A first glance into the Clearwater Refugium of northern Idaho:
a preliminary pollen record from Dismal Lake
Erin M. Herring and Daniel G. Gavin
Department of Geography, University of Oregon, Eugene, Oregon 97403
Background
Results and Discussion
5
Several recent studies have shown that during past glaciations
species distributions were not uniformly shifted to the south, but
that several northern “cryptic” refugia for warm-adapted species
occurred proximal to the ice sheets1. In the Clearwater drainage
of northern Idaho, modern distributions (including several
codistributed endemics) and genetic studies of several
herbaceous plants and amphibians support the existence of
refugia for mesic-adapted species (Figure 1)2,3,4. The Clearwater
is unique, however, because most mesic-adapted species in this
region are disjunct from their main coastal distribution, and
therefore alternative hypotheses for this disjunction involve
persistence in refugia or long-distance dispersal from the coast5.
No paleoecological studies exist in the unglaciated Clearwater
Refugium from which to assess regional vegetation changes.
Dismal Lake is a small (2.9 ha) deep (20.9 m) cirque basin
located at 1630 m elevation in Tsuga mertensiana (mountain
hemlock) forest (Figures 2 and 3). Dismal Lake is 120 km south
of the maximum extent of the Cordilleran ice-sheet (Figure 1).
a
10
15
Initial pollen analysis on the ca. 14,500-year-long record
indicates that the area around Dismal Lake underwent several
vegetation shifts since the lake was established (Figure 5).
During deglaciation, the vegetation in the region consisted of
Pinus spp., Picea spp., and Artemisia. In the early Holocene,
the percentage of Pinus decreased, while the amount of
Pseudotsuga/Larix pollen type was over 30%. After the
eruption of Mt. Mazama (7,627 cal year BP) the forest
underwent primary succession, during which Alnus and
members of the Asteraceae became established in the area.
Most of the current taxa within the Dismal Lake region became
established 5,500 cal year BP.
20
200 m
Figure 3. (a) Detailed view of Dismal Lake showing the lake’s catch basin.
(b) Bathymetric map showing the location of the core taken from Dismal
Lake. The star indicates the location the sediment core was taken.
Methods
In August 2008, a 10-m sediment core was extracted from the
center of the lake using a Livingstone piston corer and a surface
corer. A bathymetric map was constructed using a GPS unit and
a depth finder (Figure 3b). The core is being analyzed for pollen,
loss-on-ignition (LOI), and at contiguous 1-m intervals, magnetic
susceptibility and macroscopic charcoal. The chronology is
based on four AMS radiocarbon dates, three tephras, and a shift
in LOI (Table 1, Figure 4). For the top 150 cm of the core, pollen
samples were taken every 10 cm in order to detect when Tsuga
mertensiana first appears in the record.
Legend
Dismal Lake
Figure 5. Preliminary pollen summary diagram. The black line on Tsuga
heterophylla and Tsuga mertensiana represent a 10x exaggeration.
Examination of the top 150 cm of the core shows that the
vegetation has been relatively stable since 1,650 cal years BP
(Figure 6).
Tsuga mertensiana appears to be a recent
component of the forest, arriving only about 650 cal years BP in
northern Idaho. Around 200 cal years BP there is an increase in
charcoal and disturbance taxa (Alnus and Poaceae) and a
subsequent decrease in Abies spp. Increased fire at this time,
preceding the major fires of 1910, may be implicated in the
recent vegetation change, though additional age control (210Pb
and 14C) is needed.
Clearwater Refugium
Terrain
Ice sheets
Proglacial Lakes
Modern distribution of
Tsuga mertensiana
b
c
Figure 1. (a) The ice sheet extent and proglacial lakes 16ka along with the
location of Dismal Lake, the Clearwarer Refugium and the modern
distribution of Tsuga mertensiana. (b) Patterns of species richness of nine
tree and (c) 58 understory vascular plant species that occur along the coast
and in the northern Rocky Mountains5.
Figure 2. Tsuga mertensiana forest surrounding Dismal Lake.
Depth (cm)
0
7
142
331
504
593
839
881
908
1000
Median Age
(Cal. Years BP)
-58
-30
1571
3577
6421
7627
10034
11600
13550
14500
Age Basis
Top (collected 2008)
Mt. St. Helens tephra (1980)
14C
14C
14C
Mt. Mazama tephra
14C
Holocene (LOI increase)
Glacier Peak tephra
Glacial retreat (core bottom)6
Table 1. Age basis of Dismal Lake chronology
Conclusions
•
Tsuga mertensiana increased and expanded its distribution
quite recently, suggesting these forests are of “Little Ice Age”
origin.
•
Arrival of T. mertensiana is more recent than the only other
paleoecological record in inland T. mertensiana forest (ca.
1000 years ago in British Columbia)7.
•
The timing of its increase in the Dismal Lake core is
inconsistent with T. mertensiana existing within the
Clearwater Refugium.
•
During the late glacial period, our preliminary pollen
evidence suggests a sparse open early-successional cold
dry forest consisting of Pinus contorta (lodgepole pine) and
Artemisia and very little fire.
Work Cited
Figure 4. Lithology and age-depth model, with 2-sigma error bars.
1. Stewart, J. R., and A. M. Lister. 2001. Cryptic northern refugia and the origins of the
modern biota. Trends in Ecology & Evolution 16:608-613. .
2. Brunsfeld, S. J., and J. Sullivan. 2005. A multi-compartmented glacial refugium in the
northern Rocky Mountains: Evidence from the phylogeography of Cardamine constancei
(Brassicaceae). Conservation Genetics 6:895-904.
3. Nielson, M., K. Lohman, C. H. Daugherty, F. W. Allendorf, K. L. Knudsen, and J. Sullivan.
2006. Allozyme and mitochondrial DNA variation in the tailed frog (Anura: Ascaphus): The
influence of geography and gene flow. Herpetologica 62:235-258.
4. Brunsfeld, S. J., T. R. Miller, and B. C. Cartstens. 2007. Insights into the biogeography of
the Pacific Northwest of North America: evidence from the phylogeography of Salix
melanopsis. Systematic Botany 32:129-139.
5. Gavin, D. G. 2009. The coastal-disjunct flora in the inland Pacific Northwest of USA and
Canada: refugia, dispersal and disequilibrium. Diversity and Distributions 15:972-982.
6. Thackray, G.D., K.A. Lundeen, and J.A. Borgert. 2004. Latest Pleistocene alpine glacier
advances in the Sawtooth Mountains, Idaho, USA: Reflections of midlatitude moisture
transport at the close of the last glaciation: Geology 32:225-228.
7. Rosenberg, S. M., I. R. Walker, and R. W. Mathewes. 2003. Postglacial spread of hemlock
(Tsuga) and vegetation history in Mount Revelstoke National Park, British Columbia,
Canada. Canadian Journal of Botany 81:139–151.
Figure 6. Pollen summary diagram for the top 150 cm of the core. The black
line on Tsuga heterophylla, Tusga mertensiana, Betula, Chenopodaceae and
MS represent a 10x exaggeration.
Future Work
The overall goals of this research is to see how species
distributions shifted over the last 14,500 years in the mesic
forests of northern Idaho, characterize the Clearwater Refugium
and reconcile conflicting lines of evidence from genetics, modern
distributions, paleoclimate, and pollen data. Planned analyses
include 1) higher resolution pollen analyses (especially during
the Younger Dryas), 2) completion of the charcoal stratigraphy,
3) biogenic silica as a potential proxy of summer temperature.
During summer 2010 at least two more lake cores will be
collected from high and low elevation sites in the Clearwater
National Forest, including one near the area where genetic
evidence suggests the presence of a refugium2. These cores
will be analyzed in a similar fashion to Dismal Lake (pollen
analysis, magnetic susceptibility, loss-on-ignition, charcoal, and
AMS radiocarbon dates).