Quaternary Science Reviews 29 (2010) 2052e2070
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Quaternary Science Reviews
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Glacial populations and postglacial migration of Douglas-fir based on fossil pollen
and macrofossil evidence
Paul F. Gugger a, *, Shinya Sugita b
a
b
Department of Ecology, Evolution and Behavior, University of Minnesota, 100 Ecology Building, 1987 Upper Buford Circle, Saint Paul, Minnesota 55108, USA
Institute of Ecology, Tallinn University, Uus-Sadama 5, 10120 Tallinn, Estonia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2009
Received in revised form
19 March 2010
Accepted 24 April 2010
To understand how temperate forests might respond to future episodes of global warming, it is
important to study the effects of large-scale climate change brought about by rapid postglacial warming.
Compilations of fossil evidence have provided the best evidence of past plant range shifts, especially in
eastern North America and Europe, and provide a context for interpreting new molecular datasets from
modern forests. In western North America, however, such reviews have lagged even for common,
widespread taxa. Here, we synthesize fossil evidence for Douglas-fir (Pseudotsuga menziesii) from nearly
550 fossil pollen, sedimentary macrofossil, and packrat midden macrofossil sites to develop hypotheses
about the species’ late Quaternary history that can be tested with molecular phylogeographic studies. For
both the coastal and interior varieties, we identified alternative hypotheses on the number of glacial
populations and postglacial migration patterns that can be characterized as single-population versus
multiple-population hypotheses. Coastal Douglas-fir may have been subdivided into two populations at
the Last Glacial Maximum (LGM) and colonized British Columbia from populations in Washington and
Oregon. Interior Douglas-fir could have been subdivided along major topographic barriers into at least
three LGM populations and colonized British Columbia and Alberta from populations in northwest
Wyoming and/or northeast Utah. For both varieties, we calculated migration rates lower than previous
studies, which could have been as high as 100e220 m/yr if Douglas-fir reached its modern distribution
9000 cal yr BP, or as low as 50 m/yr if it reached its modern range at present. The elevational range of
populations in California and the southern Rockies shifted upslope by 700e1000 m. If there were
multiple LGM populations, these elevational shifts suggest that those populations did not contribute to
the colonization of Canada. Our findings emphasize the possibility of low-density northern LGM populations and that populations within species react individualistically in response to large-scale climate
change.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Dated fossil pollen and macrofossil records from late Quaternary
lake sediments have been compiled for many European and eastern
North American species indicating that species ranges shifted
dramatically as they tracked favorable climates. For example, Davis
(1981a, 1983) marked the first arrival of tree taxa in deglaciated
regions in eastern North America and found that thermophilous
trees spread at rates of 100e1000 m/yr into these landscapes from
small populations restricted south of the ice sheets at the Last
Glacial Maximum [LGM; 21 000e18 000 calendar years before
* Corresponding author. Tel.: þ1 862 371 2138; fax: þ1 612 624 6777.
E-mail addresses: [email protected] (P.F. Gugger), [email protected] (S. Sugita).
0277-3791/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.quascirev.2010.04.022
present (cal yr BP)]. Similarly, Huntley and Birks (1983) reviewed
the late Quaternary history of European trees, finding that many
taxa were restricted to the Iberian, Italian and Balkan peninsulas
during the LGM and expanded to their present distributions at rates
up to 2000 m/yr. Because these rates are higher than those
considered possible based on empirical estimations in modern
forests (Clark et al., 2001), rare long-distance seed dispersal by
weather or animal vectors has been proposed as a mechanism
(Davis, 1987; Johnson and Webb, 1989; Clark et al., 1998).
For many species, this relatively simple scenario in which one or
a few restricted populations concurrently and rapidly expanded
northward in response to warming has been challenged as fossil
data are re-interpreted and other independent types of data
become available for comparison (Bennett and Provan, 2008). In
particular, molecular genetic data from modern populations have
P.F. Gugger, S. Sugita / Quaternary Science Reviews 29 (2010) 2052e2070
been used to infer past changes in distribution because the effects
of past isolation and expansion on the distribution of genotypes
often endure to the present (Hewitt, 2000; Petit et al., 2003).
One key insight has been the detection of additional source
populations farther north than previously thought (Stewart and
Lister, 2001). For example, American beech (Fagus grandifolia) and
red maple (Acer rubrum) might have colonized their northern
ranges from scattered, low-density populations near the ice margin
(McLachlan et al., 2005) and not from a single, small refugium in
the South (Davis, 1981a). Low-density populations could have
escaped detection in the fossil record altogether or could have been
overlooked where pollen percentages were below thresholds used
to define presence. Similarly, molecular data suggest low-density
populations of white spruce (Picea glauca) persisted the LGM in
Alaska in addition to the known LGM populations south of the ice
(Anderson et al., 2006). In Alaska, Picea pollen accounts for less than
2% of pollen assemblages dating to the LGM (Brubaker et al., 2005).
If populations were farther north during the LGM than previously
thought, postglacial migration rates were likely much lower than
previously thought (<100 m/yr), more consistent with empirical
observations of seed dispersal (Clark et al., 2001).
Another important insight is that often only a subset of populations expands, while others remain stable or even shrink. For
example, the northern latitudes of the range of red maple
(McLachlan et al., 2005; Gugger et al., 2008) were only colonized by
populations near the ice margin and not by populations to the
south. A comparison of fossil and molecular data for European
beech (Fagus sylvatica; Magri et al., 2006) revealed that populations
from Mediterranean countries did not contribute to the colonization of central and northern Europe, as was inferred in early fossil
studies in the region (Huntley and Birks, 1983). And in some cases,
those populations may have been isolated for more than one glacial
cycle. Instead, populations from central Europe expanded to colonize middle and high latitudes.
Fossil surveys and their comparison to molecular datasets in
western North America have lagged those in Europe and eastern
North America because the climate history and topography are
more complex and generally fewer fossil sites have been analyzed.
Phylogeographic studies in the region suggest migration into
western Canada from multiple northern source populations in
Washington and the northern U.S. Rocky Mountains (Richardson
et al., 2002; Carstens et al., 2005; Mimura and Aitken, 2007;
Godbout et al., 2008; O’Connell et al., 2008), and some fossil
studies concur (Tsukada, 1982; Rosenberg et al., 2003). In the
Southwest, more complex histories of long-term isolation, elevational shifts, and migration are suggested in fossil (Spaulding et al.,
1983) and molecular (Mitton et al., 2000; Galbreath et al., 2009)
data. For temperate trees set in a topographically complex region
like western North America, LGM populations could have been
scattered and discontinuous and only some may have expanded in
response to postglacial climate warming, while others retreated to
higher elevation or migrated regionally.
Combined fossil and molecular studies have highlighted some
limitations of the fossil record, which can include difficulty
detecting low-density populations and the inability to resolve the
independent histories of different populations within a species.
However, fossils still provide the most direct, dated evidence of
a species presence or absence through time compared to the
imprecise date estimates inferred from molecular data. Without the
backdrop of fossil evidence, patterns of molecular variation cannot
confidently be attributed to particular geologic or climatic events.
Moreover, the fossil record is an important source of hypotheses as
phylogeography turns to more sophisticated statistical methods
requiring a priori hypotheses (Knowles and Maddison, 2002;
Carstens and Richards, 2007).
2053
We synthesize late Quaternary fossil records of Douglas-fir
(Pseudotsuga menziesii) for the specific purpose of estimating the
location of populations during the LGM and patterns of postglacial
range shifts that can be tested in future surveys of molecular
variation. To do so, we have compiled fossil pollen, sedimentary
macrofossil, and packrat (Neotoma spp.) midden macrofossil data
from hundreds of previously published records spanning almost
the entire modern range of Douglas-fir and extending back to just
before the LGM. Our approach emphasizes the possibility of lowdensity glacial populations and the possibility that populations had
independent histories. The various interpretations are discussed as
alternative hypotheses that can be tested with future molecular
phylogeographic studies and additional fossil evidence.
1.1. Study organism
Douglas-fir is a wide-ranging economically and ecologically
important conifer in western North America. Two varieties are
commonly recognized (Hermann and Lavender, 1990): coast
Douglas-fir (P. menziesii var. menziesii) and interior or Rocky
Mountain Douglas-fir (P. menziesii var. glauca). Coast Douglas-fir is
found from central California to central and coastal British
Columbia at elevations from sea level near the coast to 2300 m in
the Sierra Nevada. Interior Douglas-fir ranges from British
Columbia and Alberta to central Mexico, though Mexican populations are occasionally regarded as a different variety, or even
species (Martínez, 1949). Compared to coast Douglas-fir, interior
Douglas-fir inhabits drier, higher elevation sites primarily in the
montane zone but can be found from 500 m to 3200 m depending
upon latitude and local conditions. The range becomes increasingly
fragmented in the Southwest and Mexico. The two varieties meet in
a transition zone in southern, central British Columbia (Li and
Adams, 1989; Ponoy et al., 1994).
Douglas-fir’s late Quaternary history has been partially
reviewed in two previous publications (Tsukada, 1982; Hermann,
1985). Tsukada (1982) focused on the Pacific Northwest and using
fossil pollen records found that Douglas-fir migrated north from
the Willamette Valley of northern Oregon into southern British
Columbia at a rate of about 450 m/yr. Subsequent studies cast doubt
on the precise location of populations at the LGM (Barnosky, 1985a;
Worona and Whitlock, 1995). Hermann’s (1985) survey was
broader in temporal and spatial scope, encompassing most of the
range and extending back to the earliest fossil records in the
Miocene and Oligocene. Hermann’s (1985) interpretation generally
agreed with that of Tsukada (1982) in the Northwest. In the interior,
he concluded that Douglas-fir rapidly migrated into southwestern
Alberta from populations in the southern Rockies. Since then,
a number of new fossil records have been published, which may
shed new light on the late Quaternary history of Douglas-fir.
2. Regional Setting
The topography of western North America is characterized by
two sets of major north-south mountain chains: the combination of
the Rocky Mountains and Sierra Madre and the combination of the
Coast Range, Sierra Nevada, and Cascade Mountains (Fig. 1). The
arid Great Basin and Columbia Plateau separate these two sets of
mountains. Today, the climate varies substantially from cool, moist
Pacific coastal habitats to the drier, summer-monsoonal Rocky
Mountains to the intervening arid deserts.
Late Quaternary climate change in western North America is as
complex as the geography, owing to changes in circulation in
a topographically diverse region. Generally, the LGM was attained
about 18 000 cal yr BP during the maximum extent of the Fraser
(Northwest), Tioga (California), and Pinedale (Rockies) advances of
Fig. 1. Geographic features and fossil sites mentioned in the text (BB ¼ Bellingham Bog; BGL ¼ Battle Ground Lake; BKL ¼ Burnt Knob Lake; BL ¼ Bear Lake; BM ¼ Bonaparte
Meadows; BoL ¼ Bolan Lake; BRB ¼ Bogachiel River Bog; CH ¼ Cowichan Head; CL ¼ Carp Lake; CM ¼ Caledonia Marsh; CuL ¼ Cub Lake; D ¼ Dashwood; DaL ¼ Davis Lake;
DJM ¼ Dutch John Mountain; DL ¼ Dog Lake; EGC ¼ Eastern Grand Canyon; FaL ¼ Fallback Lake; FgL ¼ Fargher Lake; FL ¼ Fracas Lake; GL ¼ Gordon Lake; GLB ¼ Gold Lake Bog;
GrL ¼ Grass Lake; HL ¼ Hall Lake; HP ¼ Hedrick Pond; HRV ¼ Hoh River Valley; IPF ¼ Indian Prairie Fen; J ¼ Jasper; LL ¼ Lily Lake; LiL ¼ Little Lake; LoL ¼ Lost Lake; LDF ¼ Little
Qualicum Falls; LTPB ¼ Lost Trail Pass Bog; ML ¼ Misty Lake; MuL ¼ Muskiki Lake; OF ¼ Onion Flats; PB ¼ Poulsbo Bog; PL ¼ Posy Lake; RL ¼ Rainbow Lake; SB ¼ Silverton Bone Site
Bog; SL ¼ Sherd Lake; SwL ¼ Swan Lake; TLBC ¼ Twin Lakes, BC; TLCA ¼ Twin Lakes, CA; WC ¼ Willow Creek). Inset shows labeled sites in the Pacific Northwest.
P.F. Gugger, S. Sugita / Quaternary Science Reviews 29 (2010) 2052e2070
the Cordilleran ice sheet and mountain glaciers, some 3000 years
after the Laurentide glacial maximum in eastern North America
(Porter et al., 1983). Dry valleys and glaciated mountains predominated. Climate was generally cooler and drier than present in
the Pacific Northwest with subalpine parkland growing close to the
ice margin (Whitlock, 1992). The northern Rockies (north of Great
Divide Basin; Fig. 1) were also cooler and drier and valleys were
largely treeless with permafrost. In both regions, temperate trees
could have grown along mountain flanks. The southern Rockies and
nearby arid lands were generally cooler and wetter-than-present
(Barry, 1983; Thompson et al., 1993; Bartlein et al., 1998). By the
early Holocene (w9000 cal yr BP), warmer and wetter-thanpresent conditions prevailed in the Southwest and drier-thanpresent conditions prevailed in the Pacific Northwest (Whitlock,
1992; Thompson et al., 1993).
3. Materials and methods
Fossil pollen and macrofossil data were compiled for the parts of
western North America that currently or historically may have
supported Douglas-fir populations (Supplementary Data: Tables S1
and S2; Figs. S1 and S2 (contributed to Figs. 2e5, but not cited
elsewhere in text: Chaney and Mason, 1930; Mason, 1934; Deevey,
1944; Hansen, 1950; Martin, 1963a,b; Wright, 1964, 1976,
unpublished; Heusser, 1965, 1973, 1985; Mehringer and Haynes,
1965; Bender et al., 1966, 1967, 1970, 1971, 1977; Wells, 1966,
1976, 1983; Adam, 1967, 1985; Mehringer et al., 1967; Wells and
Berger, 1967; Baker, 1970, 1976; Stuiver, 1969; Lichti-Federovich,
1970; Bradbury, 1971; Freeman, 1972; Maher, 1972a,b; Mathewes
et al., 1972; Meyer, 1973, 1975; Mathewes, 1973; Van Devender,
1973, 1977, 1980, 1982, 1985, 1987, 1990; Wright et al., 1973;
Hansen and Easterbrook, 1974; Waddington and Wright, 1974;
Mathewes and Rouse, 1975; Andrews et al., 1975; King, 1976;
Mack et al., 1976, 1978b,c, 1979, 1983; Petersen and Mehringer,
1976; Van Devender and Mead, 1976; Phillips, 1977; Davis et al.,
1977, 1985; Hall, 1977, 1984; King and Van Devender, 1977; Van
Devender et al., 1977; Van Devender et al., 1978, 1979, 1984;
1990a,b, 1991, 1994; Mead et al., 1978; Rowland, 1978; Madsen
and Currey, 1979; Nelson et al., 1979; Van Devender and Riskind,
1979; Van Devender and Spaulding, 1979; Legg and Baker, 1980;
Spaulding, 1980, 1981, 1985, 1991, 1992, 1994; Betancourt and
Devender, 1980; Brant, 1980; Thompson et al., 1980; Adam et al.,
1981a,b; Cole, 1981, 1983, 1986; Davis, 1981b, 1990, 1992, 1999a,b;
Gottesfeld et al., 1981; Lanner and Van Devender, 1981; Mead and
Phillips, 1981; Berry et al., 1982; Bright and Davis, 1982; Leopold
et al., 1982; MacDonald, 1982, 1987, 1989; Mathewes and Clague,
1982; Mathewes and D’Auria, 1982; Mott and Jackson, 1982; Van
Devender and Toolin, 1982; Watts and Bradbury, 1982; Adam and
West, 1983; Betancourt et al., 1983, 2001; Hebda, 1983; Heusser
and Sims, 1983; Jacobs, 1983, 1985; Kearney and Luckman, 1983,
1987; Mead, 1983; Petersen et al., 1983; Thompson and Hattori,
1983; Van Devender and Toolin, 1983; Vance et al., 1983;
Anderson et al., 1984, 1985, 1989; Betancourt, 1984, 1987;
Betancourt and Davis, 1984; Carrara et al., 1984, 1986, 1991;
Hickman et al., 1984, 1990; McAndrews, 1984; Spencer et al.,
1984; Thompson, 1984, 1992; Warner et al., 1984; Brown, 1985;
Bryant and Holloway, 1985; Cole and Webb, 1985; Fall, 1985,
1995, 1997a,b; Lennstrom, 1985; Mehringer, 1985; Petersen,
1985a,b, 1988, 1994; Thomas, 1985; Van Devender and Burgess,
1985; Wells and Woodcock, 1985; Atwater et al., 1986; Beaudoin
and King, 1986, 1990; Gennett and Baker, 1986; Luckman and
Kearney, 1986; Luckman et al., 1986; Markgraf and Lennon, 1986;
Steventon and Kutzbach, 1986, 1987, 1988; White and Mathewes,
1986; Woodcock, 1986; Anderson, 1987, 1990, 1993; Brunner and
Ledbetter, 1987; Cwynar, 1987; MacDonald et al., 1987, 1991;
2055
Mead et al., 1987; Mehringer and Wigand, 1987, 1990; Zielinski
and Davis, 1987; Cinnamon, 1988; Davis and Moratto, 1988;
McAuliffe and Van Devender, 1998; McCarten and Van Devender,
1988; McGann and Brunner, 1988; Byrne and Horn, 1989;
Jennings, 1989, 1996; Jodry et al., 1989; Mathewes and King, 1989;
Reasoner and Hickman, 1989; Rypins et al., 1989; Shelley and
Altschul, 1989; Adam and Vagenas, 1990; Adam et al., 1990, 1995;
Hebda et al., 1990; McGann, 1990; Wigand, 1990; Anderson and
Carpenter, 1991; Anderson and Van Devender, 1991, 1995;
Beiswenger, 1991; Burns, 1991; Elias et al., 1991; Hickman and
Schweger, 1991; McVickar, 1991; Sauchyn and Sauchyn, 1991;
Sharpe, 1991; Beaudoin and Reasoner, 1992; Clague et al., 1992;
Davis and Shafer, 1992; Elias and Van Devender, 1992; Smith and
Anderson, 1992; Wigand and Nowak, 1992; Jennings and ElliottFisk, 1993; Lozano-García et al., 1993; Smith and Bischoff, 1993;
Cole and Liu, 1994; Dryer, 1994; Koehler and Anderson, 1994a,b,
1995; Hasbargen, 1994; Lozano-García and Ortega-Guerrero,
1994; Hutton et al., 1994; Pellatt and Mathewes, 1994, 1997;
Reasoner et al., 1994; Fall et al., 1995; Gerloff et al., 1995; Rinehart
and McFarlane, 1995; Osborn et al., 1995; Coats, 1997; Feiler et al.,
1997; Lynch, 1998; Smith and Betancourt, 1998; Cole and Murray,
1999; Mohr et al., 2000; Rhode, 2000a,b, 2002; Bennett et al.,
2001; Doerner and Carrara, 2001; Gavin et al., 2001; Pellatt et al.,
2001; Sankey et al., 2001; Brown and Hebda, 2002, 2003; Jackson
et al., 2002; Long and Whitlock, 2002; Generaux et al., 2003;
Brunelle et al., 2005; Caballero et al., 2005; Daniels et al., 2005;
Holmgren, 2005; Palacios-Fest et al., 2006; Power et al., 2006;
Arsenault et al., 2007; Galloway et al., 2007; Long et al., 2007;
Minckley et al., 2007; Koehler, unpublished data)). These include
British Columbia and Alberta, all of central and northern Mexico,
and the western states of Washington, Oregon, California, Arizona,
Utah, Idaho, Montana, Wyoming, Colorado, New Mexico and
western Texas. To simplify the dataset for the purposes of presentation and analysis, we recorded Douglas-fir fossils as “present,”
“rare,” or “absent” at 3000 calendar year intervals from before the
Last Glacial Maximum to the present (40 000e21 000;
21 000e18 000; 18 000e15 000; 15 000e12 000; 12 000e9000;
9000e6000; 6000e3000; 3000e0 cal yr BP), where for example,
3000e0 cal yr BP includes all samples with ages, ignoring error,
that fall within that interval. We chose 3000-year intervals because
many fossil sites only contained partial records through time, many
radiocarbon dates had confidence intervals of 500e1000 years, and
3000 years is considerably higher resolution than that attainable in
molecular phylogeographic studies.
Most dates of fossils were originally reported in radiocarbon
years (14C yr BP), so conversions to calendar years were made
according to Table 1. The radiocarbon age cutoffs were first
estimated by eye using the IntCal04 curve (Reimer et al., 2004)
and then verified by entering that year with a 100-year standard
error into each of two calendric age conversion programs: Calib
version 5.0.1 (http://calib.qub.ac.uk/calib) and CalPal (http://
www.calpal.de).
Table 1
Equivalence of calendar and radiocarbon year cutoffs for each 3000 calendar year
time interval.
Cal yr BP
14
0
3000
6000
9000
12 000
15 000
18 000
21 000
0
2880
5260
8050
10 260
12 640
14 670
17 600
C yr BP
2056
P.F. Gugger, S. Sugita / Quaternary Science Reviews 29 (2010) 2052e2070
3.1. Fossil pollen
For the above-mentioned regions, all 96 fossil pollen records
from lake or bog sediments were extracted from the North American Pollen Database (NAPD; http://www.ncdc.noaa.gov/paleo/
napd.html). Douglas-fir pollen percentages were calculated for
each sample by dividing the Douglas-fir pollen (plus Larix; more on
this below) count by the pollen sum, not including aquatic/bog
plants or indeterminable pollen. These were then converted to the
three categories, where “present” is greater than 0.5%, “rare” is
greater than 0% but less than 0.5%, and “absent” is 0%.
To this list, we added published fossil pollen records not found
in the database, bringing the total number of records to 316 (Table
S1, Fig. S1). All records had date controls, either radiocarbon dates
or tephra with a widely accepted date (e.g. Mazama ash,
7600 cal yr BP; Zdanowicz et al., 1999). When tephra were the only
source of date estimates (especially, Hansen, 1947; Heusser, 1960),
we did not extrapolate beyond that date to be conservative, but
when tephra were accompanied by other chronological evidence
presented by the author, we deferred to the author’s extrapolations
and interpolations (especially, Heusser, 1960, 1964). No attempt
was made to acquire the original pollen counts; rather, we simply
converted pollen percentage diagrams to present, rare, or absent for
each time period by visual inspection. Every effort was made to use
the same pollen percentage thresholds in recording presence/
absence as used for the NAPD. However, because different authors
used different presentations, we were occasionally forced to define
rare as between 0% and 1%.
Regardless of how rare is defined, very low Douglas-fir pollen
representation might indicate Douglas-fir’s presence. Douglas-fir
pollen is usually underrepresented in the fossil record due to
relatively poor production, dispersal, and preservation. Douglas-fir
produces lower quantities of pollen compared to other conifers (e.g.
Pinus) and many other wind-dispersed trees (Mack et al., 1978a;
Baker, 1983). Most pollen falls close to the tree: the 90% characteristic radius (Prentice, 1988; Sugita, 2007) of Douglas-fir pollen is
2320 m based on Prentice’s (1985) model and 1440 m based on
Sugita’s (1993) model, assuming that basin size is 10 ha, fall speed
of its pollen 0.127 m/s (Eisenhut, 1961), and wind speed 3.0 m/s.
Some evidence suggests that, once deposited, it preserves poorly
because it is highly susceptible to fungal decomposition (Goldstein,
1960), though some do not consider this an issue (Whitlock,
personal communication). Thus only one or a few pollen grains
might be considered evidence of Douglas-fir’s presence in the area
(though not necessarily locally), especially when considering the
range-wide scale of interest here (Tsukada, 1982).
In other poorly represented species, such as American beech,
a 0.5% threshold is often used to define presence, and analyses of
surface sediments have shown that false negatives are more
common than false positives with that threshold (McLachlan and
Clark, 2004). Of course, false-presences do occur at low rates due
to the occasional long-distance transport of pollen. In modern
surface samples, Douglas-fir pollen is often less than 1e2% when
present in surrounding forests (Whitlock, 1993; Williams et al.,
2006). In most cases where Douglas-fir pollen was rare (<0.5%) in
our study, it appears in multiple samples within a given time
interval, which in rare pollen types may be considered evidence of
a species presence (Smith and Pilcher, 1973; McLachlan and Clark,
2004). For these reasons, most of our analyses consider low
pollen percentages evidence of Douglas-fir’s regional presence, but
maps distinguish very low percentages (<0.5%) to capture uncertainty throughout.
In most fossil pollen studies, Douglas-fir and larch (Larix) pollen
are not distinguished. Their pollen grains are spherical and inaperturate, and similar in size and appearance under light
microscopy. This introduces uncertainty in the Canadian and
northernmost U.S Rocky Mountains, where ranges overlap.
Western larch (Larix occidentalis) and subalpine larch (Larix lyallii)
are currently and probably have been restricted to the interior
northwest. Western larch co-occurs with Douglas-fir in the
montane zones of Montana, Idaho, eastern Oregon and Washington, and the southern Canadian Rockies. This presents the most
likely source of ambiguity, and macrofossil evidence suggests both
can be represented in a pollen assemblage from a particular site
(Whitlock, 1995). Subalpine larch is rarer and restricted to very high
elevation in subalpine zones. An eastern, boreal larch (L. laricina),
enters the study area in Alberta, east and north of the Rocky
Mountains, but does not overlap the range of Douglas-fir. Records
that are thought to reflect Larix and not Pseudotsuga pollen are
noted in Table S1 and were decided based on the claims of the
original publication and/or on an unusual geographic position (e.g.
high elevation, high latitude) relative to Douglas-fir’s modern
range. Larix pollen is also underrepresented in the fossil record and
thus would be unlikely to drown out the signal of Douglas-fir
(Brubaker et al., 2005).
3.2. Macrofossils
Two-hundred sixteen packrat midden macrofossil records from
over 100 localities across western North America were retrieved
from the North American Packrat Midden Database (http://esp.cr.
usgs.gov/data/midden/; Table S2, Fig. S2). These include all
records mentioning Douglas-fir (present or absent) plus most of the
sites that do not mention Douglas-fir but clearly would have
reported it if it were present. Those not explicitly mentioning
Douglas-fir fall into three categories: 1) complete taxa lists reported, but no Douglas-fir, 2) partial taxon lists with no Douglas-fir, but
reported taxa clearly limit midden site to habitat not favorable to
Douglas-fir, such as low elevation deserts, or in one case, 3) partial
taxa lists where it is unclear if Douglas-fir was or was not present
(see Table S2). Some midden sites that did not report Douglas-fir
and were redundant in well-sampled areas, such as southern
Nevada in particular, are not included here.
In all cases, Douglas-fir is identified to the species-level (P.
menziesii). Its presence in a given midden site indicates that
Douglas-fir was likely a component of the plant community within
about 30e50 m (rarely over 100 m) of the site (Finley, 1990). A
survey of modern middens by Lyford et al. (2004) revealed that in
nearly 90% of cases where Douglas-fir was living within 50 m of
a modern midden site, it was present in the midden, and when
representing over 25% of the nearby forest cover it was always
present. False-absences occurred in several cases when Douglas-fir
was a minor component (<20%) of the surrounding forests. Falsepresences are unlikely, except if contamination across time horizons is considered.
The Packrat Midden Database already assigns samples as absent
(0), rare (1), or present (2), where rare is designated by the original
author and/or database administrators (Strickland et al., 2001) to
control for the possibility of contamination from another time
horizon. We consider these roughly comparable to our three pollen
categories, where rare indicates a high degree of uncertainty. All
samples included here were radiocarbon dated, though the material dated varies widely including various plant fragments, packrat
fecal pellets, debris, and other midden components. In cases where
multiple dates are given for a single taxa list, we chose the authorpreferred date or a date directly from a Douglas-fir macrofossil over
other dates. Where multiple dates seemed plausible, we averaged
the dates, though these were usually fairly close to each other
(noted in Table S2).
P.F. Gugger, S. Sugita / Quaternary Science Reviews 29 (2010) 2052e2070
To the midden macrofossil list, we added 14 fossil sites for which
Douglas-fir macrofossils were found in dated sediment. Seven of
these sites are from the North American Plant Macrofossil Database
(NAPMD; http://www.ncdc.noaa.gov/paleo/plantmacros.html) and
seven are from various published sources (Table S2, Fig. S2). All of
these cases strongly suggest Douglas-fir’s presence, thus rare is not
distinguished. We have not reported Douglas-fir absences from
other sites reported in the database or elsewhere because macrofossils in lake sediments are often rare or absent even when the
plant of interest is abundant as inferred from pollen abundance or
as observed in comparisons of local plant composition with surface
sediments (Jackson et al., 1997).
3.3. Migration patterns
Patterns of migration were observed by mapping Douglas-fir
fossils as present, rare, or absent at 3000-year intervals. Clusters of
sites with Douglas-fir fossils and solitary sites with macrofossils or
high pollen percentages were presumed good evidence for glacial
populations.
We assumed separate glacial populations for each of the
commonly recognized varieties because their divergence must
predate the last glacial cycle (Li and Adams, 1989; Hermann and
Lavender, 1990). Within varieties, we attempted to approximate
a number of independent LGM populations by delineating LGM
populations along large geographic gaps and/or major topographic
barriers, which assumes these geographic features isolated those
populations. This does not exclude the possibility that additional
population substructure existed.
Limited spatial and temporal resolution precluded the ability to
construct meaningful latitudinal transects beyond those already
published (Tsukada, 1982). Nonetheless approximate migration
rates from one time interval to the next were calculated by
measuring the geographic distance (WGS84 reference system) from
leading edge populations in the first interval to leading edge populations in the next and dividing by the duration of a time interval
(3000 years). An overall minimum mean rate was estimated
assuming no barriers and was calculated from the distance from the
ice margin at 18 000 cal yr BP to the present-day northern limit
divided by 18 000 years. A “maximum” mean rate was calculated
assuming Douglas-fir started from the observed northernmost LGM
population (where fossils present, not rare) at 18 000 cal yr BP and
tracked the receding ice until reaching its present range limit at
9000 cal yr BP. This ignores possible barriers and indirect routes, so
is not necessarily a true maximum.
To examine trends in elevational migration, we displayed
elevation at which Douglas-fir was present versus time in boxplots
for each of six regions: California, western Oregon/Washington
(coast to Cascades), western British Columbia (west of Canadian
Rockies), southern Rockies (south of Great Divide Basin; Fig. 1),
northern Rockies (north of Great Divide Basin to Canadian border),
and Canadian Rockies. First, all data where Douglas-fir fossils were
present and rare were included, and second only data points where
Douglas-fir fossils were present were included. Rare was excluded
in the latter approach because low amounts of wind or water
transported pollen can settle at elevations much higher or lower
than the source. To complement our regional analyses, we created
an elevational transect marking Douglas-fir presence and absence
through time according to macrofossil data in the eastern Grand
Canyon, the only locality with enough data to meaningfully do so.
Several scenarios might be meaningful: 1) Elevational range shift
should manifest as increasing minima, means (or medians), and
maxima through time; 2) Elevational range expansion would show
rising elevation maxima, constant minima, and either constant or
slightly rising means (or medians); and 3) No change (elevational
2057
stasis) would show no relationship of minima, means, or maxima
through time. Scenarios 1 and/or 2 might be expected in regions
continuously inhabited since the LGM, whereas scenarios 2 and/or
3 might be expected in regions colonized after the LGM.
In all of the above analyses, we excluded sites suspected of
having recorded Larix pollen (Table S1), but this did not alter the
results in any meaningful way (data not shown).
4. Results and discussion
4.1. Glacial populations: 40 000e18 000 cal yr BP
4.1.1. Rocky Mountain variety
Glacial populations were located throughout much of the
southern and central Rockies (Fig. 2), including strong macrofossil
evidence for populations near the Grand Canyon, along the
Mogollon Rim in central Arizona, in southern New Mexico and in
eastern Utah. It is also possible that Douglas-fir was as far north as
the Yellowstone region as suggested by very low pollen percentages
in two glacial sediment samples at Hedrick Pond (Whitlock, 1993)
and in samples from 16 700 cal yr BP at nearby Cub Lake in Idaho
(Baker, 1983). Long-distance pollen transport cannot be ruled out,
but when considering the underrepresentation of Douglas-fir
pollen, it seems unlikely that pollen found in multiple basins and
multiple sediment layers came from glacial populations in northeast Utah over 350 km away (Sugita, 1993, 1994), the nearest
Douglas-fir population supported by macrofossil evidence (Dutch
John Mountain, Jackson et al., 2005). Moreover, low Douglas-fir
pollen percentages (<1%) have been accompanied by macrofossils
at other time horizons at other nearby sites, suggesting low pollen
percentages sometimes indicate local presence (Fallback Lake;
Whitlock, 1993). Therefore, Douglas-fir populations may have
persisted the LGM in the Yellowstone region or northern Rockies
more generally. Montane species could have survived in the
northern Rockies along unglaciated mountain flanks (Thompson
et al., 1993; Whitlock, 1995).
Some mountains may have been barriers to dispersal that
separated LGM populations, and certain populations that are
presently isolated might have been continuous during the LGM
(Fig. 3) if Douglas-fir was 700 m below its present elevational range
(Fig. 4). Some high elevation barriers could include the glaciated
mountains of northwest Wyoming, Utah’s Uinta Mountains, and
the Colorado Rockies, and some prominent low elevation barriers
could include the Great Divide Basin in southwest Wyoming and
the Colorado and Green River system. These putative barriers
would have partitioned the Hedrick Pond site (northwest WY) from
the Dutch John Mountain site (northeast UT), the Dutch John
Mountain site from all sites to the south, and the sites on either side
of the Grand Canyon (Fig. 3). Unfortunately, the absence of Douglasfir between these putatively independent populations is uncertain
because intervening LGM fossil sites have not been investigated.
Phylogeographic studies in other species have found genetic breaks
along the Great Divide Basin (Demboski and Cook, 2001; Barrett
and Freudenstein, 2009), Colorado Rockies (Galbreath et al., 2009;
Jaramillo-Correa et al., 2009), and Colorado River system (Mitton
et al., 2000; Conroy and Cook, 2000). The implication of such
barriers is that only a subset of populations would have expanded
into Canada, while others remained farther south.
Several external considerations support a hypothesis of subdivided LGM populations for Rocky Mountain Douglas-fir. First, in
a survey of allozyme variation, Li and Adams (1989) found that
northern and southern Rocky Mountain populations differed, suggesting they had been isolated through at least the last glacial cycle.
Similarly, Critchfield (1984) argues that discontinuities in leaf
terpene chemistry among geographic regions suggest multiple
Fig. 2. Maps marking Douglas-fir fossils present, rare, or absent at each fossil site for each time interval. The extent of the ice sheets at each time period within a few hundred years
of the end of that interval (e.g. ice limits shown in 21 000e18 000 cal yr BP map correspond to 18 300 cal yr BP). Because ice limits are approximate and fluctuate on finer scales than
those depicted here, occasional data points appear on top of the ice, but this is not the case. For example, the point showing low pollen percentages (for Larix in this case) that
appears on top of the ice 21 000e15 000 cal yr BP is known to have been ice-free since at least 19 000 cal yr BP (Holloway et al., 1981). The congruence of fossils dated
3000e0 cal yr BP with Douglas-fir’s present range (Little, 1971) is shown in the last panel.
P.F. Gugger, S. Sugita / Quaternary Science Reviews 29 (2010) 2052e2070
2059
Fig. 3. Topography of western North America is shown to highlight possible late Pleistocene barriers (labeled features and most areas <1500 m or >2500 m) between different
glacial populations of Douglas-fir (different shapes). For example, the population on the north rim of the Grand Canyon (triangle) could have been isolated from populations south of
the Colorado River (squares), and populations south of the Colorado River could have been more interconnected during the LGM if Douglas-fir was 700 m below its present
elevation. Marked sites (stars, triangles, squares and circles) are based on fossils present 21 000e18 000 cal yr BP. Star with dotted border represents fossil from 16 700 cal yr BP (Cub
Lake, ID), and is shown to better illustrate a possible glacial population in the region.
Fig. 4. Change in elevational range of Douglas-fir through time (in thousands of cal yr BP) for six regions in the study area shown in boxplots. These patterns did not depend on
whether or not “rare” sites were included, so we only report tests where both rare and present are included. Lines mark medians, boxes capture the 25the75th quantiles, and the
whiskers capture the most extreme points not more than 1.5 times the range of the box. The y-axis scale differs among graphs.
P.F. Gugger, S. Sugita / Quaternary Science Reviews 29 (2010) 2052e2070
Pleistocene populations (Zavarin and Snajberk, 1973). Finally,
response surface models projecting the past distribution of Douglas-fir based on simulated climate from a general circulation model
suggest that the Rocky Mountain variety was fragmented (Bartlein
et al., 1998), though it is not clear where the exact partitions among
LGM populations may have been. However, this model should be
interpreted cautiously because populations were also predicted far
outside the current range (e.g. Alaska). Nonetheless, it is interesting
that the model seems to suggest low-density populations close to
the ice margin which may have been separated from the various
pockets of populations in the southern Rockies e a scenario that
may be confirmed as more fossil sites are analyzed in the northernmost U.S. Rockies.
4.1.2. Coastal variety
The fossil evidence presented here suggests two distinct populations during and before the LGM (40 000e18 000 cal yr BP): one
in unglaciated western Oregon and Washington and the other in
the San Francisco Bay area of California (Fig. 2). Those regions are
separated by the Klamath Mountains of northern California and
southern Oregon. The absence of Douglas-fir fossils in-between at
Twin Lakes (CA; Wanket, 2002; West et al., 2007), Grass Lake (CA;
Hakala and Adam, 2004), and Caledonia Marsh (OR; Hakala and
Adam, 2004) could be considered corroborating evidence. Consistent with this split, leaf terpene composition shows a break along
the Klamath Mountains, with high diversity to the south and
uniformity to the north (von Rudloff, 1972; Zavarin and Snajberk,
1973; von Rudloff and Rehfeldt, 1980; Critchfield, 1984). Moreover, a genetic break is found along the Klamath Mountains in
many plant and animal taxa (Soltis et al., 1997; Swenson and
Howard, 2005; Jaramillo-Correa et al., 2009).
However, allozyme (Li and Adams, 1989) and nuclear DNA
(Krutovsky et al., 2009) variation in coastal populations is not
strongly geographically partitioned. In addition, the models of
Bartlein et al. (1998) suggest Douglas-fir was both abundant and
continuously represented in the U.S. Pacific coastal states from
southern California to central Washington. Therefore, it is also
possible that the two putative LGM populations are an artifact of
the limited number of fossil sites (especially low elevation) in
northern California and southern Oregon from before
15 000 cal yr BP, and rather, there was a continuous population
from coastal southern California to Washington. The more consistent coastal climate since the LGM (Bartlein et al., 1998) and much
reduced topographic complexity compared to the Rockies argue
that Douglas-fir’s range was not nearly as fractured as in the
Rockies.
Regardless, the presence of glacial populations relatively close to
the ice margin is evident. Tsukada (1982) observed Douglas-fir
pollen in the Willamette and unglaciated Puget lowland in fullglacial sediments and based on his more limited dataset argued
that Douglas-fir did not likely exist north of about 46 N. This
conclusion was based on low pollen percentages in LGM sediment
at Fargher Lake, Onion Flats, and Silverton Bone Site Bog. Recurring
low pollen percentages (<1.5%) during the LGM have also been
found at Davis Lake (Barnosky, 1981) and Battle Ground Lake
(Barnosky, 1985a) in the Puget Trough. None of these sites has high
pollen percentages, which suggests either small, local populations
or larger populations elsewhere in the region.
In the region, the LGM location of large Douglas-fir populations
remains elusive. Very low pollen percentages were observed at
Carp Lake in the southwestern Columbia Basin (Barnosky, 1985b)
and possibly at Bogachiel River Bog along the coast (Heusser, 1978).
Coastal Washington has been identified as a glacial refugium for
tree taxa that currently co-occur with Douglas-fir (Heusser, 1972).
In the central Coast Range pollen and macrofossils were absent
2061
during the LGM, but present just before and after (Little Lake,
Worona and Whitlock, 1995). Sites in the central Cascades seem to
suggest a similar pattern, though no pre-LGM data are available
(Indian Prairie Fen, Gordon Lake; Sea and Whitlock, 1995; Grigg and
Whitlock, 1998). These patterns agree with those at Battle Ground
Lake, where a macrofossil marks the presence of Douglas-fir before
the Vashon Stade and high pollen percentages mark its high
abundance afterwards (Barnosky, 1985a). Low pollen percentages
throughout the region at glacial times followed by higher pollen
percentages might suggest expansion from low-density populations. Some of these small populations could have been farther
north than suggested by Tsukada (1982), for example at Davis Lake
or Bogachiel Bog. A large body of new research suggests that lowdensity, northern LGM populations were more common than
previously realized (McLachlan et al., 2005; Anderson et al., 2006;
Petit et al., 2008).
4.2. Post-LGM migration: 18 000e9000 cal yr BP
4.2.1. Rocky Mountain variety
The presence of glacial populations in the Yellowstone area or
northern Utah modifies our previous understanding of Interior
Douglas-fir’s postglacial migration. Tsukada (1982) and Hermann
(1985) have argued that Douglas-fir from the southern Rockies
colonized Canada, reaching northeastern Washington and northern
Montana and Idaho by 13 000e11 000 cal yr BP and southwestern
Alberta by 10 000 cal yr BP, implying a migration rate of nearly
200 m/yr. However, both the Dutch John Mountain (Uinta Mtns.;
Jackson et al., 2005) and Hedrick Pond/Cub Lake (Yellowstone area;
Baker, 1983; Whitlock, 1993) evidence were unknown at the time,
and now offer strong support for more northern glacial populations
that colonized the Canadian Rockies, which would reduce the rate
to about 120 m/yr over that interval. Moreover, the migration rate
could be further lowered if glacial populations are found closer to
the ice margin as more sites are analyzed, though they also could be
raised if the northern limit was reached sooner than assumed here
(9000 cal yr BP). Considering all the uncertainties, we estimate
a minimum northward migration rate of 50 m/yr and a maximum
of 220 m/yr in the Rockies (Table 2).
Douglas-fir LGM populations also appear to have expanded
eastward into the Bighorn Mountains (Wyoming) and Highwood
Mountains (Montana) in the northern Rockies and into the
northern Colorado Rockies in the southern Rockies. Mean rates
ranged from 10 to 38 m/yr, but that migration was mostly observed
12 000e9000 cal yr BP at rates of 61e114 m/yr (Table 2).
Table 2
Migration rates (m/yr) during each time interval along major migration routes.
Italicized rates were calculated across multiple time intervals and evenly distributed
accordingly, a situation forced by limited intervening data.
Migration direction
(source)
North
(coast)
North
(N. Rockies)
East
(N. Rockies)
East
(S. Rockies)
East
(CA)
Time interval
(cal yr BP)
21 000e18 000
18 000e15 000
15 000e12 000
12 000e9000
9000e6000
6000e3000
3000e0
Overall mean
Mean 18 000e9000
Minimum
“Maximum”
e
42
144
129
0
0
0
52
105
50
w100
e
0 (139)
278 (139)
87
64
64
64
93
122
58
w220
e
0
0e18
61e86
0
0
0
10e17
20e35
e
e
0
e
0
114
0
0
0
19
38
e
e
0
0
0
89
0
0
0
15
30
e
e
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If multiple LGM populations existed, then the data presented
here suggest that southern Rockies populations primarily retreated
to higher elevations rather than expanding northward (Figs. 4 and 5).
An elevational shift occurred from 18 000e6000 cal yr BP as
summarized in our regional data (Fig. 4) showing an elevation shift
of 700e900 m in the full dataset, 300 m in the fossil pollen data, and
600 m the macrofossil data. The pattern in the macrofossil data is
much clearer, shows less elevational variation at each time period,
and reflects the lower end of the elevational range of Douglas-fir in
the southern Rockies. This is consistent with the generally low
elevation habitat of packrats relative to Douglas-fir (Betancourt et al.,
1990) and the higher abundance of lakes for sediment cores at higher
elevations in the Southwest. Macrofossil evidence from the eastern
Grand Canyon locality also shows that Douglas-fir’s elevational
range shifted upward by about 500 m (Fig. 5), consistent with our
regional observations (Fig. 4). This shift is consistent with other taxa
in the region (Maher, 1961, 1963; Spaulding et al., 1983; Hall, 1985).
In contrast, northern Rocky Mountain and, once colonized,
Canadian Rocky Mountain populations may have persisted at
a similar elevational range during northward migration (Fig. 4).
Climate along elevational gradients in the northern Rockies is
determined by the balance of summer monsoonal and subtropical
high-pressure climate regimes (Whitlock and Bartlein, 1993).
Although the strength of each has varied, the boundary between
has remained relatively stable throughout the Holocene, which
might explain some of the apparent stability in the elevational
distribution of Douglas-fir in the northern Rockies. However,
confusion with larch pollen and sparse data in the region limit the
strength of this conclusion.
Larix pollen also confounds our ability to detect the time of first
colonization in the Canadian Rockies. On the basis of unusually high
elevation (>2000 m) and unusual coordinates (e.g. in the Plains)
relative to the modern range of Douglas-fir, we assume that Pseudotsuga/Larix pollen found in Alberta and the Canadian Rockies
from 21 000e12 000 cal yr BP was Larix pollen except for Twin
Lakes, BC (1100 m; Hazell, 1979) at 12 000 cal yr BP. Even if we are
wrong about the Larix sites in the Canadian Rockies, it would not
alter the overall mean migration rate or migration pathway we
conclude, because more northern sites are eventually colonized by
Douglas-fir (e.g. Jasper; Heusser, 1956; also see present range).
Douglas-fir is not thought to have ever occupied the Plains
(Hermann, 1985), which is supported by the lack of Douglas-fir
macrofossils in Alberta’s plains in general and the presence of Larix
macrofossils in at least one site, Muskiki Lake (Kubiw et al., 1989).
Fig. 5. The elevational range of Douglas-fir in the eastern Grand Canyon increased
through time. Black circles ¼ present; gray circles ¼ rare; open circles ¼ absent.
Regression line is based on sites at which Douglas-fir fossils were marked “present”.
4.2.2. Coastal variety
The migration of coastal populations is better documented and
not confounded by Larix pollen misidentification, yet the overall
pattern is consistent with that of the Rockies. California populations
underwent a pronounced elevational expansion/shift of over
1000 m (Fig. 4) as they colonized the Sierra Nevada
(12 000e9000 cal yr BP) and various mountain chains of northern
California, presumably responding to the warming and drying of
the region (Barry, 1983; Bartlein et al., 1998). Migrating populations
from Washington/Oregon LGM populations closely tracked the
receding Cordilleran ice northward into coastal and central British
Columbia (Fig. 2), accompanied by some expansion into higher
elevations (Fig. 4). Accounting for uncertainties, we estimate
minimum and maximum northward mean migration rates of
50e100 m/yr, though higher rates could have been achieved at
particular time intervals (Table 2). If glacial populations in California and western Washington/Oregon were isolated from one
another, it remains unclear from which source the intervening area
in northern California and southern Oregon was colonized. Douglas-fir macrofossils at the Willow Creek site on Santa Cruz Island
16 600 cal yr BP suggests limited southward migration with
temporary inhabitation when the island was connected to mainland California during the last glacial period (Anderson et al., 2008).
Finally, some eastward migration into the Sierra Nevada and east
Cascades also occurred from 12 000e9000 cal yr BP (Fig. 2; Table 2).
Tsukada (1982) estimated the rate of Douglas-fir migration in
the Pacific Northwest to be 450 m/yr, more than four times that
estimated here. Several considerations could explain this discrepancy. First, Tsukada (1982) only considers northern Oregon to the
southernmost edge of British Columbia spanning a roughly 2000year period. The resolution of his analyses was finer than ours, and
thus for certain periods of time higher rates could have been
possible. Our 3000-year intervals mask the fact that forests probably shifted back and forth as ice expanded and contracted on
decadal and centurial scales. However, Tsukada (1982) assumed
LGM populations south of 46 N, south of our observed northernmost LGM populations, which would produce a greater migration
distance and rate. In part, this is because the threshold value of
pollen percentage when marking time of first arrival was set higher
than ours, which means possible more northern low-density populations during the full glacial were undetected (e.g. Hall Lake,
Bellingham Bog). Additionally, we infer that populations closely
tracked ice sheet dynamics and could have persisted at the glacial
maximum in nearby Washington because populations of Douglasfir are found on Vancouver Island before the LGM during the
Olympic Interglacial in both pollen and macrofossil data (e.g.
Cowichan Head, Dashwood; 40 000e<25 000 cal yr BP; Alley,
1979). Finally, if our migration rates for the coast are off, then it
seems that they are likely too high. For example, only the Rainbow
Lake site (Heusser, 1960) marks the arrival of Douglas-fir from
12 000e9000 cal yr BP at what is near its current northernmost
coastal site. Chronological determinations for this site were based
on correlations with other radiocarbon dated sites in the region,
which may not hold after direct dating, and could reduce the
migration rate. As a result, we think that Tsukada’s (1982) estimates
are too high, and considering our upper bound migration rate
estimates, certainly could not have been maintained over millennia
as Douglas-fir expanded into Canada.
Our migration rate estimates for both varieties are roughly
consistent with rates predicted from theoretical models (80 m/yr)
based on average modern seed dispersal patterns and survival rates
(Thompson and Schorn, 1998). In addition, they are consistent with
rates estimated in western North American species with extensive
macrofossil records, such as Utah juniper (Juniperus osteosperma;
110 m/yr; Lyford et al., 2003) and single-leaf (Pinus monophylla;
P.F. Gugger, S. Sugita / Quaternary Science Reviews 29 (2010) 2052e2070
40e130 m/yr) and two-leaf (Pinus edulis; 60e80 m/yr) pinyon pines
(Thomas, 1983; Spaulding, 1984). Low migration rate estimates are
becoming more common as cryptic northern glacial populations
are discovered in combined analyses of fossil and molecular data
(McLachlan et al., 2005; Anderson et al., 2006; Godbout et al., 2008;
Petit et al., 2008).
4.3. Modern range: 9000e0 cal yr BP
By 9000 cal yr BP the modern range took shape. The northern
limit appears roughly defined, the major coastal and interior
mountains were well populated, and even isolated mountain
chains, such as those in central Wyoming (e.g. Sherd Lake, Bighorn
Mtns.; Burkart, 1976) and central Montana (Lost Lake, near Highwood Mtns.; Barnosky, 1989) were inhabited by Douglas-fir (Fig. 2).
Moreover, Tsukada (1982) and Critchfield (1984) suggest that the
two varieties may have come into contact around this time
(7700 cal yr BP) in British Columbia, though we think this remains
unclear due to insufficient sampling and lack of variety-level
resolution. Also by this time (9000e6000 cal yr BP), elevational
migration appears to have plateaued (Fig. 4). Finally, in some sites
(e.g. Dog Lake, BC, Hallett and Hills, 2006; Poulsbo Bog, WA,
Tsukada, 1982; Swan Lake, ID, Bright, 1966), pollen abundances
reach modern levels suggesting that modern Douglas-fir population densities were achieved.
However, this period was not entirely static, and in fact, there has
been a slight cooling trend since 6000e4000 cal yr BP (Wanner et al.,
2008). From 9000 to 5000 cal yr BP, a number of northern records
show higher Douglas-fir pollen abundance than the last 6000e4000
years. Examples include Misty Lake on northern Vancouver Island
(BC; Lacourse, 2005), which peaks 11 000e7500 cal yr BP; Burnt
Knob Lake (ID; Brunelle and Whitlock, 2003) from 9700 to
6000 cal yr BP; Lost Trail Pass Bog (MT; Mehringer et al., 1977) from
8000 to 5000 cal yr BP; and Lily Lake (WY; Whitlock, 1993) before
7000 cal yr BP. Together, these imply some late Holocene range
contraction or reduction in abundance at higher elevations and
northern latitudes as climate cooled. In contrast, many records in the
Pacific Northwest show a marked increase of Douglas-fir pollen after
5000e4000 cal yr BP, which corresponds with increasing moisture
in the region. These include Gold Lake Bog (Sea and Whitlock, 1995),
Bolan Lake (Briles et al., 2005), and Gordon Lake (Grigg and
Whitlock, 1998) in Oregon; Battle Ground Lake (Barnosky, 1985a),
Carp Lake (Barnosky, 1985b), and Bonaparte Meadows (Tsukada,
1982) in Washington; and Little Qualicum Falls (Heusser, 1960) in
British Columbia. One exception is the decline in Douglas-fir’s pollen
abundance of coastal Washington which is thought to reflect the
reduced role of fire and consequent rise of western hemlock
(Rosenberg et al., 2003) and western redcedar forest (e.g. Hoh Valley,
Heusser, 1974; Tsukada, 1982). In the Southwest, records are mixed
with some showing increased abundance (e.g. Posy Lake, UT, Shafer,
1989; Bear Lake, AZ, Weng and Jackson, 1999) and some showing
decreased abundance (e.g. Fracas Lake, AZ; Weng and Jackson, 1999)
depending on local changes in climate. Pollen percentages are often
difficult to use for quantifying actual changes in vegetation abundance (Sugita, 1994). Taken together, however, these changes in
Douglas-fir pollen percentages suggest that the major changes in
Douglas-fir populations over the last 9000 years were primarily
changes in abundance, rather than changes in distribution.
4.4. Support for our approach
The data from 3000 to 0 cal yr BP seem to capture the modern
distribution of Douglas-fir fairly well (Fig. 2). Poorly sampled
regions excluded, all major mountains and even many isolated
minor mountains where Douglas-fir currently resides and where
2063
fossil sites are available show the presence of Douglas-fir. Exceptions include some isolated localities in southern New Mexico and
western Texas. Moreover, the elevational range of Douglas-fir
recorded 3000e0 cal yr BP accurately reflects the known modern
elevational ranges for western British Columbia and western
Washington/Oregon (Hermann and Lavender, 1990). In the
remaining four regions Douglas-fir was observed as many as a few
hundred meters above modern elevational limits. In all cases, these
were pollen records, suggesting that pollen may have blown
upslope from nearby stands, rather than the range actually being
higher than present. In the northern U.S. and Canadian Rockies,
larch (especially subalpine larch) pollen may also explain this
discrepancy. Nonetheless, the effect is not large, and the overall
congruence of modern distribution with recent fossil evidence
lends confidence to our approach for detecting Douglas-fir’s presence at the broad scale of interest here.
4.5. Significance
Models projecting the range of Douglas-fir at the end of the 21st
century show that Douglas-fir will need to shift or expand into the
Great Basin and northern British Columbia/southeast Alaska to
survive (Shafer et al., 2001). Douglas-fir is projected to be absent
from much of its current range, including Mexico, parts of the
southern Rockies and coast, and much of central British Columbia,
indicating range shift is critical to avoid regional extinction.
However, that would require a migration distance of 100e1000 km
or greater in less than 100 years, which equals a migration rate of
1000e10 000 m/yr, far higher than anything considered possible
naturally. In other regions, this may be less of an issue, as range
shifts might be primarily elevational (Bartlein et al., 1997; Shafer
et al., 2005). Future research should emphasize understanding
the different responses to past and future climate at the population
level.
5. Conclusion: testable hypotheses and future research
The combined fossil pollen and macrofossil dataset leaves us
with a set of testable hypotheses for the late Quaternary history of
Douglas-fir. Alternative hypotheses can be characterized as singlepopulation or multiple-population during the LGM. In the former,
each variety formed a contiguous interbreeding population that
expanded northward into Canada. In the latter, two distinct glacial
populations were present on the coast and three or more were
present in the Rockies, each separated by prominent geographic
barriers. Multiple glacial populations would suggest that only
northernmost populations colonized Canada, while southern populations contracted to higher elevations. Furthermore, we
hypothesize that Douglas-fir may have persisted the LGM in the
Yellowstone region or northern Rockies based on very limited fossil
evidence.
Some of the uncertainties presented here could be resolved with
additional sampling of sites dating back to the LGM. These include
some large regions that remain sparsely studied, such as eastern
Oregon, the northern U.S. Rockies, central Utah, and southwestern
Wyoming; but also, well-sampled, topographically complex
regions that require intensive sampling to distinguish among some
of the hypotheses posed here; and finally, central British Columbia
to refine northward migration rate estimates and better estimate
the time of first contact among varieties. A number of California
fossil data remain unpublished (Adam, 1985) and would be of great
value distinguishing hypotheses on the number and distribution of
glacial populations and patterns of migration for the coast. Finally,
future fossil investigations in Mexico might reveal the Quaternary
dynamics of those poorly understood populations.
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P.F. Gugger, S. Sugita / Quaternary Science Reviews 29 (2010) 2052e2070
These hypotheses can also be tested using molecular phylogeographic approaches, which can detect population structure,
migration routes, and provide further insight into the individualistic responses of populations. Moreover, molecular methods could
reveal the relative contribution of each variety to Canadian populations of Douglas-fir. Our findings from the fossil record provide
an important dated backdrop for interpreting genetic variation in
modern Douglas-fir forests, and for understanding the ecological
and evolutionary consequences of past and present large-scale
climate change on western North American forests.
Acknowledgments
We thank L. Robertson and A. Dyke (Geological Survey of Canada)
for providing the ice sheet data used in our maps, L. Strickland (U.S.
Geological Survey) for assistance with the Packrat Midden Database,
and J. Cavender-Bares, N. Deacon, J. DeNoyer, J. Savage, R. Shaw, C.
Whitlock, R. Zink, and one anonymous reviewer for providing
constructive comments that improved this manuscript. This work
was supported by the National Science Foundation Graduate
Research Fellowship and the University of Minnesota Doctoral
Dissertation Fellowship.
Appendix. Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.quascirev.2010.04.022.
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