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Post Nipissing-phase strandplains of the Great Lakes reflect late-Holocene water levels that are linked to climate variability, glacial isostasy, and changes in basin hydrology. This study examines a strandplain sequence at Bailey's Harbor, Door County, Wisconsin, along Lake Michigan, which contains a record of water-level variations during the past 1000 years, following the separation of the Lake Michigan-Huron basin from the Lake Superior basin. Vibracores are used to define the current elevation of the foreshore facies in 25 preserved beach ridges. Optical ages for five individual beach ridges are compared with radiocarbon ages on basal peats from nine swales collected in a previous study. The optical chronology yields a ridge formation/preservation rate of 30 ± 2 yr/ridge similar to that of previous assessments. This study further demonstrates the efficacy of optical dating for deciphering late-Holocene beach-ridge sequences, providing multidecadal resolution. Lake level meets or exceeds, by at least 0.5 m, historic metrics at c. ad 1125-1225. The maximum water level after c. AD 1225 is comparable with that recorded by the instrumental data. There is a noticeable high lake level from c. AD 1560 to 1725. Dating of additional sites along Lake Michigan is required to establish local rates of glacio-isostatic adjustment, identify possible erosion of the outlets and to resolve lake-level records for individual lakes following the separation of the upper Great Lakes currently constrained to have occurred c. 2400 to 1200 years ago. [ABSTRACT FROM AUTHOR]

Aeolian sand in two small lakes within a Lake Michigan coastal-dune complex southwest of Holland, Michigan, provides a more detailed higher-resolution history of dune activity, during the later half of the Holocene, than do combined palaeosol and OSL ages from the dunes themselves. The sand signal from four cores within these lakes consists of visible sand laminae and invisible sand peaks within mud and sapropel units. Early in the history of dune growth the sand may have come from backdunes immediately adjacent to the lakes, but in the upper portions of the core aeolian sand is interpreted as having been transported from 30–40 m high parabolic dunes ∼200 m from the lakes. All but one episode of dune mobility suggested by radiocarbon-dated dune palaeosols or OSL-dated dune sand are represented by a peak in the radiocarbon-dated lacustrine sand signal. However, sand peaks occur in the lacustrine record that do not correspond to palaeosol or OSL ages, suggesting that the sand record from small lakes gives a more complete record of aeolian activity. Heterogeneity in sand deposition related to bathymetry suggests that the study of multiple cores from the same lake will yield the best results. No strong correlation between rising lake levels on the Lake Michigan relative lake level curve and aeolian activity was found, although such a correlation has been reported elsewhere. [ABSTRACT FROM AUTHOR]

The 'Qualitative Assessment of Difference' method (QAD) is proposed to objectively detect differences in the relative abundance of vegetation between paired sites using pollen percentages. This method corrects for intertaxonomic differences in pollen productivity and neutralizes influences of background pollen on pollen representation of vegetation, using an inverse form of the Extended R-value model. We test the method using modern pollen–vegetation data from small hollows in northern Michigan (6 taxa; 45 sites) and from northwestern Wisconsin (7 taxa; 43 sites) in the USA. Compared with pollen percentages, the one-tailed Fisher Exact test shows that the QAD method significantly improves the accuracy of the results for all taxa. The rank order of sites based on QAD is significantly correlated to the rank order of sites based on a survey of vegetation composition surrounding the hollow for each taxon (Spearman's rank-order correlation coefficients; p < 0.001). We apply QAD to 9000-yr pollen records from two forest hollows c. 6 km apart in northern Michigan. Among seven taxa compared, Pinus, a taxon with well-dispersed pollen and a high pollen producer, often displays discrepancies in the direction of difference between QAD and the percentage-based method, demonstrating that pollen percentages alone do not always reflect differences in vegetation composition accurately. When several similarly sized sites are available in the same vegetation zone, QAD can objectively rank-order sites for individual taxa in relation to, for example, differences in soils, topography, patterns of species invasion, natural and anthropogenic disturbances, and other factors. [ABSTRACT FROM AUTHOR]

Aeolian sand within lake sediment from Silver Lake, Michigan can be used as a proxy for the timing of high lake levels of Lake Michigan. We demonstrate that the sand record from Silver Lake plotted as percent weight is in-phase with the elevation curve of Lake Michigan since the mid-Holocene Nipissing Phase. Because fluctuations in Lake Michigan's lake level are recorded in beach ridges, and are a response to climate change, the aeolian sand record within Silver Lake is also a proxy for climate change. It appears that increases in dune activity and lake sand are controlled by similar climatic shifts that drive fluctuations in lake level of Lake Michigan. High lake levels destabilize coastal bluffs that drive dune sand instability, and along with greater wintertime storminess, increase niveo-aeolian transport of sand across lake ice. The sand is introduced into the lake each spring as the ice cover melts. [ABSTRACT FROM AUTHOR]

Recent disturbances have induced state shifts in many ecosystems. It has been proposed that human actions degrade system resilience, thus allowing state changes to occur more easily. Unless system dynamics in the absence of human influence are understood, however, we can not address two fundamental questions: are human actions producing dynamics that were previously absent in those systems? Or, are human actions simply replacing natural drivers of change in systems susceptible to state shifts? I use fossil pollen and charcoal to document Holocene vegetation and disturbance dynamics on a sand plain in Michigan to evaluate a recent anthropogenic state shift from forest to a barrens plant community. A 10 500-yr pollen record shows that a community similar to the modern barrens was present three times between 9000 and 5000 yr BP. These periods coincide with distinct peaks in the charcoal record, linking the barrens vegetation community and fire. The barrens vegetation community is thus a rare component of the Kingston Plain system, associated with both natural and anthropogenic disturbances. However, with no evidence of barrens vegetation during the last 5000 years, it appears that climate is also an important driver of vegetation change on the Kingston Plain. Ancient barrens occurred when annual precipitation was lower than current climate, and summer temperatures were warmer than current climate. Projections of decreased soil moisture and warmer temperatures in the Great Lakes region during the next century may thus imply decreased resilience of Great Lakes forests to future disturbances. [ABSTRACT FROM AUTHOR]

Previous palaeoecological studies have shown that there have been major range shifts in Tsuga (hemlock) during the middle and late Holocene that seem to indicate climatic cooling and an increase in available moisture. We examined lake-level changes and peatland growth at Glimmerglass Lake and peatland in the Sylvania Wilderness Area to determine if the mid-Holocene increase in moisture indicated by the pollen record also influenced lakes and wetlands. Sphagnum peat began accumulating at the site at 6240 cal. yr BP. Diatom-inferred depth reconstructions show that the lake level rose 2 m and the peatland expanded out of its basin at 5300 cal. yr BP. All of these changes occurred as the last remnant of the Laurentide Ice Sheet disappeared and orbital forcing reduced seasonality relative to the early Holocene. We conclude that these continental-scale factors caused increased moisture availability in the Upper Peninsula of Michigan, resulting in rising lakewater levels and peatland expansion. [ABSTRACT FROM AUTHOR]

A forested dunefield covers the Baraga Plains in western upper Michigan. The ages of five dunes, from across the dunefield, were determined by optically stimulated luminescence (OSL), employing the single-aliquot regenerative-dose (SAR) method on quartz grains. The results indicate that sand mobilization was restricted to the middle Holocene, at around 7 ka. Four environmental variables, rapid climatic oscillations, fire, strong winds and a dry climate are invoked as a combined cause of dune formation. This study provides further evidence for the wide extent of aeolian activity in the upper Midwest of North America during the middle Holocene. [ABSTRACT FROM AUTHOR]

We reconstructed the late-Holocene surface-moisture history and vegetation dynamics of a raised bog in Michigan using testate amoebae, peat humification, pollen, stomata and plant macrofossils. Our primary objective was to compare bog palaeohydrology with the water-level history of Lake Michigan, and to regional and local vegetation changes. Hydrologic histories inferred from testate amoebae and humification show similar trends, and correspond with records of past water-level variability in Lake Michigan. The bog clearly shows effective-moisture increases during the Algoma highstand (~3200-2300 cal. BP) and a later unnamed highstand (~1900-1300 cal. BP). Some higher-frequency fluctuations are also similar. The good correspondence indicates that bog hydrology and the water levels of Lake Michigan have been driven by changes in large-scale atmospheric circulation patterns for at least the past 3500 years. Climate during the Algoma highstand may have been spatiotemporally complex, with increasing moisture occurring several hundred years earlier in the northern portion of the western Great Lakes basin. Locally, the expansion and contraction of Picea populations on the bog surface was probably directly related to moisture conditions. The well-documented decline in Fagus populations in the central Great Lakes region after 1000 cal. BP is contemporaneous with a major shift towards drier conditions, suggesting that relative dryness caused the decline. [ABSTRACT FROM AUTHOR]

Eastern hemlock (Tsuga canadensis) expanded rapidly across Upper Peninsula Michigan from ∼6500 to 5500 cal. yr BP, followed by a sudden decline of hemlock pollen percentages that lasted over 1000 years. Hemlock declined throughout its range, apparently due to insect/pathogen outbreaks, which may have been affected by regional climatic shifts. Modern analogues were used to reconstruct submillennial climatic trends from fossil pollen sites spanning the western range limit of hemlock. Reconstructions from eight sites inside the range of hemlock at the time of the decline (5400 cal. yr BP) are compared with eight sites outside the range, which should be independent of variation in hemlock pollen percentages and the non-climatic effects of insects/pathogens. Expansion of hemlock before the decline occurred at a time of increasing precipitation and January temperature. Long-term precipitation averages stabilized at ∼5500 cal. yr BP and remained fairly constant throughout the period of low hemlock abundance. January temperature reconstructions decrease at the time of the hemlock decline to approximately modern values at sites inside the range, as well as at oak-dominated sites just outside the range. The lower-than-modern precipitation of the mid-Holocene may have reduced hemlock's tolerance for cold stress and winter desiccation and perhaps increased its susceptibility to insects or pathogens. January temperature reconstructions for pine-dominated sites outside the pre-decline hemlock range show little change, possibly due to the insensitivity of pine-dominated pollen assemblages to small climatic fluctuations. Hemlock began a new expansion across the region after 4000 cal. yr BP as July temperature decreased and precipitation increased. [ABSTRACT FROM AUTHOR]

Methods of interpreting pollen assemblages in sediment were examined using surface samples from 66 small forest hollows in Michigan and Wisconsin. All canopy trees in the surrounding 50 m were measured to provide detailed information about the source vegetation of each surface pollen assemblage. Basal area of trees in each forest sample was used to classify them into six stand types: hemlock-dominated, sugar maple/hemlock mixed, sugar maple-dominated, and ash-, oak-, and pine-dominated stands. Various statistical procedures were tested to learn which was most successful in sorting the pollen assemblages into appropriate vegetation groups. Two ordination techniques – detrended correspondence analysis (DCA) and canonical variate analysis (CVA) – give similar overall results, although CVA more successfully separated assemblages from hemlock stands from those of sugar maple-dominated stands. Squared chord distance ,0.05 also successfully identified samples from the same forest type. After stand dominants have been identified from pollen assemblages using multivariate methods, a further determination of stand composition is sometimes possible using ratios of pollen counts of individual taxa. Ratios can be calibrated by comparison with species abundances around surface samples. For instance, ratios of pine to hemlock pollen can indicate the abundance of pine within a stand dominated by hemlock, whereas pine pollen percentages alone are affected by variable abundance of other species. [ABSTRACT FROM AUTHOR]