Modern pollen assemblages from northern Alaska

Review of Palaeobotany and Palynology , 46 (1986): 273--291 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

273

MODERN POLLEN ASSEMBLAGES FROM NORTHERN ALASKA

PATRICIA M. ANDERSON and LINDA B. BRUBAKER

College of Forest Resources, University of Washington, Seattle WA, 98195 (U.S.A.) (Received November 5, 1984; revised and accepted April 12, 1985) ABSTRACT Anderson, P.M. and Brubaker, L.B., 1986. Modern pollen assemblages from northern Alaska. Rev. Palaeobot. Palynol., 46: 273--291. Surficial sediments from 101 lakes in northern Alaska were analyzed for their pollen content. Isopoll maps of pollen percentages show that boreal forest, mixed forest--tundra, and tundra are characterized by distinctive pollen assemblages. Pollen spectra from boreal forest contain the highest percentages of spruce (Picea) and birch (Betula) pollen, forest-tundra samples have the highest frequencies of alder (Alnus) pollen, and tundra spectra contain the highest grass (Gramineae) and sedge (Cyperaceae) pollen percentages. In addition, vegetational variations within the tundra and boreal forest are evident in the modern pollen. Differentiation of pollen from spruce and birch species accurately indicates areas (in northcentral and northeastern Alaska) where white spruce (P. glauca) and paper birch (B. papyrifera) are common. The coastal tundra is distinguished from a more interior tundra by higher percentages of grass and heath (Ericales) pollen. The increased resolution of the vegetation--pollen relationships is, in part, a function of a sampling design that employs a broad grid of sample sites instead of using isolated sites or isolated transects. The insight gained from the modern study should help in interpreting fossil pollen records and provide a more detailed picture of the development of the modern boreal forest and tundra in northern Alaska. INTRODUCTION The r e c o n s t r u c t i o n o f the past v e g e t a t i o n f r o m fossil pollen r e c o r d s in n o r t h e r n Alaska requires i n f o r m a t i o n on h o w the c o n t e m p o r a r y pollen rain is related t o the m o d e r n vegetation. The few m o d e r n pollen samples available f r o m n o r t h e r n Alaska are derived primarily f r o m m o s s polsters c o l l e c t e d at isolated sites or isolated transects o f sites (Livingstone, 1 9 5 5 ; Colinvaux, 1 9 6 4 ; M a t t h e w s , 1 9 7 0 ; Schweger, 1 9 7 6 ; Nelson, 1 9 7 9 ; A n d e r s o n , 1982). T h e p o l l e n - - v e g e t a t i o n r e l a t i o n s h i p revealed b y these pollen s p e c t r a is d i f f i c u l t t o evaluate because o f t h e u n e v e n spatial d i s t r i b u t i o n o f the m o d e r n samples. In a d d i t i o n , because m o s s polsters collect and preserve pollen d i f f e r e n t l y f r o m the w a y lake s e d i m e n t s d o (Ritchie, 1 9 7 4 ) errors can arise w h e n fossil pollen s p e c t r a derived f r o m lake m u d s are i n t e r p r e t e d b y pollen s p e c t r a f r o m m o s s polsters. T h e increased reliance o n fossil pollen samples f r o m lake cores for r e c o n s t r u c t i n g the p a l e o v e g e t a t i o n o f Alaska (Ager, 1 9 8 2 , 1 9 8 3 ; B r u b a k e r et al., 1 9 8 3 ; E d w a r d s et al., 1 9 8 5 ; A n d e r s o n , 1 9 8 5 ) m a k e s t h e n e e d clear f o r m o d e r n pollen d a t a derived f r o m lake sediments. 0034-6667/86/$03.50

© 1986 Elsevier Science Publishers B.V.

274 We therefore initiated a pollen study of surficial sediments from small lakes located north of 65°N latitude. The purposes of our study were: (1) to evaluate the ability of m o d e m pollen spectra to distinguish between boreal forest and tundra vegetation in northern Alaska; (2) to evaluate the ability of m o d e m pollen spectra to distinguish variations within either the boreal forest or the tundra, and (3) to describe a set of m o d e m pollen samples that can be used as direct guides or analogs for interpreting fossil pollen spectra from lake sediments. STUDY AREA The study area, referred to as northern Alaska, extends from approximately 65 ° to 71°N latitude and 141 ° to 166°W longitude (Figs.1 and 2). Our sample sites are located t h r o u g h o u t this region, excluding areas between the Seward Peninsula and the K o y u k u k River and in northeastern Alaska north of the Brooks Range. Landforms. The Brooks Range and its western extension, the DeLong and Baird Mountains, extend across northern Alaska from the Canadian border nearly to the Chukchi Sea (Fig.l). The elevation of these mountains increases from 350 m in the west to over 1700 m in the east. The area south of the Brooks Range consists of a series of broad lowlands and plateaux ranging in elevation from 150 to 600 m. The Arctic Slope, the area north of the crest of the Brooks Range, is divided into the Arctic Foothills and the Arctic Coastal Plain. The Arctic Foothills generally range in elevation from 100 m to 700 m. The Arctic Coastal Plain varies in elevation between sea level and 100 m. Climate. Mean annual temperatures vary from - t 2 . 5 ° C along the Arctic coast to --5°C in the central interior. Mean July temperatures are 15°C to 17°C in the interior and 5°C on the coast. Mean January temperatures are --20°C near Kotzebue Sound, --25°C on the Arctic Slope, and --27°C in the central and eastern interior. Mean annual precipitation is greatest (400 mm) in the upper Noatak and Alatna drainages. The lowest mean annual precipitation (200 mm) is registered in the Yukon Flats. Most of the precipitation t h r o u g h o u t the region falls as rain in July and August (Steinhausser, 1979). Vegetation. The vegetation of northern Alaska consists of two major formations, boreal forest and tundra (Fig.3). Despite the low diversity of plant taxa, the physiognomy and species composition of the vegetation varies greatly across the region in response to differences in climate, elevation, drainage, and disturbance regimes. We provide a brief summary of the vegetation following descriptions of the boreal forest by Viereck and Little (1972, 1975), Viereck and Dymess (1980), and Yarie (1983) and tundra by Viereck and Little (1972, 1975), Murray (1978), Viereck and Dyrness (1980), and Brown et al. (1980). Latin and c o m m o n names of plants are taken from Hult6n (1968). Boreal forest. The boreal forest extends across the interior lowlands and plateaux, reaching an upper elevational limit of about 700 m (Fig.3). White spruce (Picea glauca), black spruce (Picea mariana) and paper birch (Betula

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papyrifera) are the m o s t c o m m o n tree species, with balsam poplar (Populus balsamifera) and quaking aspen (Populus tremuloides) of secondary importance. The forest understory consists of a variety of shrubby species, including alder (Alnus crispa), willow (Salix spp.), resin birch (Betula grandulosa), and numerous heaths [e.g., Labrador tea (Ledum palustre), mealberry (Arctostaphylos uva-ursi), crowberry (Empetrum nigrum), and alpine blueberry (Vaccinium vitis-idaea)].

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Fig.3. Vegetation map of northern Alaska showing the distribution of boreal forest and tundra (after Viereck and Little, 1972; and Brown et al., 1980). Tundra has been divided into littoral, wet sedge-grass, sedge tussock-shrub, and herbaceous tundra (included in mixed tundra). The tundra of the Brooks Range is classified as mixed tundra because it is a mosaic of wet sedge-grass, sedge tussock-shrub, and herbaceous tundra types.

277 White spruce and paper birch, in mixtures or as single species, are found on warm, well-drained sites. White spruce is abundant along elevated levees and old meander banks of major rivers, especially in the Yukon Flats and along the K o y u k u k River. Paper birch establishes soon after fire and is typically succeeded by white spruce. Balsam poplar c o m m o n l y grows with willows on riparian sites and active flood plains. Quaking aspen is a minor c o m p o n e n t of the boreal forest and, unlike balsam poplar, is most often found on upland sites. Black spruce is m o s t c o m m o n on sites with cold soil temperatures. It is particularly abundant in low-lying muskegs and north-facing upland sites, both of which are characterized by permafrost close to the ground surface. Black spruce does n o t reach latitudinal treeline, located on the southern slopes of the Brooks Range, despite its extreme tolerance for cold soil and its c o m m o n occurrence at treeline on the interior plateaux (see Fig.5b). Latitudinal treeline is formed by white spruce and paper birch. Small populations of paper birch and balsam poplar, however, are found b e y o n d the limits of white spruce in the Kotzebue Sound drainage and the Arctic Foothills, respectively. In northeastern and northcentral Alaska, closed boreal forest is found near treeline, b u t in northwestern Alaska broad areas of open spruce woodland or mixed forest--tundra exist. White spruce, paper birch, and balsam poplar are the predominant tree species of this ecotone. Tree populations in mixed forest--tundra are densest in the river valleys and gradually decrease in density with increased altitude or distance from the rivers. In mid- to high-elevation sites, mixed f o r e s t - t u n d r a is replaced by sedge tussock--shrub tundra or herbaceous tundra. Tundra. Tundra communities dominate the vegetation north of 68°N latitude, b u t patches of tundra are also c o m m o n at sites exceeding 700 m elevation within the boreal forest (Fig.3). The north Alaskan tundra can be classified into four types: sedge t u s s o c k - s h r u b tundra, wet sedge--grass tundra, littoral tundra, and herbaceous tundra. The major divisions within the tundra follow important topographic, climatic, and drainage features. Sedge tussock--shrub tundra is the dominant tundra t y p e found in the mixed forest--tundra south of the Brooks Range, the Arctic Foothills, and elevated surfaces of the Arctic Coastal Plain. Tussocks are predominantly clones of cottongrass (Eriophorum vaginatum), although other sedges (Carex spp.) and grass species are frequently present. Associated shrubs include dwarf birch (Betula nana), alpine blueberry, and Labrador tea. In drier areas south of the Brooks Range, resin birch, crowberry, four-angled cassiope ( Cassiope tetragona ), and willow species (Salix glauca, Salix pulchra ) are also important members of this tundra community. Within the sedge tussock-shrub tundra c o m m u n i t y , dense shrub thickets are locally important. For example, thickets of alder and willow are c o m m o n along rivers and active flood plains of the Arctic Foothills. The Arctic Coastal Plain is dominated b y a wet sedge--grass tundra characterized by meadow-like communities that lack shrubby species. The most c o m m o n herbaceous species in these communities are Carex aquatilis and

278

Eriophorum angustifolium.

Low-lying shrub species, such as four-angled cassiope, alpine blueberry, and Labrador tea, are found on drier, elevated surfaces of the coastal plain. The littoral tundra is f o u n d on coastal strands, saline marshes, and estuaries. Rhizomatous grasses, primarily Dupontia fischeri, are the predominant life form. Sedge species such as Eriophorum angustifoliurn and Carex aquatilis are also present, but dicotyledonous herbs and shrubs are virtually absent. Herbaceous tundra communities are characteristically found on exposed slopes, scree, and fellfield sites at high elevations in the Brooks Range. Plant cover at such sites is typically low, comprising such species as Dryas

octopetala, D. integrifolia, Salix rotundifolia, Saxifraga oppositifolia, Diapensia lapponica, and Oxyria digyna. THE DATA BASE In the summer of 1982, we sampled 72 lakes (Sites 1--72) along twelve transects spaced approximately 125 km apart (Fig.2). The distance between lakes along individual transects varied depending on changes in the vegetation and the availability of suitable sites. Short cores were collected using a Phleger-type gravity sampler (Phleger, 1951). The cores come from the centers of small lake basins, generally 0.3 to 4.0 km 2 in area. Some mountainous areas and regions near the Canadian border were undersampled because of lack of sites. In areas of thermokarst topography, we were forced to sample thaw lakes, even though their sediments may be disturbed by ice-rafting (Nichols, 1967). Ten grab samples (Sites 88--97) and 19 surficial sediment samples from other cores (Sites 73--87; Sites 98--103) were added to our analysis. Of the 101 samples analyzed, 42 came from lakes located in boreal forest, 18 from mixed forest--tundra, 3 from herbaceous tundra, 29 from sedge tussock--shrub tundra, and 9 from sedge--grass tundra. METHODS Sediment samples were processed b y standard laboratory procedures (Faegri and Iversen, 1975; Cwynar et al., 1979). Pollen residues were m o u n t e d in silicone oil and counted under X 400 and X 1000 (oil immersion) magnification with a Leitz Laborlux microscope. Pollen sums consisted of at least 400 grains except in four samples in which the pollen was sparse (Sites 79, 90, 94, 101). Percentages of all pollen and spores are based on a sum of all terrestrial pollen taxa. Pollen and spore percentages of individual taxa were placed on base maps, and contour lines, or isopolls, were drawn by hand around areas of similar percentages. The diameters of 25 birch pollen grains were measured in 96 samples. The pollen of paper birch tends to be larger than that of dwarf or resin birch, b u t the size overlap among species is great (Ives, 1977). Following Ritchie (1977), we used 20 pm as an arbitrary size criterion for categorizing grains as either large (more likely to be paper birch) or small (more likely to be shrub

279 birch). This is an imprecise m e t h o d and we do n o t a t t e m p t to make quantitative estimates of the proportions of either birch type. Spatial patterns of the size categories of birch pollen were examined by plotting mean size, modal size, and ratio of grains. These maps show similar patterns; only the ratio map is presented. The corpus length, saccus length, and saccus width of 25 spruce grains were measured in samples containing 10% or more spruce pollen. These measurements were analyzed by a linear discriminant analysis (Birks and Peglar, 1980; Brubaker et al., 1983) that classified each grain as either black or white spruce. The percentages of grains classified as white spruce were then plotted on the base map and contoured. The vegetation maps used in the text were adapted from Viereck and Little (1972, 1975) and Brown et al. {1980) as modified by our own field observations. The precise distribution of many species in northern Alaska is not well known, because most areas have had only cursory vegetational surveys. However, the currently available maps, supplemented by our own field observations, suffice for the broad spatial scale of our comparisons between pollen and vegetation. RESULTS

lsopoll maps. Isopoll maps were constructed for all pollen types to define regional patterns in the pollen data (i.e., distinctive, broad-scale, spatial configurations of pollen percentages). Such patterns were seen only in five major taxa (spruce, birch, alder, sedge, and grass) and in four of the less abundant types [juniper, heaths (Ericales), sweetgale (Myrica), and Sphagnum]. No regional patterns were found in combinations of less abundant types [e.g., juniper + soapberry (Shepherdia) + bunchberry (Cornus) representing a dry shrub c o m m u n i t y ] . The pollen patterns of the individual taxa showed few anomalies (i.e., individual sites at which pollen percentages deviate from nearby sites). Most of the anomalies can be explained by pollen deposition processes or variations within the local vegetation. The regional patterns are discussed below. Sedge and grass pollen. Sedge is the predominant herbaceous pollen taxon in the study area (Fig.4a). Its isopolls show a general north--south pattern that is modified by an east--west trend in the Kotzebue Sound drainage. The area between the 10% and 20% isopolls delimits the f o r e s t - t u n d r a boundary, with less than 10% sedge pollen in areas of the boreal forest and 20% to greater than 40% sedge pollen in the tundra of the Arctic Slope. Grass pollen, though less c o m m o n than sedge pollen, is locally important in coastal areas near Kotzebue Sound and the Arctic Coastal Plain (Fig.4b). Within these regions its percentages decrease with distance from the coast. The high grass percentages along the Arctic Ocean closely correspond to the littoral tundra zone of the coastal wetlands {Fig.3). Similar grass-rich communities, although smaller in area, occur along the coast of Kotzebue Sound and may account for the high grass pollen percentages there. High percentages

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of grass pollen in the lower Colville and Kuparuk Rivers likely represent local grass-dominated communities. The pollen record in lake surface sediments appears to differentiate between the littoral tundra with high percentages of both grass and sedge pollen and a more interior tundra dominated by sedge pollen• On the Arctic Slope, the 30% sedge isopoll generally corresponds to the transition from the sedge tussock--shrub tundra to the sedge--grass tundra. Additional pollen samples, however, are needed to define this ecotone more precisely• Spruce pollen• The 10% spruce isopoll closely delimits treeline (Figs.5a and 5b). The area with 20% or greater spruce pollen corresponds to closed boreal forest, which is characterized by high densities of spruce trees and nearly continuous forest cover. The vegetation between the 10% and 20% isopolls consists primarily of open spruce woodlands or mixed forest--tundra communities•

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282 In northwestern Alaska, however, samples from the forested portions of t h e lower K o b u k and Noatak Rivers have less than 10% spruce pollen. In this area, the prevailing winds come from treeless areas to the west and northwest. The low spruce pollen percentages, therefore, may be partially explained by the lack of wind-blown pollen from forested regions and the loss of spruce pollen that is blown to the east. The spruce woodlands of northwestern Alaska are separated from the main b o d y of the boreal forest by greater distances than are treeline populations in northcentral or northeastern Alaska, and thus may receive a smaller c o m p o n e n t of wind-transported pollen from areas with dense spruce populations. A second anomaly in the 10% spruce isopoll occurs in the eastern Brooks Range where two samples from the unforested upriver portions of the East Fork of the Chandalar River exceed 10% spruce pollen. Localized up-valley winds may account for the high percentages of spruce pollen at these sites. Frequencies of white spruce pollen correspond well with the distribution of white spruce trees (Figs.5b and 5c). Four areas show greater than 50% white spruce grains. In the southeastern Brooks Range, which is b e y o n d the limit of black spruce, white spruce is c o m m o n in gallery forests of river valleys draining the mountains. Portions of the lower Noatak and K o b u k valleys also are b e y o n d the range of black spruce, and the high percentages of white spruce correspond to these westernmost populations. White spruce is abundant on stabilized levees, old meander banks, and terraces in areas that border the Yukon, Porcupine, and K o y u k u k Rivers. Black spruce woodlands and forests, however, are c o m m o n on the upland and lowland sites flanking the flood plains of these rivers. Birch pollen. The isopoll map of birch displays a complex north--south pattern. Birch percentages generally exceed 30% in the boreal forest, whereas percentages of less than 30% characterize most tundra samples (Fig.6a). The 20% contour north of the Brooks Range corresponds roughly to the northern range limit of shrub birch (Fig.6b). The high frequencies of birch pollen in the boreal forest evidently reflect the presence of both shrub and tree birch species. The distribution of birch pollen ratios (Fig.6c) suggests that tree birch increases the percentages of total birch pollen in the boreal forest. The 30% isopoll near the confluence of the Yukon and Porcupine Rivers is dashed because percentages (24-29%) here are only slightly, and probably not significantly, below 30%. The anomalously low percentages of birch in western Alaska may be a result of high percentages of sedge and, at coastal sites, high percentages of grass. Shrub birch is present in this region, and the low percentages of birch pollen are puzzling. Large birch grains are most c o m m o n in lowlands and river valleys of northcentral and northeastern Alaska (Fig.6c). This pattern is similar to that of white spruce (Fig.5c) and probably reflects the tendency of both species to colonize well-drained lowland sites and south-facing hillsides. Forests of paper birch are clearly visible from the air in the vicinity of the Yukon, Porcupine, and K o y u k u k Rivers and also in the gallery forests of the eastern Brooks Range.

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284

Some samples from the lower Noatak and lower Kobuk River valleys and the Anaktuvuk-Kuparuk drainage also have a predominance of large-size birch pollen. The sites with large birch pollen in the west reflect the presence of outlying populations of paper birch on the Noatak River. The high frequency of large birch grains in the Arctic Foothills may be the result of wind-blown paper birch pollen that originates in the John River area, is carried through Anaktuvuk Pass, and deposited in lakes north of the Brooks Range. On the other hand, the large birch pollen may be blown from the Mackenzie Delta in Canada. Nelson (1979) suggested such a source area for spruce pollen in moss polsters from eastern and central portions of the Arctic Coastal Plain. More samples are needed from the northeastern portion of our study area to determine whether the Mackenzie Delta is a likely source area. 165

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285

Alder pollen. The pattern of the alder isopolls (Fig.7a) has an important east--west c o m p o n e n t and thus departs from the predominantly north-~south patterns of birch, spruce, and sedge pollen. The highest percentages of alder pollen are f o u n d in valley and mid-elevation sites of the western and central Brooks Range. Percentages decrease to the south and east, where spruce and birch are the predominant pollen taxa, and to the north and west, where grass and sedge pollen are more frequent. The 20% isopoll of alder roughly approximates the northern limit of alder shrubs (Fig.7b). As with birch, alder pollen percentages are low in the Kotzebue S o u n d Point Hope region. Although alder shrubs are present in this region, they are restricted to a few, rather small drainages. These populations may not be large enough to make substantial contributions to the regional pollen rain. The area with highest alder percentages is characterized by the highest mean annual precipitation in northern Alaska (Steinhausser, 1979), suggesting that moisture-related factors influence the distribution of alder. This finding is consistent with the observation that alder shrubs form dense thickets along streams and seepages. Additional field studies are needed to d o c u m e n t the distribution of alder shrubs in relation to precipitation and soil moisture. Other shrubs. Pollen of heaths, juniper, and sweetgale display regional patterns (Fig.8), even though these taxa are minor components of the pollen rain. Sites in which heath pollen exceeds 5% generally are located in coastal or near coastal regions of the Arctic Slope and Kotzebue Sound. Juniper pollen, although never reaching more than 3%, is found most often in sites of the southern flanks of the Brooks Range where juniper shrubs are abundant on dry south-facing slopes. Four sites located in better drained areas of the Yukon Flats also contain trace amounts of juniper pollen. Sweetgale pollen occurs in trace amounts at sites in the extreme southern portion of the study area. Because our study area includes only the northernmost part of this species range (Hult~n, 1968), the sweetgale pattern will probably prove more informative when examined in conjunction with samples from more southerly areas of Alaska. Spores and aquatic taxa. Spores occur most frequently in sites of the K o b u k and Selawik drainages in northwestern Alaska and in northcentral Alaska between the K o y u k u k and Yukon Rivers (Fig.9). The sum of spores map is similar to the Sphagnum map because Sphagnum is the most c o m m o n spore t y p e (Figs.9a and 9b). The isolines in the Sphagnum map, however, are displaced to the south and west. The high spore frequency in northwestern Alaska may reflect the moister climates of the west and warmer temperatures south of the mountains. Pollen of aquatic taxa did not show any regional patterns. The general paucity of aquatic pollen or spores was surprising, given the frequent and widespread occurrence of various aquatic species such as p o n d w e e d (Potamogeton), water lily (Nuphar), and quillwort (Iso~tes) in lakes throughout the region. Pediastrum, an alga, was found in many of the samples and occurred in high frequencies in various areas, especially in the samples from the Arctic Slope and coastal areas of Kotzebue Sound (Fig.9c). The frequency of Pediastrum,

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Fig.8. Isopoll maps showing modern percentages of: A. heath (Ericales); B. juniper (Juniperus), and C. sweetgale (Myrica). Tick marks point in direction of higher pollen percentages.

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288

however, may be primarily a function of lake depth. A scatterplot of Pediastrum percentages against water depths shows that the highest frequencies of Pediastrum occur in lakes with depths of 4 m or less (Fig.10). SUMMARY AND CONCLUSIONS

The general conclusions of this study are: (1) The vegetation is the primary factor determining the distribution of modern pollen in northern Alaska. The pollen spectra differentiate the three major vegetation zones of tundra, mixed forest-tundra, and boreal forest. Areas of dense boreal forest are characterized by the highest percentages of spruce pollen (greater than 20%), high values of birch pollen (greater than 30%) with a predominance of paper birch-sized grains in many samples, intermediate percentages of alder (20--30%), and low values of herb pollen {less than 20%). Areas of mixed forest--tundra display lower percentages of spruce (10--20%) and birch pollen (20--30%), the highest percentages of alder pollen (greater than 40%), intermediate percentages of herb pollen (10--20%), and, in certain areas, high percentages of Sphagnum spores (greater than 10%). Tundra spectra are characterized by low percentages of spruce pollen (less than 10%), intermediate to low percentages of birch (less than 30%) and alder (less than 20%) pollen, and high percentages of herb pollen (greater than 30%), especially sedge (20-70%). Areas of coastal tundra are differentiated by relatively high percentages of grass (greater than 10%) and heath pollen (greater than 5%). ~00.

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Fig.10. S c a t t e r d i a g r a m o f percent Pediastrum a n d w a t e r d e p t h (m). Pediastrum p e r c e n t a g e s are c a l c u l a t e d b a s e d o n t h e p o l l e n sum.

289 (2) The differentiation of forest, tundra, and forest--tundra vegetation by modern pollen is evident in many arctic-subarctic areas of the Northern Hemisphere (Birks, 1973). Tundra samples are dominated by nonarboreal pollen, particularly sedge and shrub birch. Samples from boreal forests are dominated by conifer (primarily pine and/or spruce), tree birch, and, in some areas, alder pollen. F o r e s t - t u n d r a pollen samples are more variable b u t usually show percentages intermediate to those of neighboring vegetation zones. Even though modern pollen consistently differentiates these zones, the specific pollen frequencies that characterize each vegetation type vary from area to area. For example, the northern extent of spruce trees is delimited by the 10% spruce isopoll in Alaska, b u t treelines are represented by other values in northeastern Canada ( 2 5 - 3 0 % , Davis and Webb, 1975; Lamb, 1984) and northcentral Canada (20--30%, Lichti-Federovich and Ritchie, 1968). This variation is caused, in part, by differences in the character of forest--tundra and boreal forest (e.g., broad vs. abrupt ecotones, differences in tree sizes and densities in forest stands). Therefore, care must be taken when interpreting past vegetational zone boundaries from modern isopoll-vegetation relationships. The choice of an isopoll value that represents the location of past treeline must be done in light of other pollen taxa present in the pollen assemblage and the likely nature of the past vegetation. (3) Although the flora of northern Alaska is simple compared with that of lower latitudes, variations do exist within the vegetational formations. The different distributions of certain key pollen taxa (e.g., black and white spruce in boreal forest, alder in forest--tundra, grass and sedge in tundra) make it difficult to adequately summarize a vegetation zone by either averages or ranges of pollen percentages. The vegetation zones are best summarized by the mapped patterns of pollen percentages that display both the variability within and the similarity between zones. (4) The close correspondence between the modern vegetation and pollen indicates that a rather detailed history of the boreal forest and tundra in Alaska can be reconstructed from fossil pollen data. The isopoll maps also clearly demonstrate that pollen records from single sites should not be used to infer regionwide pollen frequencies. Thus, a relatively dense sample grid of fossil pollen sites is necessary to gain a detailed understanding of the vegetational history of northern Alaska. (5) Each of the major taxa that shows regional patterns in the modern data has been important historically. However, several fossil pollen spectra have no modern counterparts. For example, the modern percentages of wormw o o d (Artemisia), poplar, and juniper pollen never are as great as in the fossil samples from northern Alaska (Brubaker et al., 1983; Ager, 1983). Moreover, early Holocene pollen spectra are characterized by percentages of birch pollen that frequently exceed 50%. Such high percentages of birch pollen generally are absent from the modern pollen assemblages. We currently are making a detailed comparison of modern and fossil pollen percentages from northern Alaska to determine which modern spectra can be used as analogs for the reconstruction of past vegetation of this region.

290 ACKNOWLEDGMENTS This research was f u n d e d b y N S F G r a n t DPP 8 1 - 0 6 8 6 . Original pollen c o u n t s a n d m o r e specific d e s c r i p t i o n s o f the sites can be p r o v i d e d u p o n r e q u e s t t o t h e authors. We are grateful to M. E d w a r d s and K. M c F a r l a n e for their help in t h e field. L. G r a u m l i c h , D. N e w m a n , M. E d w a r d s and L. C h a r b o n n e a u p r o v i d e d h e l p f u l suggestions f o r i m p r o v i n g earlier versions o f this m a n u s c r i p t . D. N e w m a n d r a f t e d t h e figures. REFERENCES Ager, T.A., 1982. Vegetational history of western Alaska during the Wisconsin glacial interval and the Holocene. In: D.M. Hopkins, J.V. Matthews Jr., C.E. Schweger and S.B. Young (Editors), Paleoecology of Beringia. Academic Press, New York, N.Y., pp. 75---93. Ager, T.A., 1983. Holocene vegetational history of Alaska. In: H.E. Wright Jr. (Editor), Late-Quaternary Environments of the United States. Univ. Minnesota Press, Minneapolis, Minn., pp.128--141. Anderson, P.M., 1982. Reconstructing the Past: The Synthesis of Archaeological and Palynological Data, Northern Alaska and Northwestern Canada. Thesis. Brown University, Providence, R.I., 559 pp. Anderson, P.M., 1985. Late Quaternary vegetational change in the Kotzebue Sound area, northwestern Alaska. Quat. Res., 24. Birks, H.J.B., 1973. Modern pollen studies in some arctic and alpine environments. In: H.J.B. Birks and R.G. West (Editors), Quaternary Plant Ecology. Blackwell, Oxford, pp.143--168. Birks, H.J.B. and Peglar, S.M., 1980. Identification of Picea pollen of late Quaternary age in eastern North America. Can. J. Bot., 58: 2043--2058. Brown, J., Everett, K.R., Webber, P.J., MacLean, S.F. and Murray, D.F., 1980. The coastal tundra at Barrow. In: J. Brown, P.C. Miller, L.L. Tieszen and F.L. Bunnell (Editors), An Arctic Ecosystem: The Coastal Tundra at Barrow, Alaska. Dowden, Hutchinson and Ross, Stroudsburg, Pa., pp. 1--29. Brubaker, L.B., Garfinkel, H.G. and Edwards, M.E., 1983. A late Wisconsin and Holocene vegetation history from the central Brooks Range: implications for Alaskan paleoecology. Quat. Res., 20: 194--214. Colinvaux, P.A., 1964. The environment of the Bering Land Bridge. Ecol. Monogr., 34: 297--329. Cwynar, L.C., Burden, E. and McAndrews, J.H., 1979. An inexpensive sieving method for concentrating pollen and spores from fine-grained sediments. Can. J. Earth Sci., 16: 1115--1120. Davis, R.B. and Webb, T. III, 1975. The contemporary distribution of pollen in eastern North America: a comparison with the vegetation. Quat. Res., 5: 395--434. Edwards, M.E., Anderson, P.M., Garfinkel, H.L. and Brubaker, L.B., 1985. Late Wisconsin and Holocene vegetation history of the upper Koyukuk region, Brooks Range, Alaska. Can. J. Bot., 63: 616--626. Faegri, K. and Iversen, J., 1975. Textbook of Pollen Analysis. Hafner Press, New York, N.Y., 295 pp. Hult~n, E., 1968. Flora of Alaska and Neighboring Territories: A Manual of the Vascular Plants. Stanford Univ. Press, Stanford, 1008 pp. Ires, J.W., 1977. Pollen separation of three North American birches. Arct. Alp. Res., 9: 73--80. Lamb, H.F., 1984. Modern pollen spectra from Labrador and their use in reconstructing Holocene vegetational history. J. Ecol., 72 : 37--59.

291

Lichti-Federovich, S. and Ritchie, J.C., 1968. Recent pollen assemblages from the western interior of Canada. Rev. Palaeobot. Palynol., 7: 297--344. Livingstone, D.A., 1955. Some pollen profiles from arctic Alaska. Ecology, 36: 587--600. Matthews, Jr., J.V., 1970. Quaternary environmental history of interior Alaska: pollen samples from organic colluvium and peats. Arct. Alp. Res., 2: 241--252. Murray, D.F., 1978. Vegetation, floristics, and phytogeography of northern Alaska. InL.L. Tieszen (Editor), Vegetation and Production Ecology of an Alaskan Arctic Tundra (Ecological Studies, 29). Springer, New York, N.Y., pp.19--36. Nelson, R.E., 1979. Modern pollen rain on the Chukchi and Beaufort Sea coasts, Alaska Appendix IVb. In: R.W. Hartz and D.M. Hopkins (Editors), Environmental Assessment of the Alaskan Continental Shelf, Annual Reports of Principal Investigators for the Year Ending March, 1978. U.S. Government Printing Office, Washington, D.C., pp. 112--119. Nichols, H., 1967. The disturbance of arctic lake sediments by " b o t t o m ice": a hazard for palynology. Arctic, 20" 213. Phleger, F.B., 1951. Ecology of Foraminifera, Northwest Gulf of Mexico: Part I. Foraminifera distribution. Geol. Soc. Am. Mere., 46: 1--88. Ritchie, J.C., 1974. Modern pollen assemblages near the arctic treeline, Mackenzie Delta region, Northwest Territories. Can. J. Bot., 52 : 381--396. Ritchie, J.C., 1977. The modern and late-Quaternary vegetation of the Campbell-dolomite uplands, near Inuvik, N.W.T., Canada. Ecol. Monogr., 47: 401--423. Sehweger, C.E., 1976. Late Quaternary Paleoecology of the Onion Portage Region, Northwestern Alaska. Thesis. Univ. Alberta, E d m o n t o n , Alta., 183 pp. Steinhausser, F., 1979. Climatic Atlas of North and Central America. I. World Meteorological Organization, Geneva, 38 pp. Viereck, L.A. and Dyrness, C.T., 1980. A Preliminary Classification System for Vegetation of Alaska. U.S. Department of Agriculture, Washington, D.C., 38 pp. Viereck, L.A. and Little, Jr., E.L., 1972. Alaska Trees and Shrubs. U.S. Department of Agriculture, Washington, D.C., 265 pp. Viereck, L.A. and Little, Jr., E.L., 1975. Atlas of United States Trees Volume 2. Alaska Trees and Common Shrubs. U.S. Department of Agriculture, Washington, D.C., 103 pp. Yarie, J., 1983. Forest Community Classification of the Porcupine River Drainage, Interior Alaska, and its Applications to Forest Management General Technical Report PMW-154. U.S. Department of Agriculture, Portland, Oreg., 68 pp.