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THE NATIONAL RESEARCH COUNCIL OF CANADA
LE CONSEIL NATIONAL DE RECHERCHES DU CANADA
Volume 5
Volume 5
Number 1
March 1975
numero I
mars 1975
Structure of Subarctic Forests on Hummocky Permafrost Terrain in
Northwestern Canada
S. C. ZOLTAI
Environment Canada, Canadian Forestry Service, Northern Forest Research Centre,
5320-122 Street, Edmonton, Alberta T6H 3S5
Received May 27, 1974
Accepted September 3, 1974
ZourAt, S. C. 1975. Structure of subarctic forests on hummocky permafrost terrain in Northwestern Canada. Can. J. For. Res. 5, 1-9.
Examination of 30 stands in subarctic woodlands showed that most were even-aged, having
been established after fires. Most fires killed all trees in the stand, but in some instances some
trees survived, indicating light fires. The rare occurrence of uneven-aged stands shows that fire is
not necessary for the development of open spruce–lichen woodlands. Diameter growth is relatively rapid in the young, fire originated stands, but slows down after about 100 years. Continuous
heaving of the ground by frost action under the trees causes them to lean. In young, fire-originated
stands the trees generally grow upright, but most trees are leaning after the stands are over 100
years old.
ZOLTAI, S. C. 1975. Structure of subarctic forests on hummocky permafrost terrain in Northwestern Canada. Can. J. For. Res. 5, 1-9.
L'examen de 30 peuplements dans les forks subarctiques a revele que la majorite sont
equiennes et originent d'incendies. La plupart des incendies ont tue tous les arbres d'un peuplement, mais en certains cas des arbres ont survecu, temoins d'incendies moms severes. La rare
presence de peuplements inequiennes montre que le feu n'est pas indispensable au
developpement des groupements d'epinette a lichens. La croissance en diametre est relativement
rapide dans les peuplements jeunes, issus de feu, mais diminue apres 100 ans. Le soulevement
continu du sol par le gel sous les arbres fait qu'ils sont souvent penches. Dans les jeunes
peuplements originant d'incendies les arbres croissent generalement en position verticale mais la
plupart deviennent inclines lorsque les peuplements sont Ages de plus de cent ans.
[Traduit par le journal]
The subarctic region is often equated with
the forest tundra, a transitional zone extending
between the polar tree line and the boreal
forest region (Hustich 1969). Although the
definition of such a zone is difficult (Bluthgren
1969), it is generally considered to extend
north of the closed-crown forests of the boreal
region to the treeless tundra (Mikola 1969),
including both the forest tundra (open lichenwoodland) and the forest-and-tundra patches
Can. J. For. Res., 5,1 (1975)
(Rowe and Scotter 1973). In this region the
growing season is short and cool, reducing the
vegetative growth and restricting the reproduction of trees to favorable years (Mikola
1969). Permafrost is widespread in the subarctic region (Brown 1967).
Although a broad subarctic zone may be
recognized, the forest distribution is far from
uniform in this zone, rather it occurs in a
mosaic pattern (Hustich 1969). This is due to
2
CAN. J. FOR. RES. VOL. 5, 1975
soil, topography, and elevation differences, as
well as to the effects of fire which annually
burn large areas (Rowe and Scotter 1973).
In the absence of fires each area would support
a vegetation best suited for the particular site
and would therefore occupy it indefinitely.
However, fires are so frequent that they may
be responsible for perpetuating the very widespread spruce—lichen woodlands (Strang
1973a).
Field work was conducted in 1973 in the
Mackenzie River Valley to examine the relationships between intensively frost-churned
soils and vegetation. The soil, vegetation, and
stand data collected at 30 sample plots form the
basis of this paper. The age structure and distribution of trees, as influenced by permafrost
induced microtopography, are described, and
the role of fires in the establishment of subarctic
woodlands is examined.
Description of the Study Area
The study area lies in the valley of the Mackenzie
River, extending from latitude 61 °30'N to the tree
line near the Arctic Ocean (Fig. 1). Most of the
area is in the discontinuous permafrost zone (Brown
1967), with unfrozen areas on the well-drained uplands, on coarse textured soils and in water-saturated
wetlands. In the northern third of the area, in the
continuous permafrost zone, all land surfaces are
affected by permafrost.
The Mackenzie Valley is broad in the south, but
becomes narrow north of 62° Lat. between the Mackenzie and Franklin Mountains. The valley again
broadens north of Norman Wells until the sea is
reached. Local uplands occur throughout the valley;
some, like the Horn Plateau, extend some 300 m
above the plains. The land surface was modified by
the Laurentide ice sheet in late Wisconsin time
(Hughes 1969). Glacial lakes covered parts of the
valley, further modifying the surface.
The soils in the southern part of the area are
similar to those of the northern temperate regions:
leached Brunisols (Anonymous 1970) on sandy soils
and Luvisols on finer textured materials. Northward
permafrost becomes the dominant factor influencing
the soil forming process and churning by frost disrupts the soils. The common soils developed under
such conditions are the Cryosols (Anonymous 1973)
where the permafrost table is within 1 m of the
surface; these soils usually show signs of cryoturbation (Turbic Great Group). Some profiles on the
better drained soils have a thin brownish colored B
horizon (Brunisolic Subgroup), but others lack such
profile development (Regosolic Subgroup). Poorly
drained mineral soils show gleying due to water
saturation (Gleisolic Subgroup), as well as cryoturbation.
Earth hummocks develop on fine grained soils
underlain by permafrost (Zoltai and Tarnocai 1974).
These hummocks are small (40-50 cm high) permanent earth mounds, they are generally circular in
outline, with a diameter of 1-2 m. Their internal
structure shows buried organic layers in streaks and
lumps, indicating intensive mixing (Tarnocai et al.
1973). Earth hummocks (nonsorted nets of Washburn 1969) are believed to be formed by an upward
displacement of mineral soil in the center and a
concurrent downward movement at the borders under
the influence of frost action (Washburn 1969). It
has been estimated that in the Mackenzie Valley
north of the Arctic Circle over 80% of the mineral
soil surfaces have hummocks (Zoltai and Tarnocai
1974).
The climate is most severe in the north, with an
annual mean temperature of —9.6 °C at Inuvik and a
low total annual precipitation of 276 mm. At Norman
Wells the mean annual temperature is warmer
(-6.2 °C) and the total annual precipitation greater
(321 mm). Similarly, the mean daily temperature of
the warmest month, July, is 13.9 °C at Inuvik and
15.9 °C at Norman Wells. The mean annual total
growing degree-days above 5.5 °C is 560 just north
of Inuvik, but it increases to 890 in the Norman
Wells area (Burns 1973).
The vegetation reflects a climatic zonation, as
modified by local factors. In the lowlands of the
Fort Simpson area closed-crown boreal forests of
white and black spruce (Picea glauca (Moench) Voss
and P. tnariana (Mill.) B.S.P.) with a feather moss
carpet are common. Further north, near Wrigley, the
trees (mainly black spruce) become much shorter
and wider spaced, with reindeer lichen and moss
ground cover. This trend toward shorter trees and
wider spacing increases northward. Deciduous trees
are restricted to alluvial fioodplains and to sunny
slopes in this area. North of Inuvik the tree line is
reached when small stands of white spruce become
increasingly separated by treeless tundra until only
a few clumps of spruce remain in sheltered locations.
This transitional zone is about 50 km wide in the
study area, followed by the completely treeless
tundra to the north (Fig. 1).
Outliers of various forest types may occur in different vegetation zones. Thus closed-crown white
spruce forests, complete with a feather moss carpet,
are found as far north as the southern part of the
Mackenzie Delta, growing on rich alluvial soils.
Similarly tundra-like vegetation occurs on high mountains above the tree line, and subarctic spruce—lichen
woodlands may be found at higher elevations in the
southern part of the Mackenzie Valley.
Methods
Forests of different ages on hummocky terrain
were located by helicopter survey and a total of 30
sample plots were established. A soil pit was dug at
each location, extending at least 60 cm into the
permafrost, and the soil profiles were described and
sampled. The microtopography of at least two hummocks was mapped at each site. The location of all
trees on these hummocks was marked on the map,
3
ZOLTAI: STRUCTURE OF SUBARCTIC FORESTS
135°
125°
LEGEND
Uneven-aged stands
Even-aged stands
Two-aged stands
Even-aged stands with
some older trees
Recently burned
•••• Arctic tree line
Tundra with tree patches
100
200
km
125°
FIG. 1.
120°
Distribution of sample plots in the study area.
and the trees were cut 10 cm above the surface and
a disk taken. A detailed record of vegetation on
different microtopography was kept and the surface
cover by species estimated. A 50 m transect was run
and all trees over 1 rn tall within 1 m to either side
were recorded as to species, lean and position on
m icrotopography.
Laboratory analyses included soil analyses of texture and moisture content. The annual rings of tree
disks were counted under a 50x microscope.
Results
All plots were located on hummocky terrain
having severely cryoturbated soils. The permafrost table was within 60-90 cm of the surface
under the hummocks and 15-25 cm in the
inter-hummock troughs, the thicker active layer
being in the southern part of the study area.
The average height of hummocks, based on
4
CAN. J. FOR. RES. VOL. 5, 1975
TABLE
1. Stand data of plots with even-aged stands
Species composition
(%)
Common
Even-aged
South° bSb60
bS-90
bS-100
bS-70
bS-90
North wB-90
bS-100
Even-aged and younger
South bS-90
bS-85
bS-95
bS-95
North bS-70
bS-98
cbS-100
bS-100
`WS-100
Even-aged and older
North bS-100
bS-95
bS-100
Other
wB-30
tL-10
tL-10
tL-30
tL-10
wS-10
tL-10
tL-I5
tL-5
tL-5
wB-30
wB-2
-
tL-5
Average stump age
(years)
Other
Main age
ages
group
Average
diameter
at stump
height (cm)
Average
diameter
increment
(cm/decade)
18+ 2
2.2
1.19
63 + 4
68+ 5
86+ 6
83+4
92±9
154+5
3.5
6.1
6.6
6.4
6.6
6.6
0.56
0.89
0.75
0.77
0.72
0.43
4.1
5.2
4.5
4.9
6.8
4.6
5.0
5.0
5.8
1.12
0.61
0.32
0.42
1.02
0.61
0.55
0.36
0.22
4.5
3.5
4.6
0.64
0.39
0.34
51+3
88+16
156+5
157+4
74+10
83+4
98+4
143+6
268+7
65+3
88+ 2
132+11
28+1
65
121±8
79
48
47
71
123
243±7
93
111
158
°South: Norman Wells area; North: Inuvik area.
'Species abbreviations: bS-black spruce; tL-tamarack; B-white birch; wS-white spruce.
, Small treed patches in tundra.
10 measurements at each plot, was 46 cm, and
the average diameter was 149 cm.
Drainage classes ranged from imperfectly
drained to poorly drained slopes and flats. The
excessive microrclief, however, produced
poorly-drained conditions in the inter-hummock
troughs and relatively well-drained conditions
on tops of hummocks. The common soils were
cryoturbated soils with little horizon development (Regosolic Turbic Cryosol, Anonymous
1973), and soils with some color B horizon
(Brunisolic Turbic Cryosol). Of the 15 sites
clustered around Norman Wells and south, 10
were Regosolic Turbic Cryosols and five Brunisolic Turbic Cryosols. In the northern plots
clustered around Inuvik, only four were Regosolic Turbic Cryosols and 11 Brunisolic Turbic
Cryosols.
Floristic composition varied little in different
areas. Most black spruce - lichen (Picea
mariana - Cladonia) stands had a small (510% ), but constant tamarack (Larix laricina
(Du Roi) K. Koch) component, up to the
latitude of Inuvik. The same shrubs occur
through the area, although Labrador tea
(Ledum palustre L.) is present in the south as
subspecies groenlandicum (Oeder) Hult. and
decumbens (Ait.) Hult., but farther north the
small-leaved form (decumbens) replaces the
large-leaved form. On the whole, however, the
flora is quite uniform, the main difference is
that the trees are smaller and wider apart in
the north than in the south.
Age of Forest Stands and Radial Growth
Examining tree ages at stump height shows
four distinct types of stands (Table 1). In the
first group the trees are even-aged, with very
little variation in the ages. In the second, most
trees are even-aged, as in the first group, but a
few younger trees are also present. The third
group is also dominantly even-aged, but with
a few older trees. The fourth group represents
uneven-aged stands, with a wide spread of ages
(Table 2).
Trees growing on hummocky terrain have
5
ZOLTAI: STRUCTURE OF SUBARCTIC FORESTS TABLE
2. Stand data of plots with uneven-aged stands
Common
Uneven-aged
South° bS'-95
bS-95
bS-95
bS-90
bS-95
wB-90
North
bS-100
bS-100
Average
diameter
at stump
height (cm)
Average stump age
(years)
Species composition
(%)
Other
Oldest
Youngest
tL-5
tL-5
tL-5
tL-10
t L-5
wS-10
213
234
240
244
280
121
130
259
129
94
113
112
177
52
81
115
Average
diameter
increment
(cm/decade)
8.0
6.1
5.7
5.8
10.1
5.7
4.6
5.5
0.46
0.32
0.29
0.35
0.44
0.65
0.44
0.30
°South: Norman Wells area; North: Inuvik area.
'Species abbreviations: bS—black spruce; tL—tamarack; wB—white birch; wS—white spruce.
TABLE
Stand
3. Tree disposition on microrelief, average of plots
No. trees
per 50 m
transect
Even-aged
South°
North
Even-aged and younger
South
North
Even-aged and older
North
Uneven-aged
South
North
All stands
South
North
trees
trees on hummock
Top
Side
Trough
Upright
Leaning
71
18
5
14
39
51
56
35
80
18
20
82
67
24
6
6
41
62
53
32
48
26
52
74
21
6
50
44
38
62
53
19
5
14
52
59
43
27
16
30
84
70
65
21
5
10
43
56
52
34
53
28
47
72
°South: Norman Wells area ; North: Inuvik area.
small stump diameter. Although the largest
diameter on the plots was 10.4 cm, the average
diameters are much less (Tables 1, 2). Data
show that the average stump diameters in
stands over 70 years old is 5.6 cm and that
there is little deviation from this in older trees
regardless of the stand's age. This is supported
by the average stump diameter increment
(Tables 1, 2). The highest increment occurs
in the youngest stands and decreases gradually
in older stands.
Vegetation Distribution on Microrelief
The number of trees growing on 100 m2
transects is about three times greater on the
southern plots than on the northern plots near
Inuvik (Table 3). In the south the generally
younger even-aged stands contain more trees
than the older, uneven-aged stands. In the
northern plots, the average number of trees is
relatively constant.
The hummocky microrelief has a marked
effect on the distribution of trees within the
plots (Zoltai and Pettapiece 1974). On all
plots examined, far fewer trees grow on the
tops of hummocks than on the sides or in interhummock troughs (Table 3), although each
hummock component covers about the same
proportion of the ground. The proportion of
trees growing on the sides and in troughs is
about equal both in the south and in the north.
The distribution of lesser vegetation is also
influenced by the strong microrelief. Figure 2
shows the estimated cover of lower vegetation
6
CAN. J. FOR. RES. VOL. 5, 1975
90
80
70
60
50
Or
° 40
To
30
Tr
20
10
E 67 E 161
Trees
B
U-E
E 67
High
T Tr
F . 161
T Tr
U-E
[461
E-67
Low shrubs
shrubs
Tr
90 -.80
70 —
Tr _
Tr
60 —
50 —
—
7
30 -.20 —
7
Tr
Tr
Tr
T Tr
10 —
0
1.1
Tyr
F.67
E• 61
Dwarf shr bs
U-E
B
E•67
F . 161
U-
Fruticose lichens
1
B
E.67
F.161
UE
Mosses
FIG. 2. Distribution of vegetation on recently burned area (B), even-aged 67-year-old stand
(E-67), even-aged 161-year-old stand (E-161), and in an uneven-aged stand (U-E). The proportion of vegetation growing on hummock tops (T) and in inter-hummock troughs (Tr) is
shown.
on four selected plots in the Norman Wells
area. The plots, occurring on similar soils, form
a sequence: the first plot was burned in 1969,
the second is an even-aged stand with the oldest
trees at 67 years, the third is also even-aged
and the oldest trees are 161 years old, the
fourth plot has uneven-aged trees, with the
oldest tree at 227 years. The tree species is
black spruce, with about 10% tamarack in the
67- and 161-year-old stands. The high shrub
layer consists mainly of Salix spp, A lnus crispa
(Ait.) Pursh and Betula glandulosa Michx.
The lower shrubs are mainly Ledum palustre
ssp. groenlandicum and ssp. decumbens,
Spiraea beauverdiana Schneid. and Rosa acicularis Lindl. The dwarf shrubs consist mainly
of Vaccinium vitis-idaea L., V. uliginosum L.,
Empetrum nigrum L., Arctostaphylos rubra
(Rehd. & Wilson) Fern. and Rubus chamae-
tnorus L. The lichens are mainly Cladina alpestris (L.) Harm., C. mitis (Sandst.) Hale &
W. Culb., C. rangiferina (L.) Harm., Cladonia
uncialis (L.) Wigg., C. amaurocraea (Florke)
Schaer., Cetraria nivahs ( L.) Ach., and C.
cucullata (Bell) Ach. On recently burned
areas these lichen species are absent, in their
place Ciadonia cornuta (L.) Hoffm., C. gonecha
(Ach.) Asah., and C. coccifera (L.) Willd.
are common. The mosses in the troughs are
mainly Sphagnum fuscum (Schimp.) Klinggr.,
S. girgensohnii Russ., S. warnstorfii Russ.,
along with feather mosses as Dicranum undulatum Brid., Hylocomiutn splendens (Hedw.)
B.S.G. and Tomenthypnum nitens (Hewd.)
Loeske. On the tops of hummocks these feather
mosses, along with Pleurozium schreberi
(Brid.) Mitt. and Ptilium crista-castrensis
(Hedw.) De Not. are common.
ZOLTAI: STRUCTURE OF SUBARCTIC FORESTS 7
FIG. 3. A subarctic black spruce forest growing on hummocky terrain. Note the excessively
leaning trees. The lichen vegetation on hummock tops shows as light patches.
The selected plots show (Fig. 2) that shrubs
are common for the first few decades, but later
they diminish in importance. The dwarf shrubs
cover fairly constant areas in stands of various
ages, occurring mainly in the inter-hummock
troughs. Lichens arc completely absent in the
recently burned areas, but they become dominant on hummock tops only seven decades
after disturbance. Most mosses were killed by
the fire on the hummocks, but they survived in
the troughs. In the older stands mosses tend
to invade the hummocks to the detriment of
lichens.
Another striking feature of the subarctic
forests growing on hummocky terrain is the
large number of leaning trees (Fig. 3). This
phenomenon is most noticeable in uneven-aged
stands, where the leaning trees far outnumber
the upright trees both in the south and north
(Table 3). In the even-aged stands, however,
upright trees are more numerous in the younger
stands than in older stands. Thus in the south
in stands less than 85 years old 83% of the
trees are upright, but in older stands only 36%
of the trees remain upright. Likewise in the
north, in even-aged stands that are less than
75 years old 64% of the trees are upright, but
in older stands only 13% remain upright.
Discussion
The age structure of the stands suggests
that many originated after fires. The even-aged
stands with very little age spread resulted from
seed after a fire which exposed the seedbed
and released the black spruce seeds from the
cones (Lutz 1956). Similarly, those stands
which are essentially even-aged, but contain
some younger trees probably originated after a
fire; the younger trees represent regeneration
following the establishment of the main stand.
The forests consisting of even-aged trees, with
some older trees possibly originated after a fire
which did not burn the entire area. Small unburned patches consisting of a hummock or
part of a hummock are commonly seen in a
recently burned area.
Those stands composed of trees of different
ages probably did not originate after fires. These
stands, mainly black spruce with tamarack and
a single white birch — white spruce stand, show
a wide spread in ages (Table 2). With one
exception the oldest trees are very old for the
species (Fowells 1965), being over 200 years
for black spruce and 120 years for white birch.
In these stands cursory examination shows
very few seedlings, although they are not
totally absent. In a stand of great antiquity the
8
CAN. J. FOR. RES. VOL. 5, 1975
presence of a few seedlings at any given time
will assure an uneven-aged stand. Layering, a
common form of vegetative reproduction in
black spruce elsewhere (Place 1955), is apparently rare on hummocky terrain; most seedlings are of seed origin.
The surface layers of hummocky terrain are
subject to frost heaving, as shown by the frostchurned active layer of the soil (Zoltai and
Pettapiece 1974). Trees growing on such
surface are subject to tilting by the heaving,
as shown by the haphazardly leaning trees.
This effect is especially pronounced in unevenaged stands, which stood undisturbed for centuries and were subjected to frost heaving. The
older even-aged stands also show a great number of tilted trees, but the trees in the younger
stands still grow dominantly upright.
There are two reasons for this. Firstly, the
younger trees grow more vigorously than the
older ones, and therefore may stand upright
more readily. Secondly, fires usually induce
considerable thawing of the permafrost and a
thickening of the active layer by removing the
insulating organic mat (Zoltain and Pettapiece
1973). Later, as vegetation is reestablished, the
permafrost table rises, but initially it still contains less ice than before the fire, therefore the
surface will be relatively stable. As the trees
become older, their growth slows down, and
the ice accumulation and rising permafrost
table results in more severe surface movements.
Consequently, older stands (about 85 years
stump age in the south and 75 years in the
north) will contain many leaning trees.
The distribution of trees on hummocky
microrelief may be partially due to soil movements by frost heaving. Root damage may restrict trees and high shrubs to the sides of
hummocks and to inter-hummock troughs
(Zoltai and Pettapiece 1974). The moisture
regime of the hummocks is also important. The
hummocks, being elevated some 40-60 cm,
have a good external drainage. The active
layer is thickest under the hummocks (Zoltai
and Pettapiece 1973), allowing free internal
drainage under the highest parts of the hummocks. By contrast, the inter-hummock troughs
have a thin active layer, and are very wet. The
sides of hummocks are intermediate in moisture
regime between the very wet troughs and the
dry tops. In this area of low rainfall lack of
moisture on the hummock tops undoubtedly
contributes to the scarcity of trees on the
hummock tops.
The distribution of lesser vegetation also
reflects the moisture regime of the microtopography. Fruticose lichens are dominating
on the dry hummock tops and mosses in the
troughs. Forest fires may lay the hummocks
bare, but most mosses will survive in the wet
troughs. Feather mosses tend to cover the tops
of the hummocks if left undisturbed for a long
time, resulting in a complete moss cover.
Fires play a dominant role in the vegetation
succession on hummocky ground in the subarctic forest. Immediately after a fire liverworts
(Marchantia polymorpha L.) and mosses
(Polytrichum juniperinum Hedw. and Ceratodon purpureus (Hedw.) Brid.) may cover
extensive areas (Zoltai and Pettapiece 1973),
along with fireweed (Epilobium angustifolium
(L.)). Almost immediate establishment of black
spruce is indicated, provided a good supply of
seed is available. On some slopes white birch
may be established. As the stand matures, more
black spruce may seed in, especially under the
white birch. In the tundra—forest patches area
some of the small forest patches may consist
of white spruce, either as fire-originated or
undisturbed old stands. Generally, about 100
years after a fire the ground vegetation, the
size of trees and the tilting of trees give the
fire-originated black spruce — lichen forest the
same appearance as undisturbed stands.
The occurrence of some undisturbed stands
shows that not all spruce—lichen woodlands
originated after fires. These trees were established from seed, or possibly by layering in
existing woodlands, indicating that spruce—
lichen woodlands can perpetuate themselves
without disturbances and, under some circumstances, may be the climax communities.
The relatively rapid growth of young fireoriginated stands may suggest to the manager
that controlled fire might be a management
tool (Robinson 1972). The increased growth
is due to the thicker active layer and higher soil
temperatures on the burned areas. The results
of this study show, however, that such increased growth is not maintained through the
life of the fire-originated stand on hummocky
terrain. Diameter growth becomes negligible
after the trees pass the 75-85 year range
ZOLTAI: STRUCTURE OF SUBARCTIC FORESTS (stump height), because by this time the insulating organic layer is sufficiently thick to
reduce soil temperatures and raise the permafrost table.
Another possible management use of forest
fire also relates to the lowering of the permafrost table following the fires. As the permafrost table is lowered, much previously frozen
soil moisture is released and on most areas
this moisture drains away by seepage along the
permafrost table. This results in a relatively
stable surface over most areas, except in local
basins where thermokarst development may be
initiated, or on steep slopes where landslides
and mud flows may develop. In carefully selected areas burning may induce surface stability desirable from an engineering point of
view (Strang 1973b). Once again it must be
remembered that the induced condition is only
temporary and, barring further disturbances,
the original, highly mobile surface is reestablished within 100 years.
Data for this study were obtained from investigations carried out under the Environmental-Social
Program, Northern Pipelines, of the Task Force on
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9
FOWELLS, H. A. 1965. Silvics of forest trees of the United
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1973h. Studies of vegetation, landform and perma
frost in the Mackenzie Valley: Some case histories of
disturbance. Environ.-Soc. Program, N. Pipelines,
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73-14.
TARNOCAI, C., PETTAPIECE, W. W., and ZOLTAI, S. C.
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patterned ground in the Mesters Vig District, northeast Greenland. Meddelelser om Gronland, Bd.
176(4).
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C., and PETTAPIECE, W. W. 1973. Terrain,
vegetation and permafrost relationships in the northern part of the Mackenzie Valley and northern Yukon.
Environ.-Soc. Program, N. Pipelines, Task Force N.
Oil Develop., Gov. Canada, Rep. 73-4.
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earth hummocks. Arctic Alpine Res. 6,406-411.
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