late holocene stand‐scale invasion by hemlock …cote 1986, parshall 1999, parshall and calcote...

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Ecology, 83(5), 2002, pp. 1386–1398q 2002 by the Ecological Society of America

LATE HOLOCENE STAND-SCALE INVASION BY HEMLOCK(TSUGA CANADENSIS) AT ITS WESTERN RANGE LIMIT

TIM PARSHALL1

University of Minnesota, 100 Ecology Building, 1987 Upper Buford Circle, St. Paul, Minnesota 55108 USA

Abstract. The movement of plant range limits in the past has been clearly documentedas a response to changing climate, but the pattern and timing of local invasions that resultfrom these plant migrations are not well understood. Sedimentary evidence from smallforest hollows is used here to reconstruct the pattern of stand-scale invasion by easternhemlock (Tsuga canadensis (L.) Carr.) at its current range limit in Wisconsin. Pollen recordsfrom lakes show a slowly increasing regional hemlock population over the past 2500 yr,as its range expanded from the east in response to cooler, wetter climate. In contrast, hemlockpollen in each of the forest hollows increased abruptly within the past 500 yr, demonstratingstand-scale dynamics at a time when regional changes were small. Moreover, this abruptrise in hemlock is part of a two-phase sequence of local invasion. First, stands were initiallycolonized as much as 2000 yr ago, but the size of these local populations remained lowfor as long as 1800 yr. This pattern suggests that scattered, small populations grew beyondthe continuous range limit of hemlock, which was tens of kilometers to the east. Second,these small hemlock populations expanded in size between 100 and 500 yr ago during atime of cooler, wetter climate, which was apparently limiting population growth. Theseresults illustrate contrasting patterns of change between local and regional populations, andthe importance of outlying colonies as sources of rapid population expansion when envi-ronmental conditions become favorable.

Key words: climate change; colonization; forest hollow; forest stand invasion; plant distribution;pollen; stomata; Tsuga canadensis; Wisconsin, USA.

INTRODUCTION

Climate is the predominant factor influencing plantdistribution at subcontinental scales (Woodward 1987,Sykes et al. 1996), and changes in climate are accom-panied by shifts in plant ranges (Webb et al. 1987,Huntley and Webb 1989, Prentice et al. 1991). Thismeans that future climate change will lead to differentplant distributions than occur today (Overpeck et al.1991, Davis and Zabinski 1992, Iverson and Prasad1998). As the range of a plant spreads across a land-scape, individuals dispersing into previously unoccu-pied space must become established within the con-straints of physical site characteristics and competitionwith resident vegetation. Our understanding of this lo-cal establishment process primarily emerges from ob-servations of contemporary invasions, which have be-come a major component of recent global change (Vi-tousek et al. 1996, Pitelka et al. 1997). However, mostof these invasion studies describe species that havebeen introduced by humans to areas outside their nativeranges and, therefore, their migration or spatial spreaddoes not solely represent a range shift in response toclimate change.

Manuscript received 27 November 2000; revised 10 July2001; accepted 2 August 2001; final version received 4 Septem-ber 2001.

1 Present address: Harvard Forest, Harvard University,P.O. Box 68, Petersham, Massachusetts 01366-0068 USA.E-mail: [email protected]

Paleoecological methods document plant invasionsthat have occurred over long time intervals in responseto changing climate (Prentice 1988, MacDonald 1993,Jackson 1997). Most of these studies are restricted toplant migrations across large areas and over long timeperiods, because they use pollen preserved in lake sed-iments, which represents vegetation on landscape toregional scales (e.g., Huntley and Birks 1983, Davis etal. 1986, Bennett 1988; Huntley and Webb 1989, Jack-son et al. 1997). Very small sedimentary basins, suchas forest hollows, can be used to reconstruct past chang-es in forest composition on the scale of several hect-ares. This small spatial scale is appropriate for recon-structing colonization and population expansion of in-dividual forest stands, and a group of forest hollowswithin a relatively small area can address differencesin stand invasion in relation to range-limit dynamics(e.g., Bradshaw 1988, Bjorkman and Bradshaw 1996,Davis et al. 1998).

In this study, sedimentary records from forest hol-lows are used to reconstruct the invasion of easternhemlock (Tsuga canadensis (L.) Carr.) into foreststands at its modern western range limit in Wisconsin.Previous studies of pollen from lakes have demonstrat-ed a progressively westward movement of hemlock’srange limit over the past 6000 yr (Fig. 1) in responseto a change in climate toward cooler, wetter conditions(Bartlein et al. 1984, Brugam and Johnson 1997, Daviset al. 2000). Hemlock pollen is present by 2500 yr agoin lakes along hemlock’s western range limit, but the

May 2002 1387FOREST STAND COLONIZATION AND EXPANSION

FIG. 1. Hemlock pollen abundance in lake sediments from Wisconsin and Michigan show the westward movement ofhemlock’s range limit over the past 6000 yr: (1) Saxton (M. B. Davis, unpublished data), (2) Camp (M. B. Davis, unpublisheddata), (3) Hemlock (Davis et al. 2000), (4) Lake Mary (Webb 1974), (5) Wood (Heide 1984), (6) Crooked (Brugam et al.1997), (7) Canyon (Davis 1981), (8) Spirit (Woods and Davis 1989), and (9) Ryerse (Futyma 1982). The locations of foresthollows and Hemlock Lake are shown in relation to the presettlement distribution of forest types in Wisconsin (after Finley1976). The distribution of hemlock tree observations recorded by the U.S. General Land Office survey corresponds closelywith the distribution of the Hemlock–Pine–Hardwoods forest type.

abundance of hemlock pollen has risen very slowlysince then (Fig. 1: Davis et al. 1986). This leaves openthe question of how hemlock’s local- to landscape-scalepopulation developed as its range limit reached its pre-sent location. Were hemlock stands colonized at thesame time in the past, after which local populationsgrew slowly? Or were individual stands invaded at dif-ferent times, each contributing modestly to the regional

hemlock population? These two scenarios imply dif-ferent patterns of spread across a landscape, ultimatelyaffecting how quickly a species can respond to chang-ing climate.

STUDY DESIGN

To address these questions, I focused on the devel-opment of four hemlock stands located within 12 km

1388 TIM PARSHALL Ecology, Vol. 83, No. 5

TABLE 1. Characteristics of the forest hollow sites.

Sitename

Basinradius

(m)

Corelength(cm)

Sand–silt–clay (%)

Soiltexture

OwenDiamondCutacrossDrummond

810

56

34105

7067

69–21–1081–12–788–4–886–5–9

sandy loamloamy sandloamy sandloamy sand

Note: Soil textural characteristics were assessed by the hy-drometer method (Gee and Bauder 1986).

of each other along hemlock’s western range limit (Fig.1). Since hemlock trees are present in all four standstoday, each site represents a separate example of standinvasion as the range limit of hemlock arrived to theregion. Forest hollows provide a more local record ofvegetation change than lakes because as the size of asedimentary basin decreases, pollen produced from lo-cal sources becomes a larger component of total pollendeposited at a site (Jacobson and Bradshaw 1981, Pren-tice 1985, Jackson 1990, Sugita 1994, Jackson and Ly-ford 1999). For small lakes (20–100 m radius) likethose shown in Fig. 1, variation in pollen can distin-guish compositional differences in vegetation on theorder of 100–1000 hectares. Pollen preserved in foresthollows (,10 m radius), on the other hand, is moreclosely related to vegetation on the order of a few hect-ares. Even though 50–60% of pollen may be derivedfrom plants .100 m away, variation in forest com-position within 100 m of the hollows is discernable(Jackson and Wong 1994, Sugita 1994, Calcote 1995,1998, Jackson and Kearsley 1998).

An important consideration in this study is the abilityto detect a small hemlock population because very fewtrees might be present at a site during the initial stagesof invasion. Pollen percentages from forest hollows canresolve small, local population sizes, but only roughly.For example, hemlock pollen percentages as low as0.5% may represent the presence of trees at a site,especially when regional populations are low, but theycan also be as high as 1% in stands where hemlocktrees are absent locally, but abundant regionally (Cal-cote 1986, Parshall 1999, Parshall and Calcote 2001).Here I interpreted 1% hemlock pollen as strong evi-dence for local presence in the early stages of invasionwhen the regional population is low.

Much stronger evidence for the local presence oftrees comes from the presence of fossil stomata (Han-sen 1995, Clayden et al. 1996) because the leaves andneedles from which stomata are derived disperse veryshort distances. In fact, the presence of hemlock sto-mata in forest hollow sediments today indicates thattrees are present within 20 m (Parshall 1999), and fossilstomata are used here to define with high confidencethat trees were present within the stand in the past.

Fossil pollen assemblages were also compared topollen in surface sediments and associated forest com-position data in order to evaluate past shifts in standcomposition in relation to forests that exist today (Cal-cote 1998, Davis et al. 1998). Rather than relying ona single taxon, this approach integrates pollen from allof the important tree taxa, assuming that highly similarfossil-modern pairs are produced by similar foreststands. This method works well for areas with rela-tively homogeneous vegetation such as northwesternWisconsin (Sugita 1994, Calcote 1998, Parshall andCalcote 2001).

STUDY AREA

The study area spans the western range limit of hem-lock in northwestern Wisconsin (Fig. 1: Goder 1955,Finley 1976). In the 1850s, hemlock comprised ;25%of all trees in this area, according to the U.S. GeneralLand Office, which also provides occasional records ofhemlock trees growing .50 km to the west (Finley1976, Calcote 1986, Godman and Lancaster 1990).Other trees common in the presettlement upland forestsinclude white pine (Pinus strobus), sugar maple (Acersaccharum), yellow birch (Betula alleghaniensis),American basswood (Tilia americana), and red pine(Pinus resinosa) (Stearns 1951, Curtis 1959, Finley1976, Frelich 1995). Soil textural characteristics anddrainage of the study area are largely controlled bysandy till and outwash features of the Copper FallsFormation, deposited during the last deglaciation (Hole1976, Clayton 1984, Clayton et al. 1991).

Climate over the past 4000 yr has become progres-sively cooler and wetter in the Midwest and GreatLakes region (Webb et al. 1983, Bartlein et al. 1984,COHMAP 1988, Davis et al. 2000), leading to a risein the regional water table (Futyma and Miller 1986,Winkler et al. 1986, Miller and Futyma 1987, Winkler1988, Brugam and Johnson 1997). In addition to themigration of hemlock, the range limits of several othermesic species, including white pine and Americanbeech (Fagus grandifolia), moved westward over thepast 3000–4000 yr (Jacobson 1979, Bennett 1985, Da-vis et al. 1986, Woods and Davis 1989).

The direct impact of Europeans in northern Wiscon-sin was small until the late 1800s when logging activ-ities began (Fries 1951, Williams 1989). The Rust-Owen Lumber Company owned most of the land in thestudy area and started cutting trees in the mid-1870s(Fischer 1964). Logging peaked in the 1890s, but op-erations continued until the 1930s when the land wasacquired by the federal government and became theChequamegon National Forest.

METHODS

All four forest hollows are seasonally to continuallywet depressions, ;10 m in radius, and surrounded byclosed-canopy forests with hemlock trees growingwithin 20 m (Fig. 1, Table 1). Sediment cores werecollected from three of the hollows (Diamond, Drum-


TABLE 2. AMS (accelerator mass spectrometry) radiocarbon dates and their associated cal-endar dates identified by CALIB version 4.2 (Stuiver and Reimer 1993, Stuiver et al. 1998)at the four forest hollow study sites.

SiteDepth(cm) Material 14C age BP

Calendar age BP(2s range) Lab no.

Owen

Diamond

15291626

woodcharcoal

woodwood

350 6 502410 6 501490 6 302295 6 85

350 (510–300)2360 (2710–2340)1350 (1420–1310)2340 (2710–2120)

Beta-117377AA-21375OS-15671AA-21376

Cutacross

Drummond

28531540

woodcharcoal

woodneedles, Pinus strobus

480 6 503200 6 100

360 6 401310 6 40

520 (620–470)3430 (3680–3170)

350 (510–310)1260 (1290–1170)

Beta-117378OS-16942Beta-119673OS-15625

Note: Calender ages and ranges are from intercepts of the calibration curve (Method A) andare rounded to the nearest 10 yr.

mond, Cutacross) using a 10-cm diameter piston corer,and from one hollow (Owen) using a 6.3-cm diameteracrylic tube. The sediments in each hollow are almostentirely organic, derived primarily from plant materialshed by vegetation surrounding the basin.

The pollen record from Hemlock Lake, a closed ba-sin ;1 ha (55-m radius) in size, is representative ofother lakes in the region. The Holocene sediments aredominated by pine pollen, especially white pine, withlesser amounts of oak and birch (Davis et al. 2000).Birch pollen gradually increases ;4000 yr ago, and thefirst hemlock pollen grains appear 3000 yr ago. Con-tinuous hemlock pollen accumulation over the past2500 yr indicates the presence of hemlock trees on thelandscape (Davis et al. 1986, Davis and Sugita 1997).

Sedimentary analyses

Surface and fossil sediments were prepared for pol-len analysis using standard methods (Faegri and Iver-sen 1989). Pollen grains were identified at 4003 mag-nification and counted until at least 400 tree pollengrains were encountered. Counts of stomata and markergrains were made on the same slides assessed for pollenuntil an equivalent of 1000 pollen grains was reached.Macroscopic charcoal fragments were counted in sed-iment samples prepared by soaking in KOH for at least24 h and sieving through a 250-micrometer screen.

A date of AD 1890 was assigned to the level in thecore corresponding to the period of European settle-ment, defined stratigraphically as a reduction in pinepollen, and an increase in ragweed (Ambrosia) and oth-er herbaceous pollen. Plant macrofossils were selectedfrom sediments at important stratigraphic horizons forAMS (accelerator mass spectrometry) radiocarbonanalysis (Table 2). All dates are reported in the text ascalendar years before present (BP), where ‘‘present’’is AD 1950. Sediment ages for other depths were de-termined by linearly interpolating between radiocarbondates, the date of European settlement, and the surfaceof the core.

Modern analogues

To expand the network of surface pollen samplesfrom forest hollows already available for northwestern

Wisconsin (Calcote 1995, 1998, Parshall and Calcote2001), I collected additional paired samples of pollenand vegetation data, yielding a more inclusive rangeof modern forest stand types that exist in the region.Sites were assigned to stand types by classification ofbasal area for the 11 common upland trees, and a mea-sure of dissimilarity, squared chord distance (SCD),was calculated to compare all pollen assemblages toeach other (Prentice 1980, 1986, Overpeck et al. 1985,Grimm 1987). Fossil pollen assemblages are comparedwith this set of surface samples to infer past standcomposition.

RESULTS

Surface sample analyses

Six different stand types were identified based ontree basal area around each pollen surface sample (Fig.2). The dominant pollen types in surface sedimentscorrespond well with the dominant trees identified inthe stand type classification. Changes in past standcomposition are interpreted from fossil pollen usingthreshold SCD values identified from these data andother similar studies (Calcote 1998, Davis et al. 1998,Parshall and Calcote 2001). SCD values ,0.05 arestrong evidence that fossil pollen assemblages wereproduced by analogous modern forest types. SCD val-ues ,0.1 also provide a reliable level of interpretationand serve here as weak analogues.

Sedimentary records

Owen hollow.—Pollen and stomata from Owen Hol-low clearly demonstrate both the initial arrival of hem-lock trees and their subsequent increase in abundance(Fig. 3). Hemlock pollen first appears ;2360 yr BP,followed shortly thereafter by hemlock stomata, whichoccur in all younger sediments. Hemlock pollen per-centages increase to 10% of total pollen at 350 yr BP,and subsequent fossil pollen assemblages resemblethose of modern hemlock stands. Coincident with thisshift, ash pollen increases and pine pollen declines tem-porarily. The settlement period is represented in the topsamples by a modest increase in ragweed and otherherbaceous taxa, and a major decline in pine pollen


FIG. 2. Percentage of pollen in surface sediments from43 sites (white bars) and percentage distance-weighted basalarea of trees (gray bars) within 100 m of each pollen sample.Sites are arranged in order of stand-type groups defined bycluster analysis. Hdwds 5 hardwoods.

from 50% to 25%. Charcoal is most abundant in thedeepest sediments, when the stand was apparently dom-inated by white pine, and declines as hemlock stomataand pollen first appear. There are a few stumps presenttoday, primarily white pine, but many of the hemlocktrees at the site are .250 yr old, indicating that loggingoccurred but was not extensive.

Diamond hollow.—The stand history from DiamondHollow shows a similar pattern of development toOwen Hollow (Fig. 4). Hemlock pollen is uncommon(,0.5%) before 2340 yr BP, and reaches 1% ;1350 yrBP, suggesting local presence of trees. Hemlock sto-mata first appear around 800 yr BP, well before thesharp increase in pollen that occurs just before Euro-pean settlement. Like Owen Hollow, a shift in modernanalogue similarity toward hemlock stands occurs atthe same time that hemlock pollen percentages increasesharply. Charcoal concentrations are low throughoutthe core.

Cutacross hollow.—The core from Cutacross Hol-low was started at 13 cm below the surface owing tofibrous and woody material at the top of the sediments.Hemlock pollen is extremely low (0.3%) before 3430yr BP (Fig. 5), and surpasses 1% between 1500 and2000 yr BP. The absence of all stomata from the sed-iments between 30 and 50 cm suggests that preserva-tion was poor at this time, so hemlock may have beenpresent but undetected. Hemlock stomata first appearat 520 yr BP, as both hemlock and birch pollen increasesand white pine pollen declines. Modern analogues shifttoward hemlock stands with the rise in hemlock pollen.A layer of small fragments of bark and wood just beforethis change likely represents a canopy disturbance justbefore the development of this hemlock forest. As atOwen Hollow, charcoal abundance is very high in thelower portion of the core where pollen assemblages aredominated by pine, and declines as hemlock pollensurpasses 1%.

Drummond hollow.—Hemlock pollen is low(,0.5%) in Drummond Hollow before 1260 yr BP, anddoesn’t surpass 1% until 350 yr BP (Fig. 6). Hemlockpollen gradually increases after this time to reach 5%of total pollen by the settlement period. Stomata areonly found around the time of settlement, but the ab-sence of all stomata throughout much of the core sug-gests poor preservation conditions. Fossil pollen as-semblages before 350 yr BP are similar primarily tothe pine–oak–hardwood stand type, and show weak orno similarity to modern pine stands, which is a differentpattern than the other three sites. Although this hem-lock population has increased in size over the last threecenturies, pollen percentages fail to show an abruptincrease as seen at the other sites. Ash pollen per-centages increase sharply (.10% of total pollen) ;350yr BP, coincident with a peak in birch pollen abundanceand a brief decline in pine pollen abundance. This sug-gests that a canopy disturbance occurred just before thelocal hemlock population began to grow in size. Many


FIG. 3. Pollen, stomata, and charcoal in forest hollow sediments at Owen Hollow. Light shading is a 103 exaggeration.Dates shown are radiocarbon ages as in Table 2. Modern analogues are identified with circles representing similar fossil–modern pairs. Stand types and sites are arranged in the same order as in Fig. 2. Pollen is measured as a percentage; stomataare measured as number per milliliter; and charcoal is measured as number of fragments per 20 milliliters.

FIG. 4. Pollen, stomata, and charcoal in forest hollow sediments at Diamond Hollow. Light shading is a 103 exaggeration.Dates shown are radiocarbon ages as in Table 2. Modern analogues are identified with circles representing similar fossil–modern pairs. Stand types and sites are arranged in the same order as in Fig. 2.


FIG. 5. Pollen, stomata, and charcoal in forest hollow sediments at Cutacross Hollow. Light shading is a 103 exaggeration.Dates shown are radiocarbon ages as in Table 2. Modern analogues are identified with circles representing similar fossil–modern pairs. Stand types and sites are arranged in the same order as in Fig. 2.

FIG. 6. Pollen, stomata, and charcoal in forest hollow sediments at Drummond Hollow. Light shading is a 103 exag-geration. Dates shown are radiocarbon ages as in Table 2. Modern analogues are identified with circles representing similarfossil–modern pairs. Stand types and sites are arranged in the same order as in Fig. 2.


FIG. 7. Summary diagram of hemlock pollen percentages from each of the forest hollows and from Hemlock Lake. Theearliest evidence for the presence of hemlock trees is indicated by either ‘‘st’’ (presence of stomata) or ‘‘p’’ (hemlock pollen.1%). ‘‘Colonization’’ indicates the first evidence, either stomata or pollen, that hemlock trees were at the site, and ‘‘ex-pansion’’ represents an increase in hemlock pollen and a shift in modern analogue similarity as shown in Figs. 3–6.

of the hemlock trees surrounding the hollow are .180yr old (see also Stearns 1951), demonstrating that thestand was initially colonized by the early 1800s, beforelogging occurred. Charcoal values are low throughoutthe core, except in samples at the time of Europeansettlement, which likely represents a human-caused ig-nition since this site is near the center of logging ac-tivities at the turn of the century.

DISCUSSION

The vegetation history derived from these forest hol-lows differs from the vegetation history derived fromlakes along hemlock’s range limit. At Hemlock Lake,the gradual increase in hemlock pollen over the past2500 yr contrasts with the relatively rapid and sub-stantial increase in hemlock pollen deposited in foresthollows over the past 500 yr. These two distinct pat-terns arise from the different spatial scales representedby each basin. The forest hollows are recording changesin forest composition at the stand scale, while the lakesare recording changes in forest composition in the re-gion (Jacobson and Bradshaw 1981, Prentice 1985,Calcote 1995). Apparently, the rapid growth of localhemlock populations was not a significant regional oc-currence, since none of the lakes at hemlock’s rangelimit show much change. The small spatial represen-tation of forest hollows captures a two-stage pattern ofstand development not available from lake records, in-cluding initial colonization followed by stand expan-sion. In the following discussion I first evaluate theevidence available for this two-phase invasion se-quence, then I address ecological factors that likely

influenced both initial colonization and subsequent ex-pansion of the stands.

Evidence for colonization and stand expansion

Although hemlock pollen occurs in the older foresthollow sediments, its very low abundance and incon-sistent presence makes it doubtful that trees were grow-ing near any of the sites before 2500 yr ago. This pat-tern corroborates the evidence from lakes in the regionshowing a landscape-scale hemlock population sosmall that it is not discernable from larger distant hem-lock populations that existed to the east (Fig. 1: Daviset al. 1986, Davis 1987, Davis and Sugita 1997). Fossilstomata clearly document the earliest arrival of treesat Owen Hollow ;2000 yr ago, and the other sites showevidence of colonization at various times after this(Figs. 3–7). This agrees well with the first continuouspresence of hemlock pollen in Hemlock Lake, and sug-gests that small, dispersed populations of trees werepresent on the landscape by 2000 yr ago.

The presence of stomata is the best evidence thattrees were growing nearby, but their absence does notprove that trees were absent from a stand, because pres-ervation conditions may have been poor, or trees weregrowing too far from the forest hollow to be detected.In several instances the absence of all stomata suggeststhat the date of first colonization is probably an un-derestimate. Hemlock pollen percentages surpassing1% where stomata are absent is relatively strong evi-dence that hemlock trees were growing at Cutacrossand Diamond by 1500 yr BP, and at Drummond by 700yr BP. If both stomata and pollen evidence is consid-


ered, colonization occurred at all sites between 2000and 500 yr BP (Fig. 7).

Stand expansion, on the other hand, is documentedby a substantial rise in hemlock pollen and a shift inpollen assemblages to resemble modern hemlockstands, which includes a rise in birch and a decline inwhite pine pollen. Based on this evidence, initial col-onization clearly predated stand expansion by severalhundred to at least 1000 yr at three of four sites. A lagbetween initial colonization and population expansionis a reasonable scenario for hemlock. Hemlock’s hightolerance of shade allows saplings to survive for .100yr in a dark understory, and adult trees can persist ata site for .400 yr (Graham 1941, Hough and Forbes1943, Godman and Lancaster 1990). Therefore, oncetrees become established, only a few generations arenecessary to produce the pattern of low, persistent hem-lock populations documented in this study.

Ecological factors associated withinitial colonization

Before these hemlock populations grew in size, localpopulations were very small and the regional popula-tion was probably diffuse. How and why were theseparticular sites initially colonized? The increasingchance that seeds reached a site as hemlock’s rangelimit continued to move closer to the area must haveplayed a role. Site characteristics may have also influ-enced the chance of successful colonization, therebyaffecting the rate of range expansion because hemlock’ssmall seeds can disperse long distances (Davis et al.1986, Davis 1987, Godman and Lancaster 1990). Al-though no one site factor was identified to be importantin all four cases, several are worth discussing here,including a reduction in fire occurrence, favorablephysical characteristics, and prior forest composition.

A decline in charcoal at Owen and Cutacross Hol-lows as hemlock trees first colonized these stands isgood evidence that fire and hemlock’s initial coloni-zation were related (Figs. 3–6). There is no clear spikein charcoal abundance that would suggest a catastroph-ic fire facilitated hemlock establishment, as has beencited by some authors (Maissurow 1941, Eckstein1980). Instead, the decline in charcoal indicates thatan overall reduction in fire may have encouraged standcolonization by allowing hemlock seedlings to surviveand eventually ascend to the canopy. A cooler, wetterclimate following the mid-Holocene warm periodwould have decreased the frequency of fire, and al-though there are no long-term records of fire occurrencefrom lakes nearby, sites in south-central Wisconsinshow a reduction in fire occurrence since 3000 yr BP(Winkler 1997).

Differences in physical characteristics among sitesmay have also influenced hemlock colonization, as iscertainly true for modern outlying colonies that occurin cooler, moister northern slopes and ravines in Min-nesota and southwestern Wisconsin (Adams and

Loucks 1971, Rogers 1978, Calcote 1986). Hemlocktrees appeared first at Owen Hollow, a site located only50 m from the shore of a large lake. Lakeshores andbogs are good sites for hemlock establishment in mod-ern old-growth forests (Pastor and Broschart 1990, Fre-lich et al. 1993), and were probably among the firstareas colonized by hemlock in the past. Lakeshoreshave higher light availability, a greater amount of ex-posed mineral soil, and a more consistent source ofmoisture throughout the year, all potentially improvingthe chances for hemlock survival (Olson et al. 1959,Coffman 1978, Rogers 1978). The soils at Owen Hol-low also have a larger component of fine-grained par-ticles (Table 1), which retain water more effectivelyand reduce the chance of seedling desiccation.

The species composition of the resident forests mayhave also influenced colonization if some stand typesare more resistant to invasion than others (Elton 1958,Mooney and Drake 1986, Crawley 1987). Stands thatwere colonized first (Owen, Cutacross, and Diamond)were dominated by pine, documented by both pollenanalogues and the presence of pine stomata. In contrast,hardwoods comprised a large proportion of the standaround Drummond Hollow, the last site to be colonized.Davis et al. (1998) provide evidence for differentialstand invasibility, observing that hemlock preferen-tially invaded pine stands over hardwood stands. Thedelayed colonization of hardwood stands could be re-lated to the difficulty of hemlock establishment in hard-wood litter and competitive interactions with hardwoodseedlings and saplings (Friesner and Potzger 1934, Ka-vanagh and Kellman 1986, Frelich et al. 1993). Alter-natively, there may be some unknown physical differ-ence in site characteristics that is responsible for botha difference in stand type before hemlock arrival, andthe ability of hemlock to establish and expand at thesite.

Synchronous stand expansion suggests acommon explanation

The synchronous expansion of these hemlock standsover the past 500 yr suggests that one or only a fewfactors explain their recent development. Human im-pacts are often the cause of such a clear change in forestcomposition, but they are not the direct cause herebecause population growth was initiated before Euro-pean settlement. There is some indication that selectivelogging for white pine may have encouraged a sec-ondary expansion at the turn of the century at theDrummond and Diamond sites. More recent logging orother canopy disturbance would tend to encourage thereplacement of hemlock by hardwoods because of poorhemlock regeneration throughout forests of the GreatLakes (Tyrrel and Crow 1994, Parshall 1995, Davis etal. 1996). Deer browse is cited as one of the mainreasons for this recent lack of regeneration (Andersonand Loucks 1979, Frelich and Lorimer 1985, Alversonet al. 1988), and a decline in deer populations over the


past 500 yr could have led to an increase in hemlock.However, there is no record available to test this hy-pothesis, and other factors might also explain hem-lock’s poor regeneration recently (Mladenoff andStearns 1993).

Since all of the sites occur within a small area, arelatively large disturbance may have caused synchro-nous expansion. Fire has been suggested as a facili-tating factor in hemlock stand establishment (Mais-surow 1941, Eckstein 1980), but unlike the link be-tween fire and initial colonization, there is no evidencethat fire was associated with population expansion atthese sites. Wind is a more common disturbance agentin the Great Lakes forests (Canham and Loucks 1984,Whitney 1986, Frelich and Lorimer 1991), and light tomoderate wind disturbances over a period of time mayfavor hemlock’s eventual accession to the canopy(Hough and Forbes 1943, Rogers 1978, Abrams andOrwig 1996). Canopy disturbances probably precededlocal hemlock expansion at three sites. At CutacrossHollow, the evidence consists of a layer of woody de-bris immediately underlying sediments with higherhemlock and birch pollen percentages (Fig. 5). AtOwen (Fig. 3) and Drummond (Fig. 6) Hollows, ashand birch pollen percentages increase as pine pollendeclines, which suggests mortality of white pine, andreplacement by ash and birch, and then by hemlock.

A shift in climate toward cooler, wetter conditionscould also explain the synchronous expansion of hem-lock, since seed germination and seedling survival aresusceptible to drought (Olson et al. 1959, Godman andLancaster 1990), and tree growth is favored by lowsummer temperatures and higher precipitation (Graum-lich 1989, Cook and Cole 1991). As climate changedover the past 3000 yr, some critical threshold may havebeen crossed that increased the likelihood of local hem-lock establishment and survival, leading to the shift instand composition. The period between 500 and 150yr ago, identified in many places around the world asthe Little Ice Age (Lamb 1982, Grove 1988, Baron1995), was particularly cool and wet in the upper Mid-west (Wahl 1968, Grimm 1983, Clark 1988, 1990) andmay have further favored hemlock regeneration. Thereis no evidence for expansion of hemlock stands at otherparts of its range, but populations that are climaticallylimited may be more responsive to small changes inclimate. Furthermore, climate coupled with canopy dis-turbance would have been an effective mechanism forsynchronous expansion at these sites (Stearns 1951,Mladenoff and Stearns 1993).

CONCLUSIONS

A change in climate toward cooler, wetter conditionsover the past several millennia in the midwestern Unit-ed States led to the westward expansion of several me-sic species, and the development of the hemlock standsdescribed here provides a unique example of how thiskind of range expansion is expressed at the stand level.

While hemlock pollen from lakes documents a slowlygrowing regional population over the past 2000 yr, pol-len and stomata from forest hollows reveal that localhemlock populations were present for as long as 1800yr, and then grew very quickly in size within the past500 yr. This means that as hemlock’s range limitreached its present location, the slow growth in its re-gional population was a composite of many local col-onization and rapid population expansion events. Localstand expansion at other times over the past 2000 yrmust have occurred elsewhere on the landscape to con-tribute to the slow regional rise found in lakes, butwere not sampled here.

Before stand expansion, small populations of hem-lock trees were apparently scattered across the land-scape outside of its continuous range limit that existedtens of km to the east. Outlying colonies today occurmore than 50 km west of hemlock’s present range limit,and could serve as loci for future population expansionif the climate were to become cooler or wetter. Outlyingcolonies provide important sources of propagules forplants to extend their ranges into areas where climateis now favorable for colonization and expansion, andin doing so, plant migration can respond to changingclimate more rapidly. Long-distance seed dispersal andrapid expansion of outlying colonies must have playedsome role in the rapid plant migrations that occurredduring the early Holocene, and will likely be importantfor plant populations responding to climate change inthe future (Pitelka et al. 1997, Clark 1998, Clark et al.1998, Iverson and Prasad 1998, Higgens and Richard-son 1999, Davis 2001).

ACKNOWLEDGMENTS

The following people provided helpful comments on theproject and manuscript: R. R. Calcote, M. B. Davis, D. R.Foster, S. Jackson, F. Mitchell, D. Orwig, S. Sugita, and twoanonymous reviewers. I thank M. B. Davis for the use ofunpublished data from Saxton and Camp Lakes. This researchwas supported financially by Sigma Xi, Dayton and WilkieNatural History Funds, National Science Foundation Re-search Training Grant for Paleorecords of Global Change,and the University of Minnesota’s Graduate School and Qua-ternary Paleoecology Minor Program.

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