Late glacial and postglacial pollen and plant macrofossils from Lake West Okoboji, Northwestern Iowa

QUATERNARY

RESEARCH

12,

3588380 (19791

Late Gla’cial and Postglacial Pollen and Plant Macrofossils Lake West Okoboji, Northwestern Iowa KENT Department

of Geology,

The

VAN

from

ZANT’

of Iowa,

University

lowba City.

Iowa

52242

Received June 21, 1978 Pollen and plant macrofossils preserved in lake sediment from Lake West Okoboji, Dickinson County, Iowa, indicate how the vegetation of that area changed during the late glacial and postglacial. A closed coniferous forest, dominated by spruce and larch trees, produced the Picea -Larh pollen assemblage zone. Fir trees were a minor constituent of this forest; pine trees were probably absent. Black ash trees increased in abundance at Lake West Okoboji and by 13,500 yr ago were an important constituent of the forest. The sediment accumulation rate and the pollen influx were low throughout this time. Birch and alder pollen peaked in abundance approximately 11,800 yr ago. Pollen influx increased rapidly as birch and alder replaced coniferous trees on the uplands. A deciduous forest. containing abundant oak and elm trees, replaced the birch-alder-coniferous forest. This forest inhabited northwestern Iowa from approximately 11,000 to 9000 yr B.P. Nonarboreal species became prevalent between approximately 9000 and 7700 yr B.P. as prairie began to replace deciduous forest on the uplands. Charred remains ofAmorpha canrsc’ens and other upland species attest to the presence of prairie tires as an aid in establishing prairie and destroying the forest. The pollen influx declined. The warmest, driest part of the postglacial occurred in northwestern Iowa from approximately 7700 to 3200 yr ago. Lake level fell 9 to 10 m, and prairie extended to the edge of the lake. Wet-ground weeds inhabited areas near lake level which were alternately flooded, then dry. Pollen influx was approximately 100 grains/cm?/yr during the driest time in this dry interval. Deciduous trees, particularly oaks, returned after approximately 3200 yr B.P. Prairie continued to occupy the uplands but trees were more common in the lowlying wet areas. Settlement by Europeans in northwestern Iowa about 1865 is marked by an increase in weed pollen. Macrofossil deposition changed in 1910 in response to the stabilization of lake level.

INTRODUCTION

The late glacial and postglacial vegetational history of Iowa is poorly understood. Previous studies in Iowa are few (Lane, 1931; Brush, 1967; Durkee, 1971). These studies have suggested that pine and fir pollen and trees were important constituents of Iowa’s late glacial forests. These data conflict with information from surrounding areas (e.g., Wright, et al., 1963; Watts and Bright, 1968; Watts and Wright, 1966; J. Grtiger, 1973; King, 1973; E. Griiger, 1972). The conflict has been variously resolved (1) by suggesting that Iowa served as refugia for these species during the late glacial or by (2) speculating on the validity of the pine and fir pollen identifications from Iowa. I Present address: Department of Geology, Earlham College, Richmond, Ind. 47374.

Postglacial vegetational reconstructions are difficult due to Brush’s (1967) and Durkee’s (1971) lack of closely spaced samples, closely spaced radiocarbon dates, and, in some instances, an inadequate pollen sum. Another problem is their limited number of identified taxa. These problems have hindered regional vegetational studies and limited Quaternary paleoecological reconstructions. The purpose of the present research was to provide further information about the late glacial and postglacial vegetation in Iowa. Closely spaced pollen samples and a 300-grain pollen sum were used. Also, plant macrofossils were sieved and identified in order to add additional information. Little Millers Bay of Lake West Okoboji (Fig. 1) was selected as a coring site because of the reported presence of late gla358

0033-5894/79/060358-23$02.00/O Copy&ht @ 1979 by the University of Washington. All rights of reproduction in any form resewed.

LATE GLACIAL

AND POSTGLACIAL

POLLEN

AND PLANT

MACROFOSSILS

359

Nebraska

1

200

FIG. 1. Map of Iowa and the surrounding

km

region. Numbers refer to latitude and longitude.

cial sediments and the reported ability to penetrate the Hypsithermal sediments (Collins, 1968; Dodd, et al., 1968). Little Millers Bay is the northernmost portion of Millers Bay, a western projection of Lake West Okoboji (SW%, SW%, NE%, sec. 23, T99N, R37W, Dickinson County, Iowa) (95”12’W, 46”20’N). The coring site is 425.5 m above sea level. Approximately 2 m of ice and water were penetrated before lake sediment was reached. Lake West Okoboji is in an area where the Altamont moraine overlaps the Bemis moraine (Ruhe, 1969). Both moraines are of late Wisconsinan age and are part of the Des Moines lobe complex. The lake is surrounded by deciduous trees but farmland and prairie occur within a few hundred meters from the lake’s edge. The climate today is temperate in northwestern Iowa. A Livingstone piston corer was used to core the lake sediments in March 1975. The sediments were sampled at lo-cm intervals and processed for pollen. Lengths of 5 cm of the core from the lowest 2 m and from the top 0.5 m were washed through screens of 500- and 125~pm meshes, and plant macrofossils were picked from the residues. Samples of IO cm in length were washed for

macrofossils between lo- and 0.5-m depths. Samples that appeared to be contaminated with overlying sediment were excluded in pollen and macrofossil sampling. Ten radiocarbon dates were obtained from the core. More than 300 grains of arboreal and nonarboreal anemophilous pollen (NAP) composed the pollen sum. Eucalyptus pollen tablets were added to each sample so that the pollen influx could be calculated using Maher’s (1972) method. The sedimentation rate (cm/yr) was calculated by dividing the amount of sediment between radiocarbon-dated samples by the elapsed time between the dates. The pollen samples were processed following Faegri and Iversen (1975). Pollen identifications were made using keys in Faegri and Iversen (1975), Kapp (1969), and McAndrews et al. (1973), and by comparison to the pollen reference collection of the University of Iowa. Plant macrofossils were identified by comparison to the reference collection of the Department of Geology and the Herbarium of the University of Iowa. Pictures in Martin and Barkley (1961), Beijerinck (1947), and Katz et al. (1965) were also used for comparisons.

360

KENT

The core contains 11.68 m of lake sediment above till. The till contained an abundance of sand grains composed of green shale which were used by Matsch (1971) and Van Zant (1973) to recognize till of the Des Moines lobe. THE PICEA- LARlX POLLEN-ASSEMBLAGE ZONE

The fossil sequence is divided into eight pollen-assemblage zones. The basal zone overlies till and contains 35 -70% Picea and l-4% Lark pollen (Fig. 2). Gramineae, Cyperaceae, and Artemisia pollen each decrease in this zone from nearly 10% of the pollen rain near the base to smaller percentages near the top. Ambrosia-type pollen, however, increases throughout the zone. Fraxinus nigra pollen increases also. Bet&a, Quercus, Abies, Pinus, Ulmus, and Osttya-Carpinus pollen are rather continuously present in small percentages. Salix. Shepherdia canadensis, and Cupressaceae pollen are represented by occasional grains, as are some thermophilous trees such as Juglans nigra, J. cinerea. Celtis, and Fraxinus pennsylvanica-type (Fig. 2). The sediment accumulation rate, pollen concentration, and pollen influx are plotted in Figure 3. The sediment accumulation rate between the two lowermost radiocarbon-dated samples was the slowest recorded within the core. This sediment accumulation rate is extrapolated below the 11 lo- to 1120-cm depth in order to estimate the pollen influx at the beginning of lake sedimentation (Fig. 4). Twenty centimeters of core sediment at the base of this zone was submitted for radiocarbon dating, but too little carbon was present for analysis. The date of 13,990 F 135 yr B.P. (WIS-835) is from near the top of this zone. The pollen influx was approximately 3000-4000 grainslcm’lyr (Fig. 4) at the top of this zone. These figures are consistent with the pollen influx values for the Picea-Larix pollen assemblage zone at Rutz Lake, Minnesota (Waddington, 1969),

VAN

ZANT

the closest site to Lake West Okoboji for which pollen influx data are available. The plant macrofossils within this portion of the core are sparse (Fig. 5). The presence of seeds of Potamogeton sp. and Najas flexilis indicate that the water was deep enough to support submerged aquatic plants. The presence of spruce trees and Larix laricina within the vicinity of the lake is substantiated by needles and one Picea seed. A few seeds of Typha sp. were found within this zone. These fossils apparently represent a vegetation of closing and closed coniferous forest. The fossils compare closely with the Picea -Lark late glacial pollen assemblage zone at Sumner Bog (Van Zant and Hallberg, 1976). This pollen assemblage zone has been found in late glacial sediments from north-central Nebraska (Watts and Wright, 1966) to Ohio (Ogden, 1966), and from Minnesota (e.g., Wright et al., 1963) to Kansas (J. Griiger, 1973). The dates on this zone range from approximately 16,500 yr B.P. (full glacial in age) in southern Missouri (King, 1973), to approximately 10,000 yr B.P. in northeastern Minnesota (Wright and Watts, 1969). The date of 13,990 yr B.P. near the top of this zone at Lake West Okoboji is older than obtained at Sumner Bog (post 11,880 f 170 yr B.P.: I-1862; Ruhe. 1969). THE PICEA-FRAXlNUS POLLEN-ASSEMBLAGE

NIGRA ZONE

This pollen-assemblage zone is characterized by 36-45% Picea. 6- 15% Fraxinus nigra, and 2-3% Larix pollen (Fig. 2). Picea and Larix pollen decrease in abundance while Fraxinus nigra pollen increases. Other deciduous tree taxa (e.g., Betula, Alnus, Quercus, Carpinus, and Corvlus)

Ulmus.

Ostrya-

are well represented by consistently small pollen percentages. Abies and Pinus each account for less than 3% of the pollen sum. Pollen influx was at approximately the same rate as in the Picea-Lark assemblage zone (Fig. 4). A peak in the number of grains of Ambrosia-type pollen

,p-

POLLEN PERCENTAGE

TREES

/ / 0

Depth

2

BP

(ml

ZONES

i

“

Ambrosia i55 shells

2

1

sdty

995255

Quercus-

gyttia,

few

NAP

shells

2745

+_ 60

I

c

3240?

65

52052 Gramlneae

70

shelly

lenses 1

6 Sand

6210+_ 7.

-

I

Ambrosia

and

gravel 7

7730

+_

80

Sand 8

I

I 9075

Organic

Till

silt

I

-

Artemlsla

,

Ambrosia +

90 t

Gramineae

I

Quercus

-

I....LLLLLLLuLLL IO 30 50

70

AND

SHRUBS

-

II

HERBS

7

/-

ENTOMOPHILOUS

HERBS

t’ -d

I

r--

,

;i i I I

;

I I

1

r

r

r

:

LATE

GLACIAL

AND

POSTGLACIAL

POLLEN

AND

PLANT

MACROFOSSILS

363

364

KEN7

VAN

deposited per cm’ per yr occurs near the base of this zone (Fig. 4), just as the pollen percentages of this taxon peak at approximately the same level (Fig. 2). Sediment accumulation was low throughout this zone, and the pollen concentration and influx were essentially the same as the underlying zone (Fig. 3). The number of P&a needles deposited increased but the rate of seed deposition at the coring site remained low (Fig. 5). This pollen-assemblage zone was not found at Pickerel Lake in South Dakota (Watts and Bright, 1968), but their zone correlates to zone Aa at Kirchner Marsh (Wright et al., 1963), and to a portion of zone II at Madelia (Jelgersma, 1962). The zone represents vegetation similar to the Picea-Lark zone except that black ash trees increased in abundance. THE BETULA-ALNUS POLLEN-ASSEMBLAGE ZONE The Betula -Alnus pollen-assemblage

zone is characterized by the rapid rise and fall of Betula (from 2 to 23%) and Afnus (from 2 to 19%) pollen percentages (Fig. 2). The peaks in Bet&a and Alnus percentages are dated at 11,800 ? 110 yr B.P. (WIS836). Picea pollen percentages steadily decrease from approximately 40 to 3% of the pollen rain (Fig. 2). Lark percentages remain less than 2%. Arboreal pollen percentages increase from approximately 75% of the pollen rain to 94% (Fig. 2). Berula and Alnus pollen influx rapidly peaked, then declined in abundance, while Ulmus, followed by Quercus pollen, increased (Fig. 4). By the end of the zone the influx rate of both oak and elm pollen was greater than those of birch and alder ever were. The influx rate of spruce pollen continuously declined. The total arboreal pollen increased dramatically throughout this zone and reached its highest rate of influx recorded, almost 9000 grainsicm’iyr (Fig. 4). The pollen concentration also peaked at the end of this period, and total pollen influx was also high (Fig. 3). Seeds of Mentha ur\~ensis and Lycopus

ZANT

indicate the presence of wet meadow plants around the lake (Fig. 5). Birch seeds and scales were present and some were identified as cf. BPtrrlrr papyi,@rcr (Fig. 5). Piceo and Larix needles were deposited, but the abundance of Picea needles increased in contrast to the pollen production. At the beginning of the overlying zone, Picea pollen percentages were 2% (Fig. 2), but Picea needles peaked in abundance (Figs. 5 and 6). Three explanations seem possible: (1) Perhaps PiceLl trees died when their pollen percentages declined, but their needles were reworked and deposited somewhat later. Figure 6 indicates that birch pollen, seeds, and catkin scales coincide in abundance within the same horizons of the core that spruce pollen percentages were declining. It seems unlikely that spruce needles would be reworked when birch seed or scales were not. (2) Perhaps Picecr pollen percentages fell during this zone because the trees were no longer reproducing, but some of the trees remained alive for some time, 100 yr or so. then shed their needles when they finally died. Wright (1968) suggested that spruce trees might have remained in environments where they could no longer reproduce so long as no other plants were competing more vigorously for their ecological niche. Perhaps at Lake West Okoboji the spruce trees were not out competed until considerably after they could no longer reproduce. (3) Perhaps the most likely hypothesis is that black spruce became increasingly localized in wetlands along the lakeshore while upland spruce stands were decimated. Thus, while pollen values decrease, the abundance of needles might increase. Percentages of arboreal pollen reach their highest percentages at the top of this zone and the bottom of the overlying one (Fig. 2). This zone was produced by a mixed coniferous-deciduous forest with spruce, larch, black ash, birch, and alder as the prominent woody plants. There must have americcInus

PLANT MACROFOSSILS

Depth

(m) l-

G yttjaI

Silty gyttja, abundant

POLLEN 1% yr BP ZONE1 Ambrosia I c 390 2 55

I

2 E I

-

-

2 3 32402 4

65 __

t

------I

5205~ 70 Gramineae

5 Gyt.tja

62102 70 Ambrosia

6

n I

with

I

7 San& 8 ~-

787030’ Gramineae.

-

-

-

-

-

-

-

-

-

-

Artemisia9 IO II Ti II

L-LLuLL.JL-L0 IO

LLLLLA

1

L-J:

LLLYLL-

WET-

MARSH

-

-

I

I

1

I

PLANTS

WET-

-_

ID

I I

7

I

I -

n -

-

-

-

-

-

-

-

-

FIG.

I

5. Plant-macrofossil

diagram for Lake West Okoboji. White bars indicate conifer needles.

“e .+ : +

Seed sum

Ei

,+ + + + : T + --+t + + .+ + + I + -l + +I + E3 + + m+ + + + -,

- -LLLLLLL.LLLLLLL-

LLL

5cales

from

,L

pollen cones K. VanZant

1977

POLLEN

TREES

AND

SHRUBS

I NFLUX

Pollen Depth

lmt

I’ Gvttia Silty

yr

BP

o

Assemblage Zones

,Q’

$

Q

7

I

-,a*

d‘C

ati

r

gyttja,

abundant shell

2

Silty

gyttja,

few

shells

2745+60

32402

65

5205-c

70

4

5

Gyttj

a

with

shelly

-L NAP

3

GramineaeAmbrosia

lenses 62lOk70

6 Sand

and

gravel 7

773Ok 80

Gyttja S ant 8

Gvttia

Gramineae

-

Artemisia

-

Ambrosia Irganic

silt

1075-+ 90

PiceaTi

Lorix

II ILL-8

0

IIL-rLI-I

2 groins

3

3 $.&

x103/cm2/yr

Influx is estimated below the 1110 cm depth us,lng the sedlmentotion rate established between 1045 and Ill0 cm.

2 0”

//--

HERBS

:

c

-

I 7 LLLL---L--L-‘---.

I

4

*

(colonies

FIG.

4. Pollen-influx

I

j---L-L.LLL

x

103/cm2/yr

diagram for Lake West Okoboji.

for

Pediast

rum)

t

LATE

GLACIAL

AND

POSTGLACIAL

POLLEN

AND

PLANT

369

MACROFOSSILS

6 7 8

1.. “0

50

FIG. 6. Pollen percentages and numbers of macrofossils of Picea, from Lake West Okoboji.

been fewer, or perhaps no, open areas as in the previous vegetation, for NAP percentages and influx were very low. This zone marks the end of the late glacial and the beginning of the postglacial. The late glacial/postglacial boundary is dated at approximately 10,230 yr B.P. at Kirchner Marsh (Wright et al., 1963), at approximately 11,230 yr B.P. at Madelia (Jelgersma, 1962), and at 10,670 yr B.P. at Pickerel Lake (Watts and Bright, 1968). The birch and alder pollen peaks are not dated at Woden Bog (Durkee, 1971). The approximate age of ll,OOO-12,000 yr B.P. for this zone at Lake West Okoboji may be slightly old. At Pickerel Lake a 5% peak in Abies pollen coincided with the drop in Picea percentages and just after the peak in Betula percentages (Watts and Bright, 1968). These authors attributed this rise to the re-

,

-L_Ly %

Betula,

Amorpha,

40

20 no

and Ambrosiu

placement of spruce by fir trees and pointed out that fir is a very low pollen producer. A 5% peak in Abies pollen occurred at Lake West Okoboji, also, just before the peak in birch and alder. Possibly, fir trees were replacing spruce trees during this time period. A peak in pine pollen percentages occurs after the birch and alder peaks but before the oak and elm maxima in other diagrams from farther east (e.g., Wright ef al., 1963; West, 1961; Williams, 1974). At Pickerel Lake and Madelia, however, pine percentages remain low (Watts and Bright, 1968; Jelgersma, 1962), suggesting that south-central Minnesota, northeastern South Dakota, and northwestern Iowa are south and west of the area invaded by pine trees. Pine percentages at Lake West Okoboji never are more than 6% during this zone nor higher than 14% on the entire diagram (Fig. 2).

370

KENT

THE QUERCUS-ULMUS POLLEN-ASSEMBLAGE

VAN

ZONE

Ulm~s pollen begins a rapid increase in percentage abundance near the end of the Betda -Alnus pollen-assemblage zone (Fig. 2). Uhrs pollen eventually peaks at 40% of the pollen rain after QllerC.l(s peaks at 30%. By the top of this zone, dated at 9075 & 90 yr B.P. (WIS-832), elm and oak pollen are approximately 15% of the pollen sum (Fig. 2). This zone was deposited from approximately 11,000 to 9075 yr B.P. An 11% peak in Ostvo -Carpinus pollen, a 6% peak in Fraxinus perznsltll,anic.LI-type, and peaks of 2% each in Acer sacchrrtm and Cava pollen occur during this zone. Tilia pollen reaches percentages in excess of 1%. The percentages of arboreal pollen are greater than 90% near the base of this zone but decline to 56% by approximately 9000 yr B.P. (Fig. 2). Ambrosia-type pollen reaches 14% of the pollen rain, then Gramineae, Artemisia, and other Tubuliflorae pollen increase in abundance (Fig. 2). The arboreal pollen influx peaked at the base of this zone (Fig. 4). Influx of Ulmus pollen increased near the base of this zone then declined while the influx of Quercus pollen reached its highest levels on the diagram. The influx rate of elm pollen then peaked at its highest level and began a more rapid decline than oak (Fig. 4). Deposition of Gramineae, Cyperaceae, and Ambrosia-type pollen increased throughout this zone. The sedimentation rate was slightly more rapid throughout this zone than in the preceding ones (Fig. 3). The pollen assemblage indicates that deciduous forest, dominated by elm, oak, hickory, basswood, and ash trees inhabited the area in the early postglacial. This forest bore a pronounced resemblance to the modern deciduous forest of eastern North America. A similar assemblage zone was deposited in southeastern Minnesota (Wright et al., 1963) except that there the Pinus percentages were higher. The forest lasted for a shorter period of time at Lake West Okoboji (to approximately 9000 yr

ZAN-I

B.P.) than in southeastern Minnesota where it remained until approximately 7120 yr B.P. (Wright et trl.. 1963). The Betulu -Alrlus and Quorc.I(.s ~ Ulmus zones at Lake West Okoboji are similar to zone 2 at Pickerel Lake (Watts and Bright, 1968). The forest must have had greater openings in northeastern South Dakota. however, for the NAP percentages were considerably higher than at Lake West Okoboji. Watts and Bright (1968) described the vegetation there as a groveland with woods around depressions and on northfacing slopes and with prairie on the uplands. Elm was thought to be the dominant tree (Watts and Bright, 1968). Apparently a closed deciduous forest, dominated by elm and oak, existed in northwestern Iowa near the beginning of this time period. The uplands became progressively deforested as the climate became warmer and drier. Prairie was established over much of the uplands by 9075 yr B.P. THE GRAMINEAE-ARTEMISIA POLLEN-ASSEMBLAGE

The beginning

-AMBROSIA ZONE

of the Gramineae-Ar-

temisia -Ambrosia pollen-assemblage zone is marked by the rise in Gramineae, Artemisia. and Chenopodiaceae - Amaran-

thaceae pollen percentages and rate of influx, by greater than 10% Ambrosirr-type pollen, and by the rapid decline in Ulmus and arboreal pollen percentages (Figs. 2 and 4). Arboreal pollen decreases from 56 to 16% during this zone (Fig. 2). Artrmisia increases to nearly 20%, then declines; Gramineae percentages rise to approximately the same level throughout the zone. Ambrosia-type pollen percentages increase rapidly at the beginning and less dramatically throughout the zone. The pollen influx of Ambrosia-type pollen was sporadic, however (Fig. 4); the influx increased at the base of the zone, but fluctuated considerably before finally decreasing. The influx curve of taxa included within the pollen sum indicates a gradual

LATE

GLACIAL

AND

POSTGLACIAL

decrease in pollen deposition throughout this zone (Fig. 4). Pollen influx decreased from over 10,000 to 2000 grains/cm’/yr in this zone while the pollen concentration decreased from 180,000 to 1500 grains/cm” (Figs. 3 and 4). The decrease in pollen influx as well as pollen concentration indicates that the changes were due not only to an increase in particle size but also to a more rapid sedimentation rate (Fig. 3) and perhaps to a decrease in the number of pollen grains reaching the site of deposition per year, a potential measure of pollen production. Seeds of deeper-water aquatics, such as Najas jlexilis and Ceratophyll44m demerst4m, increase then decrease near the top of the zone as seeds of shallow-water aquatics, such as Typha sp. and Eleocharis palt4stris-type increase. Leaves, pods, and pollen of Amorpha canescens, a prairie plant, appear simultaneously (Fig. 6). The first appearance of pollen of Petalostemum sp. suggests the presence of prairie vegetation. Seeds of woodland plants are essentially absent from this zone. Seeds or fruits of Chenopodium album, cf. Panic44m capillare, C. rubrum, Potamogeton pectinatus, and Zannichellia palustris were found within this zone and were also used by Birks (1973) as indicators of prairie vegetation around the lake. Potamogeton spirillus was found within this zone at Lake West Okoboji but was found only in modern lakes surrounded by coniferous vegetation (Birks, 1973). A comparison of plant macrofossils from this zone with the water quality of lakes where they are found today (Table 1) suggests that Little Millers Bay contained slightly brackish water during deposition of this zone. Furthermore, the presence of a sand lens between 750 and 760 cm and the increase in macrofossils suggests that the coring site was nearer to the shore than it now is. The increase in prairie plant pollen, seeds, and leaves, and the decrease in woodland fossils, suggests that the climate was drier. Some seeds and Amorpha leaves

POLLEN

AND

PLANT

were charred, prairie fires.

MACROFOSSILS

371

suggesting the presence of

THE GRAMINEAE-AMBROSIA POLLEN-ASSEMBLAGE ZONE

This zone was deposited between approximately 7730 and 3240 yr B.P. and is characterized by a decrease in Artemisia pollen and the rapid increase in Ambrosia pollen (Fig. 2). Gramineae and particularly Ambrosia-type percentages vacillate throughout this zone, and sometimes do so by as much as 20% between successive samples (Fig. 2). ChenopodiaceaeAmaranthaceae pollen reaches 18% during this zone. By contrast, arboreal pollen percentages reach their lowest levels (5%) during this zone (Fig. 2). Some arboreal taxa are absent, such as Tilia, Juglans cinerea, and Platanus, while others, such as Acer saccharinum, Jt4glan.s nigra, and Celtis, are rare. Typha latifolia is more abundant toward the beginning and end of this zone. A peak in Myriophyllum pollen follows the peak in Typha latifolia toward the end of the zone (Fig. 2) and may suggest that Myriophyllum plants replaced Typha latifolia near the coring site. Perhaps water depth was increasing then. Pollen of Lemna is restricted to this zone. Equisetum spores are more abundant within this zone than elsewhere in the core (Fig. 2). Sand and gravel units, such as between 600 and 705 cm, had a low pollen concentration and influx (Fig. 3). The Ambrosiatype and Gramineae pollen influx curves (Fig. 4) are less “saw-toothed” in appearance than their percentage curves (Fig. 2). This suggests that the ratio of one type of pollen to another may have varied radically between samples but the rate of influx of each pollen species was more consistent. Seeds of shallow-water and wet-marsh plants, such as Zannichellia palustris or Scirpus validus-type, are abundant within

372

Water Slightly Vegetation

species

Fresh

quality

Moderately brackish

brackish

X

A

A

X

A

X

X

A

A

A

Brackish

Subsaline

X

A

A

A

A

X

X

X

X

X

X

A

” The range in water ly common or abundant:

X

A

X

quality is of the North Dakota potholes X = occasionally fairly common.

this zone. Wet-ground-weed Cyperus capillare,

X

odoratus-type

seeds, such as or cf. Panicurn

are abundant in certain samples. Some species’ seed abundance fluctuates dramatically between samples. This may be due to a rapidly fluctuating lake level. The sand and gravel unit between 705 and 600 cm contains fewer plant macrofossils than the surrounding gyttja. The sedimentological, pollen, and plant

(from

Stewart

X

and

Kantrud,

1972).

A = frequent-

macrofossil evidence suggest that Little Millers Bay almost dried up between approximately 7000 and 6300 yr B.P. Lake level was 9 to 10 m below its present level. Thin laminae of gyttja within the coarser sediments imply that lake level may have fluctuated continually throughout this time period. Watts and Winter (1966) suggested a similar fluctuating lake level at Kirchner Marsh during the Hypsithermal.

LATE

GLACIAL

AND

POSTGLACIAL

The 7730 -+ 80 yr B.P. (WIS-830) to 3240 + 65 yr B.P. (WIS-828) dates for this zone bracket the Hypsithermal as dated farther east (e.g., Wright et al., 1963). The forest fringe around Lake West Okoboji may have completely disappeared during this time period. Prairie probably extended to the very edge of the lake. Willow trees grew in various places around the lake’s edge in the most protected areas, and perhaps a few oak, elm, and green or white ash trees may have grown in low-lying areas. Wet-ground weeds, such as Polygonurn lapathifolium, cf. Panicum capillare. and Ambrosia artemisiifolia, grew in wet areas around the lake. Little Millers Bay probably was isolated from the rest of the lake and nearly dried up periodically during this warmest, driest part of the postglacial. THE QUERCUS-NAP ZONE

ASSEMBLAGE

This zone was deposited from approximately 3240 yr B.P. to the time of settlement by Europeans. It is characterized by higher percentages of Quercus pollen (12-28%), arboreal pollen, and lower, more consistent percentages of Ambrosia-type, Cyperaceae, Chenopodiaceae - Amaranthaceae, and Gramineae pollen (Fig. 2). The presence of Platanus pollen within this assemblage is something of an enigma. The pollen reappears soon after the Gramineae-Ambrosia pollen assemblage zone and is rather continuously present throughout the top 3 m of the core. But sycamore trees did not grow near Lake West Okoboji prior to settlement by Europeans (McBride, 1899). Fowells (1965) mapped Platanus trees only as far north and west in Iowa as Fort Dodge, 160 km south and east of Lake West Okoboji. Apparently Platanus pollen blew into the Lake West Okoboji area from the southeast or else these trees extended farther northwest in Iowa than previously thought. Pinus pollen peaked (14%) at 995 ? 55 yr B.P. (WIS-827). This percentage is too low to consider that pine trees were growing in

POLLEN

AND

PLANT

373

MACROFOSSILS

northwest Iowa at that time. Instead, these grains must have blown in from Minnesota or the Pine Ridge Escarpment in Nebraska. The distance to the nearest native pine trees in Minnesota is approximately 320 km from Lake West Okoboji (Watts and Wright, 1969); to the Pinus ponderosa stands in north-central Nebraska it is approximately 400 km (Wells, 1970). Iowa lies within the zone of dominant westerly winds, but Bryson (1966) has shown that the dominant streamlines of surface wind in April and May in Minnesota are from the north. Therefore, if Minnesota pine trees pollinated while the air was flowing southward, the chances would be greater for deposition of pine pollen at Lake West Okoboji. Both Haploxylon and Diploxylon pine pollen are present. The rate of pollen influx appears higher between 3240 and 2745 ? 60 yr B.P. (WIS829) than throughout the rest of the zone. But because each taxon increased in influx between these two dates, then uniformly declined after 2745 yr B.P., it seems likely that the values are artificial and related to the radiocarbon dates. Plant macrofossils are less common within this zone. Deeper-water aquatics, such as Najas jlexilis, increase in abundance relative to other fossils, but compared to the underlying zone, there are far fewer fossils. THE AMBROSIA POLLEN-ASSEMBLAGE

ZONE

A significant change in the pollen content occurs at the 40-cm horizon within the core (Fig. 2). Ambrosia-type pollen begins a rapid increase while pollen of other weedy plants, such as Chenopodiaceae-Amaranthaceae, increase less dramatically. Some tree pollen, such as Ulmus, Salix, and Fraxinus pennsylvanica-type pollen, increase in abundance while others, e.g., Carya, Quercus. and Pinus, decrease. This change in the pollen rain is attributed to settlement by European immigrants and the beginning of modern agriculture within the vicinity of Lake West Okoboji.

374

KENT

VAN

Census statistics from Dickinson County suggest that a significant number of acres went into crop production between 1863 and 1865 (Hull, 1883). The timber within the county at the time of settlement bordered Lake West Okoboji and Lake East Okoboji (McBride, 1899). Hull (1883) and McBride (1899) indicate that these trees were extensively cut for fuel between 1865 and 1880. The decrease in the pollen influx of Ca~a and Quercu~ (Fig. 4) are probably attributable in part to this lumbering. Salix and Fraxinus pennsylvanica-type pollen, on the other hand, increase in abundance. McBride (1899) noted that these taxa were planted by early settlers for windbreaks around farmyards. Similarly, the general increase in Ulmus pollen is no doubt related to man’s planting elm trees in farmyards and to the fugitive nature of the genus in areas of disturbed soil. A pronounced change occurred in seed deposition after the 15to-20-cm horizon. Shallow-water aquatics, such as Zannichellia palustris and Myriophyllum exalbescens, peaked at this level (Fig. 5), just after a peak in Myriophyllum pollen (Fig. 2). This change may correlate to the stabilization of lake level at 425.5 m (1396 ft) in 1910. After this horizon, macrofossils of upland plants, such as Amorpha canescens, are less common. The sediment accumulation rate and pollen influx increased dramatically during this zone (Fig. 3). The more rapid rate of sedimentation was apparently caused by the increased erosion that occurred when the prairie sod was plowed for agriculture. The increase in the pollen influx may be attributable to the number of pollen grains retained at the site of deposition rather than washed to the deeper parts of the lake, just as Waddington (1969) suggested for Rutz Lake. This idea is supported by the fact that as the macrofossils of shallow-water aquatics decline at approximately the 20-cm horizon, so too does the pollen influx (Fig. 4). PEDIASTRUM REMAINS Pediastrum, a colonial alga, is present in

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the lowest portion of the core but becomes very abundant during the Gramineae -Ambrosia pollen-assemblage zone (Fig. 2). Pediastrrrm was identified to species using Prescott (1931, 1951) and Harper (1916), and these percentages are plotted in Figure 7. P. boryanum is consistently the most abundant species. P. integrum is not absent until the top 130 cm of the core. P. duplex increases in abundance within the top 2 m of the core while P. simplex tends not to be present when P. duplex is found. The absolute abundance of Pediastrum colonies was calculated just as were pollen influx values. The figures (Fig. 7) indicate that high percentages of Pediastrum occur within the sand and gravel, between 600 and 705 cm, but the concentration and absolute abundance were small (Fig. 4). Sebestyen (1969) suggested that Pediastrum remains in lake cores are an indication of open water, perhaps with high water levels. The absolute Pediastrum abundance figures compared to the sedimentation record at Lake West Okoboji would support this idea. The rapid increase in the abundance of Pediastrum after about 7730 yr B.P. is probably not attributable to a change in water depth but instead to a change in the lake’s chemistry. RADIOCARBON GLACIAL

DATES AND THE CHRONOLOGY

The radiocarbon chronology of Lake West Okoboji does not match the glacial sequence for Iowa. Ice of Cary age is thought to have entered Iowa approximately 14,000 yr B.P., reached central Iowa soon thereafter, and retreated out of the state by 13,000 yr B.P. (Ruhe, 1969). Lake West Okoboji is located in an area where the Altamont moraine overlaps the Bemis moraine, suggesting that as the glacier ice was retreating in north-central Iowa, the ice readvanced in northwestern Iowa and overrode its previous limit. The oldest radiocarbon dates on the Lake West Okoboji core are older than one would expect for a lake located behind the Altamont moraine.

LATE

GLACIAL

AND

POSTGLACIAL

POLLEN

AND

PLANT

MACROFOSSILS

375

KENI-

376

\‘.4&

The dated samples near the base of the core may have contained older carbon from Paleozoic and Cretaceous bedrock and from mollusk shells. The basal pollen samples contained larger numbers of preQuaternary spores (plotted as “other triletes” on Fig. 2) and acritarchs. These spores reached a second abundance within the sand and gravel unit deposited during the mid-postglacial. Their presence is probably related to the age of the rock from which the sand and gravel was derived. Common sand- and silt-sized particles within the basal meter of the core were gray, calcareous pieces of shale. Radiocarbon dates from an earlier core of Little Millers Bay indicate a basal age of 12,700 yr B.P. (Collins, 1968). Pollen samples from near this depth indicate the presence of less late glacial sediment than is present in the 1975 core, suggesting that the earlier core may not have penetrated through a complete late glacial sequence. The date of 11,800 2 110 yr B.P. (WIS836) for the fall in Picra percentages is slightly older than the dating of this event at other places in the Midwest. Other dates from this core of Lake West Okoboji are in general agreement with other dated cores from the area (e.g., Wright ct al., 1963; Durkee, 1971). CORRELATION

TO OTHER IOWA

SITES IN

The late glacial sequence from Lake West Okoboji contains lower percentages of Pinus and Abies pollen than Durkee (1971) or Brush (1967) found. For example, Durkee (1971) plotted 30% pine and 15% fir in the lowermost zone at Woden Bog. Brush (1967) plotted greater than 50% fir pollen and essentially no pine pollen at McCulloch Bog. At Co10 and Jewel1 bogs there was consistently more fir than spruce pollen (Brush, 1967). At the base of Jewel1 Bog (apparently mislabeled “Co10 Bog” in Brush, 1967, Fig. 8, p. llO), A&es percentages approached 75% and were replaced by high pine percentages. At Sumner Bog in northeastern Iowa an

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open taiga vegetation was replaced by a spruce-larch forest (Van Zant and Hallberg. 1976). Fir and pine were minor constituents of the pollen rain. It seems unlikely that high percentages of fir and pine pollen could have been deposited in north-central Iowa while high spruce percentages were deposited in northeastern and northwestern Iowa. Perhaps fir and pine pollen were misidentified by Durkee (1971) and Brush (1967). COMPARISON TO THE POLLEN INFLUX SEQUENCE AT RUTZ LAKE

Rutz Lake, Minnesota (Waddington, 1969). is the closest site to Lake West Okoboji for comparison of pollen influx statistics. Rutz Lake is located in the Big Woods of Minnesota, approximately 190 km northeast of Lake West Okoboji. The method for determining the pollen concentration at Rutz Lake involved suspending exotic pollen grains in corn syrup rather than using Elrcalyptus pollen tablets. Otherwise the two methods were identical. The pollen-influx diagram from Rutz Lake compares favorably to the pollen-influx curve of Figure 3. No influx values are present for the individual species at Rutz Lake (Waddington, 1969). Generally, the pollen-influx at Rutz Lake was greater than at Lake West Okoboji. The two sequences were similar from approximately 12,000 to 8000 yr B.P. At both sites the pollen influx increased from approximately 5000 grains/cm”lyr to the lO,OOO- 15,000 grainsicm’iyr level during this time period. After approximately 8000 yr B.P. the influx decreased rapidly at Lake West Okoboji (Fig. 3) but remained fairly constant at Rutz Lake (Waddington, 1969). A peak in pollen influx of nearly 40,000 grains/cm”/yr near the top of the Rutz Lake sequence occurred approximately 50 yr ago. This peak is attributed to aquatic grasses inhabiting the area around the coring site or to limnological events (Waddington, 1969). This peak is similar to the postsettlement increase in influx at Lake West

LATE

GLACIAL

AND

POSTGLACIAL

Okoboji, but the pollen sequence at Rutz Lake contains no indication of disrupted vegetation due to European settlers. VEGETATIONAL

HISTORY

A spruce-larch forest inhabited northwestern Iowa as glacial ice melted and Little Millers Bay began to hold ponded water. This forest extended from north-central Nebraska to Ohio, and from Minnesota to Kansas. Fir trees were a minor constituent of this forest. Pine trees were probably absent; certainly they were not present in any abundance. Birch, especially Betula pupyrifera, and alder increased in abundance while spruce declined and the climate became warmer toward the end of the late glacial. Black ash was an important constituent of the forest and increased in abundance as birch and alder declined. Some spruce, probably black spruce, may have survived in protected areas, but may have ceased pollen production prior to eventually dying. A peak in fir pollen (5%) occurs just before the peak in birch and alder pollen (Fig. 2). Perhaps fir trees were replacing some spruce trees just as Watts and Bright (1968) suggested at Pickerel Lake. Pine forest did not reach northwestern Iowa during the late glacial and postglacial. Pine pollen percentages remained low at Madelia (Jelgersma, 1962), Pickerel Lake (Watts and Bright, 1968), and at Lake West Okoboji (Fig. 2). At the beginning of the postglacial, dated approximately at 11,000 yr B.P. at Lake West Okoboji, a deciduous forest inhabited northwestern Iowa. Oak and elm were dominant constituents of the forest although Ostrya -Carpinus, Corylus. Fraxinus, Carya. Juglans cinerea, J. nigra, Celtis. Acer saccharum, and Tilia were also

present in some abundance. This forest was entirely closed at the beginning of the postglacial. But as the climate became warmer and drier, herbs became more abundant as meadows opened in the forest. By 9000 yr B.P. elm trees, and to a lesser extent oak

POLLEN

AND

PLANT

MACROFOSSILS

377

trees, were rather rapidly decreasing in abundance. Other deciduous trees were declining in abundance, too, except for willow, which was persisting or establishing itself in groves around the lake’s edge. Ragweed and grass became more abundant as the forest opened and lake level fell, thereby exposing ground for fugitive species. Between 9075 t 90 yr B.P. (WIS-832) and 7730 t 80 yr B.P. (WIS-830) prairie became established on the uplands and, by the end of this period, extended to the water’s edge. Charred leaves, seeds, and fruits suggest that prairie fires aided the destruction of the remaining deciduous forest. By 7730 yr B.P. the only stands of forest were in protected, low-lying areas. Amorpha canescens and Petalostemum appeared soon after 9000 yr B.P. These plants are indicative of prairie vegetation and their presence suggests that prairie extended to the water’s edge. The decrease in water level is marked by the decrease in macrofossils of deeper water aquatics, such as Najas j7exilis, and the increase in shallow water aquatics, such as Typha sp.. at the coring site. Between about 7700 and 3200 yr B.P., Little Millers Bay became shallower and nearly dried up. Sand and gravel were deposited between approximately 6300 and 7000 yr B.P. although thin laminae of silty gyttja occur within this sediment. The erratic nature of the pollen-percentage curves of the Ambrosia pollen and other weedy species, and the silty gyttja layers suggest that lake level fluctuated, thus alternately creating and destroying sites for fugitive, weedy species. Water level must have been approximately 9 to 10 m lower than the present level. The pollen influx was very low (approximately 100 grains/cm”/yr) between 6300 and 7000 yr B.P. (Fig. 3). This low influx may be due in part to the lack of a moist collecting site as the sand was deposited. There were erratic fluctuations in pollen percentages of Ambrosia and other weedy taxa but the more constant pollen-influx rates

KENT

378

VAN

support the fluctuating lake level hypothesis. Plant macrofossils were more abundant below and above than within the 105 cm of sand and gravel (Fig. 5), also suggesting the lack of an adequate environment of preservation throughout this zone. Shallow-water aquatic plants, such as Scirpus validustype, occurred throughout the sand zone and were extremely abundant just above this zone (Fig. 5). These seeds, and those of deeper-water aquatic plants, such as Najas flexilis, may have been washed into the area. The presence of Ceratophyllum demersum seeds, however, suggests that standing water must have been present at least occasionally. These seeds would easily be destroyed by transportation. The water level increased at Lake West Okoboji at approximately 6300 yr B.P., and silty gyttja was once again deposited (Fig. 2). Oak trees became more common in low-lying areas around the lake after about 5200 yr B.P. Elm and other deciduous trees became slightly more abundant after 3240 _t 65 yr B.P. (WIS-828). Ostrya-Carpinus, Acer saccharinum, cinerea, J. nigra,

Carya,

Tilia,

Juglans

ZANT

SUMMARY

The vegetational history of northwestern Iowa can be summarized as follows: (1) A closed coniferous forest, dominated by spruce and larch, inhabited the area during the late glacial. Fir and pine pollen were never abundant, and no macrofossils of these trees were found. Fir trees were probably present in the area in small numbers. but pine trees were probably absent. (2) Black ash trees migrated into the area by 13,500 yr ago and became an important constituent of the forest. (3) Birch and alder trees migrated into the area and were dominant by 11,800 yr B.P. (4) From approximately 11,000 to 9000 yr B.P. a deciduous forest dominated northwestern Iowa. Oak and elm dominated the pollen rain. (5) Pollen and plant macrofossils of prairie species became increasingly abundant after 9000 yr ago. Charred remains of prairie plants, for example Amorpha c’anestens, suggest that tire may have been a major cause for the demise of the forest. Pollen influx declined as prairie replaced forest as the dominant vegetation. (6) From 7700 to 3200 yr B.P. prairie extended to the lake’s edge. Pollen influx decreased sharply. (7) Climatic conditions ameliorated after 3200 yr B.P., and trees reinhabited the area around the lake. (8) The settlement by Europeans and the beginning of agriculture in northwestern Iowa is marked by an increase in Ambrosia pollen.

and Celtis reappeared or increased in abundance about that time. Trees occupied the protected areas while prairie inhabited the uplands. A peak in pine pollen approximately 1000 yr B.P. is attributable to long-distance pollen transport, not to the presence of these trees at Lake West Okoboji. The settlement by Europeans in Dickinson County about 1865 increased the abundance of weedy species. The plowing of the prairie sod within historical times is recorded on the pollen diagram by the decrease in Gramineae and Tubuliflorae pollen APPENDIX: NOTES ON PLANT (Fig. 2). Some trees were cut for fuel and MACROFOSSIL IDENTIFICATIONS lumber, such as oak and hickory, and others, such as elm, willow, and ash, were Scirpus validus-type could be S. \,alidus planted in windbreaks and farmyards. or S. acutus. S. validus seems to be the The stabilization of lake level at 425.5 m most likely. Eleocharis palustris-type could be E. (1396 ft) in 1910 raised the level of Little Millers Bay and destroyed the flourishing palustris or E. Calva. Myriophyllum exalbescens population that Verbena hastata-type could be V. hasgrew near the coring site. tata or V. utricifolia.

LATE GLACIAL

AND POSTGLACIAL

Cyperus odoratus-type includes odoratus and C. ferruginescens.

POLLEN

C.

These were differentiated by size and the tip from C. strigosus (Watts and Bright, 1968, p. 875). ACKNOWLEDGMENTS R. G. Baker helped with the field work and in the laboratory, and he gave freely of his time for discussions. R. G. Baker, R. Benson, C. Lee, T. Legg, L. Moscosco, and D. Smith helped with the coring. NSF Grant BHS76-06799 paid for the radiocarbon dates. Earlham College paid for the cost of the foldout diagrams. E. Powell typed the manuscript.

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197-209. Durkee, L. H. (1971). A pollen profile from Woden Bog in northcentral Iowa. Ecology 52, 837-844. Faegri, K., and tversen, Johs. (1975). “Textbook of Pollen Analysis,” 3rd ed. Hafner, New York. Fowells, H. A. (1965). “Silvics of Forest Trees of the United States.” United States Department of Agriculture Handbook 271. Griiger, E. (1972). Late Quaternary vegetation development in south-central Illinois. Quaternary Research 2, 217-231. Grtiger, J. (1973). Studies on the Late Quaternary vegetation history of northeastern Kansas. Geological Sociefy of America Bulletin 84, 239-250. Harper, R. A. (1916). On the nature of types in Pediastrum. Garden 6,

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91- 104. Hull, J. A. (1883). “Census of Iowa for 1880.” State of Iowa, Des Moines. Jelgersma. S. (1962). A late-glacial pollen diagram from Madelia, south-central Minnesota. American Joarnal

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Kapp, R. 0. (1969). “How to Know Pollen and Spores.” Brown, Dubuque, Iowa. Katz, N. Ja., Katz, S. V., and Kipiani, M. G. (1965). “Atlas and Keys of Fruits and Seeds Occurring in the Quatemary Deposits of the U.S.S.R.” Nauka, Moscow. King, J. E. (1973). Late Pleistocene palynology and biogeography of the western Missouri Ozarks. Ecological

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Lane, G. H. (1931). A preliminary pollen analysis of the East McCuIloch peat bed. Ohio Journal of Science 31, 165-171. Maher, L. J., Jr. (1972). Absolute pollen diagram of Redrock Lake, Boulder County. Colorado. Quaternary Research 2, 531-553. Martin, A. C., and Barkley, W. D. (1961). “Seed Identification Manual.” University of California Press, Berkeley. Matsch, C. L. (1971). “Pleistocene Stratigraphy of the New Ulm Region, Southwestern Minnesota.” Unpubl. Ph.D. thesis, University of Wisconsin, Madison. McAndrews, J. H., Berti, A. A., and Norris, G. (1973). “Key to the Quaternary Pollen and Spores of the Great Lakes Region.” Royal Ontario Museum Life Science Miscellaneous Publications. McBride, T. H. (1899). Geology of Osceola and Dickinson counties. Iowa Geological Survqy Annual Reports 10, 185-239. Ogden, J. G., III. (1966). Forest history of Ohio. I. Radiocarbon dates and pollen stratigraphy of Silver Lake, Logan County, Ohio. Ohio Journal ofScience 66, 387-400. Prescott, G. W. (1931). Iowa algae. University of lowa Studies In Nataral History 13, l-235. Prescott, G. W. (1951). Algae of the western Great Lakes area. Cranbrook Institute of Sciences 31, l-946. Ruhe, R. V. (1969). “Quaternary Landscapes In Iowa.” Iowa State University Press, Ames. Sebestyen. 0. (1969). Studies on Pediastrum and cladoceran remains in the sediments of Lake BaIaton, with reference to lake history. Mitteilungen International Verein. Limnology 17, 292-300. Stewart, R. E., and Kantrud, H. A. (1972). Vegetation of prairie potholes. North Dakota. in relation to quality of water and other environmental factors. United

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585-D, Dl -D36 Van Zant, K. L. (1973). “Pleistocene Stratigraphy in a Portion of Northwest Iowa.” Unpubl. M.S. thesis, University of Iowa, Iowa City. Van Zant, K. L., and Hallberg, G. R. (1976). A lateglacial pollen sequence from northeastern Iowa: Sumner Bog revisited. Iowa Geological Survey Technical Information Series 3, 1- 17. Waddington, J. C. B. (1969). A stratigraphic record of the pollen influx to a lake in the Big Woods of Minnesota. Geological Society of America Special Paper 123,263-282.

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Watts, W. A., and Bright, R. C. (1968). Pollen, seed, and mollusk analysis of a sediment core from Pickerel Lake, northeastern South Dakota. Groloaic,nl Society of America Bulletin 79. 855-876. Watts, W. A., and Winter, T. C. (1966). Plant macrofossils from Kirchner Marsh. Minnesota-A paleoecological study. Geologica/ Society of America Bulletin Watts, W. A., and

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Wright, H. E.. Jr. (1966). Late Wisconsin pollen and seed analysis from the Nebraska sandhills. Ecology 47, 202-210. Wells, P. V. (1970). Historical factors controlling vegetation patterns and floristic distributions in the central Plains region of North America. In “Pleistocene and Recent Environments of the Central Great Plains” (W. Dort, Jr.. and J. K. Jones. Jr., Eds.), pp. 211-221. University Press of Kansas, Lawrence. West, R. G. (l%l). Late- and postglacial vegetational

history in Wisconsin, particularly changes associated with the Valders readvance. Americcrn Journal

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Williams. A. S. (1974). Late-glacial-postglacial vegetational history of the Pretty Lake region, northeastern Indiana. (/nited StaresGer&~~ic.crl Sunq Proji~ssional Paper 686, D I- D23. Wright, H. E.. Jr. (1968). The roles of pine and spruce in the forest history of Minnesota and adjacent areas. Ecology 49, 937-955. Wright, H. E., Jr.. and Watts. W. A. (1969). Glacial and vegetational history of northeastern Minnesota. Minnesota Geological Series 11. I-59.

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Wright, H. E., Jr.. Winter. R. C., and Patten, H. L. (1963). Two pollen diagrams from southeastern Minnesota: Problems in the regional late-glacial and postglacial vegetational history. Geologkal Societ) of America Bulletin 74, 1371- 1396.