The palynological record of Late-Quaternary arctic tree-line in northwest Canada

Review of Palaeobotany and Palynology, 79 (1993): 99-111

99

Elsevier Science Publishers B.V., Amsterdam

The palynological record of Late-Quaternary arctic tree-line in northwest Canada R a y W. Spear Department of Biology, State University College, Geneseo, N Y 14454, USA (Received June 29, 1992; revised and accepted July 19, 1992)

ABSTRACT Spear, R.W., 1993. The palynological record of Late-Quaternary arctic tree-line in northwest Canada. In: G.M. MacDonald, K.D. Bennett and L.C. Cwynar (Editors), Palaeoecological Perspectives and the Vegetation of Canada - A Festsehrift in Honour of J.C. Ritchie. Rev. Palaeobot. Palynol., 79:99-1 I1. The palynological records of arctic tree-lines in North America give clear evidence of large scale northward displacement of the forest limit during the early Holocene. However, small scale or local changes in forests that occurred during the mid or late Holocene have been more difficult to detect using pollen analysis. A grid of sites from the region to the east of the MacKenzie Delta, N.W.T., provides a good temporal and spatial record of tree-line (forest) movements. Detailed pollen and macrofossil analyses at three sites, Reindeer Lake, Sleet Lake and Bluffer's Pingo, which lie 50, 75, and 100 km north of the modem forest limit, respectively, provide a detailed paleoecological record. The evidence indicates that the northward displacement of forests in the early Holocene from 10,000 to 8400 yr BP was not simply a northward shift of trees but that a complex pattern of vegetation developed with white spruce populations growing north of open poplar stands. Open woodlands with black spruce grew as far north as Sleet Lake from 8400 to 3500 yr BP. These woodlands gradually retreated to just south of Reindeer Lake during the late Holocene.

Introduction

Twenty years ago J.C. Ritchie reported the results of pioneering work on the history of northern tree-lines. In collaboration with K. Hare, Ritchie published a paper on the northward displacement of tree-line in the Tuktoyaktuk Peninsula of the MacKenzie Delta, N.W.T. (Ritchie and Hare, 1971). Their palynological record at Lake Tuk 5 and macrofossil evidence (spruce stump) from the peninsula indicated that from roughly 8500 to 5000 yr BP closed crown spruce forest stood tens of kilometers north of its modern position. They hypothesized that this northward displacement of forest was a result of the movement of the July postion of the Arctic Front 350 km to the north. One of the observations made by Ritchie and Hare (1971) was that the most satisfactory evidence of Holocene vegetation (treeline) changes in northwestern North America 0034-6667/93/$06.00

came from megafossil and paleosol studies and that pollen analysis generally provided uninformative and equivocal results. Although much detailed palynological work has been done on tree-line changes in North America over the past 20 years, Ritchie and Hare's observation is still valid. The theme of this paper is how the "equivocal" results of pollen analysis in tree-line studies can be improved, as they have been in Fennoscandia (Hyvarinen 1975, 1976; Eronen 1979), through more detailed studies both spatially and temporally, new techniques, and supplemental paleoecological evidence.

The modern forest/tundra boundary Before paleoecologists can attempt to reconstruct tree-line through time, they need to define the modern boundaries between forest and tundra in North America. Larsen (1989) defined

© 1993 - - Elsevier Science Publishers B.V. All rights reserved.

100

this boundary as the point at which unbroken forest occupies less than 75% of the land surface above the water table. The remaining 25% + of uplands are occupied by a mixture of forest and tundra. An earlier definition (Larsen, 1965) defined the "forest line" as the position where forest covers less than 50% of the soil surface. These definitions are useful for delimiting forests on the scale of most aerial photographs. Payette (1983) challenged these definitions on ecological grounds. He stated that the forest/tundra boundary begins where natural openings in the forest first occur due to forest regeneration and tree growth limitations. These sites are characterized by stunted (krummholz) trees. The terms forest limit, tree-line (tree form > 5 m), and limit of tree species (krummholz) used in this paper follow those used by Hustich (t966, 1979), Black and Bliss (1978), Spear (1983), and Payette (1983). Payette (1983) made an additional and important distinction about the forest limit. He defined the physiognomic forest limit as the point where forest cover is extensive in lowland sites and regeneration is periodic. It is distinct from the forest limit or the northern limit of forest stands where hundreds of trees may occur in favorable site surrounded by tundra. The width and latitude of the forest/tundra boundary varies across North America. In Alaska it forms a narrow band of vegetation along the south slopes of the Brooks Range just north of the arctic circle. The boundary extends northward in the northern Yukon and the MacKenzie Delta region of the N,W.T., where it nearly reaches the Beaufort Sea (69°N). Here the forest/tundra boundary is roughly 100 km wide. Moving eastward across the continent the boundary broadens to 100 to 200 km wide but retreats southward. In the Keewatin district of the N.W.T. the boundary occurs at roughly 60°N latitude. Across Northern Quebec-Labrador, forest/tundra spans a distance of several hundred kilometers between 55° and 60°N latitude. Payette (1983) presented a detailed map of the boundary showing the position of boreal forest, southern forest/tundra (forest subzone) northern forest/tundra (shrub sub-zone), tundra, and the northern limit of black spruce krummholz. This map is based on aerial photographs, airplane surveys along transects (every 30'

R.W. SPEAR

of longitude) and field surveys. Payette (1983) maintained this scale of map is needed before attempting historical reconstructions. Palynological studies, which have yet to give more than general impression of shifts in the boundary, tend to be on a larger (continental) scale.

Palynological record of Late-Quaternary tree-lines Ritchie has recently summarized palynological studies of tree-line of northwestern Canada (Ritchie, 1984) and all of Canada (Ritchie, 1987). During his years of research in the Canadian North he has provided several important insights into paleoecological research along the forest/ tundra border. His summaries allow paleocologists to identify regions of Canada where the historical record of forest/tundra is poorly known and further work is needed. His advocacy of the development of denser grids of sites to move from descriptive to quantitative studies (Ritchie, 1987) may allow more detailed reconstructions of the forest/tundra region. He has emphasized the importance of having precise species identification of tree-line species, especially of white and black spruce (Ritchie, 1984). He has also recognized the importance in understanding the temporal and spatial scale of the pollen record (Ritchie, 1974, 1987; Matthews et al., 1989). Significant short-term changes in the vegetation are difficult to decipher because the pollen record shows changes on the order of 100's or 1000's of years. Lakes with annually laminated sediments provide better temporal resolution, but are rare. The spatial resolution of most pollen records is also limited by the dispersal of pollen over long distances. Consequently, major shifts in tree-line in western Canada during the early Holocene are easier to detect in the pollen record than smaller changes that may have occurred in the late Holocene (Ritchie, 1987; Matthews et al., 1989). Because eastern Canada was deglaciated after the major early Holocene changes in climate, it lacks a clear palynological record of tree-line movement. However, Ritchie (1987) pointed out that Payette and his colleagues have been successful in reconstructing the history of Northern Quebec using tree rings, fire history and plant

THE PALYNOLOGICAL RECORD OF LATE-QUATERNARY TREE-LINE IN NW CANADA

macrofossils. All these paleoecological indicators are sensitive to local or small scale change. Careful selection of peat deposits and small basins may yield a pollen record of more local origin and be able to detect small scale change (Ritchie, 1974; Matthews et al., 1989). Bradshaw and Webb (1985), Prentice (1985), Jackson (1990) and others have been working on conceptual models of pollen source areas for small deposition basins. Their work has been in forested regions where pollen production and transport processes are different from those in treeless regions.

Recent research Tree-line research in western North America that was published after Ritchie's books or outside the region of his consideration can be summarized according to two longitudinally arranged zones: Alaska Cordilleran; and western interior (N.W.T.). Tree-line occurs in the boundary between two bioclimatic zones: the low arctic subzone; and the subarctic zone. In the Alaska Cordilleran zone, the mountains of the Brooks Range eliminate the broad transition from forest to tundra. On south slopes the northern treeline occurs abruptly at low elevations. White spruce appears to occur beyond that of black spruce. Its range is matched by that of white birch and exceeded by that of balsam poplar. Cooper (1986) reported that white spruce outliers occur beyond the altitudinal tree-line in favorable sites near the continental divide. Brubaker et al. (1983) provided a vegetation history of the Walker Lake, Upper Kobuk and Alatna River region. Their study included five sites, three of which are currently beyond tree-line. They concluded that treeline never existed beyond its current position, modern boreal forest was established 5000 years ago even though spruce was present in the Brooks Range 8000 years ago, and that white spruce did not arrive at its current limit until 1500 years ago. In this region as in all of Alaska the best record of tree growth into tundra comes from the expansion of Populus during the early Holocene (Edwards et al., 1985; Edwards and Brubaker, 1986; Anderson et al., 1988; Barnosky et al., 1987). Cwynar and Spear (1991) reported on the early

101

Holocene history of forests of white spruce (9500-6500 yr BP) and then white and black spruce (6500-5000 yr BP) extending into the valley of the Blackstone River in the Southern Ogilive Ranges (Yukon Territory), a region that is now in shrub tundra. Good temporal resolution provided by well-dated sedimentary records and detailed pollen analysis at three small ponds and the qualitative separation of the pollen of white and black spruce provide good evidence for tree-line "advance". However, the strength of their study comes from the combination of the pollen record with the record of macrofossils from poplar and spruce. In the western interior of Canada, Moser and MacDonald (1990) reported an extension of black spruce forest tundra between 5000 and 3500 yr BP near Yellowknife, N.W.T. Their interpretation is based on detailed pollen analysis of lake sediments. Lacustrine sediments provide a better regional record of vegetation change than the previous studies of paleosols and of the pollen analysis of peat deposits in the region. Northwest Canada provides the best opportunity for Holocene tree-line studies because of early deglaciation and expansion of tree-line species and the sharpness of the forest/tundra boundary. This paper uses the paleoecological records at sites near tree-line in the MacKenzie Delta to demonstrate how more-detailed analysis at closely spaced sites can improve the temporal and spatial resolution of the palynological record of Holocene tree-line. The MacKenzie Delta region revisited

Introduction The MacKenzie Delta is the region of North America with the clearest record of arctic tree-line change. As advocated by Ritchie (1987), we have been working on a dense network of sites in the region to the east of the Delta on the Tuktoyaktuk Peninsula. This paper will examine the palynological records of three of these sites, Sleet Lake (Spear, 1983; Ritchie, 1984), Reindeer Lake, and Bluffer's Pingo (informal names).

Glacial History The MacKenzie Delta region was at the far northwestern edge of the Laurentide Ice Sheet. A

102

R.W. SPEAR

Late-Wisconsinan lobe of the Laurentide ice occupied the Delta and extended westward to low elevations in the Richardson Mountains (Rampton 1982). The region was covered by ice except for possibly the distal portion of the Tuktoyaktuk Peninsula (MacKay, 1963; Hughes, 1972). The MacKenzie Delta was deglaciated early and available for plant colonization prior to 14,000 yr BP (Ritchie, 1985). Following deglaciation sea level rose 15 m relative to the modern coast. As the land rebounded sea level dropped 45 m below that of today. Sea level reached its modern position by 8000 yr BP (MacKay, 1963). Despite these fluctuations in sea level, the Beaufort Sea coast-line has moved less than 100 km in the last 14,000 yr.

Methods The study sites were chosen because of their position relative to tree-line. They lie north of the modern position of tree-line just to the east of the MacKenzie Delta (Fig. 1). Figure 1 shows that the forest/tundra forms a narrow band with as few

133 °

BEAUFORT

as 25 km between the forest limit and tree-line (the southern limit of tundra). The limit of white spruce (Picea glauca) can be as much as 50 km north of tree-line. Several patches of spruce krummholz were found on the Tuktoyaktuk Peninsula near Black Ice Lake, roughly 40 km north of tree-line (Spear, 1983). Spruce krummholz is also reported from Kittigazuit which lies 10 km north of Sleet Lake and 60 km beyond tree-line. These krummholz patches are extremely rare and of low density. Reindeer Lake is 50 km north of the forest limit and 15 km north of tree-line. Sleet Lake lies 75 km northwest of the forest limit and 50 km north of treeline. Bluffer's Pingo lies still further to the north, being less than 5 km from the coast. It is 100 km north of forest limit, 75 km north of treeline and 25 km beyond the species limit. Reindeer Lake is surrounded by dwarf shrub tundra. Dwarf Birch (Betula glandulosa), heaths, willows and alder are the commonest shrubs. Spruce growing along streams comes to within a kilometer of the site. These "trees" are less than 5 m in height and thus classified as krummholz despite their upright growth form. The lake is less

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103

THE PALYNOLOGICAL RECORD OF LATE-QUATERNARY TREE-LINE IN NW CANADA

than 5 ha in area and is relatively shallow but does not freeze to the bottom. A 278 cm core was recovered from a water depth of 2 m using a modified Livingstone Piston Core (Wright, 1967) from the ice in early spring. Sleet Lake is located in the rolling hills on the southwest portion of the Tuktoyaktuk Peninsula. It is surrounded by a dwarf shrub tundra with dwarf birch, heaths and willows. Several large alders (Alnus crispa) grow on the shores of the pond. No spruce krummholz was seen within the region of Sleet Lake. The lake is very small (< 1 ha). A 394 cm core was recovered in 6.5 m of water using a modified Livingstone piston corer from a raft. Bluffer's Pingo is located in the flat, poorly drained, patterned, peatlands of the northern 2/3 of the Tuktoyaktuk Peninsula. Vegetation is a Eriophorum/Carex tundra. The sediments of a former thermokarst lake are exposed in the eroding face of a pingo. A shallow thermokarst lake is gradually exposing the core of the pingo. A 250 cm section of sediment was cleaned using a shovel. Small samples were removed every 5 cm for pollen analysis. Bulk samples were also taken every 5 cm for macrofossil analyses. Standard laboratory procedures were used to process the pollen and macrofossil samples. The pollen samples (1 ml samples measured in a brass cylinder) were macerated following the treatment outlined by Faegri et al. (1989) and concentrated using the sieving techniques of Cwynar et al. (1979). Addition of a slurry of 1 ml of an exotic marker pollen, Eucalyptus, was used t q estimate pollen concentration (Matthews, 1969; Boaney, 1972; Peck, 1974). A minimum of 300 pollen grains make up the pollen sum at Reindeer Lake and Bluffer's Pingo. The pollen sums at Sleet Lake ranged between 500 to 1000 grains. The separation of white and black spruce pollen grains followed the qualitative methods of Hansen and Engstrom (1985) which were verified using modern spruce pollen samples from the N.W.T. and Yukon. Losson-ignition (Dean, 1974) of the sediment was used to determine the amount of sediment needed for bulk radiocarbon dates. The cores were carefully scraped to remove surface contamination and seg-

ments were sent to the radiocarbon lab at the Geological Survey of Canada. Table I provides a list of dates from the three sites. The age/depth relationship for sediments at each site are shown in Fig. 2. The procedures for macrofossil analysis and identification used at Sleet Lake follow those of Spear (1989).

Results (1) The local pollen zones for the three MacKenzie Delta sites are shown in the percentage pollen diagrams (Fig. 3). Reindeer Lake has 5 pollen zones (Fig. 3a). The first zone is characterized by high percentages of Betula (40%), Gramineae (15%), Cyperaceae (15%), Artemisia (15%), Salix (13%) and herb (2%) pollen. It spans the time interval between 15,000 and 13,500 yr BP. Sediments in Zone 2 date between 13,500 and 8600 yr BP and are dominated by pollen of Betula (dwarf birch?) (80%), Populus (12%), Cyperaceae (31%) and Juniperus (2%). The third zone occurs in sediments TABLE I Radiocarbon dates from sediment cores Laboratory No.

Depth of sediment (cra)

Radiocarbon age (yr BP__ 1 SD)

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Sediment dates were corrected for ~sC/t4C fractionation.

104

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Fig. 2. Age/depth relationships for the MacKenzie Delta sites. (a) Reindeer Lake. (b) Sleet Lake. (c) Bluffer's Pingo.

dating from 8600 to 6800 yr. BP. Spruce reaches peak percentages of 24% in this zone. Other important pollen types are Betula (60%), and Myrica (1-3%) and Juniperus (2%), Alder pollen increases to above 5% at the end of this zone. In the fourth zone, which spans the interval from 6800 to 2600 yr BP, Picea pollen percentages drop from 18 to roughly 10%, Betula pollen percentages range around 30 to 40% and alder percentages go above 60%. Zone 5 spans the last 2600 years. It is characterized by low amount of Picea pollen < 5%, roughly 24% Betula and 50% Alnus pollen and increased percentages of Ericales (2%). The pollen record at Sleet Lake (Fig. 3b) can be split into 4 local pollen zones (Spear, 1983; Ritchie, 1984). The Betula (dwarf birch) zone, Zone 1, is characterized by high percentages of Betula pollen (80%) and spans the interval from 12,500 to 10,300 yr BP. Zone 2 from 10,300 to 7000 yr BP is the Picea-Betula zone. Spruce percentages average roughly 30% and reach a maximum of 36%. Birch percentages range around 60% and Myrica percentages reach a maximum of 9%. Zone 3, 7000 to 3500 yr BP, is the Picea-Betula-Alnus zone. Spruce percentages range between 5 and 10%, birch percentages range between 30 and 40% and alder percentages range between 50 and 60%. The

most recent zone 3500 yr BP to the present, is dominated by pollen of Betula (40-60%), Alnus (40%), Gramineae (> 10%), Cyperaceae (> 10%), and Ericales (4%). The pollen influx diagram (Fig. 4) from Sleet Lake shows the same pattern of zonation. The decrease in PAR values after 4000 yr BP is a result of both sediment focusing (Davis and Ford, 1982; Davis et al., 1983) and reduced pollen production. The diagram of losson-ignition and accumulation rates of inorganic and organic matter (Fig. 5) shows a increase in the percent of organic matter due to a more rapid decrease in the accumulation of inorganic matter than organic matter. The Bluffer's Pingo pollen percentage diagram (Fig. 3c) has two local pollen zones, Zone 1, characterized by birch and Cyperaceae pollen, and Zone 2, dominated by birch and alder. Spruce percentages are roughly 10 to 15% in both zones. In Zone 1, Birch percentages reach 68%, Cyperaceae ranges between from 20 to 44%, and maximum percentages of Myrica and Salix occur. Zone 1 spans the time from 10,500 to 7000 years BP. In Zone 2 (7000 to 2000 yr BP) birch percentages are roughly 40%, alder percentages range between 18 and 23% and Gramineae reaches maximum percentages of 14%.

T H E P A L Y N O L O G I C A L R E C O R D OF L A T E - Q U A T E R N A R Y T R E E - L I N E IN N W C A N A D A

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SLEET LAKE N.W.T.

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between 8000 and 6800 yr BP. PAR values are roughly 400 during this interval. Between 6800 and 2600 yr BP spruce PAR values range between 100 and 200 and reach a maximum of 206. After 2600 yr BP spruce PAR values are less than 100. The range in spruce PAR values at Sleet Lake is much larger than at Reindeer Lake. Between 10,300 and 7000 yr BP the maximum value reached is 1627 and the average range is between 500 to 1000. Between 7000 and 3500 the PAR values drop to roughly 400. The maximum value in this interval is 689. From 3500 yr BP to the present PAR values are less than 100. The range of spruce PAR values is much smaller in Bluffer's Pingo sediment running from less than 100 to around 200 in several samples dating from 9000 yr BP.

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(3) Separation of black and white spruce at the three MacKenzie Delta sites (Fig. 7).

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(2) Spruce PAR values are plotted against time in Fig. 6. The maximum spruce PAR value (450) at Reindeer Lake occurs in sediments that date

Sleet Lake shows a different white spruce profile than the other two sites. White spruce percentages rise above 10% by 10,100 yr BP, reach 32% by 9500 yr BP and remain above 10% until 7000 yr BP. In contrast percentages of white spruce do not show an early maximum at Reindeer Lake. White spruce abundance never exceeds a few percent although it does appear to be greater pilot to 5000 yr BP. White spruce percentages do not vary greatly at Bluffer's Pingo. The black spruce pollen percentage curves are remarkably similar at Reindeer and Sleet Lakes.

THE PALYNOLOGICAL RECORD OF LATE-QUATERNARY TREE-LINE IN NW CANADA

107

PICEA POLLEN ACCUMULATION RATES

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Fig. 6. Piceapollen accumulation rates at the MacKenzieDelta sites. They show the same regional rise in black spruce pollen at 8400 yr BP. Percentages of black spruce are lower at Sleet Lake, which might be expected as it is further north. The spruce profiles at Bluffer's Pingo show lower percentages and less variation than the sites to the south. Bluffer's Pingo sediments were deposited in a shallow thermokarst lake where redeposition of sediments may reduce the resolution of the pollon record. Discussion

Reindeer Lake has pollen assemblages dominated by herbs, especially Gramineae and Artemisia, from 14,500 to roughly 13,500 yr BP. The record of herb tundra is missing at both sites on the Tuktoyaktuk Peninsula, Sleet Lake and Bluffer's Pingo, whose records date from 12,500 and 9500 yr, respectively. After 13,500 yr BP dwarf birch percentages increase at Reindeer Lake, but percentages of herbacous pollen remain higher than in the birch zone at Sleet Lake. Ritchie (1985)

interpreted similar assemblages at Twin Tamarack Lake as representing herb tundra with dwarf birch. Like Twin Tamarack Lake, Reindeer Lake has peaks in poplar (12%) and juniper (2%) pollen percentages in sediments dating between 12,000 and 9500 yr BP. Open vegetation with poplar trees and juniper scrub persisted around these sites at that time. Little poplar or juniper pollen is found in the record at Sleet Lake or Bluffer's Pingo. From 12,500 to 10,000 yr BP shrub tundra with dwarf birch dominated the vegetation around Sleet Lake. High percentages and PAR values for white spruce similar to those found in closed boreal forest (Ritchie, 1974; Anderson and Brubaker, 1986; MacDonald and Ritchie, 1986) indicate that white spruce forests expanded and covered the landscape around. Sleet Lake from 10,000 to at least 7000 yr BP. A boreal forest shrub, Myrica, is also abundant ~round Sleet Lake during this interval. For a period of 1500 yr BP, boreal forests of white spruce grew near the arctic coast while further south, south of the Eskimo Lakes, open

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Fig. 7. Pollen percentages of white and black spruce at the MacKenzie Delta sites. The pollen curves show the total spruce pollen percentages. The black bars give the percentages of black spruce pollen. The dotted line gives the approximate position of tree-line through time. The bottom scale gives the distance from modern treeline.

vegetation with polar and juniper scrub persisted. Although spruce percentages and influx values are still within the lower limits of those for boreal forest in Bluffer's Pingo sediments, the lack of stratigraphy in the spruce profile and the similarity of the pollen assemblages to those in the modern tundra suggest that forest never grew as far north as Bluffer's Pingo. Spruce macrofossils (needles and ovulate cones) from the peat deposits just to the east of the village of Tuktoyaktuk (Spear, 1983) indicate that the forest limit was at least that far north in the past. Ritchie (1984) suggested that the different vegetation histories of sites on the Tuktoyaktuk Peninsula and in the Dolomite Hills near Inuvik might have been due to substrate differences. Dwarf birch and spruce may have been slow in colonizing the calcareous soils of the Dolomite Hills. However, if Ritchie's hypothesis is correct, Reindeer Lake which is on till rather than dolomite, should have a stratigraphic record of vegetation more similar to Sleet Lake than Twin Tamarack. An alternative hypothesis is that the

late-glacial marine submergence somehow influenced vegetation development on the Tuktoyaktuk Peninsula. White spruce trees grow well on well drained, calcareous soils (Drew and Shanks, 1965) and in maritime areas (Payette, 1983). Neither of these hypotheses, however, explains why sites south of the Tuktoyaktuk Penisnula have so little spruce pollen prior to 8500 yr BP when white spruce forest supposedly grew around Sleet Lake less than 70 km to the north. Perhaps early populations of spruce were established at the mouth of the MacKenzie by long distance dispersal along the river. These small populations of white spruce could have served as a point for early Holocene population expansion. It would be very interesting to see when the white spruce populations were established in the Firth River Valley of the northern Yukon. Here white spruce grows in northerly stands near the Beaufort Sea (Drew and Shanks, 1965). The expansion of black spruce is recorded synchronously in Reindeer Lake and Sleet Lake sediments at 8400 yr BP. Black spruce was abun-

THE PALYNOLOGICAL RECORD OF LATE-QUATERNARY TREE-LINE IN NW CANADA

dant in the boreal forest as far north as Sleet Lake. The percentages and PAR values of black spruce at Sleet Lake are within the values found in boreal forests through western North America (Cwynar and Spear, 1991). Percentages and PAR values of black spruce are higher at Reindeer Lake than at Sleet. Undoubtedly boreal forests of black spruce were as common in the region around Reindeer Lake as they are today and perhaps were even more dense. In contrast the forests on the Tuktoyaktuk Peninsula near Sleet were predominantly composed of populations of white spruce. Black spruce populations may have become established around Sleet Lake but were probably not very dense. The only tree species I have found on the Tuktoyaktuk Peninsula near the species limit is white spruce. A major change in the vegetation occurred 7000 yr ago. The regional rise in alder pollen percentages and PAR values occurs at this time. This rise is generally interpreted as the arrival and expansion of alder populations in the region (Ritchie, 1984). Ritchie (1984) hypothesized that a slow cooling of climate and the cumulative effects of fire opened up the northern edge of the boreal forests to form the forest/tundra. The opening of the forest allowed alder populations to increase and because alder is a prolific pollen producer its percentages rose to 50% or more. Ritchie did acknowledge that, if the early Holocene forests were predominantly white spruce and the late Holocene forest predominantly black, then changes in edaphic factors may have been important at this time interval. In central Canada (north of Yellowknife, N.W.T.) Moser and MacDonald (1990) found that the rise in alder predates that of spruce. They rejected the idea that openings in forests allowed the expansion of alder in their region and suggest that the expansion of alder was due to the development of wetter and perhaps warmer climate. At Sleet Lake the white spruce populations undergo a serious decline at about the same time as alder populations increase. The white spruce stumps that have been found on the peninsula (Ritchie and Hare, 1971; Spear, 1983) date from this period. These data support the hypothesis of a cooler, wetter climate changing edaphic conditions by promoting peat and hummock development. The

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change in regional pollen assemblages 7000 yr ago is also seen in the Bluffer's Pingo diagram, but the pollen profile of alder is subdued, indicating that the local tundra around the site probably did not change very much. From 7000 to 3500 yr BP forest/tundra may have existed as far north as Sleet Lake. Spruce needles in Sleet Lake sediments dating from 5700 yr BP indicate that spruce was still growing in close proximity to the basin. By 3500 yr BP increases in the pollen of Ericales and herbs and the decrease in spruce pollen provide solid evidence for the development of shrub tundra around Sleet Lake. As one might expect, the record at Reindeer Lake further to the south does not show as dramatic a change. Reindeer Lake has probably always been near the forest limit at the edge of the forest/tundra. The black spruce forest was retreating to its current position during this time period. Supporting evidence for climatic change 3500 yr ago comes from earth hummocks. Earth hummocks are particularly common features in the lower MacKenzie River valley. Tarnocai and Zoltai (1978) found that the percentage of ground covered by earth hummocks increases dramatically northward. Beyond treeline 95% of the ground is covered with hummocks. They attribute this rise to an increase in fine grained soils in the north due to late-glacial marine submergence, and to lower temperatures. Radiocarbon dates indicate that most of the hummocks formed after the midHolocene, about 3500 yr ago. The record of sediment lithology at Sleet Lake changes at approximately the same time. Between 4000 to 3500 yr ago (100 to 80 cm depth) the proportion of percent organic matter in the sediments begins to increase rapidly (Fig. 5) Influx data indicate that this results from a rapid decrease in the influx of inorganic matter. Cooler temperatures after 3500 yr BP probably initiated hummock development and the accumulation of organic matter in the watershed of Sleet Lake, which reduced the erosion of inorganic matter. The PAR values for spruce pollen drop to their modern levels at the same time. This decline could be indirectly related to climatic cooling. Black and Bliss (1980) reported that success

110 of black spruce reproduction is reduced by earth hummocks. No dramatic changes in the record at Bluffer's Pingo are seen during this period. The tundra vegetation probably has not varied much within the last 9000 yr. The only dramatic evidence of potential climatic change is the formation of the Pingo itself at about 2500 yr. Conclusion

The palynological record of the regions east of the MacKenzie Delta is unique. It contains a record of an early white spruce forest 70 km north of the current physiognomic forest limit. Ritchie (1984) and Ritchie and MacDonald (1986) have wondered whether the early date of arrival of white spruce may be spurious because of the rapid rate of white spurce migration that would be needed for the species to reach the arctic coast. Although the chronology would be stronger if we had accelerator dates on macrofossils from the Sleet Lake sediments, there is nothing to indicate that the radiocarbon-dates on bulk sediments at Sleet Lake dates are incorrect. None of the dates are inverted and the sediment accumulation rates determined by the age-depth curve are relatively constant. Furthermore the dates of the rises in black spruce and in alder percentages and PAR values fit the regional patterns. The Tuktoyatuk Peninsula may have well been a point for dispersal for white spruce to the east south and west. Preliminary work at additional sites on the Tuktoyatuk Peninsula and to the south of Eskimo Lakes show similar same patterns o f vegetation development. Although climatic change is undoubtedly the underlying cause for the shifting patterns of vegetation found east of the MacKenzie Delta, other factors such as changes in sea level and substrate, and the ecology of species have had a significant impact on the vegetation history of the region. Creation of an even denser grid of paeloecological study sites to the east of the MacKenzie Delta may permit even better temporal and spatial reconstruction of the history of the forest/tundra ecotone. The empirical information collected from the MacKenzie Delta region may add not only to our understanding of the history

R.W.SPEAR of treeline in the region, but may lead to a better understanding of pollen representation and of source area for pollen in arctic regions.

Acknowledgements I completed the field and laboratory portions of this study as a Postdoctoral Fellow and Research Associate in Dr. J.C. Ritchie's laboratory at the University of Toronto. The research was supported by a Natural Sciences and Engineering Council of Canada operating grant to J.C. Ritchie. The radiocarbon laboratory of the Geological Survey of Canada under the direction of W. Blake Jr. kindly provided the radiocarbon-dates for this study. Numerous colleagues in Dr. Ritchie's lab provided support for the project. I would like to specifically thank D. Hayie, J.C. Ritchie, and J.K. MacDonald for their help with the fieldwork and K. Hadden, K. Gajewski, L.C. Cwynar, and G.M. MacDonald for the help in preparation of this manuscript.

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