Environment of the Yukon

Transcription

G.G.E. SCUDDER
Department of Zoology, University of British Columbia
6270 University Blvd., Vancouver, British Columbia V6T 1Z4
Abstract. The Yukon Territory, situated in the northwest corner of Canada adjacent to Alaska, and north of 60°,
has a rugged and complex physiography and geology, dominated by mountainous terrain. There are five major
watersheds, several large rivers, and many smaller, rich river valleys. The area has innumerable small ponds and
damp depressions, but few large lakes. Yukon climate is typified by long, cold winters and short, warm summers.
The Yukon has the most variable climate in North America, and across the Territory nine distinct climatic regions
can be recognized. Most of the Yukon has either permanent, discontinuous or scattered permafrost. Cryosolic soils
dominate in the northern permanent permafrost areas, whereas Brunisolic soils occur in the south where there is
discontinuous or scattered permafrost.
The dominant vegetation types in the Yukon are arctic tundra, alpine tundra, taiga or subarctic forest, boreal
forest and subalpine-shrub forest. Arctic tundra and taiga prevail in the northern part of the Territory, whereas the
boreal forest and subalpine-shrub forest characterize the southern part. Alpine tundra occurs at higher elevations
throughout the Territory. Five distinct ecozones and 23 ecoregions are now recognized in the Yukon. In addition,
scattered throughout the Territory are a number of habitats of special interest to entomologists. These include
springs, peatlands, saline flats, sand dunes, and distinctive xeric sites with Artemisia frigida-grass communities.
During the Pleistocene much of the Yukon remained ice-free and was part of Beringia, the largest refugial area in
the north during the last ice age. As such, the Yukon is a central focus of biogeography in the Nearctic region.
Résumé. L’environnement au Yukon. Le territoire du Yukon, situé dans la partie nord-ouest du Canada, juste à côté
de l’Alaska, au nord du 60e parallèle, a une géologie et une physiographie accidentées et complexes et est dominé
par les systèmes de montagnes. On y trouve cinq bassins hydrographiques principaux, plusieurs grandes rivières et
de nombreuses petites vallées de cours d’eau plus petits. Le terrain est parsemé d’innombrables petits étangs et
dépressions humides mais compte peu de grands lacs. Les hivers sont longs et froids, les étés, courts et chauds. Le
Yukon est soumis aux conditions climatiques les plus variables en Amérique du Nord et neuf régions climatiques
distinctes peuvent être reconnues. Presque toute la surface du Yukon est recouverte de pergélisol permanent,
discontinu ou clairsemé. Les cryosols sont dominants dans les zones de pergélisol permanent du nord, alors que les
brunisols dominent dans le sud où le pergélisol est discontinu ou clairsemé.
Les principaux types de végétation du Yukon sont ceux de la toundra arctique, de la toundra alpine, de la taïga,
ou des forêts subarctique, boréale ou buissonneuse subalpine. La toundra arctique et la taïga dominent dans la partie
nord du territoire, alors que la forêt boréale et la forêt buissonneuse subalpine caractérisent la partie australe. La
toundra alpine se rencontre en altitude dans tout le territoire. On reconnaît maintenant cinq écozones distinctes et
23 écorégions au Yukon. De plus, un grand nombre d’habitats d’intérêt particulier pour les entomologistes se
retrouvent un peu partout dans le territoire: sources, tourbières, plateaux salés, dunes de sable, ainsi que de
nombreuses zones xériques à communautés Artemisia frigida-herbacées. Au cours du Pléistocène, le Yukon est
resté en grande partie déglacé et formait une partie de la Béringie, la plus grande zone refuge du nord au cours des
dernières glaciations. Le Yukon est donc une zone biogéographique d’intérêt primordial dans la région néarctique.
Introduction
The Yukon Territory occupies an area of 482 681 km2 (Oswald and Senyk 1977) in
northwestern Canada (Fig. 1). It is bounded on the north by the Beaufort Sea. In the west
along 141°W longitude it abuts Alaska, and along 60°N latitude it meets British Columbia.
The eastern boundary with the Northwest Territories is an irregular line following the height
of land from about 136°30′W longitude in the north to about 124°W longitude in the south.
The Yukon is a rugged territory of mountains, rough plateaus and valleys. Most of the
southern three-fifths of the Territory is above 914 metres in elevation (Fig. 2). Only a few
major roads traverse the complex terrain (Fig. 3). However, these roads cross the Territory
from east to west, and from north to south. In the process they provide access to the major
pp. 13 – 57 in H.V. Danks and J.A. Downes (Eds.), Insects of the Yukon. Biological Survey of Canada (Terrestrial Arthropods),
Ottawa. 1034 pp. © 1997
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FIG. 1. Geographical reference map for the Yukon.
subarctic and boreal ecozones and to more than half of the distinctive ecoregions in the
Yukon. There are only about 12 major settlements (Fig. 3), with all but one of them in the
southern half of the Territory and connected by the major roads. The one exception is Old
Crow in the north, which can be reached by air.
Physiography and Geology
The physiography of the Yukon has been considered by Bostock (1948, 1961, 1970)
and Tarnocai et al. (1993). The geology of the Territory is described in detail by Gabrielse
and Yorath (1992) and summarized in map form by Clague (1989a). There are three major
physiographic regions in the Yukon (Fig. 4), which have their own distinctive geology
(Fig. 6). The Yukon Coastal Plain is part of the Arctic Coastal Plain physiographic region,
and is formed of little-deformed sediments and volcanics, mainly of Mesozoic and Cenozoic
ages. The Yukon Coastal Plain is a narrow strip of land along the Beaufort Sea, and includes
Herschel Island (Fig. 4). The plain, which decreases in elevation to the west, is largely an
erosional surface cut into Tertiary sandstone and shale, that is covered with a thin veneer of
recent sediment. The unglaciated area west of Herschel Island is an almost flat, lacustrine
plain, incised by streams and rivers, with the coastal scarp interrupted by alluvial fans and
floodplains at the mouth of the Malcolm River and Firth River (Fig. 13). The glaciated area
to the east of Herschel Island, largely covered with late Wisconsinan morainal materials, is
spotted with lakes and ponds of thermokarst origin (Rampton 1982).
The northeastern part of the Yukon, which includes the Peel Plateau, is part of the
Interior Plains physiographic region, and is formed of thick, flat-lying Phanerozoic strata.
The Peel Plateau is a striking and rather even land surface lying between the eastern front
of the Richardson Mountains and the northern front of the Mackenzie Mountains (Fig. 5).
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FIG. 2. Yukon showing territory above 914 m elevation.
In the extreme southwest corner is a broad, shallow depression, the Bonnet Plume Basin,
lying between the Peel River and the first slopes of the mountains to the south.
The majority of the Yukon lies in the Cordilleran physiographic region, a large mountain
belt of deformed and metamorphosed sedimentary and volcanic rocks (Fig. 6) of mainly
Phanerozoic and Proterozoic age. This Cordilleran region in the Yukon is divisible into
three major physiographic subdivisions (Bostock 1948) (Fig. 4). To the north, the eastern
subdivision, composed almost entirely of folded sedimentary strata, includes the British
Mountains, the Richardson Mountains and the northern part of the Mackenzie Mountains
(Fig. 5). The British Mountains (Fig. 7), which are the eastern extension of the Brooks Range
crossing northern Alaska, reach an elevation of 1680 m near the Alaska/Yukon border, and
have steep-sided and sharp-crested peaks and ridges. The Richardson Mountains (Fig. 8)
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FIG. 3. Major settlements and major roads in the Yukon.
form a belt of closely spaced hills with smooth profiles, a profile also seen in the Canyon
Ranges section of the northern Mackenzie Mountains, so named because of the numerous
canyons contained therein.
In the southwestern part of the Yukon, the western subdivision (Fig. 4), composed of
plutonic and volcanic rocks, includes the Icefield Ranges of the St. Elias Mountains and the
Kluane Ranges (Fig. 9). The Icefield Ranges comprise the main body of the St. Elias
Mountains and include many peaks over 4265 m, with Mt. Logan attaining 6050 m. This
area is mainly snow and ice, with some nunatak areas evident in summer. Many of the
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FIG. 4. Main physiographic areas of the Yukon.
glaciers reach down into the Alsek River valley, with the Kaskawulsh Glacier sending
branches down to the Slims River valley (Fig. 10). The Kluane Ranges form a rugged frontal
ridge to the north of the St. Elias Mountains, and have narrow serrated summits rising to
over 2130 m, alpine glaciers, and steep talus slopes.
Between these eastern and western physiographic systems is the interior subdivision
(Fig. 4), composed of a mixture of volcanic, sedimentary and metamorphic rocks with
intrusive outcropping. This interior system, principally comprising the Yukon Plateau
and forming the Yukon River drainage, is composed of a central series of mountains,
namely the Ogilvie, Wernecke, Selwyn and Logan Mountains, with the Pelly Mountains
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FIG. 5. Major mountain ranges, plateaus, plains and basins of the Yukon.
somewhat offset to the south (Fig. 5). North and south of these mountains is a series of
plateaus, parts of the Yukon Plateau.
The Ogilvie Mountains are 1525–1830 m in elevation, have a rugged aspect, and as a
whole, consist of long, branching ridges connecting precipitous peaks, flanked by deep
valleys. The Wernecke Mountains, which reach over 2130 m, were glaciated and contain
part of the Mackenzie-Yukon divide; the majority drain north to the Peel River (Fig. 13).
The Selwyn Mountains, with peaks up to 2515 m, run almost parallel with the
Mackenzie Mountains and are an area of high precipitation. The Logan Mountains, especially in the south, are compact and rugged, with elevations over 2000 m, and with small
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FIG. 6. Generalized map of the geology of the Yukon (after Oswald and Senyk 1977).
glaciers and ice fields. The Pelly Mountains occupy a large, wedge-shaped area in the
southern part of the Yukon Plateau, and have a high rugged backbone some 290 km long
and 65 km wide. The highest peaks are over 2000 m in elevation, and there are small alpine
glaciers. Large U-shaped valleys run deep into all ranges of the Pelly Mountains, and some,
such as the valleys of the Big Salmon and Magundy Rivers, traverse the ranges completely.
North of the Ogilvie Mountains is the extensive Porcupine Plain and Porcupine Plateau.
The Porcupine Plateau is an area of rolling, widely spaced hills, generally above 300 m, that
lies between the Old Crow and Porcupine Rivers, and contains the Old Crow Range. The
Porcupine Plain is a long, shallow depression between the Porcupine Plateau to the south
and the British and Richardson Mountains to the north. The northern part, the Bell Basin,
interrupted by a prolongation of the Keele Ranges, forms the Old Crow Plain, a large, flat
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FIG. 7. British Mountains, Firth River valley (photograph by S.G. Cannings).
FIG. 8. Richardson Mountains, “Erebia Creek” area (photograph by S.G. Cannings).
FIG. 9. Kluane Range from Outpost Plateau (photograph by S.G. Cannings).
FIG. 10. Kluane Range and Slims River valley from Sheep Mountain (photograph by R.A. Cannings).
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area thickly spotted with lakes and ponds. The southern part of the Porcupine Plain, termed
the Eagle Plain, is an extensive flat, unglaciated area, some 190 km long and 95 km wide,
now traversed by the Dempster Highway.
The Tintina Trench runs through the centre of the Yukon Plateau, arising in the Pelly
Mountains in the southeast and then extending straight towards the northwest through Ross
River, and then just south of Mayo and just north of Dawson. Throughout its length it has
areas with well defined walls, varies in width from 4.8 km – 22.5 km, and the floor ranges
from 425–823 m in elevation.
North of the Tintina Trench, the Yukon Plateau comprises the Stewart Plateau, the
Macmillan Plateau and the Pelly Plateau. The Stewart Plateau forms a series of rather flat
tablelands between the Macmillan River and the southern fronts of the Ogilvie and Wernecke
Mountains. The steep-sided, steeply cut, broad valleys through the plateau are typical of
unglaciated terrain.
The Macmillan Plateau, which extends from near Ross River to the Macmillan River,
forms a broken, mountainous area. Pleistocene ice filled the valleys, but many of the
mountainous areas between them, particularly in the northwest, stood above the glaciers as
nunataks (Bostock 1948). The Pelly Plateau is a glaciated, rolling upland, 1220 –1525 m in
elevation, with entrenched valleys that dissect the plateau.
South of the Tintina Trench and the Pelly Mountains the southern part of the Yukon
Plateau comprises the Lewes Plateau, the Nisultin Plateau and the Teslin Plateau. The Lewes
Plateau is a broad depression south of the western ranges of the Pelly Mountains. The
Nisultin Plateau lies east of Teslin Lake, while the Teslin Plateau lies west of Teslin Lake.
The Shakwak Trench runs diagonally northwest to southeast, parallel with the Tintina
Trench, and forms the northern boundary of the Kluane Ranges, and the southern boundary
of the western part of the Yukon Plateau. The northernmost section of this part of the plateau
constitutes the largely unglaciated Klondike Plateau. The latter is a gently undulating area
with a maze of deep, narrow valleys separated, by long, smooth-topped ridges and mountains, the most conspicuous forming the Dawson Range.
The more southern section, constituting the Kluane Plateau, is divided into several
distinct physiographic parts. The southeasternmost portion, east of the Takhini Valley, is
some 1675 m in elevation, with peaks 1980 –2134 m high. Northwest of the Takhini Valley,
the plateau consists of two topographic basins, the Aishihik Basin and the Wellesley Basin.
The Aishihik Basin, at 915 –1220 m, contains Aishihik Lake and has extensive areas of
smooth rolling plateau and some isolated mountainous areas up to 1830 m. The Wellesley
Basin, at 610 m, contains Wellesley Lake, which is surrounded by innumerable kettle-holes
occupied by ponds and small lakes.
Between the Aishihik and Wellesley Basins, the Yukon Plateau forms a wide upland
composed of two broad mountainous ridges trending northwest, the Ruby and Nisling
Ranges, separated by a high, shallow trough parallel with the Shakwak Trench (Bostock
1948). The Ruby and Nisling Ranges, reaching 2134 m in the former, and 1829 m in the
latter, are composed of granitic intrusions.
Glacial History
During the late Wisconsinan, most of Canada was ice covered. The continental Laurentide ice extended from its eastern centre to cover most of north-central and northeastern
North America (Mickelson et al. 1983). To the northwest it extended no farther than a line
running roughly along the eastern slopes of the Richardson Mountains and then northwest-
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erly parallel to the Yukon coast to a point near the coast southwest of Herschel Island (Ritchie
1984; Hughes et al. 1989).
The Cordilleran ice sheet, which was centred in British Columbia, in the late Wisconsinan extended south into northern Washington, Idaho and Montana (Waitt and Thorson
1983), and covered virtually all of British Columbia (Clague 1989b), much of southern
Alaska (Hamilton and Thorson 1983), and the southern Yukon (Hughes et al. 1989).
Mountain glaciers also formed at higher elevations in northern Alaska, especially along the
Brooks Range (Heusser 1983; Porter et al. 1983).
At various times during the Pleistocene, the Laurentide and Cordilleran ice sheets were
contiguous, or nearly so, along the western margin of the Great Plains, but during many of
the glacial advances, growth of the two ice sheets evidently was out of phase (Ryder 1983;
Clague 1989c). An ice-free corridor existed between the ice sheets at various times, but this
was a variable and intermittent entity that existed only in narrow windows of Quaternary
time (Schweger 1989).
Central Yukon and central Alaska remained unglaciated throughout the Pleistocene
(Hopkins 1967; Hopkins et al. 1982), as they lie in a dry belt northeast of the high St. Elias
Mountains and Alaska Range, where most of the moisture in Pacific air masses is precipitated
(Prest 1970). This unglaciated area (Fig. 11) was by far the largest refugium in the north
during the Pleistocene. Together with the emergent Bering and Chukchi seas, at times it also
formed an unbroken corridor for exchange of Palaearctic and Nearctic flora and fauna
(Matthews and Telka 1997). The palaeoecology of this ice-age Beringian refugium is
somewhat controversial, and is discussed in detail by Schweger (1997).
Bostock (1966) inferred four advances of the Cordilleran ice sheet in the southern and
central Yukon, with each successive advance being less extensive than its predecessor. Few
readily recognizable landforms remain from the first two advances, and so the limits are
poorly known. However, marginal features of the penultimate (Reid) glaciation terminating
in the Yukon Plateau some 46 600 yr B.P. are moderately well preserved, and those of the
final late Wisconsinan (McConnell) glaciation, that terminated 13 000 –14 000 yr B.P. are
very well preserved (Hughes et al. 1989). Near the limits of successive glaciations, numerous
peaks and plateaus were isolated as nunataks (Hughes et al. 1983).
Figure 12 depicts the glacial limits of ice advance in the Yukon as summarized by
Oswald and Senyk (1977). Ice covered the whole of the southern and eastern Yukon, with
lobes extending westward to the Tintina Trench, Bonnet Plume Basin, and the Arctic coastal
plain. Alpine valley glaciers, contemporary with the continental ice masses, originated
locally in the Ogilvie, Wernecke, British, and possibly, Richardson Mountains (Hughes
1972; Hughes et al. 1969).
The unglaciated portion of the Yukon consists of most of the Klondike, Porcupine and
Arctic plateaus, the Porcupine Basin and portions of the Ogilvie, Wernecke and British
Mountains, as well as the western slopes of the Richardson Mountains (Oswald and Senyk
1977). It is these unglaciated refugial areas that are central to the discussion of Yukon
biogeography, and indeed the present patterns in the fauna and flora of Canada (Scudder
1979), and the origins of the whole North American biota (Downes and Kavanaugh 1988).
Drainage Basins
The Yukon is drained by rivers that run to both the Pacific and Arctic Oceans. There
are five major watersheds, draining to the Beaufort Sea, to the Bering Sea, to the Yukon
River, the Alsek River, and the Mackenzie River (2 subsections) (Fig. 13). The largest
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FIG. 11. Wisconsinan glaciation in eastern Beringia (redrawn after Heusser 1983 and Hopkins et al. 1982).
drainage area is the Yukon River watershed comprising about 66% of the Territory and
draining via Alaska to the Bering Sea. Major tributaries include the Stewart River, the Pelly
River, the Klondike River, the Donjek River and the White River. Many large lakes lie within
the Yukon River watershed, including Teslin Lake, Tagish Lake (Fig. 14), Bennett Lake,
Kluane Lake and Lake Laberge. The Porcupine River (Fig. 15) watershed is also part of the
Yukon system. Major tributaries include the Eagle River, Bell River, Bluefish River and Old
Crow River, the latter draining the Old Crow basin.
In the south, the Alsek River watershed occupies about 4% of the Yukon and drains to
the Pacific Ocean via Alaska. Major tributaries include the Aishihik River, the Dezadeash
River, the Kaskawulsh River and the Dusty River. Aishihik Lake, Dezadeash Lake and Pine
Lake lie in the watershed.
In the southeast corner of the Territory, the Liard River watershed drains about 12% of
the Yukon. Frances Lake, Simpson Lake and Watson Lake lie in this watershed. The Liard
River is a major tributary of the Mackenzie River.
The Peel River is another tributary of the Mackenzie River and drains most of the
Wernecke Mountains, the southwestern portion of the Richardson Mountains and the
northwestern part of the Ogilvie Mountains. The Peel watershed drains about 14% of the
Yukon, including the Bonnet Plateau Basin.
The Firth River, Malwym River, Babbage River and Blow River plus a number of
smaller streams, occur on the Arctic coastal plain and drain into the Beaufort Sea. These
and other northerly flowing rivers are frequently hampered by the freezing of water at
downstream points, often resulting in the rivers overtopping their banks, inundating the
surrounding areas. In contrast, the southerly flowing rivers usually freeze in a progressively
downstream direction.
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FIG. 12. Glacial limits in the Yukon (after Oswald and Senyk 1977).
The glacial history of these major drainage basins in the Yukon has been discussed by
Lindsey et al. (1981). They point out that most basins have had a complex postglacial
drainage history. The Peel River, for example, has had a number of drainage reversals during
the Pleistocene glaciations.
Lindsey et al. (1981) have also documented the physical and biological limnology of
many of the lakes in the Yukon. They detail the chemical composition, oxygen and
temperature regimes, morphometry, zooplankton and fish fauna of these lakes. Most of the
larger lakes are located in the southern part of the Yukon; Kluane Lake is the largest, covering
409.5 km2 with a maximum known depth of 82 m. The lakes in the northern part of the
Territory are small, usually less than 5 km2.
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FIG. 13. Major rivers, lakes and drainage basins in the Yukon (redrawn after Wahl et al. 1987).
Climate
Yukon climate is the most variable in North America (Wahl et al. 1987). It is typified
by long cold winters and short warm summers, with the annual mean daily temperature below
freezing (Fig. 16). It holds the record on the North American continent for extreme low,
−62.8°C at Snag.
January is usually the coldest month (Fig. 17) and July the warmest (Fig. 18), although
the highest maximum temperatures may occur in August. The complexity of the terrain
FIG. 14. Tagish Lake, Windy Arm (photograph by R.A. Cannings).
FIG. 15. Porcupine River near Old Crow (photograph by R.A. Cannings).
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FIG. 16. Annual mean daily temperature isotherms in the Yukon (redrawn after Wahl et al. 1987).
produces complexity in the climate, with a great deal of local and regional variation. The
many closed cells in the southern half of the Territory (Figs. 17, 18) reflect the mountainous
terrain, with the flatter terrain in the north being reflected in the more sweeping isotherms.
In general, the magnitude of the temperature extremes increases from southeast to northwest.
The general circulation pattern in the north is constant, with dominant west to east
progression of pressure systems. This results in storm tracks moving from west to east across
the whole territory. In winter, a major low pressure centre prevails over the northeastern
Pacific, and offshoots of this centre move across the Bering Strait into the Beaufort Sea to
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FIG. 17. January mean daily temperature isotherms in the Yukon (redrawn after Wahl et al. 1987).
affect the Yukon, resulting in frequent storms moving along the Arctic coast. In contrast, by
January, a major cell of high pressure prevails over the central Yukon-Mackenzie Mountain
sector (Fig. 19). This tends to stabilize the weather, giving clear skies and low temperatures.
In the summer, the most persistent feature of the pressure pattern is a large high-pressure
cell at mid-latitudes (Fig. 20), giving sunny weather and warm temperatures in the south.
The southern half of the Yukon is subject to very changeable weather in the fall and winter
as a result of storms that move in from the Gulf of Alaska across the St. Elias-Coast range.
In the southwest, the prevailing water-laden clouds moving in from the Pacific leave
most of their precipitation on the windward side of the St. Elias Mountains, so that the
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FIG. 18. July mean daily temperature isotherms in the Yukon (redrawn after Wahl et al. 1987).
leeward slopes and areas in the Yukon have little precipitation (Fig. 21). This results in the
area around Carcross and Kluane having almost desert-like conditions. In the large central
area of the Yukon, the precipitation is quite variable, depending on proximity, elevation and
position of mountains. July, August or September are normally the wettest months.
Wahl et al. (1987) recognize nine distinct climatic regions in the Yukon, as shown in
Fig. 22 and outlined below.
Arctic Slope. This is a region of low annual precipitation, the total of less than 200 mm
occurring as rain or drizzle during the summer. There is very little precipitation in the winter,
so snow depths are normally below 25 cm. Winters are characteristically long, lasting from
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FIG. 19. Mean sea-level pressure isobars for January in the Yukon (redrawn after Wahl et al. 1987).
October to early June, but minima reach only from −50° to −55°C, with February the coldest
month. Summers are cool and changeable, and dependent on whether the wind is blowing
seaward or onshore. Warm spells up to 30°C are infrequent. Winds are strong.
Northern Mountains. Annual precipitation is between 300 and 400 mm, most occurring in
summer. Winter temperatures tend to be mild owing to inversion effects. Summers tend to
have cold changeable weather because of proximity to the frozen Beaufort Sea. The region
is subject to strong winds, especially in the region of Wright Pass on the Dempster Highway
and in the headwaters of the Blow River.
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FIG. 20. Mean sea-level pressure isobars for July in the Yukon (redrawn after Wahl et al. 1987).
Porcupine-Peel Basin. In this region annual precipitation is low, and in the 200 –300 mm
range. Most precipitation is from summer convective activity, although the region often is
affected by well-developed rainstorms passing through. This region has characteristic
prolonged cold spells in winter, particularly on the valley floors. Summers are short but
variable, often with quite warm spells.
Ogilvie-Mackenzie Mountains. Annual precipitation is moderate to heavy in this region,
ranging from over 700 mm in the southeast to 400–500 mm in the Ogilvie Mountains. Winter
temperatures are moderate, and summers are cool.
Central Yukon Basin. This is a region with moderate annual precipitation, ranging from
300 – 400 mm. Most precipitation occurs in summer as showers. Temperatures are quite
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FIG. 21. Annual mean total precipitation isohyets for the Yukon (redrawn after Wahl et al. 1987).
variable, and summers can be quite warm. There can be prolonged cold spells in winter,
when well-organized storm centres tend to skirt the region.
Liard Basin. In the Liard Basin, there is a moderate annual precipitation of 400 – 600 mm,
with a substantial amount in the form of snow. This region has more days with recorded
precipitation than anywhere else in the Yukon. There can be protracted cold spells in winter,
but the summers are warm.
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FIG. 22. Climatic regions of the Yukon (redrawn after Wahl et al. 1987).
Pelly-Cassiar Mountains. This region is characterized by relatively high annual precipitation, ranging from 500 –700 mm. The heaviest precipitation occurs in fall and early winter,
and coincides with the time that active storm tracks traverse the regions. As a result, winter
temperatures are less severe than elsewhere in the Yukon, and there are frequent midwinter
mild spells. Summer temperatures are relatively cool.
Upper Yukon-Stikine Basin. This region receives less than 300 mm annual precipitation,
and many localities, such as Carcross and Kluane Lake, as little as 200 mm. The temperature
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regime is typically continental, with a great deal of variability on both a daily and seasonal
basis. The proximity to the Pacific Ocean results in more frequent midwinter mild spells
than occur elsewhere in the Yukon.
St. Elias-Coast Mountains. The climate in this region is transitional between the wet
maritime climate of the windward coastal area and the dry continental climate northeast of
the massive mountains. Annual total precipitation is up to 4000 mm on the coast, but
decreases to under 300 mm in the Shakwak Trench. Precipitation is mostly in the form of
snow above 3000 m, and major snow accumulation occurs between 1500 and 3000 m. This
results in the extensive network of glaciers in the Icefield Ranges, within the St. Elias
Mountains. Valley glaciers extend downslope from accumulation zones to the 900 –1200 m
level. At low elevations, liquid precipitation occurs from mid-May to September.
Temperatures in the region vary widely as a result of the complex topography and storm
paths from the Gulf of Alaska. The mean annual temperature within the St. Elias Mountains
above 2500 m is −10°C to −15°C. The region is one of the windiest in the Yukon.
Permafrost
Permafrost is ground (soil or rock) that remains at or below 0°C over at least two
consecutive winters and an intervening summer. In general, the overall distribution of
permafrost is related to air temperature (Hughes et al. 1983).
The extent of permafrost in the Yukon has been described by Brown (1970, 1978), Judge
(1973) and French and Heginbottom (1983), and is depicted in Fig. 23. In the extreme north,
there is a zone of continuous permafrost. Here permafrost occurs everywhere beneath the
exposed land surface, being absent only below the major rivers and lakes. In this zone, the
thickness of the frozen material is about 100 m at the southern boundary, and increases
towards the north. The active layer, the upper part that freezes in winter and thaws in summer,
generally extends to permafrost.
Most of the Yukon has either discontinuous or scattered permafrost, where the permafrost is thinner and widely distributed, but not continuous beneath the land surface. The
boundary between the continuous and discontinuous permafrost zone corresponds to a
mean annual air temperature of −8.3°C (Brown 1978). To the south there is an area of
scattered permafrost, where some areas have permafrost but others are free from permafrost.
In these latter two permafrost areas, the active layer varies in thickness, and may or may not
extend to the permafrost table.
In the extreme southwest, in the St. Elias Mountains area, there is often continuous
permafrost at high elevations, but it is discontinuous at lower elevations. The lower
elevational limit of Cordilleran permafrost coincides approximately with the −1°C average
air isotherm (Hughes et al. 1983).
Soils
The soils in the Yukon have been described by Hughes et al. (1983) and mapped by
White et al. (1992). Soil development in the northern half of the Territory has been described
in detail by Tarnocai et al. (1993), and soil classification is described by the Agriculture
Canada Expert Committee on Soil Survey (1987).
Cryosolic soils are the dominant soil type in the northern permafrost zone in the Yukon
(Fig. 24), and in the Ogilvie and Richardson Mountains, as well as in the Dawson Range of
the Yukon Plateau and other scattered higher elevations. Essentially such soils coincide with
36
G.G.E. Scudder
FIG. 23. Permafrost areas of the Yukon (redrawn after Wahl et al. 1987).
very cold temperatures throughout the year. They are characteristic of frozen organic soils
or peatland terrain, where the thick organic layers insulate the frozen soil from the warm air
of the summer.
In the north, powerful freeze-thaw cycles lead to cryoturbation of the soils, the mixing
and distortion of the soil horizons. Such Turbic Cryosols are associated with patterned
ground features, such as earth hummocks, polygons and circles, that are typical of the Arctic
slope. Zoltai and Tarnocai (1974) and Zoltai et al. (1978) indicate that cryoturbation has
been active in arctic areas since deglaciation, but probably was initially absent from the taiga
37
FIG. 24. General distribution of soil types in the Yukon (after Agriculture Canada Expert Committee on Soil Survey
1987).
or subarctic forest areas to the south. However, when climate conditions became cooler about
4500 yr ago, cryoturbation became more widespread, but over the last 200 years there have
been alternating active and dormant periods of cryoturbation (Zoltai 1975).
The coarse-textured deposits along the Arctic coast are associated with Static Cryosols,
cryosols that are mineral-rich, but not cryoturbated. Static Cryosols also dominate the
Ogilvie Mountains.
Brunisolic soils, with their typical weakly developed brown coloured B horizon,
dominate the southern half of the Territory (Fig. 24). Such Brunisolic soils occur under
coniferous forest where there is moderate soil development in association with cold temperatures, coarse parent material, and high rates of evaporation.
38
G.G.E. Scudder
Most of the southern half of the Yukon has Eutric Brunisols, which are not strongly acid
and have a well developed Ah horizon. However, in the southern Selwyn Mountains and the
Logan Mountains, and other scattered areas such as the Coast Mountains and the Cassiar
Mountains, Dystric Brunisols are found, brunisols which are strongly acid but lack a
well-developed Ah horizon. Brunisolic gray luvisolic soils, with clay accumulation in the
B horizon, are confined to the southeast corner of the Territory in the Liard River basin.
These luvisolic soils develop in cool to cold soil climates, and typically occur on fine-textured glacio-lacustrine deposits.
Chernozemic soils, with well-developed, base-rich, mineral-organic surface horizons
(Ah), are grassland soils, and are not common in the Yukon. They occur on the warm, steep
south-facing slopes in the south and central Yukon, particularly along the Yukon River
system. Regosolic soils, with very weak horizon development, are also scarce, and are largely
confined to the valley bottoms of the Peel and Yukon River system.
There are extensive areas of the Yukon that lack soil development. Such rock outcrop
areas occur mainly in the Richardson, Ogilvie, Selwyn, Logan and St. Elias Mountains.
Vegetation
The vegetation of the Yukon, like the rest of northern Canada, strongly reflects
latitudinal and elevational temperature gradients (Hughes et al. 1983). The dominant
vegetation types in the Yukon are arctic tundra, alpine tundra, taiga or subarctic forest, boreal
forest and subalpine-shrub forest (Fig. 25).
Arctic Tundra. Arctic tundra, which occurs as the dominant vegetation type in the low arctic
Yukon Coastal Plain, is treeless, with a continuous vegetative cover (Fig. 26). Typical arctic
tundra plants include Eriophorum vaginatum L. (sheathed cottongrass) and Carex aquatilis
Wahlenb. (aquatic sedge) (Cyperaceae), Salix arctica Pall. (arctic willow) (Salicaceae),
Cassiope tetragona (L.) D. Don (white arctic bell-heather) (Ericaceae), Rumex arcticus
Trautv. (arctic dock) (Polygonaceae), and Dryas integrifolia M. Vahl. (arctic avens)
(Rosaceae) (Wiken et al. 1981).
In upland areas, the vegetation consists primarily of tussocks of cottongrass (Eriophorum spp.), interspersed with trailing shrubs and heaths. In low-lying depressions, sedges
and mosses are dominant. Thus, in upland areas, such as the area between Fish Creek and
Clarence Lagoon, the elevated and well drained central portion of Herschel Island (Fig. 27),
and the higher morainal areas east of Herschel Island, the vegetation consists of expanses
of tussock tundra dominated by Eriophorum vaginatum, Ledum decumbens (Ait) Lodd.
(narrow-leaved Labrador tea) and Vaccinium vitis-idaea L. ssp. minus (Ludd.) Hult. (mountain cranberry) (Ericaceae), Betula nana L. ssp. exilis (Sukatsch.) Hult. (dwarf birch)
(Betulaceae) and mosses (Wiken et al. 1981).
Lowland areas, which are dominated by low-centre frost polygons, have cores covered
by sedges and Sphagnum spp., along with Ledum decumbens, Andromeda polifolia L. (bog
rosemary) (Ericaceae), Vaccinium vitis-idaea, Salix spp., and Betula nana forming elevated
rims. High-centre polygons have tussocks of Eriophorum vaginatum in the elevated central
portions, and mostly Carex aquatilis in the ice-wedge troughs (Wiken et al. 1981).
Alpine Tundra. Alpine tundra (Fig. 28) occurs throughout the British-Richardson Mountains, the Ogilvie, Wernecke and Selwyn Mountains, the St. Elias Mountains, and scattered
at higher elevations in the interior Cordilleran region of the Yukon, especially in the Pelly
Mountains and Dawson Ranges (Fig. 4). Typical alpine tundra plants include Dryas
39
FIG. 25. Distribution of main vegetation types in the Yukon (redrawn after Wahl et al. 1987).
octopetala L. (mountain avens), Arctostaphylos alpina (L.) Spreng. (alpine bearberry)
(Ericaceae), Saxifraga caespitosa L. (tufted alpine saxifrage) and S. tricuspidata Rottb.
(prickly saxifrage) (Saxifragaceae) and crustose lichens (Wiken et al. 1981).
Characteristics and composition of the vegetation of the arctic tundra vary with
exposure, slope, soil type, moisture gradient and snow cover. Thus in the British Mountains,
lower mountain slopes and ridge tops have patches and stripes of vegetation consisting
of Dryas octopetala, along with Salix reticulata L. (net-veined willow), Saxifraga spp.
(Saxifragaceae), Antennaria spp. (Asteraceae), Silene acaulis L. (moss campion)
40
G.G.E. Scudder
FIG. 26. Arctic tundra, Philip Creek area, Babbage River (photograph by R.A. Cannings).
FIG. 27. Well drained tundra on Herschel Island (photograph by S.G. Cannings).
41
FIG. 28. Alpine tundra, Kluane Range, Outpost Mountain (photograph by S.G. Cannings).
(Caryophyllaceae), Potentilla uniflora Lebed. (one-flowered cinquefoil) (Rosaceae),
Papaver spp. (Papaveraceae) and Senecio cymbalaria Pursh (Asteraceae). On the other hand,
toe slopes have a community with Cassiope tetragona, Dryas integrifolia, Betula glandulosa
Michx. (ground birch), Salix reticulata, other Salix spp., Vaccinium uliginosum L. (bog
bilberry) (Ericaceae), Empetrum nigrum L. (black crowberry) (Empetraceae), Dryas octopetala, and Hylocomium moss (Wiken et al. 1981).
In the south Richardson Mountains, Ritchie (1982, 1984) notes that whereas uplands
have a very characteristic, sparse, low tundra dominated by patches of Salix phlebophylla
Anderss. (skeleton willow), Arctostaphylos alpina, Dryas octopetala, Oxytropis nigrescens
(Pall.) Fisch. (Fabaceae), Betula glandulosa, Artemisia norvegica Fries. ssp. saxatilis (Bess.
ex Hook.) Hall and Clem. (Asteraceae) and Hierochloe alpina (SW.) Roem. and Schult.
(alpine holy grass) (Poaceae), calcareous rocks bear a very different tundra community. This
consists of a low vegetation, dominated by Dryas integrifolia and Carex scirpoidea Michx.
Hoefs et al. (1976) similarly note that in the alpine tundra on Sheep Mountain in the
southwestern part of the Yukon, which occurs above 1500 m, the vegetation on calcareous
soils is characterized by plant communities where Dryas integrifolia is predominant.
Stanek et al. (1981) point out that in the Richardson Mountains sedge tussock tundra
occurs on a variety of terrain types, but where the terrain is hummocky, a closed ground
cover of Sphagnum, Polytrichum and Hylocomium mosses persists under widely spaced low
shrubs and dwarf willow. On steep northern exposure slopes a lichen-shrub tundra exists,
characterized by Cassiope tetragona. On moderate slopes where soil is extremely stony and
organic layers are shallow, a lichen low-shrub tundra occurs, dominated by lichens and
Betula glandulosa.
42
G.G.E. Scudder
FIG. 29. Old Crow Flats, view to north from Old Crow Hills (photograph by R.A. Cannings).
FIG. 30. Eagle Plain at Arctic Circle. Richardson Mountains in background (photograph by S.G. Cannings).
43
FIG. 31. Boreal forest around Rainbow Lake, Shakwak Trench (photograph by R.A. Cannings).
Taiga. This subarctic forest vegetation dominates the non-mountainous areas of the Porcupine Plain and Plateau, and the Interior Plains region. In the former, it covers much of the
Old Crow Basin, Old Crow Flats (Fig. 29) and Eagle Plain regions (Fig. 30) and typically
is an open woodland with very stunted stands of Picea mariana (Mill.) B.S.P. (black spruce)
and Larix laricina (Du Roi) K. Koch (tamarack or larch) (Pinaceae) with lesser quantities
of Picea glauca (Moench) Voss (white spruce), and a ground cover of Betula nana, willow
(Salix spp.), ericaceous shrubs, cottongrass (Eriophorum spp.), lichens and mosses.
In the Eagle Plain, Stanek et al. (1981) note that on gentle western or southwestern slopes
an open canopied lichen-spruce taiga occurs, with lichen dominant in the understory and the
Picea mariana characterized by a short, ragged growth form. Complex open black spruce-tall
shrub taiga is found on steep southern exposures, with Betula glandulosa, Ledum decumbens,
and Vaccinium vitis-idaea frequent. Where hummocks are present, a variety of microsites
are present in the hummocks and inter-hummock troughs, the terrain as a whole supporting
an open spruce taiga with clumps of stunted black spruce, surrounded by low ericaceous
shrubs, particularly Ledum groenlandicum Oeder and Vaccinium vitis-idaea.
In the Interior Plains region, occupying much of the Peel River Plateau and adjacent
Fort MacPherson Plain, the taiga vegetation is similar, but with conspicuous occurrence also
of Betula papyrifera Marsh (paper birch), Populus balsamifera L. (balsam poplar) (Salicaceae) and Alnus crispa (Drylander ex Ait.) Pursh (green alder) (Betulaceae).
Boreal Forest. Boreal white and black spruce ecosystems occur as the major forest type in
the southeastern part of the Yukon (Fig. 31), and occur elsewhere in the southern half of the
Territory at lower elevations from valley floors to the bottom of the subalpine forest. The
elevational limit is at 100 m in the extreme south around Sheep Mountain in Kluane National
Park (Hoefs et al. 1976), and at 800 m along the Dempster Highway in the central Yukon
(Kojima and Brooke 1986).
44
G.G.E. Scudder
This forest is dominated by Picea glauca in most late seral and climatic climax stands.
It is usually accompanied by a number of other trees that have a broad boreal distribution
across Canada, and have moved into the Yukon from the east in postglacial time (Delcourt
and Delcourt 1987). These include Picea mariana, Populus tremuloides Michx. (trembling
aspen), P. balsamifera, Larix laricina and Betula papyrifera (these important plants are
illustrated in Stanek and Orloci 1987). However, in the Yukon, two other conifers of western
origin are major components of the white and black spruce boreal forests, namely Pinus
contorta Dougl. (lodgepole pine) and Abies lasiocarpa (Hook.) Nutt. (alpine fir) (Pinaceae).
Species composition and vegetation characteristics vary according to climate, aspect,
soil type and drainage, with dense black spruce-moss communities developing in imperfectly
drained sites (Meideniger and Pojar 1991). Stanek (1980) has described the vegetation types
in the Liard Basin, noting that here Picea glauca is dominant in primary and old secondary
stands. Populus tremuloides forms secondary stands on finer soils in warm sites, while Picea
mariana and Larix laricina are characteristic in some wetlands, but Salix shrub communities
occur in others. Betula papyrifera occurs sporadically.
Bonnor and Oswald (1989) in their reconnaissance inventory of Yukon forest resources
point out that nearly all of the land below the alpine zone in the southern Yukon has been
burnt in the last 300 years, much of it several times; because Picea glauca takes at least 25
years to produce viable seed, lacks serotinous cones, and does not reproduce by layering,
this tree is sparse or virtually absent in many areas. Indeed, some tracts of land in the southern
Yukon currently support only shrubs owing to the elimination of tree species and their
propagules as a result of repeated fires at short intervals (Oswald and Brown 1990).
Pinus contorta and Populus tremuloides owe their existence for the most part to the
occurrence of fire, because they are initial invaders in recolonization. Pinus contorta
possesses serotinous cones which release lots of seeds when opened by fire and can start
reproducing in less than 10 years; it thrives best on relatively coarse, textured, moderately
well to excessively drained soils that can have a moisture deficit during the summer (Bonnor
and Oswald 1989). Populus tremuloides, on the other hand, reproduces after fire largely by
suckering, especially on finer textured, moist or somewhat alkaline sites.
Oswald and Brown (1986), in their study of the forest around Lake Laberge, describe
over 50 distinct plant communities involving Picea glauca, P. mariana, Pinus contorta and
Populus tremuloides. Douglas (1974) described the plant communities in the boreal white
and black spruce forest ecosystem in the Alsek River region in the southwest Yukon, noting
that here is a mosaic of forest, shrub and herb communities. This mosaic results from
the variable climate within the region and frequent disturbance by both fire and geomorphological (mainly fluvial) processes. These factors create twenty-one distinct community
types, which is extremely diverse for a boreal forest area.
Subalpine Forest-shrub. The subalpine zone, with a characteristic spruce-willow-birch
ecosystem (Meidinger and Pojar 1991), occurs between the boreal white and black spruce
forest and the alpine tundra zone. Krajina (1975) recognized two elevational subzones, a
lower forested subzone and an upper shrub subzone. In the former, tree cover is like that in
the boreal white and black spruce zone, but the occurrence of Betula glandulosa in forested
as well as the non-forested upper subzone indicates the subalpine habitat.
Kojima and Brooke (1986) report that the shrub subzone on the Dempster Highway
is characterized by the shrub thickets of Betula glandulosa and Salix spp. (mostly S. planifolia Pursh ssp. pulchra (Cham.) Argus). B. glandulosa is common on moderately to
well-drained habitats near and above treeline, but is gradually replaced by Salix spp.,
45
especially S. planifolia ssp. pulchra and S. glauca L., in more moist habitats near the base
of slopes or valley bottoms, where willow may completely dominate the vegetation. In places
in the subalpine zone Picea mariana occurs as Krummholtz.
Stanek et al. (1981) describe this subalpine shrub vegetation type on the gently sloping
terrain of the Eagle Plain as a dwarf shrub tundra, noting that characteristic species in addition
to B. glandulosa and S. planifolia ssp. pulchra, and irregularly scattered P. mariana, include
Arctostaphylos alpina, A. rubra (Rehd. and Wilson) Fern., Empetrum nigrum L. (Empetraceae), Ledum decumbens, Rubus chamaemorus L. (Rosaceae), Petasites frigidus (L.)
Fries. (Asteraceae), Vaccinium uliginosum and V. vitis-idaea, sedges such as Carex spp. and
Eriophorum vaginatum, the grasses Arctagrostis latifolia (R. Br.) Griseb. and Calamagrostis
canadensis (Michx.) Beauv. (Poaceae) and lichens.
In the Alsek River region, the subalpine zone, which occurs up to 1080 –1370 m, is
dominated essentially by tall (up to 3 – 4 m) shrubs, mainly Salix spp. with scattered Picea
glauca (Douglas 1974). In the Sheep Mountain area of Kluane National Park, the subalpine
zone occurs between 1100 and 1500 m (Hoefs et al. 1976), with two distinct plant
associations. One occurs on mesic sites and is dominated by Betula glandulosa, Salix glauca,
S. reticulata and Arctostaphylos rubra. The other occurs on sites which are more hygric,
with longer snow duration, and is dominated by B. glandulosa, S. glauca, S. lanata L.,
S. reticulata, Arctostaphylos rubra, Cassiope tetragona and Vaccinium uliginosum.
Notable Habitats
A number of notable habitats in the Yukon are likely to have special and distinctive
insect faunas. These habitats include springs, peatlands, saline flats, sand dunes and xeric
Artemisia frigida-grass communities.
Springs. There are a number of major springs in the Yukon (Souther and Halstead 1969).
The hotsprings at Takhini and the sulphur springs at Engineer Creek on the Dempster
Highway are just two examples. Unfortunately, there appears to be no detailed and comprehensive study of the characteristics and biota of the Yukon springs. However, one might
expect Diptera, Coleoptera, and Trichoptera to occur in such habitats, as these are typical
spring inhabitants (Danks and Williams 1991).
Peatlands. There are extensive peatlands in the Yukon (Natural Wetlands Working Group
1988; Tarnocai et al. 1995) (Fig. 32). In the Yukon coastal plain, permafrost underlies all
the low arctic wetlands at shallow depths, and the wetlands are induced by the development
of ice wedges in the ground. These result in either low-centre polygons with soil- and
peat-formed dams around shallow pools (Fig. 33), or high-centre polygons that result from
peat accumulation (Zoltai 1987).
In the subarctic alpine tundra and taiga areas, the predominant form of the peatland is
a peat plateau, wherein a perennially frozen peatland is elevated by permafrost above the
regional water table (Zoltai 1987). Zoltai (1987) notes that such peat plateaus are dry on the
surface, except for small damp depressions, and generally have vegetation that resembles
that of the surrounding uplands. However, the surrounding fens are rich in nutrients, and
support mainly sedge and moss vegetation communities. In the boreal forest, the peatlands
are mostly in the form of fens, with bogs uncommon (Zoltai 1987). Fens are waterlogged
peatlands, with water generally rich in mineral nutrients, and with pH 5.5 – 7.5. Bogs in
contrast are peat-covered wetlands, generally lacking nutrients and with acidic waters,
usually with pH , 4.5 (Zoltai 1987).
46
G.G.E. Scudder
FIG. 32. Distribution of the major peatland areas of the Yukon (after Tarnocai et al. 1995).
Although data on occurrence of insects in fens is very scanty, a number of Odonata and
Coleoptera, in particular, are restricted to bogs (Danks and Rosenberg 1987). Because
peatlands are extensive in the Yukon, it is expected that peatland-adapted species in at least
these two groups of insects will be common in the Territory.
Saline Flats. Saline ponds (Fig. 34) and salt-crust basins occur in the Haines Junction area
(Stanek 1980). The vegetation in the immediate vicinity of these salt-crust basins contains
mostly halophytes. To survive in saline waters, insects must have special osmotic and ionic
regulatory physiological adaptations (Shaw and Stobbart 1963). Coleoptera, Diptera and
Heteroptera are the predominant insect groups in saline waters (Scudder 1969, 1976; Foster
and Treherne 1976). Although inland saline ponds are few in the Yukon, some saline tolerant
species can be expected in these habitats.
47
FIG. 33. Low-centre polygon peatlands, Bluefish Basin southwest of Old Crow (photograph by R.A. Cannings).
FIG. 34. Saline ponds west of Whitehorse (photograph by S.G. Cannings).
48
G.G.E. Scudder
Sand Dunes. The main sand-dune areas in the Yukon occur in the south near Carcross and
in the Alsek River region. Douglas (1974) describes two distinct herb communities in the
dunes near the junction of the Alsek and Dezadeash Rivers. One occurs on the stabilized
sand dunes and is characterized by a continuous but sparse cover of Carex sabulosa Turcz.
ssp. leiophylla (Mack.) Pors. The other occurs on aeolian sands in the lee of the semistabilized
dune formations and is dominated by Calamagrostis sp. Douglas (1974) found this Calamagrostis community to be the richest (floristically) herb community in the Alsek River region,
with a total of 55 plant species. Included in these species are Artemisia norvegica, Thalictrum
occidentale Gray (Ranunculaceae), Elymus calderi Barkworth (= Agropyron yukonense
auct. plur. non Scribn. and Merr.), Festuca rubra L. (Poaceae), and Hierochloe hirta
(Schrank) Borbas ssp. arctica G. Weim.
This specialized habitat is poorly collected in the Beringian area, but one lepidopteran
sand-dune restricted species is known to occur in the Carcross dunes (Lafontaine and Wood
1988). The sand-dune area may also share species with the xeric Artemisia frigida L.-grass
communities.
Xeric Artemisia frigida-grass Communities. Distinctive xeric habitats with Artemisia
frigida-grass communities occur on warm and south-facing slopes in the south and central
Yukon, particularly along the Yukon River, but also in the north on the Firth River and near
Old Crow (Scudder 1993) (Fig. 35). These grassland habitats have a distinctive insect fauna
(Lafontaine and Wood 1988; Scudder 1993).
Douglas (1974) describes such an Artemisia frigida-Poa glauca Vahl community along
the Alsek and Dezadeash River, on extremely dry and well-drained sites. The community is
recognizable by an open cover of Artemisia frigida and Poa glauca, with the addition of
Elymus calderi, Chamaerhodos erecta (L.) Bunge (Rosaceae) and Festuca brachyphylla
Schultes and Schultes. There is also a sparse xerophytic cryptogamic stratum, with the
mosses Ceratodon purpurens (Hedw.) Brid. and Tortula ruralis (Hedw.) Gaertn., Meyer and
Scherb., and the lichens Physconia muscigena (Ach.) Poelt and Cornicularia aculeata
(Schreb.) Hoffm. most prominent.
Stanek (1980) reports that the pioneer community on regosols of the south-facing arid
slopes of Sheep Mountain in Kluane National Park is an Artemisia frigida-Elymus calderiCalamagrostis purpurascens community. Further, Hoefs et al. (1976) note that such
Artemisia fridiga-Elymus calderi grassland communities on Sheep Mountain develop mostly
on very fine loess aeolian (frequently still active) regosols, in both the boreal white and black
spruce as well as the subalpine spruce-willow-birch zones.
Hoefs et al. (1976) distinguish five distinct plant associations within this community,
which vary in the significance of individual species. The main association in the boreal
white and black spruce zone on Sheep Mountain is an Artemisia frigida, Elymus calderi,
Carex filifolia Nutt., Erigeron caespitosus Nutt. (Asteraceae), Penstemon gormanii Greene
(Scrophulariaceae) association. An Artemisia rupestris L., A. frigida, Elymus calderi association occurs on colluvial material at the bottom of scree slopes in both the boreal forest
and the subalpine areas between 1065 and 1370 m, and is the dominant grassland association
at these elevations. In the subalpine zone, a Carex filifolia, Artemisia furcata Bieb.,
A. frigida, Oxytropis viscida Nutt., Calamagrostis purpurascens, Potentilla nivea L., Poa
glauca association predominates.
Similar Artemisia frigida-grass communities have been described on the arid southfacing slopes of the Yukon River in Alaska by Batten et al. (1979) and Kassler (1979).
Kassler (1979) suggests that these Artemisia frigida-grass communities are remnants of an
49
FIG. 35. Known occurrence of xeric Artemisia frigida in the Yukon (redrawn after Cody 1996): major roads are also
shown.
earlier late Pleistocene arctic steppe ecosystem. Whether this arid steppe-like community
dominated Beringia in the late Pleistocene as claimed by Guthrie (1985, 1990), and is now
representative of an extinct biome (Matthews 1976), or instead is a remnant of an earlier
tundra-steppe mosaic (Ager 1982, 1983), or an upland xeric tundra (Schweger 1982), there
is no doubt that this is a unique and special habitat in the Yukon. It contains a number of
rare plant species (Batten et al. 1979), many grassland insects with southern affinities
(Scudder 1993), and Pleistocene refugial endemic insects (Anderson 1984).
G.G.E. Scudder
50
TABLE 1. The terrestrial ecozones and ecoregions of the Yukon (Ecological Stratification Working Group 1996).
Ecozone
Southern Arctic
Taiga Plain
Taiga Cordillera
Boreal Cordillera
Pacific Maritime
Ecoregion
Yukon Coastal Plain
Peel River Plateau
Fort MacPherson Plain
Muskwa Plateau
British-Richardson Mountains
Old Crow Basin
Old Crow Flats
North Ogilvie Mountains
Eagle Plains
Mackenzie Mountains
Selwyn Mountains
Klondike Plateau
St. Elias Mountains
Ruby Ranges
Yukon Plateau-Central
Yukon Plateau-North
Yukon Southern Lakes
Pelly Mountains
Yukon Stikine Highlands
Boreal Mountains and Plateaus
Liard Basin
Hyland Highland
Mount Logan
Similar relics of the xerophytic vegetation of Beringia occur in northeastern Asia
(Yurtsev 1982). It is certain that the Artemisia frigida-grass communities on warm, arid
south-facing slopes are among the most interesting and notable habitats in the Yukon.
Ecogeographic Regions, Ecozones and Ecoregions of the Yukon
The various physiographic areas of the Yukon, together with their associated climate
and vegetation, are major reference points for understanding and interpreting the distribution
of the biota. Oswald and Senyk (1977) identified 22 ecoregions in the Yukon, areas of land
where the vegetation, soils and permafrost reflect the regional climate.
In 1987, I used the Oswald and Senyk (1977) ecoregion scheme, combined with
Bostock’s (1948) physiographic divisions of the Yukon, to produce a map of 20 ecogeographic regions that could be used in the description and analysis of the distribution of
the different insect groups in the Territory. This map (Fig. 36) is reproduced herein, because
many authors in the Insects of the Yukon have referred to it in their individual chapters.
More recently, the Ecological Stratification Working Group (1996) elaborating on
earlier schemes describing terrestrial ecozones (Wiken 1986) and ecoregions (Ecoregions
Working Group 1989), and using the methodology of Ironside (1991), have formalized an
ecological framework for the whole of Canada. This procedure identifies 5 ecozones and
23 ecoregions in the Yukon (Table 1; Fig. 37).
Ecozones are areas of the earth’s surface representative of large and very generalized
ecological units characterized by interactive and adjusting abiotic and biotic factors (Ironside
1991). They have physiographic or macro-landforms, macro-climate, and broad physiognomic vegetation types. The ecozones in the Yukon, listed in Table 1 and depicted in Fig. 37,
are briefly described below. There is major road access to the Taiga Cordillera and Boreal
Cordillera ecozones, but not to the other three ecozones.
51
FIG. 36. Ecogeographic regions of the Yukon: 1. Arctic Coastal Plain; 2. British Mountains; 3. Arctic Plateau; 4.
Porcupine Plain (incl. Old Crow Plain, Old Crow Mts., N. Porcupine Plateau); 5. Richardson Mountains; 6. Eagle
Plain (incl. S. Porcupine Plateau); 7. Peel Plateau (incl. Bonnet Plume Basin); 8. Ogilvie Mountains (incl. N. and
S. Ogilvie Mountains); 9. Wernecke/Selwyn Mountains; 10. Yukon/Tintina (incl. Lewes Plateau, part of Klondike
Plateau, and Tintina Trench); 11. Eastern Plateaus (incl. Stewart, Macmillan, and Pelly Plateaus); 12. Shakwak
Trench (incl. Wellesley Basin); 13. Western Ranges (incl. Ruby, Nisling, and Dawson Ranges, part of Klondike
Plateau). 14. Pelly Mountains; 15. Logan Mountains; 16. St. Elias/Coast Mountains; 17. Southern Lakes (incl.
Aishihik Basin, Takhini Valley, Teslin Plateau, and Nisutlin Plateau); 18. Cassiar Mountains; 19. Liard Plain (incl.
Dease Plateau); 20. Hyland/Liard Plateaus.
52
G.G.E. Scudder
FIG. 37. The ecozones and ecoregions of the Yukon as recognized by the Ecological Stratification Working Group
(1996).
The Southern Arctic ecozone (A) in the Yukon coincides with the Arctic Coastal Plain
physiographic subdivision of Bostock (1948) and was glaciated during early Wisconsinan
and, to a lesser extent, during late Wisconsinan time (Oswald and Senyk 1977; Hughes et
al. 1989). It lies within the zone of permanent permafrost and has arctic tundra vegetation
dominated by sedge and cottongrass tussocks. Small to moderate-sized lakes are common
on this Coastal Plain, and deep, medium-textured morainal material is extensive along the
coast.
53
TABLE 2. Comparison of the ecogeographic regions identified in Scudder (1987), with Yukon ecoregions in
Ecological Stratification Working Group (ESWG) (1996).
ESWG (1996)
Scudder (1987)
No.
Ecogeographic region
No.
Ecoregion
1.
Arctic Coastal Plain
1.
Yukon Coastal Plain
2.
3.
British Mountains
Arctic Plateau
5.
5.
4.
Porcupine Plain
6.
7.
Old Crow Basin
Old Crow Flats
6.
Eagle Plain
9.
Eagle Plain
7.
Peel Plateau
2.
3.
Peel River Plateau
Fort MacPherson Plain
8.
Ogilvie Mountains
8.
North Ogilvie Mountains
9.
Wernecke/Selwyn Mountains
10.
10.
Mackenzie Mountains (in part)
Mackenzie Mountains (in part)
11.
Selwyn Mountains (in part)
10.
Yukon/Tintina
15.
12.
Yukon Plateau-Central
Klondike Plateau (in part)
11.
Eastern Plateaus
16.
Yukon Plateau-North
12.
Shakwak Trench
21.
12.
Liard Basin (in part)
14.
Ruby Ranges (in part)
13.
Western Ranges
12.
14.
Ruby Ranges (in part)
14.
Pelly Mountains
18.
Pelly Mountains (in part)
15.
16.
Logan Mountains
St. Elias/Coast Mountains
11.
13.
Selwyn Mountains (in part)
St. Elias Mountains
23.
Mount Logan
17.
18.
Southern Lakes
Cassiar Mountains
17.
18.
Yukon Southern Lakes
19.
Liard Plain
18.
Hyland/Liard Plateaus
21.
22.
Liard Basin (in part)
Hyland Highland
4.
Muskwa Plateau
20.
The Taiga Plain ecozone (B) only just enters along the eastern border with the Northwest
Territories. For the most part, this ecozone is a rather flat plateau dominated by tundra,
subalpine and boreal forest.
The Taiga Cordillera ecozone (C) lies within the discontinuous permafrost zone, north
of the Tintina Trench. It is underlain by sedimentary rocks to the north and metamorphic
rocks to the south. The rugged mountainous areas have predominantly an alpine tundra
vegetation, and were largely glaciated by one or more glaciations. The plateau regions have
predominately a taiga or subarctic forest vegetation.
The Boreal Cordillera ecozone (D) lies generally south of the Tintina Trench, is within
the scattered-permafrost zone, and is underlain by a complex mixture of sedimentary,
54
G.G.E. Scudder
volcanic, intrusive and metamorphic bedrock. Most of the ecozone lies within the Yukon
drainage system, and has predominantly boreal white and black spruce forest, subalpine
spruce-willow-birch, and alpine tundra at the higher elevations.
The Pacific Maritime ecozone (E) is the extensive wet ecozone of the Pacific coast that
just reaches into the southwestern corner of the Yukon. The ecozone is predominantly very
high-elevation ice fields, alpine glaciers and summit outcrops. There is no terrestrial
vegetation or soil development in this region.
Ecoregions are subdivisions of the ecozones, characterized by distinctive large order
landforms or assemblages of regional landforms, small order macro- or mesoclimates, and
vegetational assemblages. The 23 ecoregions in the Yukon listed in Table 1 and mapped in
Fig. 37 are described by the Ecological Stratification Working Group (1996). Table 2
provides comparisons of these ecoregions with the ecogeographic regions identified in
Scudder (1987).
The Yukon in Summary
The general environmental features of the Yukon show it to be a relatively cold, rugged,
and mountainous area, much of which is covered by relatively sparse or stunted vegetation,
including arctic and alpine tundra, and subalpine and open subarctic forests. Nevertheless,
the Yukon has relatively rich river valleys and substantial areas in the south with a surprising
diversity of subtypes of forest and other vegetation. Moreover, several specific insect
habitats, such as warm, south-facing slopes, have particular interest in the context of current
faunal diversity and faunal history.
During the Pleistocene much of the Yukon remained ice-free and was part of Beringia,
the largest refugial area in the north during the last ice age. As such, the Yukon is a central
focus of biogeography in the Nearctic region.
Acknowledgements
Research for this paper was supported by grants from the National Research Council of
Canada. I am indebted to D.A. Demarchi (B.C. MELP), D. Russell (Canadian Wildlife
Service, Whitehorse) and S. Smith (Agriculture and Agri-Food Canada, Whitehorse) for
providing information and maps. R.A. Cannings and S.G. Cannings kindly provided landscape photographs. I thank L. Lucas for preparing the various illustrations and text.
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