Lake-effect snow

Records of century-scale climate variability in the Upper Midwest generally agree that moisture availability increased between 4000 and 3000 cal. yr BP (calendar years before present = 1950 CE), and that there were large, frequent droughts 1000-700 cal. yr BP followed by wetter/cooler conditions. Variability among regional sites, however, remains problematic. In this study we reconstruct climate on the Northwest Wisconsin Sand Plain (NWSP), USA, to identify potential climatic drivers of previously documented changes in vegetation and fire regimes. Oak pollen was replaced by pollen from xeric pine taxa at several sites on the NWSP ~1425 cal. yr BP, accompanied by a change to larger, less frequent charcoal peaks. Another major vegetation change occurred ~700 cal. yr BP, when pollen of the more mesic P. strobus L. (white pine) increased and charcoal influx decreased. We used a vegetationindependent lake-level record to determine whether long-term changes in moisture availability were associated with these ecosystem changes. Decreases in percent organic matter in shallow-water sediment cores from Cheney Lake indicate that the lake level decreased sharply ~1500 cal. yr BP, consistent with the interpretation that the changes in vegetation and fire regime were driven by a severe and previously undocumented drought. The lake level rose again, reaching approximately modern levels by 800-700 cal. yr BP, consistent with the hypothesis of cooler/wetter conditions in the Upper Midwest in the past ~700 years and with the expansion of mesic taxa on the NWSP 700 cal. yr BP.

A map of the vegetation of the Catskill Park, NY, was created using multi-temporal Landsat Thematic Mapper TM data and ancillary spatial data to support ecological studies in Catskill watersheds. The map emphasizes forest types defined by dominant tree species and depicts 24 vegetation classes. Mapping included a series of supervised classifications in a decision tree framework that allowed forest types to be distinguished using spectral characteristics and other environmental relationships (e.g., landscape position, elevation). Traditional contingency table analysis (based on limited ground sampling) suggests overall map accuracy ranging from 28% to 90%, depending on the level of aggregation of the original 24 map classes. Fuzzy accuracy assessment based on the same ground data suggests a 71% level of acceptable classification. The map indicates that maple-dominated forests are predominant in the Catskill region, but that beech and birch-dominated forests become more important at higher elevations. Oak-dominated forests are very important along the eastern side of the Catskills, and conifer-dominated forests are largely restricted to mountaintops and stream bottoms.

This study has investigated the spatio-temporal variability of November and March Lake Michigan snowfall for the period 1950-2013. Snowfall characteristics were assessed using time series analysis, geographic information systems, and visualization. Results indicate significant temporal decreases of November and March snowfall, peak inter-annual variability within the lake-effect belt, modest concurrent and lagged sensitivity to teleconnection indices, and strong dependence of snowfall on temperature. The decreased snowfall is in contrast to December-February snowfall and associated with a decreasing fraction of November and March precipitation days occurring as snow days, rather than changes in precipitation frequency. The decreasing fraction of precipitation days occurs as snowfall is consistent with synoptic-scale disturbances producing rain rather than snow and lake-effect rain falling in lieu of snow.

The substantial impact of Lake‐effect snow in the Laurentian Great Lakes has led to interest in the impact of climate change on snowfall in the region. A recent assessment of Lake Michigan snowfall revealed a marked decrease in November snowfall since the 1950s, associated with a warming‐induced reduction in the fraction of precipitation days occurring as snowfall. Herein, in order to identify the trend contribution from Lake‐effect snowfall, snow days from the November 1950–2012 study period are classified as primarily System or Lake‐effect, with additional options of Insignificant, Both, Remnant, and Unclear. The classification is based on the snowfall distribution and visual map inspection for synoptic‐scale forcing, and results are compared with an objective classification based on clustering of daily snowfall. The regional snowfall patterns of Lake‐effect and System snow days are markedly different, with Lake‐effect snow days exhibiting a clear Lake‐effect signature downwind of...

This study has investigated the spatio‐temporal variability of November and March Lake Michigan snowfall for the period 1950–2013. Snowfall characteristics were assessed using time series analysis, geographic information systems, and visualization. Results indicate significant temporal decreases of November and March snowfall, peak inter‐annual variability within the lake‐effect belt, modest concurrent and lagged sensitivity to teleconnection indices, and strong dependence of snowfall on temperature. The decreased snowfall is in contrast to December–February snowfall and associated with a decreasing fraction of November and March precipitation days occurring as snow days, rather than changes in precipitation frequency. The decreasing fraction of precipitation days occurs as snowfall is consistent with synoptic‐scale disturbances producing rain rather than snow and lake‐effect rain falling in lieu of snow.

The substantial impact of Lake-effect snow in the Laurentian Great Lakes has led to interest in the impact of climate change on snowfall in the region. A recent assessment of Lake Michigan snowfall revealed a marked decrease in November snowfall since the 1950s, associated with a warming-induced reduction in the fraction of precipitation days occurring as snowfall. Herein, in order to identify the trend contribution from Lake-effect snowfall, snow days from the November 1950-2012 study period are classified as primarily System or Lake-effect, with additional options of Insignificant, Both, Remnant, and Unclear. The classification is based on the snowfall distribution and visual map inspection for synoptic-scale forcing, and results are compared with an objective classification based on clustering of daily snowfall. The regional snowfall patterns of Lake-effect and System snow days are markedly different, with Lake-effect snow days exhibiting a clear Lake-effect signature downwind of Lake Michigan. The larger-scale environments also differ, with much colder conditions in the Great Lakes and higher sea-level pressure in the Great Plains on Lake-effect days. With snowfall projected onto the classifications, the decrease of snowfall east of the lake is attributable to both reductions in Lakeeffect and System snowfall, while System snowfall changes are dominant west of the lake. The trends are consistent with the sensitivity to regional temperature, as well as an increasing prevalence of rain reports (within other subregions) during snow days. Results based on the objective classification are largely congruent.

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Plants use chemical defenses to ward off herbivory. Phenolics, a carbon-based chemical defense, are present in all woody plants. There are many biotic and abiotic factors that can affect the levels of phenolics present in plant tissue. Two factors that can affect phenolic ...

In this study, the changes of long-term mean annual water levels of the Great Lakes in North America are investigated, including the potential impacts of the North Atlantic Oscillation (NAO). The levels of Lakes Superior, Michigan-Huron, St. Clair, Erie and Ontario and the hydro-climatic data, such as precipitation, evaporation and runoff, are evaluated together in view of the influence of NAO. The changes in the lake levels and hydro-climatic indicators are analysed. The annual hydro-climatic data, annual mean lake levels and the NAO indices are nondimensionalized through the nonparametric Mann-Kendall test. Then, the sequential Mann-Kendall test and paired t test are performed to gain insight into the trends of the time series and possible turning points. As a result of this study, the trend direction changes for all of the lakes are determined in the years 1965 and 1987, based on the records of the last 95 years. A similar tendency for evaporation, precipitation and runoff is found in 1982, 1935 and 1965. Regarding the effect of North Atlantic Oscillation on the changes of the lake levels, the same directional variations are found between the lake levels and the NAO FMA (February-March-April) indices. However, the directional variations between the lake levels and NAO JJA (June-July-August) indices are reversed. The absolute values of the correlation coefficients increase from west to east (from Lake Superior to Lake Ontario).

Earlier studies indicated that the general pattern of the Holocene climate in the northeastern United States changed from cool and dry (11.6e8.2 ka; 1 ka ¼ 1000 cal yr BP) to warm and wet (8.2e5.4 ka) to warm and dry (5.4e3 ka) to cool and wet (after 3 ka). A new w35-year resolution stable isotope record of endogenic calcite from a sediment core for Lake Grinnell in northern New Jersey provided a chance to examine the Holocene climate variations of the region in a finer detail. After the Younger Dryas cold climate reversal, the d 18 O fluctuated around a constant value of À7.4& until 5.8 ka, thereafter shifted to a steadily decreasing trend to the most recent value of À8.2&. Responding to this shift, the widely observed hemlock decline in the northeastern USA occurred about w350e500 (AE143.5) years later. Detrended d 18 O and d 13 C records show a clear covariance at 910-year periodicity. The amplitudes of centennial-scale d 18 O variations became much smaller after 4.7 ka. At the same time, the dominant frequency of these variations changed from 330 to 500 years. We suggest that a non-linear response of atmospheric circulation to the gradual decrease in insolation is responsible for the shift in the climate trend at 5.8 ka as indicated by the deceasing d 18 O values. A dominant frequency shift in solar forcing and the decreased seasonal contrast of insolation might have caused the change in climate variability at 4.7 ka through modulating ocean and atmosphere circulations.

Watersheds of the Catskill Mountains, New York have marked differences in nitrogen (N) dynamics among dominant tree species stands. Our objectives were to study how tree species vary in N uptake to better understand the basis for the observed variation in these forested watersheds. We conducted a 15 N tracer greenhouse study to determine NH + 4 and NO − 3 uptake of American beech (Fagus grandifolia Ehrh.), eastern hemlock (Tsuga Canadensis L.), red oak (Quercus rubra L.) and sugar maple (Acer saccharum Marsh.) seedlings. Seedlings and their native soil were collected in November 1997, over-wintered and allowed to break dormancy in spring 1998. Half of the seedlings of each tree species received 15 NH 4-NO 3 to examine NH + 4 uptake and the other half received NH 4-15 NO 3 to examine NO − 3 uptake. Plants were harvested 4 days following 15 N addition. Tree species varied in their preference for NH + 4 and NO − 3. Sugar maple and eastern hemlock seedlings took up more NH + 4 than NO − 3 per unit plant biomass, while beech was the only species to take up more NO − 3 than NH + 4. Red oak took up more NH + 4 than NO − 3 into roots, stems and leaves, but the difference between the two forms of N was not statistically significant. These results demonstrate that tree species of the Catskill Mountains vary in their capacity to take up NH + 4 and NO − 3. Coupled with stand-level studies of N dynamics, this variation can help explain some of the patterns of forested watershed N retention and loss in the Catskill Mountains shown in our field investigations (Templer, 2001; Templer et al., in press Ecosystems).

This study assesses the high-resolution, 0.11°(12 km), Canadian Regional Climate Model Version 5 (CRCM5), interactively coupled to the one-dimensional Freshwater Lake model (FLake), to predict wintertime precipitation along the Canadian snowbelts of Lake Superior and Lake Huron. CRCM5-FLake was compared against various datasets to evaluate the 20-year (1995-2014) SWE and wintertime precipitation, seven lake-induced precipitation events and lake effect snowfall (LES) predictor variables during the months of December and January. The findings of SWE along both snowbelts in December and January show MBD ≤− 10 mm and ≤− 30 mm, respectively. Similarly, precipitation results along both snowbelts in December and January show MBD ≤− 5 mm and ≤− 10 mm, respectively. The negative biases in simulated SWE and precipitation, predominantly along the snowbelts, suggest that the model may un-realistically represent lake effect processes. Comparison of lakeinduced precipitation events also indicates that the model mostly under-predicts the daily accumulated precipitation associated with each event but tends to accurately capture the timing and the general location of the squalls along the snowbelts, though not for highly localised snow bands. Furthermore, lake-wide results of LES predictor variables indicate that the model overestimates lake surface temperature (LST) for both lakes during December and January and underestimates ice cover concentrations for both lakes in December. The resultant biases could be attributed to limitations within the coupled RCM because the quality of reproducing lake-induced precipitation in this region is highly dependent on the performance of FLake.

Earlier studies indicated that the general pattern of the Holocene climate in the northeastern United States changed from cool and dry (11.6e8.2 ka; 1 ka ¼ 1000 cal yr BP) to warm and wet (8.2e5.4 ka) to warm and dry (5.4e3 ka) to cool and wet (after 3 ka). A new w35-year resolution stable isotope record of endogenic calcite from a sediment core for Lake Grinnell in northern New Jersey provided a chance to examine the Holocene climate variations of the region in a finer detail. After the Younger Dryas cold climate reversal, the d 18 O fluctuated around a constant value of À7.4& until 5.8 ka, thereafter shifted to a steadily decreasing trend to the most recent value of À8.2&. Responding to this shift, the widely observed hemlock decline in the northeastern USA occurred about w350e500 (AE143.5) years later. Detrended d 18 O and d 13 C records show a clear covariance at 910-year periodicity. The amplitudes of centennial-scale d 18 O variations became much smaller after 4.7 ka. At the same time, the dominant frequency of these variations changed from 330 to 500 years. We suggest that a non-linear response of atmospheric circulation to the gradual decrease in insolation is responsible for the shift in the climate trend at 5.8 ka as indicated by the deceasing d 18 O values. A dominant frequency shift in solar forcing and the decreased seasonal contrast of insolation might have caused the change in climate variability at 4.7 ka through modulating ocean and atmosphere circulations.

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Tree-ring characteristics in four species were examined to address whether co-occurring mature trees of different successional status respond differently to drought, and whether saplings of these species have a greater response to drought than mature trees. We examined saplings and mature trees of paper birch, yellow birch, red maple and sugar maple, which varied in successional status (shadetolerance) and co-occurred at Harvard Forest, Petersham, Mass., USA. Three drought events in 1964-1966, 1981 and 1995 were identified using climate data. For mature trees, there was no significant interspecific difference in relative changes in ring-width index (RWI) during the 1964-1966 and 1995 drought events. However, the interspecific difference was significant in the 1981 drought event. Response function analysis for mature trees showed that the radial growth of sugar maple was mainly controlled by spring and summer precipitation, red maple by spring and summer precipitation and temperature, yellow birch by winter and summer precipitation, and spring and summer temperature, and paper birch by spring and summer precipitation and spring temperature. Saplings of sugar maple and yellow birch, but not red maple and paper birch, showed significant positive correlations between RWI and annual total precipitation. In the 1995 drought event, saplings and J.-S. He ( )

Abstract: 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 2m and the peatland expanded out of its basin at 5300 cal. yr BP. All of these changes occurred as the last remnant of the A Laurentide Ice Sheet disappeared and orbital forcing reduced seasonality relative to the early Holocene. HOLOCENE We conclude that these continental-scale factors caused increased moisture availability in the Upper RESEARCH Peninsula of Michigan, resulting in rising lakewa...

Interpolated General Land Office fire occurrence notes were used to determine the spatial extent of pre-European settlement fires for 26 counties in northern lower Michigan using ordinary kriging with probability output. Best fit of a surface was achieved using a spherical model with a lag distance of 860 meters, an angular tolerance of 45 degrees, and consideration of anistropy. The interpolated data were associated with Land Type Associations to determine fire rotation and occurrence intervals for pre-European and modern day fires. The results show that modern day fire suppression efforts have curbed the size and frequency of fires.

Tallgrass prairie extends eastward in a narrowing peninsula across northern Illinois into adjacent Indiana. This vegetation reflects a longitudinal gradient of decreasing drought severity, and local vegetation patterned by interactions between fire and landscape firebreaks. Understanding how these processes shaped vegetation provides an ecological context for restoration management as well as a baseline for projecting effects of ongoing climate change on vegetation. We analyzed vegetation across this region using maps, notes and witness tree data from the Government Land Office Public Land Survey (PLS) of Lake, Porter, and LaPorte counties, Indiana, which was conducted in 1829-1835. We examined how topography, landscape firebreaks and proximity to Lake Michigan interact to mediate the transition from grassland to forest across this region and compared it to vegetation pattern in adjacent Illinois.

Recent research has indicated that snowfall in portions of the Great Lakes region subject 2 to lake-effect snow has undergone a trend reversal, with snowfall declining in recent decades. 3 This study examines the seasonal variability and trends specifically in synoptically classified 4 lake-effect snow across the eastern Great Lakes region, and investigates the mechanisms 5 responsible for observed changes. Using a synoptic climatological approach, days are identified 6 where the synoptic-scale conditions are conducive to lake-effect snowfall and the associated 7 snowfall is analyzed. Seven synoptic types over the November-March snowfall season are 8 identified with characteristics of lake-effect conditions. Snowfall from these seven lake-effect 9 synoptic types represents between 45-53% of the seasonal snowfall total along the eastern shores of Lakes Erie and Ontario with snowfall totals being highest during January. Lake-effect snowfall exhibits a 60-year increasing trend downwind of Lakes Erie and Ontario however through examination over shorter 30-year periods, a change in the trend of snowfall is occurring around 1980. While a true trend reversal is not detected, lake-effect snowfall significantly increases from 1950-1979 before exhibiting no significant trend from 1980-2009. The inter-annual variability of seasonal lake-effect snowfall is highly related to the frequency of lake-effect synoptic types where an increase (decrease) in synoptic type occurrence leads to enhanced (diminished) lake-effect snowfall totals. Depending on the period examined, long-term changes of lake-effect synoptic types' frequency and snowfall rates represent between 89-95% of the observed changes in lake-effect snow.

The influence of topography on soil development and classification and the interrelationships among soils and forest composition are poorly understood in Upper Michigan. Topographic plant-soil interrelations are important because plants are increasingly being used to indicate environmental conditions and potential forest productivity. Our objectives, therefore, were to study the effect of topography on soil development and classification, soil nutrient status, and forest composition over a classical toposequence. An 8-ha portion of a steep watershed was topographically mapped, and soils and plants were sampled and analyzed using established techniques. In addition, the distributions of selected ground flora species were mapped over the landscape. Results indicate a strong correspondence between the distribution of plants, soil development and classification, and soil nutrient status. Soils ranged from a Lithic Dystrochrept in the upper slope through a Typic Dystrochrept, Entic Haplorthod, Dystric Eutrochrept, and finally a Terric Borosaprist in the toeslope; this is considerable variability within a horizontal distance of 300 m. Soil nutrient status varied along with soil development and was strongly related to topographic position. For these coarse-textured soils, increased content of macronutrients is closely associated with increased content of soil organic matter. For example, in the surface soil of a colluvial soil profile we found that soil nitrogen was roughly 10 times greater than in the subsoil; this large difference in soil nitrogen was closely associated with large amounts of soil organic matter in the surface soil. Within the highly variable landscape, forest composition followed variability in soil development, soil organic matter, and soil nutrient status. Groups of ground flora species were closely associated with these soil and topographic variations.

Field and modeling studies were used to quantify potential successional pathways among fine−scale ecological classification units within two geomorphic regions of north−central Minnesota. Soil and overstory data were collected on plots stratified across low−relief ground moraines and undulating sand dunes. Each geomorphic feature was sampled across gradients of topography or soil texture. Overstory conditions were sampled using five variable−radius point samples per plot; soil samples were analyzed for carbon and nitrogen content. Climatic, forest composition, and soil data were used to parameterize the sample plots for use with LINKAGES, a forest growth model that simulates changes in composition and soil characteristics over time. Forest composition and soil properties varied within and among geomorphic features. LINKAGES simulations were using "bare ground" and the current overstory as starting conditions. Northern hardwoods or pines dominated the late−successional communities of morainal and dune landforms, respectively. The morainal landforms were dominated by yellow birch and sugar maple; yellow birch reached its maximum abundance in intermediate landscape positions. On the dune sites, pine was most abundant in drier landscape positions, with white spruce increasing in abundance with increasing soil moisture and N content. The differences in measured soil properties and predicted late−successional composition indicate that ecological land units incorporate some of the key variables that govern forest composition and structure. They further show the value of ecological classification and modeling for developing forest management strategies that incorporate the spatial and temporal dynamics of forest ecosystems.

We studied the variability of elemental carbon (EC) over 3 years (2009–11) in the winter snowpack of Storglaciären, Sweden. The goal of this study was to relate the seasonal variation in EC to specific snow accumulation events in order to improve understanding of how different atmospheric circulation patterns control the deposition of EC. Specifically, we related meteorological parameters (e.g. wind direction, precipitation) to snow physical properties, EC content, stable-isotope δ18O ratios and anion concentrations in the snowpack. The distribution of EC in the snowpack varied between years. Low EC contents corresponded to a predominance of weather systems originating in the northwest, i.e. North Atlantic. Analysis of single layers within the snowpacks showed that snow layers enriched in heavy isotopes coincided predominantly with low EC contents but high chloride and sulfate concentration. Based on this isotopic and geochemical evidence, snow deposited during these events had a st...

Eight new radiocarbon ages, all determined by accelerator mass spectrometry, on modern (pre-bomb) mollusks have been added to similar data provided from three samples in the Lake Michigan and Huron basins. These data confirm the existence of a substantial hard water effect correction ranging from about 250 years to 500 years in these lakes. They also show that the magnitude of these corrections form a spatially coherent pattern that can be related to the pattern of outcrop of Paleozoic (radioactively inert) carbonates that surround the basins and the pattern of circulation within the basins.

The post-glacial history of the Great Lakes has involved several changes in lake levels throughout the latest Pleistocene and Holocene, resulting from the changing position of the retreating Laurentide ice sheet, outlet 14 incision and isostatic rebound. The final lowering of lake levels occurred at approximately 7600 C yr BP, after 14 which lake levels began to rise again to the Nipissing highstand at approximately 4700 C yr BP. During this time of rising lake levels, black bands of iron sulfide were being formed in the sediments of all five of the Great Lakes. These bands signify suboxic to anoxic conditions, at least within the sediments and possibly at the sediment-water interface, during the middle Holocene warm interval. During this interval, the climate was warmer and drier than present, possibly resulting in the occasional absence of seasonal turnover in the lakes. We examined a series of piston cores from northern Lakes Michigan and Huron and found that the black bands are correlatable among cores taken from within the same basin. The observation that the banding can be correlated suggests a basin-wide cause, near-bottom or sub-bottom anoxia in the northern Michigan and northern Huron sediments during the mid-Holocene warm period. The sedimentary and geochemical processes in the Great Lakes during the middle Holocene warm interval are good indicators of possible future scenarios for the lakes as a result of global warming, st as 21-century temperatures are predicted to reach similar levels due to increased concentrations of greenhouse gases.

Strandplains of shore-parallel beach ridges bordering the Great Lakes are valuable for reconstructing histories of climate-related lake-level fluctuations. However, imprecise radiocarbon dates of ridge formation have frustrated development of dependable chronologies from which information about variation in the frequency of ridge formation and inferred climate fluctuations can be obtained. The resolution and precision of radiocarbon chronologies can be improved with AMS 14C dates of roots and rhizomes of plant species associated with the formation and growth of the sand-dune caps of breach ridges. These dates reliably estimate the timing of shore progradation when the base of the previously established beach ridge becomes inundated by the water table. An AMS radiocarbon chronology of beach-ridge formation in northern Lake Michigan shows that information about variation in the frequency of ridge formation is important for paleoclimatic interpretation.

Recent research indicates a shift in the timing and range of the seasonal lake-level cycle of Lake Michigan-Huron since 1860. The largest changes occur during the winter-spring transition. The objectives of this study are to (1) quantitatively assess seasonal variations in precipitation, runoff, and evaporation, (2) evaluate long-term trends in seasonal water supply to Lake Michigan-Huron, and (3) understand how variations in net basin supply at a seasonal timescale contributed to development of extreme lake levels recorded in Lake Michigan-Huron between 1920 and 1995. Total water input, which is the sum of overlake precipitation and runoff, has increased during autumn and winter seasons at rates of 14.6 and 9.3 mm/decade, respectively. A rise in autumn total input reflects an increase in overlake precipitation (6.3 mm/decade) and runoff (8.3 mm/decade). The increase in winter input was driven by runoff (7.5 mm/decade) at the expense of decreasing spring runoff (7.8 mm/decade). This shift to an increasing dominance of winter runoff at the apparent expense of spring runoff is most pronounced post 1965 and suggests a hydrologic response to a warming climate. Extreme lake levels principally reflect either anomalies occurring in one or more seasons or a directional shift over many years, but often with seasonal bias. Seasonal changes in overlake precipitation, runoff, and overlake evaporation lead to the formation and persistence of extreme lake levels. The effects of seasonal water supply anomalies depend on the coincidence to long-term decadal-scale variation in lake level.

Plant species distributions are generally thought to be chiefly under environmental control, although they may be affected by disturbance events or dispersion properties of the species. The relative importance of these different factors is not easy to evaluate because they often share common spatial patterns, such that an inextricable network of relationships occurs between plant distributions, environmental conditions, disturbancc events and endogenous factors such as propagule dispersion. In this paper we propose a method for untangling the common spatial component from the relationship between environmental conditions and the distribution of tree species. Using partial Mantel tests and path analysis, we test models of relationships between these data sets. Results show that in our study area, spatial patterns of species associated with hydric conditions remain largely correlated with environmental conditions. However, mesic sites show more complex forest covers, in which a significant spatial component persists when environmental variation is statistically controlled for. This remaining spatial variability suggests that other factors possessing spatial structure partly explain species distributions.

Radiocarbon dates Arboreal assemblage A mid-Holocene buried organic layer 10 to 60 cm thick is present along the Lake Michigan shoreline in southeastern Wisconsin. Named the Southport forest bed for its location in Kenosha County, the unit has yielded abundant wood specimens, including large logs, branches, twigs, stumps, and root material. Roots of truncated in situ oak (Quercus) and elm (Ulmus) trees extend into the underlying till of the late Wisconsin Oak Creek Formation. Nearshore lacustrine sand above the organic layer contains abundant driftwood, overlain by 2 to 3 m of cross-bedded dune sand. More than 50 wood samples have been identified; the assemblage is that of a mixed hardwood forest dominated by oak (Quercus) and hickory (Carya), which account for about 60% of the assemblage. Although the site has been mostly concealed for many years, it was beautifully exposed in the 1960s and early 1970s, when it was initially studied by Phil Sander, who discovered the site and documented it with field notes and numerous photographs. Here we report several new (unpublished) radiocarbon dates and details of the stratigraphy and arboreal flora. Radiocarbon dates suggest that the Southport forest probably lived for 900-1000 14 C years, possibly longer, reaching its climax at about 5300 B.P. The site is of particular importance because of its proximity to the Nipissing shoreline and provides a significant point on the Nipissing transgression time curve. The unusually large number of identified specimens affords an accurate evaluation of the warm and relatively dry climate that characterized the southern Great Lakes region during the mid-Holocene.

Background/Question/Methods Climate-driven treeline movement and forest encroachment into alpine meadows has been documented at many locations, and may influence surface albedo, snow melt patterns, carbon stores, and species habitat. Dynamic vegetation model scenarios suggest large-scale changes in response to climate, but treeline movement and meadow encroachment are modified by a variety of environmental factors at local spatial scales. The relative importance and interactive effects of these fine-scale environmental controls on tree establishment is poorly understood. The objective of this study was to quantify patterns of tree establishment in a subalpine forest/meadow parkland in relation to topographic variability of late season snow persistence, distance from potential seed sources, climate variation, and a debris flow disturbance event. The study occurred in the subalpine parkland of the Mount Jefferson Wilderness, Oregon, U.S.A. Field plots were established and geo-referenc...

Tree invasions have been documented throughout Northern Hemisphere high elevation meadows, as well as globally in many grass and forb-dominated ecosystems. Tree invasions are often associated with large-scale changes in climate or disturbance regimes, but are fundamentally driven by regeneration processes influenced by interactions between climatic, topographic, and biotic factors at multiple spatial scales. The purpose of this research was to quantify spatiotemporal patterns of meadow invasion; and how climate, larger landforms, topography, and overstory trees have interactively influenced tree invasion. We combined airborne Light Detection and Ranging (LiDAR) characterizations of landforms, topography, and overstory vegetation with historical climate, field measurements of snow depth, tree abundance, and tree ages to reconstruct spatial and temporal patterns of tree invasion over five decades in a subalpine meadow complex in the Oregon Cascade Range, USA. Proportion of meadow occupied by trees increased from 8 % in 1950 to 35 % in 2007. Larger landforms, topography, and tree canopies interactively mediated regional climatic controls of tree invasion by modifying depth and persistence of snow pack, while tree canopies also influenced seed source availability. Landscape context played an important role mediating snow depth and tree invasion; on glacial landforms tree invasion was negatively associated with spring snowfall, but on debris flows tree invasion was not associated with snow fall. The importance of snow, uncertain climate change impacts on snow, and mediation of snow by interacting and context dependent factors in complex mountain terrain poses substantial hurdles for understanding how these ecotones may respond to future climate conditions. Keywords Topography Á Oregon Cascades Á Snow Á Subalpine meadow Á Tree invasion Á LiDAR Electronic supplementary material The online version of this article (

Eight new radiocarbon ages, all determined by accelerator mass spectrometry, on modern (pre-bomb) mollusks have been added to similar data provided from three samples in the Lake Michigan and Huron basins. These data confirm the existence of a substantial hard water effect correction ranging from about 250 years to 500 years in these lakes. They also show that the magnitude of these corrections form a spatially coherent pattern that can be related to the pattern of outcrop of Paleozoic (radioactively inert) carbonates that surround the basins and the pattern of circulation within the basins.

1. BACKGROUND Lake-effect snow storms are important components of the climate of the U.S. Upper Midwest, resulting in some locations near the Great Lakes receiving double the average wintertime snowfall of locations several 10s of km inland (e.g., Scott and Huff 1996, Braham and Dungey 1984). The economic and societal impacts of these storms depend critically on the timing of their occurrence relative to activities of the 35 million people living in the Great Lakes basin (GLERL 2002). This study utilizes hourly precipitation data collected in the snow belt regions of Lakes Superior and Michigan to determine the typical diurnal behavior of lake-effect precipitation occurrence. While much is known about the causes of lake-effect precipitation, little research has been reported in the scientific literature on the diurnal evolution of such storms. Passarelli and Braham (1981), for example, speculated that snowfall rates in Lake Michigan lake-effect storms would be expected to be greater...

In landscapes dominated by late-successional plant communities, early-successional species may lead a tenuous existence, persisting only as fugitives or relying on refuges in marginal habitats to provide a persistent seed source. The objective of this study was to relate fine-scale distributions of earlysuccessional tree species in hemlock-hardwood forests of northern Wisconsin, USA to potential landscape persistence strategies. A special emphasis was placed on eastern white pine (Pinus strobus), a restoration priority in the region. Witness tree data from nineteenth century US Public Land Survey records (encompassing 40,610 km 2 and 106,790 trees) were used with modern environmental data to relate species distributions to habitat characteristics. Early-successional tree species had strong positive associations with marginal habitats such as inclusions of sandy soil and margins of lakes, wetlands, and rivers. Marginal habitats occupied *44 % of the landscape, which may help account for the abundance of early-successional species in our study area relative to other hemlock-hardwood forests. Populations of early-successional species in marginal habitats could also have provided important seed sources for the upland mesic landscape matrix, as [70 % of the landscape was within 200 m of these habitats. The degree to which early-successional species were limited to marginal habitats largely followed predictions based on species lifehistory characteristics, except that white pine was more common than expected in upland mesic habitats. These findings illustrate the potential importance of landscape heterogeneity for persistence of earlysuccessional species in late-successional forest landscapes and provide baseline information on habitat associations and landscape dynamics that will be useful in restoration efforts.

Plant species distributions are generally thought to be chiefly under environmental control, although they may be affected by disturbance events or dispersion properties of the species. The relative importance of these different factors is not easy to evaluate because they often share common spatial patterns, such that an inextricable network of relationships occurs between plant distributions, environmental conditions, disturbancc events and endogenous factors such as propagule dispersion. In this paper we propose a method for untangling the common spatial component from the relationship between environmental conditions and the distribution of tree species. Using partial Mantel tests and path analysis, we test models of relationships between these data sets. Results show that in our study area, spatial patterns of species associated with hydric conditions remain largely correlated with environmental conditions. However, mesic sites show more complex forest covers, in which a significant spatial component persists when environmental variation is statistically controlled for. This remaining spatial variability suggests that other factors possessing spatial structure partly explain species distributions.

We report on pollen, plant macrofossils, and associated lithostratigraphy of a sediment core extracted from the base of Silver Lake, a kettle in northern Lower Michigan, USA, which reveal a complex deglacial scenario for ice block melting and lake formation, and subsequent plant colonization. Complementary multivariate statistical and squared-chord distance analyses of the pollen data support these interpretations. The basal radiocarbon age from the core (17,540 cal yr BP) is rejected as being anomalously old, based on biostratigraphic anomalies in the core and the date’s incongruity with respect to the accepted regional deglaciation chronology. We reason that this erroneous age estimate resulted from the redeposition of Middle Wisconsin- age fossils by the ice sheet, mixed with the remains of plants that existed as the kettle lake formed at ca. 10,940 cal yr BP by ice block ablation. Thereafter, the kettle lake became a reliable repository of Holocene- age fossils, documenting a mature boreal forest that existed until 10,640 cal yr BP, followed by a pine-dominated mixed forest, an early variant of the mixed conifer- hardwood forest that persists to the present day. Our study demonstrates that researchers investigating kettle lakes, a common depositional archive for plant fossils in deglaciated landscapes, should exercise caution in interpreting the basal (late Pleistocene/early Holocene- age) part of lake sediment cores.

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.

Variations in the oxygen-isotope composition of paleo-water bodies in the Lake Superior Basin provide information about the timing and pathways of glacial meltwater inflow into and within the Lake Superior Basin. Here, the oxygen-isotope compositions of Lake Superior have been determined using ostracodes from four sediment cores from across the

Variations of temperature, precipitation and snow depths during the last 50-100 years, up to 1984, are investigated for mean cold season values (November to April) for 110 locations in the Former Soviet Union (FSU). Following quality checking, statistical analyses are performed on the data. There is cold season warming after 1953 or after 1966 over almost all the FSU apart from eastern Siberia, but temperature variations significantly affect the mean snow depths only in some sensitive areas of the Caucasus, central Asia, and the western European territory. Mean snow depth time series reveal contrasting local trends. Higher precipitation and snow depths are measured in eastern European Russia from 50° to 65 °N and in some areas of western Siberia at the beginning of the twentieth century. Otherwise, precipitation variations have little influence on the mean cold season snow depths over much of the FSU after 1930. Some predominant periodicities are detected for mean cold season temperature, but not for precipitation and snow depths.