Northwestern North America

This article was originally published in the Encyclopedia of Quaternary Science published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial Gavin D.G., and Hu F.S. (2013) Northwestern North America. In: Elias S.A. (ed.) The Encyclopedia of Quaternary Science, vol. 4, pp. 124-132. Amsterdam: Elsevier. © 2013 Elsevier Inc. All rights reserved. Author's personal copy Northwestern North America D GGavin , University of Oregon, Eugene, OR, USA FSHu , University of Illinois, Urbana, IL, USA ã 2013 Elsevier B.V. All rights reserved. Introduction shade-tolerant conifers up to 60 N. In the high-snow areas along the coast, treeline declines from 1500 m in western The vegetation history of northwestern North America (NWNA) Washington to 500 m in south-central Alaska. East of the following the last glaciation is marked by several major transi- coastal ranges, summer moisture increases with latitude due tions that are consistent with a rich and complex climate his- to increased summer convective precipitation, especially in tory. The region today supports very heterogeneous vegetation mountainous areas. From south-central British Columbia corresponding to broad-scale gradients in moisture availability northward, the inland vegetation transitions from sagebrush and temperature and fine-scale topography in mountainous steppe to pine forests, then to subboreal and boreal spruce areas (Figure 1). This heterogeneity presents a major challenge forest (i.e., the Cold Desert to Western Cordillera to Boreal for understanding the regional vegetation history because local Cordillera transition on Figure 1.). An exception to this overall geomorphic and climatic conditions can modify species re- pattern occurs to the west of the continental divide from sponses to climate change at any particular site (Whitlock, northern Idaho to central British Columbia, where high pre- 1992). Nevertheless, some common patterns have emerged cipitation results in a moist interior forest, often called the from more than five decades of paleoecological studies in this interior wet belt, dominated by Tsuga heterophylla (western region. hemlock) and Thuja plicata (western redcedar) that are disjunct Our aim in this article is not to present a comprehensive from their main coastal distribution (Gavin, 2009). In central summary of the wide range of findings from the hundreds of Alaska, the boreal forest is mainly composed of black and pollen records from NWNA, much of which would reiterate white spruce, with aspen and birch on warm and disturbed previous articles (Table 1). Rather, we aim to highlight robust sites. In general, treeline in the interior forests is hundreds of findings from a subset of records representative of broad vegeta- meters higher and declines at a slower rate with latitude com- tion zones. This discussion focuses on the area north of 42 N pared to the maritime forests. The forest limit in Alaska abuts and west of the continental divide. We first describe the broad the southern flanks of the Brooks Range and is limited west- climatic gradients and the corresponding ecosystems across ward toward the cold-maritime climates near the Bering Sea. NWNA. We then summarize the major vegetational changes For detailed accounts of vegetation–climate relationships in based on key pollen profiles from several subregions. The history this region, we refer readers to detailed monographs focused of each subregion is discussed with respect to three major cli- on Oregon and Washington (Franklin and Dyrness, 1988), matic periods: the Last Glacial Maximum (LGM, broadly defined British Columbia (Meidinger and Pojar, 1991), or Alaska as the interval 25–18 ka; ka, thousands of years before present), (Gallant et al., 1995; Viereck et al., 1992). the last glacial–interglacial transition (14.5–11.6 ka), and the Holocene (11.6–0 ka). All ages are based on calibrated radiocar- bon dates. Alaska This section should be read in conjunction with Northern Broad Patterns of Climate and Vegetation North America. The dominant westerly flow of moisture results in a major Last Glacial Maximum longitudinal moisture gradient due to coastal mountain ranges that produce heavy orographic precipitation to the west and a Despite the fact that much of the region was unglaciated during rain shadow to the east. Thus, the primary vegetation gradient the last glaciation, most of the Alaskan pollen records do not is characterized by a maritime moist conifer forest west of the extend beyond the past 14000 years. At all of the sites where crest of the coastal ranges and a mixture of sagebrush steppe, full-glacial sediments have been recovered, the pollen spectra mixed conifer, and pine forests east of the coastal ranges. In of the LGM are dominated by Poaceae, Cyperaceae, and Arte- the southern half of NWNA, little rainfall occurs from July misia (Eisner and Colinvaux, 1990; Livingstone, 1955, 1957; to September, due to the dominance of a subtropical high- Oswald et al., 1999). These pollen assemblages are difficult to pressure system centered over the Pacific. This results in a interpret because of their broad ecological tolerances (e.g., seasonally dry maritime forest along the coast in Oregon and Cwynar, 1982). However, minor taxa such as Encalypta rhapto- Washington, and very dry shrub steppe sagebrush in the inte- carpa and Selaginella siberica suggest that sparse, xeric tundra rior. Further to the east, beyond the continental divide, sum- prevailed under the cold, dry climate of the late Pleistocene. mer moisture increases from monsoon-like precipitation. In An issue that remained controversial for several decades was the northern half of NWNA and generally north of the sub- whether some of the boreal-forest tree species existed in eastern tropical high pressure, increasing onshore flow results in a Beringia (Alaska and adjacent Canada) during the LGM hypermaritime coastal climate dominated by dense forests of (Hopkins et al., 1981; Hulte´n, 1937). Based on the spatial 124 Encyclopedia of Quaternary Science, (2013), vol. 4, pp. 124-132 Author's personal copy POLLEN RECORDS, POSTGLACIAL | Northwestern North America 125 1000 km (a) (b) Figure 1 (a) Shaded-relief map of northwest North America using a color scheme that differentiates humid (green) and xeric (brown) regions. Ecoregions differentiate the major vegetation types and climatic regions. The map shows all eight ecoregions occurring within our area of focus from the ecoregions of North America (Commission for Environmental Cooperation, 1997). (b) Extent of the Cordilleran ice sheet, other ice fields, pluvial lakes, and proglacial lakes at 18000 cal. years before present (Dyke et al., 2003). Table 1 Previously published regional syntheses of et al., 2006), offering unambiguous evidence that spruce did survive the LGM in Alaska. Together, the DNA and pollen data paleovegetational change from northwest North America. This table is limited to papers published since 1990 suggest that tundra prevailed on the regional landscape during the LGM, but small populations of trees exited in isolated Region Citations sheltered areas. Alaska Anderson and Brubaker (1994), Transition from the Pleistocene to the Holocene Bigelow et al. (2003), Brubaker et al. (2005), Edwards et al. (2005) Between 15 and 12 ka, pollen records indicate that herb tundra Yukon Cwynar and Spear (1995), Vermaire and Cwynar (2010) transitioned to shrub tundra dominated by Betula (birch) in Washington, Oregon, Idaho, and Thompson and Anderson (2000), response to increases in temperature and effective moisture. Western Montana Thompson et al. (2003), Whitlock Betula, Salix (willow), and Cyperaceae became the dominant (1992), Whitlock et al. (2008) taxa, and tundra vegetation cover became more continuous Columbia River Basin (Interior Mehringer (1996), Walker and than previously. Associated with these vegetational changes Washington, Idaho, and Pellatt (2008) are the stabilization of eolian dunes, aggradation of streams, British Columbia) and formation of organic peat deposits (Mann et al., 2002). British Columbia Hebda (1995), Mathewes (1991) A number of pollen diagrams from Alaska display prominent Southern Vancouver Island Brown and Hebda (2003) peaks of Populus balsamifera, suggesting that this species formed Coastal forests Hebda and Whitlock (1997) woodlands in at least some local habitats (Figure 2). A recent synthesis of the regional pollen diagrams shows a cluster of Populus peaks between 13 and 10 ka (Edwards et al., 2005). patterns of individual pollen types, Brubaker et al. (2005) These are thought to be evidence of warmer-than-present cli- concluded that the common tree and shrub taxa in the Alaskan matic conditions during the early Holocene, presumably in pollen records survived the last LGM in eastern Beringia. This response to peak values of summer insolation (Kaufman includes Picea (spruce),

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