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Representation of Tundra Vegetation by Pollen in Lake Sediments of Northern Alaska W

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Representation of Tundra Vegetation by Pollen in Lake Sediments of Northern Alaska W Journal of Biogeography, 30, 521–535 Representation of tundra vegetation by pollen in lake sediments of northern Alaska W. Wyatt Oswald1,2*, Patricia M. Anderson2, Linda B. Brubaker1, Feng Sheng Hu3 and Daniel R. Engstrom41College of Forest Resources, 2Quaternary Research Center, Box 351360, University of Washington, Seattle, WA 98195, USA, 3Departments of Biology and Geology, University of Illinois, Urbana, IL, USA and 4St Croix Watershed Research Station, Science Museum of Minnesota, St Croix, MN, USA Abstract Aim To understand better the representation of arctic tundra vegetation by pollen data, we analysed pollen assemblages and pollen accumulation rates (PARs) in the surface sediments of lakes. Location Modern sediment samples were collected from seventy-eight lakes located in the Arctic Foothills and Arctic Coastal Plain regions of northern Alaska. Methods For seventy of the lakes, we analysed pollen and spores in the upper 2 cm of the sediment and calculated the relative abundance of each taxon (pollen percentages). For eleven of the lakes, we used 210Pb analysis to determine sediment accumulation rates, and analysed pollen in the upper 10–15 cm of the sediment to estimate modern PARs. Using a detailed land-cover map of northern Alaska, we assigned each study site to one of five tundra types: moist dwarf-shrub tussock-graminoid tundra (DST), moist graminoid prostrate-shrub tundra (PST) (coastal and inland types), low-shrub tundra (LST) and wet graminoid tundra (WGT). Results Mapped pollen percentages and multivariate comparison of the pollen data using discriminant analysis show that pollen assemblages vary along the main north– south vegetational and climatic gradients. On the Arctic Coastal Plain where climate is cold and dry, graminoid-dominated PST and WGT sites were characterized by high percentages of Cyperaceae and Poaceae pollen. In the Arctic Foothills where climate is warmer and wetter, shrub-dominated DST, PST and LST were characterized by high percentages of Alnus and Betula pollen. Small-scale variations in tundra vegetation related to edaphic variability are also represented by the pollen data. Discriminant analysis demonstrated that DST sites could be distinguished from foothills PST sites based on their higher percentages of Ericales and Rubus chamaemorus pollen, and coastal PST sites could be distinguished from WGT sites based on their higher per- centages of Artemisia. PARs appear to reflect variations in overall vegetation cover, although the small number of samples limits our understanding of these patterns. For coastal sites, PARs were higher for PST than WGT, whereas in the Arctic Foothills, PARs were highest in LST, intermediate in DST, and lowest in PST. Main conclusion Modern pollen data from northern Alaska reflect patterns of tundra vegetation related to both regional-scale climatic gradients and landscape-scale edaphic heterogeneity. Keywords Alaska, Arctic Coastal Plain, Arctic Foothills, modern pollen, North Slope, palynology, paleoecology, pollen accumulation rates, tundra. *Correspondence: W. Wyatt Oswald, Quaternary Research Center, Box 351360, University of Washington, Seattle, WA 98195, USA. E-mail: [email protected] Ó 2003 Blackwell Publishing Ltd 522 W. W. Oswald et al. 1998), but the ability of pollen to represent the landscape- INTRODUCTION scale heterogeneity related to edaphic variability has not Understanding the history of the tundra biome has long been tested. been an important objective in paleoecology (Iversen, 1952; The tundra region of northern Alaska is well suited for a Livingstone, 1955; Colinvaux, 1964; Ritchie, 1984). During study of the modern relationship between pollen data and past glacial intervals, tundra was extensive not only in the patterns of vegetation. The distribution of different tundra Arctic, but also in lower latitude areas of North America and types is influenced regionally by climatic gradients and at the Eurasia that are currently forested (e.g. Jackson et al., 2000; landscape scale by variation in substrate (Muller et al., Prentice & Jolly, 2000). Knowledge of the nature of the 1999). In general, shrub-dominated tundra prevails in the vegetation during those times is relevant to a variety of relatively warm and wet Arctic Foothills in the southern research areas, including the modern floristic geography of portion of this region, whereas graminoid species dominate the Arctic, human migration and settlement patterns, and the Arctic Coastal Plain in the north where climate is colder the ecology of Pleistocene fauna (e.g. Fernald, 1925; Hulte´n, and drier (Walker et al., 1998; Muller et al., 1999). The 1937; Hopkins et al., 1981; Guthrie, 1990). Furthermore, composition of the vegetation also varies at smaller spatial studies of past changes in tundra ecosystems help us to scales, primarily in response to heterogeneous soil drainage understand how tundra responds to environmental change, a (Walker et al., 1994, 1995; Walker, 2000). For example, topic that has become prevalent as evidence builds that the shrubs occur on the coastal plain in areas where slightly Arctic will be especially sensitive to future changes in climate higher topography affords drier soils (Walker & Everett, (e.g. Overpeck et al., 1997; Serreze et al., 2000). 1991; Muller et al., 1999), and the coarse-textured soils of There is particular interest in understanding the small- recently glaciated areas of the Arctic Foothills are occupied scale (1–100 km) heterogeneity of past tundra ecosystems. by different plant communities than the surrounding land- Studies of modern arctic tundra show that ecosystem scape (e.g. Jorgenson, 1984). processes and plant community composition are spatially Based on pollen percentages in surface sediments of heterogeneous in response to both regional-scale (100– twenty-seven lakes in this region, Anderson & Brubaker 1000 km) climatic gradients and landscape-scale edaphic (1986) showed that the primary gradient of vegetation and variability (e.g. Walker, 2000), and many authors have climate is represented by variation in the percentages of the hypothesized that past tundra also varied according to local major taxa. Coastal sites were characterized by high per- variations in topography or substrate. For example, con- centages of Cyperaceae and Poaceae pollen, whereas inland flicting interpretations of the full-glacial vegetation of the sites were characterized by high percentages of Betula and Bering Strait region (e.g. Cwynar, 1982; Guthrie, 1985) were Alnus pollen. This study improves upon the previous work in reconciled by the notion that the vegetation was a mosaic of two ways. First, we added forty-three pollen sites to that different vegetation types (Schweger, 1982; Anderson, 1985; data set, thus enabling us to better define regional-scale Eisner & Colinvaux, 1992). Valley bottoms and lowlands patterns, as well as to explore variability within the major would have supported continuous, mesic tundra commu- tundra zones. In addition, we used a recently created land- nities, while uplands featured xeric, sparsely vegetated tun- cover map of northern Alaska (Muller et al., 1999) to clas- dra. Additionally, there is growing evidence that edaphic sify each pollen site as one of five tundra types. Because the variability is likely to influence the response of tundra eco- map presents the regional vegetation in more detail than systems to future climate change (e.g. Hobbie & Gough, was previously available (100-m pixel size), it allowed us to 2002), and paleoecological studies may provide insights to examine the variability in pollen assemblages within the this question (Oswald, 2002). broad coastal and inland vegetation zones and to assess Reconstructing the subregional heterogeneity of past whether that variability is related to the small-scale patterns tundra vegetation, however, has proven difficult. Palynol- of plant communities. We used discriminant analysis to ogy, the method most commonly used in Quaternary define trends in the pollen data, and we compared those paleoecology, is viewed as a Ôblunt instrumentÕ for recon- trends with climate and vegetation data to explore how the structing small-scale patterns of tundra (Colinvaux, 1967). pollen data represent vegetational patterns associated with Pollen assemblages (as represented by percentage data) are both climatic forcing and edaphic controls. Secondly, we difficult to interpret because key species are over- or under- analysed pollen in surface cores dated with 210Pb to estimate represented, pollen types cannot be differentiated within PARs for eleven sites representing the five tundra types. many genera or families, and the spatial resolution of the These analyses show that modern pollen assemblages reflect data is unclear (Nichols, 1974; Anderson et al., 1994; Ga- variations in tundra associated with the latitudinal gradient, jewski et al., 1995). Pollen accumulation rates (PARs) and indicate that the landscape-scale variations in tundra ) ) (number of pollen grains cm 2 year 1) were explored as a related to edaphic heterogeneity also are recorded by pollen possible metric of past variations in vegetation cover data. (e.g. Cwynar, 1982), but their utility has been disputed (e.g. Guthrie, 1985, 1990). Modern calibration studies have STUDY AREA shown that pollen percentage data reflect the major, regional-scale variations in tundra (Ritchie, 1974; Anderson There are two major physiographical zones in Alaska north & Brubaker, 1986; Short et al., 1986; Brubaker et al., of the Brooks Range (Wahrhaftig,
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