Margaret B. Davis

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This collection includes the full text of papers written by University of Minnesota Regents' Professor Margaret Davis.

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    Three pollen diagrams from central Massachusetts
    (1958) Davis, Margaret B.
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    On the theory of pollen analysis
    (1963) Davis, Margaret B.
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    Patch formation and maintenance in an old-growth hemlock-hardwood forest
    (1993) Frelich, Lee E.; Calcote, Randy R.; Davis, Margaret B.; Pastor, John
    Cause of patch formation was investigated on a 7.2 ha study area in Sylvania Wilderness Area, a primary forest remnant in Upper Michigan comprising a mosaic of hemlock, sugar maple, and mixed-forest patches. Spatial autocorrelation analysis of the stem map indicated that, although most species pairs have a neutral association between canopy trees and understory trees of other species, hemlock and sugar maple canopy trees both have strong positive self association and negative reciprocal association with each other. No species pairs have a positive reciprocal association on regeneration with each other. MOSAIC, a Markov simulation model in which transition probabilities depend on neighborhood species composition, shows that the negative reciprocal association between hemlock and sugar maple of the intensity observed in this study, could lead to spatial separation into monodominant patches over long time periods (3000 yr). The mixed-forest patches are along spatial continua of varying steepness between sugar maple and hemlock patches. Interactions sugar maple and hemlock overstory and understory trees, along with the pattern of invasion of hemlock, provide a reasonable explanation for the patch structure. Pedological, topographical, and disturbance history differences do not coincide with the location of patches within upland forests on the study area.
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    Climatic changes in southern Connecticut recorded by pollen deposition at Rogers Lake
    (1969) Davis, Margaret B.
    Rates of deposition of pollen grains throughout late- and postglacial time were determined from the pollen concentration in radiocarbon-dated sediment. Changes by a factor of 5 or more for all except rare pollen types from one level to the next were considered significant indication of changes in the pollen input to the lake, reflecting changes in the pollen productivity of the surrounding vegetation. Low pollen deposition rates in the oldest sediments reflect the prevalence of tundra vegetation between 14,000 and 12,000 years ago. An increase in the rate for tree pollen occurred 12,000 years ago, when boreal woodland became established. The rates continued to increase until a sudden sharp rise for white pine, hemlock, poplar, oak, and maple pollen 9,000 years ago marked the establishment of forest, similar perhaps to modern forests of the northern Great Lakes region. Pine pollen rates declined 8,000 years ago, and deciduous tree pollen became dominant. Ragweed pollen was deposited at relatively high rates 8,000 years ago, reflecting changes in the vegetation associated with the "prairie period" recorded in the Great Lakes region at this time. Subsequent changes in pollen deposition rates reflect the immigration of beech (6,500 years B.P.), hickory (5,500 years B.P.), and chestnut (2,000 B.P.) to southern Connecticut. During the past few hundred years pollen deposition rates reflect changes in the vegetation caused by disturbance by European settlers. Throughout much of postglacial time the pollen assemblages deposited at Rogers Lake are different from assemblages known from modern sediment. This makes climatic interpretation difficult and suggests that the forest associations of the region as they are recognized now are of quite recent origin.
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    Paleoecology of range limits: beech in the Upper Peninsula of Michigan
    (1989) Woods, Kerry D.; Davis, Margaret B.
    Fossil pollen was used to map American beech (Fagus grandifolia) populations in Wisconsin and Michigan during the last 8000 yr. Among questions addressed were: (1) What were routes and rates of range expansion? (2) Did range expansion follow a @'wave-front@' model or occur by coalescene of outliers? (3) Has the range of beech been determined by environmental tolerance or by dispersal rate? Presettlement distributions were mapped from survey records. Range expansion was reconstructed using pollen diagrams from 34 sites, most in a 30-50 km grid. We show beech moving through southern Michigan into Wisconsin and Michigan's Upper Peninsula. During much of the last 8000 yr the range of beech was relatively stable and presumably environmentally controlled. Brief delays may have been occasioned by dispersal barriers. Range expansions occurred during the last millennium, as well as 3000-2500 yr ago and 8000-5000 yr ago. Several separate colonizations across Lake Michigan formed temporary outliers. No other past outliers were detected, although modern outliers are known; however, even with closely spaced sites, outliers may have been undetected.
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    Determination of absolute pollen frequency
    (1966) Davis, Margaret B.
    Changes in technique have simplified and made more reliable a method for determining the absolute numbers of pollen grains in sediment samples. The prepared sample is suspended in its entirety in the volume of fluid from which measured portions are taken for counting; tertiary butyl alcohol, instead of benzene with dissolved silicone fluid, is used as the suspending fluid.
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    Holocene variation in spatial scales of vegetation pattern in the upper Great Lakes
    (1993) Graumlich, Lisa J.; Davis, Margaret B.
    While continental-scale patterns of vegetation change during the Holocene clearly record the influence of climatic change, the factors governing change at the landscape scale are less clearly defined. In order to characterize the scales of processes determining vegetation patterns during the Holocene, we analyzed a network of 52 pollen sites in the upper Great Lakes region. Pollen percentage data for three dominant tree genera (pine, Pinus; oak, Quercus; birch, Betula) were interpolated from samples bracketing four target years (500, 2500, 4500, and 6500 yr BP). Smoothed isopoll maps of taxon abundance for each target year show broad trends in pollen abundance that correspond to climatic gradients. Residuals, representing the deviation of each pollen datum from the smoothed valued, indicate the amount of spatial variation in pollen abundance independent of that already modeled as a broad gradient. The three genera differ in the magnitude and pattern of residual variation. Oak residuals are relatively small in magnitude, while pine and birch residuals are relatively large and show greater local variability in sign and magnitude. This indicates local variability in tree abundance, as pollen of all three taxa is readily dispersed by wind. Spatial correlograms, which summarize the strength of spatial autocorrelation as a function of distance between pairs of sites for a given taxon, were calculated separately for each target year and allow the quantification of the dominant scale of variability of each taxon. Oak correlograms corroborate the mapped data in indicating the dominance of region-wide trends. In contrast the birch and pine correlograms indicate that factors operating at scales of 150 to 300 km are as important as region-wide trends in governing pollen abundances. The structure of the correlograms for birch and pine pollen changes through time, with birch showing a more patchy spatial pattern in the mid-Holocene (4500 and 6500 yr BP) as compared to the late-Holocene (500 and 2500 yr BP). Pine, in contrast, shows a more strongly autocorrelated pattern in the mid-Holocene. Our results suggest that substrate, an environmental constraint on vegetation at scales of tens to hundreds of kilometers, has been important in governing the spatial distribution of birch and pine in the upper Great Lakes region. The changing distribution of birch and pine is attributed, in part, to changes in the relative abundance of ecologically dissimilar species within these genera. Further, these observations suggest that spatial scales of tree abundances are dynamic and that constraints imposed by substrate vary in importance in response to long-term climatic variation.
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    Pollen in laminated sediments provides evidence for a mid-Holocene forest pathogen outbreak
    (1986) Allison, Taber D.; Moeller, Robert E.; Davis, Margaret B.
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    Plant migration and climate change
    (1997) Pitelka, Louis F.; Plant Migration Workshop Group
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    Outbreaks of forest pathogens in Quaternary history
    (1981) Davis, Margaret B.
    Several outbreaks of forest pathogens have occurred in North America in historic time. The chestnut blight (1904-1950), which virtually exterminated Castanea dentata, has left a clear fossil record. In subsurface sediment from Connecticut lakes chestnut pollen drops from 7% to less than 1% and just above these levels first birch pollen (probably from Betula lenta, a successional species) and then pollen from oak (a com· petitor of chestnut) increase in abundance (Brugam, 1975). A decline in chestnut pollen can be seen everywhere where chestnut was abundant in the forest. A similar phenomenon occurs in sediment deposited 4800 radiocarbon years ago. Tsuga pollen declined precipitously throughout the geographical range occupied by the species at that time. In New Hampshire it dropped from 30% to 5% within a century. In some regions it reg\ined its previous abundance 2000 years later, but in other regions the forest community changed through immigration of new species, and hemlock never recovered full abundance. The synchroneity of the hemlock fall at 4800 radiocarbon years, at. sites from New Brunswick to Upper Michigan (a distance of 1500 km) despite different abundances of hemlock and different forest communities is persuasive evidence for the widespread outbreak of a pathogen on hemlock. The swiftness of the decline and its widespread occurrence argue against climatic change as the causal factor. Neither could human activity have been important simultaneously over such a wide region. In each locality, a series of changes in tree pollen abundances follow the hemlock decline; these reflect a successional sequence that differed in each forest community. Although disease may have played a part in hastening the local demise of other tree species at times when climate changed, there is no evidence elsewhere in the Holocene pollen record for an outbreak of a paihogen. The elm decline in Europe 5000 years ago is the sole exception but it is often attributed to climate, or to human influence because it was simultaneous with the advent of agriculture. The rarity of sudden sharp declines in pollen abundances over wide regions implies that pathogen outbreaks have been infrequent phenomena in Quaternary history.
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    Late Quaternary history of low-and mid-elevation vegetation in the White Mountains of New Hampshire
    (1994) Spear, Ray W.; Davis, Margaret B.; Shane, Linda C.K.
    Pollen and plant macrofossil analyses of sediments from an altitudinal series of lakes in the White Mountains of New Hampshire, USA were used to reconstruct the history of vegetation on the mountain slopes and to identify the factors responsible for vegetation change. Six sites, Mirror Lake (213 m), Lost Pond (625 m), Little East Pond (793 m), Lonesome Lake (831 m), Carter Notch Pond (1004 m), and Lake of the Clouds (1538 m), provide paleoecological records of changes in the altitudinal limits of species, the species abundances within communities, and the vegetation zones. These are supplemented by previously published data from three high-elevation sites (Spear 1989). Although past plant communities were different from modern ones, differentiation of vegetation along the slopes has always existed. At low elevations the sequence of vegetation change was: 13 700-11 500 yr BP, tundra; 11 500-9000 yr BP, transitional mixed-conifer woodlands of first spruce (Picea) and then fir (Abies balsamea), larch (Larix laricina), poplar (Populus), and paper birch (Betula papyrifera); 9000-7000 yr BP, forests dominated by pine (Pinus) and oak (Quercus); 7000 yr BP-present, mixed-hardwood forests. No late-glacial paleoecological records exist at mid-elevation sites (700-1200 m). The steep slopes at these elevations stabilized by 10 000 yr BP and an early woodland of spruce was replaced by fir, larch, and paper birch. The altitudinal limits of both white pine (Pinus strobus) and hemlock (Tsuga canadensis) expanded to mid-elevations during the mid-Holocene, suggesting greater warmth 6000-4000 yr BP. The modern spruce/fir forests of mid-elevations became established 2000 yr ago as spruce expanded at all elevations, suggesting cooler, moister climate similar to today. A tree line dominated by balsam fir and black spruce (Picea mariana) was established at its modern position 10 000 yr ago and has varied little since then, although it appears to have been slightly higher than now during the early Holocene. Changes in the vegetation at low and mid-elevations have not been synchronous with those at tree line and result from a more complex set of environmental factors and climatic variables acting on several different species. At low and mid-elevations disturbance by wind and frost action was important during the late-glacial. Disturbance by fire was important during the period of spruce woodland and later in the early Holocene pine and oak forest. From 7000 yr BP to the present the primary factor disturbing New Hampshire forest was again probably wind, especially from 7000 to 4000 yr BP when higher temperatures than present may have been associated with increased frequency and intensity of thunderstorms and tropical hurricanes. The vegetation history reveals that different lapse rates have occurred along the mountain slopes. This provides evidence that the source of air masses reaching the White Mountains has varied. The boundary between alpine tundra and subalpine fir forest (tree line) most likely has always been governed temperature (summer insolation) and wind. The montane plant communities result from individual species response rather than community response to the numerous climatic forces that have affected the mountains over the past 14 000 yr.