Vegetation history of northcentral Alaska: A mapped summary of late-quaternary pollen data

Quaternary Science Reviews, Vol. 13, pp. 71-92, 1994. Copyright© 1994 ElsevierScience Ltd. Printed in Great Britain. All rights reserved. 0277-3791/94 $26.00

Pergamon

0277-3791(94)E0017-5

VEGETATION HISTORY OF NORTHCENTRAL ALASKA: A MAPPED SUMMARY OF LATE-QUATERNARY POLLEN DATA PATRICIA M. A N D E R S O N * and L I N D A B. B R U B A K E R t *Quaternary Research Center, AK-60, University of Washington, Seattle, WA 98195, U.S.A. "[College of Forest Resources, AR-IO, University of Washington, Seattle, WA 98195, U.S.A. Abstract - - Fossil pollen data, as illustrated by isopoll and isochrone maps, document the complex late Quaternary history of tundra and boreal forest development in northcentral Alaska. Major plant taxa behaved independently over time, resulting in substantial differences in the vegetation history of eastern and western regions. Major vegetation changes are in general agreement with GCM simulations and confirm the importance of the continental ice sheet and insolation variations in determining late Quaternary climatic trends. Herb-dominated tundra characterized the vegetation between 18 and 14 ka BP, with mesic graminoid tundra in lower elevations of western areas and more xeric, sparse tundra communities in the east and at higher elevations. Moist Betula tussock tundra rapidly replaced the western herb tundra ca. 14 ka BP. However, Betula shrubs expanded more slowly in the east, establishing relatively dry shrub tundra as the predominant regional vegetation by ca. 12 ka BP. River valleys and south-facing slopes supported Populus woodlands between 11 and 9 ka BP, but shrub tundra continued to dominate most upland sites. AInus shrubs first expanded in the southwestern Brooks Range between 10 and 9 ka BP, spreading rapidly throughout the entire region between 8 and 7 ka BP. Picea glauca populations also expanded between 10 and 9 ka BP, but from source areas in northwestern Canada. The P. glauca forests were most abundant in riparian settings, but isolated stands probably also established in the shrub tundra. P. glauca reached the central Brooks Range by ca. 8 ka BP, followed by an apparent population decline between 8 and 7 ka BP. P. mariana became the dominant tree species ca. 6 ka BP, when it invaded non-riparian P. glauca forests in eastern and central areas and moist shrub tussock tundra in the west. The modern distribution of communities in northcentral Alaska was achieved between 6 and 4 ka BP.

INTRODUCTION

QSR

dated pollen records extrapolated over large regions, presumed a simple northward movement of intact modern communities from ill-defined glacial refugia (e.g. Livingstone, 1955, 1957; Colinvaux, 1967). Such interpretations imply very simple and uniform responses of terrestrial systems to post-glacial climatic amelioration. An expanded grid of well-dated fossil pollen records, however, reveals that the major arcto-boreal taxa responded individualistically to climatic changes (Ritchie, 1987; Anderson and Brubaker, 1993) and, at times, formed communities that lacked analogs on the modern landscape (Anderson et al., 1989). Computer models that simulate climatic responses to changing external factors (i.e. boundary conditions) provide further evidence that the simple latitudinal movement of present-day vegetation zones was unlikely during the late Quaternary. General Circulation Models (GCMs), for example, demonstrate that subcontinental regions will respond differently to changes in global circulation (COHMAP, 1988), with a corresponding degree of spatial variation in simulated paleovegetation (e.g. Guetter and Kutzbach, 1990; Webb and Bartlein, 1988; Webb et al., 1987). This paper examines the development of modern boreal forest and tundra communities in one region of northern North America. The late Quaternary vegetational history

Despite many years of concentrated research in the North, scientists only now are beginning to fully comprehend the intricacies of terrestrial and atmospheric systems interactions (BOREAS, 1991). The urgency to improve the understanding of these linkages has greatly increased in response to concerns about the possible adverse impacts of global warming on boreal and tundra environments. For example, coupled atmosphere-ocean models predict warmer seasonal temperatures (6-- 12°C in summer and 2-6°C in winter) at northern high latitudes with increased levels of atmospheric CO2 (Schlesinger and Mitchell, 1987). The potential impact of CO,-induced climate change on vegetation is also pronounced, with studies suggesting that wet regions of the boreal forest will convert to cool temperate forests and dry areas to cool temperate steppe (Emanuel et al., 1985; see also D'Arrigo et al., 1987; Pastor and Post, 1988). Paleoecologists working in northern North America likewise have begun to realize the complex development of modern arctic and subarctic communities (Ritchie, 1987; Barnosky et al., 1987; MacDonald, 1987a,b; Lamb and Edwards, 1988). The earliest paleovegetation reconstructions, which relied on the results of a few poorly 71

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Quaternar)' Science Reviews: Volume 13

of northcentral Alaska is summarized through a series of pollen percentage maps based on 212 modern and 25 fossil lacustrine records. These maps provide a regional integration of individual pollen sequences and update previous overviews of the Alaskan literature (Ager, 1983: Ager and Brubaker, 1985; Barnosky et al., 1987; Lamb and Edwards, 1988; Anderson and Brubaker, 1993). We also review ecological studies and computer simulations that aid interpretations of the pollen data. The vegetational changes described for northcentral Alaska should not be used simplistically as a direct means of predicting responses of the current vegetation to future climatic changes. However, these data improve the understanding of climate-vegetation relationships at high latitudes, and thereby add important information for resolving questions of the potential consequences of global warming.

Mean January temperatures are n e a r - 2 0 ° C in the Kotzebue Sound area but decrease to less than -27°C in the eastern Intermontane region (Bowling, 1979). Summer temperatures similarly reflect moderating maritime influences, as mean July temperatures are 10°C near the coast and greater than 15°C in the interior. Mean July precipitation is greatest in the central Brooks Range (> 75 mm) and lowest in the Yukon Flats (< 25 ram). Mean January precipitation exceeds 20 mm in the central and eastern Brooks Range, whereas mean values vary between 10 and 20 mm in the western Brooks Range and Intermontane region. Permafrost and Soils

Permafrost is continuous throughout the Brooks Range proper, but its extent is variable to the south (Ferrians, 1965). Within the Intermontane system, it is generally continuous west of 153°N and discontinuous to the east. The regional distribution of permafrost is controlled by mean annual temperature (Brown, 1960), but its local occurrence is strongly affected by vegetation, topography and lithology (Brown and P6w6, 1973; Osterkamp and Payne, 1981 ). The presence of permafrost and the annual freeze-thaw cycle have a marked effect on the landscape, resulting in a number of periglacial features such as frost heaves, patterned ground, pingos, palsas and thaw lakes (Washburn, 1980). Soils within the study area are developed in a variety of parent materials, although generally limestone dominates all other lithologies (Ping, 1985). Soils in the uplands of the Intermontane region are developed in micaceous siltloams developed in loess, which ranges from less than a meter to more than 30 m deep in some areas (Viereck et al.. 1986), Soils in valleys are developed in alluvial material (sands or silts) and on glacial or eolian deposits,

THE STUDY AREA

Physiography and Climate The study area, referred to as northcentral Alaska, includes the crest and southern flanks of the Brooks Range and northeastern portions of the Yukon River drainage (Fig. 1). Its major physiographic features are the east-west trending Brooks Range in the north and a series of alternating Intermontane plateaus (Yukon-Tanana Upland. Porcupine Plateau) and lowlands (Kobuk-Selawik Lowlands, Koyukuk Flats, Kanuti Flats. Yukon Flats) to the south (Wahrhaftig, 1965). The Noatak, Kobuk and Selawik Rivers flow from the western Brooks Range into Kotzebue Sound. The central and eastern Brooks Range are drained by the Koyukuk, Chandalar and Porcupine systems, which along with the Tanana River to the south, form the major tributaries of the Yukon River. These waters ultimately drain into the Bering Sea. 165

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P.M. Anderson and L.B. Brubaker: Vegetation History of Northcentral Alaska Parent material in the Brooks Range is glacial till or local bedrock (Beikman, 1980). Chemical weathering is slow due to cold climatic conditions and the morphological development of soil profiles is slight throughout the study area. Most soils are classified as Inceptisols, Entisols or Histosols (U.S.D.A., 1979). In the Fairbanks region, well-drained soils are generally Alfic Cryochrepts on uplands and Typic Cryofluvent on alluvial sites, whereas poorly drained soils are typically Histic Cryaquepts and Histic Pergelic Cryaquepts. Soils in the central Brooks Range are predominantly Pergelic Dystric Cryochrepts, Pergelic Cryothents, Pergelic Cryoborolls, and Pergelic Cryofibrists. In the Coastal Plains, soils are poorly drained and classified as Pergelic Cryaquepts and Pergelic Histic Cryaquepts. On polygonized terrain these soils become Pergelic Ruptics indicating that the continuum of the soils on the landscape is interrupted by ice-wedges. Late Pleistocene Glacial Geology and Paleogeography Although most of northcentral Alaska remained icefree during the late Quaternary, Itkillik II (25-11.5 ka BP) glaciers covered high elevations throughout the Brooks Range (Porter et al., 1983; Hamilton, 1986). In the western Brooks Range, ice was limited to valley glaciers in the eastern Baird Mountains and a small ice cap in the Delong Mountains. Central and eastern Brooks Range glaciers were more extensive, but ice streams still terminated well within the range front (Hamilton, 1986).

73

The Itkillik glacial chronology is best known in the southcentral Brooks Range, where glaciers advanced between 25 and 24 ka BP, retreated slightly ca. 22-20 ka BP, then readvanced to their terminal zones (Hamilton, 1982, 1986). Fluctuating glacial retreat may have begun as early as 17 ka BP, but glaciers evidently readvanced in some valleys between 13 and 12.5 ka BP (Hamilton, 1982). In the southern Brooks Range, this event may have been limited to upper valleys because lower elevation moraines were revegetated by 13.5 ka BP (Brubaker et al., 1983). Between 11.8 and 11.5 ka BP, glacial ice disappeared from all but the highest elevations in the eastern Brooks Range. Shifting sea levels caused dramatic changes in the geography of Alaska during the late Quaternary (Hopkins, 1972, 1973, 1979, 1982). Seas were approximately 70-90 m lower than present during the Itkillik II maximum, ca. 18 ka BP, exposing vast areas of the Bering platform joining North America and Asia (McManus and Creager, 1984; Hopkins, 1979). Original analyses of deep sea cores suggested that the sea level rose to-55 m ca. 16 ka BP. The continents remained joined until ca. 14 ka BP, when a shallow seaway west of St. Lawrence Island breached the Bering platform. By ca. 12 ka BP (sea levels at -30 m), St. Lawrence Island was isolated from mainland Alaska, and by ca. l0 ka BP only the shallowest areas remained exposed along the Alaskan coast. Near modern shoreline was established in most areas of western Alaska by ca. 8 ka BP. Portions of this scenario, however, recently were called into question when new radiocarbon datg_s obtained

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FIG. 2. Vegetation map of northern Alaska showing the distribution of boreal forest and tundra (after Anderson and Brubaker, 1986). Tundra is subdivided into types as described in the text.

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Quaternar3' Science Reviews: Volume 13

from macrofossils preserved in the non-marine layers of cores taken from the Chukchi platform suggested that the bulk sediments originally used to date the flooding of the Land Bridge may be as much as 3800 years too old (Elias et al., 1993). MODERN VEGETATION Northcentral Alaska is dominated by boreal forest and tundra (Viereck and Little, 1975: Viereck et al., 1986; nomenclature follows Anderson, 1974 and Hutt6n, 1968) (Fig. 2). The global distribution of these biomes is related to climatic factors such as mean seasonal temperatures (Hopkins, 1959; Larsen, 1965, 1980), air masses (Bryson, 1966; Hare and Ritchie, 1972; Larsen, 1971; K6ppen, 1936), net radiation (Hare and Ritchie, 1972), potential evapotranspiration (Hare, 1950), and combined temperature-moisture-air mass indices (Tuhkanen, 1984). The relationship of vegetation and climate patterns in Alaska differs somewhat from other arctic/subarctic regions of North America. For example, the general north-south distribution of boreal forest and tundra in North America is modified in western Alaska by east-west vegetation gradients that reflect maritime influences (Hare and Ritchie, 1972; Fig. 2). Specific climate values associated with vegetation boundaries also differ between Alaska and other regions. For example, treeline corresponds to net radiation of 18-19 kilolangleys in Canada but 16 kly in Alaska (Hare and Ritchie, 1972). Canadian and Eurasian treelines parallel the July 13°C isotherm, whereas Alaskan treeline approximates the 10°C July isotherm (Hopkins, 1959). Boreal Forest

The boreal forest communities dominate the Intermontane lowlands and plateaus (Fig. 2), reaching approximately 450 m and 900 m in elevation in the western and eastern Brooks Range, respectively. The forest-tundra transition is abrupt in the central and eastern Brooks Range, where continuous Picea glauca forests extend almost to the absolute limit of tree growth. In these areas, P. glauca have upright forms with full crowns and regenerate by seed (Goidstein, 1981). Open P. glauca woodland or mixed forest-tundra form a broad transitional zone in the western Brooks Range. Here continuous forests are confined to river valleys, and tree densities decrease markedly with altitude or distance from rivers. Betula papyrifera var. humilis and Populus balsamifera are important on well drained soils in this ecotone. They also grow in small populations beyond the limits of P. glauca in the Kotzebue Sound drainage and in the northern foothills of the Brooks Range (Viereck and Little, 1975). Forest cover is nearly continuous below 760 m within the Intermontane region, although tree densities are generally lower in western than eastern areas (Viereck and Little, 1975). These forests are characterized by three conifer and three hardwood tree species that have markedly different life-history characteristics, responses to disturbance and soil requirements. The interaction of

species with variations in topography, soils and disturbance results in distinct forest mosaics (e.g. Viereck et al., 1986: Bonan and Shugart, 1989; Van Cleve and Viereck, 1981 ; Van Cleve etal., 1991). We summarize the ecological characteristics of these tree species below because an understanding of their ecologies is essential for reconstructing past vegetation patterns and for inferring processes that caused past vegetation changes. Picea mariana is the most common conifer in northcentral Alaska (Van Cleve and Dymess, 1983a, b). It grows primarily in pure stands on poorly drained clays and clay loams on north-facing upland sites and on thick organic soils in low-lying muskegs (Viereck et al., 1986; Van Cleve et al., 1991). P. mariana is particularly tolerant of cold soils and can occur where depth to permafrost is less than 25 cm (Van Cleve and Yarie, 1986; Ritchie, 1987). It has the lowest nutrient requirements of all boreal forest species (Van Cleve et a/., 1983a, b; Van Cleve and Yarie, 1986; Chapin, 1986). P. mariana has a flexible reproductive strategy as it can establish by layering and is adapted to fire by semiserotinous cones, which provide an on-site seed source for reestablishment as long as 8 years after fire (Viereck, 1983). It regenerates by seed on both mineral and organic seedbeds (Zasada, 1986; Van Cleve et al., 1991). P. mariana's ability to reproduce by layering and seeds allows it to inhabit a variety of site conditions, including recent bums (Black and Bliss, 1978, 1980), thick organic soils (Viereck et al., 1986) and deep snow patches (Payette et al., 1985). P. glauca generally grows in single-species stands or in mixtures with B. papyrifera on warm, well-drained sites. It is less tolerant of cold, poorly drained soils than is P. mariana (Van Cleve and Yarie, 1986). P. glauca is often restricted to south-facing slopes and alluvial soils (Ritchie, 1987; Viereck et al.. 1986; Van Cleve et al., 1991). It is most abundant and attains largest sizes along elevated levees and old meander banks of major rivers, especially in the Yukon Flats and along the Porcupine River (Yarie and Van Cleve, 1983). Seed germination and seedling establishment is best on mineral soils exposed after fires or floods (Zasada, 1986). P. glauca is not fire adapted and must recolonize burned sites by seeds from nearby populations (Van Cleve et al., 1991). Asexual reproduction is exceedingly rare. Tree growth, seed production and seedling establishment are favored by warm summer temperatures (Garfinkel and Brubaker, 1980; Goldstein, 1981; Jacoby and Cook, 1981; Jacoby et al., 1985: Zasada et al., 1978). Larix laricina is common only in the extreme southcentral portions of the study area, where it occurs on poorly drained sites with P. mariana (Viereck and Little, 1975; Viereck et al., 1986). Its pollen is conspicuously absent from fossil records, and we assume LarL~ was never common in the study area during the period of our study; consequently we will not consider it further in this paper. B. papyrifera grows in pure stands or in mixtures with P. glauca on light textured soils, typically on southfacing slopes and alluvial sites (Viereck et al.. 1986).

P.M. Anderson and L.B. Brubaker: Vegetation History of Northcentral Alaska Mosaics of B. papyrifera and P. glauca stands are particularly common in uplands of the eastern study area. Like other hardwood species, B. papyrifera requires warmer soils and higher nutrient regimes than both P. glauca and P. mariana (Van Cleve and Yarie, 1986). B. papyrifera regenerates vigorously after fire by stump sprouts as well as by dispersal of abundant winged seeds (Viereck and Little, 1975). P. balsamifera is common with Alnus rugosa and Salix spp. on riparian sites and active floodplains throughout the boreal forest (Walker et al., 1986; Viereck et al., 1986). In the central Brooks Range, it also occurs on steep southfacing slopes beyond the limit of P. glauca, often in association with Juniperus communis (Viereck and Little, 1975; Odasz, 1983; Lev, 1987). P. balsamifera is shade intolerant and requires moist but well drained soils. It reproduces both by abundant, wind-dispersed seeds that require mineral soil for germination and by root suckering (Viereck and Little, 1975). Root suckers are the primary mode of reproduction in clonal stands beyond treeline (Lev, 1987). Radial growth on both alluvial and treeline sites is enhanced by warm, early summer temperatures (Edwards and Dunwiddie, 1985; Lev, 1987). Populus tremuloides is most common on warm, dry upland sites (Viereck and Little, 1975). It is shade intolerant and regenerates by wind-dispersed seeds and vigorous root suckers (Viereck and Little, 1975). Large clonal populations are common within eastern and central portions of the study area. P. tremuloides regenerates abundantly after fire and, without disturbance, is typically replaced by P. glauca (Viereck et al., 1986). Tundra Alaskan tundra belongs to the low arctic biogeographical zone (Bliss, 1975; see also Rosswall and Heal, 1975; Bliss et al., 1981). Tundra communities are continuous north of ca. 68°N latitude, west of ca. 162°W longitude, at high altitudes (> ca. 900 m) and on extremely cold, poorly drained lowland soils (Fig. 2). The variable composition and distribution of Alaskan tundra has resulted in several different tundra classifications (Bliss, 1975, 1981; Murray, 1978). Five tundra types, as modified from Bliss (1981), are important to this study: low shrub heath, tall shrub, tussock shrub, graminoid, and cushion plant tundra. The distribution of these types is determined primarily by local drainage, snow cover, and temperature. However, regional zonal patterns in tundra also occur. For example, large areas of low shrub heath tundra lie adjacent to forest-tundra, and wet, graminoid tundra is common along coastlines (Bliss, 1981). Low shrub heath is the most common tundra type of northcentral Alaska occurring on the rolling hills beyond latitudinal and altitudinal treeline (Fig. 2). Low shrubs, primarily Betula nana/glandulosa and Salix spp., form an open canopy with a ground cover of Carex bigelowii, Eriophorum vaginatum, Gramineae, forbs and sub-shrubs (10-20 cm in height) composed primarily of Ericaceae (e.g. Vaccinium vitis-idaea, V. uliginosum, Empetrum

75

nigrum, Ledum palustre, Arctostaphylos rubra and Cassiope tetragona). Tall shrub tundra is usually restricted to well drained sites bordering rivers and streams, although it is also found on sheltered slopes that retain spring snow cover (Fig. 2). It is characterized by dense thickets of B. glandulosa, Alnus crispa and Salix spp. These communities can be succeeded by low shrub or tussock shrub tundra with increased plant cover and shallow active layers (Bliss and Cantlon, 1957). Shrub tussock tundra is found on upland soils of intermediate drainage and may be locally abundant, especially in far northern and western Alaska (Fig. 2). It also occurs in shallow basins associated with drained thermokarst lakes. Cyperaceae (particularly Carex bigelowii, C. lugens, and Eriophorum vaginatum) commonly form tussocks with several low shrubs (ericads and Betula) growing on tussock sides. Arctagrostis latifolia and forbs are sometimes important community members. Sphagnum is locally abundant in poorly drained areas.

Graminoid tundra is dominated by Cyperaceae (e.g.

Carex rariflora, C. ramenskii, C. mackenziei) and Gramineae (e.g. Calamagrostis deschampsioides, Elymus arenanius, Poa eminens), sometimes in combination with low shrubs such as Empetrum nigrum, Salix ovalifolia and S. fuscescens (Fig. 2). This is the wettest tundra type in Alaska and extends along much of the coastlines of southwestern and northern Alaska. Cushion plant communities are locally restricted to dry windswept hill tops. Dryas octopetala and D. integrifolia dominate with graminoids and Salix arctica. Plant cover at these sites is typically low, with much barren ground. This community type is most abundant in the Brooks Range. Two Alnus species occur in the study area. Alnus crispa is shade intolerant and prefers disturbed mesic sites, where it may be favored by its ability to fix nitrogen (Viereck and Little, 1975). In the Brooks Range, A. crispa thickets fill mountain draws and form broad bands between treeline and alpine tundra. A. crispa reproduces vegetatively from stem sprouts and sexually by abundant winged seeds. The northern range of A. crispa lies 200 km beyond Picea treeline. A. rugosa is a lowland shrub most often found on seasonally flooded alluvial soils (Viereck and Little, 1975). Its reproduction is similar to that ofA. crispa. It is rapidly replaced by P. balsamifera in alluvial successional sequences (Walker et al., 1986). B. glandulosa is the most common shrub Betula in northcentral Alaska. It occurs in openings of both P. glauca and P. mariana forests and is an important member of most shrub tundra communities. It is most abundant on moderately drained soils, although it is also an important component of tussock communities. Reproduction is by seeds and stem sprouts. B. glandulosa readily hybridizes with B. papyrifera, and hybrids ranging from shrub to tree sizes are common in many areas. Salix species are an important component of tundra communities throughout the study area. Their growth forms and site requirements vary greatly among species.

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Tall Salix, such as S. alexensis and S. glauca, torm dense thickets along streams, lake shores and locally moist sites. Smaller species, such as S. fuscescens, are common in shrub tussock tundra. Numerous mat-forming species, including S. rotundifolia, S. phlebophylla and S. chamissonis, are common on extreme microsites. Juniperus communis occurs at scattered locations on rocky, south-facing slopes in both tundra and boreal forest (Viereck and Little, 1975). Its modern abundance is rare compared to other shrub taxa, however, and its occurrence in the fossil record does not show regional patterns. We, therefore, do not address its history.

COMPUTER SIMULATIONS: AIDS FOR INTERPRETING FOSSIL POLLEN RECORDS Fossil data describe the temporal and spatial variations in past environments, but the causative mechanisms driving the observed changes cannot be directly inferred. In contrast, computer simulations provide physically consistent responses of natural systems to changes in boundary controls and are powerful tools for understanding why past environments may have changed in specific ways (Webb and Bartlein, 1988: Bartlein and Prentice, 1989). However, the models are also limited, because they cannot evaluate if these possible environments actually existed. Consequently, many scientists are applying approaches that combine computer simulations and field data to gain a more complete knowledge of past and present climatic and ecological systems. Applications differ between GCMs and vegetation models. The emphasis in the latter has been the examination of possible responses of modern ecosystems to proposed global wanning. GCMs have also been used to address future climatic scenarios. However, many studies have focused on the comparisons of the simulated paleoclimates to trends inferred from the fossil records to assess the accuracy of the models and the interpretations of the paleo-data (e.g. Kutzbach and Wright. 1985: Kutzbach and Street-Perrot, 1985; Spaulding and Graumlich, 1986; Webb et al., 1987; Barnosky e t a / . , 1987: COHMAP, 1988; Harrison et al., 1992). The emphasis in this paper, of course, is with historical questions, and the relevancy of the GCMs to paleoclimatic interpretations is clear. Simulations of Alaskan forest dynamics, while not directly applicable to our work, also hold great potential for interpreting the fossil pollen records. Thus we include a brief review of results from both types of models. General Circulation Models

General circulation models describe the effects of large-scale boundary conditions (e.g. insolation, extent and volume of continental ice sheets, extent of sea ice, sea surface temperatures, atmospheric composition) on global and regional climates. The National Center for Atmospheric Research Community Climate model (NCAR CCM, in particular CCM0; Kutzbach and Guetter, 1986) is the most commonly used model for comparisons

to fossil data with simulations of global climates available at 3000 year intervals over the past 18 ka BP (COHMAP, 1988). Other models (e.g. UKMO Mitchell et al., 1988; LRMO Kutzbach and Gallimore, 1988; Kutzbach et al., 1991 ) provide additional information about the role of sea ice, sea surface temperatures and snow cover on regional climates. Bartlein et al. (1991) and Barnosky et al. (1987) described the late Quaternary climates of Alaska from the GCM simulations. The computer models indicated that net insolation and the size of the North American ice sheet were the primary mechanisms determining the late Quaternary climates of Alaska. The ice sheet exerted its greatest influence in the 18 ka BP models, where cold, dry climates were simulated. With the eastward retreat of the continental ice, variation in summer insolation seems to be the primary influence on Alaskan climates, although feedbacks related to sea ice extent and sea surface temperatures probably were also important (Bartlein et al., 1991). High global ice volume and relatively low summer insolation, atmospheric CO 2 and sea levels resulted in a cold continental climate in Alaska ca. 18 ka BP. A 'glacial' anticyclone was centered over the large North American ice sheet, the Aleutian low was intensified, and two branches of the jet stream flowed to the north and south of the ice sheet. Circulation anomalies generated stronger than present southeasterly January winds into Alaska and stronger southwesterly July winds. Advective warming, caused by winds descending from the ice sheet, somewhat mitigated the general cooling effects with eastern Alaska experiencing more moderate winters than western Alaska. Simulations, however, indicate that summers in eastern Alaska were cooler and drier than in the west. The extent of these effects, however, is strongly determined by the configuration and volume of the continental ice. By 12 ka BP, the ice sheet retreated eastward, the intensity of the Aleutian low diminished and the jet stream rejoined. Southwesterly winds prevailed across Alaska. Although net summer insolation approached its maximum at this time, summer temperatures rose less sharply due to the effects of a still cool North Pacific Ocean. Thus, seasonal conditions during the transition from full- to interglacial climates were warmer than previous but probably still cooler than present. The North American ice sheet was sufficiently reduced to exert little influence on the Alaskan climate by 9 ka BP. Net insolation was greater than today, and simulated summers were warmer than present. The temperature increase in Alaska, however, was not as great as northwestern Canada, where southwesterly winds associated with the remnant ice sheet enhanced the effects of the high summer insolation. Models with interactive ocean, sea ice and snow cover indicate that the greater summer warmth influenced fall and winter conditions by delaying the formation and limiting the extent of winter sea ice (Kutzbach and Gallimore, 1988). Feedbacks associated with sea surface temperature, sea ice and snow cover resulted in warmer winters than present, even though winter insolation was lower than today. Between 9 ka BP

P.M. Anderson and L.B. Brubaker: Vegetation History of Northcentral Alaska

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Quaternar3' Science Reviews: Volume 13

and present, summer insolation gradually decreased. Simulations suggest a general trend toward cooler, wetter summers.

Models of Boreal Forest Dynamics Stand simulation models increase our understanding of environmental factors and ecological processes controlling forest dynamics (e.g. Shugart, 1984; Botkin et al., 1972; Bonan, 1989a; Bonan and Korzuhin, 1989). Even though boreal forests have few tree species compared to other ecosystems, boreal forest models are relatively complex, because they must include feedbacks between vegetation, soil and the atmosphere to successfully simulate the observed forest behavior (Bonan et al., 1990; Bonan, 1989a, b; Pastor and Post, 1981). Furthermore, experiments which consider warmer and wetter than present climates (Bonan et al., 1990) emphasize that the Alaskan forests probably respond more strongly to combined effects of several variables (e.g. precipitation-evaporation) than to any single climatic variable (e.g. air temperature). Although boreal forest models have not been explicitly joined with paleovegetational data as in other regions (e.g. Solomon et al., 1981; Solomon and Shugart, 1984; Overpeck and Bartlein, 1989; Bonan and Hayden, 1990), they do offer insights into the ecological processes underlying the vegetational changes revealed in the fossil pollen maps. Simulations of successional sequences following disturbance are the most important to interpreting the Alaskan fossil record (e.g. Van Cleve et al., 1991; Van Cleve et al., 1986; Bonan, 1989a, b). Both models and field data indicate that P. tremuloides, B. Impyrifera, and P. glauca are the first tree species to reestablish by seed on all but the coldest sites. With the loss of moss and organic layers, heat flux to the soil increases, resulting in deep active layers and well-drained substrates. Warm, well-drained soils enhance the decomposition of nutrient-rich and easily degraded litter of hardwood species. Hardwood species, however, do not persist on such sites, because poor seedbed quality of the forest floor prevents the establishment of new individuals I Van Cleve et al., 1991 ). P. glauca grows more slowly than the hardwoods and becomes an important part of the canopy 100 years or more after disturbance (Walker et al., 1986). Because P. ghmca litter degrades more slowly than that of hardwoods /Flanagan, 1986), increased dominance of P. glauca results in thicker organic layers and reduced heat flux to the soil. Thick mats of feather mosses with low thermal conductivity develop and further insulate the soil from heat uptake from the air. Soil temperatures are also reduced, because the dense crowns of P. glauca decrease radiation reaching the forest floor (Bonan and Shugart, 1989). Over time, permafrost rises and reduces soil drainage (Bonan and Shugart, 1989; Oechel and Van Cleve, 1986; Van Cleve etal., 1991). Without disturbance, litter and moss thicknesses continue to increase and make the site less suitable for P. glauca (Van Cleve et al., 1991). In these late stages,

permafrost reaches close to the soil surface and severely impedes soil drainage. Soils are cold, wet and nutrientpoor. These conditions are most common on north-facing slopes and on extensive lowlands away from active floodplains. P. mariana, sometimes in mixtures with l_zwix, is the most important tree species in such settings. Fires or other disturbances are seldom intense enough to remove the entire soil organic layer, consequently favoring the continued dominance of P. mariana forests. M E T H O D S A N D DATA

Pollen Data Our analysis is based on fossil pollen data from 25 lakes (Fig. 1; Table 1), complemented by modern pollen from mud-water interfaces of 212 Alaskan lakes (Fig. 4). All but two of the fossil sites were counted in our laboratory, T.A. Ager kindly provided original pollen counts for Birch Lake. The George Lake data were obtained by digitizing the published percentage diagram (Ager, 1975). Pollen percentages were calculated from a sum of all identified, unknown and unidentifiable pollen grains with the latter two components generally comprising less than 2% of the pollen assemblage. Pollen sums exceed 300 grains in most fossil samples and 400 grains in modern samples. Data plotted on the maps represent actual or interpolated percentages for each 1000 year interval (see Dating). The proportion of P. glauca vs. P. mariana pollen was determined in selected levels using a maximum likelihood discrimination technique (Brubaker et al., 1987). Modern pollen-vegetation relationships in the study area suggest P. glauca pollen is under-represented compared to P. mariana (Brubaker et al.. 1987). We infer that when the proportion (theta) ofP. glauca pollen is estimated to be 0.8 or greater, it is the exclusive Picea species in the vicinity of the lake. Mixed P. glauca-P, mariana or predominantly RADIOCARBON DATES

0

5

K"

lit.[]

m

g

10

O

a15

0

2

4

6

8

10

12

14

Number of Dates per Time Interval

FIG. 4. Histogram showing age distribution of radiocarbon dates used to construct maps of fossil pollen data. Twelve dates > 22

ka BP are not shown.

79

P.M. Anderson and L.B. Brubaker: Vegetation History o f Northcentral Alaska TABLE I. Site information and key to Fig. 1 Site

Site name

Location

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Kaiyak Niliq Squirrel Etivlik Headwaters Redondo Joe Kollioksak Selby Minakokosa Ruppert Angal Ranger Redstone Screaming Yellowlegs Rebel Sakana Crowsnest Seagull Sithylemenkat Sands of Time Tiinkdhul Ped Birch George

68°09'N 67°52"N 67°06'N 68°08'N 67°56'N 67°43'N 66°46'N 66°58'N 66°5 I'N 66°55'N 67°04'N 67°08'N 67°09'N 67°15"N 67°35'N 67°25'N 67°26'N 68°20'N 68°16'N 66°07'N 66°02'N 66°35'N 67°12'N 64°19'N 63°47'N

161°25'W 160°26'W 160°23'W 156°02'W 155°02'W 154°33'W 157°13"W 156°27"W 155°43'W 155°02"W 154°15'W 153°54'W 153°39'W 152°36'W 151°25"W 149°48'W 147°51'W 146°29"W 145°13'W 151 °26'W 147°31'W 143°09'W 142°04'W 146°50'W 144°30'W

Elevation (m)

Age*

190 274 91 631 820 460 183 213 145 122 210 820 820 914 650 914 640 881 637 213 250 189 211 274 389

18" 14 18; 14 11 5 18" 14 18" 18" 12 14 18* 14 14 18" 13 I0 14 13 18" 18" 12 15 17

Reference Anderson (1985) Anderson (1988) Anderson (1985) Anderson (unpublished) Brubaker et al. (1983) Brubaker et al. (1983) Anderson (1988) Anderson (unpublished) Anderson (unpublished) Anderson (unpublished) Brubaker et al. (1983) Brubaker et al. (1983) Brubaker el al. (1983) Edwards et al. (1985) Edwards et al. (1985) Edwards el al. 0985) Bruhaker (unpublished) Anderson (unpublished) Bruhaker (unpublished) Anderson et al. (1990) Lamb and Edwards (1988) Anderson et al. (1988) Edwards and Bruhaker (1986) Ager (1975) Ager (1975)

*Maximum age (years BP x 1000) used in mapping based on linear interpolation of radiocarbon dates. i'Maximum core age exceeds 18 ka BP.

165

170

160

155

150

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72

A

140 r2

Gramlneae Isopoll (18 000 BP)

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145 "

140 I 72

Cyperaceae Ilmpoll ( l e 00o BP)

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|

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11~

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FIG. 5. Isopoll maps showing pollen percentages for 18 ka BP: A. Gramineae, B. Cyperaceae, C. A r t e m i s i a , D. Salix.

140

Quaterna~ Science Reviews: Volume 13

80 170 72 A

165

160

•

'

155 150 ' G r a m l n e a e 1 0 % I s o c h m n e (18 000-12 0 0 0 BP)

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BP)

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7ot

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FIG. 6. Isochrone maps for: A. 10c/c Gramineae pollen (18-12 ka BP), B. 20% Cyperaceae pollen (18-9 ka BP), C, 10qc Salix pollen (18-12 ka BP), D. 0% ericaceous pollen ( 14-9 ka BP). Tick marks point to areas of greater percentages.

P. mariana forests are indicated when theta values fall below 0.8. Because P. glauca is under-represented, even low theta values could represent a substantial presence of this species in the local vegetation. Shrub (B. nana/glandulosa) and tree species (B. papyrifera) of Betula were separated based on measurements of pollen diameters (Ritchie, 1984a; lves, 1977). Pollen grains > 20 microns in diameter are classified as B. papyrifera, whereas smaller grains are considered B. nana/glandulosa. Such assignments are tentative at best, as no comprehensive study of Betula pollen dimensions has been made. A recent investigation by Edwards et al. (1991), suggests that Betula pollen diameters are quite variable within and among species and interpretations based on measurements may not be valid. Dating A total of 114 radiocarbon dates from 21 sites were used to construct the isopoll maps (Fig. 4). Clearly anomalous dates were eliminated, and core tops were assumed to be 0 ka BP. Cores from Rebel, Crowsnest and George Lakes, which had poor radiocarbon control, were dated by pollen stratigraphic correlation to nearby well dated sites. Age assignments for sample levels in all cores were calculated by simple linear interpolation between radiocarbon dates.

Other methods (linear regression, spline) resulted in similar age assignments. When discrepancies existed among the methods, linear interpolation generally provided the most reasonable age assignments. Intervals of 1000 years were chosen to summarize major temporal patterns in the data, because slow sedimentation rates in many cores prevented finer temporal resolution. Most dates fall between 15-5 ka BP with the largest number between 11-5 ka BP (Fig. 4), the period of major interest in this paper. The last 2000 years and the period 15-19 ka BP are poorly represented, although six dates of > 19 ka BP increase the dating reliability of the latter period.

Maps We examined general temporal and spatial patterns in the data by plotting pollen percentages of selected taxa on base maps and drawing contours of equal pollen percentages (isopolis) by hand. Isopoll maps were constructed for all major taxa at 1000 year intervals between 18 ka BP and present. We then summarized these maps with isochrone maps, which show the locations of "key' isopolls at each time interval. Key isopolls represent pollen percentage values that correspond to plant range limits or population centers. To choose these values we

P.M. Anderson and L.B. Brubaker: Vegetation History of Northcentral Alaska 170 72

165

160

155

150

140 72

145

BP)

Betula 40% Isochrone (14 ~

170 72

165 B

160

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Betula 40% I s o c h r m ~ (80OG.SO00

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BP)

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N

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105

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FIG. 7. Isochrone maps for: A. 40% Betula pollen (14-9 ka BP), B. 40% Betula pollen (8-5 ka BP), C. 20% Alnus pollen (8-7 ka BP), D. 40% Alnus pollen (8--4 ka BP). Tick marks point to areas of greater percentages. compared isopoll maps for the 2 ! 2 modern sites with plant range maps. Isopolls that approximate range limits are: Picea, 10% Picea pollen; shrub Betula, 20% Betula pollen; Alnus, 20% Alnus pollen (Fig. 3). Areas with relatively dense populations of boreal forest species display > 20% Picea and often > 40% Betula pollen (including an important component of B. papyrifera pollen). Tundra is indicated by > 30% Cyperaceae pollen and coastal areas display > 10% Gramineae pollen. The isochrone maps (Figs 6-7, 9) summarize the major changes in pollen data. Locations of isochrones were drawn by hand and some locations are tentative where few sites exist. Although not indicated graphically, these locations are easily recognized. We excluded a few obviously anomalous samples from the data when drawing the isochrone maps. These instances are: Sithylemenkat with 12% Salix pollen at 12 ka BP, 50-80% Betula pollen from 14-9 ka BP at Kaiyak and Kollioksak Lakes, and 48% Betula at 7 ka BP at Headwaters Lake. For the period 18-15 ka BP, we used isopoll rather than isochrone maps to summarize major features of the pollen data, because few sites.are available and dating control is poor for these times (Fig. 4). Percentages of Ericaceae and Populus pollen vary greatly over time and space and are thus difficult to contour. Consequently, we summarized these data with presence/absence maps.

DESCRIPTION OF P O L L E N MAPS We chose 20 maps to illustrate changes in pollen percentages for the period 18-0 ka BP. Hopkins (1982) divided the late Quaternary history of Beringia into three intervals: Duvanny Yar (25-14 ka BP), Birch (14-9 ka BP) and Holocene (9 ka BP to present). We use the following modification: (1) Late Duvanny Yar Interval (18-14 ka BP) represented by mapped data for 18 ka BP; (2) Early Birch Interval (14-11 ka BP) represented by data for 14 and 12 ka BP; (3) Late Birch Interval (11-9 ka BP) represented by data for 9 ka BP; (4) Spruce Interval (9 ka BP to present) represented by data mapped at 1000 year intervals between 9 and 4 ka BP. Late Duvanny Yar Interval (18-14 ka BP) Pollen spectra are dominated by herbaceous taxa, particularly Gramineae, Cyperaceae, and Artemisia (Fig. 5A, B, C). Salix is the only common shrub taxon (Fig. 5D) and tree pollen is rare. At 18 ka BP, Gramineae percentages exceed 30% in the west and decrease to approximately 20% to the east. Artemisia percentages are greater than 20% at northern sites in the Brooks Range and decline to less than 10% in the south. Cyperaceae percentages are generally above 20% throughout the

Quaternar3' Science Reviews: V o l u m e 13

82 170 72

165

160

155

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~45 "

140 72

170 7~

A Po~u~ (t4 oooaP)

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155 ,

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POpUlUS (5000 BP)

Populu$ ( 9 0 0 0 BP)

r- ~

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1(I0

156

150

145

64 140

D

~

M

66

64 17~

185

~

1~10

1~

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1M

140

FIG. 8. Maps indicating the presence or absence of Populus pollen at: A. 14 ka BP, B. I 1 ka BP, C. 9 ka BP, D. 5 ka BP. Filled circles indicate presence: triangles indicate absence. region and exceed 30% in the central Brooks Range. Salix pollen, although never abundant, is most c o m m o n at southern sites. These major patterns in the pollen data persist until ca. 14 ka BP.

Early Birch Interval (14-11 ka BP) Gramineae, Cyperaceae, A r t e m i s i a a n d Salix percentages generally decrease, and Ericaceae and Betula percentages increase during the Early Birch Interval (Figs 6 and 7A, B). By 14 ka BP, Artemisia pollen has declined to modern values (< 10%), which represents local variations in plant abundances that cannot be mapped on a regional scale. Gramineae pollen exceeds 10% throughout most of the study area at 14 ka BP, but the 10% isochrone shifts westward to the western Brooks Range by 12 ka BP (Fig. 6A). Cyperaceae percentages decrease slightly in eastern areas, resulting in a minor shift in the 20% Cyperaceae isochrone by 12 ka BP (Fig. 6B). Similarly, the 10% Salix isochrone shows a modest shift, predominantly in the far western and eastern Brooks Range (Fig. 6C). Declines in the above percentages primarily reflect increases in Betula pollen, as the pollen accumulation rates of most herb taxa do not decrease at 14 ka BP. By 14 ka BP, Betula pollen reaches 40% in the east (Fig. 7A) and 20% in the west

(not illustrated). By 12 ka BP, Betula percentages are above 40% across nearly the entire region (Fig. 7A). Pollen accumulation rates (see papers listed in Table 1) indicate a different pattern, with the first major increase of Betula pollen accumulation occurring in the western study area ca. 14 ka BP and a more gradual increase in the east between 14 and 12 ka BP. We interpret Betula pollen during this time to be predominantly or exclusively from shrub species (e.g. Brubaker et al., 1983: Edwards et aL, 1985; Anderson, 1985). Ericaceae pollen, though never abundant, is present at sites in the western and central Brooks Range by 14 ka BP and in some southeastern sites by 12 ka BP (Fig. 6D). Populus pollen appears consistently at a regional scale for the first time during this period (Fig. 8A).

Late Birch Interval (11-9 ka BP) Isochrones of Gramineae, Cyperaceae and Salix approximate modern locations by 9 ka BP (Fig. 6A, B, C). Betula pollen (Fig. 7A) remains 40--60% throughout the region, and Ericaceae pollen is present in most of the area by 9 ka BP (Fig. 6D). Despite its relatively low percentages, Populus is a consistent component of pollen assemblages at many sites, reaching maximum values ca. 9 ka BP (Fig. 8B, C).

83

P.M. Anderson and L.B. Brubaker: Vegetation History of Northcentral Alaska leg

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(O < 0.e)

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FIG. 9. Maps showing Picea pollen data for: A. 10% Picea isochrone (9-7 ka BP), B. 10% Picea isochrone (excludingAlnus from the pollen sum: 9-7 ka BP), C. 10% Picea isochrone (7-4 ka BP), D. dates indicating times at which the proportion of P. glauca pollen decreased to less than 0.8 theta values, indicating a change from predominantly P. glauca to a mixture of P. glauca and P. mariana in the vegetation.

Spruce Interval (9 ka B P to Present) Betula pollen remains above 40% at all but far western sites until ca. 8 ka BP and then decreases steadily between 8 and 5 ka BP (Fig. 7B). These percentage decreases are largely due to increases in Picea and Alnus pollen that begin ca. 9 ka BP (Figs 7C and 9A), since pollen accumulation rates of Betula remain relatively constant over this period at most sites. Populus pollen disappears from most sites between 9 and 5 ka BP (Figs 8C, D). Picea isochrones show complex patterns during the Hoiocene (Fig. 9). Picea percentages exceed 10% at sites in the Tanana and Yukon valleys by 9 ka BP and are greater than 10% at sites in the Porcupine Plateau and the eastern and central Brooks Range by 8 ka BP (Fig. 9A). Between 8 and 7 ka BP, Picea pollen decreases to less than 10% at most sites, causing a major eastward shift in the 10% isochrone. These declines are caused in part by a contemporaneous increase in Alnus pollen (Fig. 7C). However, when Alnus was removed from the pollen sum, the 10% Picea isochrone still shifts eastward at 7 ka BP, though not so markedly (Fig. 9B). Pollen accumulation

data also suggest a decline in Picea pollen between 8 and 7 ka BP (Brubaker et al., 1983; Anderson et al., 1990). Between 7 and 6 ka BP, Picea percentages and accumulation rates increase in the central and western Brooks Range, and the 10% isochrone shifts westward reaching its present location by ca. 4 ka BP (Fig. 9C). From 9 to 7 ka BP, Picea pollen is primarily or exclusively P. glauca (theta > 0.8; Fig. 9D). After 6 ka BP, most sites show predominantly P. mariana pollen. Alnus pollen reaches 20% in western and central sites by 8 ka BP (Fig. 7C) and attains similar values in eastern areas by 7 ka BP. Alnus percentages are consistently highest in the western Brooks Range, which exceed 40% as early as 8 ka BP (Fig. 7D). The 40% isochrone reaches its maximum extent in the Brooks Range by 6 ka BP. After this time, Alnus pollen decreases and modern values are in place by 4 ka BP. VEGETATION HISTORY This section interprets vegetation history from the mapped pollen data (Figs 5-9). Figure 10 summarizes our interpretations, as described below.

Quaternar3, Science Reviews: Volume 13

84

WEST

south-facing slopes and along large braided rivers. Small thickets of Betula nana/glandulosa and Salix spp. may also have survived by vegetative propagation in such settings.

EAST

0 2

P. GLAUCA FORESTS

4 mariana

6 8 AI

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qauca 10 12 14

BETULA TUNDRA / POPULUSWOODLAND MOIST BETULA TUNDRA Be Ma m MOIST HERB

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u/us

DRY BETULA

DRY HERB

16 TUNDRA

TUNDRA

18

FIG. 10. Cartoon of late Quaternary vegetation history of northcentral Alaska based on fossil pollen data.

Late Duvanny Yar Interval (18-14 ka BP) The poor species resolution of Gramineae, Cyperaceae and Artemisia pollen and the sparsity of well dated pollen diagrams for this period have resulted in a variety of vegetation reconstructions ranging from extensive grasslands, steppe or steppe tundra to sparse polar desert (see Hopkins et al., 1982 and Guthrie, 1990 for a review of these interpretations). We believe that the presence of minor pollen taxa with strong arctic affinities (e.g. Oxyria, Saxifraga, Dryas, Saussurea), the minor presence of shrub taxa (Yurtsev, 1981), and the statistical similarity of Duvanny Yar spectra to modern arctic pollen assemblages (Anderson et al., 1989) argue for the presence of herbdominated tundra rather than grassland or steppe. The composition of these tundra communities varied both locally and regionally. Graminoid communities, with moist herbs such as Rubus chamaemorus, Polygonum amphibium and Saxifraga hirculus, covered lowlands and valleys of the western study area. These areas were probably relatively productive habitats that provided important feeding grounds for Pleistocene megafauna. Vegetation cover was less continuous in upland sites of the west and across most of the eastern study area, where herb taxa adapted to xeric and/or frequently disturbed sites (e.g. Artemisia, other Compositae and Chenopodiaceae) were more common. Plant cover probably became very sparse with increasing altitude and proximity to ice sheets in the east, and cushion plant communities may have occupied extensive windswept uplands in these areas. With the exception of Salix, shrub taxa were rare on the full-glacial landscape. Salix was probably locally abundant along streamsides and in snow bed communities. Betula nana/glandulosa may have persisted on the Land Bridge in much smaller and more restricted populations than Salix. Of the boreal tree species, Populus balsamS[era was the most likely to have survived the rigors of fullglacial climate in eastern Beringia, but pollen and macrofossil evidence for this is negligible (Hopkins et aL. 1981). Glacial-age populations of this species would probably have persisted by vegetative propagation on

Early Birch Interval (14-11 ka BP) Betula nana/glandulosa shrub tussock tundra replaced herb tundra as the dominant vegetation in the western study area by ca. 14 ka BP. Salix remained abundant in riparian and snow bed sites. Tall Salix and Betula glandulosa shrubs may have formed dense thickets along streamsides, perhaps in association with P. balsamifera. The development of tussocks is indicated by increases in Ericaceae pollen and Sphagnum spores, as these taxa characterize lake sediments in areas of modern Alaskan shrub tussock tundra (Anderson and Brubaker, 1986). Shrub tussock tundra was probably widespread in upland areas with intermediate drainage and in locally drained thermokarst basins. These communities may have persisted in the far west for nearly 8000 years, until the development of P. mariana muskegs ca. 6 ka BP. In contrast to the west, herb tundra probably remained common in the eastern study area until ca. 12 ka BP. This interpretation is reasonable even though Betula pollen percentages rise to over 40% at eastern sites as early as 14 ka BP, because Betula PARs increased only gradually between 14 and 12 ka BP. The percentage increase in Betula pollen at 14 ka BP is related to the low pollen productivity of sparse herb tundra communities. Even slight increases in PARs result in a large percentage increases when background pollen productivity is low. Under these circumstances PARs are considered a more reliable indicator of plant abundances. Thus, the gradual rise in Betula PARs in the east suggests a slow expansion of Betula shrubs in that region. In addition to the gradual increase in Betula shrubs, mesic herbs probably became more abundant in eastern herb tundra communities between 14 and 12 ka BP, and xeric herb species may have expanded into areas previously too harsh for plant growth. However, after 12 ka BP Betula shrub tundra probably dominated the eastern landscape. The absence of ericads and Sphagnum suggests dry shrub communities without tussocks, perhaps similar to modern tall Betula glandulosa tundra occupying well drained sites of the Alaska Range and eastern Intermontane region. Late Birch Interval (11-9 ka BP) Betula shrub tundra continued to dominate the landscape, with tussock communities in the west and drier, tall shrub assemblages in the east. However, the expansion of Populus, ericads, Typha and Myriophyllum caused striking changes in the vegetation of local habitats (Anderson et al., 1988; Ritchie et al., 1983). Populus became common throughout the study area, probably as gallery forests along rivers and as cional stands on southfacing, well drained hillslopes (Anderson et aL, 1988). Its rapid population increase undoubtedly involved both vegetative expansion of existing clones and the

P.M. Anderson and L.B. Brubaker: Vegetation History of Northcentral Alaska establishment of new populations by seed. Geomorphological evidence for an increase in streambed aggradation due to increased flooding (Carter and Hopkins, 1982) suggests a greater availability of riparian habitats during the Late Birch Interval. Such sites would have favored the establishment of P. balsamifera and provided efficient avenues for dispersal. The expansion of ericads from western to eastern Alaska implies the development of moist shrub tussock tundra in thermokarst terrain of the eastern lowlands (e.g. Yukon and Kanuti Flats) and/or the presence of local heath tundra communities on uplands. Range extensions of Typha and Myriophyllum suggest changes in aquatic environments and perhaps indicate a regional lowering of lake levels and/or rise in water temperature. Overall, changes in the pollen record for the Late Birch Interval indicate less pronounced vegetation gradients than for earlier times.

Spruce Interval (9 ka BP to Present) P. glauca invaded the Tanana and upper Yukon River drainages between 10 and 9 ka BP, reaching the central Brooks Range by 8 ka BP. P. glauca forests in northwestern Canada likely were the seed sources for the colonization of eastern Alaska (Ritchie, 1977, 1982, 1984a, b, 1985a, b; Spear, 1983; Cwynar and Spear, 1991), with broad river valleys providing extensive migration corridors and excellent sites for seedling establishment and tree growth. These newly established gallery forests dominated floodplains and river courses and provided seeds for further Picea expansions into Betula shrub communities on south-facing slopes and finer textured lowland soils away from rivers. The greater abundance of P. glauca in fossil compared to modern pollen data argues that P. glauca was common in lowlying areas during the Early Holocene, in contrast to today where P. mariana muskegs are more typical. Local soil conditions during the Early Holocene may have been more suitable for P. glauca colonization in these areas, because low litter accumulation in dry Betula tundra communities and high summer temperatures would have allowed significant soil warming. Even during the Early Holocene, however, loess soils were probably less favorable than alluvial soils for P. glauca, and lowlands away from rivers may have supported open woodlands rather than dense forests. Hardwood species were probably also important components of these woodlands and forests. Populus likely remained common along rivers and on south-facing slopes, although its abundance declined after 8 ka BP. Macrofossils document the presence of Betula papyrifera in the vicinity of Fairbanks (Hopkins et al., 1981) and beyond its current range on the Seward Peninsula (McCulloch and Hopkins, 1966). As today, it probably grew on warm, well-drained soils. Alnus shrubs began expanding in the far northwestern Brooks Range beginning ca. 10 ka BP, but Betula tussock tundra and Populus woodlands continued to dominate the landscape until ca. 8 ka BP. Initial Alnus populations grew slowly and shrub thickets were probably restricted to

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mountain slopes and floodplains. Between 8 and 7 ka BP, however, Alnus spread rapidly throughout the entire study area, reaching greatest abundance in the central and western Brooks Range ca. 6 ka BP. Extensive thickets established on mountain slopes and along stream courses in the Betula shrub tundra and forest-tundra ecotones. Alnus also invaded Picea forests and woodlands. P. glauca populations declined across much of the central Brooks Range and Intermontane area between 8 and 7 ka BP, but the precise extent of this reduction is difficult to assess. Fossil Picea percentages (Fig. 9) decreased below values that are typically found at modern treeline, although they were similar to percentages in modern samples of the lower Kobuk and Noatak Rivers where gallery forests extend into shrub tundra (Figs 2 and 3A). Therefore, P. glauca forests may have disappeared completely or scattered stands may have persisted in the central Brooks Range ca. 8-6 ka BP. In eastern Alaska, the decline of P. glauca forests was much less pronounced. Floodplain forests in this region probably remained relatively unchanged, but P. glauca woodlands on fine-textured lowland soils may have declined. A second major forest expansion occurred ca. 6 ka BP, when P. mariana replaced P. glauca as the dominant forest species in the central and eastern study area, and both P. glauca and P. mariana invaded the western lowlands. The spread of P. mariana across Alaska was contemporaneous with a major population expansion in northwestern Canada (Cwynar and Spear, 1991; MacDonald, 1987a; Ritchie, 1987) and, similar to P. glauca, initial seed sources likely were located in northwestern Canada. P. mariana probably entered declining P. glauca woodlands on non-riparian sites in central and eastern portions of the study area and invaded cold, organic-rich soils of Betula tussock tundra communities in the west. The westward expansion of P. glauca was restricted to riparian and south-facing sites with warm, well-drained soils. Picea treeline was near its present location by ca. 4 ka BP. With the widespread establishment of P. glauca and P. mariana forests by 5 ka BP, Betula shrub tundra disappeared as the dominant vegetation type in westcentral Alaska. Alnus was probably more abundant than.present in the central and western Brooks Range ca. 6 ka BP. However, populations apparently declined slightly between 6 and 4 ka BP. B. papyrifera and P. balsamifera/tremuloides may have also reached their modern distributions within the boreal forest between 5 and 4 ka BP. Thus, by 4 ka BP the composition and distribution of modern boreal forest communities in northcentral Alaska was achieved. CLIMATE HISTORY

Paleoclimate Inferred from Fossil Data The prevalence of herb-dominated tundra suggests conditions were cooler and drier than present during the Duvanny Yar Interval. The inferred decrease in vegetative cover from western to eastern portions of the study area

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may indicate regional climatic gradients of decreasing precipitation or precipitation-minus-evaporation, as moisture availability strongly limits productivity of modern high arctic communities (Bliss, 1977). The beginning of post-glacial climatic amelioration is indicated by the transition from herb to Betula shrub tundra. An effective moisture gradient apparently continued during the Early Birch Interval, as indicated by the establishment of Betula-ericads-Sphagnum communities in western Alaska and the persistence of more xeric herb-Betula tundra in eastern areas. The wetter conditions in the west may have been further enhanced by the initial flooding of the Land Bridge which provided a more local moisture source. The appearance of the first forested environments during the Late Birch Interval is further evidence of the general trend towards warmer climates during the latest Pleistocene. The widespread expansion of Populus forests across northcentral Alaska may indicate decreasing temperature and precipitation gradients, in contrast to the previous 7000 years. Range extensions of Populus, Typha and Myriophyllum suggest that summer conditions were somewhat warmer and drier than present (Ritchie et al., 1983; Edwards and Dunwiddie, 1985: Lev, 1987). However, the fossil evidence for a significant post-glacial warming in Alaska is not as great as in adjacent northwestern Canada, where Picea treeline was beyond its current limits at several locations ca. 9 ka BP (Cwynar and Spear, 1991; Ritchie et al., 1983). The spread of P. glauca in eastern and central portions of the study area indicates summers remained relatively warm in the early Spruce Interval. Higher summer temperatures would have favored P. glauca directly through enhanced tree growth, seed production, and seedling establishment and indirectly by increasing soil warmth and dryness. The absence of P. glauca forests in northwestern Alaska may reflect cooler summer conditions caused by the moderating influences of a maritime climate. However, the extension of B. papyrifera beyond its modern limit on the Seward Peninsula ca. 8 ka BP suggests early Holocene temperatures were also warmer than present near Kotzebue Sound (Hopkins et al., 1981). The decline of P. glauca and the rapid expansion of P. mariana and Alnus during the middle Spruce Interval suggest an increase in effective moisture throughout the study area. The widespread appearance of P. mariana following the decline in P. glauca is often interpreted to indicate cooler conditions (e.g. Edwards and Brubaker, 1986), but it seems unlikely that a significant decrease in summer temperatures would have favored the rapid population of Alnus (Anderson et al., 1991) nor the continued slow expansion ofP. glauca. A slight cooling in summer temperatures, however, could have resulted in greater effective soil moisture, thereby contributing to the spread of both Alnus and P. mariana and the decline of suitable sites for P. glauca. Modern or near-modern temperature and precipitation regimes were apparently established between 6 and 4 ka BP.

Comparison to Computer Simulations The above paleoclimatic interpretations inferred from the pollen data are in general agreement with the CCMO simulations. The severest conditions, indicated by herbdominated tundra, correspond to times when the ice sheet exerted its strongest influence on the northern hemisphere circulation patterns (18 ka BP). In addition, the west to east trend in decreasing vegetative cover is in accord with simulations of colder, drier climates in eastern Alaska. The establishment of Betula shrub tundra corresponds to the simulated post-glacial warming trend resulting from higher summer insolation and the diminished glacial anticyclone, although this trend is modified by continued cool sea surface temperatures ( 12 ka BP experiment). With increased July net insolation and the absence of circulation anomalies associated with the ice sheet (9 ka BP experiment), warmer than present summer temperatures are simulated. These conditions would have favored the widespread establishment of Populus forests and range extensions of several species. Little change in July temperatures is simulated between 9 ka BP, 6 ka BP, and 3 ka BP, although summer insolation decreases during this period. In contrast to the relatively constant simulated temperatures during the Holocene, major vegetational changes occurred over this period (i.e. the rise and decline of P. glauca forests, the rapid spread of P. mariana and Alnus). This apparent discrepancy between simulated and inferred paleoclimates may reflect the lack of an interactive ocean in the model and/or the absence of sea surface temperature, sea ice and snow cover feedbacks (Bartlein, pers. commun.). Nonetheless, the overall agreement of simulated and inferred paleoclimates is striking, and these results support the idea that the fluctuations in insolation and the circulation anomalies associated with the North American ice sheet are key factors determining late Quaternary climates in Alaska. In addition to regional climatic change, successional processes might have contributed to the shift from P. glauca to P. mariana dominated forests between 8 and 6 ka BP. Stand simulation models (Bonan, 1989a: Bonan and Korzukin, 1989) and field observations (e.g. Van Cleve et al., 1991) indicate that the replacement of P. glauca by P. mariana would eventually lead to thick organic soils which, in turn, would favor the continued dominance by P. mariana. Geochemical data from lake sediments in southeastern Labrador suggest that the replacement of P. glauca, first by Abies balsamea and then by P. mariana, was associated with the development of cool, wet and nutrient-poor soils (Engstrom and Hansen, 1985). The duration of this transition (ca. 2000 years) is similar to that for the shift from P. glauca to P. mariana forests in Alaska. However, forest model simulations and field observations in northcentral Alaska indicate that this change can occur within hundreds of years (Bonan, 1989a; Van Cleve and Viereck, 1981; Walker et al., 1986), suggesting that the mid-Holocene vegetation changes should not be attributed solely to successional processes that occur within present day climatic conditions.

P.M. Anderson and L.B. Brubaker: Vegetation History of Northcentral Alaska DISCUSSION The Tundra

The first paleoenvironmentai interpretations of the Duvanny Yar Interval suggested that a uniform vegetation characterized Eastern Beringia (Matthews, 1976). Later interpretations emphasized the presence of small-scale mosaics of herb-dominated communities whose patterns were controlled primarily by variations in local topography and drainage (Schweger and Habgood, 1976; Schweger, 1982; Guthrie, 1982). We present a slightly different concept that emphasizes regional vegetational gradients. That is, communities dominated by xeric adapted species were more common in eastern Alaska, whereas more mesic graminoid communities typified the western Alaskan lowlands. This idea does not conflict with interpretations of small-scale vegetational mosaics, because topographically controlled variations in local plant communities undoubtedly occurred in all parts of the study area. Unfortunately, large numbers of pollen and/or macrofossil sites are needed to test any model of vegetational variation for the Duvanny Yar Interval. The data presented in the isopoll maps, while an improvement over previous full-glacial data sets, can only be considered as a skeletal framework for inferring the nature of regional vegetational gradients. Regional differences also occurred in the development of shrub tundra during the Early Birch Interval, suggesting earlier interpretations of a uniform Betula shrub tundra (e.g. Ritchie, 1984a; Anderson, 1985) were overly simplistic. However, like the Duvanny Yar Interval, the environment of this period remains poorly understood. The transition from glacial to interglacial conditions marks a major reorganization of plant and animal communities, yet scant attention has been paid to this Interval. In fact, little is currently understood about the development of modern tundra in Alaska. In part, this reflects few records from sites presently in the tundra and the difficulty in interpreting tundra pollen spectra. Clearly a concerted effort is needed to examine the history of this key biome and to develop new techniques for reconstructing past tundra communities. The Boreal Forest

The present day composition of Alaskan boreal forests was built gradually over a period of 5000-6000 years. The first wooded environments occurred during the Late Birch Interval. The Populus balsarn~fera (and possibly tree Betula) forests of this period offer a striking contrast to the present day conifer-dominated boreal forests of North America. Comparisons between fossil and modem pollen assemblages indicate that these early hardwood forests did not resemble modern vegetation in subarctic areas of North America. In fact, the similarity between the Late Birch Interval and modern pollen spectra is even lower than between Duvanny Yar and modern assemblages (Anderson et al., 1989). Thus, the hardwood forests/woodlands at the glacial/interglacial transition

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represent one of the most unusual vegetation types over the past 20,000 years. The expansion of P. glauca between 10 and 9 ka BP marked the beginning of the Spruce Interval and established the coniferous character of the Alaskan boreal forests. Modem forest composition, however, was not achieved in northcentral Alaska until ca. 6 ka BP with the widespread establishment of P. mariana. The expansions of both Picea species in Alaska coincided with larger range extensions in western Canada (Ritchie and MacDonald, 1986; Ritchie, 1987; MacDonald, 1987a, b; Moser and MacDonald, 1990; Wang and Geurts, 1991a, b; Cwynar and Spear, 1991; Keenan and Cwynar, 1992). For example, the arrival of P. glauca was part of an extremely rapid movement of this species from southern Alberta to northcentral Alaska and northwestern Canada. By 9 ka BP, P. glauca spread into Alaska and beyond its modem limits on the Tuktoyaktuk Peninsula, but the northernmost Canadian populations retreated southward to their present range limits by ca. 6 ka BP (Ritchie et al., 1983; Spear, 1983; Ritchie, 1984a; Cwynar and Spear, 1991). P. mariana migrated northward and westward at the same time as P. glauca, but did not spread beyond modem treeline during the early Holocene (MacDonald, 1987a; Cwynar, 1988; Wang and Geurts, 1991a,b). A second major fluctuation of Picea populations occurred in the mid- to late Holocene. The range limit of P. mariana remained relatively stationary near Great Bear Lake in the western Northwest Territories until ca. 7-6 ka BP. At this time, P. mariana spread rapidly into much of the Yukon Territory (Cwynar, 1988; Cwynar and Spear, 1991), the lower MacKenzie basin (Ritchie, 1985) and northcentral Alaska, approximating its modern distribution in these regions. In northcentral Canada, however, P. mariana populations expanded beyond modem limits ca. 5 ka BP, but retreated to modem locations ca. 3 ka BP (Moser and MacDonald, 1990). Thus, the dispersal histories of both P. glauca and P. mariana have included periods of rapid advances, still stands, and retreats. The above Picea history suggests climate rather than differential migration rates, dispersal barriers, or soil development as the primary control of treeline location. GCMs indicate that the decreasing effect of the continental ice sheet and increasing effect of insolation anomalies resulted in considerable regional variation in climates in northern North America between 9 and 3 ka BP (COHMAP, 1988; Kutzbach, 1987). The 9 ka BP ice sheet still produced a weak glacial anticyclone which, when coupled with higher summer insolation, caused significantly warmer than present conditions immediately to the west of the ice sheet (Mitchell et al., 1988; Kutzbach, 1987). In contrast, the warming effects of the insolation anomaly at 9 ka BP in Alaska were moderated by the intensified eastern Pacific subtropical high and possible sea ice feedbacks (Kutzbach, 1987). Such differences could account for the large Early Holocene northward expansion of treeline in northwestern Canada and the less extensive treeline advance in Alaska (Bartlein, pets. commun.). With the eastward retreat of the continental ice between 9 and 6 ka BP, circulation-

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enhanced warming in northwestern Canada diminished, resulting in a regional cooling and retreat ofPicea treeline. This retreat was probably further helped by the gradually decreasing summer insolation. The northward displacement of P. mariana treeline in northcentral Canada may reflect warming associated with the remnant ice sheet, but the mid- to late Holocene climatic controls of P. mariana distribution are not clear.

Computer Simulations Although comparisons of fossil records to paleoclimatic and forest stand simulations are an important approach for understanding the causes of high latitude vegetation change, such juxtapositions are not without problems (Harrison et al., 1991; Harrison et al., 1992; Kutzbach et al., 1991). Some of the more important limitations include differences in the spatial and temporal scales of the models and fossil data, model simulations that do not include all potentially important variables, and the general, incomplete understanding of the relationships between climatic conditions and specific controls of the vegetation. The uncertainty in data-GCM comparisons in Alaska arise from several factors (see Harrison et al., 1991, for general discussion of GCM limitations). The coarse spatial' resolution of the models (e.g. 7.5 ° longitude by 4.4 ° latitude in CCM0) results in smoothing of topographic features. Such features as the Alaska and Brooks Ranges are not represented in the model, but clearly play an important role in determining regional climatic patterns. Another important limitation of the model's coarse resolution is the depiction of the Beringia subcontinent. Changing sea levels are inadequately represented, thereby minimizing the consequent effects of maritime conditions on regional climates. Perhaps most important for arctic regions, few experiments include interactive oceans, sea ice and snow albedo, all of which are important feedbacks in high latitude climates (Kutzbach et al., 1991 : Bartlein et al., 1991 ). Forest dynamic models provide different, albeit equally significant, limitations when applied to fossil records. Like the GCMs, they suffer from temporal and spatial limitations. However, in this case the spatial scales are too small (1/12 ha) and temporal scales too brief (typically a few hundred years) for direct comparison to the pollen data. The vegetational changes represented by these models fall below the resolution of the lake's depositional area (ca. 20 km radius; Prentice, 1988) and, for all practical purposes, below the resolution of radiocarbon dates in the slowly sedimenting Alaskan lakes. Despite these limitations, experiments that simulate responses of inferred paleovegetation to variations in climatic or edaphic factors would be invaluable aids to understanding the development of late Quaternary vegetation.

Future Directions Despite the richness of the interpretive base used in this paper (i.e. a relatively large number of fossil and modern pollen sites, paleoclimatic and forest stand simulations,

and ecological field studies), many of our interpretations remain speculative. Several problems require further research before the history, of northcentral Alaska can be understood more clearly. For example, we need a comprehensive investigation of peatland development in order to evaluate the potential role of paludification in the expansion of P. mariana during the mid-Holocene. The histories of tree and shrub Betula are largely speculative due to poor species resolution of the pollen data (Edwards et al., 1991). As a result, our vegetation interpretation of such major periods as the Betula Interval need corroboration with macrofossil evidence of the predominant Betula species. Similarly, we have no empirical data on the importance of fire in the vegetation history of northcentral Alaska. Such information is particularly critical to our understanding of processes shaping past boreal forest communities, because fires are important determinants of modern forest composition and distribution. The generally good agreement between pollen data and GCM simulations suggests substantial understanding of controls over high latitude environments on very broad temporal and spatial scales. However, current models cannot be used to examine shorter term, smaller scale vegetational variations that are recorded by the pollen data (e.g. Picea 8-6 ka BP). In addition to the development of mesoscale models that can address such fluctuations, future experiments need to incorporate more detailed input of late Quaternary sea levels, sea surface temperatures and sea ice cover. Another as yet unconsidered feedback is permafrost, which is an important control of both regional and local vegetation. Its historical importance, however, is difficult to evaluate from currently available fossil records, and we have little idea of its role in past vegetational changes, despite our rather detailed knowledge of the vegetational history of northcentral Alaska. Implications f o r Questions o f Global Change at Northern High Latitudes

The potential inaccuracies in using paleovegetational records dating to earlier warm periods as exact analogues for vegetation existing under future climates are clear. As discussed above, fossil pollen data and GCM simulations indicate paleotemperatures have been higher than present in northern North America during the Holocene. However, the boundary conditions responsible for this warmer than present climate (e.g. increased summer insolation, circulation anomalies associated with continental ice sheets) are dissimilar to conditions (i.e. changing atmospheric composition) that will be the primary determinants of future climate systems. Thus the previous warm interval may not be a good analog for future conditions and consequent vegetational changes at northern high latitudes. Even though fossil studies may not yield data for making exact predictions, they do provide insights about the impact of climatic change on the vegetation. Such information should prove useful for anticipating possible consequences of future global warming on arcto-boreal vegetation. For example, the

P.M. Anderson and L.B. Brubaker: Vegetation History of Northcentral Alaska Alaskan pollen data indicate that tundr a and boreal forest communities did not migrate as intact units from glacial refugia in response to post-glacial climatic amelioration. It is doubtful, therefore, that these modern communities will persist intact if climatic conditions change substantially in the future. Furthermore, it is unlikely that the boreal forest will respond similarly across the entire continent, as comparisons between the Alaskan and Canadian forest histories indicate strong regional vegetational responses to global climatic changes.

ACKNOWLEDGEMENTS The University of Washington data were collected and analyzed under research grants sponsored by the National Science Foundation (DPP76-23041, DPP81-06806, DPP8403598, DPP86-19214, DPP87-22005 and DPP89-22491). Logistical support was provided in part by the National Park Service. We would like to thank the National Park Service, the Bureau of Land Management, U.S. Fish and Wildlife Service, NANA Regional Corporation, and Doyon Ltd. for allowing us access to lakes. We are especially grateful for the patience and help of Pat Bartlein, Mary Edwards, Dave Hopkins, Jim Ritchie and Tom Webb. We also appreciate the helpful comments of Terry Chapin, Charlie Schweger, Feng Sheng Hu and an unnamed reviewer on earlier drafts of this manuscript.

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Anderson, P.M., Reanier, R.E. and Brubaker, L.B. (1990). A 14,000-year pollen record from Sithylemenkat Lake, northcentral Alaska. Quaternary. Research, 33, 400---404. Anderson, P.M., Bartlein, P.J., Brubaker, L.B., Gajewski, K. and Ritchie, J.C. (1991 ). Vegetation-pollen-climate relationships for the Arcto-Boreal region of North America and Greenland. Journal of Biogeography, 18, 565-582. Barnosky, C.W., Anderson, P.M. and Bartlein, P.J. (1987). The northwestern U.S. during deglaciation: vegetational history and paleoclimatic implications. In: Ruddiman, W.F. and Wright, H.E., Jr (eds), North America and Adjacent Oceans during the Last Deglaciation, pp. 289-321. Geological Society of America, Boulder. Bartlein, P.J. and Prentice, I.C. (1989). Orbital variations, climate and paleoecology. Trends in Ecology and Evolution, 4, 195-199. Bartlein, P.J., Anderson, P.M., Edwards, M.E. and McDowell, P.F. (1991). A framework for interpreting paleoclimatic variations in eastern Beringia. Quaternary International, 10-12, 73-83. Beikman, H.M., Compiler. (1980). Geologic Map of Alaska. U.S. Geological Survey, Washington D.C. Black, R.A. and Bliss, L.C. (1978). Recovery sequence of Picea mariana-Vaccinium uliginosum forests after fire near Inuvik, Northwest Territories, Canada. Canadian Journal of Botany, 56, 2020-2030. Black, R.A. and Bliss, L.C. (1980). Reproductive ecology of Picea mariana (Mill.) Bsp., at tree line near Inuvik, Northwest Territories, Canada. Ecological Monographs, 50, 331-354. Bliss, L.C. (I 975). Tundra grasslands, herblands, and shrublands and the role of herbivores. Geoscience and Man, 10, 51-79. Bliss, L.C. (1977). Truelove Lowland, Devon Island, Canada: A High Arctic Ecosystem. University of Alberta Press, Edmonton. Bliss, L.C. (1981). North American and Scandinavian tundras and polar deserts. In: Bliss, L.C., Heal, O.W. and Moore, J.J. (eds), Tundra Ecosystems: A Comparative Analysis, pp. 8-24. Cambridge University Press, Cambridge. Bliss, L.C. and Cantlon. J.E. (1957). Succession on river alluvium in northern Alaska. American Midland Naturalist, 58, 452-569. Bliss, L.C., Heal, O.W. and Moore, J.J. (eds) (1981). Tundra Ecosystems: A Comparative Analysis. Cambridge University Press, Cambridge. Bonan, G.B. (1989a). Environmental factors and ecological processes controlling vegetation patterns in boreal forests. l_xmdscape Ecology, 3, I 11-130. Bonan, G.B. (1989b). A computer model of the solar radiation, soil moisture, and soil regimes in boreal forests. Ecological Modeling, 45, 275-306. Bonan, G.B. and Hayden, B.P. (1990). Using a forest stand simulation model to examine the ecological and climatic significance of the late Quaternary pine-spruce pollen zone in eastern Virginia, U.S.A. Quaternary Research, 33, 204-218. Bonan, G.B. and Korzuhin, M.D. (1989). Simulation of moss and tree dynamics in the boreal forests of interior Alaska. Vegetation, 84, 31-44. Bonan, G.B. and Shugart, H.H. (1989). Environmental factors and ecological processes in boreal forests. Annual Review of Ecology and Systematics, 20, 1-28. Bonan, G.B., Shugart, H.H. and Urban, D.L. (1990). The sensitivity of some high-latitude boreal forests to climatic parameters. Climatic Change, 16, 9-29. BOREAS Science Steering Committee. (1991). Charting the boreal forest's role in global change. EOS, 72, 33-35. Botkin, D.B., Janak, J.F. and Walles, J.R. (1972). Some ecological consequences of a computer model of forest growth. Journal of Ecology, 60, 849-873. Bowling, S.A. (1979). Alaska's weather and climate. In: Weller, G. (ed.), Alaska's Weather and Climate, pp. 1-25. Geophysical Institute, University of Alaska, Fairbanks.

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