Changes in the parkland-boreal forest boundary in northwestern Ontario over the Holocene

Quaternary Science Reviews 30 (2011) 1232e1242

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Changes in the parkland-boreal forest boundary in northwestern Ontario over the Holocene Melissa T. Moos*, Brian F. Cumming Paleoecological Environmental Assessment and Research Lab (PEARL), Department of Biology, Queen’s University, Kingston, ON, Canada K7L 3N6

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 August 2010 Received in revised form 23 February 2011 Accepted 28 February 2011 Available online 8 April 2011

Changes in vegetation were tracked from a well-dated sediment core from a boreal lake, Lake 239, at w200-year resolution over the Holocene. This presently oligotrophic lake is located w100-km east from the present-day parkland-forest ecotone in northwestern Ontario. Near-shore sediment core transects from Lake 239 have previously shown this lake was at least 8-m lower than present in the mid-Holocene, or w58% less lake volume in comparison to today. Large shifts were expected in the terrestrial vegetation if the low lake levels were related to climate. The core from Lake 239 shows increases in the relative abundance and concentration of pollen such as Cupressaceae and Ambrosia, indicating a more open boreal forest between w4500e8000 cal yr BP. Pollen-based inferences of average, summer and winter temperatures suggest that temperatures were on average up to 1e2
C warmer than today, with winter temperatures up to 4
C warmer. The pollen inferences also suggest enhanced precipitation, likely in the summer, but with an overall increase in evaporation and evapotranspiration resulting in reduced effective moisture. To assess regional climate changes, pollen-based reconstructions of temperature and precipitation were developed and synthesized from sediment cores from eight previously published lakes, from which pollen sites were available to both the west and east of Lake 239, spanning present-day prairie lakes to forested lakes up to 300 km east of the prairie-boreal ecotone. All sites show shifts in pollen assemblages that indicate a warm mid-Holocene period; prairie sites west of the Experimental Lakes Area (ELA) show mid-Holocene decreases in precipitation relative to today, whereas sites near or east of ELA show consistent increases in precipitation, but with increased temperatures and enhanced evaporation during the mid-Holocene. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Experimental lakes area Holocene Pollen Prairie-Boreal ecotone Paleoclimate Paleolimnology Northwestern Ontario

1. Introduction Boreal forests are thought to have dominated much of Ontario since the end of the last glacial period. Investigating changes in vegetation structure during the mid-Holocene, a time during which warmer periods have been inferred (Yu et al., 1997; Clark et al., 2002; Laird et al., 2003; Nelson et al., 2006; Nelson and Hu, 2008; Williams et al., 2009) may provide an analog to future anthropogenic climate conditions. Predicting the impacts of climate change on landscape structure depends on our ability to put these changes in context with local and regional natural variability (Umbanhowar Jr., 2004; Nelson et al., 2006). Although the midHolocene may not be a perfect analog for future warming, as forcing mechanisms for the projected warming will be different, the resultant climate impacts may be similar (e.g., Pienitz and Vincent,

* Corresponding author. Tel.: þ1 905 395 4668. E-mail address: [email protected] (M.T. Moos). 0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2011.02.013

2000; Lynch et al., 2004; Asselin and Payette, 2005; Nelson et al., 2006; Schindler and Donahue, 2006; St. Jacques et al., 2010). Analysis of pollen assemblages preserved in lake sediments is one of the most commonly used techniques for understanding how vegetation has changed over time, because pollen is well preserved, and pollen records represent an integrated terrestrial signal from the surrounding landscape. Paleoclimatic transfer functions using pollen are calibrated by datasets using modern environmental variables with observed contemporary pollen assemblage data from surficial sediments, thereby allowing reconstruction of past environments using pollen assemblages preserved in lake sediment cores. These techniques have been widely used to quantitatively reconstruct temperature and precipitation over time (e.g. Bartlein and Whitlock, 1993; Whitmore et al., 2005; Viau and Gajewski, 2009). Understanding changes in vegetation becomes even more relevant at ecotonal boundaries, i.e. the transition from one vegetation zone to another. Ecotonal areas are climatically sensitive, with complex interactions between climate and localized factors such as fire and land-use controlling the spatial shifts in these

M.T. Moos, B.F. Cumming / Quaternary Science Reviews 30 (2011) 1232e1242

regions (Grimm, 1983, 2001; Camill et al., 2003; Asselin and Payette, 2005; Williams et al., 2009). Understanding the dynamics of the prairie-forest boundary has been an area of intense interest to ecologists, paleoclimatologists, and atmospheric scientists (Williams et al., 2009) especially in light of expected future changes from anthropogenic warming. In a recent study, Williams et al. (2009) provide a spatio-temporal summary of shifts in the prairie-forest ecotone in central Canada and the north-central US in response to past climate changes. Williams et al. (2009) used modern pollen assemblages crossreferenced with remote sensing data to reconstruct percent woody cover using a Modern Analog Technique (MAT). Consistent with previous studies, a rapid eastward expansion of prairie occurred between 10,000 and 8000 cal yr BP, with the prairie reaching its maximum extent by 6000 cal yr BP. Williams et al. (2009) noted the consistency of these changes in conjunction with other independent climate proxies including increased aeolian activity, declines in lake level, and other biological proxies. After 6000 cal yr BP, percent woody cover increases at many sites suggesting a reexpansion of forests westward. Here, we investigate changes in pollen assemblages in dated sediment cores from the prairies and northwestern Ontario, a region with a notable absence of sites in Williams et al. (2009), but a region that had large changes in water availability (Laird and Cumming, 2008) and water quality (Moos et al., 2009) in the mid-Holocene. In the recent synthesis of Williams et al. (2009), little change in the location of the forest-prairie boundary around ELA was observed, based on percent woody cover, although changes in vegetation structure from other studies show clear changes to more open-canopy forest during the warm early-tomid-Holocene (McAndrews, 1982; Björck, 1985). It is not just the forest-prairie boundary that is of interest, but changes to transitional areas between the prairies and the dense stands of boreal forest. These so-called transition areas are of considerable ecological and paleoclimatological interest to environmental scientists, policy makers and the general public. Our main study lake, Lake 239, is located in the Experimental Lakes Area (ELA) just east (w100 km) of the present-day prairieforest ecotone. Northwestern Ontario contains few well-dated, whole-Holocene sites (Williams et al., 2009) and therefore studies such as this one are important to determine the extent of change in

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the prairie-parkland-boreal ecotone. This region is climatically sensitive, with a slow increase in temperature over the last 20 years (Parker et al., 2009), and marked limnological changes during the droughts of the 1970s and 1980s (Schindler et al., 1997; Schindler, 1998; Findlay et al., 2001). Previously studied areas in and around northwestern Ontario indicate warming during the mid-Holocene, such as in the prairies to the south and west (Dean et al., 1996; Laird et al., 1996), Manitoba (Lewis et al., 2001), and the Great Lakes region (Yu et al., 1997). Diatom analysis of near-shore (Laird and Cumming, 2008) and central cores (Moos et al., 2009) from Lake 239 provide empirical evidence for lowered lake levels and nutrient enrichment, respectively, during the early-to-mid-Holocene. Changes in diatom assemblages during the mid-Holocene indicate reductions in lake level by at least w8 m (Laird and Cumming, 2008) and a shift in the trophic status of Lake 239 from an oligotrophic to a mesotrophic system (Moos et al., 2009). Based on the location of the lake and large inferred changes in lake level and water quality, Lake 239 was chosen as an important site to investigate changes to the prairie-boreal forest ecotone during the mid-Holocene. This study investigates the changes in pollen assemblages during the Holocene with focus on the warmer early-to-midHolocene as a potential analog to future warming. Using pollen extracted from a sediment core from Lake 239, we establish changes from a closed-canopy boreal forest to a more open forest/ parkland, indicative of modern sites to the west of the ELA. Further, this study synthesizes available lower-resolution records across the prairie-boreal ecotone from Manitoba through northwestern Ontario to provide insights into vegetational changes and their paleoclimatic significance. 2. Materials and methods 2.1. Site description Lake 239 is located within the ELA in northwestern Ontario (49
40ʹN, 93
44ʹW) (Fig. 1). This region is currently dominated by boreal tree species including jack pine (Pinus banksiana) and black spruce (Picea mariana), and is characterized by shallow mosscovered soils underlain by Precambrian shield (Davies and Pryslak, 1967; Brunskill and Schindler, 1971; Schindler et al., 1996). Lake 239

Fig. 1. Location map for the Experimental Lakes Area (ELA) (A) and bathymetry for Lake 239 (B) showing the approximate site of the piston core location. Bathymetric map modified from Brunskill and Schindler (1971).

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is a headwater lake with a surface area of w56 ha, a catchment area of w240 ha and a maximum depth of w30 m; it is part of the Lake of the woods, Hudson Bay drainage system. The ELA is located in a region that has been relatively undisturbed by human activities, with meteorological and limnological variables continually monitored since 1968 (Johnson and Vallentyne, 1971). Records of mean annual temperature and precipitation from Kenora, Ontario (w50 km west of ELA) are available beginning in the early 1900s and are highly correlated (r ¼ 0.97 and 0.83, respectively) with the shorter ELA meteorological records (Moos et al., 2005). Mean annual, summer, and winter temperatures for the period 1968 to 2005 are 2.4
C, 17.8
C, and 14.9
C, respectively, and mean annual precipitation over this period has been 730 mm. 2.2. Field sampling and sample preparation Two overlapping cores were retrieved from the central basin of Lake 239 in July 2004 using a 1.0-m Livingstone square-rod piston corer with an internal diameter of 5.08 cm (Glew et al., 2001). Each 1-m section was split, described, and sectioned into 0.5-cm intervals which were then placed into 16-oz WhirlpakÒ bags and stored at 4
C for later analysis. Core 2 was chosen for detailed analysis, with a cumulative length of 1135 cm, while Core 1 is archived for future use. Sub-samples (1.0 cm3) of sediment from Lake 239 were sent to the Limnological Research Centre (LRC), University of Minnesota, Minneapolis for pollen preparation following their standard procedures, including digestion of silicates with hydrofluoric acid. Briefly, samples were spiked with 1.0 ml of synthetic microspheres of a similar density, diameter, and chemical resistance compared to pollen, heated in 10% potassium hydroxide (KOH), rinsed with deionized water (dH2O), sieved through a 160-mm sieve to remove particulates, and captured on a second 7-mm sieve to help concentrate the pollen. The samples were then heated in hydrofluoric acid (HF) to remove silicates, and then hydrochloric acid (HCl) and acetolysis solution, followed by rinses with dH2O, glacial acetic acid (GAIc) and ethanol (EtOH), before being stored in silicon oil. Pollen was mounted using silicon oil onto microscope slides for enumeration. A LeicaÒ light microscope equipped with a 40 Differential-Interference-Contrast (DIC) objective was used to identify and enumerate pollen grains along transects. For each interval, 400 pollen grains were identified and classified to the lowest taxonomic level using published references (Bassett et al., 1978; McAndrews et al., 1978; Kapp et al., 2000). 2.3. Geochronology The geochronology of the central core was based on radiocarbon dates obtained by Accelerator Mass Spectrometry (AMS), at Lawrence Livermore National Labs, from seven pollen samples extracted from w5 g of sediment by the Limnological Research Center, Minnesota, and 1 macrofossil (wood) sample, dated by IsoTrace, Ontario (Table 1 in Moos et al., 2009). The radiocarbon date based

on macrofossil material (9110  60 cal yr BP) was consistent with the pollen sample (9175  170 cal yr BP) from the same interval, suggesting that dates from pollen concentrates are as reliable as macrofossils. The uppermost sediments at the top of the piston core were assessed for 210Pb activity using gamma spectroscopy techniques outlined by Schelske et al. (1994). The gamma results suggest the recent sediments were successfully recovered. Sediment at the 3.0-cm interval in the piston core matches the 210Pb and 137Cs activities of sediment at 5.25e5.5 cm in the gravity core from the same location that dated to 1958 using the Constant-Rateof Supply (CRS) dating model (Binford, 1990), suggesting that only w2 cm was lost from the piston core. The overall age-depth model for Lake 239 was based on a second-order polynomial with a strong age-depth fit (r2 ¼ 0.99, Moos et al., 2009), thus giving adequate chronology to address the questions associated with centennial-tomillennial climate change. 2.4. Synthesis and data analysis Pollen was graphed using the computer program C2 v.1.6.3. (Juggins, 2003). Deviation from average annual temperature (Tavg), annual precipitation (Pann) and Evapotranspiration (ET) were estimated from fossil pollen records using the Modern Analog Technique (MAT). The MAT infers environmental variables by matching pollen species assemblages in the core to modern sites with known environmental variables (Ortiz and Mix, 1997). The MAT of Tavg, Pann, and ET were based on 387 modern sites sub-sampled from the larger North American modern pollen database (Whitmore et al., 2005; Williams et al., 2006, version 1e7) to form a regional calibration dataset appropriate for lakes in this region (Fig. 2). Sites were chosen between 45e60
N and 90e105
W to encompass the ELA area and prairie-forest ecotone (Fig. 2). Fossil pollen was well represented in this modern database, thus it was chosen for the pollen-inferred environmental reconstructions. A squared-chord distance was used as the measure of similarity and the MAT model was based on the 5 best unweighted matches for each sample. The squared-chord distance was used since it is considered the best distance measure for pollen assemblage data (Overpeck et al., 1985; Gavin et al., 2003; Viau et al., 2006; Viau and Gajewski, 2009). Average annual temperature (Tavg), average annual precipitation (Pann) and average evapotranspiration (ET) were reconstructed for Lake 239 with the MAT approach described above (bootstrapped r2 (r2boot) ¼ 0.91, root-mean-squared-error of prediction (RMSEP) ¼ 1.3
C; r2boot ¼ 0.81, RMSEP ¼ 70 mm; boot r2boot ¼ 0.82, RMSEP ¼ 50 mm, for Tavg, Pann, and ET,respectively) using the computer program C2 (Juggins, 2003). Mean winter (DJF) and summer (JJA) temperature and precipitation were also reconstructed using the MAT approach specified above (Winter T: r2boot ¼ 0.92, RMSEP ¼ 1.5
C; Winter P: r2boot ¼ 0.78, RMSEP ¼ 4 mm; Summer T: r2boot ¼ 0.91, RMSEP ¼ 0.9
C; Summer P: r2boot ¼ 0.78, RMSEP ¼ 9 mm). Pollen concentrations were calculated based on known microsphere concentrations in the core

Table 1 Summary of location, date range, and investigator of the eight additional lakes used in this study, as well as Lake 239. Lake name

Location

E Lake Sewell Lake Glenboro Lake Mordsger Lake Hayes Lake Myrtle Lake Peggy Lake Oliver Pond Lake 239

50.41.29 49.50.30 49.26.00 51.23.00 49.35.00 47.59.00 49.28.48 48.25.20 49.39.40

N N N N N N N N N

99.39.30 99.34.40 99.17.00 94.15.00 93.45.00 93.23.00 92.00.00 89.19.26 93.43.24

W W W W W W W W W

Manitoba Manitoba Manitoba Ontario Ontario Minnesota Ontario Ontario Ontario

Age range (Yr BP)

Investigator/Published reference

0e11400 (9 dates) 0e10700 (2 dates) 0e12000 (6 dates) 20e9310 (3 dates) 100e10000 (5 dates) 0e11000 (7 dates) 20e8840 (3 dates) 0e10300 (4 dates) 0e10000 (8 dates)

Ritchie, J.C./Ritchie (1964, 1969, 1976) Ritchie, J.C./Ritchie (1976) Ritchie, J.C./Ritchie and Lichti-Federovich (1968) McAndrews, J.H./McAndrews (1986) unpublished MNR report McAndrews, J.H./McAndrews (1982) Janssen, C.R./Janssen (1968, 1984) McAndrews, J.H McAndrews, J.H./Julig et al. 1990 Moos, Melissa T.

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Fig. 2. Location map for the calibration lakes (closed circles), eight additional study sites (closed squares) and Lake 239 (star).

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Changes in vegetation across the region were inferred using a series of eight existing whole-Holocene pollen sites to the north, east and west of the ELA region that were obtained from the National Oceanic and Atmospheric Administration (NOAA) North American Pollen Database (Fig. 2, Table 1). Sites were chosen to create an eastwest transect, using only whole-Holocene records, from the prairies

La

Pi ce

Pi nu

a

s

following methods first described by Benninghoff (1962), and further developed by Matthews (1969) and Bonny (1972). PCA axis1 scores were calculated, using the program Canoco for Windows  version 4.5 (Ter Braak and Smilauer, 2002), to synthesize the main directions of variation in the pollen assemblages for both relative abundance and concentration.

0 1000

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2000 3000 4000 5000 6000

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Relative Abundance (%) Fig. 3. The relative abundance of dominant pollen taxa (>5%) found in the central basin piston core from Lake 239. Reconstructed variables and principal components analysis (PCA) axis-1 scores are shown to the right of the diagram. Graphed based on calibrated accelerator mass spectrometry (AMS) 14C (cal yr BP  1 sigma) dates. Pollen zones are based on a stratigraphically constrained cluster analysis.

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well into the boreal forest. Chronologies for the sites are based on calibrated 14C dates provided in the NOAA database; the chronological control on these sites is not as strong as the chronology for Lake 239, with chronologies based on between 2 and 9 dates; Sewell Lake has the weakest chronology and is based on only one 14C and one recent date assumed for the top of the core (Table 1). Average annual temperature (Tavg), and winter temperature (Tdjf), as well as annual precipitation (Pann) and summer precipitation (Pjja) were calculated for the eight existing Holocene study lakes using the MAT approach outlined earlier. Performance for the eight lakes ranged for Tavg: r2boot ¼ 0.79e0.91, RMSEP ¼ 1.2e1.3
C and for Pann: r2boot ¼ 0.78e0.80, RMSEP ¼ 68e72 mm. Summary figures for the eight study lakes were produced using OriginÒ v. 6.1.

and Artemisia pollen in low abundances (Figs. 3 and 4). During this period, the forest transitions into a pine (Pinus) dominated forest, which remains the dominant taxon for the Holocene. Reconstructions of Tavg, Pann, and ET suggest a cool climate with low rates of evapotranspiration during this period, as well relatively low amounts of precipitation (Fig. 3). Reconstructed summer and winter temperatures are the lowest of the entire record (Fig. 5). 3. Early-to-mid-Holocene (w8600e4500 cal yr BP) From w8600 to 4500 cal yr BP (Zone B) the pollen assemblage was quite different from the early Holocene, with some marked changes in species assemblage. Picea abundances are much lower than in the early Holocene and remain low throughout this zone. There is an increase in Cupressaceae (most likely Juniper in this region) and Ambrosia, indicative of a more open boreal forest. Pinus relative abundances decline relative to other taxa at the start of the midHolocene from abundances of upwards of 80% to its lowest value of 50%, with a gradual increase back to current values throughout Zone B (Fig. 3). However, Pinus concentrations continue to increase in this zone, reaching high levels of abundance (Fig. 4). Reconstructed climate variables increase sharply around w8600 cal yr BP stabilizing during Zone B, at temperatures that are 1e2
C higher than modern and a w20% increase in precipitation (Fig. 3). Pollen-inferred summer precipitation increases in the mid-Holocene more than winter precipitation and both summer and winter temperatures are elevated compared to periods prior to and following the mid-Holocene, with a larger increase in winter months (Fig. 5).

3. Results Thirty-four pollen types were identified from 47 samples from the sediments of Lake 239 over the last w10,000 years (Figs. 3 and 4). Those with 5% or greater relative abundance are included in pollen diagrams with the rest grouped into larger taxonomic categories (e.g. other deciduous trees, other herb/flowering plants, and aquatic/emergent macrophytes). Holocene pollen zones were defined using CONISS (stratigraphically constrained cluster analysis) in Tilia (Grimm, 2004) and their placement is similar to zones defined based on diatoms from the same core (Moos et al., 2009). Similar patterns of change are identified in both the pollen relative abundance (Fig. 3) and concentration data (Fig. 4) for individual taxa (r ¼ 0.69 to 0.98, p < 0.05). Total pollen concentrations are dominated by the genus Pinus (Fig. 4). In Lake 239, trends in concentrations are similar to accumulation rates because sediment age is nearly linear with respect to sediment depth (r ¼ 0.95).

3.3. Late Holocene (w4500 cal yr BP - present) The dominant taxon for the most recent sediments (Zone A) is Pinus, maintaining abundances of up to 70% of the assemblage (Fig. 3). There is a slight increase in both the relative abundance and concentration of Picea indicating a return to a closed-canopy boreal forest structure and a decline in mid-Holocene open-canopy types

3.1. Early Holocene (w11,600e8600 cal yr BP)

e or sc

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The initial post-deglaciation pollen assemblage (Zone C) mostly indicates a spruce (Picea) dominated forest, with deciduous tree

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Pollen Concentration (x 10 ) Fig. 4. The total concentration of dominant pollen taxa (x103) found in the central basin piston core from Lake 239. Principal components analysis (PCA) Axis-1 scores are shown to the right of the diagram. Graphed based on calibrated accelerator mass spectrometry (AMS) 14C (cal yr BP  1 sigma) dates. Pollen zones are based on a stratigraphically constrained cluster analysis.

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M.T. Moos, B.F. Cumming / Quaternary Science Reviews 30 (2011) 1232e1242

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C 75

85

95

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(mm)

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(mm)

40 15

18

(°C)

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-14

-10

(°C)

Fig. 5. Average Summer (June, July, August) and Winter (December, January, February) precipitation and temperature reconstructions for Lake 239. Graphed based on calibrated accelerator mass spectrometry (AMS) 14C (cal yr BP  1 sigma) dates. Zones are matched to pollen zones and based on a stratigraphically constrained cluster analysis.

such as Cupressaceae (Figs. 3 and 4). Pollen-based inferences of Tavg, Pann, and ET return to levels lower than those in the early-to-midHolocene but not as low as in the early Holocene. Pollen-based inferences of winter temperatures and precipitation decrease during this period to modern levels, while summer temperatures change only slightly, and precipitation declines to modern levels (Fig. 5).

3.4. Regional paleovegetation and paleoclimatic analyses and syntheses Changes in pollen across the region were synthesized based on eight whole-Holocene pollen sites surrounding the ELA (Fig. 6). There is a shift from a spruce-dominated to pine-dominated pollen assemblage between w10,000e8500 cal yr BP (Fig. 6A) along with shifts in herbaceous and open-canopy taxa between w85004500 cal yr BP (Fig. 6B). Prairie lakes show a dramatic increase in herbaceous pollen, such as Poaceae and Ambrosia during the earlyto-mid-Holocene, while the eastern lakes show changes to Cupressaceae and Pinus (Fig. 6). Inferences of temperature and precipitation from these sites show changes similar to Lake 239, with generally higher annual and winter temperatures in the midHolocene (Fig. 7), with slightly higher annual and summer precipitation except for the three western prairie lakes (E-Lake, Glenboro, and Sewell), where precipitation is lower (Fig. 8). Changes in these lakes are dependent on location, with the severity of changes more evident in prairie lakes and more muted in the present-day boreal forest lakes to the east of ELA.

shifts in vegetation structure indicative of changes across the ecotonal boundary. These changes will be further discussed by zone. 4.1. Early Holocene (w11,600e8600 cal yr BP): post-deglaciation changes Following deglaciation in this region, local areas of tundra developed within the Lake Agassiz region which lasted until w10,000 14C yr BP (w11,600 cal yr BP) (McAndrews, 1982; Bajc et al., 2000; Dyke, 2005). Early boreal forest species began to colonize following the tundra period (Dyke, 2005), with spruce being the dominant tree species, a common pattern in many boreal regions, including Europe and the central interior of North America (Kaufman et al., 2004; Teed et al., 2009). Changes in the additional eight study lakes also follow a similar pattern to Lake 239, with a change from spruce-dominated to pine-dominated boreal forests during the early Holocene. Reconstructed temperature and precipitation in Lake 239 are lowest in the early Holocene, following deglaciation, and stabilized within the first couple of hundred years. Early Holocene climate simulations using the community climate model (CCM1), as well as studies based in Europe, show similar trends to Lake 239 (Kutzbach et al., 1998; Antonsson et al., 2006). Kutzbach et al. (1998) suggested that cooler winter temperatures were common in the early Holocene using a global integrated model of climate and insolation. Antonsson et al. (2006) suggest remarkably low summer temperatures in Sweden at the start of the early Holocene, reconstructed from fossil pollen and chironomids. Temperature and precipitation began a steady increase towards the end of the early Holocene (w8600 cal yr BP), reaching maximum levels in the mid-Holocene thermal maximum (Fig. 3).

4. Discussion Pollen analysis from the sediments of Lake 239 provides insights into the vegetation history of the ELA region, while the west to east transect of sites helps provide regional context for these changes. Pollen-inferred reconstructions of temperature and precipitation suggest climate-related changes during the Holocene, as well as

4.2. Mid-Holocene (w8600e4500 cal yr BP): boreal forest to opencanopy boreal forest Forest structure changed significantly in the mid-Holocene. A decrease in the relative abundance of Pinus and an increase in Cupressaceae and Ambrosia indicate a more open forest around

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Fig. 6. Summary of relative pollen abundance through time for the eight additional study lakes (A) PiceaePinus complex, (B) Open-canopy/prairie taxa. Only relevant species are shown. Lakes are presented based on their locations from west to east. Horizontal lines represent pollen stratigraphic zones (A,B,C) from Lake 239.

Lake 239. Studies east of the ELA indicate these changes were widespread in the region (McAndrews, 1982; Björck, 1985). Williams et al. (2009) also found a change in woody cover during the mid-Holocene with a southward shift in the prairie-forest boundary. Although ELA is east of the prairie-forest ecotone, changes in forest structure with changing climate have occurred in the past. Mid-Holocene changes in the other eight study lakes show similarly consistent patterns. Changes in pollen assemblage were noted in all eight study lakes but the actual species found in each lake vary with location; changes in Poaceae are more apparent in prairie lakes, while easterly sites show an increase in Cupressaceae. Reconstructed temperature in Lake 239 suggests an increase in summer, winter and annual temperatures from w8000 to 4500 cal yr BP which is consistent with other paleoclimate studies in North America and Europe (Davis et al., 2003; Viau et al., 2006; Viau

and Gajewski, 2009), Viau et al. (2006) found pollen-inferred July temperature increased in North America to a maximum between 6000 and 3000 cal yr BP. The additional eight study lakes also show a similar increase in annual temperature to that seen in Lake 239. Changes in insolation contributed to this warming in the midHolocene resulting from changes in the Earth’s orbital parameters and associated changes in large-scale circulation patterns (Yu et al., 1997; Ruddiman, 2001, 2005); summer insolation in the Northern Hemisphere was as much as w8% higher than present (COHMAP 1988). Changes in the mid-Holocene included increases in summer temperature (Ganopolski et al., 1998; McFadden et al., 2005) and increased seasonality (Prentice et al., 1991; Allen et al., 2007). Although forcing mechanisms for future climate changes will be different (anthropogenic-induced warming), the resultant climatic impacts to Lake 239 and the terrestrial environment may be similar.

M.T. Moos, B.F. Cumming / Quaternary Science Reviews 30 (2011) 1232e1242

A

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C 12000 -6-4-2 0 2 4 6 -4 -2 0 2 4 -4 -2 0 2 4 6 -2

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-15

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-15

-10

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-20 -15 -10

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Temperature °C Fig. 7. Pollen-inferred changes in annual average temperature for the eight additional study lakes (A) and average winter temperature (B). Lakes are presented based on their locations from west to east. Horizontal lines represent pollen stratigraphic zones (A,B,C) from Lake 239.

Changes in precipitation were also estimated from pollen assemblages in Lake 239 during the mid-Holocene. A larger increase in summer precipitation (JJA) and only a slight increase in winter precipitation (DJF), compared to modern values, suggests a change in seasonality with more accumulation in the summer months compared to the winter months (Fig. 8). Parker et al. (2009) found similar trends in modern meteorological data from the ELA (wlast 20 years), with a general trend towards increased summer precipitation, and an increase in annual and winter temperatures. The additional study lakes around Lake 239 show a similar increase in precipitation during the mid-Holocene with the exception of the prairie lakes (E-Lake, Glenboro, and Sewell). Although interesting, the increase in precipitation must be considered with caution, as the change is small and errors on the inferred values are quite large. However, Nelson and Hu (2008) found a similar decrease in aridity from west to east across three study lakes in Minnesota, suggesting western sites are more arid than eastern sites. Pollen-inferred precipitation and temperature from boreal sites in Europe show similar trends in the mid-Holocene, with warmer temperatures and an increase in summer precipitation, but an overall reduction in moisture availability due to increased summer evaporative demand (Allen et al., 2007). Diatom-based salinity reconstructions from northern Minnesota and the Canadian prairies provide further evidence for reduced effective moisture in the mid-Holocene, with increases in salinity and reductions in lake levels (Laird et al., 1996; Michels et al., 2007).

Reconstructed evapotranspiration in the Lake 239 watershed increased during the warmer early-to-mid-Holocene to levels unprecedented in the rest of the historical record (Fig. 3), and may explain the lake level decrease with potential increases in precipitation. ET models generally predict an increase in Potential Evapotranspiration (PET) with climate warming; however, there is continued uncertainty in how lakes will change with anthropogenic climate change due to large discrepancies between mechanistic models used to predict changes in PET (Kingston et al., 2009). Empirical evidence from pollen reconstructions may help improve PET models by model comparisons with the mid-Holocene. Long-term data, over the last 40 years, suggests evaporation and evapotranspiration (6% increase per degree Celsius) can outstrip small increases in precipitation, resulting in decreased stream flows and lowered lake levels (Schindler, 2006). Therefore, small to medium increases in precipitation are likely not enough to counteract increases in evapotranspiration as suggested by the w8-m drop in lake levels in Lake 239 during the mid-Holocene (Laird and Cumming, 2008). Empirical evidence from Lake 239 suggests that with climate warming there will be reduced effective moisture in the region and a shift from a closed to a more open boreal forest. Shifts in vegetation can also be linked to climate changes associated with the collapse of the Laurentide Ice-Sheet w8.2 ka (Shuman et al., 2002; Nelson and Hu, 2008). The additional study lakes show changes beginning around this time with the exception

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Precipitation(mm) Fig. 8. Pollen-inferred changes in annual precipitation for the eight additional study lakes (A) and summer precipitation (B). Lakes are presented based on their locations from west to east. Horizontal lines represent pollen stratigraphic zones (A,B,C) from Lake 239.

of the three prairie lakes (Sewell, Glenboro, and E-lake). Precipitation and temperature shifts in Lake 239 began around w8100 cal yr BP following the ice-sheet collapse and drainage of Lake Agassiz, which likely contributed to the regional climate and vegetation changes, and changes to regional hydrology (Hu et al., 1997; Clarke et al., 2003). The discrepancy in the western study lakes are likely related to a rapid drying in the continental interior (Nelson and Hu, 2008) resulting in a decrease in precipitation, contrary to the increase seen in the present-day boreal lakes. A decrease in conifers and increase in herbaceous and open-canopy species in Lake 239, as well as in the additional study lakes, are evidence of these rapid climate changes. 4.3. Late Holocene (4500 cal yr BP - present): onset of modern pollen assemblages During the late Holocene the most dominant taxon was Pinus maintaining abundances of up to 70% for the last w4500 years. The onset of modern pollen assemblages for this region, similar to estimates by Dyke (2005) made using pollen and plant macrofossils, occurred around w3000 years ago. The pollen analyses in Lake 239 support this, with a slow increase in Picea over the late Holocene period and a sharp decline in Cupressaceae around w4500 cal yr BP. Elk Lake, Minnesota (Bradbury and Dieterich-Rurup, 1993) shows similar changes in vegetational shifts from Pinus to Picea as Lake 239. The eight study lakes also show similar trends to Lake 239 and Elk

Lake, with the onset of modern conditions. In the eastern-most lakes, Pinus reaches abundances of up to 60%, while in the 3 prairie lakes (E-Lake, Glenboro, and Sewell) there is a return to cooler and moister conditions, indicated by small increases in Picea and small declines in Ambrosia and Cupressaceae. Reconstructed temperature, precipitation and evapotranspiration decline in the late Holocene to levels higher than those in the early Holocene, but much lower than mid-Holocene values (Figs. 3 and 5). The return to increased forest cover in the late Holocene is likely a result of increased moisture availability with the reduction in evapotranspiration and temperature. This is consistent with other studies that found increased effective moisture after w6 ka (Williams et al., 2009), along with higher lake levels (Laird et al., 1996; Laird and Cumming, 2008) and lower temperatures (Viau et al., 2006). 4.4. Future Implications The mid-Holocene warmed to levels 1e2
C warmer than present, a result not unlike future warming scenarios under increased CO2 (IPCC, 2007). Future trends remain uncertain, as climate models have predicted both increases and decreases in precipitation with current warming trends, but agree there will be increased hydrologic variability (Williams et al., 2009). Current meteorological records from the ELA region suggest an upward trend and changes in the seasonal distribution of precipitation over

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the last 40 years (Parker et al., 2009). Additional well-dated wholeHolocene studies can help improve current process-based models and provide more consistency in predicting future climate scenarios. Furthermore, models generally do not include the influence of fire, pests and pathogens, which can be influenced by climate (Williams et al., 2009). Further studies on the linkages between vegetation and fire-climate-pathogen interactions may help improve our knowledge in this important area. 5. Conclusions The results from this study show unequivocal changes in forest structure, with shifts to a more open boreal forest during the warmer early-to-mid-Holocene. The mid-Holocene warmed to levels at the low end of future warming scenarios under increased CO2 (IPCC, 2007) and may provide one possible analog for future anthropogenic warming. Empirical evidence from pollen reconstructions can help improve models on evapotranspiration and moisture availability, allowing better insights of possible future change. This study also helps fill a knowledge gap in NW Ontario as indicated by previous studies (Williams et al., 2009). Current meteorological trends at the ELA (over the last 40 years) include increased summer precipitation, and an increase in annual and winter temperatures (Parker et al., 2009). Evidence from ELA suggests evapotranspiration can outstrip precipitation leading to changes in stream flow and lake levels (Schindler, 2006). This study provides support that such changes occurred in the past under warmer conditions, and consequently similar changes may occur in the future. Further studies are needed to better understand the timing of precipitation and changes to precipitation and evapotranspiration under warming scenarios. Acknowledgements We thank ELA staff for logistical support while conducting fieldwork and for providing long-term data, Dan Selbie, Chris Lorenz, and Lauren Forrester for help with fieldwork. We thank Amy Myrbo and associates at the LacCore facilities (University of Minnesota) for pollen digestions, and Tom Brown at LLNL for processing our 14C samples. This project was funded by a NSERC Discovery grant to B.F. Cumming, and an OGSST grant to M.T. Moos. References Allen, J.R.M., Long, A.J., Ottley, C.J., Pearson, D.G., Huntley, B., 2007. Holocene climate variability in northernmost Europe. Quaternary Science Reviews 26, 1432e1453. Antonsson, K., Brooks, S.J., Seppä, H., Telford, R.J., Birks, H.J.B., 2006. Quantitative paleotemperature records inferred from fossil pollen and chironomid assemblages from Lake Gilltjärnen, northern central Sweden. Journal of Quaternary Science 21, 831e841. Asselin, H., Payette, S., 2005. Late Holocene opening of the forest tundra landscape in northern Quebec, Canada. Global Ecology and Biogeography 14, 307e313. Bajc, A.F., Schwert, D.P., Warner, B.G., Williams, N.E., 2000. A reconstruction of Moorhead and Emerson Phase environments along the eastern margin of glacial Lake Agassiz, Rainy River basin, northwestern Ontario. Canadian Journal of Earth Sciences 37, 1335e1353. Bartlein, P.J., Whitlock, C., 1993. Paleoclimatic interpretation of the Elk Lake pollen record. In: Bradbury, J.P., Dean, W.E. (Eds.), Elk Lake Minnesota: Evidence for Rapid Climate Change in the North-central United States. Geological Society of America Special Paper 276, pp. 275e293. Bassett, J., Crompton, C.W., Parmlee, J.A., 1978. An Atlas of Airoborne Pollen Grains and Common Fungus Spores in Canada. Canada Department of Agriculture, Ministry of Supply and Services, Canada. Benninghoff, W.S., 1962. Calculation of pollen and spore density in sediments by addition of exotic pollen in known quantities. Pollen Spores 4, 332e333. Binford, M.W., 1990. Calculation and uncertainty analysis of 210Pb dates for PIRLA project lake sediment cores. Journal of Paleolimnology 3, 253e267. Bonny, A.P., 1972. A method for determining absolute pollen frequencies in lake sediments. New Phytologist 71, 393e405. Bradbury, J.P., Dieterich-Rurup, K.V., 1993. Holocene diatom Paleolimnology of Elk Lake, Minnesota. In: Bradbury, J.P., Dean, W.E. (Eds.), Elk Lake, Minnesota:

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