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Accepted 20 May 2008
Pacific Northwest
Siskiyou Mountains
Vegetation history
Biological diversity controls
Climate change
ort
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Palaeogeography, Palaeoclimatology, Palaeoecology 265 (2008) 159–169
Contents lists available at ScienceDirect
Palaeogeography, Palaeocli
e lsSteep elevational gradients created by mountains transform large-
regional scale climate patterns into site-specific microclimates in
which different aspects and elevations strongly influence vegetation
and fire across the landscape. The Siskiyou Mountains of north-
western California and southwestern Oregon, a subrange of the
Klamath Mountains, are known for their heterogeneous landscape
and extraordinary gamma diversity (i.e., total number of species
across the region). This diversity is attributed to spatial variability
in soil, climate, and disturbance regime (DellaSala et al., 1999);
however, the role of long-term climate change or stability in shaping
climate fluctuations on millennial and shorter time scales (Kaufman
et al., 2004; Vacco et al., 2005). Here we compare the postglacial
vegetation and fire history in the northern Siskiyou Mountains from
two sites located along a moisture gradient to further describe the
nature of environmental change over the last 14,000 years following
deglaciation of the region. The two sites are within 20 km of each
other on similar substrates, but the site closest to the coast, Sanger
Lake, is slightly wetter than the more inland site, Bolan Lake, and has a
well established population of endemic conifers including Picea
breweriana (Brewer's spruce) and Chamaecyparis lawsoniana (Port-vegetation and fire across this complex lan
to be addressed (West, 1989; Mohr et al., 2
Daniels et al., 2005).
The Siskiyou Mountains have tradition
regionwhere the climate has remained stable
⁎ Corresponding author. Tel.: +1 406 994 6856; fax: +
E-mail addresses: cbriles@montana.edu, christybrile
0031-0182/$ – see front matter © 2008 Elsevier B.V. Al
doi:10.1016/j.palaeo.2008.05.007change in mountainous
d (Shafer et al., 2005).
became cooler and drier elsewhere in the Pacific Northwest (Whit-
taker, 1960). This notion of stability is contradicted by evidence from
the Quaternary which suggests that the region experienced significantThe environmental response to climate
regions is complex and not well-understoo1. IntroductionSynchronya moisture gradient to reconstruct the vegetation, fire and climate history. The late-glacial period was
characterized by subalpine parkland and infrequent fire at both sites. During the late-glacial/Early Holocene
transition period, subalpine parkland was replaced by a closed forest of Pinus, Cupressaceae, Abies and
Pseudotsuga and more frequent fires a 1000 years earlier at the wetter site, and it is likely that reduced
Pacific Ocean upwelling created warmer drier conditions at the coast. In the Early Holocene, Pinus,
Cupressaceae were less abundant and fire less frequent at the coastal site during a period of increased coastal
upwelling and fog production. In the Late Holocene, Abies, Pseudotsuga, Pinus, and Quercus vaccinifolia
increased in the forest at both sites suggesting a widespread response to cooling. Fewer fires at the wetter
site may account for the abundance of Picea breweriana within the last 1000 years. The comparison of the
two records implies that large-scale controls in climate during the last 14,000 cal yr BP have resulted in major
changes in vegetation and fire regime. Asynchrony in the ecosystem response of wetter and drier sites arises
from small-scale spatial variations in effective moisture and temperature resulting from topographically-
influenced microclimates and coastal-to-inland climate gradients.
© 2008 Elsevier B.V. All rights reserved.
thereby providing a refuge for temperate forest taxa as conditionsKeywords:regime of the Siskiyou forests, pollen, charcoal, and lithological evidence was examined from two lakes alongReceived 1 December 2007
Received in revised form 3 May 2008
high diversity of conifers th
historical factors. To evaluate how past climate variability has influenced the composition, structure and fireRegional and local controls on postglacial
Siskiyou Mountains, northern California,
Christy E. Briles a,⁎, Cathy Whitlock b, Patrick J. Bartle
a Department of Geography, University of Oregon, Eugene, OR 97403-1251 USA
b Department of Earth Sciences, Montana State University, Bozeman MT 59717 USA
A B S T R A C TA R T I C L E I N F O
Article history: The Siskiyou Mountains of n
j ourna l homepage: www.dscape has only begun
000; Briles et al., 2005;
ally been considered a
during the Cenozoic Era,
1 406 994 2067.
s@gmail.com (C.E. Briles).
l rights reserved.egetation and fire in the
A
a, Philip Higuera b
hwestern California and southwestern Oregon are a floristic hotspot, and the
likely results from a combination of geological, ecological, climatological and
matology, Palaeoecology
ev ie r.com/ locate /pa laeoOrford cedar) unlike themore inland site (see site descriptions below).
One objective of this study was to determine whether the present
vegetation differences arise from distinct vegetation, fire and climate
histories or merely reflect local site differences. Another objective was
to determine the geographic extent of early-Holocene trade-offs
between haploxylon Pinus (likely Pinus monticola and Cupressaceae
(likely Calocedrus decurrens)), that were associated with changes in
fire frequency recorded at Bolan Lake (Briles et al., 2005).

2. Study area
2.1. Study sites
Sanger Lake (Lat. 41°54′06″N, Long.123°38′49″ W,1550m elevation,
4 ha) is located in a late-Pleistocene cirque (Fig. 1). The bedrock around
the lake is diorite, and upslope and downslope of Sanger Lake are
serpentine deposits. The lake has no permanent inflowing streams and
is mainly fed by groundwater. Sanger Lake lies in the Abies concolor
(white fir) Vegetation Zone (1300–1900 m elevation) (Franklin and
Dryness, 1988) (all botanical nomenclature is based on Hickman, 1993).
The dominant conifers around the lake include A. concolor, Picea
breweriana, Chamaecyparis lawsoniana, Pinus monticola (western white
pine), and Pseudotsuga menziesii. C. lawsoniana is restricted to and
dominates mesic serpentine slopes within the basin. The dominant
shrubs around the lake include Quercus vaccinifolia (huckleberry oak),
Chrysolepis chrysophylla (golden chinquapin), Lithocarpus densiflora
(tanoak), and Ceanothus velutinus (snowbrush).
The modern climate (based on elevation adjusted interpolated
station data; Bartlein and Shafer, unpublished data, 2007) at Sanger
Lake is characterized bycoolwinters (1 °Cmean temperature) andmild
summers (12.7 °C mean temperature). The annual precipitation is
about 1300 mm with 48% received in the winter and 15% in summer.
Sanger Lake is slightlymore humid in the summer,with lower absolute
maximum temperatures, and slightly warmer wetter winters than at
Bolan Lake.
Young Pseudotsuga (b20 years old) on the south side of the lake
have charred bark and Chamaecyparis lawsoniana on the southeast
slope of the lake have fire scars, both attesting to recent fire in the
watershed. The north slope of the lake supports a young cohort of
trees b100 year old, whereas the east and south slopes have trees
N300 years old. Fire return intervals (FRI) for Abies concolor forests in
the region range between 25 and 64 years (Agee, 1993; Skinner et al.,
2006), but the Sanger Lake forests likely have a longer FRI as a result of
wetter cooler conditions.
Bolan Lake (Lat. 42.023, Long. 123.458, 1638 m elevation, 5 ha) is
located in southwestern Oregon on the California/Oregon border
about 20 km northeast of Sanger Lake (Fig. 1). Like Bolan Lake, it also
occupies a late-Pleistocene cirque basin underlain by diorite and
serpentine and has no inflowing streams. The forest at Bolan Lake is
more open than at Sanger Lake and is dominated by Abies concolor,
Pinus monticola and Pseudotsuga menziesii. Picea breweriana and
Chamaecyparis lawsoniana do not grow at the site today. The warmer,
drier setting, with 190 mm less annual precipitation (mostly in winter
as snow), cooler winters (−0.5 °C mean temperature), more snow, and
warmer summers (13 °C mean temperature) at Bolan Lake than at
Sanger Lake (Bartlein and Shafer, unpublished data), is likely due to its
more inland location.
160 C.E. Briles et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 265 (2008) 159–169Fig. 1. Map showing location of Sanger and Bolan lakes. Aerial perspective is from late winter for Bolan Lake and summer for Sanger Lake (from Google Earth).