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

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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 discon- tinuous 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 domin- ated by Poaceae and Cyperaceae communities with few shrubs.
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