles, selected at random in the country north of Lake Athabaska, proved to have more than 100 ponds and small lakes of suf– ficient size to appear on a map scaled at 4 miles to the inch. The number of smaller bodies of water can only be conjectured, but a conservative estimate would be another 100. They are extremely variable in shape, size, elevation, depth, and mode of origin. However they were formed, or whatever their history, one is forced to consider their marginal floras as one of the most important elements in the vegetation of the region. An idea of their vegetational variability can be gathered from the fact that of a total of 161 species of vascular plants listed at pond margins in various parts of the pre-Cambrian areas, no less than 58 (36%) were noted as of primary importance in at least one association. These 58 primary species can be grouped into 83 different combinations or associations which they dominate. This lack of uniformity and continuity in the distribution of primary species is one of the most striking aspects in the lake and pond shore vegetation of the region. There is a strong suggestion that the shore-line communities are extremely young from a successional standpoint, and are still in the processs of becoming aggregated from among migrating species.
The lake and pond shore communities can be divided roughly into three groups: aquatic, wet meadow, and shrub, The principal species in the aquatic communities are water lilies of the genus Nuphar , pondweeds of the genus Potamogeton , and bur reeds of the genus Sparganium . Aquatic buttercups ( Ranun–culus spp.) are also widespread, as well as water milfoils ( Myriophyllum spp.). In the wet meadows, several species of Carex , notably C. aquatilis , C. atherodes , C. rostrata , and C. canescens , are of wide occurrence. At the drier margins of the meadows, Calamagrostis canadensis is very common. In the shrub communities,

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various Ericaceae such as leatherleaf ( Chamaedaphne calyculata ), bog blue– berry ( Vaccinium uligincsum ), and bog rosemary ( Andromeda sp.) are abundant as are also the scrub birch ( Betula glandulosa ), various willows, and occa– sionally alders.
In the central part of the Mackenzie Basin, it is extremely difficult to draw up a logical organization of the pond and lake shore communities owing to the extreme variability noted above. There is, however, a limited degree of geographical expression which is coincident with the nature of the underlying rocks. The principal division is between lakes which overlie Pale– ozoic and younger sediments on one hand and those which overlie pre-Cambrian rocks on the other. Within the latter it is possible to make a further sub– division between lakes on granitic rock and those on metamorphosed sediments such as dolomites and pre-Cambrian sandstones and quartzites. These distinc– tions ap ^ p ^ ear in all three of the major elements in the shore-line communities. The genus Potamogeton , for instance, has its greatest concentration over the Paleozoic and younger sediments, and over the dolomites of the Canadian Shield. Sparganium is most abundant over the pre-Cambrian sandstones and quartzites. Ponds on the granitic rocks are extremely poor in both these groups.
In the wet meadow communities, Carex atherodes is almost unknown in the pre-Cambrian country but is extremely common over Paleozoic rocks. The same is true of such associated species as the bulrush ( Scirpus validus ) and the cattail ( Typha latifolia ). Within the pre-Cambrian area the most complex wet meadow communities are on metamorphosed sediments, while over granitic rocks they are reduced to a few species of which Carex squatilis and C. rostrata are among the most common.

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In the shrub communities, leatherleaf and other ericaceous shrubs are most characteristic of the pre-Cambrian areas. Over younger sediments their place is taken largely by scrub birch and such muskeg shrubs as Salix glauca and S. myrtillifolia .
The vegetation on the shores of large lakes is almost as varied as that around ponds and small lakes, but it is controlled by a group of factors which do not affect smaller bodies of water. It ranges from broad meadows on gently sloping shores of sand and mud to steep rocky shores upon which grow chilly lichens that can cling to the rocks.
The principal factors that appear to govern the incidence of plant com– munities on the lake shores are wind, wave action, and ice-push. Added to these is a great variability in water levels in the large lakes. Lake Atha– baska, for instance, has been known to vary as much as 9 feet in level over a period of less than 10 years. How often these changes occur is unknown, but there is some evidence that the high levels are not reached more than once in 40 or 50 years.
Many of the shores of the larger lakes, particularly those underlain by pre-Cambrian sandstones, are sandy and shelving, and have been greatly affected by duning of the sand. Wave action on these shores is also highly effective in limiting the development of plant communities — except in protected embay– ments. Much less affected by winds and waves are shores composed of stones and boulders derived from glacial till and outwash. These shores, like those of steep rock, are inhabited predominantly by communities of lichens.
The third major factor mentioned above, that of ice-push, is almost univer– sal on the larger lakes except in some protected harbors. The ice in the larger

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lakes except in some protected harbors. The ice in the larger lakes in spring first disappears around the shores, so that large floating masses are free to be blown by the wind. Under strong winds these masses are driven up on the shores with great force, pushing sand and boulders into ridges sometimes many feet in height. If the ridges are of stones and boulders they retain [: ] their form more or less permanently, but when com– posed of sand they are usually soon reduced by wind and wave action to a fraction of their original size. As might be expected, ice-push is highly destructive to vegetation deve [: ]^ lo ^ ping on lake shores.
Any consideration of the development of vegetation along the shores of the larger lakes is conditioned by the fact that there has been a long– term lowering of general water levels in postglacial times. Well-defined shore lines with their familiar forms due to waves, wind, and ice-push are to be found in the hills around the lakes — often hundreds of feet above the present water lines. These ancient shore lines are continuous with those of the present lakes, suggesting that the latter are still receding. Off– shore bars and barrier beaches now in process of formation point to the same conclusion.
In attempting to outline the development of vegetation on a given shore, therefore, it becomes necessary to determine its position with regard to the physical factors affecting it. Well-defined zonal arrangements of communities are common. They may have been developed successfully within the existing regime of wind and wave action, but they are subject to destruction at any time by ice-push. They are also subject to great modification by temporary rises in the general level of the lakes. Forests of pine and spruce, even with individual trees having a diameter of six inches, have been observed to

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be drowned by such flooding. Areas on the higher parts of the beaches ap– proach stability, but in view of the hazardous nature of line on the lower beaches it is nearly impossible to describe, on the basis of existing zonal pehnomena, how they have developed. Still another factor governing this development is long-term change in climate. Forests that now exist on the upper beaches may have had their beginnings under climates which no longer obtain.
Sand Dunes
Sand dunes occur, in our region, principally in areas where the shores of large lakes overlie the pre-Cambrian sandstones and quartzites. There are a few such areas around the eastern arm of Great Slave Lake, at various places on the northern shores of Lake Athabaska, and on the shores of other lakes on the Canadian Sheild. They have their largest development, however, on the south side of Lake Athabaska. Most of this shore is underlain by sandstone, as are also wide areas south of the lake. Sand dunes of characteristic form are to be found wherever the lake-shore sand is exposed to strong winds. The most active dunes are on the south side of Lake Athabaska, for this shore is exposed to dry northwesterly winds.
As Lake Athabaska has gradually been drained, its southern shore has slowly moved northward, and great expanses of sand have been left on the abandoned shore lines. In areas within a few miles of the present shore, where the lake winds are effective, there are large tracts wherein the sand has not been sta– bilized by vegetation, and active dunes still exist. Immediately behind William Point, about midway along the south shore of the lake, there is one area alone comprising about 200 square miles of these active dunes. Some of them are as

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much as 40 feet high, and at the edges of the open sand there are rapidly ad– vancing dune fronts which are overwhelming forests of Banksian pine. Farther away from the lake are large dunes in all stages of stabilization. They even– tually come to support forests of Banksian pine.
Active dunes, whether immediately on the lake shores or on the larger ex– panses of sand, are partially and temporarily stabilized by open communities of plants that have elaborate rhizomes, large masses of fibrous roots, or mat– forming habits of growth that are able to withstand progressive submergence in sand. The most important of these sand binders are two species of grasses ( Bromus pumpellianus and Elymus arenarius var. villosus ), the false heather ( Hudsonia tomentosa ), and the crowberry ( Empetrum nigrum ). Other plants that are locally abundant and partially effective in holding the dunes are Festuca rubra , Salix silicicola , S. turnorii , S. brachycarpa var. psammophila , Vac–cinium uliginosum , and Tanacetum huronense var. floccosum . More stabilized dunes develop a mat of fruticose lichens and trailing heaths such as Arcto–staphylos uva-ursi and Vaccinium vitis-idaea var. minum . This community is in– vaded by Banksian pines in less exposed situations, and by white spruce on the higher parts of exposed beaches.
Subalpine Scrub
Throughout most of the region under discussion, the subalpine areas on the mountains are dominated by more or less dense growths of scrub birch ( Betula glandulosa ) and willows ( Salix glauca vars.). The vertical range of the scrub varies with slope, exposure, and the nature of the substratum, being sometimes as much as a thousand feet. Toward the upper edge of the scrub, the crowberry ( Empetrum nigrum ) and the alpine form of the bog blueberry ( Vaccinium uliginosum

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var. alpinum ) may predominate. Openings in the scrub are dominated by such grasses as Festuca altaica and several sedges of the genus Carex . In the south– western part of our area, in the mountains on either side of the Peace River pass, Rhododendron albiflorum predominates in many parts of the subalpine scrub.
The scrub vegetation descends far below timber line in gullies and areas where snowslides are frequent. In these situations it is commonly mixed with the shrub flora of the forest zones, notably with several species of Salix and with Alnus crispa .
Alpine Plant Communities
Alpine vegetation in the Rocky and Mackenzie Mountains is highly variable, being almost incapable of organization into clear-cut communities. In some places the plants form a fairly continuous turf within which ecological inter– relationships are probably developed among the species, but over most of the alpine surfaces such interdependencies are difficult or impossible to define.
The nature of the alpine plant cover depends in large measure upon the stability of the soil and the amount of water at or near the surface. Both of these factors in turn are dependent for their expression upon the relative friability of the underlying rock, the steepness of slope, the degree of con– solidation of the mat [: ]^ eri ^ al if it is a glacial till, and the length of time since the surface was exposed by glaciers.
The most complex alpine communities are in “damp meadows” which are watered by melting snow and ice. These are usually dominated by grasses and sedges, though they may show a colorful variety of other flowering plants. Drier meadow communities occur on stabilized slide rock. At the other extreme are the scat– tered plants which grow in soils that are frequently in motion owing to sliding

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on steep slopes, or in soils that are kept in motion by solifluction. By far the largest portion of the alpine zone of the mou ^ n ^ tains is underlain by such unstable soils. The commonest plants here include species of Dryas , Vaccinium (dwarfed alpine forms), trailing willows, and matforming saxifrages. Among these flowering plants is a variety of fruticose lichens. Closely related to the flora of the unstable soils is that of crevasses in outcropping rock.

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9. Raup, Hugh M. Botanical Investigations in Wood Buffalo Park . Ottawa, Patomoude, 1935. Nat.Mus.Can. Bull . no.74. Biol.Ser . no.20.

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13. ----, “----. II. Forests,” Ibid. vol.27, pp.1-85, 1946.

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EA-II. Raup. Vegetation

15. Tyrrell, J.B. “Ice on Canadian lakes,” Canad.Inst.Min.Motall. Trans . vol.9, pp.13-21, 1910.

16. ----. Report on the Country Between Athabasca Lake and Churchill River . Ottawa, Dawson, 1896. Can.Geol.Surv. Ann.Rep . vol.8, 1894, pt.D.

17. ----. Report on the Doobaunt, Kazan and Ferguson Rivers, and the North– West Coast of Hudson Bay; and on Two Overland Routes from Hudson Bay to Lake Winnipeg . Ottawa, 1897. Can.Geol.Surv. Ann.Rep . vol.9, 1896, pt.F.

18. Tyrrell, J.W. Report on an Expolratory Survey Between Great Slave Lake and Hudson Bay Districts of Mackenzie and Keewatin . Ottawa, 1902. Can.Dept.Int. Ann.Rep . 1900-1901, pp.98-131.

Hugh M. Raup