Vegetation Ecotone Dynamics in Southwest Alaska During the Late Quaternary Linda B

Quaternary Science Reviews 20 (2001) 175}188

Vegetation ecotone dynamics in Southwest Alaska during the Late Quaternary Linda B. Brubaker! *, Patricia M. Anderson", Feng Sheng Hu# !College of Forest Resources, University of Washington Box 352100, Seattle WA 98195, USA "Quaternary Research Center, University of Washington Box 351360, Seattle WA 98195, USA #Department of Plant Biology, University of Illinois, Urbana IL 61801, USA

Abstract

To examine Late Quaternary vegetation change across the modern vegetation gradient from continuous boreal forest (central Alaska) to Betula shrub tundra (Bristol Bay region), pollen records from Idavain and Snipe Lakes are described and compared to those of four other sites in southwest Alaska. Major features of the vegetation history at Idavain Lake include herb-dominated tundra (ca. 14}12 ka BP), mixed herb/Betula shrub tundra (ca. 12}8 ka BP), and Alnus/Betula shrub tundra (8 ka BP to present). The Snipe Lake record reveals a brief period of herb tundra ('12 ka BP), Betula shrub tundra (ca. 12}8.5 ka BP), and Picea forest mixed with Alnus/Betula shrub tundra (ca. 8 ka BP to present). Comparisons with other pollen records indicate that southwest Alaska has been the location of major vegetation ecotones throughout the last 12 ka years. Northern areas have consistently been dominated by larger growth forms (shrubs or trees) than have southern areas. During the Betula period (12}8 ka BP), a dense Betula shrubland occupied central Alaska, changing to a mixed low-Betula shrub and herb tundra in the south. In the Alnus/Picea period (8 ka BP to present), Picea and Betula trees were more common to the north; Alnus and Betula shrubs more abundant to the south. Vegetation dynamics have been complex at individual sites and across the region. Each site shows both long- and short-term shifts in major taxa, but the magnitude of these changes varies across the transect. In addition, some pollen changes appear to be synchronous among sites (within the constraints of existing chronologies), whereas others are strikingly time transgressive across the region. Similar vegetation dynamics at all sites are: (1) long-term decreases in herb taxa during the Betula period, (2) short-term oscillations between Betula shrubs and herbs during the Betula period, and (3) major increase in Alnus shrubs ca. 8 ka BP. Signi"cant di!erences among sites include: (1) major expansion of Populus trees in northern but not southern areas during the Betula period, (2) progressively later expansion of P. glauca at northern sites than at southern sites (ca. 9.5}4.5 ka BP), and (3) #uctuation of P. glauca populations in extreme northern areas during the early Alnus/Picea period. ( 2000 Published by Elsevier Science Ltd. All rights reserved.

1. Introduction three continuous lake pollen records and a few discontinuous peat sections from the Yukon-Kuskokwim Delta Southwest Alaska encompasses extensive lowlands and Bristol Bay region (Ager, 1982; Hu et al., 1995, 1996). and scattered uplands extending from central Alaska Consequently, little is known about the long-term history of to the coasts of Bristol Bay and southern Bering Straits the area currently occupied by the broad ecotone between (Fig. 1). The region is currently characterized by promin- continuous boreal forest and southwest coastal tundra. ent vegetation and climatic gradients (Mock et al., 1998; Several recent studies have emphasized that the Viereck et al., 1992; Wahrhaftig, 1965). Continuous late Quaternary vegetation of Beringia was marked boreal forests occupy central Alaska, where summers are great spatial and temporal variability (Lozhkin et al., relatively warm and dry (Table 1), and shrub- or herb- 1993; Anderson and Brubaker, 1994; Hu et al., 1995). Not dominated tundra in coastal and near-coastal areas, surprisingly, the location of major vegetation ecotones where summers are cooler and wetter. Although the Late (e.g., tundra versus forest) has varied over time. As a re- Quaternary vegetation history of central Alaska is rela- sult, sites that are presently within the same vegetation tively well understood (Anderson and Brubaker, 1994), association may have been dominated by contrasting past vegetation of southwest Alaska is known from only vegetation types in the past, and vice versa. De"ning patterns of vegetation change at a variety of spatial scales * Corresponding author. has become an important goal of paleoenvironmental E-mail address: [email protected] (L.B. Brubaker). research because such patterns can provide critical

0277-3791/01/$- see front matter ( 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 1 2 4 - 4 176 L.B. Brubaker et al. / Quaternary Science Reviews 20 (2001) 175}188

Fig. 1. Regional study area showing locations of rivers, mountain ranges (shaded), pollen sites (ⅷ), and weather stations listed in Table 1 (1: King Salmon, 2: Puntilla, 3: McGrath). Approximate distribution limits of Picea is indicated by dashed lines. information for evaluating hypotheses about causal fac- uplands (Fig. 1). This region is bordered on the east by tors (Anderson and Brubaker, 1994; Bartlein et al., 1998). the Alaska Range and on the west by the Kuskokwim However, despite the increase in paleoecological research and Aklund Mountains. Late Wisconsinan glaciers over the past two decades (e.g., this issue), late Quater- covered the Aklund Mountains, the Alaska Range and nary vegetation patterns remain poorly known across possibly isolated peaks of the Kuskokwim Mountains, large areas of Beringia, and the description of these but did not extend into the lowland areas (Hamilton and patterns remains a high research priority. Thorson, 1983; Porter et al., 1983). The climate of this The overall goal of this paper is to describe the vegeta- region is characterized by a north}south gradient from tion history of a northeast-to-southwest trending region continental conditions in central Alaska (min Jan from central Alaska to Bristol Bay (Fig. 1). We "rst (! 153C, max July &203C, and annual precipitation describe pollen records from continous lacustrine cores ca 30 cm) to maritime conditions in the Bristol Bay at two new sites (Snipe and Idavain Lakes) in the modern region (min Jan '53C, max July &10}123C, and an- forest}tundra ecotone in the Upper Kuskokwim and nual precipitation ca 50 cm) (Table 1). Nushagak Lowlands. These data are then combined The major vegetation gradient in this region is with previously analyzed lake records to interpret vegeta- a transition from predominantly closed boreal forest in tional changes across the larger study region. Snipe Lake the north to shrub tussock or graminoid tundra in the lies near the southern limit of the continuous boreal south (Viereck et al., 1992). However, at all locations forest zone and Idavain Lake is located in moist shrub along the gradient, plant communities form distinct mo- tundra ca. 50 km from the coast of Bristol Bay. When saic patterns that re#ect the e!ect of soil and landform on placed in the context of pollen records from other coastal local growing conditions. In northern areas ('ca. areas and from interior areas to the north, these records 300 km from coast), closed conifer and/or hardwood reveal a complex history of vegetation change in south- forests cover most landscape locations, with Picea west Alaska over the last 12 ka BP. mariana communities on poorly drained lowlands and Picea glauca or mixed P. glauca-Betula papyrifera and Populus tremuloides stands on well-drained upland sites. 2. Regional setting Shrub B. glandulosa or alpine tundra occupies small areas at highest elevations. At intermediate locations The regional study area encompasses the Upper (50}300 km from coast), P. glauca and P. mariana are Kuskokwim Lowlands, Nushagak Lowlands and nearby generally less common, but occupy similar sites as further L.B. Brubaker et al. / Quaternary Science Reviews 20 (2001) 175}188 177

Table 1 (ca 6}8 m water depth). Regional vegetation is Betula Climatic data for four stations indicated in Fig. 1 (data from Western shrub tundra, with extensive Alnus thickets on nearby hill Regional Climate Center) slopes. Station name July Max. Jan. Min. Total precipitation (cm) Temp. (3C) Temp. (3C) 3.2. Snipe Lake

1. King Salmon 15.3 !13.4 48.6 Snipe Lake (60338N, 154317W, 579 m asl) lies in the 2. Puntilla 17.4 !20.7 41.9 western foothills of the Alaska Range (Fig. 1, Table 2). 3. McGrath 20.2 !27.4 40.6 4. Tanana 21.6 !27.8 32.0 The glacial history of this region is poorly known (Hamilton and Thorson, 1983; Porter et al., 1983), but late-Wisconsinan ice is thought to have receded by ca. 13 ka BP (P. Lea, personal communication). The lake north. Alnus crispa, B. glandulosa,orSalix are present in has a relatively complex bottom, with three small basins shrub tundra communities on most hill slopes. Herb- (8, 16, and 19 m water depths) surrounded by #at shelves dominated tundra occurs on the wettest, lowlying sites. ca. 5 m in depth. Three small inlet streams drain nearby At the southern end of the transect, the regional vegeta- slopes, but no outlet streams exist. The lake lies within tion is tall shrub B. glandulosa-Salix tundra, with dense the broad ecotone between boreal forest to the north and A. crispa thickets on hillsides. Isolated stands of P. glauca shrub tundra to the south. Isolated stands of P. glauca and Populus balsamifera occur on riparian sites and occur in mixtures with B. glandulosa shrubs and A. crispa southfacing, low-elevation hill slopes, but P. mariana is thickets within the Snipe Lake watershed. not present. Poorly drained, coastal lowlands are dominated by Poaceae and Cyperaceae communities with few shrubs. 4. Field and laboratory methods

Sediment cores were retrieved in summer from the 3. Study sites #at shelves of each lake using a modi"ed Livingstone square-rod sampler (Wright et al., 1984). Attempts to 3.1. Idavain Lake core deep basins were unsuccessful. Coring in each lake was stopped by sti!, clay-rich sediments. Changes in Idavain Lake (58346N, 155357W, 223 m asl) is located color and the organic, sand, silt, and/or clay content of in the southern foothills of the Alaska Range on the sediments were described in the "eld and whole-core southeastern margin of the Nushagak Lowlands, ca. magnetic susceptibility was measured in the laboratory 90 km northwest of Mount Katmai (Fig. 1, Table 2). The using a Bartington Instruments M.S.1 m with a 2-cm- late-Wisconsinan glacial history of the region is complex, thick loop. Magnetic susceptibility readings are re- with at least four ice advances originating from the ported without correction for sediment bulk density. Alaska Range between 26 and 10 ka BP (Brooks Lake Subsamples (1 cm) were removed and processed for pol- Glaciation; Stilwell and Kaufman, 1996). The Idavain len and spore identi"cation according to standard pro- Lake basin was deglaciated between the `early phasea of cedures for arctic sediments (PALE, 1994). Lycopodium the Brooks Lake and the Newhalen Stades (bracketing spore tablets (Stockmarr, 1972) were added to each ages, 26}16 and 14}12 ka BP), which deposited terminal sample prior to preparation. For both lakes, pollen of moraines at the western end of the lake and 2 km east of vascular plants and spores of Equisetum, Filicales, the lake, respectively. Idavain Lake is approximately Lycopodium, and Sphagnum were counted until a pollen 8 km long and 1.5 km wide, with two deep basins (20 and sum of at least 350 terrestrial pollen grains was reached. 21 m water depth) surrounded by extensive #at shelves Pollen and spore identi"cation was based on reference

Table 2 Site characteristics: latitude, longitude, elevation and watershed vegetation

Lake name Latitude Long Elevation (m) Vegetation

Wien Lake 64320 151316 305 P. glauca and P. mariana forest Farewell Lake 62333 153338 320 P. glauca and P. mariana forest Snipe Lake 60338 154317 579 P. glauca boreal forest/Betula shrub tundra Idavain Lake 58346 155357 223 Betula shrub tundra Grandfather Lake 59348 158331 142 sparse P. glauca forest/Betula shrub tundra Ongivinuk Lake 59334 159322 163 Betula shrub tundra with Populus stands 178 L.B. Brubaker et al. / Quaternary Science Reviews 20 (2001) 175}188 collections housed at the University of Washington and in magnetic susceptibility correspond to visible bands of identi"cation keys in Faegri and Iversen (1992). At coarse sand- and silt-sized material, respectively. Another Idavain Lake, spores of true mosses (Bryidae) were also interval of high magnetic susceptibility ('200 SI units, counted using morphological criteria describe by 750}800 cm) corresponds to a gray sediment band with (Brubaker et al., 1998). At Snipe Lake, percentages of increased silt content. Below 975 cm, the clay content P. glauca and P. mariana pollen were estimated at se- and magnetic susceptibility increase sharply and remain lected levels using maximum likelihood analysis of pollen high to the base of the core. grain measurements (25 grains per sample, Brubaker Although a detailed tephra analysis was not done, it is et al., 1987). Pollen and spore percentages were cal- likely that some of the distinctive sediment layers in this culated as percent of total terrestrial pollen. Percentage core originated from Alaska or Aleutian Range vol- diagrams were constructed and zones established by canoes. For example, a band of sand-sized particles at visual inspection. Radiometric C dates of bulk sedi- 1089}1093 cm consists entirely of glass shards with ments and AMS dates of unidenti"ed plant fragments minerology matching the Lephe tephra (Jim Riehle, per- ('500 lm) sieved from sediment were obtained for each sonal communication, U.S. Geological Survey Anchor- core. All dates are reported in uncalibrated C years, age) found extensively near Illiamna Lake, ca. 50 km following conventions by (Stuiver and Polach, 1977). The north of Idavain (Kaufmann and Stilwell, 1997). In addi- age-depth curves "t to radiocarbon dates (see below) and tion, a compacted, light-brown sediment band at the concentration of non-fossil Lycopodium spores were 8}10 cm core depth is probably tephra from the 1912 used to calculate pollen accumulation rates (PARs) eruption of Mt Katmai (Fierstein and Hildreth, 1992). (Faegri and Iversen, 1992). In total, 11 AMS C dates on unidenti"ed plant fragments and 1 radiometric C date on bulk sediment were obtained for the core (Table 3). A date at 5. Sediment description and chronology 144}145 cm (5.28 ka BP) is clearly anomalous compared to the age-depth trend established by the remaining dates 5.1. Idavain Lake and was deleted from consideration. Although the date at 210}215 cm (1.96 ka BP) is somewhat inconsistent with The 1245-cm long core from Idavain Lake is predomi- other dates and minor dating reversals occur in the lower nantly silts and clays, interrupted by narrow bands of half of the core, we used all of the remaining dates to sand-sized particles. The upper ca. 1000 cm consists of establish the age-depth curve. This decision was made moderately organic-rich silts and numerous layers of because curves using di!erent subsets of dates (e.g., pres- clastic material with variable texture and thickness. Mag- ence or absence of 1.96 ka BP date) were indistinguish- netic susceptibility is generally (150 SI units, but values able. Sediment-age assignments were based on a second- #uctuate greatly from 22 to 800 SI units (Fig. 2). The order polynomial "t to 11 dates and the assignment of most prominent feature of this portion of the magnetic 0 ka BP to the mud-water interface (Table 3). This curve susceptibility curve is an interval of higher values assigned a date of 13,135 ka BP to the Lephe tephra layer (100}150 SI units, 120}210 cm), bounded by two distinct at 1089}1093 cm, broadly similar to a published date of peaks (485 and 800 SI units). The upper and lower peaks 12.6 for this tephra (Pinney and Beget, 1991).

Fig. 2. Magnetic susceptibility of sediment cores from: (a) Idavain Lake; (b) Snipe Lake. L.B. Brubaker et al. / Quaternary Science Reviews 20 (2001) 175}188 179

Table 3 of the core (ca 0.0027 cm yr). Unfortunately, there was Radiocarbon dates for Idavain and Snipe Lakes not enough organic material to obtain another date for this level. Without additional information, we decided to Depth (cm) Date (ka BP) Laboratory number retain the date at 145}150 cm and to establish the age- Idavain Lake depth curve by linear interpolation, because this curve 144}145 5.28$0.06 CAMS 29159 option makes fewest assumptions about sedimentation 210}215 1.96$0.06 CAMS 29160 regimes. Considering these dating uncertainties, the 280}287 4.27$0.05 CAMS 29161 Snipe Lake chronology should be considered tentative 530}540 8.54$0.06 CAMS 18372 600}608 9.32$0.08 CAMS 26226 prior to ca 7.0 ka BP. 710}711 10.18$0.06 CAMS 26227 805}808 11.41$0.06 CAMS 26228 870}880 12.05$0.07 CAMS 18371 6. Vegetation history: Idavain and Snipe Lakes 952}954 12.28$0.06 CAMS 26229 1035}1045 12.07$0.49 Beta 6156 1196}1199 14.1$0.06 CAMS 29162 The following vegetation interpretation of the pollen 1201}1208 13.44$0.09 CAMS 30330 diagrams for Idavain and Snipe Lakes (Figs. 3 and 5) is based primarily on qualitative comparisons of their fossil Snipe Lake 39}40 3.11$0.06 CAMS 26224 pollen assemblages with spectra in mud}water interface 85}95 4.38$0.08 Beta 61561 samples from lakes in boreal and arctic regions of Alaska 122}123 5.08$0.06 CAMS 26225 (Anderson and Brubaker, 1986, 1994). The modern data 125}135 5.09$0.11 Beta 59072 set consists of more than 200 lakes, including each of the $ 142}145 6.33 0.06 CAMS 30329 lakes discussed in this paper. Comparisons between the 145}155 8.95$0.09 Beta 59073 170}180 12.71$0.14 Beta 61562 Idavain and Snipe Lake pollen records are made in the 190}200 11.48$0.15 Beta 59074 discussion section, as part of a larger assessment of late- glacial and Holocene vegetation gradients in the regional study area.

5.2. Snipe Lake 6.1. Idavain Lake

The stratigraphy of the 275-cm long core from Snipe 6.1.1. Zone ID-1 (1250}900 cm; ca 14}12 ka BP) Lake is complex. The upper ca. 55 cm of the core is The palynological assemblage is dominated by dominated by dark, organic-rich silts with low magnetic Cyperaceae, Poaceae, Artemisia, Salix, and several minor susceptibility (10}50 SI units) (Fig. 2). Sediments between herbs (e.g., Thalictrum, Pedicularis, Lamiaceae, Cary- 55 and 150 cm are alternating bands of light and dark ophyllaceae, Brassicaceae) and true mosses (Bryidae). silts (30}50 SI units) interrupted by discrete lenses of Picea, Betula and Alnus pollen reach 2}4% in basal sedi- sand-sized particles with higher magnetic susceptibility ments. Total pollen accumulation rates are low (ca. 500 (100}400 SI units). A distinctive interval of light-yellow grains/cm/yr). sediment with silty texture and near-zero magnetic sus- The diverse habitats represented by pollen taxa in this ceptibility occurs at 150}185 cm. This sediment reacted zone indicate that a wide range of plant communities strongly with weak HCl. A similar sediment layer at occupied di!erent landscape locations near Idavain Farewell Lake (Fig. 1) consisted primarily of calcium Lake. For example, pollen of Thalictrum, Rumex/Oxyria, carbonate. Based on an extensive examination of geo- Lamiaceae and Salix (the only common shrub) is evid- chemistry and biotic indicators, the interval at Farewell ence for moist microsites, possibly in topographic de- Lake was interpreted to represent a period of high aqua- pressions with late snowmelt. The abundance of tic productivity (Hu et al., 1996). Below 185 cm the Snipe Cyperaceae pollen may indicate the presence of meadow Lake core is comprised of alternating bands of gray silts communities with su$cient moisture to support continu- and blue clays with increasing, but variable magnetic ous plant cover, but this interpretation must be con- susceptibility (higher SI values for clays than for silts). sidered tentative because members of this family occur in The core chronology is provided by 3 CAMS a wide variety of habitats. Evidence for discontinuous dates on unidenti"ed plant fragments and 5 radiometric plant cover and soil erosion include high percentages of C dates on bulk sediment (Table 3). The AMS C Artemisia pollen, low PARs, and high magnetic suscepti- date at 122}123 cm and the radiometric C date at bility of sediments. Pollen of Picea, Betula and Alnus may 125}135 cm are nearly identical (5.08 and 5.09 ka BP, have been wind blown from distant sources, reaching respectively), indicating no major discrepancy between minor percentages in the late-glacial sediments due to the dating methods for this portion of the core. The date at low pollen productivity of the regional vegetation. 145}150 cm (8.95 ka BP) may be anomalous, as it results Non-vascular plants were also an important part of in an unusually slow sedimentation rate for this portion the regional #ora, because Bryidae spores percentages 180 ..Buae tal. et Brubaker L.B. / utraySineRe Science Quaternary v es2 20)175 (2001) 20 iews } 188

Fig. 3. Pollen and spore percentage diagram for Idavain Lake. L.B. Brubaker et al. / Quaternary Science Reviews 20 (2001) 175}188 181 are relatively high throughout this period. Although munities invaded some microsites. Overall, Betula and most moss spores belong to morphological groups that Cyperaceae percentages in Zone ID-2a are similar to include species from a wide range of ecological settings, modern assemblages from Alaskan Betula-shrub tussock one spore type has strong indicator value. Encalypta tundra (Anderson and Brubaker, 1986, 1994). However, rhaptocarpa, produceds distinctive spores (Brubaker percentages of Artemisia and minor herbs are higher and et al., 1998) and is presently restricted to dry, wind swept Ericales and Alnus lower than in modern tundra spectra sites in arctic and alpine regions (Horton, 1983; Vitt et al., (Anderson and Brubaker, 1986, 1994). These compari- 1988). The consistent presence of its spores in the lower sons imply that Zone ID-2a vegetation contained a lar- portion of Zone ID-1, therefore, agrees with the inter- ger component of herbs than does the current Betula- pretation of discontinuous plant cover based on pollen shrub tundra of Alaska. assemblages and on physical sediment characteristics. Artemisia pollen and Equisetum, Sphagnum, and 6.1.4. Subzone ID-2b (810}730 cm, 11.5}10.2 ka BP) Bryidae spores increased and Cyperaceae pollen declined This subzone is de"ned by abrupt declines in Betula in the uppermost Zone ID-1 sediments. This period cor- pollen and Bryidae spores and increases in Cyperaceae responds roughly to the end of the Newhalen Stade, and Salix pollen. These changes probably indicate a re- when the Brooks Lake glacier receded from areas east of duction in Betula shrubs and an expansion in herb Idavain Lake (Stilwell and Kaufman, 1996). The decline tundra communities near Idavain Lake. Nevertheless, in magnetic susceptibility and sediment clay content Betula shrubs probably continued to be a component of (975}950 cm) suggests a reduction in the input of glacial the regional tundra, because Betula percentages do not meltwater during this period. As before, the pollen and decline below those found at the modern range limit of spore assemblages probably re#ect plant communities Betula shrubs in Alaska (Anderson and Brubaker, 1986). growing on di!erent landscape locations. The increases The extent of Bryidae ground cover declined dramati- in Artemisia pollen may re#ect the invasion of Artemisia cally. The pronounced increase in magnetic susceptibility species onto well-drained, recently deglaciated substrates between 800 to 750 cm suggests that slope erosion in- east of Idavain Lake (e.g., Oswald, 1998). In contrast, creased, possibly due to an overall reduction in plant elevated percentages of Equisetum, Sphagnum, and pos- cover in the Idavain Lake watershed (Fig. 4). sibly some species of Bryidae are probably evidence of increased plant cover in moist settings and the onset of 6.1.5. Subzone ID-2c (730}550 cm, 10.2}8kaBP) peat accumulation at the wettest microsites. This period is characterized by a series of percentage peaks in Betula (730}690 cm), Poaceae (690}600 cm), Poly- podiaceae (590}550 cm) and Sphagnum (590}550 cm), 6.1.2. Zone ID-2 (900}550 cm; 12}8kaBP) Increased percentages of Betula pollen mark the beginning of this zone. Total PARs vary between ca. 2000 and 5000 grains/cm/yr. Fluctuations in the percentages of Betula, Cyperaceae, and a number of other taxa (e.g., Poaceae, Equisetum, Polypodiaceae) are the basis of iden- tifying three subzones that indicate substantial changes in vegetation composition (see Section 7 for additional interpretations of these #uctuations).

6.1.3. Subzone ID-2a (900}810 cm, 12}11.5 ka BP) This subzone begins with increases in percentages of Betula, Aconitum and Apiaceae pollen and Lycopodium annotinum spores and declines in pollen/spore percentages of other taxa (Cyperaceae, Poaceae, Artemisia, Cary- ophyllaceae, Brassicaceae, Thalictrum and Bryidae). The increase in Betula pollen probably represents the expansion of Betula shrubs (rather than trees) into the regional herb tundra, because macrofossil analyses at other sites in the Bristol Bay region indicate the presence of B. glandulosa/nana during this period (Hu et al., 1995). The general decline in herb pollen and Bryidae spores suggests that the overall extent of herb and Bryidae cover declined, although increases in Aconitum, Apiaceae Fig. 4. Magnetic susceptibility (thin line) and Betula pollen percentages and Lycopodium annotinum suggest that moist herb com- (thick line) at Idavain Lake, 600}900 cm core depth. 182 L.B. Brubaker et al. / Quaternary Science Reviews 20 (2001) 175}188 and a general decline in Cyperaceae, Artemisia and mentation regime of the coring site during this period, Bryidae percentages. Populus and Salix percentages but mechanisms that might have produced the observed increase at the upper Zone ID-2c boundary. Betula changes in the pollen and sediment are unclear. shrubs apparently expanded brie#y and then declined. Members of Poaceae, Polypodiaceae, and Equisetum 6.2. Snipe Lake probably became abundant in moist habitats, and Sphag- num increased on wet sites with organic soils. This inter- 6.2.1. Zone SN-1 (220}200 cm; ca 12}11.5 ka BP) pretation of an expansion of moist habitats is supported No pollen or plant fragments were found below by increased percentages or "rst occurences of pollen of 220 cm core depth. Pollen assemblages in this zone are several minor taxa (Apiaceae, Aconitum, Sanguisorba, and dominated by herbs, and PARs are extemely low ( 50 Galium) that currently occupy moist stream banks and grains/cm/yr) (Fig. 5). depressions with late snow melt (Viereck et al., 1992). The The lack of organic material, high clay content and rise in Populus and Salix percentages at the end of this high magnetic susceptibility of basal sediments suggest period suggests that Populus trees and Salix shrubs were that these sediments were deposited by meltwater from more common than at present on #ood plains and/or late-Wisconsinan glaciers, prior to plant colonization south-facing slopes. P. balsamifera is the most likely near Snipe Lake. Moderate percentages of Poaceae, Cy- species in this period, because it is the only Populus that peraceae, Artemisia, and Salix pollen and the presence of currently grows in Alaskan shrub tundra (Viereck et al., several minor herb types (e.g., Thalictrum, Dryas, 1992). Rumex/Oxyria) suggest that a variety of herb tundra communities became established in the area. Artemisia and 6.1.6. Zone ID-3 (850}0cm; 8 ka BP to present) Dryas probably occupied sparsely vegetated mountain This zone begins with an increase in Alnus pollen slopes and rocky substrates. Cyperaceae and mesic herbs percentages accompanied by decreases in percentages of (Thalictrum, Rumex/Oxyria) may have formed more con- several pollen and spore taxa (e.g., Salix, Poaceae, Ar- tinuous plant cover on less harsh microsites, and Salix temisia, Equisetum, Polypodiaceae, and Sphagnum). Picea shrubs were probably restricted to moist sites and snow- pollen percentages increase to above trace amounts near beds. the middle of the zone (ca 4.2 ka BP). In addition, Alnus pollen percentages decrease and several herb and spore 6.2.2. Zone SN-2 (200}150 cm; 11.5}9kaBP) percentages (e.g., Cyperaceae, Artemisia, Polypodiaceae, The transition from Zone SN-1 to Zone SN-2 is char- and Sphagnum) increase between 120 and 210 cm. Total acterized by increases in pollen or spore percentages of PARs increase slightly at the begining Zone ID-3, and Betula, Ericales, Lycopodium, Sphagnum, and Equisetum are generally 2000}5000 grains/cm/yr. plus decreases in pollen of Cyperaceae, Artemisia and The major change in regional vegetation at the begin- several herbs (e.g., Thalictrum, Dryas, Rumex). In contrast, ning of this period was the widespread increase in Alnus pollen percentages of Poaceae and Salix remain relatively shrubs on mountain slopes and along stream banks. constant across the lower boundary. Populus pollen Although Cyperaceae and Poaceae communities may percentages increase slightly near the Zone SN-2/3 have declined somewhat, they probably remained com- boundary. The sediment interval with low magnetic sus- mon in the regional landscape. The increase in Picea ceptibility (150}185 cm) occurs within this zone. pollen probably re#ects the expansion of P. glauca trees The increases in Betula and Ericales pollen and into the northern Bristol Bay region (Hu et al., 1995), Lycopodium, Sphagnum, and Equisetum spores probably where they became established as isolated stands or trees record the establishment of Betula shrubs (e.g., Hu et al., on well-drained stream sides and south-facing slopes. 1996) and the expansion of moist plant communities on However, trees were probably never common near organic soils. Betula shrub densities may have been Idavain Lake, because Picea pollen percentages and greater than those in modern Alaskan Betula-shrub tun- PARs never exceeded those in near-surface sediments. dra, because Betula percentages exceeded values current- The short-term decline in Alnus pollen and increase in ly found in this vegetation zone (Anderson and Brubaker, herb pollen and spore percentages (ca 3.5}2.5 ka BP) 1986, 1994). Although Betula shrubs may have displaced corresponds to a sediment interval of increased magnetic herb communities across much of the landscape, the susceptibility. Although these sediment and pollen cha- relatively high percentages of Poaceae, Salix and Ar- nges might be evidence of a decrease in shrub cover, temisia suggest that these components of the Lateglacial expansion of herbs, and increase in slope erosion, it is tundra remained common. Herb tundra communities also possible that this interval represents a period of probably persisted at cold, dry sites at higher elevations anomalous sedimentation at Idavain Lake. Reversals in near Snipe lake, whereas Betula shrub communities ex- radiocarbon dates (Table 1), two distinct peaks in mag- panded at lower elevation sites with warmer temper- netic suseptibility (485 and 800 SI), and an unusual peak atures and moister soils. Slight declines in Betula in PARs (ca 8000/cm/yr) suggest a change in the sedi- and increases in Poaceae percentages between 170 and L.B. Brubaker et al. / Quaternary Science Reviews 20 (2001) 175}188 183

Fig. 5. Pollen and spore percentage diagram for Snipe Lake.

180 cm may indicate a minor #uctuation in shrub and Picea pollen indicates the local presence of trees, Picea herb cover in the regional vegetation (see Section 7). The populations established near Snipe Lake sometime between minor peaks in Populus pollen at the SN-2/3 boundary 7 and 5 ka BP. The assignment of this broad time interval suggest that P. balsamifera expanded brie#y on riparian for the arrival of trees is also justi"ed given the uncertainty sites in the Snipe Lake area. of dating during this portion of the core.

6.2.3. Zone SN-3 (150}0cm; 8.9 ka BP to present) 6.2.5. Subzone SN-3b (125}0cm, 5 ka BP to present) A sharp increase in Alnus and declines in Poaceae, Picea pollen percentages reach 10% at the beginning Salix, and Artemisia pollen percentages mark the begin- of this period and ca 20% ca 4 ka BP. Picea pollen ning of this zone. Picea pollen percentages increase grad- initially was predominantly P. glauca, but P. mariana ually during the middle of the period. The pollen record became more common than P. glauca ca 4 ka BP. Thus is divided into two subzones based primarily on changes P. glauca trees apparently increased in overall abund- in Picea percentages. ance in the regional vegetation until ca 4 ka BP, when P. mariana began to expand on poorly drained, low-lying 6.2.4. Subzone SN-3a (150}125 cm, 8.9}5kaBP) sites. This vegetation change probably marks the estab- The sharp increase in Alnus pollen re#ects the regional lishment of the current mosaic of plant communities near expansion of Alnus shrubs, which probably formed dense Snipe Lake. thickets on mountain slopes near Snipe Lake, and the decreases in Poaceae and Artemisia pollen probably re- #ects the regional decline of herb tundra communities. 7. Discussion Although the gradual increase in Picea glauca percentages between 140 and 110 cm provides evidence for the 7.1. Vegetation gradients in Southwest Alaska spread of this species into the general region, the date for the "rst arrival of trees near Snipe Lake is di$cult Recent studies at Wien, Farewell, Grandfather, and to determine from such slow percentage increases. For Ongivinuk Lakes (Fig. 1, Table 2; Hu et al., 1993, 1995, example, investigations of Picea pollen and macrofossils 1996) provide several continuous palynological records in Holocene lake sediment (Hu et al., 1993, 1995, 1996) that can be compared with the pollen diagrams from and the patterns of Picea pollen percentages in modern lake Idavain and Snipe Lakes. These comparisons reveal spa- sediments (Anderson and Brubaker, 1986; Anderson et al., tial and temporal patterns of late Quaternary vegetation 1994) suggest that Picea pollen reaches 3}10% where Picea extending from central Alaska to the Bristol Bay region. trees are present in lake watersheds. Assuming that 3}10% Of the six sites, Farewell and Snipe Lakes are the most 184 L.B. Brubaker et al. / Quaternary Science Reviews 20 (2001) 175}188

Fig. 6. Pollen percentage diagrams (12 ka BP to present) for major pollen taxa at Wien, Farewell, Snipe, Idavain, Grandfather, and Ongivinuk Lakes. Horizontal line at 8 ka BP separates Betula and Alnus/Picea periods.

`isolated,a because they are each '150 km from other 7.1.1. Betula period (12}8kaBP) lakes with palynological records. Overall, the discussion The Betula period is characterized by strong, north- below should be considered a preliminary picture of the east-to-southwest gradients in both the composition vegetation history of this region. We emphasize only and dynamics of vegetation (Figs. 6 and 7). Pollen major features of past vegetation gradients, interpreting percentages of Betula are consistently higher and herbs trends in the "ve most important pollen types (Betula, (predominantly Poaceae and Cyperaceae) lower at Alnus, Picea, Populus, and sum of Cyperaceae, Poaceae northern than at southern sites, indicating a gradient and Artemisia) for two broad periods (Betula period, from shrub-dominated tundra in central Alaska to 12}8 ka BP, and Alnus/Picea period, 8 ka BP to present) graminoid-dominated tundra in the Bristol Bay region. (Fig. 6). For these comparisions, we have recalculated High Betula percentages at Farewell and Wien Lakes original pollen percentage at each site based on the indicate that northern portions of the transect were occu- pollen sum of the common taxa: Betula, Alnus, Picea, pied by dense shrublands, possibly similar to modern Cyperaceae, Poaceae, and Artemisia. Populus percentages high-shrub Betula communities in the central Alaska were based on the pollen sum of these taxa. Populus was Range (Viereck et al., 1992). Intermediate herb and Betula not included in the sum, because pronounced #uctu- percentages at Snipe Lake suggest that this site was near ations in Populus at northern sites strongly in#uenced the center of the vegetation gradient. The southern end pollen percentages of other taxa. Deleting Populus thus of the study area was occupied by graminoid tundra facilitated the comparison of pollen records among communities with some Betula shrubs, as Betula pollen sites. percentages were similar to those in present-day tussock L.B. Brubaker et al. / Quaternary Science Reviews 20 (2001) 175}188 185

Fig. 7. Pollen percentage diagrams (12}8 ka BP) for Betula and herb pollen at Wien, Farewell, Snipe, Idavain, Grandfather, and Ongivinuk Lakes. Shaded rectangles indicate period of Betula and herb pollen percentage #uctuations.

tundra with Betula shrubs (Anderson and Brubaker, preceded the maximum in Populus percentages at each 1986; Anderson et al., 1994). site. Di!erences in the timing and magnitude of the The pollen records at all sites indicate a #uctuation in Populus pollen rise among sites con"rm growing evidence shrub and herb cover, followed by an increase in Populus for marked spatial and temporal variability of Populus (Figs. 6 and 7). However, the magnitude of these vegetation expansions in Alaska at the late-glacial/Holocene changes varied along the transect, with the decline transition (Bartlein et al., 1995). in Betula shrubs and increase in herbs most pronounced in The last major vegetation change during the Betula the Bristol Bay region (see Section 7) and the subsequent period occurred only in the far northern part of the study expansion of Populus trees greatest in the north. Changes in area. Speci"cally, the rise in pollen percentages and Betula and Populus pollen percentages are minor at Snipe the "rst presence of macrofossils indicate the arrival of Lake compared to other sites, suggesting that the vegeta- P. glauca trees at Wien Lake ca. 9.5 ka BP. Wien Lake tion in the central portion of the study region remained must have been close to the western boreal forest-tundra relatively stable despite major changes in northern and ecotone, because at this time Picea pollen is common in southern areas. Alternatively, larger #uctuations in Betula most pollen diagrams to the east of Wien Lake, but and Populus pollen might have gone undetected because of absent or rare in records to the west (Hu et al., 1993; the coarse sampling resolution at this site. Anderson et al., 1994). Macrofossil records at Wien Lake The maximum percentages of Populus pollen are also indicate that Betula papyrifera trees arrived at Wien higher in northern than southern portions of the study Lake ca 9.5 ka BP. region. In the north, percentages exceed 40%, suggesting that Populus stands occupied a variety of upland and 7.1.2. Alnus/Picea period (ca 8 ka BP to present) riparian sites, resulting in a regional vegetation of mixed As in the Betula period, pollen assemblages during the Populus woodlands and Betula shrub tundra. In contrast, Alnus/Picea period indicate strong northeast-to-south- Populus percentages rise to only 5}10% at southern sites, west gradients in vegetation composition and dynamics. indicating minor population expansions in restricted All sites show increases in Alnus and Picea and decreases habitats. In addition, the timing of Populus expansion in Betula and herb pollen; however, Picea and Betula appears to have been earlier in the north than in the percentages are consistently higher and Alnus and herb south. Populus percentages peak 10}12 ka BP at Wien percentages lower at northern than southern sites. and Farewell Lakes, but do not increase until 8}9kaBP Alnus pollen percentages increase sharply at most at Snipe Lake in the central area and at Idavain, Grand- lakes 8.5}7.5 ka BP, indicating that Alnus shrubs spread father and Ongivinuk Lakes near Bristol Bay. The #uctu- rapidly across the region. Although the rise in Alnus ations of Populus and herb/Betula are clearly two percentages is slightly earlier at Idavain and Snipe Lakes separate `eventsa in the overall regional vegetation his- than at other sites, these di!erences may re#ect dating tory, because the oscillation in herb/Betula percentages uncertainties, particularly at Snipe Lake. 186 L.B. Brubaker et al. / Quaternary Science Reviews 20 (2001) 175}188

In contrast to the Alnus rise, dates for the initial in- pollen obscured variations in percentages of other taxa. crease in Picea pollen (P. glauca at all sites examined) Removing Populus from the pollen sum has made #uctu- show a trend of progressively younger dates from north- ations between Betula and herb percentages easier to ern to southern sites. P. glauca percentages decreased for detect. Recent, detailed pollen analyses at Windmill and a brief period after their initial rise at Wien Lake. This Birch Lakes, located in the foothills of the northern decline has been observed at other locations in central Alaska Range and in the Tanana Valley, respectively, Alaska (Anderson and Brubaker, 1994), but is not evident document short-term oscillations between Betula and at other sites along our transect. The date for the increase Artemisia pollen percentages (Bigelow, 1997; Bigelow and in P. glauca pollen at Farewell Lake (ca 8 ka BP) sug- Edwards, 2001). Thus pollen evidence at a number of gests that P. glauca trees continued to spread southward sites suggests that a roughly synchronolus #uctuation during the period of declining abundance in central between shrubs and herbs extended across much of Alaska (Hu et al., 1993). From ca 8 to 6 ka BP, mixed southwest and central Alaska. P. glauca/B. papyrifera forests dominated northernmost Several di!erences among sites suggest that the taxa parts of the transect near Wein Lake, P. glauca forests involved in these vegetation changes varied geographi- without B. papyrifera were common near Farewell Lake, cally. The vegetation shift on Kodiak Island is character- and Betula shrub tundra with Alnus thickets characterized by a sharp decrease in ferns and an increase in ized areas to the south. The appearance of P. glauca Empetrum. The dominant feature of vegetation change at macrofossils at Grandfather Lake at 4 ka BP suggests all other sites was a decline in Betula shrubs, but the that this species had spread to its modern range limit response of herb taxa di!ered among sites. For example, between 6 and 4 ka BP. Pollen percentages for P. mariana Cyperaceae increased in the southern Alaska Range (not shown) indicate that this species also spread from (Idavain Lake), Artemisia and other Asteraceae became north to south, becoming a common tree near Wien, more common in the Aklund Mountains (Grandfather Farewell, and Snipe Lakes ca 6.5, 4 and 4 ka BP, respec- and Ongivinuk Lakes), whereas Poaceae increased on tively. Macrofossil evidence suggests that B. papyrifera the Yukon Delta (Tungak Lake). Salix shrubs appear to expanded near Farewell Lake ca. 6 ka BP, but B. papyrif- have increased slightly near Idavain but decreased or era trees may never have been present at Snipe Lake. remained constant in the Aklund Mountains and Yukon Most features of the modern vegetation gradient were Delta. The magnitude of Betula shrub #uctuations also established by ca 4.5 ka BP, when P. glauca reached might have di!ered somewhat across the region, because the Aklund Mountains and P. mariana established near the recovery of Betula appears to have been more pro- Farewell and Snipe Lakes. However, P. glauca pollen nounced in the Aklund Mountains and Yukon Delta percentages continued to increase slowly at Grandfather than in the southern Alaska Range. Lake, indicating that this species reached modern abund- Overall, the brief widespread decline in Betula shrubs ances in the area ca 2 ka BP (Hu et al., 1995). and increase in herb tundra suggests that a short interval of colder temperatures interrupted the general climatic 7.2. Glacial-interglacial transition: evidence for a Younger warming at the late-glacial/Holocene transition. The Dryas type event? magnitude and duration of this cooling appears to have been greater in coastal southwest Alaska than Several paleoecological records in near-coastal areas in central Alaska. Unfortunately, the pollen stratigraphies of southwest Alaska show strong evidence of short-term and radiocarbon chronologies are not su$ciently vegetation #uctuations during the last glacial}inter- detailed to date this interval precisely at most sites. In glacial transition. Pollen records from Idavain, Grand- addition, the low organic content of sediments has father and Ongivinuk Lakes suggest a short term, resulted in inconsistent dating at some lakes (Hu et al., regional decline in the cover of Betula shrubs and an 1995). Despite these limitations, existing radiocarbon expansion of herbs in the Bristol Bay region (Fig. 6). chronologies indicate that the timing of the Betula/ herb Similar #uctuations are present in a pollen record from oscillation is remarkably similar among lakes. Age Tungak Lake in the Yukon Delta (Ager, 1982), and assignments for these #uctuations are ca 10}11 ka BP at a recent, well-dated sediment record from Phalarope Ongivinuk, Grandfather and (possibly) Snipe Lakes, ca Lake on Kodiak Island shows major #uctuations in 10.5}11.5 ka BP at Idavain, Farewell and Wien Lakes pollen and plant-macrofossil assemblages (Peteet and (Fig. 7), and 10}11 ka BP at Birch and Windmill Lakes Mann, 1994). Fluctuations in Betula and herb pollen at (Bigelow, 1997). A relatively secure chronology exists for Snipe, Farewell and Wien Lakes, though less pronounced the record from Kodiak Island, based on tephra ages and than at southern sites, also indicate short-term vegeta- on AMS C dates of plant macrofossils, which place the tion changes in central and northern portions of the vegetation reversal between 10.8 and 10.0 ka BP (Peteet study area (Fig. 6). These #uctuations were not pre- and Mann, 1994). These chronologies indicate that viously discussed at Farewell and Wien Lakes (Hu, 1994; the oscillation is roughly contemporaneous with the Hu et al., 1996), in part because #uctuations in Populus Younger-Dryas event in the North Atlantic region. L.B. Brubaker et al. / Quaternary Science Reviews 20 (2001) 175}188 187

Several other studies (Kallel et al., 1988; Matthewes, etation history revealed by pollen records in southwest 1993; Kotilainen and Shackleton, 1995) have Alaska suggests that throughout the past 12 ka yr this documented evidence of short-term cooling in the North region has been in#uenced by a heirarchy of climatic Paci"c region that appears contemporaneous with the controls acting at di!erent spatial scales (e.g., Mock et al., Younger Dryas event. In a recent simulation of a short- 1998). Bigelow and Edwards (2001) also discuss the im- term shut down of North Atlantic Deep Water formation portance of such interactions in controlling vegetation with a coupled ocean}atmosphere general circulation history at Windmill Lake in central Alaska. The identi- model under modern conditions (e.g., no ice sheet and "cation of speci"c mechanisms controlling late-Quater- modern sea-surface temperatures), the interruption in nary climate change in southwest Alaska requires deep-water formation caused a Younger Dryas type additional documentation of the spatial pattens in veg- cooling in Europe as well as decreases in atmospheric etation and other climate indicators (e.g., beetles, stable temperature in the North Paci"c region (Milkolajewicz isotopes) in conjunction with conceptual and explicit et al., 1997). The e!ects of cooling in the North Atlantic climate models that include feedbacks at di!erent spatial were transmitted primarily by atmospheric mechanisms scales. a!ecting the Aleutian low-pressure system and cyclonic activity in the eastern North Paci"c. However, simulation experiments using other models (e.g., Peteet et al., Acknowledgements 1997) suggest that a regional cooling in southwest Alaska could have been caused by decreased North Paci"c sea- We thank Chris Earle for invaluable "eld assistance, surface temperatures. To evaluate alternative causal and Darrel Kaufman and James Reihle for help in the mechanisms represented by such computer simularions, identi"cation of the Lephe tephra. This paper is contribu- additional high-resolution pollen records (e.g., Bigelow tion C114 of Paleoclimates from Arctic Lakes and Es- and Edwards, 2001) are needed to document the presence tuaries (PALE). The research was supported by NSF or absence of oscillations in Alaskan vegetation during grants DPP8722005 and DPP8922491. the glacial}interglacial transition.

References 8. Conclusion Ager, T.A., 1982. Vegetation history of western Alaska during the Southwest Alaska has been characterized by complex Wisconsin glacial interval and the Holocene. In: Hopkins, D.M., patterns of vegetation composition and dynamics over Matthews, C.E., Schweger, C.E., Young, S.B. (Eds.), Paleoecology of the past 12 ka yr. Initially, a major vegetation a Beringia. Academic Press, New York, pp. 75}95. transition occurred from Betula shrub tundra in the Anderson, P.M., Bartlein, P.J., Brubaker, L.B., 1994. Late-Quaternary history of tundra vegetation in northwestern Alaska. Quaternary north to mixed herb-Betula shrub tundra in the south. Research 9, 306}315. After ca 9 ka BP, boreal forests spread gradually from Anderson, P.M., Brubaker, L.B., 1986. Modern pollen assemblages north to south, resulting in the present gradient of con- from northern Alaska. Review of Palaeobotany and Palynology 46, tinuous boreal forest in the north, forest}tundra ecotone 273}291. in central areas, to shrub tundra with sparse trees in the Anderson, P.M., Brubaker, L.B., 1994. Vegetation history of northcen- tral Alaska: a mapped summary of late}Quaternary pollen data. south. Thus despite a major biome shifts from shrub Quaternary Science Reviews 13, 71}92. tundra to forest or near-treeline at all locations, there has Bartlein, P.J., Anderson, K.H., Anderson, P.M., Edwards, M.E., Mock, always been a regional vegetation gradient characterized C.J., Thompson, R.S., Webb, T.W.I., Whitlock, C., 1998. Paleocli- by greater dominance of woody growth forms in north- mate simulatins for North America over the past 21,000 years: ern than southern areas (shrubs during the Betula period, features of the simulated climate and comparisons with paleoenvir- nomental data. Quaternary Science Reviews 17, 549}584. trees during the Alnus/Picea period). In addition to the Bartlein, P.J., Edwards, M.E., Shafer, S.L., Barker Jr., E.D., 1995. compositional and physiognomic gradients, the region Calibration of radiocarbon ages and the interpretation of paleoen- has experienced marked gradients the dynamics of veg- vironmental records. Quaternary Research 44, 417}424. etation change (e.g., more prominent Betula/herb #uctu- Bigelow, N.H., 1997. Late Quaternary vegetation and lake}level chan- ations in south than north; larger Populus expansions in ges in central Alaska. Unpublished Doctoral Dissertation Thesis, University of Alaska Fairbanks. north than south; and earlier expansion of P. glauca and Bigelow, N.H., Edwards, M.E., 2001. A 14,000 yr paleoenvironmental P. mariana in north than south). record from Windmill Lake, central Alaska: evidence for high-fre- The above vegetation patterns suggest that a broad quency climatic and vegetation #uctuations. Quaternary Science climatic gradient extended across southwest Alaska Reviews 20, 203}215. throughout the past 12 ka yr, with warmer growing sea- Brubaker, L.B., Anderson, P.M., Murray, B., Koon, D., 1998. A Palynological investigation of true moss (Bryopsida) spores: sons in northeastern than southwestern areas, and that morphology and occurrence in modern and Late-Quaternary lake the e!ects and/or occurrence of shorter-term climate sediments of Alaska. Canadian Journal of Botany 76 (12), #uctuations di!ered across the region. The complex veg- 2145}2157. 188 L.B. Brubaker et al. / Quaternary Science Reviews 20 (2001) 175}188

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