Patterns and drivers of Holocene vegetational change near the prairie-forest ecotone in Minnesota: revisiting McAndrews' transect.

Abstract

Holocene vegetational dynamics along the prairie-forest border of Minnesota were first documented in McAndrews' classic work. Despite numerous subsequent paleo-studies, a number of questions remain unanswered about the vegetation history of the region. Here, pollen, stable-isotope, mineral, and charcoal data are described from three lakes near McAndrews' sites. These data were compared with other paleoenvironmental records to reconstruct vegetation, aridity, and fire. The climate was relatively wet with increasing summer temperatures before approximately 8000 yr before present (BP). The rates of changes were asymmetric for the onset and termination of middle-Holocene aridity, with an abrupt increase at approximately 8000 yr BP and a gradual, but variable, decline from approximately 7800 to 4000 yr BP. Early-Holocene coniferous forests changed to mixed-grass prairie without an intervening period of tallgrass prairie or deciduous forest, whereas the retreat of prairie was characterized by transitions from mixed-grass to tallgrass prairie to deciduous forest and finally to coniferous forest. Within the middle Holocene, the composition and structures of grass-dominated vegetation varied both temporally and spatially. Fire primarily responded to changes in climate and fuel loads. Vegetation was more strongly influenced by climatic changes than by fire-regime shifts.

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Blackwell Publishing Ltd
Patterns and drivers of Holocene vegetational change near
the prairie–forest ecotone in Minnesota: revisiting
McAndrews’ transect
David M. Nelson1 and Feng Sheng Hu2,3,4
1Institute for Genomic Biology, University of Illinois, 1206 West Gregory Drive, Urbana, IL 61801, USA; 2Department of Plant Biology, 3Program in Ecology
and Evolutionary Biology, and 4Department of Geology, University of Illinois, 265 Morrill Hall, Urbana, IL 61801, USA
Summary
• Holocene vegetational dynamics along the prairie–forest border of Minnesota
were first documented in McAndrews’ classic work. Despite numerous subsequent
paleo-studies, a number of questions remain unanswered about the vegetation
history of the region. Here, pollen, stable-isotope, mineral, and charcoal data are
described from three lakes near McAndrews’ sites. These data were compared with
other paleoenvironmental records to reconstruct vegetation, aridity, and fire.
• The climate was relatively wet with increasing summer temperatures before
~8000 yr before present (BP). The rates of changes were asymmetric for the onset
and termination of middle-Holocene aridity, with an abrupt increase at ~8000 yr BP
and a gradual, but variable, decline from ~7800 to 4000 yr BP.
• Early-Holocene coniferous forests changed to mixed-grass prairie without an
intervening period of tallgrass prairie or deciduous forest, whereas the retreat of prairie
was characterized by transitions from mixed-grass to tallgrass prairie to deciduous
forest and finally to coniferous forest. Within the middle Holocene, the composition
and structures of grass-dominated vegetation varied both temporally and spatially.
• Fire primarily responded to changes in climate and fuel loads. Vegetation was
more strongly influenced by climatic changes than by fire-regime shifts.
Key words: climate, fire, Holocene, McAndrews’ transect, Minnesota, pollen,
prairie–forest border.
New Phytologist (2008) 179: 449–459
© The Authors (2008). Journal compilation © New Phytologist (2008)
doi: 10.1111/j.1469-8137.2008.02482.x
Author for correspondence:
David M. Nelson
Tel: +1 217 333 4376
Fax: +1 217 333 0508
Email: dmnelson@life.uiuc.edu
Received: 18 February 2008
Accepted: 20 March 2008
Introduction
In the mid-1960s, John McAndrews pioneered the ‘transect’
concept in paleo-biogeography by analyzing pollen assemblages
in sediments from four ponds across the prairie–forest border
in northern Minnesota (Fig. 1). His results demonstrated that
during the middle Holocene the prairie–forest border was
~100 km east of its location during the early and late
Holocene (McAndrews, 1966, 1967). These results implied
that the middle Holocene was warm and dry, although the
timing of prairie expansion and contraction was equivocal
because it was constrained by a total of only five 14C dates on
bulk sediments. McAndrews’ work was followed by numerous
paleoecological and paleoclimatic studies in Minnesota (e.g.
Watts & Winter, 1966; Wright & Watts, 1969; Jacobson &
Grimm, 1986; Brugam et al., 1988; Bartlein & Whitlock, 1993;
Clark etal., 2001; Camill etal., 2003; Nelson etal., 2004; Wright
etal., 2004) and surrounding regions (e.g. Wright, 1968; Baker
et al., 1992; Dorale et al., 1992; Laird et al., 1996a; Nelson et al.,
2006). Many of these studies involved pollen analysis, and a sum-
mary map of ‘prairie forbs’ confirmed that the pronounced
movement of the prairie–forest border occurred during the
middle Holocene (Webb et al., 1983). Recently, geochemical,
sedimentological, and diatom analyses have also been used to
examine Holocene climatic changes in the midwestern USA
and the Great Plains (e.g. Valero-Garcés et al., 1997).

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Despite the tremendous influence of McAndrews’ transect
on numerous paleoenvironmental studies over the past four
decades, the patterns and drivers of the pronounced shift of
the prairie–forest border have not been evaluated in light of
new paleorecords from northern Minnesota. One reason is
that pollen records with secure chronologies are scarce from
sites near McAndrews’ transect (Wright et al., 2004). In
addition, pollen-independent climatic and fire reconstructions
remain limited; such reconstructions are necessary to avoid the
circularity of using pollen data as an indicator of environmental
changes and also as a measure of the response of vegetation to
environmental changes (Webb et al., 2003). Deciphering how
climate and fire influence Holocene vegetational fluctuations
can help to address longstanding issues concerning the controls
of middle-Holocene prairie expansion. Although this expansion
has often been attributed to warm and dry conditions (e.g.
McAndrews, 1966, 1967; Bartlein et al., 1984; Shuman et al.,
2002) other evidence suggests that such conditions may be
less important. For example, paleorecords from Minnesota
(e.g. Grimm, 1984) and Illinois (Nelson et al., 2006), as well
as historical data from Illinois (e.g. Gleason, 1913), highlight
fire as a key control on the relative abundance of woody
versus herbaceous vegetation along the prairie–forest border.
In addition to the relative roles of climate and fire, other
important questions remain unanswered. For example, how
do aridity and temperature interact to affect vegetational
dynamics? What was the spatial and temporal variability of
climate, vegetation, and fire within the middle Holocene?
To help elucidate Holocene changes in vegetation, climate,
and fire regimes along the prairie–forest border, here we syn-
thesize records with multiple indicators from the well-dated
Holocene sediments of three lakes in northwestern Minnesota
located near McAndrews’ original west–east transect. We include
our previously published data (Hu et al., 1997; Hu et al.,
Fig. 1 Pre-European-settlement vegetational
map of Minnesota and adjacent states
(adapted from Küchler, 1964) with locations
of some of the sites mentioned in the text.
Sites used in Fig. 4 and Supplementary
Material Fig. S1 are represented by squares
(ML, Moon Lake; WOL, West Olaf Lake; DL,
Deep Lake; SL, Steel Lake). Sites in Minnesota
from McAndrews’ study (1966, 1967) are
represented by circles, as are sites in Iowa
(LO, Lake Okoboji; CL, Clear Lake), which are
also shown in Fig. 5.

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1999; Nelson et al., 2004; Wright et al., 2004) as well as
results of new analyses from the three study sites. We also
compare our data with paleoecological and paleoclimatic records
from adjacent areas and highlight recent studies that provide
insights into Holocene environmental changes in the region.
Materials and Methods
Study sites
The three primary sites discussed in this paper span the
major vegetational formations of Minnesota before European
settlement, as delineated by the Marschner map based on
land-survey records (Heinselman, 1974). Steel Lake (46°58′N,
94°41′W, 23 ha, 415 m above sea level (asl)) is in coniferous
forest (dominated by Pinus spp.), with local stands of Populus
tremuloides Michx. and Betula papyrifera Marsh. resulting from
fires (Wright et al., 2004). Deep Lake (47°41′N, 95°23′W,
4 ha, 411 m asl) is just inside coniferous forest, but near a
narrow band of oak woodland separating coniferous forest from
tallgrass prairie. West Olaf Lake (46°37′N, 96°11′W, 58 ha,
393 m asl) is in a mix of tallgrass prairie and oak woodland.
We compare data from these sites with pollen assemblages
from Moon Lake (46°51′N, 98°09′W, 35ha, 444 m asl), located
in eastern North Dakota in mixed-grass prairie (Fig. 1).
The lakes (18–21 m deep) contain varved sediments (e.g.
Tian et al., 2005), although varves are discontinuous during
the middle Holocene. Mean January temperature varies little
(−16 to −15°C) among our sites. Mean July temperature increases
along a north–south gradient, from 19°C at Deep Lake to
21°C at West Olaf Lake. Mean annual precipitation and
precipitation minus evaporation display strong east–west
gradients across the region, ranging from 66 and −10 cm,
respectively, near Steel Lake, to 48 and −30 cm, respectively,
near Moon Lake (Fig. 1; Winter & Woo, 1990; Grimm, 2001).
Corresponding to the east–west moisture gradient, magne-
sium:calcium (Mg:Ca) ratios in surface lake-water collected
in July 2002 increase from 0.3 at Steel Lake to 0.7 at Deep
Lake and 1.7 at West Olaf Lake. This pattern suggests that
lakes in regions with drier conditions generally have greater
Mg:Ca ratios in their waters.
Core sampling and analysis
Sediment cores were obtained from the deepest portion of
each lake with a modified Livingstone piston sampler (Wright,
1991). Multiple cores from each lake were correlated on the
basis of distinct lamination patterns. Sediment was sieved to
recover macrofossils for accelerator mass spectrometry (AMS)
14C analysis, as described in Nelson et al. (2004). For Steel Lake,
we used the chronology of Wright et al. (2004), which is based
on 14C ages calibrated using CALIB 4.2 (http://calib.qub.ac.uk/
calib/) with the atmospheric 14C data set (Stuiver et al., 1998).
For West Olaf and Deep lakes, 14C dates were converted to
calibrated ages using CALIB 5.0.2 (http://calib.qub.ac.uk/
calib/) with the atmospheric 14C data set. Use of 14C ages
calibrated with CALIB 5.0.2 instead of 4.2 has minimal
influence on the Steel Lake chronology; the maximum difference
is only 10 yr for any of the calibrated ages. Unless otherwise
noted, all ages are reported as calibrated 14C yr before AD
1950 throughout this paper.
Methods for the analyses of pollen assemblages, sediment
mineral composition, carbonate δ18O, and charcoal influx are
described in Nelson et al. (2004). The mineral composition of
bulk sediment was measured at each site, and we used the
aragonite:calcite ratio to infer changes in aridity at West Olaf
and Deep lakes. Low aragonite:calcite values suggest wet
climatic conditions, and high values suggest dry conditions,
because aragonite (a polymorph of calcite) forms preferentially
over calcite as magnesium ions become concentrated in lake
water when evaporation exceeds precipitation. δ18O was not
used to infer climatic patterns throughout the Holocene at
these two sites because aragonite, which is abundant in the
middle-Holocene sediments, has an isotopic fractionation
factor different from that of the coexisting calcite and thus
would complicate climatic inferences from δ18O. By contrast,
calcite is the only form of carbonate in the sediments of Steel
Lake; thus we used carbonate δ18O to infer aridity at this site.
Although δ18O may potentially indicate factors (e.g. precipitation
seasonality and moisture source) other than aridity, previous
studies (e.g. Nelson et al., 2004; Tian et al., 2006) have shown
that, at Steel Lake, δ18O primarily indicates aridity variations
within the middle and late Holocene.
Detrended correspondence analysis (DCA) was used to
compare temporal variations in pollen assemblages among
sites. Only taxa with ≥ 2% abundance in at least one sample
from at least one site were included in the DCA. Pollen
percentages of taxa with < 2% abundance were excluded from
the pollen sum, and percentages of taxa with ≥ 2% abundance
were recalculated from the raw pollen counts before the
analysis. DCA analysis was performed using past (version 1.44;
Hammer et al., 2001). Canonical correspondence analysis
(conducted in PC-ORD 3.01), a constrained ordination
technique (ter Braak, 1986), was used to relate temporal
patterns in pollen assemblages to climate indicators and charcoal
accumulation rates (environmental variables) at each site.
Results
Radiocarbon chronologies
The chronological controls at our sites are based on a com-
bination of new 14C dates (Supplementary Material Table S1)
and 14C dates reported in previous studies (Hu et al., 1997,
1999; Nelson et al., 2004; Wright et al., 2004). The age–depth
model for West Olaf Lake was constructed by fitting a
third-order polynomial to 10 AMS 14C dates (Fig. 2). An age
reversal occurs between the second- and third-youngest dates

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at this site. Both dates were included in the model because the
reason for this discrepancy is unclear. The West Olaf Lake
record begins ~9100 yr before present (BP), which is the age
of the oldest sediments recovered from this site.
The age–depth model for Deep Lake was constructed
from seven AMS 14C dates that were fit with a second-order
polynomial (Fig. 2). We only present data spanning 10 000 to
3700 yr BP because our cores were initially taken for a study
focusing on the late Glacial and early Holocene (Hu et al., 1997,
1999), and the late-Holocene sections were not properly
correlated with one another on the basis of lamination patterns.
The age–depth model for Steel Lake was constructed using
26 AMS 14C dates with a locally weighted polynomial
regression (Fig. 2; Wright et al., 2004). The Steel Lake record
spans the entire Holocene.
Aridity fluctuation
Our climatic reconstruction focuses on spatial and temporal
variations of aridity, which encompass the effects of temperature
on evapotranspiration and are a key control on the distribution
of prairie and forest (Changnon et al., 2002). At West Olaf
Lake, the aragonite:calcite ratio is low, fluctuating around a
mean of 0.34, before ~8000 yr BP (Fig. 3), which suggests a
generally wet climate with drier episodes. At Deep Lake aragonite
first appears ~9300 yr BP, but the aragonite:calcite ratio remains
negligible (~0.02) until ~8000 yr BP. These results suggest
that the regional climate became slightly drier c. ~9300 yr BP
but remained wet compared with just after 8000 yr BP. This
interpretation is supported by the stratigraphic patterns of
detrital minerals and δ18O at Steel Lake. The abundance of
quartz and feldspars, detrital minerals that were probably
transported to the lake through eolian erosion of dry soils (e.g.
Dean et al., 1996), increases slightly after ~9400 yr BP but
remains low before ~8000 yr BP. Similarly, δ18O values are
relatively low before ~8000 yr BP (Fig. 3).
A sharp increase in aridity occurred at or just after 8000 yr
BP, as suggested by the abrupt increase in each indicator of
Fig. 2 Age–depth models. Depth is from sediment surface. 2σ ranges
of calibrated ages are shown. The seven oldest dates from West Olaf
Lake were presented in Nelson et al. (2004), and the Steel Lake
chronology was first presented in Wright et al. (2004). cal. ka BP,
thousands of calibrated 14C years before present.
Fig. 3 Oxygen-isotope and mineral records used to infer aridity
changes at West Olaf, Deep, and Steel lakes. cal. ka BP, thousands of
calibrated 14C years before present.

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aridity to peak or near-peak values (Fig. 3). This change is also
recorded at other sites in central and southeastern Minnesota
(e.g. Brugam et al., 1988; Keen & Shane, 1990; Shuman et al.,
2002; Camill et al., 2003), as well as eastern North Dakota
(Laird et al., 1996a). The regional increase in aridity may have
resulted from diminishing influence of the ice sheet on the
regional atmospheric circulation (Shuman et al., 2002),
possibly through enhanced atmospheric subsistence (e.g.
Diffenbaugh et al., 2006) or increased eastward penetration of
dry Pacific air (e.g. Booth et al., 2006). Following this aridity
peak, each indicator fluctuates along generally declining
trends, suggesting effective-moisture variation superimposed
on a long-term increase. By ~3500 yr BP the regional climate
was again relatively wet in northwestern Minnesota, as inferred
from the near absence of aragonite at West Olaf Lake and
generally low values of detrital minerals and δ18O at Steel Lake.
Together these results indicate that the middle Holocene was
the driest period since the last deglaciation, as suggested by
McAndrews’ pollen data (McAndrews, 1966, 1967) and verified
by subsequent studies using pollen-independent approaches
(e.g. Brugam et al., 1988; Laird et al., 1996a). Within the
middle Holocene, the intensity of aridity varied temporally
and spatially. For example, at West Olaf Lake episodes of
aridity occurred between ~6500 and ~5800 yr BP and between
~5300 and ~4900 yr BP, in addition to the aridity peak just
after 8000 yr BP that indicates the driest conditions of the
Holocene (Fig. 3). Aridity generally decreased from west to east
across the study region within the middle Holocene, similar
to the spatial pattern at present (Grimm, 2001). For example,
the aragonite:calcite ratio was overall higher at West Olaf Lake
than at Deep Lake during the early and middle Holocene, and
aragonite was absent from the Steel Lake sediments.
Vegetational change
Pollen assemblages indicate that, at Deep and Steel lakes,
herbaceous taxa (including Ambrosia, Artemisia, and Poaceae)
and Quercus began replacing Pinus banksiana/resinosa-dominated
forests ~9400yr BP (Fig4. and Supplementary Material Fig. S1;
Wright et al., 2004). At Deep Lake, herbaceous taxa increased
sharply between 9400 and ~9100 yr BP and then fluctuated
slightly until ~8100 yr BP. By contrast, the expansion of
herbaceous taxa was gradual at Steel Lake, extending from
~9400 to 8100 yr BP (e.g. Fig. 4 and Supplementary Material
Fig. S1; Wright et al., 2004). DCA results confirm these
patterns. At 9000 yr BP, Deep Lake has a more negative DCA1
score than Steel Lake (Fig. 5), indicating a greater abundance,
and earlier expansion, of herbaceous taxa at Deep Lake.
Vegetation at these sites between ~9400 and 8000 yr BP trended
toward mixed-grass prairie communities of the Dakotas before
European settlement, but with a generally greater abundance
of woody species than in the Dakotas (Fig. 5). We cannot
pinpoint the timing of prairie development at West Olaf Lake
because the recovered sediment core has a basal age of ~9100 yr
BP. However, it is likely that herbaceous taxa were already
established near West Olaf Lake before their expansion at Deep
Lake, as suggested by the greater abundance of nonarboreal
pollen at West Olaf Lake at 9000 yr BP (Fig. 4a). Vegetation
near West Olaf Lake 9000 yr BP was similar to that at 10 000 yr
BP at Moon Lake (Fig.1) where mixed-grass prairie, characterized
by abundant Artemisia and a moderate abundance of Poaceae
(Hoyt, 2000; Grimm, 2001), was already established (Fig. 5;
Laird et al., 1996a). Together these results indicate that the
expansion of prairie from eastern North Dakota to Minnesota
was time-transgressive (Wright et al., 2004).
As pine percentages decreased following the initial expansion
of herbaceous taxa into northern Minnesota, Ambrosia and
Artemisia increased sharply to their maximum abundance just
Fig. 4 Percentages of the key pollen types. (a) Nonarboreal pollen
(NAP); (b) Ambrosia; (c) Poaceae; (d) Artemisia; (e) Quercus; (f)
Pinus. ML, Moon Lake; WOL, West Olaf Lake; DL, Deep Lake; SL,
Steel Lake. The order of site names is reversed in (e) and (f). The pollen
sum includes all terrestrial pollen types. Three-sample moving
averages are used for all plots. Pollen data from Moon Lake were
obtained from the North American Pollen Database. More detailed
pollen diagrams are available in Supplementary Material Fig. S1. cal.
ka BP, thousands of calibrated 14C years before present.