Holocene vegetation, fire and climate history of the Sawtooth Range, central Idaho, USA

Quaternary Research 75 (2011) 114–124

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Quaternary Research j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / y q r e s

Holocene vegetation, fire and climate history of the Sawtooth Range, central Idaho, USA Cathy Whitlock a,⁎, Christy E. Briles b, Matias C. Fernandez c, Joshua Gage a,1 a b c

Department of Earth Sciences, Montana State University, Bozeman, MT 59715, USA Department of Anthropology, Texas A&M University, College Station, TX 77843, USA Ecology, Evolution, and Environmental Biology Department, Columbia University, New York, NY 10027, USA

a r t i c l e

i n f o

Article history: Received 11 January 2010 Keywords: Holocene Fire history Vegetation history Climate history Charcoal Pollen Sawtooth Range Central Idaho Western US

a b s t r a c t The paucity of low- and middle-elevation paleoecologic records in the Northern Rocky Mountains limits our ability to assess current environmental change in light of past conditions. A 10,500-yr-long vegetation, fire and climate history from Lower Decker Lake in the Sawtooth Range provides information from a new region. Initial forests dominated by pine and Douglas-fir were replaced by open Douglas-fir forest at 8420 cal yr BP, marking the onset of warmer conditions than present. Presence of closed Douglas-fir forest between 6000 and 2650 cal yr BP suggests heightened summer drought in the middle Holocene. Closed lodgepole pine forest developed at 2650 cal yr BP and fires became more frequent after 1450 cal yr BP. This shift from Douglas-fir to lodgepole pine forest was probably facilitated by a combination of cooler summers, cold winters, and more severe fires than before. Five drought episodes, including those at 8200 cal yr BP and during the Medieval Climate Anomaly, were registered by brief intervals of lodgepole pine decline, an increase in fire activity, and mistletoe infestation. The importance of a Holocene perspective when assessing the historical range of variability is illustrated by the striking difference between the modern forest and that which existed 3000 yr ago. © 2010 University of Washington. Published by Elsevier Inc. All rights reserved.

Introduction Recent drought conditions in the western US have resulted in low lake levels and stream flow (Barnett et al., 2008), increased fire activity (Westerling et al., 2006), and increased insect infestations and tree mortality (Logan and Powell, 2001; van Mantgem et al., 2009). Drought has generated conflict, legal suits, and costly water projects to alleviate water shortages. In addition, the combination of tree mortality and increased fire hazard has become an issue of concern for forest management planning, local economies, and landowners in the region. Questions remain about the precedence of the current drought as well as its long-term consequences on the region's natural resources. The answers require information on past climate variability and ecosystem response. We report here the first information about the long-term vegetation, fire and climate history at middle elevations in the Sawtooth Range of central Idaho and compare these findings with reconstructions from other low- and middle-elevation sites in the northern Rocky Mountains to provide a regional paleoecological synthesis. These data help describe the historical range of variability in past vegetation and fire regimes that, in turn, provides a better understanding of the natural resilience of lowelevation forests to climate change. The study was conducted on a small lake (named here Lower Decker Lake; 44°04.161N, 114°53.359W, 2167 m ⁎ Corresponding author. E-mail address: [email protected] (C. Whitlock). 1 Current address: American Wildlands, P.O. 6669, Bozeman, MT 59715, USA.

elev) in the Sawtooth National Forest in Custer County. Lower Decker Lake lies in a small closed basin on one of the impressive late-Pinedale moraines that extend from the Sawtooth Range (Fig. 1; Thackray et al., 2004; Breckenridge et al., 1988 after Williams, 1961). The lake is currently located in homogeneous lodgepole pine (Pinus contorta) forest, in which Douglas-fir (Pseudotsuga menziesii) and limber pine (Pinus flexilis) are present but relatively rare. Ponderosa pine (Pinus ponderosa), a common species in low-elevation forests throughout the Rocky Mountains, is absent from southeastern Idaho, including from the east side of Sawtooth Range. Below ~1800 m elevation, lodgepole pine forest sharply gives way to sagebrush (Artemisia tridentata) steppe with Idaho fescue (Festuca idahoensi), bluebunch wheatgrass (Agropyron spicatum), and perennial herbs. Steppe vegetation is prevalent on glacial outwash deposits within the Sawtooth Valley of the upper Salmon River valley on the east side of the Sawtooth Range. Valley-floor riparian communities consist of willow (Salix spp.), gray alder (Alnus incana), black cottonwood (Populus balsamifera) and quaking aspen (P. tremuloides). Aspen, alder, and willow are also present on the lower slopes in areas of moist soil. Above ~2300 m elevation, lodgepole pine forest is replaced by forests of Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa) and whitebark pine (Pinus albicaulis), but spruce and fir also extend to lower elevations in areas of cold-air drainage. Alpine tundra is present on the highest peaks above ~2900 m elev (Arno, 1979). Stanley, Idaho, 17 km north of Lower Decker Lake, is one of the coldest locations in the US. January temperatures average −3.4°C, with average minimum January temperature of −18°C. July and August

0033-5894/$ – see front matter © 2010 University of Washington. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.yqres.2010.08.013

C. Whitlock et al. / Quaternary Research 75 (2011) 114–124

a)

115

b) MT

Foy Lake

e ang hR toot Saw

WA

Pintler Lake

L. Decker Lake

Slough Creek Lake McCall Fen

L. Red Rock Lake L. Decker Lake

S. Payette River LO1

Sawtooth Valley

10 km

Cygnet Lake Hedrick Pond

Blacktail Pond

c) Decker Creek riparian zone

Grays Lake

abandoned outlet

N OR ID

100 km

WY

L. Decker Lk catchment

50 m

abandoned inlet

N

Figure 1. a) Location map of sites discussed in text; b) Lower Decker Lake in the Sawtooth Range; c) landscape features around Lower Decker Lake.

Methods

Truspec C:N machine, which combusted each sample in pure oxygen and analyzed the resulting gas for its carbon and nitrogen content. Magnetic susceptibility was measured in contiguous 0.5-cm intervals in the core to assess changes in inorganic allochthonous sediment (Gedye et al., 2000). Measurements were made using a Bartington magnetic susceptibility meter, and results were reported in cgs units. Organic and carbonate contents of the lake sediments were determined from weight-loss after ignition at 550 and 900°C for 2 h (at each temperature) on samples of 1 cm3 volume taken at 1.5-cm intervals (Dean, 1974).

Field

Pollen

A 3.00-m-long sediment core was obtained with a modified Livingstone piston sampler, and a 60-cm-long short core was obtained with a Klein piston corer from Lower Decker Lake from an anchored platform in the deepest water (Wright et al., 1983). Core segments were wrapped in plastic and aluminum foil and transported to the lab where they were refrigerated. Cores were cut in half, described, and subsampled at 0.5-cm intervals in the lab.

Pollen analysis provided information on the vegetation history. Pollen samples of 0.5 cm3 volume were prepared using methods of Bennett and Willis (2002), except a Schulze procedure was substituted for acetolysis to oxidize organics (Doher, 1980). A Lycopodium tracer was added to calculate pollen concentration (grains cm−3). Pollen samples were taken at core intervals ranging from 50 to 100 yr. Pollen grains were identified at magnifications of 500× and 1250×, and N300 terrestrial grains and N100 non-Pinus grains were counted per sample. Pollen was identified to the lowest taxonomic level possible using reference collections and atlases (e.g., Kapp et al., 2000; Moore and Webb, 1978). Pinus grains were separated into Haploxylon- and Diploxylon-types, and those missing a distal membrane were identified as “Undiff. Pinus.” Vegetation was reconstructed based on changes in pollen percentages, pollen ratios, and pollen accumulation rates (PARs; grains cm−2 yr−1), as well as a comparison of fossil assemblages and modern pollen data (Baker,

temperatures reach 25.6°C on average. Mean annual precipitation averages 33.4 cm, with 80% of that received in winter and spring as snow (http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?idstan). July through October is the driest period, and summer moisture deficits make the region vulnerable to late-summer drought and fire. This area would be classified as summer-dry (sensu Whitlock and Bartlein, 1993), given that the ratio of July/annual precipitation is low (.045).

Chronology and lithology Plant macrofossils from the cores were submitted for AMS radiocarbon dating. Carbon/nitrogen (C/N) analysis provides a measure of how the local vegetation has changed over time by comparing the relative contributions of terrestrial versus algal nutrients in the lake sediments (Kaushal and Binford, 1999). Samples with a dried weight of 0.2 g were taken at 1.5-cm intervals in the cores and analyzed in a LECO

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Table 1 Uncalibrated and calibrated

14

C and

210

Pb ages for Lower Decker Lake.

Depth (m)a

Predicted age (cal yr BP; med. Prob.)b,c

0.00 0.01 0.02 0.03 0.05 0.07 0.09 0.13 0.17 0.75 0.87 1.13 1.23 1.38 1.47 1.56 1.75 1.92 2.08 2.16 2.34 2.47 2.59 2.76 2.98

−56 −50 −46 −40 −27 −10 7 50 105 1452 1869 2819 rejected 3585 3845 4132 4770 5243 5697 5985 6878 7685 8450 9404 10378

Upper age range (cal yr BP)d

1552 1973 3001 rejected 3767 3997 4267 4920 5404 5820 6089 6965 7767 8520 9467 10543

Lower age range (cal yr BP)d

1379 1778 2662 rejected 3451 3733 4031 4648 5097 5557 5869 6802 7618 8374 9304 10290

14 C age (14C yr BP)

1520 1922 2860 881 3660 3380 3716 4437 4599 4489 5071 5966 6845 7762 8477 9139

±

Material dated

Referencee 210

36 43 120 32 170 60 39 37 69 66 58 40 50 44 54 48

gyttja gyttja gyttja gyttja gyttja gyttja gyttja gyttja gyttja seed wood seed pine needle pine needle Mt. St. Helens Y Ashf gyttja wood pine needle pine needle cone parts pine needle Mazama Ashf pine needle charcoal wood

Pb Pb 210 Pb 210 Pb 210 Pb 210 Pb 210 Pb 210 Pb 210 Pb AA70039 AA70031 AA70038 AA70032 AA70040 Sarna-Wojcicki et al., 1983 AA80797 AA70033 AA70041 AA70034 AA80796 AA70035 Bacon, 1983 AA70036 AA80798 AA70037 210

a

Depth below mud surface. C calibrated ages derived using a Monte Carlo approach based on the probability distribution function of all 14C ages (two sigma error) in the age–depth model (see Methods; Calib 5.0.2; Stuiver et al., 2005). c 210 Pb dates were adjusted for the 56 yr (the core was taken in 2006) since 1950 AD before being considered with the radiocarbon dates. d 99% confidence interval based on 2000 runs. e 210 Pb ages from Flett Research Ltd. of Winnipeg, Manitoba; 14C age determinations from NSF-University of Arizona AMS Radiocarbon Lab. f Tephra identifications provided by Steven Kuehn, Concord University, using microprobe analysis. b 14

1976; Whitlock, 1993; Minckley et al., 2008). Percentages of terrestrial upland taxa were based on a sum of pollen from all trees, shrubs, herbs, and pteridophytes. Percentage data were divided into zones by use of a constrained cluster analysis (CONISS; Grimm, 1988). The ratio of Diploxylon-type Pinus (inferred to be lodgepole pine) to Pseudotsugatype pollen (inferred to be Douglas-fir) was also calculated (Lp/Df ratio). PARs were determined by dividing pollen concentrations by deposition times (yr cm−1).

Results Chronology and lithology Long and short cores were correlated based on lithology and charcoal stratigraphy to create a single age–depth model (Fig. 2). The chronology was based on thirteen 14C AMS dates from the long core, two known tephra ages, and eight 210Pb age determinations from the

Charcoal Charcoal particles were extracted from 1 cm3 samples at contiguous 0.5-cm intervals with standard sieving methods (Whitlock and Larsen, 2002). Our analysis focused on large particles (N125 μm in diameter), which provide a record of high-severity (i.e., standreplacing) fires occurring within a few kilometers of the study site (Higuera et al., in press). Conversion of charcoal concentration data to charcoal accumulation rates (CHAR, particles cm−2 yr−1) employed CharAnalysis software (Higuera et al., 2008), which separates the long-term trends (i.e., background CHAR or BCHAR) from positive deviations (i.e., charcoal peaks) in the CHAR time series. Charcoal peaks represent fire episodes (i.e., one or more fires occurring during the time span of the peak), and the time span between peaks is the fire-episode return interval (FeRI). Charcoal concentrations and deposition times were binned into 17-yr intervals, the median sampling resolution, before converting the data to CHAR. A locally weighted 1000-yr mean, robust to outliers, was used to define background CHAR. Peaks represent the positive residuals after the background component is removed. FeRIs were summarized by smoothing the time intervals with a tricubic locally weighted regression (1000-yr window) to display long-term trends in fire activity.

Figure 2. Age-versus-depth curves and deposition times based on radiocarbon and 210Pb age determinations and tephrochronology. Gray shading represents range of dates and deposition times and black lines indicate the 50th (i.e., median age) percentile of all runs. The 50th (circle), 2.5th and 97.5th (bars) percentiles of the probability distribution function of calibrated dates are shown. See Table 1 for age determinations.

C. Whitlock et al. / Quaternary Research 75 (2011) 114–124

short core (Table 1). Mount St. Helens Y and Mazama ash layers were identified from microprobe analyses at Washington State University and assigned ages of 3380 ± 60 14C BP and 6845 ± 50 14C BP (SarnaWojcicki et al., 1983; Bacon, 1983). An age determination at 123 cm depth was out of chronological order and left out of the age model. Age–depth relations were developed from calibrated radiocarbon ages (Stuiver et al., 2005; Reimer et al., 2004) and modeling techniques, using MCAgeDepth software (Higuera et al., 2008). Age–depth models were constructed with a cubic smoothing spline and a Monte Carlo approach that allowed each date to influence the age model through the probability density function of the calibrated age (two sigma error; Stuiver et al., 2005; Higuera et al., 2008). The final age–depth model and dating error were based on 2000 iterations. The chronology suggests that Lower Decker Lake was formed at ca. 10,500 cal yr BP. The sediment deposition time ranged between 25 and 34 yr cm−1 between 10,500 and 6350 cal yr BP and 15–20 yr cm−1 thereafter (Fig. 2). Core lithology consisted of a basal unit (3.05–2.57 m depth; 10,500–8330 cal yr BP) of clay and finely laminated silt layers (Fig. 3). From 2.57 to 2.00 m depth (8330–5460 cal yr BP), sediment was composed of organic clay, and this unit was overlain by inorganic clay from 2.00 to 1.74 m depth (5460–4740 cal yr BP). The uppermost 1.74 m consisted of fine-detritus gyttja. Organic content was low (b10%) below 2.57 m depth and between 2.00 and 1.74 m depth, and it increased above 1.74 m depth reaching 53%. In general, carbonate content was low (b7%) and varied little. Magnetic susceptibility values were high (between 3 and 5 × 10−6 cgs) below 2.21 m depth (10,500–6200 cal yr BP), suggesting more allochthonous input in the early period, and decreased (~ 1 × 10−6 cgs) above 2.21 m depth. High magnetic susceptibility was also registered for the two volcanic ashes and a laminated silt layer between 2.76 and 2.69 m depth (9400– 9040 cal yr BP). Carbon/nitrogen (C/N) ratios from 2.47 to 2.00 m depth (7685– 2352 cal yr BP) ranged between 9.7 and 11.8, with a low value of 8.6 at 2.21 m depth (6200 cal yr BP). Between 2.00 and 1.00 m depth (5457– 2352 cal yr BP), C/N ratios were slightly higher on average and ranged between 10.5 and 13.2, with peak ratios occurring at 1.94, 1.54, and 1.07 m depth (5295, 4065, and 2609 cal yr BP, respectively). Ratios were slightly lower, between 10.0 and 11.8, between 1.00 and 0.40 m depth (2352–543 cal yr BP) and dropped to 9.5 above 0.40 m depth. Pollen record The pollen record was divided into four pollen zones (Fig. 4). Zone DEC-1 (3.00 to 2.55 m depth; ca. 10,500 to 8420 cal yr BP) featured high percentages of Pinus (40–75%), moderate percentages of Chenopodiaceae (formerly Chenopodiaceae-Amaranthaceae; http:// www.efloras.org/), Asteraceae and Rosaceae (2–9% each) and low to moderate percentages of Artemisia (8–25%), Pseudotsuga-type (5– 15%), Salix (b2%) and Alnus (b3%). PARs were moderate (~1000–4500 grains cm−2 yr−1) until 6250 cal yr BP. The Lp/Df ratio was moderate (~15). Zone DEC-2 (2.55–2.16 m depth; 8420–5990 cal yr BP) was characterized by increased percentages of Artemisia (12–40%) and Pseudotsuga-type (6–35%) pollen and decreased percentages of Diploxylon-type Pinus (18–65%) and Chenopodiaceae (2–5%). The lowest percentages of Diploxylon-type Pinus and highest percentages of Artemisia and Pseudotsuga-type of the record occurred between 8420 and 7840 cal yr BP. Salix percentages remained unchanged from the previous zone. Arceuthobium (dwarf mistletoe) percentages were relatively high (3–6%) between 7270 and 6130 cal yr BP. PARs were low in this zone (650–2000 grains cm−2 yr−1), and the Lp/Df ratio was low (~ 10). Zone DEC-3 (2.16–1.08 m depth; 5990–2650 cal yr BP) had increased percentages of Diploxylon-type Pinus (40–75%) and decreased percentages of Artemisia (15–25%) and Pseudotsuga-type (6–17%). Haploxylontype Pinus percentages increased slightly from before but remained low

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(b2%). Percentages of Chenopodiaceae and Asteraceae (b3% each) were the lowest of the record. Picea and Abies became abundant for the first time (up to 4% each). A significant drop in Diploxylon-type Pinus (from 60% to 21%) occurred between 3580 and 2920 cal yr BP and was associated with increased percentages of Pseudotsuga-type (~10%), Rosaceae (~3%) and Chenopodiaceae (~6%). In addition, the highest percentages of Arceuthobium occurred between 3350 and 2540 cal yr BP (up to 7%). Salix pollen was nearly absent. PARs were the highest of the record (3000–6000 grains cm−2 yr−1), but the Lp/Df ratio remained low (~10). Zone DEC-4 (1.08–0 m depth; 2650 cal yr BP–present) had the highest percentages of Pinus (50–80%) and lowest percentages of Pseudotsuga-type pollen (1–6%) of the record. Artemisia (10–26%) and Chenopodiaceae (3–6%) percentages increased slightly from the previous zone. PARs decreased to moderate levels (1200 grains cm−2 yr−1), with a single spike of high PARs (16,500 grains cm−2 yr−1) in the 20th century, mostly from Pinus PARs. The Lp/Df ratio was high, reaching 47 at 1010 cal yr BP. Charcoal Background CHAR reflects levels of arboreal fuel biomass, which is related to the amount of forest cover as well as the size and severity of the fires that produce charcoal (Marlon et al., 2006). At Lower Decker Lake, values fluctuated between 0.0003 and 0.0048 particles cm−2 yr−1 since 10,500 cal yr BP. Background CHAR was low (from 0.003 to 0.001 particles cm−2 yr−1) between 10,500 and 5990 cal yr BP, increased slightly (from 0.001 to 0.0024 particles cm−2 yr−1) between 5990 and 3420 cal yr BP, and declined to low levels (b0.001 particles cm−2 yr−1) between 3420 and 1450 cal yr BP. Values increased significantly after 1450 cal yr BP to 0.0048 particles cm−2 yr−1 at present. Fire-episode return intervals (FeRIs) refer to the number of years between fire episodes, and the charcoal peaks themselves tend to record high-severity forest fires that produce greater amounts of charcoal than low-severity surface fires (e.g., Whitlock et al., 2004). FeRIs fluctuated between 100 and 700 yr, and charcoal peaks ranged between 0.05 and 0.25 pieces cm−2 peak−1, with the exception of large peaks at ca. 9770 and ca. 2450 cal yr BP. The lowest fire activity (FeRIs of 400–600 yr) occurred between 10,500 and 5550 cal yr BP. FeRIs were moderate (200–300 yr) between 5550 and 3380 cal yr BP. Fire activity decreased (FeRIs of 300–400 yr) between 3380 and 1450 cal yr BP and increased after 1450 cal yr BP (FeRIs b100 yr). Discussion Lacustrine, vegetation, and fire history of Lower Decker Lake Lower Decker Lake is surprisingly young (~ 10,500 yr old), considering its location on a prominent late-Pinedale moraine complex where local deglaciation occurred soon after 14,000 cal yr BP (Thackray et al., 2004). The depression containing the lake probably originated as a buried ice block, which melted and created an early channel of Decker Creek. The creek later shifted its course northwards, and the lake formed in the abandoned depression at 10,500 cal yr BP (Fig. 1). A small delta fan on the western side of the basin suggests several flood events early in the lake's history. The complex lithostratigraphy at the base of the core is explained by its geomorphic history. The basal inorganic clay and finely laminated silt layers imply intermittent ponding during the abandonment of the early Decker Creek channel. After 8330 cal yr BP, the lake was stable and modestly productive, as evidenced by the deposition of organic clay. A period of flooding may have occurred between 5460 and 4740 cal yr BP, explaining deposition of inorganic clay. With final closure of the basin at 4740 cal yr BP, the organic, carbonate, and nitrogen content of sediments increased, indicating that the lake became more productive. C/N ratios (ranging between 9 and 13) measure changes in the source of

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Figure 3. Lithologic, geochemical, and charcoal concentration data for Lower Decker Lake.

organic matter, with algae having a C/N ratio between 4 and 10, and terrestrial organic matter having a C/N N 20 (Kaushal and Binford, 1999). Lower Decker Lake shows dominantly an algal contribution and little fluctuation in lake level to affect the source of the organic component. Low magnetic susceptibility after 6000 cal yr BP (with the exception of the Mount St. Helens Y ash) also suggests stabilized vegetated slopes. Water level is currently at a low stage, as evidenced by the exposed margin on the east side that is colonized by young conifers. This low water stand appears to be a fairly recent. Four assumptions underlie the vegetation reconstruction. First, most of the unidentified Pinus pollen is ascribed to Diploxylon-type, which far surpasses the abundance of identifiable Hapoxylon-type Pinus pollen. Of the Diploxylon types, lodgepole pine is considered the major contributor, based on its dominance in the lower forest today and the fact that ponderosa pine does not grow on the east side of the Sawtooth Range. Ponderosa pine may have expanded its range during past warm periods, although macrofossil evidence elsewhere does not support such an expansion (Pierce et al., 2004). Haploxylon-type Pinus is attributed to whitebark pine, which is found at high elevations in the Sawtooth Range at present, or possibly limber pine, which also grows in the region. Second, Pseudotsuga-type pollen is attributed to Douglas-fir, as opposed to western larch (Larix occidentalis), which is absent from the Sawtooth region but is found in northern Idaho. Douglas-fir does not produce much pollen and its pollen is not dispersed far from the tree

(Sugita, 1994). Modern pollen data from the Rocky Mountains show that percentages of Pseudotsuga-type pollen N5% occur when Douglasfir is locally abundant (Baker, 1976; Whitlock, 1993). The opposite is true of lodgepole pine, which is among the most prolific pollen producers in the region and broadcasts its pollen widely (Minckley et al., 2008). A third assumption is that substrate differences have always influenced the boundary between forest and steppe. Well-drained outwash gravels underlying the Sawtooth Valley favor sagebrush steppe over forest, and it is unlikely that conifer forest grew on the valley floor in the past. Fourth, the relative abundance of lodgepole pine and Douglas-fir in the past, inferred from the Lp/Df ratio, is ascribed to shifts in climate and disturbance regime (Steele et al., 1981). Lodgepole pine tolerates low temperatures better than Douglas-fir (Thompson et al., 1999), and in this region, it is a seral species that regenerates soon after disturbance (Burns and Honkala, 1990). Stand-replacing fires and insect outbreaks help maintain lodgepole pine's dominance in even-aged stands (e.g., Romme et al., 1998). Lodgepole pine forests become less flammable as the interval between fires increases, because late-successional forests lack a welldeveloped understory and ladder fuels for fire spread. Douglas-fir is moreshade tolerant than lodgepole pine, and it develops a thick fire-resistant bark as it matures. In the absence of severe disturbance, lodgepole pine may be replaced by Douglas-fir at low elevations and by subalpine fir and Engelmann spruce at higher elevations (Fischer and Clayton, 1983).

C. Whitlock et al. / Quaternary Research 75 (2011) 114–124 Figure 4. Charcoal and pollen data for selected taxa from Lower Decker Lake. Also shown is the 2000 PDSI reconstruction for central Idaho (Site 69, Cook et al., 2004). PDSI values range between −8 and + 6 and have been clipped in the figure to emphasize the variations in the moving average (black line) (http://www.ncdc.gov/paleo/newpdsi.html). The open black rectangle in the PDSI indicates a time of high fire activity, identified by Pierce et al. (2004) from west-central Idaho, which is part of a dry interval during the Medieval Climate Anomaly.

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Therefore, we assume that past periods of Douglas-fir dominance would be associated with relatively long FeRIs during times of high-severity fires, creating a relatively closed forest, or with short FeRIs during times of lowseverity surface fires, creating an open forest. Intervals of lodgepole pine dominance would occur with frequent high-severity fires and other types of disturbance (Steele et al., 1981). In the early Holocene (10,500–8420 cal yr BP), the lower slopes of the Sawtooth Range supported a forest of lodgepole pine and Douglasfir, and the frequency of high-severity fires was initially low but steadily increased (decreasing FeRIs). The watershed was unstable at this time, reflecting frequent flooding of the basin, and fire and flooding may have contributed to the abundance of seral lodgepole pine. Between ca. 8420 and 5990 cal yr BP, pollen data suggest an open forest dominated by Douglas-fir with less lodgepole pine. Persistent Rosaceae, Chenopodiaceae and abundant Artemisia pollen imply forest openings as well as sagebrush steppe on the valley floor. FeRIs were generally long, ranging from 400 to 600 yr, with the longest intervals at ca. 7400–7000 and 6300–5700 cal yr BP. Infrequent fire episodes would have favored Douglas-fir over lodgepole pine and promoted development of a shrub understory that included rosaceous taxa (e.g., Amelanchier, Prunus, Potentilla spp.). The shortest FeRIs occurred between 8950 and 8040 cal yr BP, with fire episodes at ca. 8840, 8720, 8481, and 8040 cal yr BP. Highelevation conifers (spruce, fir, whitebark pine) are poorly represented in the pollen record before 5990 cal yr BP, perhaps because upper treeline was higher than at present. Beavers were plentiful in the Sawtooth Valley at the time of European Contact (Miller, 1965), and their activities probably contributed to well-developed riparian communities of Salix and Alnus in the early Holocene. One can envision a landscape with strong environmental gradients in which steppe and shrubs grew in the lowland, open Douglas-fir and shrubs were present on dry south-facing exposures, lodgepole pine forest occupied north-facing slopes, and mesophytic subalpine conifers were confined to high elevations. This vegetation persisted through the middle Holocene (ca. 5990 to 2650 cal yr BP), with abundant Douglas-fir, as shown by low Lp/Df ratios. Increased PARs and CHAR at this time suggest more vegetation cover and fuel biomass, implying effectively wetter conditions than before. Low values of Rosaceae and Chenopodiaceae and decreased Artemisia percentages suggest a more closed forest. The relatively low representation of lodgepole pine and sagebrush, both prolific pollen producers, may account for the increased percentages of Picea and Abies pollen, which would have been restricted to higher elevations and cool mountain valleys. Salix and Alnus pollen were nearly absent at this time, implying restricted riparian cover. In the middle Holocene, FeRIs were generally shorter than before, ranging between 200 and 375 yr, and were b200 yr from 4540 to 4230 cal yr BP and 3750 to 3600 cal yr BP. The low magnitude of the charcoal peaks suggests generally small fires, which would have promoted Douglas-fir. During the last 2650 yr, lodgepole pine has become the dominant conifer and the role of Douglas-fir has greatly diminished, as evidenced by the increasing Lp/Df ratios. The abundance of Artemisia, Chenopodiaceae and Salix pollen suggests well-developed steppe and riparian vegetation. A drop in Pinus abundance after 600 cal yr BP coincides with a rise in Artemisia and Chenopodiaceae indicating dry conditions or an upslope shift in steppe vegetation. The limited role of Douglas-fir after 2650 cal yr BP is consistent with cold winters and reduced growing degree days (Thompson et al., 1999). High background CHAR suggests increased fuel biomass, and low FeRIs imply frequent, probably stand-replacing fires, which would favor even-aged stands of lodgepole pine. Five intervals of low Pinus pollen percentages are evident at Lower Decker Lake: 10,500–9700 cal yr BP, 8230–7850 cal yr BP, at 6450– 6300 cal yr BP, 3580–2920 cal yr BP, and at 600–500 cal yr BP. We interpret these intervals as drought periods. Considering that

lodgepole pine is a prolific pollen producer and its pollen grains travel long distances (Minckley et al., 2008), these Pinus declines represent significant loss of pine forest in the region. High percentages of Arceuthobium and Rosaceae pollen are associated with the declines. Mistletoe infestations occur during periods of moisture stress and can result in reduced tree growth, premature tree mortality, poor seed and cone development, reduced wood quality, and increased susceptibility of the host tree to pathogen and/or insect attack (Hawksworth and Wiens, 1996). The most sustained period of Pinus pollen decline (8230–7850 cal yr BP) coincides with the well-known 8.2 ka event, a cold event in the North Atlantic (Alley et al., 1997) and also an inferred wet period at Bear Lake (Utah/Idaho) 290 km south of Decker Lake (Dean et al., 2006). The interval is preceded by fire episodes at ca. 8840, 8720, 8480, and one episode occurred at 8040 cal yr BP, implying a series of fires leading up to and following that particular Pinus decline. A concurrent interval of heightened fire activity is noted in a charcoal record from the Bitterroot Range of southwestern Montana, and the presence of mountain pine beetle remains (Dendroctonus ponderosae) in lake sediments there suggests an insect infestation (Brunelle et al., 2007). No insect remains were found in the Lower Decker Lake cores, but a combination of drought, fire, mistletoe and insect outbreaks may have led to a drastic decline in pine forest throughout the northern Rocky Mountains between 8230 and 7850 cal yr BP, including in the Sawtooth Range. This combination of disturbances is similar to the situation in the region at present. Regional comparisons with other low-elevation sites Few paleoecologic records have been described from low- and middle-elevation forests in the northern Rocky Mountains, but together with the Lower Decker Lake data, they add to our understanding of Holocene environmental changes across the region (see Fig. 1 for site locations). Sites in Figure 5 are generally arrayed from west to east along a gradient of decreasing annual precipitation and temperature (climate data: http://www.wrcc.dri.edu/CLIMATEDATA.html), and all but Slough Creek Lake and Blacktail Pond are located in summer-dry regions (discussed below). For each record, increases in pollen types of xerophytic plant taxa, insights from modern pollen-climate relations (Minckley et al., 2008), and charcoalbased reconstructions of fire were used to infer the timing of maximum warmth and aridity (gray shading, Fig. 5). McCall Fen (44°56.078′N, 116°2.501′, 1608 m elev; 133 km NW of Lower Decker Lake) in west-central Idaho provides a pollen-based vegetation history from mixed conifer forests of ponderosa pine, lodgepole pine and Douglas-fir, and nearby steppe vegetation (Doerner and Carrara, 2001). From the South Fork of the Payette River (~50 km west of Lower Decker Lake), fire-history information comes from a study of fire-triggered sedimentation events for the last 7500 cal yr BP (Pierce et al., 2004) (not shown in Fig. 5). Cores from Foy Lake (48°11.635 N, 114°37.880′W, 1218 m elev; 455 km N of Lower Decker Lake) at the forest-steppe ecotone have been analyzed for pollen, isotopes, diatoms, and lake-level fluctuations (Power, 2006; Power et al., 2006; Stone and Fritz, 2006; Stevens et al., 2006; Shuman et al., 2009). Pintler Lake (45°50.461′N, 113°26.435′W, 1924 m elev; 225 km NNE of Lower Decker Lake) in the Pintler Range is surrounded by closed lodgepole pine and fir forest, and Brunelle et al. (2005) describe both pollen and charcoal records from the site. Lower Red Rock Lake (44°37.722′N, 115°50.318′W, 2015 m elev; 245 km ENE of Lower Decker Lake) in the Centennial Valley of southwestern Montana lies in sagebrush steppe adjacent to Douglasfir and pine forest and has a pollen record spanning the last 20,000 yr (Mumma, 2009). Pollen data from Grays Lake (43°04.079′N, 111° 26.590′W, 1931 m elev; 298 km ESE of Lower Decker Lake) in southeastern Idaho extends back to full-glacial time (Beiswenger, 1991). Today, Grays Lake is surrounded by sagebrush steppe with pine

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Figure 5. Comparison of vegetation history at Lower Decker Lake with other low- and middle-elevation sites in the northern Rocky Mountains.

and juniper forest on nearby hills. Hedrick Pond (43°45.262′N, 100° 34.444′W, 2048 m elev; 346 km E of Lower Decker Lake) in Jackson Hole, Wyoming, is a pollen site located in open mixed-conifer forest (Whitlock, 1993). In northern Yellowstone National Park, Slough Creek Lake (44°55.474′N, 110°21.180′W, 1887 m elev; 349 km ENE of Lower Decker Lake) and Blacktail Pond (44°57.264′N, 111°36.242′W, 2036 m elev: 369 km ENE of Lower Decker Lake) are surrounded by steppe and open Douglas-fir forest. Cygnet Lake (44°39.813′N, 110° 36.786′W, 2531 m elev; 357 km ENE of Lower Decker Lake) in central Yellowstone National Park lies in an open meadow within lodgepole pine forest (not shown on Fig. 5). The postglacial vegetation and fire history at the three Yellowstone sites has been described (Whitlock and Bartlein, 1993; Millspaugh et al., 2000, 2004; Huerta et al., 2009). Of longstanding interest is the Hypsithermal (or Holocene climatic optimum) in the western US, which was originally defined as a timestratigraphic interval by Deevey and Flint (1957) but is now understood as a time-transgressive response to climate changes induced by the amplification of the seasonal cycle of insolation between 12,000 and 6000 cal yr BP (Bartlein et al., 1998). Higher-than-present summer insolation in the early and middle Holocene coupled with lower-thanpresent winter insolation had direct and indirect effects on climate throughout the western US. Directly, the summer insolation maximum increased summer temperatures and evapotranspiration. Indirectly, it enhanced the northeastern Pacific subtropical high-pressure system, which suppressed summer precipitation, and concurrently strengthened onshore flow of monsoonal moisture and convection from the Southwest (Bartlein et al., 1998). The indirect climate effects thus promoted effectively drier-than-present conditions in some regions, whereas others became wetter. These two precipitation regimes (socalled summer-wet and summer-dry) display considerable spatial heterogeneity in the northern Rocky Mountains at present, and the contrast between regimes was even sharper in the early and middle Holocene (Whitlock and Bartlein, 1993; Shafer et al., 2005).

Throughout the region, a shift from spruce-pine parkland to pinedominated forest occurred between 11,000 and 9500 cal yr BP, and it is likely that spruce-pine parkland also grew in the Sawtooth Range during the late-glacial period prior to the formation of Lower Decker Lake. Between 10,500 and 8420 cal yr BP, pine and Douglas-fir were present on the lower slopes of the Sawtooth Range as part of a broad expansion of temperate conifers and steppe taxa at low and middle elevations in the northern Rocky Mountains. These taxa are evidence of warmer conditions than before, consistent with rising summer insolation. The middle Holocene (broadly defined from 8000 to 3000 cal yr BP) was a transition at many sites between the warm dry conditions of the early Holocene and subsequent cool wet conditions of the late Holocene (Fig. 5). This progression in regional climate, inferred primarily from pollen data, tracks the decrease in summer insolation after 9000 cal yr BP to present values. Grays Lake, for example, shows a shift from steppe with nearby juniper woodland in the early Holocene to steppe with nearby juniper, pine, and Douglas-fir forest after 8000 cal yr BP, suggesting the onset of cooler, effectively wetter conditions in the middle Holocene. Pollen data from Lower Red Rock Lake show the establishment of steppe and pine forest at 10,500 cal yr BP, followed by an increase of Douglas-fir and pine forest on the slopes after 7100 cal yr BP. Hedrick Pond featured open forest of pine and Douglas-fir at 9500 cal yr BP and a closed mixed forest after 7000 cal yr BP. McCall Fen, Pintler Lake and Foy Lake registered open pine and/or Douglas-fir forest from 11,000 or 10,000 to 6000 cal yr BP. At McCall Fen and Pintler Lake, these communities were replaced by a closed mixed-conifer forest at 6000 cal yr BP, and at Foy Lake by a mixture of pine forest and grassland until 2400 cal yr BP and open Douglas-fir forest thereafter. Pierce et al.'s (2004) LO1 site on the western side of the Sawtooth Range also suggests that Douglas-fir was more abundant in the middle Holocene (from 7400 to 6800 cal yr BP) than at present. At Lower Decker Lake, open forest dominated by Douglas-fir developed relatively late at 8420 cal yr BP and persisted until 6000 cal yr BP. The Douglas-fir forest was more closed

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between 6000 and 2650 cal yr BP, which based on the vegetation and short FeRIs, marks the driest period. Shuman et al. (2009) suggest protracted drought at this same time, based on low water levels at Foy Lake. These records suggest that the timing of the Hypsithermal was highly variable in the northern Rocky Mountains, with some sites showing early-Holocene warm dry conditions, and others (including Lower Decker Lake) indicating that the middle Holocene was somewhat drier. At sites in extremely cold settings, like Lower Decker Lake, the effects of colder-than-present winters (driven by the winter insolation minimum in the early Holocene) may have limited available moisture, forest cover, and fires despite higher-than-present summer insolation. It is also important to note that differences among sites may reflect the combination of climatic and nonclimatic controls that govern lower treeline position in particular locations. These controls include local temperature extremes, growing-degree days, and effective moisture, as well as substrate and topography (Daubenmire, 1974). The different vegetation history at Slough Creek Lake and Blacktail Pond in northern Yellowstone National Park is attributed to its summer-wet location, which is, and was, more strongly affected by summer monsoonal circulation than the other sites. The summerwet/summer-dry hypothesis (Whitlock and Bartlein, 1993) posits more mesophytic vegetation in summer-wet regions during the early Holocene when higher-than-present summer insolation strengthened monsoonal circulation. The hypothesis further proposes that the summer-wet/summer-dry boundary in the Rocky Mountains did not shift substantially during the Holocene, because of the strong role that topography plays in guiding summer precipitation patterns. As a result, summer-wet regions became wetter, and summer-dry regions were drier in the early Holocene, with the two regimes remaining relatively close to their present locations. In northern Yellowstone National Park, the vegetation history fits summer-wet predictions of progressively drier conditions: development of closed pine-juniper forest occurred between 11,000 and 8000 cal yr BP and was followed by the appearance of Douglas-fir at ca. 8000 cal yr BP and establishment of an open Douglas-fir, juniper and pine forest after 4000 cal yr BP. An associated increase in fire frequency in the middle and late Holocene at Slough Creek Lake (although not at Blacktail Pond) also suggests that summers became progressively drier. In general, however, evidence of enhanced fire activity in the early Holocene is better registered in the monsoonal regions of the southwestern US (Anderson et al., 2008), and the signal becomes more subtle and spatially variable farther north in the summer-wet regions of the northern Rocky Mountains (Whitlock et al., 2008). Continued cooling in the late Holocene explains the decline of Douglas-fir and the development of closed lodgepole pine forest after ca. 2650 cal yr BP near Lower Decker Lake. A similar shift towards closed forest and mesophytic conifers is evident in the pollen records from most summer-dry sites (Fig. 5). The exceptions are Hedrick Pond and Lower Red Rock Lake where the forests have become more open in recent millennia. The dramatic increase in fire activity at Lower Decker Lake in the last 1450 cal yrs benefited lodgepole pine at the expense of Douglas-fir (high Lp/Df ratios). More fires may have been related to multi-decadal droughts from 2000 to 1700 cal yr BP and ca. 1200 to 700 cal yr BP (Cook et al., 2004, Grid 69; Fig. 4) inasmuch as FeRIs were shortened to 100– 200 yr at 2000 cal yr BP and dropped to b100 yr after 1450 cal yr BP. A similar increase in fire activity was noted at Foy Lake (Power et al., 2006) and at other sites in the northern Rocky Mountains (Mehringer et al., 1977; Marlon et al., 2006). Charcoal dates of ~1000 and 400 cal yr BP from fire-triggered sedimentation events in the Sawtooth Range near Lower Decker Lake also fall within drought periods (J. Pierce, unpublished data, 2009). In addition, Pierce et al. (2004) document small fire-related sedimentation events in west-central Idaho at ~30002800, 1500, 1200, and 350 cal yr BP, and a period of large fire events at ~1000-750 cal yr BP during the most severe period of aridity in the

Medieval Climate Anomaly (Cook et al., 2004) (see Fig. 4). The increase in fire activity may be related to human-set fires, drought, and disturbance interactions between fire and mountain pine beetle infestations in recent centuries (Raffa et al., 2008). In addition, Native American populations in the northern Rocky Mountains were at their highest numbers in the last two millennia, and frequent burning of valley floors may have contributed to elevated charcoal levels (Mehringer et al., 1977; Barrett and Arno, 1999). The combination of severe droughts during the Medieval Climate Anomaly and wetter conditions leading to fuel build-up during the Little Ice Age (ca. 150– 550 cal yr BP; Carrara, 1987) may have further intensified the severity of fire regardless of the ignition source. Conclusions and implications The Lower Decker Lake study contributes information on the environmental history in the northern Rocky Mountains in several ways that are relevant to our understanding of forest ecology at present. First, prior to 2650 cal yr BP, Douglas-fir was at least as abundant as pine in the Sawtooth Range, and the vegetation was more open. These early communities were likely a response to warmer drier conditions during the early- and middle-Holocene summer insolation maximum. The degree of change in forest composition and structure in the last few millennia is astonishing considering the pervasiveness of closed lodgepole pine forest at lower elevations today and the relatively limited role of Douglas-fir. Second, maximum warmth at Lower Decker Lake is registered by xerophytic vegetation and high fire frequencies in the middle Holocene (8240–2650 cal yr BP), somewhat later than at other summer-dry sites. The delayed vegetation signal may have been a consequence of extremely cold winters in the early Holocene, caused by lower-thanpresent winter insolation. In other words, severe winter conditions may have overwhelmed the benefits of warmer early-Holocene summers as a result of the summer insolation maximum. The middle Holocene featured intermediate conditions at many Rocky Mountain sites, in terms of vegetation and climate. However, at Lower Decker Lake, this was the time of warmest, effectively driest conditions, with open Douglas-fir forest and high fire activity. The predominance of lodgepole pine at Lower Decker Lake after 2650 cal yr BP suggests a shift to cooler conditions, shorter growing seasons, greater fuel loads, and more standreplacing fires. Third, dramatic declines in pine forest at ca. 10,500–9700 cal yr BP, 8230–7850 cal yr BP, 6450–6300 cal yr BP, and 3580–2920 cal yr BP are evidence of drought episodes. In two cases, the decline was associated with more fires, and in all cases the declines occurred with increases in dwarf mistletoe, suggesting moisture-stressed conifer stands. Lower Decker Lake contained no insect remains to link these events to past outbreaks of mountain pine beetle, as has been described elsewhere in the northern Rocky Mountains (Brunelle et al., 2007), but such disturbance synergisms would not be surprising. Fourth, the charcoal data suggest a fire-regime shift at about 1450 yr ago, when fire frequency increased. This shift may be explained by a combination of greater fuel buildup than before, a series of multidecadal droughts, and possibly a new ignition source (humans). The increase in lodgepole pine after 2650 cal yr BP may have facilitated a shift to frequent stand-replacing fires. Finally, paleoecological data provide unique and important insights about the sensitivity of forests to climate change that should be considered in assessments of the historical range of variability. The magnitude of past changes in vegetation and fire regime in the Sawtooth Range, for example, could not have been inferred from recent observations or tree-ring-based studies that span the last few centuries. Without the pollen and charcoal data, we would not know that the present-day closed lodgepole pine forest was established at 2650 cal yr BP, or that the current forest structure probably developed in response to a sharp increase in fire activity ca. 1450 yr ago. Warmer annual

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temperatures, increased fire activity, and continued infestation of mountain pine beetle are projected for the future in this region (Westerling et al., 2006; Logan and Powell, 2001). Repeated beetle outbreaks will likely promote seral lodgepole pine and greater synchronization of fire events (Raffa et al., 2008). Persistence of this disturbance regime may reduce lodgepole pine's dominance and lead to open forests of Douglas-fir and residual lodgepole pine. The conditions of the early and middle Holocene would seem to provide a useful analogue for understanding and managing forests for the decades ahead. Acknowledgments This research was supported by the Inland Northwest Research Alliance, USDA Forest Service Cooperative Agreement and National Science Foundation grant EAR-0818467. Reviews by J. Pierce, G. Thackray, and P. Bartlein are greatly appreciated. We thank R. Garwood (Sawtooth National Forest) for logistical support and advice; W. Brawner, M. Huerta, S. Mumma, and D. McWethy for field assistance; and S. Kuehn (Concord University) for tephra identifications. References Alley, R.B., Mayewski, P.A., Sowers, T., Stuiver, M., Taylor, K.C., Clark, P.U., 1997. Holocene climatic instability: a large, widespread event 8200 years ago. Geology 25, 483–486. Anderson, R.S., Allen, C.D., Toney, J.L., Jass, R.B., Bair, A.N., 2008. Holocene vegetation and fire regimes in subalpine and mixed conifer forests, southern Rocky Mountains, U.S.A. International Journal of Wildland Fire 17, 96–114. Arno, S., 1979. Forest regions of Montana. USDA Research Paper INT-218. Bacon, C.R., 1983. Eruptive history of Mount Mazama and Crater Lake caldera, Cascade Range, U.S.A. Journal of Volcanology and Geothermal Research 18, 57–115. Baker, R.G., 1976. Late Quaternary vegetation history of the Yellowstone Lake basin, Wyoming. U.S. Geological Survey Professional Paper 729-E: E1–E48. Barnett, T.P., Pierce, D.W., Hidalgo, H.G., Bonfils, C., Santer, B.D., Das, T., Bala, G., Wood, A.W., Nozawa, T., Mirin, A.A., Cayan, D.R., Dettinger, M.D., 2008. Human-induced changes in the hydrology of the western United States. Science 319, 1080–1083. Barrett, S.W., Arno, S.F., 1999. Indian fires in the Northern Rockies: ethnohistory and ecology. In: Boyd, R.T. (Ed.), Indians, Fire and Land in the Pacific Northwest. Oregon State University Press, Corvallis, OR, pp. 50–64. Bartlein, P.J., Anderson, P.M., Anderson, K.H., Edwards, M.E., Thompson, R.S., Webb, R.S., Webb III, T., Whitlock, C., 1998. Paleoclimate simulations for North America over the past 21,000 years: features of simulated climate and comparisons with paleoenvironmental data. Quaternary Science Reviews 17, 549–585. Beiswenger, J.M., 1991. Late Quaternary vegetational history of Grays Lake, Idaho. Ecological Monographs 61, 165–182. Bennett, K.D., Willis, K.J., 2002. Pollen. In: Smol, J.P., Birks, H.J.B., Last, W.M. (Eds.), Tracking Environmental Change Using Lake Sediments. : Terrestrial, Algal, and Siliceous Indicators, 3. Kluwer Academic Publishers, Dordrecht, pp. 5–32. Breckenridge, R.M., Stanford, L.R., Cotter, J.F.P., Bloomfield, J.M., Evenson, E.B., 1988. Glacial geology of the Stanley Basin. In: Link, P.K., Hackett, W.R. (Eds.), Guidebook to the Geology of Central and Southern Idaho: Idaho Geological Survey Bulletin, 27, pp. 209–211. Brunelle, A., Whitlock, C., Bartlein, P.J., Kipfmuller, K., 2005. Postglacial fire, climate, and vegetation history along an environmental gradient in the Northern Rocky Mountains. Quaternary Science Reviews 24, 2281–2300. Brunelle, A., Rehfeldt, G.E., Bentz, B., Munson, A.S., 2007. Holocene records of Dendroctonus bark beetles in high elevation pine forests of Idaho and Montana, USA. Forest Ecology and Management 255, 836–846. Burns, R.M., Honkala, B.H., 1990. Silvics of North America, volume 1, conifers. USDA Forest Service Agricultural Handbook 654. Carrara, P.E., 1987. Holocene and latest Pleistocene glacial chronology, Glacier National Park Montana. Canadian Journal of Earth Sciences 24, 387–395. Cook, E.R., Woodhouse, C.A., Eakin, C.J., Meko, D.M., Stahle, D.W., 2004. Long-term aridity changes in the western United States. Science 306, 1015–1018. Daubenmire, R.F., 1974. Plants and Environment: A Textbook of Plant Autecology. John Wiley & Sons, Inc. 432 pp. Dean, W.E., 1974. Determination of carbonate and organic matter in calcareous sediments by loss on ignition comparison to other methods. Journal of Sedimentary Petrology 44, 242–248. Dean, W.E., Rosenbaum, J.R., Forester, R.M., Colman, S.M., Bischoff, J.L., Lie, A., Skipp, G., Simmons, K., 2006. Glacial to Holocene evolution of sedimentation in Bear Lake, Utah-Idaho. Sedimentary Geology 185, 93–112. Deevey, E.S., Flint, R.F., 1957. Postglacial hypsithermal interval. Science 125, 182–184. Doerner, J.P., Carrara, P.E., 2001. Late Quaternary vegetation and climatic history of the Long Valley area, west-central Idaho, U.S.A Quaternary Research 56, 103–111. Doher, L.I., 1980. Palynomorph preparation procedures currently used in the paleontology and stratigraphy laboratories, 830. U.S. Geological Survey Circular, Washington, DC. Fischer, W.C., Clayton, B.D., 1983. Fire ecology of Montana forest habitat types east of the Continental Divide. USDA Forest Service Intermountain Forest and Range Experiment Station General Technical Report INT-141.

123

Gedye, S.J., Jones, R.T., Tinner, W., Ammann, B., Oldfield, F., 2000. The use of mineral magnetism in the reconstruction of fire history: a case study from Lago di Origlio, Swiss Alps. Palaeogeography, Palaeoclimatology, Palaeoecology 164, 101–110. Grimm, E.C., 1988. Data analysis and display. In: Huntley, B., Webb III, T. (Eds.), Vegetation History. Kluwer Academic, Dordrecht, Netherlands, pp. 43–76. Hawksworth, F.G., Wiens, D., 1996. Dwarf Mistletoes: Biology, Pathology, and Systematics. USDA Forest Service, Agriculture Handbook 709. Higuera, P.E., Brubaker, L.B., Anderson, P.M., Brown, T.A., Kennedy, A.T., Hu, F.S., 2008. Frequent fires in ancient shrub tundra: implications of paleo-records for arctic environmental change. PLoS ONE 3, e0001744. Higuera, P., Whitlock, C., Gage, J., in press. Fire history and climate-fire linkages in subalpine forests of Yellowstone National Park, Wyoming, U.S.A., 1240–1975 AD. The Holocene. doi:10.1177/0959683610374882. Huerta, M., Whitlock, C., Yale, J., 2009. Holocene Vegetation–Fire–Climate Linkages in Northern Yellowstone National Park, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 271, 170–181. Kapp, R.O., Davis, O.K., King, J.E., 2000. Pollen and Spores, The American Association of Stratigraphic Palynologists2nd Edition. Texas A&M University, College Station, Texas. 279 pp. Kaushal, S., Binford, M.W., 1999. Relationship between C:N ratios of lake sediments, organic matter sources, and historical deforestation in Lake Pleasant, Massachusetts, USA. Journal of Paleolimnology 22, 439–442. Logan, J.A., Powell, J.A., 2001. Ghost forests, global warming, and the mountain pine beetle (Coleoptera: Scolytidae). American Entomologist, Fall 2001, 160–172. Marlon, J., Bartlein, P.J., Whitlock, C., 2006. Fire–fuel–climate linkages in the northwestern U.S. during the Holocene. Holocene 16, 1059–1071. Mehringer Jr., P.J., Arno, S.F., Peterson, K.L., 1977. Postglacial history of Lost Trail Pass Bog, Bitterroot Mountains, Montana. Arctic and Alpine Research 9, 345–368. Miller, M., 1965. The history of Sawtooth and Virginia City. Idaho Yesterdays 9, 11–16. Millspaugh, S.H., Whitlock, C., Bartlein, P.J., 2000. Variations in fire frequency and climate over the last 17,000 years in central Yellowstone National Park. Geology 28, 211–214. Millspaugh, S.H., Whitlock, C., Bartlein, P., 2004. Postglacial fire, vegetation, and climate history of the Yellowstone-Lamar and Central Plateau provinces, Yellowstone National Park. In: Wallace, L. (Ed.), After the Fires: The Ecology of Change in Yellowstone National Park. Yale University Press, pp. 10–28. Minckley, T.A., Bartlein, P.J., Whitlock, C., Shuman, B.N., Williams, J.W., Davis, O.K., 2008. Associations among modern pollen, vegetation, and climate in western North America. Quaternary Science Reviews 27, 1962–1991. Moore, P.O., Webb, J.A., 1978. An Illustrated Guide to Pollen Analysis. John Wiley and Sons, New York. Mumma, S.A., 2009. A 20,000-yr-old record of vegetation and climate from Lower Red Rock Lake, Centennial Valley, southwestern Montana. M.S. Thesis. Montana State University, Department of Earth Sciences, Bozeman, MT, 72 pp. Pierce, J.L., Meyer, G.A., Jull, A.R., 2004. Fire-induced erosion and millennial-scale climate change in northern ponderosa pine forests. Nature 432, 87–90. Power, M.J., 2006. Recent and Holocene fire, climate, and vegetation linkages in the Northern Rocky Mountains, USA. PhD Dissertation, University of Oregon, Eugene, OR, 243 pp. Power, M.J., Whitlock, C., Bartlein, P.J., Stevens, L.R., 2006. Fire and vegetation history during the last 3800 years in northwestern Montana. Geomorphology 75, 420–436. Raffa, K.F., Aukema, B.H., Bentz, B.J., Carroll, A.L., Hicke, J.A., Turner, M.G., Romme, W.H., 2008. Cross-scale drivers of natural disturbances prone to anthropogenic amplification: dynamics of biome-wide bark beetle eruptions. Bioscience 58, 501–517. Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Bertrand, C.J.H., Blackwell, P.G., Buck, C.E., Burr, G.S., Cutler, K.B., Damon, P.E., Edwards, R.L., Fairbanks, R.G., Friedrich, M., Guilderson, T.P., Hogg, A.G., Hughen, K.A., Kromer, B., McCormac, F.G., Manning, S.W., Ramsey, C.B., Reimer, R.W., Remmele, S., Southon, J.R., Stuiver, M., Talamo, S., Taylor, F.W., van der Plicht, J., Weyhenmeyer, C.E., 2004. IntCal04 Terrestrial radiocarbon age calibration, 26–0 ka BP. Radiocarbon 46, 1029–1058. Romme, W.H., Everham, E.H., Frelich, L.E., Moritz, M.A., Sparks, R.E., 1998. Are large, infrequent disturbances qualitative different from small frequent disturbances? Ecosystems 1, 524–534. Sarna-Wojcicki, A.M., Champion, D.E., Davis, J.O., 1983. Holocene volcanism in the conterminous United States and the role of silicic volcanic ash layers in correlation of latest-Pleistocene and Holocene deposits. In: Wright Jr., H. (Ed.), Late Quaternary Environments of the United States 2. University of Minnesota Press, Minneapolis, pp. 52–77. Shafer, S.H., Bartlein, P.J., and Whitlock, C. 2005. Understanding the spatial heterogeneity of global environmental change in mountainous regions. In: Huber, U.M., Bugman, H.K.M., Reasoner, M. (Eds.), Global Change and Mountain Regions: An Overview of Current Knowledge. Kluwer Academic, Dordrecht, pp. 21– 31. Kluwer. Shuman, B., Henderson, A.K., Colman, S.M., Stone, J.R., Fritz, S.C., Stevens, L., Power, M.J., Whitlock, C., 2009. Holocene lake-level trends in the Rocky Mountains, U.S.A Quaternary Science Reviews 28, 1861–1872. Steele, R., Pfister, R.D., Ryker, R.A., Kittams, J.A., 1981. Forest habitat types of central Idaho. USDA Forest Service General Technical Report, INT-114. Intermountain Research Station. 138 pp. Stevens, L.R., Stone, J.R., Campbell, J., Fritz, S.C., 2006. A 2200-year record of hydrologic variability from Foy Lake, Montana USA, inferred from diatom and geochemical data. Quaternary Research 65, 264–274. Stone, J.R., Fritz, S.C., 2006. Multi-decadal drought and Holocene climate instability in the Rocky Mountains. Geology 34, 409–412. Stuiver, M., Reimer, P.J., Reimer, R.W., 2005. CALIB 5.0. [WWW program and documentation].

124

C. Whitlock et al. / Quaternary Research 75 (2011) 114–124

Sugita, S., 1994. Pollen representation of vegetation in Quaternary sediments: theory and method in patchy vegetation. Journal of Ecology 82, 881–897. Thackray, G.D., Lundeen, K.A., Borgert, J.A., 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. Thompson, R.S., Anderson, K.H., Bartlein, P.J., 1999. Atlas of relations between climatic parameters and distributions of important trees and shrubs in North America. U.S. Geological Survey Professional Paper 1650 A&B. van Mantgem, P.J., Stephenson, N.L., Byrne, J.C., Daniels, L.D., Franklin, J.F., Fule, P.Z., Harmon, M.E., Larson, A.J., Smith, J.M., Taylor, A.H., Veblen, T.T., 2009. Widespread increase of tree mortality rates in the western United States. Science 323, 521–524. Westerling, A.L., Hidalgo, H.G., Cayan, D.R., Swetnam, T.W., 2006. Warming and earlier spring increase western US forest wildfire activity. Science 313, 940–943. Whitlock, C., 1993. Postglacial vegetation and climate of Grand Teton and southern Yellowstone National parks. Ecological Monographs 63, 173–198.

Whitlock, C., Bartlein, P.J., 1993. Spatial variations of Holocene climatic change in the Yellowstone region. Quaternary Research 39, 231–238. Whitlock, C., Larsen, C.P.S., 2002. Charcoal as a fire proxy. In: Smol, J.P., Birks, H.J.B., Last, W.M. (Eds.), Tracking Environmental Change Using Lake Sediments:. : Terrestrial, Algal, and Siliceous indicators, 3. Kluwer Academic Publishers, Dordrecht, pp. 75–97. Whitlock, C., Skinner, C.N., Minckley, T.A., Mohr, J.A., 2004. Comparison of charcoal and tree-ring records of recent fires in the eastern Klamath Mountains California, USA. Canadian Journal of Forest Research 34, 2110–2121. Whitlock, C., Marlon, J., Briles, C., Brunelle, A., Long, C., Bartlein, P.J., 2008. Long-term relations among fire, fuel, and climate in the north-western US based on lakesediment studies. International Journal of Wildland Fire 17, 72–83. Williams, P.L., 1961. Glacial geology of the Stanley Basin: Moscow, Idaho Bureau of Mines and Geology, Pamphlet, 123. 29 pp. Wright Jr., H.E., Mann, D.H., Glaser, P.H., 1983. Piston cores for peat and lake sediments. Ecology 65, 657–659.