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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.

Northwestern North America D GGavin , University of Oregon, Eugene, OR, USA

FSHu , University of Illinois, Urbana, IL, USA

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

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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), a dominant taxon in the modern bo- et al., 2004). This age range likely reflects that many of the 14 real forests, as evidenced by the fact that trace amounts of pollen diagrams are based on bulk-sediment C dating, spruce pollen are common in the LGM sediments from a which tends to yield anomalously old dates. Several recent number of sites. This hypothesis cannot be tested rigorously pollen records with AMS-14C ages on terrestrial plant macro- using fossil data; spruce macrofossils dated to the LGM have fossils suggest that the Populus interval was post-Younger not been found. However, a recent analysis of chloroplast DNA Dryas. Furthermore, Populus peaks occurred at different times in extant trees revealed unique haplotypes and a higher diver- at different sites, and midge-based temperature reconstructions sity of haplotypes in Alaska than areas farther south (Anderson from three sites in interior Alaska provide compelling evidence

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Author's personal copy 126 POLLEN RECORDS, POSTGLACIAL | Northwestern North America that the early Holocene was not warmer, probably as a result of West East ocean–atmosphere–land interactions associated with the states 0 of the Arctic Oscillation and El Nin˜ o Southern Oscillation (Clegg et al., 2011). Pollen records from Alaska indicate that climate cooling 2 Picea glauca during the Younger Dryas Chronozone (YDC; 12.9–11.6 ka) altered the species composition and decreased the cover of Spruce forest terrestrial vegetation in at least some areas of Alaska. Evidence 4 includes decreased tree cover in southeastern Alaska (Engstrom et al., 1990 ) and the expansion of herb taxa at the expanse of

Betula shrubs in south-central and southwestern Alaska (Hu 6 et al., 1995, 2002; Peteet and Mann, 1994). In northern Alaska, Picea mariana Birch tundra Mann et al. (2002) report a contraction in the range of Populus with alder balsamifera , presumably in response to a return to cold and dry conditions. Other evidence supporting YDC-related climatic 8 fluctuations in Alaska includes high-resolution analysis of bio- Alnus genic silica (diatom remains) in lake sediments (Hu et al., 2002, 2006; Kaufman et al., 2010), geomorphic and paleoeco- present before ϫ 1000 Years 10 Populus logical records of stream down-cutting, eolian dune activity, Poplar woodland (11.5–10?) and reduced peat formation (Mann et al., 2002), and glacier fluctuations (Briner et al., 2002). 12

Birch tundra

Betula The Holocene Herb tundra 14 Pollen records from several sites with high-quality chronolog- Figure 2 Schematic diagram showing the spatiotemporal dynamics of ical control indicate the early Holocene expansion of Populus postglacial vegetation in Alaska, based on fossil-pollen analysis. Lines balsamifera. This expansion may have resulted from climatic indicate the approximate timing of population expansions of select major warming and/or from the presence of extensive disturbed min- taxa in the Alaskan pollen records. eral soils during the early postglacial interval (Hu et al., 1993). In interior Alaska, Betula tree species such as Betula neoalaskana, a common component of early-successional stands on the similar to those we see today became established around modern landscape, may have also gained importance at this 6.5 ka. Associated with this vegetational change was a pro- time. Although studies of modern pollen samples indicate that nounced increase in forest fire frequency, which was probably separation of B. neoalaskana pollen from shrub-Betula species is driven by the high flammability of P. mariana forests (Higuera possible (Clegg et al., 2005; Edwards et al., 1991), few fossil- et al., 2009; Lynch et al., 2003). Pollen percentages and accu- pollen studies have attempted to distinguish these species in mulation rates in lake sediments suggest that the westward

migration of sparse Picea stands halted around 8 ka, presum- Alaska (Brubaker et al., 1983; Tinner et al., 2008). However, existing pollen and macrofossil records of tree-Betula seeds ably because of an episode of climatic cooling (Hu et al., reveal local presence no later than 9 ka (Hu et al., 1993; Tinner 1993). Picea then resumed its westward expansion to reach its et al., 2006 ). modern western range limit during the late Holocene (Ager Pollen assemblages with a substantial amount of Picea and Sims, 1981; Brubaker et al., 1983). A striking spatial began to occur as early as 10 ka in the eastern and southern pattern based on the dense network of Holocene pollen re-

Alaska (Figure 2). The first spruce species to expand in the early cords from interior Alaska is that in contrast to the rapid spread Holocene pollen assemblages is Picea glauca (white spruce), across the eastern sites, spruce expanded extremely slowly from which probably formed woodland with tree birch between 10 central to southwestern Alaska, and there is not a clear climatic and 9 ka in interior Alaska. Pollen profiles that separate Picea or ecological explanation for this contrast (Kaltenrieder et al., pollen to the species level indicate that both P. glauca and 2011). As a result of the late expansion of spruce in southwest-

P. mariana , the two common species of today’s Alaskan boreal ern Alaska, birch tundra was the prevalent vegetation type forests, were both present on the landscape by 9 ka in eastern for 10000 years, until 3–4 ka when P. glauca invaded the Alaska (Tinner et al., 2006). The expansion of white spruce region. A recent study showed that the abundance of birch appears to have occurred from the east to west (Anderson and shrubs fluctuated with climatic changes possibly associated Brubaker, 1994), and the rates of population increases exhibit with solar-irradiance fluctuations and ocean–atmosphere in- majorspatiotemporal patterns. In eastern Alaska, Picea pollen teractions at hemispheric scales (Hu et al., 2003). percentages reached near-modern values within 600 years of The establishment of spruce forests similar to those of today initial appearance. Several pollen records from central Alaska occurred between 6.5 and 5 ka in interior Alaska (Anderson and display a rise in Picea pollen percentages at about the same Brubaker, 1994). Picea pollen percentages began to increase time as the eastern sites, but the abundance of Picea pollen between 6 and 4 ka (Anderson and Brubaker, 1994)nearthe remained substantially below modern abundance until 5 to modern treelines in northwestern Alaska and around 4 ka in 6 ka. In central Alaska, boreal forests dominated by P. mariana southwestern Alaska (Brubaker et al., 2001). Pollen abundance

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reached modern values 2–4 ka, suggesting that the modern Eriophorum vaginatum, Ericaceae species, and Sphagnum mosses forest-tundra ecotone became established during this interval (Walker et al., 1994) around 7500 years ago. In contrast, at a (Anderson and Brubaker, 1994; Brubaker et al., 2001; nearby site on the Itkillik II surface (14000 years old), pollen Hu et al., 1995; Kaltenrieder et al., 2011). Spruce pollen analysis revealed that prostrate-shrub tundra communities never exceeded 10% of the terrestrial pollen sum during the prevailed on the landscape since the early Holocene, although late Holocene in these regions, suggesting that population vegetation cover may have increased around 7500 years ago densities have never been higher after the development of (Oswald et al., 2003). Oswald et al. attributed this vegetation- the modern tundra-forest ecotone in western Alaska. How- history contrast to differences in soil texture and geomorphol- ever, this assessment may be a result of the lack of high- ogy between the two geomorphic surfaces. Specifically, as temporal-resolution pollen records from western Alaska. effective moisture increased from early to middle Holocene,

The first detailed palynological study of lake sediments the fine-texture soils and gentle topography of the older of the past 700 years revealed pronounced fluctuations Sagavanirktok surface promoted buildup of soil organic matter, of altitudinal treeline in the Copper River Basin of southeast- shallowing of permafrost table, and development of water- ern Alaska (Tinner et al., 2008). Tinner et al. (2008) con- logged soils, which would have favored the development of ducted multiproxy analyses of lake sediments at decadal-to- tussock tundra. These changes probably did not occur on the centennial timescales to infer ecosystem and climatic fluctu- younger Itkillik II surface because of better drainage associated

ation associated with the Little Ice Age (LIA; 500–150 years with coarser-texture and less-acidic soils. ago). Their pollen data revealed diminished abundance of major tree taxa, including P. glauca, P. mariana,andBetula neoalaskana , as a result of decreases in temperature and effective moisture during the LIA. These changes were accompa- Marine West Coast Forest nied by the expansions of shrub and herbaceous species, Last Glacial Maximum including Alnus viridis, Betula glandulosa/nana,andEpilobium. Alnus (alder) is a major taxon in the many Holocene pollen A 7.7-m peat core from coastal western Washington, located diagrams from Alaska, reaching 70–80% of the pollen spectra south of the maximum extent of the Juan de Fuca lobe of the at somesites. The earliest expansion of alder occurred at sites in Cordilleran ice sheet, revealed a coarse vegetation history that the northwestern Brooks Range around 9500 years ago, and may extend to the previous interglacial (Heusser et al., 1999). alder appears to have expanded southward and eastward in The full-glacial pollen assemblages are comprised mainly of interior Alaska. It became a major component of the pollen Pinus (pine, mainly the shore-pine variant of P. contorta), Tsuga diagrams at many sites around 8000 cal BP, suggesting the mertensiana (mountain hemlock), and Poaceae, suggesting cold rapid expansion of this taxon across of the landscape around open parkland, in contrast to dense Picea sitchensis (Sitka this time. Because alder is a nitrogen fixer, its Holocene expan- spruce)/T. heterophylla (western hemlock) at the site today. sion probably markedly increased nitrogen availability and Similar pollen assemblages, but with more warm-adapted accelerated the rate of nitrogen cycling in Alaska, especially T. heterophylla pollen, were found in the low elevations of the where alder is a prevalent component of the terrestrial plant coastal ranges of western Oregon and southwest Washington communities such as southwestern Alaska. The N effects of the (Grigg and Whitlock, 2002; Grigg et al., 2001). In the unglaciated

Holocene alder expansion have been documented with the western Cascade Range of central Oregon, a debris flow deposit > analyses of lake sediments for biogenic silica, C/N ratio, and dated to 35 ka contained abundant macrofossils of tree species nitrogen and carbon isotopes (Hu et al., 2001), although it common today 1000 m higher in elevation, including Picea remains unknown how the increased N availability affected engelmannii (Engelmann spruce) and Abies lasiocarpa (subalpine other species that are associated with alder. fir). Further to the east, the High Cascades were covered by an ice

In the pollen records from the North Slope of Alaska, Picea sheet at the time (Figure 1). Taken together, these records indi- and Alnus are present in the Holocene sediments, but the abun- cate that forest cover occurred within 10s to 100s of kilometers dance is low for both taxa. Spruce trees have never been locally from the ice sheet maximum, that subalpine forest types ex- present in this region based on the late Quaternary fossil re- tended to low elevations, but also that warm-adapted conifer cords, and the presence of spruce in the pollen diagrams from tree species persisted at some sites in Oregon. The most abundant the North Slope represents long-distance transport from areas warm-adapted species in the region today, Pseudotsuga menziesii south of the Brooks Range. In contrast, Alnus expanded in the (Douglas-fir), was very rare but likely present in Oregon during Brooks Range and southern Arctic Foothills between 8 and 7 ka the last glacial (Gugger and Sugita, 2010). (Bergstrom, 1984; Eisner, 1991; Livingstone, 1955; Oswald Ice-free areas existed in certain locations north of the et al., 1999, 2003). Tundra communities similar to those of southern ice sheet margin during the LGM. In combination today became established during the early–middle Holocene with lower sea level, many headlands along the British Colum-

(Oswald et al., 1999, 2003). A detailed palynological study bia and Alaska coast remained ice-free allowing for persistence (Oswald et al., 2003) demonstrated that the trajectories of of plant and animal populations in refugia. Refugia have Holocene vegetation development differed between two major been identified on the basis of pollen and macrofossils geomorphic substrates related to the Quaternary glacial history on the Queen Charlotte Islands, Alexander Archipelago, and of the region. On the Sagavanirktok surface (>500000 years Vancouver Island (Carrara et al., 2007; Hebda, 1997; Warner old), relatively dry and sparse tundra dominated by prostrate et al., 1982). These findings are supported by phylogeographic shrubs transitioned to tussock tundra dominated by Betula nana, studies (reviewed in Shafer et al., 2010) and findings of tree

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Author's personal copy 128 POLLEN RECORDS, POSTGLACIAL | Northwestern North America macrofossils appearing in offshore postglacial sediments near The YDC (12.9–11.6 ka), as recorded by several nonvegeta- Vancouver Island (Lacourse et al., 2003). tion climate proxies in this region, was a distinct cold and dry interval (Barron et al., 2003; Heine, 1998; Vacco et al., 2005), but its expression in the vegetation record is variable. Pollen records show little change in forests in western Oregon directly Transition from Pleistocene to Holocene associated with the YDC, possibly reflecting the fact that At 14.5 ka, a rapid warming initiated the retreat of many alpine warm-adapted vegetation was well established before the YDC glaciers, while at the same time, the Cordilleran ice sheet limit and that many species (e.g., Pseudotsuga) have broad climatic was moving northward. In the Oregon Coast Range, this tran- tolerances (Grigg and Whitlock, 1998). From Vancouver sition involved a change from subalpine parkland to lowland Island northward, pollen assemblages from coastal sites show tree species (Alnus rubra (red alder) and Pseudotsuga menziesii). subalpine-adapted species (Tsuga mertensiana) and low pollen The abrupt increase of Pseudotsuga menziesii is notable, given its influxes coeval with the YDC (Brown and Hebda, 2002; rarity or absence during much of the glacial maximum (Grigg Lacourse, 2009; Mathewes, 1993). and Whitlock, 1998; Worona and Whitlock, 1995). Further north, from Puget Sound northward, the initial colonizer of deglaciated landscapes was almost entirely Pinus contorta, fol- The Holocene lowed by a mix of subalpine species (Abies, Picea, and Tsuga mertensiana ) between 14 and 13 ka. Here, the transition out of The early Holocene is marked by increased summer insolation this parkland vegetation occurred later (at 12 to 11 ka) than at (8% more relative to modern at 50 N) that strengthened the sites further south, typically close in time to the first major Pacific Subtropical high-pressure system, creating warm stable increase of Pseudotsuga menziesii (Cwynar, 1987). and dry air in coastal regions. Throughout the Pacific

500 km South North Battle Ground Little Lake, OR Lake, WA Mineral Lake, WA Hall Lake, WA Kirk Lake, WA East Sooke Fen, BC Pixie Lake, BC Whyac Lake, BC 44° 16.72¢ N 45° 08.00¢ N 46° 71.81¢ N 47° 80.81¢ N 48° 24.36¢ N 48° 35.19¢ N 48° 59.64¢ N 48° 67.22¢ N ° ¢ ° ¢ ° ¢ ° ¢ ° ¢ ° ¢ ° ¢ 123 58.39 W 122 49.17 W 122° 17.69¢ W 122 30.81 W 121 63.28 W 123 68.17 W 124 19.67 W 124 84.44 W 210 m a.s.l 154 m a.s.l. 433 m a.s.l. 104 m a.s.l. 194 m a.s.l. 155 m a.s.l. 70 m a.s.l. 15 m a.s.l. 0 Douglas-fir/ Cedar/pine/ Douglas-fir/ 1000 red alder/ western Western hemlock/ hemlock/ fir/ cedar red alder western hemlock/ cedar nonarboreal Western hemlock/ cedar 2000 Douglas-fir/cedar/ Western hemlock/ Western hemlock/ red alder/western cedar/Douglas-fir cedar/Douglas-fir 3000 hemlock/fir Douglas-fir/red Western hemlock alder/cedar 4000 Douglas-fir/ western hemlock/ Cedar/Douglas-fir/ cedar Spruce/ 5000 Western hemlock Western hemlock/ cedar 6000 Douglas-fir/cedar Douglas-fir/cedar 7000 Douglas-fir/ Western hemlock/ western hemlock spruce/fir

8000 Douglas-fir/oak/ nonarboreal Douglas-fir/alder/ Spruce/ Age (cal year BP) Douglas-fir/red 9000 bracken Douglas-fir/alder/ Western hemlock/ alder/bracken/ alder oak Douglas-fir/alder Douglas-fir/alder/ bracken bracken bracken 10 000 Douglas-fir/red Pine/spruce/fir/ alder/western western hemlock hemlock/bracken Pine/spruce/fir/ 11 000 Pine western hemlock Pine 12 000 Douglas-fir/fir/ Pine/spruce/fir/ red alder/bracken western hemlock Pine/spruce/fir/ Pine/spruce/fir/ 13 000 alder alder Pine Douglas-fir/ Pine/spruce/fir/ Pine/spruce/ Pine red alder/pine fir/sitka alder sitka alder 14 000 Pine/spruce/ Pine/poplar/ mountain hemlock mountain hemlock 15 000

High fire activity Moderate fire activity Low fire activity No fire activity No charcoal data

Figure 3 Pollen zones and inferred fire activity for eight low-elevation sites in the Marine West Coast Forest ecoregion from south (left) to north (right):

Little Lake, OR (Long et al., 1998; Worona and Whitlock, 1995), Battle Ground Lake, OR (Barnosky, 1985; Walsh et al., 2008), Mineral Lake and Hall Lake, WA (Sugita and Tsukada, 1982), Kirk Lake, WA (Cwynar, 1987), East Sooke Fen, Pixie Lake, and Whyac Lake, BC (Brown and Hebda, 2002). Reproduced from Walsh MK, Whitlock C, Bartlein PJ (2008) A 14,300-year-long record of fire–vegetation–climate linkages at Battle Ground Lake, southwestern Washington. Quaternary Research 70: 251–264.

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Northwest, the early Holocene is marked by hotter summers increased Artemisia coincided with the YDC, though Artemisia and increased drought and forest fire (Figure 3). Records from was already increasing prior to a tephra marker at 13.1 ka insect remain in lake sediments in southern British Columbia (Mack et al., 1978; Walker and Pellatt, 2008). Peak Artemisia quantified July temperatures at 3 C warmer than present during the YDC was followed by increased tree (Pinus and (Gavin et al., 2011), in agreement with a temperature recon- Alnus) pollen until 10.5 ka. Further south in Idaho and western struction based on pollen data from southeast Oregon Montana, a Picea parkland predominated at higher elevations (Minckley et al., 2007). In western Oregon and Washington, before 13 ka, which changed to a closed pine forest during the increased fire and summer warmth fostered widespread early- YDC (Brunelle et al., 2005). successional Pseudotsuga–Alnus rubra forests with abundant Pteridium spores indicating full-sun conditions after severe for- The Holocene est fires ( Brown and Hebda, 2002; Sugimura et al., 2008). There is mounting evidence, however, that the early Holocene was not Increased warm-adapted taxa occur throughout the Western uniformly warm and dry, but was marked by distinct century- Cordillera during the early Holocene. At lower elevations, scale intervals of increased moisture. The remnants of the wan- increased Poaceae and decreased tree pollen suggest an uphill ing ice sheet to the north (which had its last major collapse at retreat of the forest-grassland ecotone (Mack et al., 1978). In

8.5 ka) may have still been influencing the path of the jet stream the mountains of central Idaho and western Montana, cool- across western North America (Gavin et al., 2011). adapted conifers (Picea and Abies) declined with increased The transition out of the early Holocene was marked by Pseudotsuga/Larix pollen from 11.5 to 7.5 ka (Brunelle et al., increasing representation of shade-tolerant late-successional 2005). In British Columbia, recently deglaciated land was species ( Tsuga and Abies), the decline of Pseudotsuga and Alnus usually colonized by Pinus cf. contorta, though pollen records rubra, and a decrease in fire. This transition began as early as indicate a fairly rapid primary succession to a more diverse

8 ka, but modern pollen assemblages were not reached until forest (Gavin et al., 2011). Modern forest conditions generally some time between 6 and 3 ka. The wide range in timing of the developed by 4 ka with an increase in mesic taxa, such as Picea establishment of modern assemblages may be due to progres- and Abies in subalpine forest sites. sive development of ecosystem properties that favor An event during the early Holocene that deserves special acid-tolerating species such as T. plicata, a species that arrived mention is the eruption of Mount Mazama that created Crater Æ comparatively recently north of Vancouver Island (Hebda and Lake 7627 150 years ago (Zdanowicz et al., 1999). The ash Mathewes, 1984). In southeast Alaska, the cooler and wetter deposits from this eruption blanketed the northwest, especially climatewas marked by several vegetation changes, including east of the Cascade Range. This ash layer created the seed bed the expansion of Tsuga and development of paludified soils for today’s forests and altered the trajectories of forest change. (Hansen and Engstrom, 1996). One reversal (warming) in the In some areas, extremely thick ash deposits led to ‘tephra

Neoglacial cooling trend was a period of increased fire found at plains’ that remain very dry and sparsely vegetated today. several sites in British Columbia and Washington, beginning at In other areas, ash deposits thickened soils and may have 2.5 ka and possibly attributable to human-set fires (Hallett et al., allowed for greater water retention. Colonization of forest on 2003; Prichard et al., 2009; Sugimura et al., 2008). steep coarse rock talus in the mountains may have required the addition of tephra to function as a mulch and thus changed the

character of the steeper parts of the landscapes (Mehringer

Western Cordillera et al., 1977). Last Glacial Maximum In wetter subzones of the Western Cordillera, forests con- tain many species disjunct from their main coastal distribu- Few sediment records extend to the glacial maximum in tion. Originally hypothesized to be the result of a Miocene the unglaciated terrain of eastern Washington and Oregon vicariance (following Cascadian orogeny), this biogeographic

(Bartlein et al., 2010). In the rain shadow of the Cascade pattern and its potential root in a northern Idaho refugium Range, near the modern forest–steppe boundary, the LGM have received much focus from geneticists and paleoecologists vegetation was dominated by Artemisia, Poaceae, and herb (Carstens et al., 2005; Daubenmire, 1968). The emerging taxa (Whitlock et al., 2000). Modeling of paleoclimate and picture is that each species has a unique history, that many species distributions over western North America for the plant species dispersed during the Holocene from the coast, glacial maximum is consistent with the inferences from the and that the modern mesic-forest composition in northern few existing geologic records: contraction of warm-adapted Idaho may be a very recent feature of the landscape (Gavin, conifers (e.g., Pseudotsuga) and expansion of cold-adapted 2009; Gavin and Hu, 2006; O’Connell et al., 2008; Shafer et al., Picea and Artemisia (Bartlein et al., 1998). 2010). Further north within the belt of mesic forests, the earliest detection of the modern forest dominant (T. hetero-

Transition from Pleistocene to Holocene phylla) occurred at 4.5 ka, coincident with the decline of Betula and transition from a continental to a cool and wet maritime Most of the Cordilleran ice sheet persisted in valley bottoms climate (Gavin et al., 2011). until the Holocene; therefore, few pollen records in inland In the Western Cordillera, forests dominated by pine and British Columbia encompass the late Pleistocene. East of the mixed conifers support a complex mixed-severity fire regime.

Cascade Range, Artemisia and Pinus dominated the pollen as- Vegetation and fire histories are of great importance for land semblages, indicating a more open and drier vegetation type managers who are setting restoration targets based on the than occurs today. At Waitts Lake in northeastern Washington, historical range of variation in vegetation and disturbance

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Author's personal copy 130 POLLEN RECORDS, POSTGLACIAL | Northwestern North America rates. Pollen and charcoal records from the Western Cordillera transform entire ecosystems. The existence of pockets of ende- are summarized in several papers (Gavin et al., 2007; Marlon mism today suggests a long-term history of isolation and per- et al., 2009; Whitlock et al., 2008). Fires are a long-term pres- sistence. Applying both genetic and fossil investigations into ence from wet coastal forests to dry interior basins, and the the history of patterns of disjunction and endemism is a pow- Holocene vegetation history is more precisely interpreted erful biogeographic approach to understand the origin of mod- alongside independent fire-history data. Some especially high- ern patterns of diversity. There remains much potential by resolution studies have demonstrated a highly episodic history merging these lines of evidence in the study of vegetation in of fire, showing that any reference to disturbance regimes from the Pacific Northwest. the past 300 years alone does not describe the degree of vari- The paleoecological record can also inform managers in ability that occurs over longer periods, including events of high setting restoration target points. Large areas of dry forest in severity and low frequency that may be considered highly un- the Pacific Northwest have ‘missed’ several cycles of fire since desirable today (Colombaroli and Gavin, 2010). Over the entire the period of effective fire suppression. Disagreement on the Holocene, the regional coherence of paleorecords indicates that character of the historical range of variability of fire frequency increased fire during the early Holocene thermal maximum and fire severity exists despite decades of research on the topic. promoted disturbance-adapted vegetation. There is some evi- The sediment record, where it is of sufficiently high resolution, dence that east of the continental divide increased insolation is a powerful tool to describe the historical range of variability resulted in increased summer moisture, more mesic vegetation, in natural disturbances and its link to climate change. and reduced fire (Whitlock et al., 2008).

See also: Pollen Analysis, Principles. Chironomid Records: Cold Deserts Chironomid Overview. Glaciations: Late Quaternary in North America. Paleobotany: Paleophytogeography. Plant Macrofossil

In general, the pollen assemblages from this region are domi- Records: Holocene North America. Pollen Methods and Studies: nated by broadly dispersed and highly productive pollen taxa. Changing Plant Distributions and Abundances. Pollen Records, Thus, pollen records from these regions reflect broad areas. In Late Pleistocene: Northern North America. addition, these drought-adapted taxa have wide ecological am- plitudes, resulting in more stable populations through short- term climate change. However, the vegetation changes that are detected in these regions are concordant with the paleoclimatic References inferences from the coastal region. For example, a well-studied core from Bear Lake confirms the increased extent of cold- and Ager TA and Sims JD (1981) Holocene pollen and sediment record from the Tangle dry-adapted steppe taxa during glacial maxima (Jimenez- Lake area, central Alaska. Journal of Palynology 5: 85–98. Moreno et al., 2007). Anderson PM and Brubaker LB (1994) Vegetation history of northcentral Alaska – A mapped summary of late-Quaternary pollen data. Quaternary Science Reviews Pollen data from eastern Oregon and Washington indicate 13: 71–92. less dense vegetation than present at 14 ka (Mehringer, 1996; Anderson LL, Hu FS, Nelson DM, et al. (2006) Ice-age endurance: DNA evidence of a Minckley et al., 2007). The pollen assemblages from the cold white spruce refugium in Alaska. Proceedings of the National Academy of deserts show little sensitivity to the YDC, though all sites show Sciences of the United States of America 103: 12447–12450. Barnosky CW (1985) Late Quaternary vegetation near Battle Ground Lake, southern increased herb taxa after 11 ka, indicating warmer and drier Puget Trough, Washington. Geological Society of America Bulletin 96: 263–271. conditions. Decreased fire at this time likely reflects reduced Barron JA, Heusser L, Herbert T, and Lyle M (2003) High-resolution climatic evolution biomass continuity (Minckley et al., 2007). After 7 ka, Pinus of coastal northern California during the past 16,000 years. Paleoceanography

18: 1020. http://dx.doi.org/10.1029/2002PA000768. forest increased at high elevation sites and modern pollen Bartlein P, Anderson K, Anderson P, et al. (1998) Paleoclimate simulations for North assemblages were reached at most sites shortly thereafter. America over the past 21,000 years: Features of the simulated climate and comparisons

with paleoenvironmental data. Quaternary Science Reviews 17: 549–585.

Conclusions Bartlein PJ, Harrison SP, Brewer S, et al. (2010) Pollen-based continental climate reconstructions at 6 and 21 ka: A global synthesis. Climate Dynamics 37: 775–802. Bergstrom MF (1984) Late Wisconsin and Holocene History of a Deep Arctic Lake, North- The vegetation history of NWNA is becoming more fully Central Brooks Range, Alaska. M.S. Thesis, Ohio State University, Columbus, OH. understood due to a large number of recent studies. The region Bigelow NH, Brubaker LB, Edwards ME, et al. (2003) Climate change and Arctic is poised to provide information on postglacial vegetation ecosystems: 1. Vegetation changes north of 55 degrees N between the last glacial maximum, mid-Holocene, and present. Journal of Geophysical Research- dynamics similar to that available from eastern North America. Atmospheres 108(D19): 8170. http://dx.doi.org/10.1029/2002JD002558. Maps of taxa and community types through the late Quater- Briner J, Kaufman D, Werner A, et al. (2002) Glacier readvance during the late glacial nary may soon be possible in this region. This rich data set will (Younger Dryas?) in the Ahklun Mountains, southwestern Alaska. Geology further our understanding of the millennial-scale controls of 30: 679–682. vegetation change, while high-resolution pollen analysis has Brown KJ and Hebda RJ (2002) Origin, development, and dynamics of coastal temperate conifer rainforests of southern Vancouver Island, Canada. Canadian begun to reveal coherent decadal-to-century-scale changes. Journal of Forest Research 32: 353–372. These latter efforts provide important information on ecolog- Brown KJ and Hebda RJ (2003) Coastal rainforest connections disclosed through a Late ical responses to rapid climate change. Quaternary vegetation, climate, and fire history investigation from the Mountain The paleoecological record from the Cordillera provides an Hemlock Zone on southern Vancouver Island, British Colombia, Canada. Review of Palaeobotany and Palynology 123: 247–269. example of how populations persist on the landscape through Brubaker LB, Anderson PM, Edwards ME, et al. (2005) Beringia as a glacial refugium for periods of rapid climate change and how combinations of boreal trees and shrubs: New perspectives from mapped pollen data. Journal of climate change and natural disturbances may rapidly Biogeography 32: 833–848.

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