W oodland-to-forest transition dur ing prolonged drought in Minnesota …€¦ · from those...

Ecology, 90(10), 2009, pp. 2792–2807! 2009 by the Ecological Society of America

Woodland-to-forest transition during prolonged droughtin Minnesota after ca. AD 1300

BRYAN SHUMAN,1,2,3 ANNA K. HENDERSON,2 COLIN PLANK,2 IVANKA STEFANOVA,2 AND SUSY S. ZIEGLER1

1Department of Geography, University of Minnesota, 414 Social Sciences Building, 267 19th Avenue South,Minneapolis, Minnesota 55455 USA

2Limnological Research Center, University of Minnesota, 220 Pillsbury Hall, 310 Pillsbury Drive SE,Minneapolis, Minnesota 55455 USA

Abstract. Interactions among multiple causes of ecological perturbation, such as climatechange and disturbance, can produce ‘‘ecological surprises.’’ Here, we examine whetherclimate–fire–vegetation interactions can produce ecological changes that differ in directionfrom those expected from the effects of climate change alone. To do so, we focus on the ‘‘BigWoods’’ of central Minnesota, USA, which was shaped both by climate and fire. Thedeciduous Big Woods forest replaced regional woodlands and savannas after the severity ofregional fire regimes declined at ca. AD 1300. A trend toward wet conditions has long beenassumed to explain the forest expansion, but we show that water levels at two lakes within theregion (Wolsfeld Lake and Bufflehead Pond) were low when open woodlands weretransformed into the Big Woods. Water levels were high instead at ca. 2240–795 BC whenregional fire regimes were most severe. Based on the correlation between water levels and fire-regime severity, we infer that prolonged or repeated droughts after ca. AD 1265 reduced thebiomass and connectivity of fine fuels (grasses) within the woodlands. As a result, regional fireseverity declined and allowed tree populations to expand. Tree-ring data from the region showa peak in the recruitment of key Big Woods tree species during the AD 1930s drought andsuggest that low regional moisture balance need not have been a limiting factor for forestexpansion. The regional history, thus, demonstrates the types of counterintuitive ecosystemchanges that may arise as climate changes in the future.

Key words: Big Woods, Minnesota, USA; drought; ecological surprises; fire; forest expansion; fossilpollen analysis; lake levels; prairie–forest ecotone; radiocarbon dating; woodlands.

INTRODUCTION

Studies of modern ecological dynamics indicate thatunanticipated ecological changes (ecological surprises),including unexpected shifts in species abundances andvegetation composition, may result when an ecosystemis affected by two or more simultaneous perturbationslike climate change and severe disturbance (Paine et al.1998). Ecological surprises can also result from dynam-ics that have never been observed, such as those drivenby large climate changes (Doak et al. 2008), and there-fore may be especially common in response to ongoingglobal changes. Paleoecological studies have increasedknowledge about the responses to climate change, butthey have also been limited in ways that may prohibitthe recognition of surprising ecological dynamics: boththe causes and responses of past environmental changehave often been inferred from the same data sets (e.g.,fossil pollen data). The underlying relationships (e.g.,

between vegetation, climate, and disturbance regimes)have to be assumed, which leaves little room forrecognizing the unexpected and has led to uncertaintiesabout how rapidly and directly vegetation has respond-ed to climatic change in regions such as the fire-proneecotone between the prairies and deciduous forests ofcentral North America (e.g., Camill et al. 2003,Umbanhowar et al. 2006, Nelson and Hu 2008).To study potentially surprising ecological responses to

multiple perturbations (i.e., climatic and fire regimechanges), we evaluate independent reconstructions ofclimate, fire, and vegetation over multiple centurieswhen processes such as postfire succession and responsesto century-scale climate changes could have interacted.In doing so, we focus on a region shaped by theinteractions among climate, fire, and vegetation at aprairie–forest ecotone: central Minnesota’s Big Woods.During early European settlement in Minnesota in the

AD 1850s, Acer saccharum Marsh., Tilia americana L.,and Ulmus spp. dominated a biogeographical island ofdense deciduous forest, known as the Big Woods, whichthen covered ;6500 km2 amid woodlands and prairies(Fig. 1). The forest also contained significant popula-tions of Fraxinus, Ostrya, Populus, and Quercus species,and grew where topography and surface water bodies

Manuscript received 28 May 2008; revised 16 January 2009;accepted 27 January 2009. Corresponding Editor: A. H. Lloyd.

3 Present address: Department of Geology and Geophys-ics, University of Wyoming, Laramie, Wyoming 82071 USA.E-mail: [email protected]

2792

protected the tree populations from severe prairie fires(Daubenmire 1936, Marschner 1974, Grimm 1984).However, closed forest in the area of the Big Woodsonly began to develop several centuries earlier when theseverity of the regional fire regime decreased after ca.AD 1300, and did not become composed of Acer–Tilia–Ulmus communities until ca. AD 1500 (McAndrews1968, Grimm 1983, Umbanhowar 2004). Prior to AD1300, the region was broadly similar to the Quercuswoodlands and prairies that historically surrounded theBig Woods to the east and north (Grimm 1983, 1984)where sedimentary charcoal data record high-severityfire regimes (Umbanhowar 2004). Indeed, Quercuswoodlands had been extensive in the region between2300 and 500 BC when charcoal data show that regionalfire regimes had been most severe (Camill et al. 2003,Umbanhowar et al. 2006).This paper focuses on how the regional climate

changed when the Big Woods formed to elucidate theclimate–fire–vegetation dynamics involved. The forest

developed after a period of severe regional aridity andlow regional forest cover from ca. 6000–2000 BC (Fig. 2;e.g., Bartlein et al. 1984, Bradbury and Dean 1993,Clark et al. 2001, Camill et al. 2003), but much less isknown about how climate changed after 2000 BC.Grimm (1983) presented a widely cited hypothesis (e.g.,Clark 1988, Laird et al. 1996, Brugam and Swain 2000,Fritz et al. 2000) that an increase in regional precipita-tion rates decreased fire frequencies and encouraged treerecruitment. Yet, the key Big Woods taxa had toleratedthe driest portions of the Holocene (Fig. 2D–F), andUmbanhowar (2004), paralleling ideas presented byClark et al. (2001, 2002), has proposed an alternative(and more ‘‘surprising’’) hypothesis that drought en-couraged forest expansion by lowering the abundance offine grassland fuels, thus reducing fire severity.

The two alternative hypotheses have never beenexplicitly tested. We evaluate a prediction made byGrimm (1983:347) that ‘‘the history of vegetation shouldbe consistent with any detailed history of climate

FIG. 1. (A, B) Wolsfeld Lake (WL) and Bufflehead Pond (BP) lie in the northeastern portion of the Big Woods forest, which issituated in the north-central United States, at the boundary between forest and prairie in south-central Minnesota. (C, D) Aerialphotographs show that the lakes were low and surrounded by large areas of dry lake bottom in AD 1937. (C) Water at BuffleheadPond is shown as a dark area containing a ring of aquatic vegetation. Lake extents were much greater in AD 2003 (dashed lines).Locations of semi-submerged stumps (X), cores (circles), and the GPR profiles (lines) are shown. (D) Solid white lines depict thebathymetry of Wolsfeld Lake at 1.5-m intervals; 1-m intervals are used on (E) the bathymetric map of Bufflehead Pond (maximumdepth 7.3 m). The location of Elk Lake (EL), Minnesota, is also shown (in panel B) because data from the lake are presented inFig. 2.

October 2009 2793DROUGHT-TRIGGERED FOREST EXPANSION

reconstructed from water-level changes.’’ To do so, wereconstruct the water-level history of two small lakes inthe Big Woods region (Fig. 1). In addition, we use recentforest history data to study moisture–vegetation rela-tionships over the last few centuries. The resultsunderscore both the complications related to projectingfuture vegetation responses to climate change, and the

importance of using multiple lines of evidence tounderstand past climate–ecosystem interactions.

STRATEGY AND STUDY SITES

We use stratigraphic evidence to document past shiftsin lake-shoreline elevations and thus reconstruct mois-ture history to compare with the regional vegetation and

FIG. 2. Long-term context of the formation of the Big Woods. (A) Diatom ratios of Stephanodiscus minutulus/Fragilariacrotonensis, (B) quartz content, and (C) Ambrosia pollen percentage data from Elk Lake in north-central Minnesota (Bradbury andDean 1993) show that similarities exist between the mid-Holocene and the last millennium. (D–F) Pollen data from Kimble Pond(dashed line; Camill et al. 2003), Pogonia Bog Pond (solid line; Brugam and Swain 2000), and Wolsfeld Lake (gray line; Grimm1983) in the Big Wood region show high relative abundance of several key Big Wood taxa (D, Ostrya-type; E, Tilia sp.; F, Acer sp.)during the mid-Holocene as well as the last millennium. The period when the Big Woods formed is marked by the vertical gray bar.Sediment quartz content is presented as determined from the relative height (in mm) of intensity peaks measured by X-raydiffraction (XRD).

BRYAN SHUMAN ET AL.2794 Ecology, Vol. 90, No. 10

fire-history data. The approach builds on the well-established concept that sandy sediments and littoralplant remains tend to accumulate closer to the center ofa lake when water levels are low, and climate conditionsdry, than when water levels are high and conditions wet(e.g., Dearing 1997, Digerfeldt 1986, Shuman 2003).When lake levels drop, wave action can prohibit or slowthe accumulation of fine-grained, low-density sedimentsnear shore, and can thus form a break (unconformity) inthe nearshore lake-sediment sequence. Conversely, fine-grain sediments typically deposited in deep water can bedeposited close to shore during times when water levelsare high.To assess the past lake-level history according to this

framework, we used ground-penetrating radar (GPR)profiles and transects of sediment cores to detectchanges in (1) the geometry of sediment accumulation(i.e., unconformities and episodes of shoreward sedi-mentation), (2) the types of sediments (i.e., sand vs. silt)in different locations within each lake, (3) the sedimentaccumulation rates (i.e., evidence of slowed accumula-tion near past shorelines), and (4) the distribution oflittoral and nearshore terrestrial vegetation. We alsoexamined fossil pollen in the sediment cores.The work was conducted in two Minnesota Scientific

and Natural Areas (SNAs) that contain areas of BigWoods forest that were never cleared after Euro-American settlement (Fig. 1). In the Wolsfeld WoodsSNA, we examined the water-level history of WolsfeldLake (4580001800 N, 9383401500 W; 291 m; 13.75 ha),where Grimm (1983) initially dated the vegetationhistory of the region. In the Wood-Rill SNA, 3 kmfrom Wolsfeld Woods, we studied Bufflehead Pond(4485901300 N, 9383201000 W; 295 m elevation; 2 ha), aswell as the local forest history. Bufflehead Pond has nosurface inlets or outlets and a maximum depth of 7.3 m.Wolsfeld Lake has surface inlets and outlets, but has asimilar maximum depth of 7.9 m. Both lakes formedwithin a moraine deposited by the Des Moines lobe ofthe Laurentide Ice Sheet. The water levels of similarlakes have been widely shown to track the balance ofprecipitation minus evapotranspiration (moisture bal-ance) in a region, and their histories tend to beregionally consistent with each other (e.g., Harrisonand Digerfeldt 1993).

FIELD AND LABORATORY METHODS

At each lake, we collected subsurface profiles using anSIR-3000 GPR (ground-penetrating radar; GeophysicalSurvey Systems, Salem, New Hampshire, USA) with a200-MHz antenna, and transects of sediment cores usinga simple piston corer with 10 cm diameter polycarbonatetubes. Because the Big Woods developed recently, onlyshort (0.5–2 m length) surface sediment cores werecollected. All cores are labeled here by the first letter ofthe lake’s name and then by a number that is the core’swater depth in centimeters. At Wolsfeld Lake, weobtained three cores (W180, W200, W210) in AD 2007

at 30, 32.5, and 35 m south from the north shore. AtBufflehead Pond, four cores (B60, B80, B100, B110)were collected in AD 2005 at 5, 10, 15, and 25 m northfrom the south shore. We compared the sedimentaryand macrofossil record of lake-level change in thesecores with fossil pollen from a core (B690) collected atthe center of Bufflehead Pond, as well as with publishedpollen and charcoal data from the region. B690 is acomposite of two separate cores collected within 50 cmof each other: one contains undisturbed modernsediments, but the second extends to a greater depththan the first. Aerial photos archived at the JohnBorchert Map Library at the University of Minnesotaprovide evidence of recent lake levels, as do the ages andelevations of two semi-submerged tree stumps atBufflehead Pond.

Two macrofossil samples from a core at WolsfeldLake, six macrofossil and charcoal samples from threeBufflehead Pond cores, and two wood samples from theBufflehead Pond stumps were dated by AMS (acceler-ator mass spectrometry) radiocarbon analysis at BetaAnalytic (Miami, Florida, USA) and the Keck AMSfacility at the University of California–Irvine (Table 1).All radiocarbon ages were calibrated to account forvariations in the atmospheric concentration of 14Cthrough time using CALIB 5.0 (Reimer et al. 2004).CALIB 5.0 also calculates probabilities of the samples’ages falling within sub-ranges of the analytical uncer-tainty of each radiocarbon date; we present theseprobabilities where relevant. One recent sample wascalibrated using CALIBomb, which accounts for theradiocarbon produced by atomic weapons testing(Reimer and Reimer 2004). Thirteen .50-cm3 samplesfrom the upper 50 cm of core B690 at Bufflehead Pondwere also dated by 210Pb and 137Cs activity measured byTeledyneBrown Engineering (Huntsville, Alabama,USA).

Pollen was analyzed in B690 and in one nearshorecore (B100) from Bufflehead Pond at 5–10 cm intervals,using standard techniques (Faegri and Iversen 1989).Pollen percentages are presented based on a sum of allterrestrial pollen types; we counted 286–365 terrestrialpollen grains per sample. Sedimentary data wereobtained at the National Lacustrine Core Repositorycore lab (LacCore) at the University of Minnesota, usinga multi-sensor core logger (Geotek, Daventry, North-ants, UK) to obtain gamma-ray attenuation bulkdensity and magnetic susceptibility measurements (Lastand Smol 2002), a DMT CoreScanner (Essen, Germany)to collect digital images of cores (Last and Smol 2002),and a muffle furnace to conduct loss-on-ignition analysis(Shuman 2003).

Aquatic and nearshore macrophyte macrofossilsprovide an additional line of evidence for reconstructingthe lake history (Digerfeldt 1986, Dearing 1997). Wepicked and identified macrofossils from contiguous 10-cm3 samples from cores B690 and B100 that were wetsieved through a 125-lm mesh. Each macrofossil sample


spanned 5 cm of core length. To aid interpretation, wealso analyzed macrofossils in 26 surface sedimentsamples collected with an Ekman dredge at 50 cm depthintervals across Bufflehead Pond. Ten cm3 sedimentvolumes were analyzed in the surface samples, but weconfirmed our results by analyzing larger (50 cm3)volumes in a subset of samples. Additional informationwas obtained from pollen-slide counts of Nymphaceaetrichosclereids, cells from the bases of leaf hairs onNymphaea and Nuphar spp.To further aid the interpretation of our data, we

examined local forest demographics over the last twocenturies near Bufflehead Pond. The Wood-Rill SNA,which surrounds the pond, is a 60-ha preserve ofdeciduous Acer–Tilia–Quercus forest on the westernedge of the Twin Cities metropolitan area and was oncepart of the more extensive Big Woods. Our analysisincluded crossdating tree cores extracted at a height of 1m from 40 Acer saccharum trees of closed-canopy forestnear Bufflehead Pond, and obtaining the pith ages of thetrees. The cores were collected, prepared, and analyzedfollowing standard dendrochronological procedures(Stokes and Smiley 1996). We compared the resultswith similar work (Fig. 3; Ziegler et al. 2008) from open-canopy plots of Quercus–Fraxinus savanna in the HelenAllison Savanna SNA on the Anoka Sand Plain in east-central Minnesota (4582205800 N, 9381000000 W; 52 kmnortheast of Bufflehead Pond). Helen Allison Savanna

SNA contains 20 ha of preserved Quercus savanna, andis located just south of Cedar Creek Ecosystem ScienceReserve LTER. Trees studied there were cored at aheight of 0.30 m (Ziegler et al. 2008).

RESULTS

Historical changes at the study sites

Aerial photographs beginning in AD 1937 show thatBufflehead Pond and Wolsfeld Lake have responded tohistorical droughts (Fig. 1). Water levels declined duringthe AD 1930s ‘‘Dust Bowl’’ drought and have been highsince the AD 1940s when regional moisture balanceincreased.Partially submerged stumps (each .40 cm in diame-

ter) ring each of the lakes at water depths of up to 60 cm(Fig. 1). These stumps may represent tree growth duringlow water levels in the AD 1930s, but for several reasonsthey have a high probability of having grown duringperiods of low water as early as AD 1663. First, trees areabsent from stump locations at Bufflehead Pond in AD1930s aerial photos (Fig. 1). Second, the radiocarbonages (Table 1) of wood from the center of two stumps inBufflehead Pond indicate that one of the trees, if itestablished in the 20th century, could have grown onlybetween AD 1944 and 1950 (17.4% probability).However, water levels had already begun to rise at lakesin the area by AD 1941 (Fig. 3A). The second stump

TABLE 1. Radiocarbon ages (6SD) of samples from Wolsfeld Lake and Bufflehead Pond in the Big Woods forest, Minnesota,USA.

Sample location,laboratory, and

sample no.Depth incore (cm) Material

14C age,yr beforeAD 1950

Calibrated age, BC/AD!

Minimum Median Maximum

Stumps at Bufflehead Pond

UCIAMS 26033 2 center of 41 cm diameter stump 150 6 25 AD 1672 AD 1777 AD 1952UCIAMS 26034 4 center of 35 cm diameter stump 205 6 15 AD 1663 AD 1783 AD 1950

Bufflehead Pond, B80

Beta 204472 14–15 deciduous leaf fragments 153.1% 6 0.8%modern

AD 1963.20 AD 1971.79

UCIAMS 26035 43–45 0.23 mg C from .125-lmcharcoal

760 6 15 AD 1260 AD 1265 AD 1275


Beta 204473 32–34 .125-lm charcoal pieces 1270 6 40" AD 684 AD 736 AD 773Beta 204474 40–41 .125-lm charcoal pieces 1000 6 40 AD 989 AD 1031 AD 1147UCIAMS 26036 85–86 0.020 mg C from .125-lm

charcoal1870 6 50 AD 80 AD 143 AD 213


Beta 204475 115–116 bark fragments 410 6 40 AD 1437 AD 1479 AD 1615

Wolsfeld Lake, W2210

UCIAMS 35480 23 0.063 mg C from .125-lmcharcoal

2600 6 25 803 BC 794 BC 785 BC

UCIAMS 35481 102 .125-lm charcoal pieces 3800 6 15 2280 BC 2242 BC 2201 BC

Note: ‘‘Beta’’ refers to Beta Analytic, and ‘‘UCIAMS’’ to the W. M. Keck Carbon Cycle Accelerator Mass SpectrometryLaboratory at University of California–Irvine.

! Dates were calibrated using CALIB 5.0.2 (Reimer et al. 2004), except for sample Beta 204472, which was calibrated with sub-annual resolution using CALIBomb (Reimer and Reimer 2004). Radiocarbon ages are calculated based on isotopic values relativeto AD 1950. Samples that are younger than AD 1950 have .100% of the ‘‘modern’’ radiocarbon concentration and can becalibrated with sub-annual (monthly) precision (decimals) using CALIBomb (Reimer and Reimer 2004).

" Denotes out-of-order age.


could have grown in the 20th century between AD 1919and 1952 (21.3% probability).In the uplands around Bufflehead Pond, dominant

tree species, such as Acer saccharum, continued to berecruited to .100 cm height during the AD 1930s

drought (Fig. 3D). Increment cores from trees near thepond indicate peak annual recruitment of A. saccharum(four trees) in AD 1934, and that 21 of the 40 treessampled recruited from AD 1930–1941. A similar peakin tree recruitment was recorded in pith dates at 30 cm

FIG. 3. Tree recruitment during historical droughts in central Minnesota. (A) Historical lake-level observations fromMinnetonka Lake (4.5 km from Wolsfeld Lake and 2.5 km from Bufflehead Pond). (B) July–August Palmer Drought SeverityIndex (PDSI) reconstruction for east-central Minnesota (Cook and Krusic [2004]; grid point 198) shows decadal droughts since AD1800. (C) Cross-dated pith dates of 75 trees in the Helen Allison Savanna Scientific and Natural Area (Ziegler et al. 2008). (D) Pithdates of 40 Acer saccharum trees in the Wood-Rill Scientific and Natural Area, surrounding Bufflehead Pond, show that trees grewto coring height during droughts. Pith dates indicate the age of the innermost, cross-dated tree ring in cores of each tree. Verticalgray bars show decadal-scale droughts, which are also indicated by the 11-year smooth of the PDSI reconstruction (heavy line).Vertical dashed lines mark years before AD 1900 with tree establishment and below-average PDSI. Helen Allison Savanna datainclude: 25 Quercus macrocarpa trees, including nine trees from before AD 1900 and four trees from AD 1930–1937; seven Q.ellipsoidalis trees since AD 1963; and 43 Fraxinus pennsylvanica trees since AD 1900.


height at the Helen Allison Savanna SNA (Ziegler et al.2008). Indeed, at both locations the only years when thegrowth to coring height of the modern living treesexceeded two trees per year were during the 1930sdrought, and 10 of the 11 trees that date from before AD1900 established during decadal droughts in the earlyand mid-19th century (Fig. 3).

The data suggest that severe drought did not precluderecruitment of numerous small trees of four importantBig Woods species. The 1930s peak in tree recruitmentat the Helen Allison SNA includes both Fraxinuspennsylvanica Marsh. and Quercus macrocarpa Michx.Likewise, three of four Tilia americana trees (not shown)along with nine A. saccharum trees sampled at theWood-Rill SNA have pith dates from 1933 to 1936.Although the proximate causes of these recruitmentepisodes are unclear, land use was not the same in thetwo settings (one a deciduous forest, the other an opensavanna), and is unlikely to explain the similar

recruitment patterns. Regional expansion of importanttree populations, therefore, does not appear limited byregional moisture balance as low as the most severedrought of the 20th century.

Late-Holocene lake-level changes

Nearshore unconformities.—GPR profiles from bothWolsfeld Lake and Bufflehead Pond show evidence ofsubmerged paleoshorelines associated with the trunca-tion of older units of lacustrine sediment. The WolsfeldLake GPR data (Fig. 4) show that the sedimentsequence is truncated near the water–sediment interfacein water depths of ,2.1 m. We refer to the truncatedsediments as unit 1. The radar reflector that truncatesunit 1 is also buried by more recent sediment (units 2and 3) ;35–40 m from the modern shoreline. AtBufflehead Pond (Fig. 5), the most recent sedimentsare uniformly stratified, and form a well-defined on-lapping sequence (shoreward expansion of progressively

FIG. 4. Ground-penetrating radar (GPR, in nanoseconds) and core data fromWolsfeld Lake. (A) A profile of GPR data showsthe modern lake bathymetry as well as the stratigraphy of the underlying lake sediments; sedimentary features fade at depth as theradar signal becomes attenuated. Core locations are shown by arrows. (B) A schematic of our interpretation of the GPR datadenotes the three major sediment units, which are numbered and delineated by heavy lines. Arrows show locations of reflectortermination (e.g., onlapping over the underlying unit). (C) Magnetic susceptibility (dashed lines) and sediment density (solid lines)are shown for all three cores. Magnetic susceptibility is a unitless ratio of the induced magnetization of the sediments to the strengthof the field used to induce the magnetization. Short horizontal bars to the right show the location of calibrated radiocarbon ages.


younger sediments) above a paleoshoreline deposit,which is not well stratified. An associated unconformitytruncates older, evenly stratified sediments. The presenceof the paleoshorelines in the GPR data from the twolakes indicates that in the past the lakes were lower thanthey are today, whereas evidence of truncated units ofancient sediments indicates that earlier water levels wereonce as high as or higher than they are today.

Nearshore sand layers.—Consistent with the paleo-shorelines and unconformities in the GPR profiles,sediment cores collected near to shore at Wolsfeld Lakecontain 30–80 cm of organic-rich silts (correlative withGPR unit 1), which are bounded by dense magnetic sandlayers (Fig. 4). All of the cores are capped by sand layers5–15 cm thick, and contain dense inorganic gray claysand silts below or interbedded with the sand layer thatlies below unit 1. The dense gray clays and silts areprobably of Pleistocene age, and the overlying sandsrepresent subsequent low water levels. The sedimentsequence confirms, however, that the lake was high

enough during a portion of the Holocene for fine-grained sediment to accumulate where sand currentlyaccumulates in shallow water. Unit 1 probably repre-sents the wettest period of the Holocene.

At Bufflehead Pond, we also find evidence of a recentlow water-level interval. Cores and surface samplesindicate that the modern lake sediments consist of anorganic ooze, which extends from the deepest portionsof the lake to water depths of ,50 cm where the sandcontent of the modern sediments becomes high. CoresB60, B80, and B100, however, each contain a layer oflight-colored sand like that found near shore today; acorrelative interval of sandy organic sediment (but not avisible sand layer) exists at 27–36 cm depth in B110 (Fig.5). The sandy sediments abruptly interrupt otherwisecontinuous organic-rich sediments in B80 and B100.Organic sediments above the sand layer in B100 are also.0.1 g/cm3 (.15%) less dense than the organicsediments below the sand (Fig. 6). The sequence ofdense, organic-rich sediments overlain by a sand layerand then low-density organic oozes is consistent with theunconformity in the GPR data, and probably indicatesthat the lakeshore was once closer to the center of thelake. Likewise, the uppermost organic-rich sediments inB100, B80, and B60 become progressively thinner andyounger toward shore (beginning at AD 1850–1890 inB100, and at AD 1963–1970 in core B80; see nextsection), and are thus consistent with on-lapping in theGPR profiles and a recent expansion of the lake area.

Timing and rate of sand deposition.—At WolsfeldLake, in core W210, two radiocarbon ages at 88 and 9cm depth bracket the interval of organic-rich siltdeposition (Unit 1; Fig. 4C). The lower date calibratesto 2280–2201 BC, and the upper date to 803–785 BC.Sediments accumulated at the core site at a netaccumulation rate (possibly including the effects oferosion, compaction, and diagenesis) of 52.8–56.6cm/kyr (kiloyears ! 1000 years) between ca. 2240 and795 BC, but the last several millennia are represented inonly 9 cm (;3.2 cm/kyr). Webb and Webb (1988)identified the threshold for ‘‘nonconstant accumula-tion,’’ such as is expected in shallow environments, atnet sediment accumulation rates of ,10 cm/kyr. Basedon the low net accumulation rate, the water depths ofthe core locations have been sufficiently shallow tominimize net sediment accumulation since 795 BC, butthe water was previously deep enough to facilitatecontinuous sediment accumulation and preservation forseveral millennia.

At Bufflehead Pond, the sand layer in B100 isbracketed by a calibrated AMS radiocarbon date ofAD 990–1150 from a depth of 40–41 cm in the core, andby the presence of .10% Ambrosia pollen at 30–31 cm,which indicates local land clearance by Euro-Americansbetween AD 1850 and 1890 (Figs. 5 and 6). Based onthese ages, the sand layer in B100 was deposited in 900–700 years at a net accumulation rate of 11.4–14.3cm/kyr, which is close to Webb and Webb’s (1988)

FIG. 4. Continued.


threshold. By contrast, sediments below the sand inB100 have a net accumulation rate of 41.1–57.5 cm/kyrafter AD 80–213, and those above the sand accumulatedat a net rate of 194–270 cm/kyr since ca. AD 1850(based on dates in Table 1 and the historical Ambrosiarise). A calibrated AMS age of AD 773–684 on charcoalfrom 32 to 34 cm depth (near the top of the sand) inB100 (Table 1) is out of sequence because the date comesfrom within the Ambrosia rise.In B80, an AMS radiocarbon age on macroscopic

charcoal from a depth of 43–45 cm in the core andbelow the sand layer calibrates to AD 1260–1275,whereas an AMS analysis of terrestrial plant leafmacrofossils from above the sands at 14–15 cm containsradiocarbon levels attributable to atomic weaponstesting (Fig. 5). The upper age calibrates to AD 1963–1971, and is therefore notably younger than the

radiocarbon dates of the two semi-submerged treestumps. The result indicates that the trees grew duringthe period of sand deposition, and confirms theinterpretation that the sand represents low water levels.Pollen data.—At Bufflehead Pond, a comparison of

the pollen stratigraphies from B690 and B100 indicatesthat the sand layer in B100 accumulated when the BigWoods formed (Fig. 6). The lowermost pollen zone(BH-1) in B690 is dominated by 40–60% Quercus pollenwith .5% Ostrya-type, .2.5% Ambrosia, and .2.5%Poaceae pollen also present. Acer, Ulmus, and Tiliapollen occur at ,2%. Above these sediments, beginningat 135 cm in B690, the Big Woods interval can bedivided into two pollen zones. BH-2 contains highpercentages of Ulmus (.5%) and Tilia (.2%) pollen,and BH-3 is marked by the addition of high percentagesof Ostrya-type (.10%) and Acer saccharum pollen

FIG. 5. Core and ground-penetrating radar (GPR) data from Bufflehead Pond. (A) GPR data from Bufflehead Pond show keyfeatures of the lake stratigraphy: a poorly stratified unit truncates older stratified units and is overlain by a younger stratified unit.(B) A schematic diagram shows the major features of the radar profile. (C) A shallower but higher resolution profile than in panel(A) highlights the dark sand layer that extends out from shore. (D) The stratigraphies of cores are consistent with the GPR data,with low loss-on-ignition (LOI; white lines) and visible light-colored sand (inset) corresponding with the dark, poorly stratifiedGPR unit. White outlined boxes indicate core intervals that were the source of radiocarbon-dated material.


(.2.5%). The increase in Acer pollen percentages at thebase of BH-3 in B690 is dated by a calibratedradiocarbon age of AD 1437–1615 on charcoal at 115cm depth. The rise of Ambrosia pollen percentages to.10% at 80 cm in B690 denotes the onset of pollen zoneBH-4 (Fig. 6).In B100, parts of the Big Woods pollen zones (e.g.,

.10% Ostrya-type pollen in the lower portion of zoneBH-3) are absent, and the remainder of the zones (i.e.,.2.5% Acer saccharum pollen in the upper portion ofzone BH-3) coincides with the sand layer above 42 cmdepth (Fig. 6B). Organic sediments above the sandcontain the high Ambrosia pollen percentages of zoneBH-4. Pollen in the organic-rich sediments below thesand layer is consistent with radiocarbon ages that areolder than the lowest portion of B690. Ostrya-typepollen percentages are lower and Poaceae pollenpercentages are higher in the base of B100 than in the

base of B690. Therefore, the 20-cm sand layer in B100represents the same interval of time as the lower 90 cmof B690.

Littoral plant remains.—Both aquatic pollen types andthe macrofossils of littoral and shoreline plants record aspatial shift in their patterns of accumulation over time,which is consistent with the inferred changes in waterlevel. Nymphaea odorata Aiton grows today in a ring ofwater depths of ;0.5–1.5 m at Bufflehead Pond.Nymphaea pollen occurs at .0.5% in modern sedimentsin B100, but occurs at ,0.5% in B690. Nymphaea pollenpercentages are ,0.5% in the upper 80 cm of B690, butare .1% in most samples above the sand in B100(Fig. 7). During the Big Woods pollen zones, BH-2 andBH-3, however, Nymphaea pollen percentages increaseto .2% in B690 indicating that the lake was shallowenough to allow Nymphaea to grow near the core site.Consistent results are found in B100, where Nymphaeapollen decreases to ,1% in samples bracketing thevisible sand layer. Below the Big Woods pollen zones(BH-2 and BH-3), Nymphaea pollen percentages aresimilar to those found today (,1% in B690 and .2% inB100). The abundance of Nymphaceae trichosclereidsconforms to the patterns observed in Nymphaea pollenpercentages (Fig. 7), and supports their interpretation.

The macrofossils of several littoral and nearshoreplant species also show a basin-ward shift in accumu-lation during the Big Woods pollen zone. Seeds andfruits of Carex scoparia Schkuhr ex Willd., Cyperuserythrorhizos Muhl., C. strigosus L., and Polygonumlapathifolium L. are common in the historical (post-1850AD) sediments of B100, but do not occur in historicalsediments from B690. Today, these macrofossils arefound predominantly in surface sediment samplescollected in water depths of ,4 m (Fig. 8). Macrofossilsof these species were found, however, within the BigWoods pollen zones, BH-2 and BH-3, in B690, and wereabsent from the sand layer in B100 (Fig. 7). Below theBig Woods pollen zones, the species are again absentfrom B690 and present in B100. These littoral andnearshore plant remains, like the Nymphaceae pollenand trichosclereids, corroborate the inference that thewater level of Bufflehead Pond was low when the BigWoods formed.

DISCUSSION

Evidence of the Big Woods drought

When the Big Woods formed, water levels atBufflehead Pond were lower than at any other time formore than 1800 years. This inference is based onnearshore sand layers and unconformities, which (1)are bracketed by calibrated radiocarbon ages and otherage markers spanning from before AD 1275 to after AD1850, (2) contain the Big Woods pollen zones, and (3)coincide with the presence of littoral-plant remains inthe center of the pond. Indeed, the presence of fine-grained sediments (unit 1) near shore at Wolsfeld Lakeindicates that the highest water levels of the Holocene

FIG. 5. Continued.


were likely reached before the Big Woods developed.These fine-grained sediments accumulated at WolsfeldLake from 2240 to 795 BC when pollen data from ElkLake, Minnesota indicate the highest annual precipita-tion rates of the Holocene (Bartlein and Whitlock 1993)and when sedimentary charcoal from Big Woods’ lakesindicate peak biomass burning (Camill et al. 2003).Because the date of AD 1265 (1275–1260) in B80 is

the youngest of the dates below the sand layer atBufflehead Pond, we assume that the water levels in thelake did not drop until after that time. Littoral plantremains in B690 indicate that the lowest water levelswere reached by AD 1480 (when the Big Woods wasforming). Some sediment erosion or sediment redeposi-tion associated with the low water levels could accountfor the age of ca. AD 1030 below the sand in B100.Six lines of evidence eliminate run-off (storm or

snowmelt) events or slumps as the source of the sandlayers and unconformities at both Wolsfeld Lake and

Bufflehead Pond. First, littoral plant remains indicatethat the sand layers were deposited at Bufflehead Pondwhen the lake was shallower than today. Second, the lowslope of the lake margins (typically 0.01–0.05 m/m basedon the ratio of the core water depths and distances toshore; see also Figs. 4 and 5) precludes slumping orother mass movement. Third, the unconformities ringthe basins (e.g., Fig. 5A, B), which would be unlikely fora localized slump or channelized surface runoff. Fourth,the low sedimentation rates associated with the sandsare inconsistent with a rapid depositional event (e.g.,Noren et al. 2002), and probably represent nonconstantdeposition in shallow water (Webb and Webb 1988).Fifth, runoff cannot explain the sharp density differencebetween the organic silts above and below the sand layerin B100 (Fig. 6B), which is explained well by a period oflow water levels that enabled the dewatering andcompaction of older sediments. Finally, subsequentorganic-rich accumulation would not expand shoreward

FIG. 6. Pollen data from Bufflehead Pond cores B690 and B100. High Ostrya-type, Ulmus, Tilia, and Acer saccharum pollenpercentages, diagnostic of the Big Woods (Grimm 1983, Umbanhowar 2004), define two pollen zones (BH-2 and BH-3; gray bars)(A) between 135 and 80 cm in B690 and (B) within the sand layer from 41 to 30 cm in B100. Low loss-on-ignition (thin line) andhigh bulk density (thick line) mark the sand layer in B100; note also a significant drop in bulk density from below to above the sandlayer. Solid circles mark the positions of AMS (accelerator mass spectrometry) radiocarbon dates; the short-dashed line in panel(A) marks the position of the lowermost sediments with high Cs137 concentration (indicative of hydrogen weapons testingbeginning in 1954) in B690. Unlike other cores, core B690 was collected in two segments; section 1 (black shading) contained themost recent sediments and the water–sediment interface; section 2 (medium gray shading) extended more deeply.


time-transgressively over a slump or runoff deposit.Instead, new sediments would evenly drape over suchdeposits, which is inconsistent with the on-lappingsediments above the sands in both the cores atBufflehead Pond (i.e., the shoreward decrease in theage of the organic sediments above the sand layers; Fig.5D) and in the radar data at both lakes (Figs. 4 and 5).Our results also do not stand alone. Nine detailed

studies of Minnesota paleoclimates have inferred epi-sodes of drought or low annual precipitation ratesbeginning at AD 1340 6 150 yr (estimated age 6 SD)and ending by AD 1650 6 170 yr; we found no detailedresearch from Minnesota that did not record extraordi-nary drought during this period. The evidence fordrought derives from fossil pollen data (at AD 1325–1650, Gajewski 1988; AD 1300–1400 and 1660–1710, St.Jacques et al. 2008), fire history data (AD 1325–1650,Clark 1988), oxygen isotopes (e.g., AD 1150–1350 and1500–1700, Tian et al. 2006), peat deposits (e.g., a hiatusin peat accumulation from AD 1340–1810, Booth et al.2006), diatoms (e.g., AD 1450–1750, Dean et al. 1994),

and aeolian deposition in lakes (e.g., after AD 1150,Dean 1997, Nelson and Hu 2008; see Fig. 2B). Likewise,dunes and loess deposits show evidence of repeatedaeolian activity and no evidence of soil formation (i.e.,wet conditions) in the dune areas of the Great Plainsfrom AD 1200–1850 (e.g., Miao et al. 2007).

Only one vegetation-independent data set from theregion supports the hypothesis of high regional moisturebalance when the Big Woods formed. Laird et al. (1996)used fossil diatoms to document a decline in thealkalinity of Moon Lake, North Dakota, beginningabout AD 1200, which has been interpreted as the resultof an increase in regional moisture balance. However, adiatom record from Coldwater Lake, also in easternNorth Dakota, documents an opposite trend in lakesalinity at the same time (Fritz et al. 2000). Likewise,fossil diatom assemblages similar to those from the aridmid-Holocene have been consistently found after AD1300 in Minnesota and Wisconsin (see Fig. 2A; Farris1981, Bradbury 1986, Brugam et al. 1988, Bradbury andDieterich-Rurup 1993, Dean et al. 1994). Even at Moon

FIG. 7. Aquatic and nearshore macrophyte data from Bufflehead Pond cores B690 and B100. Major pollen types are alsoshown for comparison. Pollen data are presented as percentages, and all other data are given as counts. Macrofossil counts (bars)are presented per 10 cm3 of sediment. Gray bars mark the Big Woods pollen zones (BH-2; BH-3) in B690, as well as the sand layerin B100.


Lake, Ambrosia pollen percentages, which are expectedto be high during droughty intervals, increased duringthe period of Big Woods development at the expense ofPoaceae pollen percentages, which are expected to behigh when annual precipitation is high (Clark et al. 2001,Grimm 2002).Local evidence from published data at Wolsfeld and

other Big Woods lakes also appears consistent with ourinterpretations. Grimm’s (1983) lake sediment core fromWolsfeld Lake has similar sediment characteristics (i.e.,high mineral content, high density) during both the drymid-Holocene and the Big Woods period, as has beenfound at other sites in Minnesota (e.g., Bradbury andDean 1993, Nelson and Hu 2008; Fig. 2B). Umbanho-war (2004) also determined that the magnetic charac-teristics of lake sediments (potentially related todrought) in central Minnesota during the Big Woodsperiod were like those during the dry mid-Holocene (seeCamill et al. 2003). Grimm (1983) attributed the high-mineral content of the Big-Woods-age sediments to

erosion from runoff during a wet period, but we findthat when the Big Woods formed, the water levels ofWolsfeld Lake must have been lower than they werefrom 2240 to 795 BC.We conclude that the most parsimonious interpreta-

tion of the data is that the Big Woods developed duringa time of regionally extensive drought, but alternativeinterpretations of the data include the following. First,the low water levels could represent hydrologic re-adjustment during an uninterrupted trend toward highmoisture balance if rising groundwater levels affectedhydrologic baselines by causing erosion of lake outlets.Bufflehead Pond, however, does not have an outlet, andthis hypothesis does not explain evidence of drought atnumerous other locations. Second, the transition fromopen woodlands to dense Quercus forest and then theBig Woods could have increased transpiration to thedegree that local groundwater levels fell (e.g., Jackson etal. 2005). Such a change in transpiration could locallyamplify the effects of drought on Wolsfeld Lake andBufflehead Pond, but evidence for drought is also foundin both prairie and long-forested areas where transpira-tion rates were unlikely to have been increased byvegetation change (e.g., Booth et al. 2006, Miao et al.2007). Finally, a shift in the seasonality of precipitationcould have increased fuel moisture during fire seasons(i.e., in the fall and spring when senesced biomasstypically provides dry fuels). Such a shift in precipita-tion, however, would also contribute to high lake levelsbecause evapotranspiration is low during these seasons(see e.g., Shuman and Donnelly 2006).

Comparison of climate, vegetation, and fire histories

Data from Umbanhowar (2004) show that Poaceaepollen percentages and sedimentary charcoal influxdeclined at 14 lakes in the region at AD 1300 6 160 yrand AD 1310 6 180 yr, respectively (see histograms ofthese ages in Fig. 9). These ages overlap with the date ofAD 1260–1275 from below the sand layer in B80, whichis consistent with the hypothesis that a decline inregional moisture balance altered fire regimes and thusthe vegetation (Fig. 9). Likewise, the coincidence of highwater levels at Wolsfeld Lake at 2240–795 BC with peakcharcoal accumulation rates in south-central Minnesotaat 2300–500 BC (Camill et al. 2003, Umbanhowar et al.2006) is consistent with observations that grass produc-tivity and its spatial variability are strongly influencedby moisture balance in modern tallgrass prairies (Knappet al. 2001).Low charcoal inputs into prairie lakes at the margins

of the Big Woods after ca. AD 1300 (Umbanhowar2004) support the interpretation that drought inducedchanges in grassland biomass and thus burning. Wetgrowing seasons would have been expected to increaseprairie biomass (Knapp et al. 2001) and thus fire severity(Clark et al. 2001), and as noted above wet fire seasonswould likely have raised water levels. Additionally, asimultaneous albeit small regional increase in Ambrosia

FIG. 8. Modern distributions of the macrofossil remains ofkey aquatic and nearshore macrophytes at Bufflehead Pond.Circles show the depths and macrofossil content of all surfacesediment samples collected across the pond; light gray circlesindicate that no remains of the specific species were found in thesample. Vertical lines mark the depths of B100 (left) and B690(right).


pollen percentages (see e.g., Figs. 2C and 9; Whitlock etal. 1993, Brugam and Swain 2000, Clark et al. 2001,Umbanhowar 2004, Nelson and Hu 2008, St. Jacques etal. 2008) may denote breaks in the continuity ofgrassland fuels, which helped to reduce the spread ofsevere fires after AD 1300. High Ambrosia pollenpercentages can indicate areas of bare ground (Grimm2002).The regional reduction in fire severities probably

facilitated forest composition changes after ca. AD1300, including an initial increase in Quercus spp.abundance and the subsequent development of extensiveUlmus–Ostrya and Acer–Tilia–Ulmus communities, de-spite persistent or repeated droughts that lowered lakelevels (Fig. 9). Similarly, populations of these treespecies had tolerated droughts during the mid-Holocene(Fig. 2D–F) and had recruited new saplings during theAD 1930s in the Wood-Rill and Helen Allison Savanna

SNAs (Fig. 3). The Big Woods drought also coincidedwith the low temperatures of ‘‘the Little Ice Age’’ fromAD 1300 to 1850 (Fagan 2000), which extended intoMinnesota (Gajewski 1988) and probably contributed tothe woodland-to-forest transition because Acer, Tilia,and Ulmus spp. can tolerate lower temperatures thanQuercus spp. (Thompson et al. 1999, Williams et al.2006). Indeed, the onset of low temperatures wasinferred from fossil pollen data (Gajewski 1988) andtherefore may contribute to many of the simultaneousforest changes throughout Minnesota and Wisconsin(e.g., Almendinger 1992, Hotchkiss et al. 2007).

CONCLUSIONS

As noted by Doak et al. (2008), ecological surprisesarise because species have multidimensional require-ments (e.g., climatic tolerances arising from multipletraits and life stages) and context-dependent sensitivities

FIG. 9. Drought, reduced grassland biomass and fire severity, and forest development over time. The Big Woods drought isrecorded by a decline in lake levels, indicated here by the percentage loss-on-ignition (LOI) data from Bufflehead Pond (B100).Regionally representative pollen data (from Pogonia Bog Pond, near Bufflehead Pond; Brugam and Swain 2000) and sedimentarycharcoal data (Umbanhowar 2004) then show (from bottom to top in the figure): (1) a decline in Poaceae pollen percentages (andsubsequent small rise in Ambrosia pollen percentages), (2) a regional decline in sedimentary charcoal influx (fire severity), and (3)sequential increases in Quercus, Ostrya-type, and Acer pollen percentages. Histograms show the timing of the decline in Poaceaepollen percentages and charcoal influx at sites studied by Umbanhowar (2004).


to shifting abiotic conditions (e.g., species climatetolerances may depend on interactions with otherstresses). The indication that drought facilitated thereplacement of Quercus woodlands by the Big Woodsforest demonstrates both possibilities. As proposed byGrimm (1983), multiple states have existed in Minneso-ta’s forest–grassland mosaic depending on the combi-nation of climate and fire regimes. Regional moisturebalance probably was too low to sustain closeddeciduous forests at 6000–2000 BC, and severe firesfavored open woodlands over dense forest even whenmoisture balance was most positive from ca. 2200 to 800BC. The Big Woods deciduous forest therefore devel-oped under the particular combination of moisturebalance, temperatures, and fire regimes that existed afterAD 1300, even though that combination included lowermoisture balance than during preceding millennia.Similar combinations of multiple factors have probablymediated other undetected ecological ‘‘surprises’’ inresponse to past climatic changes, and will almostcertainly yield other counterintuitive outcomes in thefuture.

ACKNOWLEDGMENTS

Work was supported by Grants-in-Aid of Research from theUniversity of Minnesota Graduate School to B. Shuman and S.Ziegler, as well as by a University of Minnesota McKnightLand-Grant Professorship held by B. Shuman. We thank EdCushing and an anonymous reviewer for useful critiques of thesubmitted manuscript, A. Myrbo, A. Noren, and K. Brady atthe National Lacustrine Core Repository (LacCore) forassistance with lake-core analyses, J. Rauchfuss for processingtree cores, and the Minnesota Department of NaturalResources for access to the Wood-Rill and Wolsfeld WoodsSNAs. We also thank J. E. Almendinger, A. Berland, E.Grimm, G. Jacobson, T. Webb III, and J. Williams for usefuldiscussions of our results and manuscript.

LITERATURE CITED

Almendinger, J. C. 1992. The late Holocene history of prairie,brush-prairie, and jack pine (Pinus banksiana) forest onoutwash plains, north-central Minnesota, USA. Holocene 2:37–50.

Bartlein, P. J., E. C. Fleri, and T. Webb III. 1984. Holoceneclimatic change in the northern Midwest: pollen derivedestimates. Quaternary Research 22:361–374.

Bartlein, P., and C. Whitlock. 1993. Paleoclimatic interpreta-tion of the Elk Lake pollen record. Pages 275–293 in J. P.Bradbury and W. Dean, editors. Elk Lake, Minnesota:evidence for rapid climate change in the north central UnitedStates (Special Paper 276). Geological Society of America,Denver, Colorado, USA.

Booth, R. K., M. Notaro, S. T. Jackson, and J. E. Kutzbach.2006. Widespread drought episodes in the western GreatLakes region during the past 2000 year: geological extent andpotential mechanisms. Earth and Planetary Science Letters242:415–427.

Bradbury, J. P. 1986. Effects of forest fire and otherdisturbances on wilderness lakes in northeastern MinnesotaII. Paleolimnology. Archiv fur Hydrobiologie 106:203–217.

Bradbury, J. P., and W. E. Dean. 1993. Elk Lake, Minnesota:evidence for rapid climate change in the north central UnitedStates (Special Paper 276). Geological Society of America,Denver, Colorado, USA.

Bradbury, J. P., and K. V. Dieterich-Rurup. 1993. Holocenediatom paleolimnology of Elk Lake, Minnesota. Pages 215–

237 in J. P. Bradbury and W. Dean, editors. Elk Lake,Minnesota: evidence for rapid climate change in the northcentral United States (Special Paper 276). Geological Societyof America, Denver, Colorado, USA.

Brugam, R. B., E. C. Grimm, and N. M. Eyster-Smith. 1988.Holocene environmental changes in Lily Lake, Minnesotainferred from fossil diatom and pollen assemblages. Quater-nary Research 30:53–66.

Brugam, R. B., and P. Swain. 2000. Diatom indicators ofpeatland development at Pogonia Bog Pond, Minnesota,USA. Holocene 10:453–464.

Camill, P., C. E. Umbanhowar, Jr., R. Teed, C. E. Geiss, J.Aldinger, L. Dvorak, J. Kenning, J. Limmer, and K. Walkup.2003. Late-glacial and Holocene climatic effects on fire andvegetation dynamics at the prairie–forest ecotone in south-central Minnesota. Journal of Ecology 91:822–836.

Clark, J. S. 1988. Effects of climate change on fire regimes innorthwestern Minnesota. Nature 334:233–235.

Clark, J. S., E. C. Grimm, J. J. Donovan, S. C. Fritz, D. R.Engstrom, and J. E. Almendinger. 2002. Drought cycles andlandscape responses to past aridity on prairies of thenorthern Great Plains, USA. Ecology 83:595–601.

Clark, J. S., E. C. Grimm, J. Lynch, and P. G. Mueller. 2001.Effects of Holocene climate change on the C-4 grass-land/woodland boundary in the Northern Plains, USA.Ecology 82:620–636.

Cook, E. R., and P. J. Krusic. 2004. The North Americandrought atlas. Tree-Ring Laboratory, Lamont-DohertyEarth Observatory of Columbia University, Palisades, NewYork, USA.

Daubenmire, R. F. 1936. The ‘‘Big Woods’’ of Minnesota: itsstructure, and relation to climate, fire, and soils. EcologicalMonographs 6:233–268.

Dean, W. E. 1997. Rates, timing, and cyclicity of Holoceneeolian activity in north-central United States; evidence fromvarved lake sediments. Geology 25:331–334.

Dean, W. E., J. P. Bradbury, R. Y. Anderson, L. R. Bader, andK. Dieterich-Rurup. 1994. A high-resolution record ofclimatic change in Elk Lake, Minnesota for the last 1500years. Open File Report 94-578. U.S. Geological Survey,Denver, Colorado, USA.

Dearing, J. A. 1997. Sedimentary indicators of lake-levelchanges in the humid temperature zone: a critical review.Journal of Paleolimnology 18:1–14.

Digerfeldt, G. 1986. Studies on past lake-level fluctuations.Pages 127–143 in B. E. Berglund, editor. Handbook ofHolocene palaeoecology and palaeohydrology. John Wileyand Sons, New York, New York, USA.

Doak, D. F., et al. 2008. Understanding and predictingecological dynamics: are major surprises inevitable? Ecology89:952–961.

Faegri, K., and J. Iversen. 1989. Textbook of pollen analysis.Fourth edition. John Wiley and Sons, Chichester, UK.

Fagan, B. 2000. The Little Ice Age: how climate made history1300–1850. Basic Books, New York, New York, USA.

Farris, D. P. 1981. The recent historical limnology of MirrorLake, Waupaca County, Wisconsin (U.S.A.) evidenced bythe diatom stratigraphy. University of Michigan, Ann Arbor,Michigan, USA.

Fritz, S. C., E. Ito, Z. Yu, K. R. Laird, and D. Engstrom. 2000.Hydrologic variation in the northern Great Plains during thelast two millennia. Quaternary Research 53:175–184.

Gajewski, K. 1988. Late Holocene climate changes in easternNorth America estimated from pollen data. QuaternaryResearch 29:255–262.

Grimm, E. C. 1983. Chronology and dynamics of vegetationchange in the prairie–woodland region of southern Minne-sota, U.S.A. New Phytologist 93:311–350.

Grimm, E. C. 1984. Fire and other factors controlling the BigWoods vegetation of Minnesota in the mid-19th century.Ecological Monographs 54:291–311.


Grimm, E. C. 2002. Trends and palaeoecological problems inthe vegetation and climate history of the northern GreatPlains, U.S.A. Biology and Environment: Proceedings of theRoyal Irish Academy 99B:1–18.

Harrison, S. P., and G. Digerfeldt. 1993. European lakes aspalaeohydrological and palaeoclimatic indicators. Quaterna-ry Science Reviews 12:233–248.

Hotchkiss, S., R. Calcote, and E. Lynch. 2007. Response ofvegetation and fire to Little Ice Age climate change: regionalcontinuity and landscape heterogeneity. Landscape Ecology22:25–41.

Jackson, R. B., E. G. Jobbagy, R. Avissar, S. B. Roy, D.Barrett, C. W. Cook, K. A. Farley, D. C. le Maitre, B. A.McCarl, and B. C. Murray. 2005. Trading water for carbonwith biological carbon sequestration. Science 310:1944–1947.

Knapp, A. K., J. M. Briggs, and J. K. Koellinker. 2001.Frequency and extent of water limitation to primaryproduction in a mesic temperate grassland. Ecosystems 4:19–28.

Laird, K. R., S. C. Fritz, K. A. Maasch, and B. F. Cumming.1996. Greater drought intensity and frequency before 1200A.D. in the Northern Great Plains. Nature 384:552–554.

Last, W. M., and J. P. Smol. 2002. Tracking environmentalchange using lake Sediments. Volume 1. Kluwer Academic,Dordrecht, The Netherlands.

Marschner, F. 1974. The original vegetation of Minnesota(map). Technical Report, USDA Forest Service, NorthCentral Forest Experiment Station, St. Paul, Minnesota,USA.

McAndrews, J. H. 1968. Pollen evidence for the protohistoricdevelopment of the ‘‘Big Woods’’ in Minnesota (U.S.A.).Review of Palaeobotany and Palynology 7:201–211.

Miao, X., J. A. Mason, J. B. Swinehart, D. B. Loope, P. R.Hanson, R. J. Goble, and X. Liu. 2007. A 10,000 year recordof dune activity, dust storms, and severe drought in thecentral Great Plains. Geology 35:119–122.

Nelson, D. M., and F. S. Hu. 2008. Patterns and drivers ofHolocene vegetational change near the prairie–forest ecotonein Minnesota: revisiting McAndrews’ transect. New Phytol-ogist 179:449–459.

Noren, A. J., P. R. Bierman, E. J. Steig, A. Lini, and J.Southon. 2002. Millennial-scale storminess variability in thenortheastern United States during the Holocene epoch.Nature 419:821–824.

Paine, R. T., M. J. Tegner, and E. A. Johnson. 1998.Compounded perturbations yield ecological surprises. Eco-systems 1:535.

Reimer, P. J., et al. 2004. IntCal04 terrestrial radiocarbon agecalibration, 26–0 ka BP. Radiocarbon 46:1029.

Reimer, P. J., and R. W. Reimer. 2004. Discussion: reportingand calibration of post-bomb 14C data. Radiocarbon 46:1299–1304.

Shuman, B. 2003. Controls on loss-on-ignition variation incores from two shallow lakes in the northeastern UnitedStates. Journal of Paleolimnology 30:371–385.

Shuman, B., and J. P. Donnelly. 2006. The influence of seasonalprecipitation and temperature regimes on lake levels innortheastern United States during the Holocene. QuaternaryResearch 65:44–56.

St. Jacques, J.-M., B. F. Cumming, and J. P. Smol. 2008. A 900-year pollen-inferred temperature and effective moisturerecord from varved Lake Mina, west-central Minnesota,USA. Quaternary Science Reviews 27:781–796.

Stokes, M. A., and T. L. Smiley. 1996. An introduction to tree-ring dating. University of Arizona Press, Tucson, Arizona,USA.

Thompson, R. S., K. H. Anderson, and P. J. Bartlein. 1999.Atlas of relations between climatic parameters and distribu-tions of important trees and shrubs in North America. U.S.Geological Survey Professional Paper 1650-B, Denver,Colorado, USA.

Tian, J., D. M. Nelson, and F. S. Hu. 2006. Possible linkages oflate-Holocene drought in the North American midcontinentto Pacific Decadal Oscillation and solar activity. GeophysicalResearch Letters 33:L23702.

Umbanhowar, C. E., Jr. 2004. Interaction of fire, climate andvegetation change at a large landscape scale in the Big Woodsof Minnesota, USA. Holocene 14:661–676.

Umbanhowar, C. E., P. Camill, C. E. Geiss, and R. Teed. 2006.Asymmetric vegetation responses to mid-Holocene aridity atthe prairie-forest ecotone in south-central Minnesota. Qua-ternary Research 66:53–66.

Webb, R. S., and T. Webb III. 1988. Rates of sedimentaccumulation in pollen cores from small lakes and mires ofeastern North America. Quaternary Research 30:284–297.

Whitlock, C., P. Bartlein, and W. A. Watts. 1993. Vegetationhistory of Elk Lake. Pages 251–274 in J. P. Bradbury and W.Dean, editors. Elk Lake, Minnesota: evidence for rapidclimate change in the North Central United States (SpecialPaper 276). Geological Society of America, Denver, Colo-rado, USA.

Williams, J. W., et al. 2006. An atlas of pollen–vegetation–climate relationships for the United States and Canada.American Association of Stratigraphic Palynologists Foun-dation, Dallas, Texas, USA.

Ziegler, S. S., E. R. Larson, J. Rauchfuss, and G. P. Elliott.2008. Tree recruitment during dry spells at an oak savanna inMinnesota. Tree-Ring Research 64:47–54.


W oodland-to-forest transition dur ing prolonged drought in Minnesota …€¦ · from those...

Documents

Transcript of W oodland-to-forest transition dur ing prolonged drought in Minnesota …€¦ · from those...