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316 JOURNAL OF CLIMATE VOLUME 22

Reconstructing Millennial-Scale, Regional Paleoclimates of Boreal during the Holocene

A. E. VIAU AND K. GAJEWSKI Laboratory for Paleoclimatology and Climatology, Department of Geography, University of , Ottawa, , Canada

(Manuscript received 26 November 2007, in final form 2 July 2008)

ABSTRACT

Regional paleoclimate reconstructions for Canada quantify Holocene climate variability on orbital and millennial time scales and provide a context to better understand the current global warming. The reconstructions are based on available pollen diagrams from the boreal and low zones of Canada and use the modern analog technique (MAT). Four regional reconstructions document the space–time evolution of the climate during the Holocene. Highest summer and winter temperatures anomalies are found in during the early Holocene. was relatively cool in the early Holocene, whereas central Canada was warmest at that time. was relatively dry in the early to mid-Holocene during which time Canada was relatively moist. Millennial-scale temperature varia- tions, especially the Medieval Warm Period and Little Ice Age are seen across the , with some suggestion of time-transgressive changes from west to east. At the millennial scale, precipitation anomalies are of opposite signs in eastern and . The results herein indicate that modern increases in temperatures in far exceed natural millennial-scale climate variability.

1. Introduction during the more recent Little Ice Age (LIA) and Me- dieval Warm Period (MWP) of the past 1000 yr (Ga- Pollen-based climate reconstructions have been ex- jewski 1987; Viau et al. 2006). Thus, regional millennial tensively used to document climate variability on sev- to centennial climate changes are important, particu- eral time and space scales. However, most of the pre- larly because they are relevant to understanding the vious work has consisted of mapping certain time inter- current global warming issues. However, regional syn- vals, such as 6 ka (e.g., COHMAP Members 1988; theses of the time–space evolution of the climate are Gajewski et al. 2000; Sawada et al. 2004), or producing lacking. time series from one or several sites (e.g., Gajewski Much of the recent interest in millennial-scale cli- 1988; Sawada et al. 1999; Kerwin et al. 2004). Using key mate variability has centered on analysis of ice core and time intervals, data–model comparisons show strong re- marine sediments; terrestrial evidence, such as pollen gional responses to orbital forcing (e.g., COHMAP sequences from lake sediments, has been less often Members 1988). studied in this context (Gajewski et al. 2007). The large Radiocarbon-dated pollen (and other) records, how- number of available pollen diagrams, each a multivari- ever, show variability on suborbital scales. Studies have ate time series, have resisted easy synthesis, in part be- shown that vegetation responds rapidly to climate cause of the low temporal resolution of the data and change (Webb 1986; Gajewski 1987) and that there is ambiguities of the chronologies. Although climate evidence of a synchronous vegetation response to change is invoked as an explanation for vegetation abrupt climate changes during the late glacial period change in pollen diagrams, most palynological analysis and Holocene (Grimm et al. 1993; Williams et al. 2002; has focused on the response of plants and vegetation to Viau et al. 2002, 2006; Gajewski et al. 2006) as well as climate variability on orbital scales (e.g., Wright et al. 1993). However, significant changes in pollen se- quences across and occur syn- Corresponding author address: A. E. Viau, Laboratory for Pa- leoclimatology and Climatology, Department of Geography, Uni- chronously, arguing for a response to changes in the versity of Ottawa, Ottawa, ON K1N 6N5, Canada. climate regime at both a century and millennial scale E-mail: [email protected] (Gajewski 1987, 2000; Viau et al. 2002; Gajewski et al.

DOI: 10.1175/2008JCLI2342.1

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2006, 2007, and references therein). In addition, this sible to reflect the physiographic and climate zones of relative lack of interest in lake sediments has been Canada, but they were constrained by the need to en- partly based on a perception that vegetation responds sure sufficient sites to permit the computation of re- slowly to , a conclusion that has, how- gional averages. Modern July temperatures across the ever, been questioned for many years (Gajewski 1988; range from 88C in central Canada to 10.18Cin Viau et al. 2002; Williams et al. 2002; Gajewski et al. Labrador, modern January temperatures range from 2007). By computing regional averages (Viau et al. 218.98C in Labrador to 227.28C in central Canada, 2006), we can attempt a continental-scale synthesis of and annual precipitation ranges from 529 mm in central the boreal zone of Canada and mitigate against some of Canada to 935 mm in Labrador. Because of the lower these problems. available site density, the central Canadian Here we explore the use of regional paleoclimatic spans 408 of longitude, compared to the roughly 208 averages to study Holocene changes within the modern span of the other three regions. There are only four boreal zone of Canada. Although this region has fewer sites from southern available in the available sites than in more temperate regions, we focus database, so the Mackenzie region primarily contains on higher latitudes because they are most affected by sites from the and upper Mackenzie valley; that the current global warming, mainly because of strong is, the boreal portion of this region. This desire for snow/ice albedo feedbacks. Additionally, Viau et al. objective site selection with no exclusion means that (2006) have shown that the development of regional our analysis includes one site on Baffin Island and an- composite reconstructions permits higher-temporal- other from West from the Arctic zone. resolution paleotemperature estimates through time– At each site, January and July temperature and an- space averaging, which partly counteracts the low tem- nual precipitation were estimated from the fossil pollen poral resolution and sparse density of sites in some re- records using the modern analog technique (MAT; gions. Overpeck et al. 1985). The modern pollen database In this paper, we quantify Holocene paleoclimates used for calibration consists of 4590 sites and associated across northern Canada between 508 and 708N, encom- climate data, as described in Sawada et al. (2004), which passing the boreal and low Arctic regions. Through this includes data from most regions of North America, in- determination of regional-scale patterns of climate vari- cluding the Arctic and Greenland regions. Approxi- ability we can provide a context for global warming. To mately 75% of the ‘‘climate space’’ of North America is accomplish this, we use a network of fossil pollen represented by the modern pollen site distribution records that has the advantage of reducing the temporal (Whitmore et al. 2005; Sawada et al. 2004). The 14C and spatial uncertainties associated with site-specific age–depth relation of each pollen time series was re- uncertainties. The data were extracted from the North modeled using linear interpolation between dates based American Pollen Database (NAPD) and represent the on the author’s approved dates. Calendar-year age– most extensive network of Holocene terrestrial proxy- depth models were then derived using internationally climate data available for North America (Grimm ratified calibration curves (INTCAL98) (Stuiver et al. 2000). Temperature and precipitation reconstructions 1998). In this paper, we refer to dates as ka (1000 yr for each region are discussed on both orbital [low-fre- before A.D. 1950). quency (104) variations of the past 12 ka] and millennial b. Paleoclimate reconstructions [higher-frequency (103) variations of the past 12 ka] time scales. We used the modern analog technique to estimate January and July temperatures and annual precipita- 2. Data and methods tion, following the methodology of Viau et al. (2006). Essentially, this technique successively compares each a. Data fossil pollen sample of every core to all modern pollen We extracted 117 fossil pollen records from the samples in the database. The modern pollen samples NAPD (see the appendix; Grimm 2000) including all that have the smallest dissimilarity, measured by the sites available from northern Canada between 508 and squared chord distance (SCD), represent the best ana- 708N and between 508 and 1408W. The pollen sum used logs (Overpeck et al. 1985). While debate remains over in this study consists of 89 taxa, comprising all abundant how many analogs should be retained or averaged taxa and those important in boreal and regions. (Viau and Gajewski 2007; Shuman et al. 2007; Williams We then divided the region into four (Fig. and Shuman 2008; Viau et al. 2008, hereafter VIA), we 1) to examine the spatiotemporal patterns of change simply retained the one least dissimilar modern pollen through time. The regions were chosen as much as pos- sample and used the July and January temperature and

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FIG. 1. Location of pollen diagrams used in this study (references in the appendix). annual precipitation values at the location of the best the climate data from these widely separated areas analog as the best estimate for the reconstructions (e.g., would result in an assigned climate different from both see Viau et al. 2006). While averaging several analogs regions. Using both an expanded pollen set and the best reduces the impact that an outlier point may have on analog should better separate different regional cli- the climate assigned to the fossil data point, the spatial mates (Sawada et al. 2004), at least for boreal Canada. averaging in this study also accomplishes this goal. The application of the MAT in this context is discussed However, a threshold is needed that specifies what is an in Sawada et al. (2004) and Viau et al. (2006). acceptable analog, and there is at present no concrete Once either the temperature or precipitation was es- agreement upon methods to do this, especially because timated for all sample points of each time series, we this is variable in time and space (Sawada et al. 2004; estimated the value of each time series at 100-yr inter- Williams and Shuman 2008; VIA), as well as a function vals using a simple linear interpolation. Because the of taxonomic diversity (Waelbrock et al. 1998). In this original pollen diagrams have an average 250-yr reso- study, we use the single best analog that is the modern lution, this interpolation has minimal effect on the po- sample that most resembles the fossil data point. Av- sition of the peaks and troughs found in the original eraging several analogs produces temperature and pre- time series. We then averaged across all sites in each cipitation estimates with more or less variability, de- region to create four composite time series, one for pending on the site density of the modern calibration each region, with a resolution of 100 yr. samples. Moreover, the averages may be based on c. Dating control and temporal resolution samples spanning a wide region. For example, in the boreal forest, ‘‘good analogs’’ are found across a wide The temporal resolution of these composite recon- geographic region (Anderson et al. 1989) and averaging structions is based on hundreds of dates and thousands

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FIG. 2. July and January temperature and annual precipitation anomalies for the Labrador region (508–708N, 508–658W) for the past 12 000 and 2000 yr. Site density, chronological control, and sampling interval are also presented. Shaded area represents less confident reconstructions resulting from a decreasing number of sites. of samples. The 117 individual pollen diagrams used in use. The pollen-sampling interval of any individual core this study contain an average of 4.5 radiocarbon dates varies greatly from site to site, with a mean value of for a total of 490 radiocarbon dates for chronological approximately 10 cm, which translates to a median control. This translates to a radiocarbon date every value of approximately 250 yr between samples. How- ;100 yr for each regional curve, although the dates are ever, in the composite curve, any 250-yr period contains not uniformly distributed (Viau et al. 2002). Although many samples and dates upon which to compute an individual pollen records may not be sampled at a 100- average value (see Figs. 2–5). yr resolution, this does not necessarily exclude their The resulting temporal resolution of our paleorecon-

FIG. 3. July and January temperature and annual precipitation anomalies for the northern region (508–708N, 658–808W) for the past 12 000 and 2000 yr. Site density, chronological control, and sampling interval are also presented. Shaded area represents less confident reconstructions resulting from a decreasing number of sites.

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FIG. 4. July and January temperature and annual precipitation anomalies for the central Canada region (508–708N, 808–1208W) for the past 12 000 and 2000 yr. Site density, chronological control, and sampling interval are also presented. Shaded area represents less confident reconstructions resulting from a decreasing number of sites. structions remains difficult to determine but appears to troughs remain in the same place; therefore, any de- be a function of the sampling interval, age–depth rived signal must be in the original data. The converse, model, and especially the spatiotemporal correlation. however, is less desirable, because interpolating at Nevertheless, the presence of spatial and temporal co- lower intervals may mask out millennial-scale fre- herency ensures that the climate signal is preserved in quencies because of aliasing. The use of a network of the composite records. Interpolating to higher intervals sites has the advantage of reducing the temporal and than the sample depths will not add any new informa- spatial uncertainties associated with site-specific uncer- tion to the resulting time series because peaks and tainties.

FIG. 5. July and January temperature and annual precipitation anomalies for the MacKenzie region (508–708N, 1208–1408W) for the past 12 000 and 2000 yr. Site density, chronological control, and sampling interval are also presented. Shaded area represents less confident reconstructions resulting from a decreasing number of sites.

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TABLE 1. Holocene climate variability for regions shown in Fig. 1. For orbital scales, the range between the minimum and maximum temperature (T) and precipitation (P) anomaly during the past 12 ka is presented. For the millennial scale and past 2 ka, numbers are the standard deviation of the first differences of T and P anomalies.

Time scale Labrador Quebec Central Canada Mackenzie 104 (orbital) T July (25.0)–(11.0) (23.0)–(11.5) (0.0)–(13.0) (22.5)–(10.5) Min/max (ka) (11.2)–(3.6) (9.0)–(8.0) (2.0)–(10.0) (11.1)–(7.7) T January (214.0)–(11.0) (22.0)–(11.5) (0.0)–(14.5) (21.5)–(12.3) Min/max (ka) (11.5)–(1.5) (6.0)–(8.0) (0.1)–(11.8) (7.4 –(11.1) P annual (2600)–(160) (2140)–(150) (2130)–(155) (231)–(1200) Min/max (ka) (11.5)–(1.5) (8.8)–(1.6) (11.3)–(5.5) (11.6)–(8.5) 103 T July 0.3 0.3 0.3 0.3 (millennial) T January 0.7 0.5 0.5 0.4 P annual 32 18 14 19 Last 2 ka T July 0.19 0.11 0.18 0.14 T January 0.62 0.41 0.28 0.34 P annual 28 14 10 24 d. Mapping space–time patterns of change and precipitation anomalies are used to synthesize the regional composite records of Figs. 2–5 on two time The temperature and precipitation reconstructions scales of variability: the past 12 000 yr, as a way to for each region are discussed on both orbital [low- visualize low-frequency changes (orbital scale), and a frequency (104) variations of the past 12 ka] and mil- more detailed depiction of the last 2000 yr, as a way to lennial [higher-frequency (103) variations of the past 12 visualize higher-frequency changes (millennial scale) ka] time scales. We separately discuss the reconstruc- across boreal Canada. Anomalies are with reference to tions of the past 2 ka, resulting from the interest in that the 0-ka reconstruction. time period as a context for understanding global warming using paleoclimate records under similar boundary conditions to those of today. To compare the 3. Results range of variability among the regions on 104 time scale we simply compared the maximum and minimum val- a. Orbital scale 3 ues of the estimated temperatures; the variability on 10 1) LABRADOR time scales is presented as the standard deviation of the rate of change between adjacent points (i.e., the differ- July and January temperatures increased and precipi- ence between each time interval). tation generally increased between 12 ka and the Site density decreases in older time periods and this present in the Labrador region (Fig. 2). July tempera- is associated with larger temporal variability in the re- ture anomalies were nearly 118C circa 4 ka, and have constructions. A decrease in site density at 0 ka does slightly decreased subsequently. January temperature not represent a problem because the fossil pollen anomalies peaked at 118C around 1.5 ka. Annual pre- sample represents modern-day conditions. In general, cipitation anomalies closely parallel January tempera- when the regional reconstructions are based on very tures, with maximum anomalies of 160 mm occurring few sites they become less reliable; the decrease in site at 1.5 ka. Although we have less confidence for the density is quite abrupt because of the pattern of degla- period between 12 and ;7 ka, the range over the entire ciation of the area. Although we retained the complete period for July temperature anomalies is from 258 to reconstructions in the figures, as is the practice in den- 118C, for January temperature anomalies from 2148 to droclimatic studies, for example, we do not consider the 118C, and for annual precipitation anomalies from older periods when site density was too low. ;2650 to 160 mm (Table 1). Site density decreased To summarize the regional climate reconstructions, between 6 and 8 ka; the extreme negative values in the we use a Hovmo¨ ller diagram (contour plots with space reconstructions, especially before 10 ka, may be disre- and time as axes) approach, which is commonly used in garded. meteorology, to illustrate the broad-scale evolution of 2) QUEBEC climate patterns through time and across northern Canada. This approach has the advantage of synthesiz- Site density in the Quebec region is low before 7 ka, ing the results without relying on an exhaustive map- and large fluctuations in reconstructed temperatures ping exercise. The space–time diagrams of temperature before that time are not reliable. Results show a general

Unauthenticated | Downloaded 09/25/21 02:55 PM UTC 322 JOURNAL OF CLIMATE VOLUME 22 progression toward a warmer and moister climate from 6 to 2 ka (Fig. 3). July temperature anomalies peak at 10.358C at 3.2 ka, and January temperature anomalies increased abruptly around 3.7 ka, with maximum values of 10.668C around 1.8 ka. This was followed by an abrupt decrease in temperature, reaching minimum value between 0.4 and 0.5 ka (20.968C). Annual pre- cipitation anomalies show maximum values of 150 mm occurring between 1.6 and 1.8 ka. The range of the temperature reconstruction is lower than that of Lab- rador (Table 1).

3) CENTRAL There was a general progression toward a cooler and drier climate since 8 ka in central Canada. From 8 to ;5.5 ka, summer temperature anomalies were rela- tively stable (mean of 10.628C) after which tempera- ture decreased toward modern values (Fig. 4). January temperature anomalies show a similar pattern; from 9 to ;;3 ka, January temperature anomalies fluctuated around a mean of 11.48C and then decreased to mod- ern values. Annual precipitation anomalies were maxi- mum (154 mm) at 5.5 ka. In summary, in this region the early Holocene was warm and dry and the middle Holocene relatively warm and humid. January and July temperatures decreased during the late Holocene.

4) MACKENZIE The available pollen records from this region tend to be longer, so site density remains high for nearly 12 ka. July temperatures increased between 11 and 9 ka, and remained high until after 5 ka (Fig. 5). January tem- perature anomalies gradually decreased since 11 ka and precipitation has decreased from maximum values at approximately 8.5 ka.

5) SPACE–TIME PATTERNS To summarize the above results, space–time dia- grams are presented illustrating the anomalies of tem- perature and precipitation of the past 12 ka (Fig. 6). Warmest temperature anomalies are found in central Canada (Fig. 6) and, as expected, the had maximum warmth occurring earlier than in the FIG. 6. Space–time patterns of (a) July temperature (8C), (b) January temperature (8C), and (c) annual precipitation (mm) eastern region (Fig. 6). This is not surprising because anomalies during the Holocene for the regions in Fig. 1. Area deglaciation occurred much earlier in the west while the above (i.e., older than) bold line represent less reliable recon- remnants of the (LIS) still influ- structions resulting from a decreasing number of sites. Note that enced local to regional climates in the east until at least for the Quebec region, values from 9 to 12 ka represent interpo- 8000 yr ago. During the mid-Holocene, western Canada lations. was wetter than present, whereas eastern Canada was drier.

Unauthenticated | Downloaded 09/25/21 02:55 PM UTC 15 JANUARY 2009 V I A U A N D G A J E W S K I 323 b. Millennial scale anomalies are in phase and show oscillations with a period of around 500 yr. January temperatures were 1) LABRADOR cool and annual precipitation was lower 1000 yr ago Millennial-scale variability during the past 12 000 yr than today, when July temperatures were relatively in the Labrador region, quantified as the standard de- warm. The average range of the anomalies for the past viation of the first difference, is estimated at 60.288C 2000 yr is 60.198C for July temperatures, 60.628C for for July temperature anomalies, 60.698C for January, January temperatures, and 628 mm for annual precipi- and 632 mm for annual precipitation anomalies (Table tation (Table 1). 1). Millennial-scale variability appears greater during the period from 12 to ;7 ka (Fig. 2); however, the lower 2) QUEBEC site density is a contributing factor. For the Quebec region, a decreasing trend in July temperatures between 2 and 0.8 ka was interrupted by 2) QUEBEC a brief warming centered on 1 ka, corresponding to the Millennial-scale variability during the past 8 ka in the Medieval Warm Period (Fig. 3). July temperatures have Quebec region is estimated at 60.288C for July tem- been increasing since 0.5 ka. January temperatures perature anomalies, 60.508C for January, and 618 mm were low between 0.5 and 1.5 ka, again with relatively for annual precipitation anomalies (Table 1). The warm anomalies around 1.1 ka. Annual precipitation greatest and most abrupt millennial-scale variability ap- was higher prior to 1 ka. Higher-frequency annual pre- peared during winter, with large transitions at 2 and cipitation anomalies are out of phase with those from 3.7 ka. Labrador (Figs. 2, 3). The average range of the anoma- lies is less than that in Labrador (Table 1). 3) CENTRAL 3) CENTRAL Millennial-scale variability in central Canada region during the past 12 ka is estimated at 60.328C for July July temperatures in central Canada remained simi- temperature anomalies, 60.448C for January, and 614 lar to today during the past 2 ka, except for a brief mm for annual precipitation anomalies (Table 1). Vari- period of warmer conditions centered at 1 ka (Fig. 4). ability of this scale was greater during the winter. January temperatures were relatively high prior to 1 ka, and steadily decreased during the past 1000 yr. Annual 4) MACKENZIE precipitation anomalies vary about zero, with relatively dry periods centered on 0.5 and 1.4 ka. The reconstructions for the Mackenzie region are marked by abrupt and large changes. For example, 4) MACKENZIE January temperatures show several abrupt coolings, and annual precipitation shows several peaks of in- July temperature anomalies in the Mackenzie region creased precipitation. Variability is comparable to that decreased between 1.2 ka until around 0.4 ka, and then measured in the Quebec and central Canada regions, increased (Fig. 5). January temperature anomalies were but is less than that in Labrador (Table 1). generally below the present-day, except for a large and abrupt peak around 1.1 ka. Annual precipitation was 5) SPACE–TIME PATTERNS generally similar to today, again except for higher val- ues centered on 1.1 ka. Interestingly, the January tem- On the millennial time scale, both July and January perature anomaly curve for the Mackenzie region re- temperature anomaly patterns show smaller oscillations sembles the July curve from the central region and vice superimposed on the orbital-scale trends during the versa (Figs. 4, 5). Holocene (Fig. 6).

5) SPACE–TIME PATTERNS c. The last 2000 yr Warming during the Medieval Warm Period, around 1) LABRADOR 1 ka, and cooling during the Little Ice Age, centered on During the last 2000 yr, temperature anomalies in the 0.5 ka, are seen across northern Canada. The magni- Labrador region varied between 10.268Cat1.1ka,cor- tude of warming of the MWP was, as would be ex- responding to the MWP and 20.48C at 0.5 and 1.7 ka, pected, greater in central Canada because of its conti- corresponding to the LIA and Dark Ages cool periods nentality, whereas the LIA appears to be more strongly (Fig. 2). January temperature and annual precipitation expressed in eastern and western Canada (Fig. 7). How-

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4. Discussion and conclusions

Despite questions that Quaternary scientists have concerning site density/distribution and chronology of terrestrial (i.e., pollen) records, it is still possible to get usable quantitative estimates of Holocene millennial- to centennial-scale climatic changes using these data. We have identified coherent patterns among the recon- structions across the study area, suggesting large-scale forcing of vegetation change at orbital to centennial time scales. The results of the analyses for the past 2000 yr (Fig. 7) suggests that even relatively brief climate variations cause responses in regional pollen produc- tion and vegetation patterns (Gajewski 1988, 2000; Wil- liams et al. 2002; Gajewski et al. 2007), and further that that these are coherent across large areas. Because the temperature reconstructions are averages based on a number of sites from relatively large regions, the par- ticular sequence of pollen changes at any one site dif- fers across the region. For example, cooling could mani- fest itself as a decrease in spruce pollen at the (at the expense of pollen from tundra plants) but an increase in spruce at the southern portion of the boreal forest (at the expense of pollen from deciduous trees). Both would reconstruct a cooling using the statistical method, and our results can be reasonably related to transitions in the vegetation as interpreted by the au- thors of the original pollen studies. In Labrador, many sites record a relatively long pe- riod of tundra pollen following deglaciation (Short and Nichols 1977; Lamb 1980), and this leads to cold tem- peratures being reconstructed for the early Holocene (Figs. 2 and 6). A transition from herb tundra to shrub tundra in northern Labrador (Short and Nichols 1977), and a decrease in fir and white spruce at the expense of black spruce in southern Labrador (Engstrom and Hansen 1985) centered around 6.5 ka, resulted in an abrupt warming in winter temperature to be recon- structed. Maximum summer temperatures between 4 and 2.5 ka correspond to maximum spruce percentages and pollen accumulation rates in southeastern (Lamb 1980) and central (Short and Nichols 1977) Labrador. Decreasing pollen influx and sediment organic matter and increasing values of herbaceous pollen were inter- preted as being due to late-Holocene cooling (Lamb

FIG. 7. Space–time patterns of (a) July temperature (8C), (b) 1980, 1985; Short and Nichols 1977), which is evident in January temperature (8C), and (c) annual precipitation (mm) the reconstruction of summer, but not winter tempera- anomalies during the past 2000 yr for the regions in Fig. 1. tures. In northwestern Quebec, spruce percentages in- ever, the magnitude of change between 1 and 0.5 ka is creased between 6 and 2 ka, depending on region comparable across all regions (approximately 20.78C). (Richard 1979; Richard et al. 1982; Gajewski et al. 1993; Annual precipitation is out of phase between Labrador Leitner and Gajewski 2004), and this leads to a recon- and the Mackenzie region. struction of warming temperatures and increasing pre-

Unauthenticated | Downloaded 09/25/21 02:55 PM UTC 15 JANUARY 2009 V I A U A N D G A J E W S K I 325 cipitation. If the period prior to 7 ka in Quebec is ig- Millennial-scale variations in temperature are - nored because of low site density, the summer tempera- servable across Canada during the Holocene (Fig. 6); ture curves from Labrador and Quebec closely however, the lower number of data before 9 ka makes resemble each other. However, late-Holocene winter it more difficult to determine the synchronicity during temperatures and annual precipitation decreased in the early Holocene. Winter temperatures are warm Quebec, but continued to increase in Labrador. In both across all of Canada between 6 and 4 ka. In general, Labrador and , the annual precipita- millennial-scale changes occur slightly earlier in west- tion reconstruction closely parallels that of winter tem- ern Canada than in the east. peratures. In the last 2000 yr, the pollen records document The location of the tree line in Quebec has not varied variations in temperature and precipitation of both the greatly in the past 5000 yr; climate changes have im- Little Ice Age and Medieval Warm Period. This warm- pacted the vegetation through alterations of the density ing and subsequent cooling are identified across and pollen production of the trees and shrubs that com- Canada in both winter and summer, although perhaps prise the various zones of the broad forest–tundra that beginning slightly earlier in the Mackenzie region (Fig. comprises the transition in this region (Gajewski et al. 7). The greatest amount of warming during the MWP 1993). This impact on the vegetation is particularly no- was in central Canada, whereas the greatest cooling ticeable in the winter, and to a lesser extent in the during the LIA occurred in northern Quebec. Precipi- summer temperature reconstruction (Fig. 3), where the tation changes are out of phase between eastern and period of maximum temperatures began when alder western Canada, as had been observed in the northern pollen (characteristic of the forest–tundra) decreased (Gajewski 1988). This out-of-phase rela- and ended when spruce pollen (characteristic of lichen tion is seen in the overall postglacial curves as well as woodland) began to decrease (Gajewski et al. 1993, during the most recent 2000 yr (Fig. 6). In general, the 1996; Gajewski and Garralla 1992; Leitner and Gajew- pollen changes on century to millennial scales represent ski 2004). Interpretation of the tree line in Labrador is changes in the relative abundance of taxa rather than more difficult because of the presence of both altitudi- replacement of one pollen type by another. Thus, these nal and latitudinal gradients, but the cooling tempera- are responses of the pollen production in the already tures reconstructed for the past 3 ka corresponds to an established vegetation patterns rather than migration of increase in tundra vegetation (Lamb 1985) taxa from one region to another. Site density is lower in central Canada, so more work Contrary to the results presented by Smol et al. is needed here to better quantify the Holocene climates (2005), and also reported in ACIA (2005, p. 370), there of the region. Further, many records are short, and we is no evidence of ‘‘climate stability’’ in Quebec/ have less confidence in the reconstructions prior to 9 Labrador during the past 2000 yr. The assertion of sta- ka. The maximum in temperature and precipitation bility was apparently based on several short diatom during the mid-Holocene is more pronounced here records, but these pollen-based paleoclimate estimates than in eastern Canada, as expected in the central part confirm climate and ecosystem variability at several of a continent. In this region, a movement northward of scales, as was indicated by the analysis of the pollen tree line has been documented between 6 and 4 ka records (Gajewski 2000). Our interpolated pollen (Moser and MacDonald 1990; MacDonald and Gajew- records do not, however, have the resolution to deter- ski 1992; Gajewski and MacDonald 2004). South of the mine the recent warming impacts on the vegetation. tree line, increases in the relative proportions of alder Although the Little Ice Age has long been known to and spruce pollen at the expense of shrub birch are not be a global event (Matthews and Briffa 2005), there are as pronounced, but are still observable (e.g., Mac- still questions about the nature of the Medieval Warm Donald 1987a). Period (Crowley and Lowery 2000). Our results indi- In western Canada (the Mackenzie region), maxi- cate that across northern Canada, it was relatively mum temperatures are reconstructed between approxi- warm at this time. Work is needed to develop high- mately 10 and 6 ka. During this time, the tree line was resolution multiproxy-based reconstructions to better located farther north than today in the Mackenzie delta specify the nature of this time period. region, and increases in several pollen taxa indicated Our results show that at no time during the Holocene warmer temperatures (Ritchie et al. 1983; Spear 1993). have millennial-scale temperature variations exceeded These changes have been associated with orbital warm- 10.78C in boreal Canada (Table 1). These results there- ing and atmospheric circulation changes caused by the fore show that presently observed temperature in- Laurentide ice sheet to the east (Ritchie et al. 1983). creases in northern Canada far exceed natural variabil-

Unauthenticated | Downloaded 09/25/21 02:55 PM UTC 326 JOURNAL OF CLIMATE VOLUME 22 ity found in this study (Solomon et al. 2007), providing Canadian Foundation for Climate and Atmospheric paleoclimatic support for human cause of the present- Sciences (CFCAS). We gratefully acknowledge the day global warming. contributors to the North American Pollen Database (NAPD) and North American Modern Pollen Data- Acknowledgments. This study is a contribution of the base (NAMPD). We thank an anonymous reviewer for Stability Network (PCSN) funded by the comments and suggestions. APPENDIX Sites Used in This Study

Site ID Site name Lat (8) Lon (8) Elev References 3 Alexander Lake 53.33 260.58 143 Jordan (1975) 4 Alexis Lake 52.52 257.03 200 Lamb (1978) 6 Aliuk Pond 54.58 257.37 25 Jordan (1975) 11 Attawapiskat Lake 53.00 285.17 100 Terasmae (1969) 20 Bereziuk 54.05 276.12 205 Richard (1979) 29 Border Beacon 55.33 263.20 470 Lamb (1985) 31 Boundary Lake 55.25 267.40 525 Stravers (1981) 41 Caribou Hill 55.67 263.25 475 Lamb (1985) 45 Chism 1 54.80 276.15 340 Richard (1979) 46 Chism 2 53.09 276.34 273 Richard (1979) 48 Churchill Falls North 53.60 264.32 404 Morrison (1970) 49 Churchill Falls South 53.58 264.30 404 Morrison (1970) 51 Clearwater Lake 50.87 2107.93 686 Mott (1973) 56 Coppermine Saddleback 67.83 2115.32 43 Nichols (1975) 65 Cycloid Lake 55.27 2105.27 369 Mott (1973) 67 Daumont 54.88 269.40 607 Richard et al. (1982) 69 Delorme 2 54.42 269.93 538 Richard et al. (1982) 73 Diana 375 60.99 269.96 114 Richard (1981) 80 Eagle Lake 53.23 258.55 400 Lamb (1980) 83 Ennadai Lake, 1972 site 61.24 2100.95 325 Nichols (1975) 84 Ennadai Lake 61.17 2100.92 168 Bender et al. (1965) 96 Grand Rapids 53.00 298.25 350 Ritchie and Hadden (1975) 98 Gravel Ridge 55.03 262.63 565 Lamb (1985) 106 Hebron Lake 58.20 263.03 170 Lamb (1984) 109 Hopedale Pond 55.47 260.28 76 Short and Nichols (1977) 119 Kanaaupscow 54.03 276.64 200 Richard (1979) 124 Kogaluk Plateau Lake 56.07 263.75 535 Short and Nichols (1977) 128 Lac Hamard 54.80 267.50 564 Stravers (1981) 135 Lake A 53.24 2105.73 440 Mott (1973) 136 Lake B 53.80 2106.08 553 Mott (1973) 148 Lake Hope Simpson 52.45 256.43 295 Engstrom and Hansen (1985) 152 Long Lake 62.63 2101.23 259 Kay (1979) 155 Lynn Lake 56.84 2101.04 340 Bender et al. (1966) 173 Moraine Lake 52.27 258.05 385 Engstrom and Hansen (1985) 177 Nain Pond 56.53 261.82 92 Short and Nichols (1977) 179 Nedlouc 57.65 271.65 330 Richard (1981) 181 Nicol Lake 61.58 2103.48 348 Kay (1979) 188 Northwest River Pond 53.52 260.17 29 Jordan (1975) 193 Paradise Lake 53.05 257.75 180 Lamb (1980) 210 Pyramid Hills Lake 57.63 265.17 381 Short and Nichols (1977) 216 E Lake 50.72 299.65 724 Ritchie (1969) 217 R Lake (Ontario) 54.31 284.56 147 McAndrews et al. (1982) 218 Rivie`re-aux-Feuilles 1 58.23 272.07 205 Richard (1981) 219 Lac des Roches Moutonne´es 56.77 264.82 410 McAndrews and Samson (1977) 229 Sandy Cove Pond 54.40 257.72 100 Jordan (1975) 235 ‘‘LD’’ Lake 50.14 267.13 122 Mott (1976) 244 Slow River 63.03 2100.75 265 Kay (1979) 245 Snow Lake 56.63 263.88 522 Lamb (1982) 246 Sona Lake West 53.58 263.95 436 Morrison (1970) 251 Saint John Island Pond 53.95 258.92 137 Jordan (1975)

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Site ID Site name Lat (8) Lon (8) Elev References 267 Track Lake 55.77 265.17 442 Short and Nichols (1977) 269 Tunturi Lake 55.02 267.50 610 Stravers (1981) 270 Ublik Pond 57.38 262.05 122 Short and Nichols (1977) 282 Whitney’s Gulch 51.52 257.30 98 Lamb (1978) 295 Honeymoon Pond 64.63 2138.40 1160 Cwynar and Spear (1991) 383 Delorme 1 54.42 269.92 513 Richard et al. (1982) 384 Brisay 2 54.36 270.36 595 Richard et al. (1982) 425 Baie du Diana 60.78 269.83 50 Richard (1977) 427 Cristal Lake 52.12 290.08 355 Steventon and Kutzbach (1983) 451 Wild Spear Lake 59.25 2114.15 880 MacDonald (1987b) 452 Lac Demain 62.05 2118.70 745 MacDonald (1987b) 453 Lac Me´le`ze 65.22 2126.12 650 MacDonald (1987b) 454 Snowshoe Lake 57.45 2120.67 900 MacDonald (1987a) 455 Lone Lake 56.72 2119.72 1100 MacDonald et al. (1991) 458 Toboggan Lake 50.77 2114.60 1480 MacDonald et al. (1991) 459 Lac Ciel Blanc 59.52 2122.17 651 MacDonald et al. (1991) 463 Eaglenest Lake 57.77 2112.10 725 Vance (1986) 475 Lake BI2 57.12 276.38 210 Gajewski and Garralla (1992) 476 Lake LT1 58.14 275.15 150 Gajewski and Garralla (1992) 477 Lake LR3 58.58 275.25 160 Gajewski and Garralla (1992) 487 Lac Faribault 58.87 271.72 250 Richard (1981) 495 Rivie`re-aux-Feuilles 2 58.22 271.95 225 Richard (1981) 497 Tourbie`re de la 58.23 272.07 200 Richard (1981) Rivie`re-aux-Feuilles 529 Queens Lake 64.12 2110.57 480 Moser and MacDonald (1990) 530 McMaster Lake 64.13 2110.58 480 Moser and MacDonald (1990) 531 Yesterday Lake 56.77 2119.48 1050 MacDonald (1984) 534 Lake EC1 56.28 275.10 250 Gajewski et al. (1993) 535 Lake GB2 56.10 275.28 300 Gajewski et al. (1993) 536 Lake LR1 58.58 275.25 170 Gajewski et al. (1993) 584 Wilcox Pass 52.24 2117.22 2355 Beaudoin and King (1990) 586 Lake LB1 57.92 275.62 200 Gajewski et al. (1993) 806 Mordsger Lake 51.38 294.25 400 J. H. McAndrews (1986, personal communication) 807 Beach Lake 65.22 2127.05 Rowe et al. (1975) 848 Pinecrest Lake 50.50 2121.50 320 Mathewes and Rouse (1975) 854 Compass Pond 50.03 256.20 236 Dyer (1986) 871 Lake 31 67.05 250.47 115 Eisner et al. (1995) 972 L’Anse aux Meadows Road 51.60 255.53 0 Davis (1980) Cut Bog 974 Ra Lake 65.23 2126.42 330 MacDonald (1987a) 1073 Lac Patricia 56.67 264.68 538 Samson (1983) 1080 Twin Lakes (British 50.75 2116.33 1100 Hazell (1979) Columbia) 1140 Natla Bog 63.02 2128.80 1380 MacDonald (1983) 1152 Crowfoot Lake 51.65 2116.42 1940 Osborn et al. (1995) 1259 Hanging Lake 68.38 2138.38 500 Cwynar (1982) 1281 Lake O’Hara 51.36 2116.35 2015 Reasoner and Hickman (1989) 1282 Opabin Lake 51.34 2116.31 2280 Reasoner and Hickman (1989) 1332 Louise Pond 53.42 2131.75 650 Pellatt and Mathewes (1994) 1345 SC1 Pond 54.42 2131.91 550 Pellatt and Mathewes (1997) 1346 Shangri-La Bog 53.27 2132.41 595 Pellatt and Mathewes (1997) 1450 Andy Lake 64.65 2128.08 1360 Szeicz et al. (1995) 1451 Keele Lake 64.17 2127.62 1150 Szeicz et al. (1995) 1452 Bell’s Lake 65.02 2127.48 580 Szeicz et al. (1995) 1549 Maria Lake 68.27 2133.47 105 Ritchie (1977) 1550 Sweet Little Lake 67.65 2132.02 Ritchie (1984) 1551 Tuktoyaktuk 5 69.05 2133.45 60 Ritchie and Hare (1971) 1552 Lateral Pond 65.95 2135.93 615 Ritchie (1982) 2300 Tonquin Creek 52.73 2118.37 1935 Kearney (1981) 2301 Watchtower Basin 52.78 2117.08 Kearney (1981)

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Site ID Site name Lat (8) Lon (8) Elev References 2302 Maligne Lake 52.73 2117.62 1675 Kearney (1981) 2304 Excelsior Basin 52.78 2117.12 Kearney (1981) 3170 Lily Lake () 59.20 2135.40 230 Cwynar (1990) 3179 Candelabra Lake 61.68 2130.65 1040 Cwynar (1995) 3180 Hail Lake 60.03 2129.02 690 Cwynar (1995) 3500 Sleet Lake 69.28 2133.58 Spear (1993) 3501 Lake 69.12 2132.17 Spear (1993)

REFERENCES ——, and S. Garralla, 1992: Holocene vegetation histories from three sites in the tundra of northwestern Quebec, Canada. ACIA, 2005: Arctic Climate Impact Assessment. Cambridge Uni- Arct. Alp. Res., 24, 329–336. versity Press, 1042 pp. ——, and G. M. MacDonald, 2004: Palynology of North American Anderson, P., P. Bartlein, L. B. Brubaker, K. Gajewski, and J. C. Arctic Lakes. Long-Term Environmental Change in Arctic Ritchie, 1989: Modern analogues of late-Quaternary pollen and Lakes, R. Pienitz, M. S. V. Douglas, and J. P. J. Bio- spectra from the western interior of North America. Smol, Eds., Springer, 89–116. geogr., 16, 573–596. ——, S. Payette, and J. C. Ritchie, 1993: Holocene vegetation Beaudoin, A. B., and R. H. King, 1990: Late Quaternary vegeta- history at the boreal-forest–shrub-tundra transition in north- tion history of Wilcox Pass, , . western Que´bec. J. Ecol., 81, 433–443. Paleogeogr. Paleoclimatol. Paleoecol., 80, 129–144. ——, S. Garralla, and V. Milot-Roy, 1996: Postglacial vegetation Bender, M. M., R. A. Bryson, and D. A. Baerreis, 1965: Univer- at the northern limit of lichen woodland in northwestern sity of Wisconsin radiocarbon dates I. Radiocarbon, 7, 399– ´ Ge´ogr. Phys. Quat., 407. Quebec. 50, 341–350. ——, ——, and ——, 1966: University of Wisconsin radiocarbon ——, and Coauthors, 2000: The climate of North America and dates II. Radiocarbon, 8, 522–533. adjacent waters ca. 6 ka. Can. J. Sci., 37, 661–681. COHMAP Members, 1988: Climatic changes of the last 18,000 ——, A. E. Viau, M. C. Sawada, D. E. Atkinson, and P. Fines, years: Observations and model simulations. Science, 241, 2006: Synchronicity in climate and vegetation transitions be- 1043–1052. tween Europe and North America during the Holocene. Cli- Crowley, T. J., and T. S. Lowery, 2000: How warm was the Me- matic Change, 78, 341–361, doi:10.1007/s10584-006-9048-z. dieval warm period? Ambio, 29, 51–54. ——, ——, and ——, 2007: Millennial-scale climate variability in Cwynar, L. C., 1982: A late-Quaternary vegetation history from the Holocene—The terrestrial record. Climate Variability Hanging Lake, northern Yukon. Ecol. Monogr., 52, 1–24. and Change: Past, Present and Future: John E. Kutzbach ——, 1990: A late Quaternary vegetation history from Lily Lake, Symp., Madison, WI, University of Wiscosnin—Madison, Chilkat Peninsula, southeast Alaska. Can. J. Bot., 68, 1106– 133–154. 1112. Grimm, E. C., cited 2000: North American pollen database. WDC ——, 1995: Paleovegetation and paleoclimatic changes in the for Paleoclimatology, NCDC. [Available online at http://www. Yukon at 6 ka BP. Ge´ogr. Phys. Quat., 49, 29–35. ncdc.noaa.gov/paleo/napd.html.] ——, and R. W. Spear, 1991: Reversion of forest to tundra in the ——, G. L. Jacobson, W. A. Watts, B. C. S. Hansen, and K. central Yukon. Ecology, 72, 202–212. Maasch, 1993: A 50 000 year record of climate oscillations Davis, A. M., 1980: Modern pollen spectra from the tundra-boreal from Florida and its temporal correlation with the Heinrich forest transition in northern , Canada. Boreas, events. Science, 261, 198–200. 9, 89–100. Hazell, S., 1979: Late Quaternary vegetation and climate of the Dyer, A.K., 1986: A palynological investigation of the Late Qua- Dunbar Valley, British Columbia. M.S. thesis, Department of ternary vegetational history of the Baie Verte Peninsula, Botany, University of , 101 pp. Northcentral Newfoundland. M.S. thesis, Department of Ge- Jordan, R. H., 1975: Pollen diagrams from Hamilton Inlet, central ography, Memorial University of Newfoundland, 221 pp. Labrador, and their environmental implications for the Eisner, W. R., T. E. To¨ rnqvist, E. A. Koster, O. Bennike, and J. F. northern maritime Archaic. Arct. Anthropol., 12, 92–116. N. van Leeuwen, 1995: Paleoecological studies of a Holocene Kay, P. A., 1979: Multivariate statistical estimates of Holocene lacustrine record from the Kangerlussuaq (Søndre Strømf- vegetation and climate change, forest-tundra transition zone, jord) region of West Greenland. Quat. Res., 43, 55–66. NWT, Canada. Quat. Res., 11, 125–140. Engstrom, D. R., and B. C. S. Hansen, 1985: Postglacial vegeta- Kearney, M. S., 1981: Late Quaternary vegetational and environ- tional change and soil development in southeastern Labrador mental history of Jasper National Park, Alberta. Ph.D. dis- as inferred from pollen and chemical stratigraphy. Can. J. sertation. University of Western Ontario, 318 pp. Bot., 63, 543–561. Kerwin, M. W., J. T. Overpeck, R. S. Webb, and K. H. Anderson, Gajewski, K., 1987: Climatic impacts on the vegetation of eastern 2004: Pollen-based summer temperature reconstructions for North America for the past 2,000 years. Vegetatio, 68, 179– the eastern Canadian boreal forest, , and Arctic. 190. Quat. Sci. Rev., 23, 1901–1924. ——, 1988: Late-Holocene climates of eastern North America Lamb, H. F., 1978: Post-glacial vegetation change in southeastern estimated from pollen data. Quat. Res., 29, 255–262. Labrador. M.S. thesis, Department of Ecology, University of ——, 2000: Climate of the last millennium in eastern North Minnesota, Minneapolis, 101 pp. America and impacts on the vegetation. Pr. Geogr., 107, 101– ——, 1980: Late Quaternary vegetational history of southeastern 104. Labrador. Arct. Alp. Res., 12, 117–135.

Unauthenticated | Downloaded 09/25/21 02:55 PM UTC 15 JANUARY 2009 V I A U A N D G A J E W S K I 329

——, 1982: Late Quaternary vegetational history of the forest- ——, and ——, 1997: Holocene tree line and climate change on tundra ecotone in North-Central Labrador. Ph.D. disserta- the Queen Charlotte Islands, Canada. Quat. Res., 48, 88–99. tion, University of Cambridge. Reasoner, M. A., and M. Hickman, 1989: Late Quaternary envi- ——, 1984: Modern pollen spectra from Labrador and their use in ronmental change in the Lake O’Hara region, Yoho National reconstructing Holocene vegetational history. J. Ecol., 72, Park, British Columbia. Paleogeogr. Paleoclimatol. Paleo- 37–59. ecol., 72, 291–316. ——, 1985: Palynological evidence for postglacial change in the Richard, P. J. H., 1977: Histoire post-wisconsinienne de la ve´ge´- position of tree limit in Labrador. Ecol. Monogr., 55, 241– tation du Que´bec me´ridional par l’analyse pollinique. Service 258. de la Recherche, Direction Ge´ne´rale des Foreˆts, Ministe`re Leitner, R., and K. Gajewski, 2004: Modern and Holocene sto- des Terres et Foreˆts du Que´bec, Publications et Rapports mate records of tree-line variations in northwestern Quebec. Divers, Vol. 1, xxiv1312 pp.; Vol. 2, 142 pp. Can. J. Bot., 82, 726–734. ——, 1979: Contribution a` l’histoire postglaciaire de la ve´ge´tation MacDonald, G. M., 1983: Holocene vegetation history of the up- au nord-est de la Jame´sie, Nouveau-Que´bec. Ge´ogr. Phys. per Natla River area, , Canada. Arct. Quat., 33, 93–112. Alp. Res., 15, 169–180. ——, 1981: Pale´ophytoge´ographie postglaciaire en Ungava par ——, 1987a: Postglacial development of the subalpine-boreal l’analyse pollinique. Collection Pale´o-Que´bec, No. 13, 154 transition forest of western Canada. J. Ecol., 75, 303–320. pp. ——, 1987b: Postglacial vegetation history of the ——, A. C. Larouche, and M. A. Bouchard, 1982: Age de la de´gla- Basin. Quat. Res., 28, 245–262. ciation finale et histoire postglaciaire de la ve´ge´tation dans la ——, and K. Gajewski, 1992: The northern treeline of Canada. partie centrale du Nouveau-Que´bec. Ge´ogr. Phys. Quat., 36, Geographical Snapshots of North America, D. G. Janelle, 63–90. Ed., Guilford Press, 34–37. Ritchie, J. C., 1969: Absolute pollen frequencies and carbon-14 ——, R. P. Beukens, and W. E. Kieser, 1991: Radiocarbon dating age of a section of Holocene lake sediment from the Riding of limnic sediments: A comparative analysis and discussion. Mountain area of . Can. J. Bot., 47, 1345–1349. Ecology, 72, 1150–1155. ——, 1977: The modern and late Quaternary vegetation of the Mathewes, R. W., and G. E. Rouse, 1975: Palynology and paleo- Campbell-Dolomite Uplands, near , N.W.T. Canada. ecology of postglacial sediments from the lower Ecol. Monogr., 47, 401–423. canyon of British Columbia. Can. J. Earth Sci., 12, 745–756. ——, 1982: The modern and Late-Quaternary vegetation of the Matthews, J. A., and K. R. Briffa, 2005: The ‘‘Little Ice Age’’: Re- Doll Creek Area, North Yukon, Canada. New Phytol., 90, evaluation of an evolving concept. Geogr. Ann., 87A, 17–36. 563–603. McAndrews, J. H., and G. Samson, 1977: Analyse pollinique et ——, 1984: A Holocene pollen record of boreal forest history implications arche´ologiques et ge´omorphologiques, lac de la from the Travaillant Lake area, Lower Mackenzie River Ba- Can. J. Bot., Hutte Sauvage (Mushuau Nipi), Nouveau-Que´bec. Ge´ogr. sin. 62, 1385–1392. ——, and F. K. Hare, 1971: Late-Quaternary vegetation and cli- Phys. Quat., 31, 177–183. mate near the arctic tree line of northwestern North ——, J. L. Riley, and A. M. Davis, 1982: Vegetation history of the America. Quat. Res., 1, 331–342. Lowland: A postglacial pollen diagram from the ——, and K. A. Hadden, 1975: Pollen stratigraphy of Holocene Sutton Ridge. Nat. Can., 109, 597–608. sediments from the Grand Rapids area, Manitoba, Canada. Morrison, A., 1970: Pollen diagrams from interior Labrador. Can. Rev. Paleobot. Palyn., 19, 193–202. J. Bot., 48, 1957–1975. Moser, K. A., and G. M. MacDonald, 1990: Holocene vegetation ——, L. C. Cwynar, and R. W. Spear, 1983: Evidence from north- change at treeline north of , Northwest Territo- west Canada for an early Holocene Milankovitch thermal ries, Canada. Quat. Res., 34, 227–239. maximum. Nature, 305, 126–128. Mott, R. J., 1973: Palynological studies in central : Rowe, J. S., D. Spittlehouse, E. Johnson, and M. Jasieniuk, 1975: Pollen stratigraphy from lake sediment sequences. Geologi- Fire studies in the Upper Mackenzie Valley and adjacent cal Survey of Canada Paper 72-49, 18 pp. precambrian uplands. Department of Indian and Northern ——, 1976: A Holocene pollen profile from the Sept-Iles area, Affairs, Ottawa, INA Publication QS-8045-000-EE-A1, 128 pp. Que´bec. Nat. Can., 103, 457–467. Samson, G., 1983: Prehistoire du Mushuau Nipi, Nouveau- Nichols, H., 1975: Palynological and paleoclimatic study of the Quebec: Etude du mode d’adaptation a l’interieur des terres late Quaternary displacements of the boreal forest-tundra hemi-arctiques. Ph.D. dissertation, University of Toronto, ecotone in Keewatin and Mackenzie, N.W.T., Canada. Insti- 810 pp. tute of Arctic and Alpine Research, University of Colorado, Sawada, M., K. Gajewski, A. deVernal, and P. Richard, 1999: Occasional Paper 15, 87 pp. Comparison of marine and terrestrial Holocene climates in Osborn, G., C. Clapperton, P. T. Davis, M. A. Reasoner, D. T. eastern North America. Holocene, 9, 267–278. Rodbell, G. O. Seltzer, and G. Zielinski, 1995: Potential gla- ——, A. E. Viau, G. Vettoretti, W. R. Peltier, and K. Gajewski, cial evidence for the Younger Dryas event in the cordillera of 2004: Comparison of North American pollen-based tempera- North and . Quat. Sci. Rev., 14, 823–832. ture and global lake-status with CCCma AGCM output at 6 Overpeck, J. T., T. Webb III, and I. C. Prentice, 1985: Quantitative ka. Quat. Sci. Rev., 23, 225–244. interpretation of fossil pollen spectra: Dissimilarity co- Short, S. K., and H. Nichols, 1977: Holocene pollen diagrams from efficients and the method of modern analogs. Quat. Res., 23, subarctic Labrador-Ungava: Vegetational history and cli- 87–108. matic change. Arct. Alp. Res., 9, 265–290. Pellatt, M. G., and R. W. Mathewes, 1994: Paleoecology of post- Shuman, B., P. Bartlein, and T. Webb III, 2007: Response to glacial tree line fluctuations on the Queen Charlotte Islands, ‘‘Comments on: ‘The magnitude of millennial- and orbital- Canada. Ecoscience, 1, 71–81. scale climatic change in eastern North America during the

Unauthenticated | Downloaded 09/25/21 02:55 PM UTC 330 JOURNAL OF CLIMATE VOLUME 22

Late-Quaternary’ by Shuman et al.’’ Quat. Sci. Rev., 26, 268– ——, ——, P. Fines, D. E. Atkinson, and M. C. Sawada, 2002: 273. Widespread evidence of 1500 yr climate variability in North Smol, J. P., and Coauthors, 2005: Climate-driven regime shifts in America during the past 14 000 yr. Geology, 30, 455–458. the biological communities of arctic lakes. Proc. Natl. Acad. ——, ——, M. C. Sawada, and P. Fines, 2006: Millennial-scale Sci. USA, 102, 4397–4402. temperature variations in North America during the Ho- Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. locene. J. Geophys. Res., 111, D09102, doi:10.1029/ Averyt, M. Tignor, and H. L. Miller, Eds., 2007: Climate 2005JD006031. Change 2007: The Physical Science Basis. Cambridge Univer- ——, ——, ——, and J. Bunbury, 2008: Low-and-high-frequency sity Press, 996 pp. climate variability in Eastern during the past 25,000 Spear, R. W., 1993: The palynological record of Late-Quaternary years. Can. J. Earth Sci., in press. arctic tree-line in northwest Canada. Rev. Paleobot. Palyn., Waelbrock, C., L. Labeyrie, J.-C. Duplessy, J. Guiot, M. Labrach- 79, 99–111. erie, H. Leclair, and J. Duprat, 1998: Improving past sur- Steventon, R. L., and J. E. Kutzbach, 1983: University of Wiscon- face temperature estimates based on planktonic fossil faunas. sin radiocarbon dates XX. Radiocarbon, 25, 152–168. Paleoceanography, 13, 272–283. Stravers, L. K. S., 1981: Palynology and deglaciation history of the Webb, T., III, 1986: Is vegetation in equilibrium with climate? central Labrador-Ungava Peninsula. M.S. thesis, Department How to interpret late-Quaternary pollen data. Vegetatio, 67, of Geology, University of Colorado, 171 pp. 75–91. Stuiver, M., and Coauthors, 1998: INTCAL98 radiocarbon age calibration, 24000-0 cal BP. Radiocarbon, 40, 1041–1083. Whitmore, J., and Coauthors, 2005: A North American modern Szeicz, J. M., G. M. MacDonald, and A. Duk-Rodkin, 1995: Late pollen database for multi-scale paleoecological and paleocli- Quaternary vegetation history of the central Mackenzie matic applications. Quat. Sci. Rev., 24, 1828–1848. Mountains, Northwest Territories, Canada. Paleogeogr. Pa- Williams, J. W., and B. Shuman, 2008: Obtaining accurate and leoclimatol. Paleoecol., 113, 351–371. precise environmental reconstructions from the modern ana- Terasmae, J., 1969: A discussion of deglaciation and the boreal log technique and North American surface pollen dataset. forest in the northern region. Proc. Entomol. Quat. Sci. Rev., 27, 669–687. Soc. Ont., 99, 31–43. ——, D. M. Post, L. C. Cwynar, A. F. Lotter, and A. J. Levesque, Vance, R. E., 1986: Pollen stratigraphy of Eaglenest Lake, north- 2002: Rapid and widespread vegetation responses to past cli- eastern Alberta. Can. J. Earth Sci., 23, 11–20. mate change in the North Atlantic region. Geology, 30, 971– Viau, A. E., and K. Gajewski, 2007: Comments on ‘‘The magni- 974. tudes of millennial- and orbital-scale climatic change in east- Wright,H.E.,J.E.Kutzbach,T.WebbIII,W.F.Ruddiman, ern North America during the Late Quaternary’’ by Shuman F. A. Street-Perrott, and P. J. Bartlein, Eds., 1993: Global et al. 2005. Quat.Sci.Rev.,26, 264–267, doi:10.1016/ Climates since the . University of Min- j.quascirev.2006.07.010. nesota Press, 569 pp.

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