The timing and nature of Late Quaternary vegetation ...yansa/Yansa 2006-QSR spruce.pdf · The timing and nature of Late Quaternary vegetation changes in the northern Great Plains,

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Quaternary Science Reviews 25 (2006) 263–281

The timing and nature of Late Quaternary vegetation changesin the northern Great Plains, USA and Canada:

a re-assessment of the spruce phase

Catherine H. Yansa�

Department of Geography, Michigan State University, East Lansing, MI 48824, USA

Received 17 September 2004; accepted 21 February 2005

Abstract

This paper revises the chronology for the northward migration of Picea glauca (white spruce) across the northern Great Plains,

following the recession of the Laurentide Ice Sheet, and reinterprets the species composition and structure of the late-glacial

vegetation on the basis of pollen and plant-macrofossil analysis. The timing of spruce migration is based on 26 14C ages obtained

from Picea macrofossils. The date for the appearance of white spruce in southern South Dakota, USA, remains unchanged, 12,60014C yrBP (ca 15,000 cal yr BP), but its arrival in southern Saskatchewan, Canada, by 10,300 14C yr BP (ca 12,100 cal yr BP) is about

1500 years later than previously estimated based on an organic sediment date. Picea glauca thus migrated northwards at an average

rate of 0.38 km/14C year (0.30 km/calendar year), significantly slower than the previously published rate of 2 km/14C year. White

spruce trees probably inhabited lake shorelines, whereas prairie, parkland, and boreal plants occupied both lowlands and uplands,

forming an open white spruce parkland. This interpretation differs from a previous reconstruction of a boreal-type spruce forest and

thus offers another paleoclimatic interpretation. Precipitation was probably low and summer temperatures relatively mild, averaging

about 19 1C.

r 2005 Elsevier Ltd. All rights reserved.

1. Introduction

Picea glauca (white spruce) is today an importantconstituent of the boreal forest biome in northern NorthAmerica. The southern range limit of this taxon atpresent follows the mean summer isotherm of 18 1C inCanada (Fig. 1), where the precipitation is greater than39 cm/year (Ritchie and Harrison, 1993). White sprucemay also be found at lower latitudes in high-elevationsettings, which have cool and humid climates similar tothat of the boreal forest. Numerous reports of sprucefossil deposits throughout North America indicate thatPicea glauca occupied areas farther south during full-glacial Wisconsinan time (21,000–14,000 14C yrBP,equivalent to 24,900–16,800 cal yrBP) (e.g., Webb, 1987;Jacobson et al., 1987). Populations close beyond the ice

e front matter r 2005 Elsevier Ltd. All rights reserved.

ascirev.2005.02.008

353 3910; fax: +1 517 432 1671.

ess: [email protected].

front were the probable seed sources for the spruce thatcolonized recently deglaciated terrain, following thenorthward recession of the Laurentide Ice Sheet (Bakerand Waln, 1985; Wright, 1987). The late-glacial north-ward migration of white spruce to form what laterbecame the boreal forest biome in Canada has been atopic of research by paleoecologists for decades (e.g.,Wright, 1968; Ritchie and MacDonald, 1986; Ritchie,1987; McLeod and MacDonald, 1997; MacDonald,2002).

The last survey of Late-Pleistocene white sprucemigration across the northern Great Plains was pro-vided by Ritchie and MacDonald (1986), who summar-ized previously published 14C chronologies. Accordingto their scenario, white spruce colonized southernmostSouth Dakota, USA, by 12,600 14C yrBP (ca 15,000cal yr BP) (Watts and Wright, 1966), arrived in southernSaskatchewan, Canada, by about 11,700 14C yr BP(ca 13,600 cal yr BP) (Ritchie and DeVries, 1964), and

ARTICLE IN PRESS

14,15

MONTANADAKOTANORTH

Missouri Coteau

Prairie Coteau

SWPicea glauca (modern range)

BH

BH Black Hills

CH Cypress Hills

SW Spruce Woods

CH

MINNESOTA

DAKOTASOUTH

MANITOBA

SASKATCHEWAN

ALBERTA

Missouri

River

Areashown

km

0 100 200

Lake Agassiz(maximum extent)

2

34

5

7

9

71

91

1110

0

12

16

18

1

6

8

13

2

21 222

Fig. 1. Locations of Late Pleistocene spruce fossil localities on the northern Great Plains in relation to physiography and the modern distributions of

Picea glauca in the boreal forest, Black Hills, Cypress Hills, and Spruce Woods. See Table 1 for information about these sites (#1–22).

C.H. Yansa / Quaternary Science Reviews 25 (2006) 263–281264

reached what is today the southern limit of the borealforest in Manitoba, Canada, by 11,500 14C yr BP(ca 13,600 cal yr BP) (Ritchie, 1969). Ritchie and Mac-Donald’s (1986) chronology, which is based on 14C agesobtained from terrestrial plant macrofossils as well asorganic sediments, has been perpetuated in the literature(e.g., Baker and Waln, 1985; Ritchie, 1987; Barnoskyet al., 1987; Grimm, 2001; MacDonald, 2002; Webbet al., 2004). Organic sediments are well known toprovide erroneously older 14C ages (e.g., MacDonaldet al., 1991), so the chronology presented in this paper isbased solely on 14C ages obtained from terrestrial plantmacrofossils. This revised chronology dates the arrivalof white spruce in southern Saskatchewan about 1500calendar years later than previously thought. This paperalso provides a more realistic estimate of the possiblespeed of future white spruce migration northward as aconsequence of global warming, baring habitat frag-mentation and other anthropogenic factors.

This paper also presents the results of recentinvestigations of several pollen and plant-macrofossillocalities in North Dakota, USA, and southern Sas-katchewan, Canada, which provide a re-assessment ofthe species composition and structure of this anomalousLate-Pleistocene vegetation. Also used in this survey arepollen data published by other researchers and 14Cchronologies (based on terrestrial macrofossils) reportedfor late-glacial spruce fossil sites in the glaciatedportions of South Dakota and North Dakota as wellas southern Saskatchewan and southwestern Manitoba.This paleovegetation reconstruction has implications for

other regions of North America and Europe that havesimilar histories of deglaciation and Picea colonization.

2. Study area

The study area is that portion of the northern GreatPlains covered by the Laurentide Ice Sheet during thelast (late-Wisconsinan) glaciation, which can be roughlydefined as the area north and east of the Missouri River(Fig. 1). The topography of the study area is level torolling. The Missouri Coteau (700–800masl) and PrairieCoteau (530–625masl) uplands are hilly (hummockymoraine) with numerous kettle lakes and ponds (Fig. 1)(Clayton and Moran, 1982). To the east and northeastof these uplands, respectively, lies the Glaciated TillPlain (about 330–600masl), which has large shallowlacustrine basins that were once glacial lakes drained bymeltwater channels (Holliday et al., 2002). Farther eastis located the Red River Valley Lowland (about240–320masl), which was formerly occupied by glacialLake Agassiz (Fig. 1; Teller, 2004).

Plant and animal fossils of Late-Pleistocene andHolocene age are commonly found in these kettle lakesand other wetland sites. The regional water level wasvery high during deglaciation due to the melting ofglacial ice, but it lowered as a result of severe Holocenedroughts (Yansa, 1998; Grimm, 2001). Significant eolianand colluvial deposition in lakes and ponds during theHolocene partly in filled these basins, thereby reducingtheir water volumes (Yansa and Basinger, 1999; Grimm,

ARTICLE IN PRESSC.H. Yansa / Quaternary Science Reviews 25 (2006) 263–281 265

2001). Consequently, only a few lakes on the northernGreat Plains are deep enough to bear complete fossilsequences and the rest have become ephemeral wetlandsor saline playas (Barnosky et al., 1987; Grimm, 2001).Yansa (1998) demonstrated that some shallower lakeand pond basins contain well-preserved fossils in anoxicsediments below the water table, providing valuable,although truncated, fossil records dating from the Late-Pleistocene to the mid-Holocene. Intact fossil siteslocated farther south in the unglaciated portion of theGreat Plains are even rarer, because of the prevailingarid climatic regime throughout much of the Holocene.

The modern climate of the northern Great Plains ischaracterized by spatial and temporal variability inprecipitation and temperature, which affect not onlylake levels and chemistries but also vegetation. MeanJuly temperatures range from 24 1C in South Dakota to19 1C in southern Saskatchewan, with the north–southtemperature gradient being greater during the coldwinters (Owenby and Ezell, 1992; Environment Canada,1993). Precipitation grades from about 49 cm in easternNorth Dakota to 30 cm in eastern Montana because ofreduced westward penetration of the humid Gulf ofMexico air mass and a rainshadow effect of the RockyMountains (Bryson et al., 1970; Owenby and Ezell,1992).

This precipitation gradient is reflected in the nativegrassland communities of the northern Great Plains(which have been largely altered by agriculture andlivestock grazing), which grade east-to-west from tall-grass prairie to short-grass prairie. This paper focuseson the area characterized by mixed-grass prairie in theeastern Dakotas and southern Saskatchewan, which iscomposed of a mixture of tall (C3) and short (C4) grassesand forbs. Prairie marsh plants inhabit the numerousshallow wetlands and are adapted to fluctuating waterlevels (Kantrud, 1989). Moist soils along rivers andaround perennial (deep) lakes within the northern GreatPlains support Populus tremuloides (aspen poplar),Quercus macrocarpa (bur oak), and other tree and shrubtaxa more common in the forests to the east.

Stands of Populus and Betula (birch) are moreabundant farther north in the aspen parkland ofsouthern Manitoba, central Saskatchewan, and south-eastern Alberta, where the climate is slightly morehumid and cooler. The aspen parkland belt forms atransitional ecozone between the grassland and southernboreal forest. The tree flora of the southern portion ofthe boreal forest is composed of Picea glauca (whitespruce), Picea mariana (black spruce), Pinus banksiana

(jack pine), Abies balsamea (balsam fir), Larix laricina

(tamarack), Populus balsamifera (balsam poplar), Popu-

lus tremuloides (aspen poplar), and Betula papyrifera

(paper birch). The northern range limit of Fraxinus nigra

(black ash) occurs at the southeastern edge of the borealforest.

The absence of native white spruce stands on thenorthern Great Plains since the Late-Pleistocene hasbeen attributed to insufficient soil moisture and tosummer warmth, both of which inhibit white spruceseedling establishment and growth (Ritchie and Mac-Donald, 1986; MacDonald and Case, 2000). Today, therange of Picea glauca is restricted to the boreal forestand higher elevations of the Black Hills of SouthDakota (41250masl) and Cypress Hills of southernSaskatchewan and Alberta (41200masl) where theclimate is cool and moist compared to the adjacentplains (Fig. 1). One exception is the anomalous ‘‘SpruceWoods’’ population in southeastern Manitoba, wherestands of Picea glauca occupy a deltaic deposit of sand(along the paleoshoreline of glacial Lake Agassiz) withinthe aspen parkland, about 110 km south of the borealforest (Fig. 1).

3. Study design

3.1. Radiocarbon chronologies

The chronologies for several previous paleovegetationreconstructions, including an earlier assessment of thepostglacial spread of white spruce in northern NorthAmerica (i.e., Ritchie and MacDonald, 1986), aresuspected here to be too old, because they are basedon the 14C dating of organic sediments containingradioactively dead carbon. Other researchers havediscussed the problems associated with dating suchsediments (see Barnosky et al., 1987; MacDonald et al.,1991; Grimm and Jacobson, 2004). Radiocarbon agesobtained from organic sediments (including gyttja andmarl) differ from those acquired from the dating ofterrestrial plant macrofossils significantly, 1000–800014C years for paired samples from the same stratigraphiclevels (e.g., Clayton and Moran, 1982; MacDonald etal., 1991; Grimm, 2001).

Consequently, the chronology used in this paper isbased on the 14C dating of wood and spruce needles(Table 1). Several of these ages occur at 14C plateaus,which provide multiple possible calendar ‘‘dates’’ for asingle 14C age (Lowell and Teller, 1994). 14C ages areincluded in this paper along with calibrated datesobtained from the INTCAL98 dataset (Stuiver et al.,1998), so that the former may be recalibrated in thefuture when the calibration curve is further improved.

3.2. Fossil data sources

The dataset assembled here includes pollen percen-tages and plant-macrofossil abundances paired withreliable chronologies, which together document the firstarrival and spread of spruce across the northern GreatPlains of the USA and Canada. The ‘spruce rise,’ a

ARTICLE IN PRESS

Table

1

Radiocarbonandcalendarages

forlate-Pleistocenespruce

fossillocalities

inSouth

Dakota

(SD),NorthDakota

(ND),Saskatchew

an(SK),andManitoba(M

B)

Site#

(see

Fig.

1)

Site/location

14C

AgeyrBP(calyrBPa)

14C-D

atedmaterials

Pollen

study

Plant

macro-

fossil

study

*P

icea

pollen

%

(Fig.2)

References(C

omments)

1Rosebudsite,SD

(431130 N

,1001500 W

)

12,6307160b(14,990);

Pic

eawood(twosamples,

50cm

apart

indepth)

OO

OWattsandWright(1966);NAPD

c

12,5807160b(14,930)

2Strouckel

site,SD

(441400 N

,981580 W

)

12,2507150(15,020or

14,300)

Pic

eawood

Martin

andKlukas(1989)(associatedwith

P.

gla

uca

cones;at14C

plateauwithtw

opossible

calibrateddates)

3CottonwoodL.,SD

(441500 N

,991540 W

)

11,6907180(13,690)

Pic

eaneedles

OO

Barnoskyet

al.(1987);E.Grimm

(unpublished,

available

from

NAPD

c)(spruce

arrival

estimatedat12,300

14Cyrs

BP)

4MedicineL.,SD

(441490 N

,971210 W

)

10,9407135(13,040)

Pic

eawood

ORadleet

al.(1989)(notshownin

Fig.2,because

only

onewood

14C

date

atbase)

5Pickerel

L.,SD

(451300 N

,971190 W

)

12,070740(14,130)

Pic

eaneedles

DeanandSchwalb

(2000)(geochem

istrystudy)

10,6707140(12,700)

Pic

eawood

OO

OWattsandBright(1968);NAPD

c(P

icea

seeds,

possibly

of

P.

gla

uca)

6ColdwaterL.,ND

(461010 N

,991050 W

)

10,790770(12,950)

Pic

eawood

OO

OThisstudy;Yansa

(2002)(P

.g

lau

caseeds

identified)

7Wendel

site,ND

(461250 N

,981200 W

)

11,550765(13,560)

11,550790(13,570)

Core

WE-1:

Pic

eaneedles

&wood(both

samples;

20cm

apart

indepth)

OO

Thisstudy;Yansa

(2002)(P

.g

lau

caseeds

identified)

8Tappen

site,ND

(461460 N

,991350 W

)

11,4807300b(13,550)

Pic

eawood

Moir(1957)(notedpresence

of

P.

gla

uca

cones)

9MoonL.,ND

(461510 N

,981090 W

)

11,830780(13,830)

Woodytw

ig—

Pic

ea?

OO

Lairdet

al.(1996,1998);Grimm

(2001);data

available

from

NAPD

c

10,7807100(12,920)

Pic

eaneedle

10

JohnsLakesite,ND

(471400 N

,1001200 W

)

10,8207150(12,930)

Pic

eawood

AshworthandSchwert(1992)(paleo-

entomologystudy;noted

P.

gla

uca

cones)

11

RiceL.,ND

(481000 N

,1011310 W

)

9,240790(10,450)

Pic

eawood

OLairdet

al.(1996,1998);NAPD

c(endofspruce

phase;pollen

%notin

Fig.2)


ARTICLE IN PRESS12

KettleL.,ND

(481360 N

,1031370 W

)

11,080780(13,090)

Wood-probably

Pic

eaO

Clark

etal.(2002);(pollen

analysisof

Pic

ea

zonenotyet

published)

13

Scrim

bitsite,SK

(491460 N

,1051110 W

)

10,0007250b(11,620)

Pic

eawood(upright

stump)

Dew

(1959)

14

Andrewssite,SK

(501200 N

,1051520 W

)

10,2007140(11,870)

10,200780(12,000)

10,2307140(11,920)

Pic

eawood(trunk)

OThisstudy;Yansa

(1998);Yansa

andBasinger

(1999)(P

.g

lau

cacones

andseedsassociated

withwood;

Pic

eaneedle

litter

associatedwith

theroot.TheAndrewssite

islocated1.6km

northoftheHafichuksite)

Pic

eawood(trunk)

Pic

eawood(root)

15

Hafichuksite,SK

(501200 N

,1051520 W

)

11,6507150b(13,600)

organic

sedim

ent(basal

age)

OO

ODeV

ries

(1963),andRitchieandDeV

ries

(1964)

(see

Figs.1–3)

16

Neufeld

site,SK

(501200 N

,1071430 W

)

10,3007140(12,170)

Pic

eawood(trunk)

OThisstudy;Yansa

(unpublished

data)(P

.g

lau

ca

seedsandcones

identified)

17

Herbertsite,SK

(501230 N

,1071130 W

)

10,0507300b(11,900)

Sa

lix(w

illow)wood

(30cm

above

Pic

eacone)

OKupsch(1960)(pollen

data

andstratigraphic

positionof14C

ageshownin

Fig.3B)

18

Kyle

site,SK

(501530 N

,1071500 W

)

10,300790(12,100)

Pic

eawood(trunk)

OThisstudy;Yansa

(unpublished

data)(P

.g

lau

ca

cones

identified)

19

Clearw

aterL.,SK

(501530 N

,1071540 W

)

9980770(11,500)

Pic

eacf.

P.

gla

uca

needles

OLast

etal.(1998)(latter

part

ofthespruce

phase)

20

Beechysite,SK

(501550 N

,1071400 W

)

10,300790(12,100)

Pic

eawood(trunk)

OThisstudy;Yansa

(unpublished

data)(P

.g

lau

ca

cones

identified)

21

Dinsm

ore

site,SK

(511060 N

,1071290 W

)

10,3007140b(12,170)

Pic

eawood

McC

allum

andWittenberg(1962)

22

RidingMountain

site

(ELake),MB

(501410 N

,991390 W

)

95707130b(10,930)

11,1407200b(13,210)

Pic

eawood

McC

allum

andWittenberg(1962);(w

ood

associatedwith

P.

gla

uca

cones)

Organic

sedim

ent

ORitchie

(1969);(heestimatedthearrivalof

spruce

atca

11,500

14C

yrBPbyextrapolating

downcore

from

thedatedlevel)

See

Fig.1forthelocationsofthesesites.

aAges

werecalibratedwithCALIB

4.4.2

usingtheIntcal98dataset(Stuiver

etal.,1998);calendarages

are

the50%

meanprobabilitybasedonthe2srange.

b14C

ages

notcorrectedford1

3C

fractionation.

cData

from

theNorthAmericanPollen

Database

(NAPD;www.ngdc.noaa.gov/paleo/napd.htm

l).

C.H. Yansa / Quaternary Science Reviews 25 (2006) 263–281 267

http://www.ngdc.noaa.gov/paleo/napd.html

ARTICLE IN PRESSC.H. Yansa / Quaternary Science Reviews 25 (2006) 263–281268

notable upcore increase in Picea pollen values, iscommonly thought to confirm site colonization andpopulation expansion of this taxon (e.g., Davis, 1986;Ritchie and MacDonald, 1986). Plant macrofossils(seeds, buds, leaves, and wood) and stomata also verifythe local presence of taxa (unless there is evidence forredeposition) and most of these permit species-levelidentifications (Birks, 2001).

Pollen and plant-macrofossil data from two sites insoutheastern North Dakota and plant-macrofossilinformation from four sites in southern Saskatchewan(Table 1; Fig. 1) provide the primary data for this paper.The sites in North Dakota include the Wendel site,a shallow wetland situated on the Glaciated TillPlain, and Coldwater Lake, located on the MissouriCoteau upland. The Andrews, Kyle, and Beechy sitesin southern Saskatchewan are also located on theMissouri Coteau, near the northern limit of this belt

PERCENTAGES OF P

14

C A

GE

(yr

B.P

. x 1

000)

10.2

DAKONORT

DAKOTSOUTH

10.3

12

11 1

10.3

10.3

20 40 60

12

11

10

9

8

13

Rosebudsite, SD (# 1)

CottonwoodLake, SD (# 3)

20 40 60

12

11

10

9

8

13

Pickerel Lake,SD (# 5)

20 40 60 8012

11

10

9

8

Moon LaND (# 9)

2012

11

10

9

8

ALBERTA

SASKATCHEWAN

km

0 100 200

Missouri Coteau

Prairie Coteau

Picea glauca (modern range)

122

x 1000 14C yr BP

1441618

68

21

Fig. 2. Minimum ages for the earliest arrival (� 1000 14C yrBP) of Picea gla

pollen percentage curves plotted against 14C age (chronologies based on t

represented in the pollen records. See Table 1 for information about these sit

dates for spruce.

of hummocky moraine, and, like Coldwater Lake, arekettle lakes or ponds. The nearby Neufeld site is locatedalong the edge of a meltwater channel (spillway) thatincised this upland.

Methodologies used to analyze these pollen and plant-macrofossil samples followed standard procedures(i.e., Faegri and Iverson, 1975; Birks, 2001), and thefossils were compared to modern plant materials fortaxonomic identifications. Percentages of Picea pollenfrom the Late-Pleistocene to early Holocene are shownfor the Wendel site and Coldwater Lake in Fig. 2 toillustrate the timing of spruce occupation of thesesites. Counts of a minimum of 400 pollen grains ofupland taxa per sample were used to calculate thesepercentages. The taxonomy and modern habitat infor-mation used in this paper are derived from theGreat Plains Flora Association (1986) and Loomanand Best (1987).

ICEA POLLEN

TAH

MINNESOTA

A

.6

12.33

12.1

11.811.55

10.80 8

ke,

40 60

Wendel site,ND (# 7)

20 40 6012

11

10

9

8

ColdwaterLake, ND (# 6)

20 4011

10

9

8

20 40 6011

10

9

8

Hafichuk site,SK (# 15)

probably redepositedredeposited

MANITOBA

9.6

2

5

9 88

100

22

1

uca at fossil localities (site # inside the tree icon). Also shown are Picea

errestrial plant fossils) for those sites where the spruce phase is well

es, the data used in constructing these pollen % curves, and the arrival


Additional pollen and plant-macrofossil data used inthis paper are derived from studies by other researchers.Diagrams showing percentages of Picea pollen abun-dance for the Rosebud site, Cottonwood Lake, andPickerel Lake in South Dakota and Moon Lake inNorth Dakota (Table 1, Figs. 1 and 2) are based ondata available from the North American Pollen Data-base website (www.ngdc.noaa.gov/paleo/napd.html)and published sources (Watts and Wright, 1966; Wattsand Bright, 1968; Barnosky et al., 1987; Laird et al.,1996, 1998; Grimm, 2001). Also included in Table 1 andFig. 1 are all published reports of 14C ages obtainedfrom spruce macrofossils for the northern Great Plains,for even though they lack any associated study of plantmacrofossils or pollen the wood documents the presenceof spruce at these sites. Those 14C ages considered todate the arrival of spruce in different areas of thenorthern Great Plains are shown in Fig. 2. One sitelocated north of the study area (the Riding Mountainsite in Manitoba) is included in this dataset, because it isthe only fossil record with a basal wood date from what

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

5.2

HAFICHUK SITE

3.2

3.6

3.4

3.8

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

5.8

5800 ± 80

7700 ± 80

8800 ± 140

10,200 ± 140

10,200 ± 90

10,230 ± 140

14C

Age

s (u

ncal

ibra

ted,

obt

aine

d fr

om te

rres

tria

l pla

nt m

acro

foss

ils)

Lithology Depth Picea (% pollen) (m)

Depth Lithology (m)

ANDREWS SITE

20 40 60

ZZ

Z Z

charcoallayer

sandy clay marlsilty clay

gyttyasilty c

(A)

Populus (leaves) ZPicea glauca

(cones)

Fig. 3. (A) stratigraphic and fossil correlation between the Andrews site and t

reliable ages (from the Andrews site) for the pollen record of the Hafichuk si

sediments). Fig. 2 shows this pollen stratigraphy based on the 14C chronology

Saskatchewan (modified from Kupsch (1960)), which correlate with the up

Hafichuk sites.

is now the southern limit of the boreal forest (Table 1,Figs. 1 and 2; McCallum and Wittenberg, 1962; Ritchie,1969).

3.3. Correlation of datasets: The Hafichuk, Andrews, and

Herbert sites

A reliably dated and detailed late-Quaternary pollenstudy (that documents the spruce phase) is lacking forthe Canadian prairie ecoregion. Only one study, that ofthe Herbert site in southern Saskatchewan, has a 14C ageobtained from wood (Fig. 3), but its pollen record isbased on the analysis of only seven samples (Kupsch,1960). The three remaining pollen records from southernSaskatchewan and southwestern Manitoba have chron-ologies based on organic sediment dates. The beststudied of these is the Hafichuk site in southernSaskatchewan (Table 1; Fig. 1), which has been usedas a key site in regional paleovegetation reconstructions(e.g., Ritchie and MacDonald, 1986; Barnosky et al.,1987; MacDonald, 2002). DeVries (1963) and Ritchie

10,270

10,630

11,650

14C A

ges (uncalibrated, obtained from organic sedim

ents)

diamicton(till)lay

Piceaneedle layer

gyttyastony clay

(B)

4.0

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

20 40 6010,050 ± 300

HERBERT SITELithology Depth Picea (% pollen) (m)

14C

Age

(un

calib

rate

d, o

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ned

from

terr

estr

ial p

lant

mac

rofo

ssils

)

Zannichellia palustris (fruits)

Salix (wood)

he nearby Hafichuk site in southern Saskatchewan provide estimates of

te (previously dated by Ritchie and DeVries (1964) using bulk organic

of the Andrews site. (B) Pollen record and 14C age of the Herbert site,

per part (gyttja silty clay) of the stratigraphies of the Andrews and

http://www.ngdc.noaa.gov/paleo/napd.html


and DeVries (1964) interpreted the pollen, plantmacrofossils, and an organic-sediment date from theHafichuk site as indicating the establishment of aboreal-type spruce forest at 11,6507150 14C yrBP (ca13,600 cal yr BP). In contrast, 10 14C ages obtained fromwood and needles from eight nearby sites in southernSaskatchewan (Table 1; Fig. 2) indicate that insteadwhite spruce arrived in southern Saskatchewan by10,300–10,200 14C yrBP (12,100–11,870 cal yr BP) andpersisted until about 9980 14C yrBP (11,500 cal yr BP).

A revised chronology for the pollen record of theHafichuk site, shown in Fig. 2, is presented here basedon lithostratigraphic and fossil correlation with thereliably dated macrofossil record of the nearby Andrewssite (Fig. 3A) for which an available pollen record islacking. Correlation between these sites is robust, forthey are located only 1.6 km apart, share identicaltopographic settings (kettle ponds within the same trackof hummocky moraine), possess similar plant-macro-fossil records, and share several distinctive marker beds(charcoal layers, lithology, fossil beds, etc.) (Fig. 3A).The detailed descriptions by DeVries (1963) and Ritchieand DeVries (1964) of the sediment (MunsellTM color,texture, % organic carbon content) and fossils at 10-cmintervals for the Hafichuk site facilitated this correla-tion. Picea cones and Populus (poplar) leaves wereespecially useful for this correlation, as was the presenceof fruits of Zannichellia palustris (horned pondweed),which indicates increased salinity within these closedbasins (Kantrud, 1989; Yansa, 1998).

High Picea pollen values and abundant needles above2.9m at the Hafichuk site (Ritchie and DeVries, 1964)correlate with the mid-Holocene-age section of theAndrews site (Fig. 3A). These fossils were probablyeroded and redeposited during rainstorms followingdroughts, as have been interpreted for other sprucefossils recovered from mid-Holocene age sediments(Laird et al., 1998; Yansa, 2002). The stratigraphicsection sampled at the Hafichuk site is less complete(5.1m) than that of the Andrews site which extends intodiamicton (5.8m), so exact basal correlation betweenthese two sections is not possible. However, the bottomof the Hafichuk pollen record is unlikely to predate thebasal age of the Andrews site substantially, given theuniform geological setting and lithostratigraphy of thesenearby kettle sites.

This correlation (Fig. 3A) is further strengthened bycomparison with the pollen record of the Herbert site(Fig. 3B), which has one 14C age of 10,0507300 yr BP(11,900 cal yr BP) obtained from Salix (willow) woodpositioned 0.3m above a Picea glauca (white spruce)cone (Kupsch, 1960). The Herbert site pollen record,lithostratigraphy, and 14C date represent the latter partof the spruce phase (Fig. 3B), which roughly correlateswith the upper part of the gyttja silty clay units of boththe Hafichuk and Andrews sites (Fig. 3A). Peak values

of about 60% at the bases of both the Hafichuk andHerbert pollen records when correlated with thechronology of the Andrews site indicate that Picea hadarrived at these sites before 10,230 14C yrBP. AtCottonwood Lake, South Dakota, Picea pollen valuesincreased from about 15% (basal sample) to 60% over300 14C years (Fig. 2), which suggests that sprucearrived in southern Saskatchewan (Kyle, Beechy andNeufeld sites) several centuries before 10,300 14C yr BP(12,100 cal yr BP; Table 1). However, spruce did notappear in southern Saskatchewan as early as11,6507150 14C yrBP, as proposed by Ritchie andDeVries (1964) based on an organic sediment date, sincethis age predates spruce colonization of North Dakota(Fig. 2) and the currently accepted chronology forthe deglaciation of southern Saskatchewan by about11,300 14C yr BP (ca 13,250 cal yr BP) (Clayton andMoran, 1982).

4. White spruce migration, colonization, and extirpation

4.1. Deglaciation and soil development

The Late-Pleistocene was a time of rapid environ-mental changes on the northern Great Plains. The finalmelting of the Laurentide Ice Sheet created an unstablewater-saturated landscape. One would expect tundraherbs and shrubs to be the first vascular plants to havecolonized this recently deglaciated terrain, since ecolo-gical studies of modern glacial moraines, such as thosein the Yukon, have documented this stage in primaryplant succession (Birks, 1980). A pioneering tundra-likevegetation dating to the late-glacial has been reportedfrom pollen and plant-macrofossil studies conducted incentral Minnesota and eastward (e.g., Birks, 1976;Maher et al., 1998). In contrast, there is no evidencefor such a vegetation in the fossil records of the northernGreat Plains, and instead the earliest plant communityreconstructed was composed of white spruce trees,deciduous shrubs, and herbs.

A plausible explanation for the absence of tundrafossils is that during the early phases of deglaciationthere was a lack of stable depositional sites to preservethe remains of this pioneering vegetation (Florin andWright, 1969; Maher et al., 1998). Meltwater drainageon the Glaciated Till Plain and in the Lake Agassizbasin, as well as solifluction associated with the meltingof stagnant ice blocks buried under till on the Missouriand Prairie coteaus might not have been conducive forpreserving plant materials long after ice retreat (Wright,1976; Clayton and Moran, 1982). This tundra phasemay have been short-lived, since ecological studies ofplant colonization of newly formed moraines in theYukon indicate that succession from tundra to spruceforest can occur as quickly as 120 years (Birks, 1980).

ARTICLE IN PRESS

bedrock

tilltill

stagnant ice stagnant icelake sediment

Fig. 4. Schematic depiction of the open white spruce parkland which existed on the northern Great Plains during the late-glacial (adapted from

Tuthill et al., 1964). Picea glauca probably occupied the shores of lakes and ponds and the uplands were covered by shrubs and herbs. These plants

grew in till overlying buried stagnant ice, which slowly melted to form these waterbodies (not drawn to scale).


Fossil and lithostratigraphic data from the northernGreat Plains suggest that white spruce trees and otherplants became established in till overlying buriedstagnant ice on the Missouri and Prairie coteaus priorto and during the formation of lakes and ponds (Fig. 4).Wright (1976) described how this buried ice tookthousands of years to melt, during which time plantdetritus accumulated in temporary supraglacial poolsand were redeposited with the gradual downwasting ofthe ice beneath. Only when all of the ice melted didstable lake basins form, preserving spruce ‘‘trash layers’’at the bottom of many of these basins. In the case of theAndrews site in southern Saskatchewan, however, 10-cmPicea litter layer penetrated by in situ spruce roots waslaid down intact and flooded (Yansa, 1998).

The timing of basin establishment, which determinedthe starting point for preservation of plant remains,varied across the northern Great Plains, thereby makingthe exact arrival of spruce and other plants uncertain(Clayton and Moran, 1982; Yansa, 1998, 2002). Conse-quently, some sites within the region have morecomplete records of the spruce phase (e.g., Moon Lake)than others (e.g., Coldwater and Rice Lakes), and the14C dates for the spruce at all of these sites should beconsidered to be minimum ages (Table 1; Figs. 1 and 2).

The extent of soil development prior to spruceestablishment is unknown. Picea, however, is betterable to germinate and grow in mineral soils than mostother trees, because its roots can uptake nutrients fromnitrogen-fixing ectomychorrhizal fungi (Miller et al.,1994; MacDonald, 2002). Co-occurrences of fossilmychorrhizae and Picea root fragments from Late-Pleistocene sediments of the Andrews and Wendel sites(Yansa, unpublished data) indicate that this symbioticrelationship was also important during spruce coloniza-tion of the northern Great Plains.

4.2. Late-glacial climate

The changing range expansion of spruce on thenorthern Great Plains occurred during the transitionfrom glacial to interglacial climates. The cool climateduring deglaciation was offset by greater (�7%)summer insolation than modern, as determined fromastronomical calculations (e.g., Kutzbach and Webb,1991; Bartlein et al., 1998). Ritchie and MacDonald(1986) proposed that these relatively warm summerswould have aided in the seed germination and saplinggrowth of spruce. However, photosynthesis and hencegrowth rates of these plants were probably slowed by thelow partial pressure of carbon dioxide in the atmo-sphere, compared to modern conditions (Cowling andSykes, 1999).

The late-glacial climate was probably not uniformacross the northern Great Plains and neighboringregions. Rather, the northeastward retreat of theLaurentide Ice Sheet probably caused steep east–westand north–south thermal gradients across north-centralNorth America (Barnosky et al., 1987; Webb et al.,1987; Bartlein et al., 1998). The eastern part of thisregion, closer to the retreating glacier, probablyexperienced cooling both from albedo and from colddry winds generated from a persistent high-pressure cellcentered over the ice sheet (Barnosky et al., 1987;Kutzbach and Webb, 1991). This anticyclonic flow mayhave reduced the penetration of moist Gulf airmass intonorth-central North America from about 14,000 to10,000 14C yr BP (ca 16,800–11,600 cal yr BP) (Mac-Donald and Case, 2000; Hostetler et al., 2000). Theorientation of sand spits along the shore of a glacial lakein northern Michigan confirm the prevalence of east andsoutheast winds between 11,800 and 10,500 14C yr BP(ca 13,900–12,500 cal yr BP) just south of the Laurentide


Ice Sheet (Krist and Schaetzl, 2001). This anticycloniccirculation (with outward air flow in all directions)explains how the winged seeds of Picea glauca and otherplants were transported northwards into deglaciatedterrain (Ritchie and MacDonald, 1986). The remainderof the flora spread their seeds readily by interconnectingwaterways and by water fowl transporting seeds on theirmuddy feet.

Pollen data also support these paleoclimate inter-pretations. At the western limit of the northern GreatPlains, prairie colonized northwestern Montana soonafter deglaciation, indicating the presence of warmerand drier conditions away from the ice front (Barnoskyet al., 1987; Barnosky, 1989). Similarly, drought-tolerant Populus (poplar) has been identified as theearliest arboreal colonist in southeastern Alberta to thenorth (MacDonald and Case, 2000; Beaudoin andOetelaar, 2003). Thus, on the plains of Montana andsouthern Alberta, areas deglaciated first, the Late-Pleistocene vegetation directly responded to increasedsummer insolation shortly after deglaciation, comparedto areas farther east (Barnosky, 1989; MacDonald andCase, 2000). Picea never colonized these areas duringthe Late-Pleistocene, for evidence presented here putsthe western limit of this taxon on the Missouri Coteau.Spruce colonization of the eastern portion of northernGreat Plains, in other words, was a product of aglacially influenced climate, and the pace of its northernmigration was controlled primarily by the speed of ice-margin recession, the development of a suitable climate,and the rate of seed dispersal.

4.3. White spruce migration chronology

After deglaciation, Picea glauca migrated time-trans-gressively by moving northwards across the northernGreat Plains over 800 km from the South Dakota-Nebraska border to reach its present range in thesouthern boreal forest of Manitoba and Saskatchewan(Table 1, Figs. 1 and 2). Spruce occupied the areaimmediately south of the late-Wisconsinan ice limit, butthe extirpation of this taxon in Iowa, Kansas, andNebraska is poorly dated, occurring sometime duringthe late-glacial (Van Zant, 1979; Baker and Waln, 1985).Picea colonized the Rosebud site situated on the SouthDakota-Nebraska border (Table 1, Figs. 1 and 2),immediately south of the late-Wisconsinan glacial limit,by 12,600 14C yrBP (14,990 cal yr BP) (Watts andWright, 1966; Wright, 1992).

There are several spruce fossil localities within theglaciated portion of the northern Great Plains (sites areshown in Table 1 and Fig. 1 and dates for sprucecolonization are also depicted in Fig. 2). Plant macro-fossils verify the local occurrence of white spruce on theMissouri Coteau (Strouckel site) in northern SouthDakota at 12,250 14C yrBP (which has two possible

calendar ages of 15,020 and 14,300 yr BP, because of a14C plateau) (Martin and Klukas, 1989). Picea, probablyPicea glauca, inhabited the nearby Cottonwood Lakearea at about 12,300 14C yrBP (ca 14,300 cal yr BP)(Barnosky et al., 1987; Grimm, 2001). White sprucearrived on the Prairie Coteau upland in northeasternSouth Dakota (Pickerel Lake) by 12,070 14C yr BP(14,130 cal yr BP) (Dean and Schwalb, 2000). Picea alsodominated the earliest part of the pollen record ofnearby Medicine Lake (Radle et al., 1989), which has abasal wood date of 10,940 14C yrBP (13,040 cal yr BP).

Several localities contain spruce fossils in the MissouriCoteau of North Dakota. Picea occupied ColdwaterLake on the North Dakota–South Dakota border priorto 10,790 14C yrBP (12,950 cal yr BP), since only the tailend of the peak of spruce pollen is recorded at this site(Yansa, 2002). Picea must have colonized this part ofthe Missouri Coteau earlier, for farther north at theTappen site in central North Dakota an excavatedspruce stump dating to 11,480 14C yrBP (ca 13,550cal yr BP) was found in growth position and associatedwith cones of Picea glauca (Moir, 1957). Sprucecolonized the Johns Lake site in central North Dakotaby 10,820 14C yrBP (cal 12,930 yr BP), based on theidentification of Picea wood associated with conifer-bark beetle fossils (Ashworth and Schwert, 1992).Farther north, spruce occupied the shores of KettleLake by 11,080 14C yrBP (13,064 cal yr BP), and sprucepersisted at Rice Lake until at least 9240 14C yr BP(10,450 cal yr BP) (Laird et al., 1996, 1998; Grimm, 2001;Clark et al., 2002).

On the Glaciated Till Plain of North Dakota, east ofthe Missouri Coteau, spruce became established at theWendel site by 11,550 14C yr BP (ca 13,560 cal yr BP)(Yansa, 2002). Also in this area, spruce occupied thevicinity of Moon Lake by 11,830 14C yrBP (ca13,830 cal yr BP) (Laird et al., 1996, 1998). Farther east,meanwhile, glacial Lake Agassiz drained southward intothe Mississippi system (Teller, 2004), thereby restrictingthe northwestward migration of plants from the Mid-west. During a temporary low-water stand of this lake(caused by a drainage diversion), scattered standsof spruce and deciduous hardwoods colonized theswampy lake plain from 10,230 to 9900 14C yr BP(ca 11,960–11,320 cal yr BP) in the vicinity of Fargo,North Dakota (site not shown in Fig. 1; Yansa andAshworth, 2005).

In the Canadian portion of the northern Great Plains,Late-Pleistocene spruce fossil localities are restricted tothe Missouri Coteau, except for the Neufeld site (Table1, Figs. 1 and 2). Because the Neufeld site is located onthe edge of a meltwater channel that incised theMissouri Coteau, the wood was derived either fromthe coteau (where it was redeposited down slope) or thewood was rafted to the site by meltwater. Uprightspruce stumps recovered from the Scrimbit site (Dew,


1959) and the Andrews site (Yansa, 1998; Yansa andBasinger, 1999) provide conclusive evidence for treegrowth in situ at these sites. Radiocarbon ages from alleight spruce localities in southern Saskatchewan rangebetween 10,300 and 9980 14C yrBP (ca 12,100–11,500cal yr BP) (Table 1). The clustering of these ages doesnot necessarily indicate that the spread of spruce wasmore rapid in Saskatchewan than earlier in the Dakotas,since these ages occur at a well-documented 14C plateau(Lowell and Teller, 1994; Stuiver et al., 1998).

The plant-macrofossil records of the Andrews site(Yansa, 1998; Yansa and Basinger, 1999) and nearbyClearwater Lake (Last et al., 1998) and the pollen recordof the Herbert site (Fig. 3B; Kupsch, 1960) indicate thatspruce persisted in southern Saskatchewan until about9980 14C yrBP (ca 11,500 cal yr BP). A stump of spruceassociated with other macrofossils of Picea glauca froma site within the Riding Mountain National Park,Manitoba, dates to 95707130 14C yrBP (ca 10,930cal yr BP) (Table 1; McCallum and Wittenberg, 1962).This site, unlike the others mentioned, is located todaywithin the southernmost range limit of white spruce inthe boreal forest (Figs. 1 and 2). This basal wood date ofca 9600 14C yr BP (ca 10,950 cal yr BP) for the RidingMountain site (McCallum and Wittenberg, 1962)provides a minimum age for the initial formation ofwhat later became the southern boreal forest, because ofuncertainty whether the wood came from the earliestcolonists. However, this age is closer to the actual timingof spruce arrival than the age of ca 11,500 14C yr BP(ca 13,500 cal yr BP; Table 1) proposed by Ritchie (1969,1987) based on the dating of organic sediment from theRiding Mountain site (for the reasons discussed inSection 3.1). This age of 11,500 14C yrBP, furthermore,predates the accepted timing of deglaciation in southernManitoba (Clayton and Moran, 1982).

4.4. Estimated migration rate for white spruce

An average migration rate in 14C (and calendar) years(Table 2) was calculated based on the results of thisstudy (Table 1). The dates used in these calculations areminimum ages, since the wood samples used to obtainthese 14C ages may not have been from the first treeswhich colonized these sites. Another assumption is that

Table 2

Migration rate for Picea in both 14C and calendar years per kilometer, ass

Dakota, northwest to the Andrews site, Saskatchewan (based on data in Ta

Distance (km) between

(direction from the Ros

Rosebud site, SD to Andrews site, SKa 920 (northwest, �51 lon

aThe oldest ages were used for the Rosebud site (12,630 14C yrBP, equ

equivalent to 11,920 cal yr BP).

the spread of white spruce was more-or-less northwardsfrom populations south of the Laurentide Ice Sheet (i.e.,the Rosebud site area). This assumption is supported bygeomorphic evidence indicating that routes from theeast and southeast were blocked by glacial LakesDakota and Agassiz and associated meltwater streamsfrom about 12,000 to 10,900 14C yrBP (ca 14,000–13,000 cal yr BP) (Clayton and Moran, 1982; Teller,2004).

With these caveats in mind, spruce migrated from theSouth Dakota–Nebraska border (Rosebud site) north-west along the Missouri Coteau upland to reachsouthern Saskatchewan (Andrews site) at an averagerate of 0.38 km/14C year (0.30 km/calendar year)(Table 2). This rate is far less than the rate of2 km/14C year or about 1 km/calendar year previouslycalculated for the northward spread of white spruce innorth-central North America (Ritchie and MacDonald,1986; Bartlein et al., 1995). However, the rate presentedin this paper agrees with those obtained for theHolocene range expansions of spruce (0.25 km/14C year)and other coeval trees (0.1–0.5 km/cal yr) in easternNorth America (Davis, 1981; Grimm and Jacobson,2004), and is similar to the late-glacial migration rateof another species of Picea in Europe (Huntley andBirks, 1983).

The northward spread of white spruce on the north-ern Great Plains during the late-glacial was controllednot only by climate change, but also by the timing ofdeglaciation and the local availability of seeds. Conse-quently, it took about 3000 calendar years for late-glacial white spruce to spread 900 km. Even after takinginto account periodic lags in spruce migration, becauseof the influences of non-climate factors, this late-glacialmigration rate is still far slower than the predicted600–700 km northward advance of the boreal forestbiome over the next 50 years as a result of globalwarming (Kojima, 1994). The boreal forest biome willundoubtedly shift northwards in the future, but thismigration will probably lag behind warming tempera-tures, as in the past. Picea glauca will probably becomelocally extinct along its southernmost range, wheredeciduous trees will become more common, agreeingwith the predictions of other paleoecologists (e.g., Daviset al., 2000; MacDonald, 2002).

uming a constant speed of movement from the Rosebud site, South

ble 1)

sites

ebud site)

Picea migration rate

(km/14C yr)

Picea migration rate

(km/cal yr)

gitude) 0.38 0.30

ivalent to 14,990 cal yr BP) and the Andrews site (10,230 14C yrBP,


4.5. Open white spruce parkland

The late-glacial vegetation and climate interpretationspresented here for the northern Great Plains differ fromthose of other pollen and plant-macrofossil researchers.Some have described the species composition andstructure of this vegetation as being like the borealforest (Ritchie and DeVries, 1964; Watts and Bright,1968; Ritchie, 1987; Radle et al., 1989; Last et al., 1998).Other researchers have shied away from the term‘‘boreal forest,’’ because they recognized that thisLate-Pleistocene vegetation on the northern GreatPlains lacked Pinus banksiana (jack pine) and otherspecies which constitute this forest today, and insteadreconstructed a ‘‘spruce forest’’ for the northern GreatPlains (Watts and Wright, 1966; Laird et al., 1996, 1998;Grimm, 2001).

Ritchie and DeVries (1964) and Ritchie (1969, 1987)further considered this forest to have been closedcanopy, and to have included both Picea mariana (blackspruce) and Picea glauca. Their identification of Picea

mariana was based on the morphology of needles, butthis is considered by Baker and Drake (1994) to not be areliable character. All female cones and seeds of sprucerecovered from several sites on the northern GreatPlains have been identified as those of Picea glauca

(Table 1). Furthermore, Yansa and Basinger (1999)confirmed that white spruce was the only conifer at theAndrews site by stomatal analysis of several needlesfrom a buried litter layer. Differentiating between thesespecies has paleoenvironmental implications, for todaythese taxa typically inhabit different habitats. Picea

glauca prefers moderate to well-drained soils, whereasPicea mariana is more tolerant of both wet and acidicsoil conditions, such as in bogs (Ritchie, 1987; McLeodand MacDonald, 1997).

This paper reconstructs an open white spruce park-land (Fig. 4), rather than a spruce forest, based onseveral lines of evidence. First, the spatial coverage andabundance of white spruce in the Late-Pleistocenevegetation of the northern Great Plains were previouslyoverestimated. Typical pollen spruce maxima of40–60% for Late-Pleistocene sites within the region(see the pollen % curves in Fig. 2), should not becompared to the pollen spectra of modern forests, sincePicea glauca was the dominant, if not the sole, treeduring the late-glacial. Moreover, some of these sprucepollen grains were probably redeposited with otherdetritus as ‘‘trash layers’’ in kettle basins (described inSection 4.1), thereby inflating Picea values compared tothe regional pollen rain.

Associated with spruce pollen peaks are low values ofPinus (pine; o10%), which have been interpreted asderived from long-distance dispersal by this prolificpollen producer (Watts and Wright, 1966; Whitlocket al., 1993). Pollen values of Larix (tamarack) are low

(1–2%) and macrofossils of this taxon have only beenreported at a few sites (e.g., Watts and Bright, 1968).Pollen percentages of Fraxinus nigra-type (black ash) aremoderate (2–5%) for this species and indicate that blackash may have arrived sometime during the spruce phase,although macrofossils of this taxon have not yet beenrecovered (Yansa, 2002). The stratigraphic positions ofmacrofossils and pollen of Populus spp. (poplar) andBetula spp. (birch) in relation to those of spruce atseveral sites in the study area suggest that thesedeciduous trees arrived near the end of the spruce phase(Laird et al., 1998; Yansa, 1998).

Late-glacial white spruce dominated the arborealpollen spectra and swamped the pollen signal ofherbaceous plants, as observed today along forest-prairie ecotones (Baker and Drake, 1994). Non-arborealpollen (NAP) values of 20–40%, largely consisting ofArtemisia (sage), Poaceae (grass), Cyperaceae (sedgefamily) and, to a lesser extent, Ambrosia-type (ragweed),are significant for these late-glacial fossil sites. Thesevalues are comparable to those of the present-daygrassland and aspen parkland (Baker and Drake, 1994).

Second, macrofossils of the herbs and shrubs asso-ciated with those of white spruce from sites on thenorthern Great Plains have been previously under-appreciated or misinterpreted. Some of the plantmacrofossils identified at the Hafichuk site (DeVries,1963; Ritchie and DeVries, 1964), Pickerel Lake (Wattsand Bright, 1968), and Clearwater Lake (Last et al.,1998) were interpreted as boreal taxa. Upon closerexamination of the modern geographic ranges of thesetaxa, most of these ‘‘boreal’’ plants, such as the shrubArctostaphylos uva-ursi (bearberry), occupy not only theboreal forest but also the aspen parkland and mixed-grass prairie (see Table 3 for a complete list of speciesassociated with white spruce). In fact, the majority ofspecies identified from plant macrofossils at theselate-glacial sites today inhabit all three ecozones. Thisfossil assemblage includes only a handful of speciesrestricted today to the boreal forest and an equalnumber of species presently limited to the grasslandecozone (Table 3). This interpretation is supported byFredlund and Tieszen’s (1997) study of phytoliths andstable carbon isotopes in South Dakota, which recon-structed that a modern-like mixed-grass prairie becameestablished on the lower-elevation slopes of the BlackHills by 10,500 14C yrBP (ca 12,500 cal yr BP).

Third, additional data supporting this reconstructionof an open spruce parkland on the northern GreatPlains are provided by the limited abundance of buriedspruce logs, as well as coeval paleosols and mammothfossils. Excavations of entire kettle basins (�30mdiameter and �5m deep) at the Andrews, Kyle, andBeechy sites in Saskatchewan (for livestock-wateringholes) exposed about 3–5 logs or rooted stumps in eachbasin, suggesting that a few white spruce trees occupied

ARTICLE IN PRESS

Table 3

Presence (*) of taxa associated with white spruce at all plant-macrofossil localities investigated within the region

Macrofossil taxa Range Site (#)

1 5 6 7 14 15 16 18 19 20

Trees:

Betula sp. (birch) G, A, B *

Betula cf. B. occidentalis (river birch; water birch) G, A, B * * *

B. papyrifera (paper birch) G, A, B *

Larix laricina (tamarack) B *

Picea sp. (spruce) B * * * * * * * * * *

P. glauca (white spruce) A, B * * * * * * *

P. mariana (black spruce) B *

Populus tremuloides (aspen poplar; quaking aspen) G, A, B * * * *

Shrubs:

Andromeda polifolia (bog rosemary) B *

Arctostaphylos uva-ursi (bearberry) G, A, B * * *

Cornus canadensis (bunchberry) G, A, B *

C. stolonifera (C. alba) (red-osier dogwood) G, A *

Juniperus communis (low juniper; common juniper) G, A, B * *

Rubus idaeus (red raspberry) G, A, B * * * * * *

R. pubescens-type (dewberry; creeping blackberry) G, A, B * *

Salix sp. (willow) G, A, B *

Shepherdia canadensis (buffaloberry; rabbitberry) G, A, B * * *

Mudflat and Upland Herbs:

Aralia nudicaulis (wild sarsaparilla) G, A, B *

Aster sp. (aster) G, A, B * * *

Aster cf. A. novae-angliae (New England aster) G, A, B *

Asteraceae (aster family) G, A, B *

Brassicaceae (Cruciferae) (mustard family) G, A, B * *

Caltha palustris (marsh marigold) G, A, B *

Chenopodium berlandieri (pitseed goosefoot) G * * * * *

C. capitatum (strawberry blite) G, A, B *

C. rubrum (red goosefoot; alkali blite) G, A * * * * *

C. salinum (saline goosefoot) G, A * * * * * *

Cirsium sp. (thistle) G, A, B *

Cirsium cf. C. muticum (swamp thistle) G, A, B *

Epilobium sp. (willowherb) G, A, B *

Epilobium cf. E. ciliatum (northern fireweed) G, A, B *

Erigeron sp. (daisy fleabane) G, A, B *

E. philadelphicus (Philadelphia fleabane) G, A, B *

Fragaria vesca (American wild strawberry) G, A, B *

F. virginiana (wild strawberry) G, A, B * * * *

Geranium cf. G. bicknellii (Bicknell’s cranes-bill) G, A, B *

Lycopus americanus (water hoarhound) G, A, B * * * * * * *

Mentha arvensis (wild mint) G, A, B * * * * *

Poaceae (grass family) G, A, B * *

Polygonum sp. (smartweed) G, A, B *

Polygonum amphibium (water smartweed) G, A, B * *

P. lapathifolium (pale smartweed) G, A, B * *

Potentilla anserina (silverweed) G, A, B *

Potentilla gracilis (graceful cinquefoil) G, A, B *

Potentilla millegrana (P. rivalis) (brook cinquefoil) G *

Potentilla norvegica (rough cinquefoil) G, A, B * * * * * *

Potentilla palustris (marsh cinquefoil) A, B * *

Ranunculus gmelinii (small yellow buttercup) G *

Ranunculus pennsylvanicus (bristly buttercup) G, A, B *

Ranunculus sceleratus (ditch buttercup) G, A, B * * * * *

Ranunculus section Batrachium (buttercup) G, A, B * * *

Rorippa sp. (yellow cress) G, A, B *

Rorippa cf. R. truncata (yellow cress) G, A, B * *

Rumex maritimus (golden dock) G, A, B * * * * * * * *

Solidago sp. (goldenrod) G, A, B *


ARTICLE IN PRESS

Table 3 (continued )

Macrofossil taxa Range Site (#)

1 5 6 7 14 15 16 18 19 20

Stachys sp. (hedge nettle) G, A, B * *

Stellaria crassifolia (fleshy stitchwort) G, A, B *

Triglochin maritima (seaside arrow-grass) G, A, B * * *

Urtica gracilis-type (U. dioica) (stinging nettle) G, A, B * *

Viola sp. (violet) G, A, B * *

Aquatic-Emergents:

Alisma– planto-aquatica (water-plantain) G, A, B * *

Carex sp. (sedge) G, A, B * *

Carex aquatilis (water sedge) G, A, B * * *

Carex cf. C. atherodes (awned sedge) G, A, B * * * *

Carex cf. C. aurea (golden sedge) G, A, B *

Carex cf. C. lenticularis (lens-fruited sedge) B *

Carex cf. C. rostrata (beaked sedge) G, A, B * * * * * *

Carex cf. C. sychnocephala (long-beaked sedge) G, A, B * * * *

Carex retrosa (turned sedge) G, A, B *

Carex tenera (slender sedge) G *

Carex vulpinoidea (fox sedge) G, A, B *

Eleocharis sp. (spike-rush) G, A, B *

Juncus sp. (rush) G, A, B *

Sagittaria latifolia (broad-leaved arrowhead) G, A *

Sagittaria rigida (sessile-fruit arrowhead) G *

Scirpus americanus (three-square bulrush) G, A * * * * * *

S. cyperinus (wool-grass) G, A, B *

S. nevadensis (Nevada bulrush) G, A *

S. paludosus (prairie bulrush) G, A, B *

S. validus (common bulrush) G, A, B * * * * * * * *

S. cf. S. atrovirens (green bulrush) G, A *

Sparganium sp. (bur-reed) G, A, B * *

Sparganium chlorocarpum (stemless bur-reed) G, A, B * *

Subularia aquatica (water awl-wort) B * *

Typha latifolia (broad-leaved cattail) G, A, B * * * * * * * *

Aquatics:

Hippuris vulgaris (mare’s tail) G, A, B * * * *

Lemna minor (lesser duckweed) G, A, B *

Lemna trisulca (ivy-leaved duckweed) G, A, B * * * *

Myriophyllum sp. (water-milfoil) G, A, B * * * *

M. verticulatum (whorled water-milfoil) G, A, B * * * *

Najas flexilis (slender naiad) G, A, B * * * *

Potamogeton filiformis (slender pondweed) G, A, B * * * * * *

P. gramineus (variable pondweed) G, A, B *

P. natans (floating-leaved pondweed) G, A, B *

P. obtusifolius (blunt-leaved pondweed) A, B * *

P. pectinatus (sago pondweed) G, A, B * * * * *

P. pusillus (P. berchtoldii) (small pondweed) G, A, B *

P. richardsonii (Richardson’s pondweed) G, A, B * *

P. strictifolius (straight-leaved pondweed) G

P. vaginatus (sheathed pondweed) G, A, B * * *

P. vaseyi (Vasey’s pondweed) G, A, B *

P. cf. P. foliosus (leafy pondweed) G, A, B *

Utricularia vulgaris-type (greater bladderwort) G, A, B *

Wolffia arrhiza (water-meal) G, A, B * *

Non-Vascular Plants:

Chara sp. (green alga) G, A, B * * * * * * *

Drepanocladus sp. (sickle-branch moss) G, A, B * * * * * *

Equisetum sp. (horsetail) G, A, B * * *

Included are the geographic ranges where these taxa currently exist: grassland (G); aspen parkland (A); and boreal forest (B). See Table 1 and Fig. 1

for the names and locations of these sites.



the shorelines of each of these ponds. This paleovegeta-tion interpretation is supported by the lack of spodichorizon development in the sediments underlying spruce-needle layers at the Andrews, Kyle, and Beechy sites(Yansa, 1998; unpublished data) and in the buried soilsof eastern North Dakota (Hopkins and Running, 2000).Moreover, the recovery of Mammuthus primigenius

(woolly mammoth) fossils on the northern Great Plains,some dating to about 11,500 14C yrBP (Harington andAshworth, 1986), have an autecology that agrees withthe interpretation presented here for uplands coveredprimarily by herbaceous plants.

The vegetation that colonized the northern GreatPlains after deglaciation was composed of an unusualassemblage of prairie, aspen parkland, and borealplants, forming a spruce parkland environment. Thisparkland interpretation was originally proposed for theTappen site (Moir, 1957) and a mollusk site (Tuthillet al., 1964) in North Dakota and for the Herbert site inSaskatchewan (Kupsch, 1960), but it was supplanted bythe more popular reconstruction of a boreal forest byRitchie (1969, 1987) and others. This paper supportsthe spruce parkland interpretation, which is depictedin Fig. 4.

This paper is also in agreement with the interpretationof an anomalous parkland of Picea, Fraxinus nigra

(black ash), Artemisia, and Cyperaceae occupying theeastern USA during the late-glacial (Webb et al., 1983,2004; Webb, 1987; Jacobson et al., 1987; Williams et al.,2001). Their reconstruction of scattered spruce treessurrounding numerous sedge wetlands is identical tothat presented in this paper for the northern GreatPlains during the same time period, the only exceptionbeing that Fraxinus nigra pollen values are higher forsites in the eastern USA. Furthermore, both of theseinterpretations differ from the ‘‘classic’’ interpretationof a spruce forest comprising the late-glacial vegetationof the eastern USA (e.g., Wright, 1968; Davis, 1981;Delcourt and Delcourt, 1991; Whitlock et al., 1993), andthe northern Great Plains (e.g., Ritchie, 1969, 1987;Laird et al., 1996, 1998; Grimm, 2001).

The absence of a suite of plants now restricted to theboreal forest in the fossil records of the northern GreatPlains (Table 3) indicates that the climate was not socold as proposed in an earlier interpretation of a borealforest (Ritchie and DeVries, 1964; Ritchie, 1987;MacDonald, 2002). Similarly, Grimm and Jacobson(2004) proposed that during the late-glacial spruce phasein the Midwest the regional climate was somewhatwarmer than that of the boreal forest today. There is noevidence for Younger Dryas cooling in the fossil recordsof the northern Great Plains, although such coolinghas been reported in the eastern USA (e.g., Shumanet al., 2002).

An exact modern analog is lacking for the open whitespruce parkland that once existed on the northern Great

Plains. The closest facsimile, however, is the vegetationof the Spruce Woods Provincial Park of southernManitoba, where pockets of Picea glauca occupy low-lying locations surrounded by upland prairie within theaspen parkland belt. Picea glauca is more droughttolerant than Picea mariana (black spruce) and Pinus

banksiana (jack pine) (Ritchie, 1987), which may explainwhy the former, rather than one of the latter, colonizedthe northern Great Plains under a low precipitationregime (as discussed above in Section 4.2). White sprucetrees may have supplemented their moisture require-ments from the moist lakeshore soils during the late-glacial (Fig. 4).

The modern climate of the Spruce Woods and aspenparkland of southern Manitoba and south-centralSaskatchewan (Shay, 1999) provides the best approx-imation for the late-glacial climate of the Dakotasand Canadian prairies based on the fossils identified(Table 3) and their relative abundances. The meansummer temperature during the late-glacial in thisregion was probably about 19 1C, a degree warmer thanthat of the southern limit of the boreal forest today. Thispaleotemperature reconstruction is also 3 1C and 5 1Ccooler than present-day mean July temperatures inNorth Dakota and South Dakota, respectively (Owenbyand Ezell, 1992).

4.6. White spruce extirpation and plant-community

succession

The late-Quaternary fossil records of the northernGreat Plains suggest that during the Pleistocene–Holo-cene transition Picea glauca was replaced by deciduoushardwoods, primarily Ulmus (elm) and Quercus (oak) inthe Dakotas, and Populus (poplar) and Betula (birch) inSaskatchewan (Grimm, 2001; Yansa, 2002). The ex-tirpation of spruce was more-or-less time-transgressive,occurring at: 11,200 14C yr BP (ca 13,200 cal yr BP) innorthern South Dakota (Cottonwood Lake; Barnoskyet al., 1987); 10,600 14C yrBP (ca 12,750 cal yr BP) insouthernmost North Dakota (Coldwater Lake; Yansa,2002); 10,300 14C yrBP (ca 12,100 cal yr BP) in south-eastern North Dakota (Moon Lake; Laird et al., 1996,1998); and about 9980 14C yrBP (ca 11,500 cal yr BP) insouthern Saskatchewan (Clearwater Lake; Last et al.,1998).

The shift from spruce parkland to deciduous parklandon the northern Great Plains occurred at about the sametime as the transition from spruce parkland (or forest) topine woodland in neighboring Minnesota to the east(Shay, 1967; Whitlock et al., 1993; Williams et al., 2001).These regional vegetation changes have been attributedto greater summer warmth and reduced soil moisture,which would have desiccated white spruce seedlingsand stressed older trees (e.g., Bartlein et al., 1995;Grimm and Jacobson, 2004). Summer droughts became


common during the early and mid-Holocene in north-central North America. They were probably caused byzonal westerly (Pacific) airflow blocking the penetrationof moist Gulf air (Kutzbach and Webb, 1991; Dean andSchwalb, 2000).

Besides climate, the drainage and evaporation ofmeltwater on the landscape during the late-glacial andearly-postglacial would also have been responsible forthe drying of soils within the region. On the broaderscale, the regional water table lowered, affecting thevegetation. On the local scale, basins within an areareceived variable groundwater inflow depending upon(1) their landscape positions relative to the water table(Smith et al., 1997), and (2) volumes of stagnant iceblocks buried under local tills, the meltwater of whichseeped into overlying basins over 1000–3000 years(Wright, 1976; Clayton and Moran, 1982). Yansa(2002) referred to this as the ‘‘residual meltwater effect,’’because it probably supported trees around waterbodiesindependently of the climatic regime for a time.

These changes in climate and hydrology during thePleistocene–Holocene transition would have favoredgreater fire frequency. Charred Picea logs and needles atthe Andrews site (Yansa, 1998), and reports of abundantcharcoal associated with declining spruce pollen valuesat several other sites (Watts and Bright, 1968; Yansa,2002; Clark et al., 2002) suggest that fire probablycontributed to the demise of spruce trees at these fossillocalities. Species of Populus and Betula colonized theseburned areas, as seen today after fires and otherdisturbances in the boreal forest and aspen parkland.

Although these fires reflected a regional drying trendduring the transition from the late-glacial to early-postglacial, they were probably localized. Spruce per-sisted at Rice Lake, for example, until shortly after 924014C yrBP (ca 10,450 cal yr BP) (Laird et al., 1996, 1998),long after it disappeared from nearby sites (Table 1).The timing of Picea glauca extirpation varies from site-to-site because these trees are long-lived (up to 500years) in the absence of local fires and other distur-bances (Szeicz and MacDonald, 1994). White sprucetrees once established can also tolerate warm summers(418 1C mean July temperatures) so long as precipita-tion or soil moisture is high (Webb et al., 1993). Grimmand Jacobson (2004) suggested that a moist climate andlack of fires favored Picea during the late-glacial.Alternatively, I contend that it was not high precipita-tion but a meltwater-saturated landscape that inhibitedfires and prolonged spruce growth during this period.Only when the landscape began to dry out in the early-Holocene under a warm-dry climatic regime did firesbecame common and deciduous trees appear. Thearrival of deciduous trees has also been correlated withlowering of the local water tables and higher lakesalinities in the Dakotas and Saskatchewan (e.g., Last etal., 1998; Dean and Schwalb, 2000).

White spruce therefore became extinct on the north-ern Great Plains for a variety of reasons, many of whichare inter-related. These causal agents probably includeddesiccation, fire, and being outcompeted by deciduoustrees for limited resources. The prairie grasses and forbsthat had occupied the drier soils during the spruce phaseprobably expanded in their coverage during this regionalwarming and drying trend. This aridity continued, andabout a millennium later the deciduous trees disap-peared and prairie expanded to form a widespreadgrassland, as seen today in this region (Laird et al., 1996,1998; Yansa, 1998, 2002; Grimm, 2001).

5. Summary

This paper reassesses the timing and communitycomposition of the white spruce vegetation thatcolonized and spread across the northern Great Plainswith the recession of the Laurentide Ice Sheet. Availablefossil data indicate that white spruce was the mainconstituent of this vegetation along with prairie, park-land, and boreal shrubs and herbs and that a precedingtundra-like vegetation was absent. This paper confirmsthe results of other pollen researchers that the north-ward spread of white spruce was time-transgressive butoffers a more accurate chronology for this migrationbased on the 14C dating of terrestrial plant macrofossils.Previous chronologies had integrated 14C ages obtainedfrom spruce wood with those ages obtained fromorganic sediments (gyttja).

White spruce existed on the South Dakota-Nebraskaborder, immediately south of the Late-Wisconsinanglacial limit, at 12,600 14C yrBP (ca 15,000 cal yr BP).The onset of Picea glauca migration into glaciated SouthDakota is poorly dated. However, by 12,250 14C yr BP(15,020–14,300 cal yr BP) white spruce had reachednorthern South Dakota, by 10,820 14C yrBP (12,930 calyr BP) had arrived in central North Dakota, by 10,30014C yrBP (12,100 cal yr BP) had migrated to southernSaskatchewan, and sometime before 9570 14C yr BP (ca10,930 cal yr BP) reached what is now the southernboreal forest in southern Manitoba. White sprucemigrated northwest along the Missouri Coteau uplandinto southern Saskatchewan at an estimated rate of0.38 km/14C year (0.30 km/calendar year). This rate issubstantially slower than the previously published rateof 2 km/14C year (1 km/calendar year) for the postglacialspread of white spruce in the study area, but iscomparable to published rates for Holocene treemigration in eastern North America.

In addition, this paper proposes that this vegetationwas an open white spruce parkland, rather than a spruceforest or boreal forest, as previously interpreted by otherresearchers. Evidence supporting a more open characterto this late-glacial vegetation includes: (1) Picea pollen


percentages being inflated, compared to modern pollenspectra, due to the low species diversity of this unusuallate-glacial vegetation and redeposition of spruce fossilsduring the formation of kettle basins; (2) absence of a‘‘boreal’’ macrofossil flora and the presence of severalgrassland and aspen parkland plants; (3) spruce logs andtrunks exposed during excavations of kettle pondsbeing few in number; (4) the lack of buried Spodosols,which would have formed under coniferous forestcover; and (5) localities of grazing woolly mammoth ofages comparable to those of white spruce. An openwhite spruce parkland was previously interpreted bya few other authors decades ago, but this paper isthe first to reconstruct this vegetation using detailedplant-fossil analysis. Moreover, this paleovegetationreconstruction concurs with an interpretation recentlyproposed by other pollen researchers for a spruce-sedgeparkland existing in the eastern USA during thesame time.

This interpretation of a spruce parkland on thenorthern Great Plains agrees with paleoclimate modelsimulations for reduced precipitation during the late-glacial. White spruce trees today inhabit well-drainedsoils, but during the late-glacial they apparently wererestricted to the shorelines of lakes and ponds where soilmoisture may have offset the limited precipitationreceived. The association of prairie and parkland shrubsand herbs with spruce trees and a few boreal plants alsosuggests that the temperatures were not so cold asproposed earlier for a boreal forest, but were still coolerthan present on the northern Great Plains, as expectedfor a glacially influenced climate. Mean summertemperatures on the northern Great Plains during thelate-glacial were probably about 19 1C, like that of apocket of white spruce parkland which exists todaywithin the aspen parkland ecozone of southern Mani-toba. Continued warming during the early-Holoceneresulted in the extirpation of white spruce in this region,starting before 11,200 14C yrBP (ca 13,200 cal yr BP) inSouth Dakota and ending at about 9980 14C yrBP (ca11,500 cal yr BP) in southern Saskatchewan. This sprucevegetation was replaced by a parkland of deciduoustrees and prairie, which about a millennium later wassupplanted by extensive grassland that comprises thenative vegetation of this region today.

Acknowledgments

Reviews of this paper by Tom Webb III and HerbWright Jr. have been very helpful in improving the finalproduct. Earlier versions of this paper have benefitedfrom the comments of Vance Holliday, Jim Knox, LeeClayton, Ben LePage, and Jim Basinger. I also thankEric Grimm for providing the sediment cores fromColdwater Lake and Pietra Mueller for conducting

pollen counts on some of the samples from these cores.As well, I acknowledge Ed Murphy for coring theWendel site and Dennis Wendel for allowing us access tothis site. The Palliser Triangle Global Change Project(Geological Survey of Canada) provided funds to collectsamples from the Andrews, Beechy, Kyle, and Neufeldsites and to obtain 14C ages for fossils from these sites.The 14C ages for the Wendel site and some of thoseobtained for Coldwater Lake were provided free by theUniversity of Arizona AMS 14C Laboratory.

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