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Canadian Journal of Forest Research Journal canadien de la recherche forestifte Published by Publie par 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 Northern Oil Development, Government of Canada. ANONYMOUS 1970. The system of soil classification for Canada. Can. Dep. Agric., Queen's Printer, Ottawa, Canada. 1973. Report of the working group on northern soils: Tentative classification system for Cryosolic soils. Proc. 9th Meet. Can. Soil Surv. Comm., Saskatoon, May 16-18,1973, pp. 346-358. BLUTHGREN, J. 1969. Problems of definition and geographical differentiation of the Subarctic with special regard to northern Europe. Proc. Symp. Ecol. Subarctic Regions, Helsinki, 1966, UNESCO, Ser. Ecology and Conservation No. 1, pp. 11-33. BROWN, R. J. E. 1967. Permafrost in Canada. Geol. Surv. Can. Map 1246A, first edition. BURNS, B. M. 1973. The climate of the Mackenzie Valley Beaufort Sea. Vol. 1. Environ. Canada, Atmosph. Environ., Climatological Studies No. 24. 9 FOWELLS, H. A. 1965. Silvics of forest trees of the United States. U.S. Dep. Agric., For. Serv., Agric. Handbook No. 271. HUGHES, 0. L. 1969. Surficial geology of northern Yukon Territory and northwestern District of Mackenzie, Northwest Territories. Geol. Surv. Can., Pap. 69-36. HUSTICH, I. 1969. On the study of the ecology of subarctic Regions. Proc. Symp. Ecol. Subarctic Regions, Helsinki, 1%6, UNESCO, Ser. Ecology and Conservation No. 1, pp. 235-240. LUTZ, H. J. 1956. Ecological effects of forest fires in the interior of Alaska. U.S. Dep. Agric., Tech. Bul. 1133. MIKOLA, P. 1969. Forests and forestry in subarctic regions. Proc. Symp. Ecol. Subarcitc regions, Helsinki, 1966, UNESCO, Ser. Ecology and Conservation No. 1, pp. 295-302. PLACE, I. C. M. 1955. The influence of seedbed condition in the regeneration of spruce and balsam fir. Can. Dep. N. Aff. Natl. Res., For. Branch, Bul. 117. ROBINSON, J. M. 1972. The deleterious effect of muskeg on the Canadian north. Proc. 14th Muskeg Res. Conf., Kingston, Ontario, 1971, pp. 169-184. ROWE, J. S., and SCOTTER, G. W. 1973. Fire in the boreal forest. J. Quatern. Res. 3,444-464. STRANG, R. M. 1973a. Succession in unburned subarctic woodland. Can. J. For. Res. 3,140-143. 1973h. Studies of vegetation, landform and perma frost in the Mackenzie Valley: Some case histories of disturbance. Environ.-Soc. Program, N. Pipelines, Task Force N. Oil Develop., Gov. Canada, Rep. 73-14. TARNOCAI, C., PETTAPIECE, W. W., and ZOLTAI, S. C. 1973. Report on cryoturbed soils in northern Canada. Proc. 9th Meet. Can. Soil Surv. Comm., Saskatoon, 1973, pp. 96-116. WASHBURN, A. L. 1969. Weathering, frost action, and patterned ground in the Mesters Vig District, northeast Greenland. Meddelelser om Gronland, Bd. 176(4). ZOLTAI, S. 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. 1974. Tree distribution on perennially frozen earth hummocks. Arctic Alpine Res. 6,406-411. ZOLTAI, S. C., and TARNOCAI, C. 1974. Soils and vegetation of hummocks. Environ.-Soc. Program, N. Pipelines, Task Force N. Oil Dev., Gov. Canada, Rep. 74-5.