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Imaging of charcoal particles extracted from lake sediments provides an important way to understand past fire regimes. Imaging of large numbers of particles can be time consuming. In this note we explore the effects of subsampling and extrapolation of area on estimates of sum charcoal area, using resampling of real and simulated data sets and propose a protocol in which all particles are counted with only the first 100 encountered being imaged. Extrapolated estimates of sum total area of charcoal for 40 real samples were nearly identical to actual values, and error introduced due to subsampling was low (Coefficient of variation

This research has been supported by the Leverhulme Trust (grant no. RC-2018-023), the European Research Council (grant no. 694481), the German Research Foundation (grant no. FE-1096/6-1), the Swiss Government Excellence Postdoctoral Scholarships (grant no. FIRECO 2016.0310), the National Science Centre of Poland (grant no. 2015/17/B/ST10/01656), the SCIEX Scholarship Fund (grant no. PSPB-013/2010), and the Estonian Research Council (grant no. MOBJD313)., Sedimentary charcoal records are widely used to reconstruct regional changes in fire regimes through time in the geological past. Existing global compilations are not geographically comprehensive and do not provide consistent metadata for all sites. Furthermore, the age models provided for these records are not harmonised and many are based on older calibrations of the radiocarbon ages. These issues limit the use of existing compilations for research into past fire regimes. Here, we present an expanded database of charcoal records, accompanied by new age models based on recalibration of radiocarbon ages using IntCal20 and Bayesian age-modelling software. We document the structure and contents of the database, the construction of the age models, and the quality control measures applied. We also record the expansion of geographical coverage relative to previous charcoal compilations and the expansion of metadata that can be used to inform analyses. This first version of the Reading Palaeofire Database contains 1676 records (entities) from 1480 sites worldwide. The database (RPDv1b – Harrison et al., 2021) is available at https://doi.org/10.17864/1947.000345., Leverhulme Trust RC-2018-023, European Research Council (ERC) European Commission 694481, German Research Foundation (DFG) FE-1096/6-1, Swiss Government Excellence Postdoctoral Scholarships FIRECO 2016.0310, SCIEX Scholarship Fund PSPB-013/2010, Estonian Research Council MOBJD313

Can we cluster bacterial sediment communities based on lake size, depth, and trophic status? Or, are bacterial microbial communities an emergent property of their geography, integrating regional physical and climatic conditions? Lakes in Minnesota are uniquely situated to address these questions because of their wide geographic range and variability in size and basin land-use. In this study, we selected twenty lakes with varying morphological and chemical properties across four ecological regions of Minnesota. Our objectives were to (i)) evaluate the diversity and spatial variation of the bacterial community at the sediment-water interface and (ii) determine how lake location and watershed land-use impact aqueous chemistry and influence community structure. Our data indicate that sediment communities from similar depth intervals are more likely to cluster by ecological region rather than any individual lake properties (e.g., trophic status, TP concentration, lake depth). However, composition is tied to a given lake, wherein samples from the same core were more alike than samples at similar depths across lakes. Our results illustrate the diversity within lake sediment microbial communities and provide insight into relationships between taxonomy, physicochemical, and geographic properties of north temperate lakes.

Sedimentary charcoal records are widely used to reconstruct regional changes in fire regimes through time in the geological past. Existing global compilations are not geographically comprehensive and do not provide consistent metadata for all sites. Furthermore, the age models provided for these records are not harmonised and many are based on older calibrations of the radiocarbon ages. These issues limit the use of existing compilations for research into past fire regimes. Here, we present an expanded database of charcoal records, accompanied by new age models based on recalibration of radiocarbon ages using INTCAL2020 and Bayesian age-modelling software. We document the structure and contents of the database, the construction of the age models, and the quality control measures applied. We also record the expansion of geographical coverage relative to previous charcoal compilations and the expansion of metadata that can be used to inform analyses. This first version of the Reading Palaeofire Database contains 1681 records (entities) from 1477 sites worldwide. The database (DOI: 10.17864/1947.319) is available from https://researchdata.reading.ac.uk/id/eprint/319.

Small peat-forming ecosystems in arctic landscapes may play a significant role in the regional biogeochemistry of high-latitude systems, yet they are understudied compared to arctic uplands and other major peat-forming regions of the North. We present a new data set of 25 radiocarbon-dated permafrost peat cores sampled around eight low arctic lake sites in northern Manitoba (Canada) to examine the timing of peat initiation and controls on peat accumulation throughout the Holocene. We used macrofossils and charcoal to characterize changes in the plant community and fire, and we explored potential impacts of these local factors, as well as regional climatic change, on rates of C accumulation and C stocks. Peat initiation was variable across and within sites, suggesting the influence of local topography, but 56% of the cores initiated after 3000 B.P. Most cores initiated and remained as drier bog hummock communities, with few vegetation transitions in this landscape. C accumulation was relatively slow and did not appear to be correlated with Holocene-scale climatic variability, but C stocks in this landscape were substantial (mean = 45.4 kg C m−2), potentially accounting for 13.2 Pg C in the Taiga Shield ecozone. To the extent that small peat-forming systems are underrepresented in peatland mapping, soil organic carbon (SOC) stocks may be underestimated in arctic regions. Mean fire severity appeared to be negatively correlated with C accumulation rates. Initiation and accumulation of soil C may respond to both regional and local factors, and substantial lowland soil C stocks have the potential for biogeochemical impacts on adjacent aquatic ecosystems.

The late-Quaternary paleoenvironmental history of the western Hudson Bay region of Subarctic Canada is poorly constrained. Here, we present a regional overview of the post-glacial history of eight lakes which span the forest-tundra biome in northern Manitoba. We show that during the penultimate drainage phase of Lake Agassiz the lake water had an estimated pH of ∼6.0, with abundant quillwort (Isoetes spp.) along the lakeshore and littoral zone and some floating green algae (Botryococcus spp. and Pediastrum sp.). Based on multiple sediment proxies, modern lake ontogeny in the region commenced at ∼7500 cal yrs BP. Pioneering diatom communities were shaped by the turbid, higher alkalinity lake waters which were influenced by base cation weathering of the surrounding till following Lake Agassiz drainage. By ∼7000 cal yrs BP, soil development and Picea spp. establish and the lakes began a slow trajectory of acidification over the remaining Holocene epoch. The natural acidification of the lakes in this region is slow, on the order of several millennia for one pH unit. Each of the study lakes exhibit relatively stable aquatic communities during the Holocene Thermal Maximum, suggesting this period is a poor analogue for modern climatic changes. During the Neoglacial, the beginning of the post-Little Ice Age period represents the most significant climatic event to impact the lakes of N. Manitoba. In the context of regional lake histories, the rate of diatom floristic change in the last 200–300 years is unprecedented, with the exception of post-glacial lake ontogeny in some of the lakes. For nearly the entire history of the lakes in this region, there is a strong linkage between landscape development and the aquatic ecosystems; however this relationship appears to become decoupled or less strong in the post-LIA period. Significant 20th century changes in the aquatic ecosystem cannot be explained wholly by changes in the terrestrial ecosystem, suggesting that future changes to the lakes of N. Manitoba will be strongly influenced by direct climatic effects to the lakes.

The direction of pan-Arctic ecosystem shifts in response to climate warming is relatively well understood; however, landscape-level linkages among terrestrial, wetland, and lake ecosystems significantly influence the dynamics of each, making it difficult to generalize about lake responses to warming across the Arctic, and at times difficult to interpret paleoclimate records from lake sediments. To investigate differences in lake responses to recent climate change at the boreal-tundra ecotone, we conducted a 2-year survey of diatom assemblages from lakes with varying catchment characteristics in northern Manitoba, Canada. We investigated whether catchment geomorphology and landscape, including slope and vegetative cover, result in characteristic water chemistries and hence diatom assemblage signatures, which could then be used in paleolimnological studies to infer past changes in the catchment. Forty-four lakes were sampled for water chemistry and catchment vegetation was characterized using Landsat Imagery. Lake catchments were generally small (median 702 ha) and dominated by peat (Sphagnum) with or without lowland forest (Picea-Larix), or open tundra, with different amounts of exposed rock/till, upland forest/woodlands, and burn recovery area. Lakes were generally nutrient-poor, with lower nutrient and DOC concentrations in tundra-dominated catchments, and higher nutrients and DOC in catchments with greater forest cover. A diatom-based transfer function for pH (R2 = 0.72, \( {\text{R}}_{\text{boot}}^{2} \) = 0.54) was developed and compared with diatom assemblage turnover and sediment geochemistry in cores from eight lakes to reconstruct limnologic conditions over the past ~200 years. Most cores showed similar increases in biogenic silica and carbon burial, beginning around AD 1880 in the tundra lakes and about 1920 in the more forested catchments, likely in response to regional warming. In contrast to lakes in other Arctic regions, our lakes showed only minor pH changes in recent decades. The shift, however, was more pronounced in higher-latitude lakes with less forest cover, suggesting small-scale watershed influence on lake response to climate, even on short time scales. Diatom assemblages did not follow previously published models of climate-linked community change seen in circum-Arctic and sub-Arctic lakes. Translating local changes detected in the paleolimnological record to the regional level requires an understanding of how different catchment properties mediate the response of lakes, and their diatom assemblages, to climate change.

Fossil pollen assemblages provide information about vegetation dynamics at time scales ranging from centuries to millennia. Pollen-vegetation models and process-based models of dispersal typically assume stable relationships between source vegetation and corresponding pollen in surface sediments, as well as stable parameterizations of dispersal and productivity. These assumptions, however, are largely unevaluated. This paper reports a test of the stability of pollen-vegetation relationships using vegetation and pollen data from the Midwestern region of the United States, during a period of large changes in land use and vegetation driven by Euro-American settlement. We compared a dataset of pollen records for the early settlement-era with three other datasets of pollen and forest composition for two time periods: before Euro-American settlement, and the late 20th century. Results from generalized linear models for thirteen genera indicate that pollen-vegetation relationships significantly differ (p Fagus, Betula, Tsuga, Quercus, Pinus , and Picea . The estimated pollen source radius for the 8 km gridded vegetation data and associated pollen data is 25–85 km, consistent with prior studies using similar methods and spatial resolutions. Hence, the rapid changes in land cover associated with the Anthropocene affect the accuracy of ecological predictions for both the future and the past. In the Anthropocene, paleoecology should move beyond the assumption that pollen-vegetation relationships are stable over time. Multi-temporal calibration datasets are increasingly possible and enable paleoecologists to better understand the complex processes governing pollen-vegetation relationships through space and time.

Many ecosystem processes that influence Earth system feedbacks – vegetation growth, water and nutrient cycling, disturbance regimes – are strongly influenced by multidecadal- to millennial-scale climate variations that cannot be directly observed. Paleoclimate records provide information about these variations, forming the basis of our understanding and modeling of them. Fossil pollen records are abundant in the NE US, but cannot simultaneously provide information about paleoclimate and past vegetation in a modeling context because this leads to circular logic. If pollen data are used to constrain past vegetation changes, then the remaining paleoclimate archives in the northeastern US (NE US) are quite limited. Nonetheless, a growing number of diverse reconstructions have been developed but have not yet been examined together. Here we conduct a systematic review, assessment, and comparison of paleotemperature and paleohydrological proxies from the NE US for the last 3000 years. Regional temperature reconstructions (primarily summer) show a long-term cooling trend (1000 BCE–1700 CE) consistent with hemispheric-scale reconstructions, while hydroclimate data show gradually wetter conditions through the present day. Multiple proxies suggest that a prolonged, widespread drought occurred between 550 and 750 CE. Dry conditions are also evident during the Medieval Climate Anomaly, which was warmer and drier than the Little Ice Age and drier than today. There is some evidence for an acceleration of the longer-term wetting trend in the NE US during the past century; coupled with an abrupt shift from decreasing to increasing temperatures in the past century, these changes could have wide-ranging implications for species distributions, ecosystem dynamics, and extreme weather events. More work is needed to gather paleoclimate data in the NE US to make inter-proxy comparisons and to improve estimates of uncertainty in reconstructions.

To better understand aquatic–terrestrial linkages in the sub-Arctic, and specifically the relative importance of landscape position versus land cover, we surveyed lakes, soils, land cover, and lake/basin characteristics in a 14 000 km 2 region of acidic forest–tundra landscape near northern Manitoba, Canada (59.56°N, 97.72°W) in 2009. We analyzed 39 different biological, chemical, and physical variables for lakes and soils. We used a remote-sensing– based classification to determine that the landscape was 21% water, 46% peat-forming lowland, and 24.9% open tundra, and we assigned lake order to all lakes based on the order of the outlet stream for each lake. Lakes were oligotrophic to mesotrophic (median total phosphorus: TP = 11.8 µg L −1 ), N-limited (median dissolved inorganic nitrogen: TP = 1.6), acidic (median pH 5.7), and had moderate amounts of dissolved organic carbon (median DOC = 5.2 mg L −1 ). We identified 2 principle groups of variables represented by DOC and conductivity/ cations, respectively, that captured major axes of lake variation. DOC, 2 measures of DOC quality (a 250 /a 365 [a proxy for molecular weight and aromaticity] and specific ultraviolet absorbance), and Fe and were significantly correlated with percent cover of lowland forest, but conductivity/cations were not correlated with variation in land cover. Soils were generally acidic (pH 2.7–4.4) and nutrient-poor, and wetland soils contained more carbon and higher concentrations of calcium, magnesium, and other cations than upland open tundra. Landscape position of lakes (measured as lake order) did not capture systematic differences in land cover or lake biogeochemistry. Our results highlight the importance of lowland export of DOC to lakes and further suggest the need for additional regional studies of aquatic–terrestrial connections in Arctic and sub-Arctic landscapes.

Prairie remnants often serve as targets for restoration despite little knowledge of how they have altered over the past +150 years due to changes in the biotic and abiotic environment including suppression of fire, increased inputs of exogenous nitrogen, extirpation of bison, climate warming associated with the mid-19th century termination of the ‘Little Ice Age’ and 20th century anthropogenic forcing, and invasive introduced species. In this paper I assess the utility of prairie remnants as targets for restoration and management of prairies by examining compositional change of native plants in western Minnesota over the past~170 years. This analysis is made possible by the recent discovery of an unpublished manuscript by Charles A. Geyer as well as the relocation of 100s of herbarium specimens collected by Geyer. Geyer was a botanist with the 1838–1839 expeditions of J.N. Nicollet in western Minnesota and the eastern Dakotas. Comparison of the Geyer data with modern lists taken from the Natural Heritage Information System Releve Database indicates that there has been surprisingly little change in composition of native species. This suggests that, in the aggregate, remnants retain much of their original flora and provide a good target for prairie restoration and management.

Intensified warming in the Arctic and Subarctic is resulting in a wide range of changes in the extent, productivity, and composition of aquatic and terrestrial ecosystems. Analysis of remote sensing imagery has documented regional changes in the number and area of ponds and lakes as well as expanding cover of shrubs and small trees in uplands. To better understand long-term changes across the edaphic gradient, we compared the number and area of water bodies and dry barrens (>100 m2) between 1956 (aerial photographs) and 2008–2011 (high-resolution satellite images) for eight ∼25 km2 sites near Nejanilini Lake, Manitoba (59.559°N, 97.715°W). In the modern landscape, the number of water bodies and barrens were similar (1162 versus 1297, respectively), but water bodies were larger (mean 3.1 × 104 versus 681 m2, respectively) and represented 17% of surface area compared with 0.4% for barrens. Over the past 60 years, total surface area of water did not change significantly (16.7%–17.1%) despite a ∼30% decrease in numbers of small (2) water bodies. However, the number and area of barrens decreased (55% and 67%, respectively) across all size classes. These changes are consistent with Arctic greening in response to increasing temperature and precipitation. Loss of small water bodies suggests that wet tundra areas may be drying, which, if true, may have important implications for carbon balance. Our observations may be the result of changes in winter conditions in combination with low permafrost ice content in the region, in part explaining regional variations in responses to climate change.

Fossil pollen assemblages are widely used to reconstruct past vegetation community composition at time scales ranging from centuries to millennia. These reconstructions often are based on the observed relationships between the proportions of plant taxa in the source vegetation and the proportions of the corresponding pollen types in pollen assemblages collected from surface sediments. Pollen-vegetation models rely upon parameters whose values typically are assumed to be stable through time, but this assumption is largely unevaluated, due in part to the rarity of comprehensive forest data, particularly for earlier time periods. Here we present a new dataset of early settlement-era pollen records for the upper Midwest of North America and combine it with three other pollen and forest composition datasets to assess the stability of the relationship between relative pollen composition and relative abundances of tree genera for two time periods: immediately prior to Euro-American settlement, and the late 20th Century. Over this time interval, Euro-American settlement resulted in widespread forest clearance for agriculture and logging, producing major changes to forest composition and structure and the pollen assemblages produced by these forests. These major changes provide an opportunity to test the constancy of the relationship between pollen and forest vegetation during a period of large vegetation change. Pollen-vegetation relationships are modeled, using a Generalized Linear Model, for thirteen upper Midwestern tree genera. We find that estimates of pollen source radius for the gridded mesoscale data are 25-85 kilometers, consistent with prior studies. Pollen-vegetation relationships are significantly altered for several genera: Fagus, Betula, Tsuga, Quercus, Pinus, and Picea (p < 0.05). The use of contemporary pollen-vegetation relationships to model settlement era community composition significantly under-predicts the presence of Fagus, Betula, Tsuga, Quercus and Picea at all tree densities. Pinus is over-predicted at low relative proportions (

The low-Arctic region of western Hudson Bay in interior Canada is one of the most poorly described areas of North America in terms of Holocene climate history. Here, we present new data from a well-dated lake sediment core from northern Manitoba, Canada. We assemble one of the richest multi-proxy datasets to date for a low-Arctic lake and characterize terrestrial and lake processes and exchanges between them. These proxies include fossil pollen and diatom assemblages, charcoal, magnetic properties (susceptibility and remanance), mineral grain size, bulk density, organic-matter content, elemental geochemistry, sediment cation (K+, Mg2+, Ca2+, Fe2+/Fe3+) and macronutrient (P, N, C) contents, biogenic-silica content, basal peat dates (wetland initiation), and stable isotopes (δ13C, δ15N). The sediment proxies record both broad- and fine-scale (millennial and sub-millennial) climate change. We find indirect evidence for a cool and dry post-glacial period from 9,000 to 6,500 cal yr BP, a warm and moist mid-Holocene period from 6,500 to 2,500 cal yr BP, and a cool and moist late-Holocene period from 2,500 cal yr BP to present. High-resolution geochemical data suggests 300- to 500-year-long dry periods at ~6,500–6,100, 5,300–5,000, 3,300–2,800, and 400–0 cal yr BP. These results suggest that terrestrial and aquatic ecosystem dynamics in the western Hudson Bay region are sensitive to past climate change and are likely to respond to future changes in temperature and precipitation.

Few studies have assessed the relative importance of landscape, land use history, climate, and regional heterogeneity on lake ecosystem processes, despite the likelihood that interactions among these factors must be important for controlling lake dynamics. We used 14 sediment measures from 20 lake cores in a climatically sensitive region of the prairie-forest border in southern Minnesota to (1) assess relationships between modern lake productivity (Carlson’s Trophic State Index [TSI]), modern land-use, catchment, and lake morphometry, and (2) contrast regional responses to climatic transitions from the ‘Medieval Climatic Anomaly’(1000–1350) to the ‘Little Ice Age’ (1350–1800) to ‘Modern’(~1980–1996 AD). TSI was significantly positively correlated with modern sedimentation rate, and accumulation rates of organic matter (OM), biogenic silica (BSi), and total phosphorus (TP). TSI was not significantly correlated with “modern” land cover, catchment, or lake morphometry characteristics, but total organic N(N) was negatively correlated with percent cultivation in the catchment area and negatively correlated with δ15N of bulk organic matter in sediment. Regional, among-lake heterogeneity was high over the past 1,000 years, but Little Ice Age (LIA) cooling appeared to result in an approximately 20% decline in OM, BSi and TP accumulation, while warming and cultural eutrophication of the past 150 years corresponded to a 200–400% increase in accumulation rates as well as an 80% increase in carbonates and a small but significant 10% drop in C/N ratios consistent with greater in-lake productivity. Our results indicate that climate does have regional effects on lake ecosystems but that among-lake variability is high, reflecting the importance of local factors and suggesting a need for (1) more explicit definition of what ‘regional’ means, (2) a focus on degree as well as direction of change, and (3) estimating accumulation rates based on multiple lakes and multiple cores within lakes.

The Valley of the Great Lakes in western Mongolia is a unique ecosystem comprising a wide variety of terrestrial and aquatic habitats. Relatively little is known about the long-term climate history of the area or the relative impact of changes in climate and land use in the region. Tree-ring records have established an understanding of regional temperature fluctuations; however, the few records of moisture balance from Mongolia offer little insight into potential changes in the precipitation-to-evaporation balance in the region with warming temperatures. Modern and paleoclimatic records from across arid Central Asia show substantial temporal and spatial variability in the correlation between temperature and moisture availability and climate models are inconsistent in predictions for future changes with continued warming. In order to refine the understanding of moisture balance in the region, we developed six diatom-inferred records of lake salinity between 200 and 2000 years in length, demonstrating a negative correlation between temperature and effective moisture (warm-dry and cool-wet conditions) over most of this time period. In addition to climate warming, Mongolian grasslands have seen substantial human impacts over the last several decades. Recent core records show evidence of eutrophication and must be interpreted carefully to understand the individual and combined impacts of climate and land use on sedimentary records and the potential for human—environment interactions to confound inferences about climate changes.

Climate warming and major land-use changes have profoundly affected the Mongolian landscape in the past several decades. Previous studies have recognized the impacts of a warmer, more arid climate and Mongolia’s 1991 transition from a command to a market economy on terrestrial ecosystems, including impaired sustainability of subsistence herding and threats to wild animals. In this study, we examined the combined effects of changing climate and herding practices on lake eutrophication in Western Mongolia. We sampled 65 lakes for modern nutrients and found the majority of lakes were eutrophic to hyper-eutrophic. Sediment cores were taken from five of the lakes to compare current lake status to paleolimnologial measures of lake eutrophication over the past 100–2000 years, including changes in diatom assemblages, diatom-inferred total phosphorus, biogenic silica, organic matter, and sediment accumulation rates. Variance partitioning analysis showed that recent shifts in diatom assemblages were related to changes in both climate and herding practices. The results presented here demonstrate a need for further study and long-term monitoring of water quality in Mongolia to understand the complicated interactions of climate and land use on aquatic resources and to preserve water quality in this remote and ecologically important region.

Fires are common in grassland regions of the world, and the frequency and severity of fire is linked to climate-driven changes in fuel loads. Western Mongolia is dominated by grasslands but the fire history of this region is largely unknown. We reconstructed modern fire (48 lakes) and historical fires (9 lakes) using sediment charcoal. Modern fuel loads were estimated using a combination of clipped plots, satellite-based estimates of annual aboveground net primary productivity (NPP) and NPP modeled from annual temperature and precipitation. Loss-on-ignition and environmental magnetics of lake sediments were analyzed as proxies for climate. We found little evidence for modern or historical fire in the landscape, as charcoal was absent from the surface sediments of 34 of 48 lakes. Charcoal influxes were uniformly low, averaging from 0.002 to 0.028 mm2/cm2 per yr, over the past 1200 years at nine lakes, and the past 6000—5000 years at two of the lakes with longer sediment records. In the modern landscape, livestock grazing has eliminated most of the fuels necessary to carry a fire, as measured fuel loads (27.3±4.9 g/m2) were only ~20% of aboveground annual NPP estimated using MODIS Imagery or modeled from climate data. The historical absence of fire may indicate a longer history of intensive grazing than sometimes assumed, and cultural prohibitions against burning may also play a role. Regional summary indicated a >50% decrease in charcoal influxes since AD 1600 at most sites which may be related to lower temperatures or greater aridity during the `Little Ice Age'. Alternatively this decrease in charcoal influxes may reflect increases in livestock numbers or increased local concentrations because of restrictions on the movement of animals coincident with the establishment of Manchu rule in the late seventeenth century.

It is widely accepted, based on data from the last few decades and on model simulations, that anthropogenic climate change will cause increased fire activity. However, less attention has been paid to the relationship between abrupt climate changes and heightened fire activity in the paleorecord. We use 35 charcoal and pollen records to assess how fire regimes in North America changed during the last glacial–interglacial transition (15 to 10 ka), a time of large and rapid climate changes. We also test the hypothesis that a comet impact initiated continental-scale wildfires at 12.9 ka; the data do not support this idea, nor are continent-wide fires indicated at any time during deglaciation. There are, however, clear links between large climate changes and fire activity. Biomass burning gradually increased from the glacial period to the beginning of the Younger Dryas. Although there are changes in biomass burning during the Younger Dryas, there is no systematic trend. There is a further increase in biomass burning after the Younger Dryas. Intervals of rapid climate change at 13.9, 13.2, and 11.7 ka are marked by large increases in fire activity. The timing of changes in fire is not coincident with changes in human population density or the timing of the extinction of the megafauna. Although these factors could have contributed to fire-regime changes at individual sites or at specific times, the charcoal data indicate an important role for climate, and particularly rapid climate change, in determining broad-scale levels of fire activity.

The mechanical behavior of frozen twigs may be important in understanding twig breakage during winter owing to wind or snow loading. While below 0 °C temperatures are known to increase the stiffness of wood, few studies have examined the effects of cold temperatures on the biomechanical properties of twigs of both coniferous and deciduous woody plants. In this study, we compared the effects of –11 to –14 °C temperatures on the Young’s modulus (E) of fine twigs of 14 species of woody plants. Twigs were 13%–304% stiffer when frozen than when thawed at room temperature (21 °C), with conifers showing the greatest percentage increase in stiffness. In general, more flexible twigs (when thawed) showed the greatest percentage increase in E when frozen. Greater stiffening of more flexible twigs may in part be the result of higher water contents but may also reflect differences in the relative importance of supercooling and extracellular freezing, with greater frozen stiffness associated with freezing. Our work suggests a direct link between cell physiology and whole organ biomechanics, and highlights reasons for differences in susceptibility to breakage due to wind or snow loading.

The mid-Holocene (ca. 8000–4000 cal yr BP) was a time of marked aridity throughout much of Minnesota, and the changes due to mid-Holocene aridity are seen as an analog for future responses to global warming. In this study, we compare the transition into (ca. 9000–7000 yr ago) and out of (ca. 5000–2500 yr ago) the mid-Holocene (MH) period at Kimble Pond and Sharkey Lake, located along the prairie forest ecotone in south-central Minnesota, using high resolution (∼ 5–36 yr) sampling of pollen, charcoal, sediment magnetic and loss-on-ignition properties. Changes in vegetation were asymmetrical with increasing aridity being marked by a pronounced shift from woodland/forest-dominated landscape to a more open mix of grassland and woodland/savanna. In contrast, at the end of the MH, grassland remained an important component of the landscape despite increasing effective moisture, and high charcoal influxes (median 2.7–4.0 vs. 0.6–1.7 mm2cm− 2yr− 1at start of MH) suggest the role of fire in limiting woodland expansion. Asymmetric vegetation responses, variation among and within proxies, and the near-absence of fire today suggest caution in using changes associated with mid-Holocene aridity at the prairie forest boundary as an analog for future responses to global warming.

Sediment magnetic properties of a short core from Sharkey Lake, MN, record the effects of Euroamerican settlement and climate change over the last 150 yr. The onset of European-style farming led to increased erosion, reflected in high values of concentration-dependent parameters such as magnetic susceptibility (ĸ), Isothermal Remanent Magnetization (IRM), and Anhysteretic Remanent Magnetization (ARM). These high values are only partially due to increased supply of terrigenous material to the lake, and recent sediment contains an additional component of authigenic fine (single-domain) magnetite, most likely magnetosomes from magnetotactic bacteria. High organic productivity in the lake during the 1920s to 1940s drought increased this authigenic component resulting in highly magnetic fine-grained sediment. A comparison with older Holocene sediment from the same lake shows that, over time, most of the fine magnetic signal is lost after deposition, leading to decreases in magnetization and a bimodal grain size distribution of ultrafine, superparamagnetic grains and coarser multidomain particles, evident from measurements of ARM/IRM ratios, hysteresis measurements, and low-temperature analyses. The effects of dissolution and the superposition of climate and land-use signals complicate the use of recent sediments as modern analogs for sediment magnetic analyses.

The Big Woods region of Minnesota is on the prairie-forest border and is a much studied model for the interaction of climate, fire and vegetation. The purpose of this study was (a) to document the extent and timing of changes in vegetation and fire over the past 2000 years and (b) to examine the link between charcoal influx and vegetation during the eighteenth and early nineteenth centuries. To reconstruct changes in vegetation, fire and climate, sieve charcoal (120 gm), pollen, environmental magnetism and loss-on-ignition were used. Charcoal deposition at prairie lakes was 3-4 times greater than for woodland or forest lakes, consistent with current understanding of the fire dependence of prairies, but charcoal influxes to forests were considerably higher than reported elsewhere, suggesting that ground fires were more common than assumed. Regionally, charcoal deposition at a subset of 17 lakes began to change at cal. AD 1100, dating to the period of the ‘Mediaeval Climate Anomaly’ (MCA; cal. AD 900-1250) but preceding the ‘Little Ice Age’ (LIA; cal. AD 1250). For seven of 14 lakes the MCA was characterized by a significant increase in charcoal deposition. This was followed by a decrease with the onset of the LIA, which for most sites continued into the nineteenth century. Relative timing of changes in charcoal and pollen and the other proxies differed from site to site, suggesting no one single response to climatic change. This variation most likely derives from local differences in fuels and topography, as compounded by strong positive feedbacks between fuels and fire.

The relationship of fire and climate in explaining the origin and maintenance of the grasslands of the northern Great Plains has long been of interest. I examined the hypothesis that burning was more frequent during wet years using charcoal to reconstruct fire histories near Coldwater Lake in southcentral ND, and Rice Lake in northcentral ND and published records of ostracod Mg/Ca ratios as a proxy for climate at these same two sites. Over the past 10 000 years, charcoal influx rates for Coldwater (0.04–5.68 mm 2 cm −2 year −1 ) greatly exceeded influx rates for Rice ( 2 cm −2 year −1 ). Both sites showed strong, significant periodicities of ∼1500–1800, ∼800–900 and ∼130–140 years but charcoal maxima and minima were only similar at ∼4200 cal years BP. Charcoal influx during the past 2000 years also revealed strong periodicities of between 310–400 and 140 years at both sites. Comparison of smoothed charcoal influxes and ostracod Mg/Ca ratios suggests that both proxies are responding to changes in climate. When smoothed with a 320–400-year window, increases in charcoal influx typically preceded rises in ostracod Mg/Ca ratio by ∼50–100 years, and further smoothing (140-year window) of the residuals suggested a variable relationship between Mg/Ca ratios and charcoal influx. These results do not clearly support or reject the fuel limitation hypothesis, and this lack of clear support may result from (a) climate-driven shifts in the mix of C 3 and C 4 grasses resulting in maximum productivity at intermediate moisture levels, (b) possibly different responses of charcoal and Mg/Ca ratios to summer vs. winter precipitation, or (c) ground-water driven lags in the response of Mg/Ca ratios to shifts in climate. Comparison of the results from this study with other studies suggests that links between fire and fuels at finer time scales (0–100 years) may be broadly constrained by longer term (∼500–2000 years) patterns of climate in the northern Great Plains.

Establishing management targets for lake nutrient inputs remains a major challenge to limnologists and resource managers concerned with the cultural eutrophication of lakes. In this study we used multiple sediment cores to reconstruct pre-Euroamerican (1650–1850) phosphorus (P) loading to Lake Volney, a hypereutrophic lake in Le Sueur County, Minnesota, and compared changes in P inputs to inferred changes in lake productivity based on biogenic silica (bSi). Euroamerican changes in land-use, as abstracted from census and tax records, were also compared with sedimentary proxies for soil erosion – loss-on-ignition and environmental magnetism. Whole-basin P accumulation in Lake Volney ranged from 0.31 – 0.39 g m−2 yr−1 prior to the arrival of Euroamerican agriculture in the 1850s. There-after P accumulation rose nearly three-fold largely due to increased fluxes of organic and non-apatite inorganic P. P inputs were highest (3.9 g m−2 yr−1) when organic, apatite and non-apatite P all peaked. Modern P a...

Summary 1 Treeline ecotones, such as the prairie‐forest boundary, represent climatically sensitive regions where the relative abundance of vegetation types is controlled by complex interactions between climate and local factors. Responses of vegetation and fire to climate change may be tightly linked as a result of strong feedbacks among fuel production, vegetation structure and fire frequency/severity, but the importance of these feedbacks for controlling the stability of this ecotone is unclear. 2 In this study, we examined the prairie‐forest ecotone in south-central Minnesota using two lake sediment cores to reconstruct independent records of climate, vegetation and fire over the past 12 500 years. Using pollen, charcoal, sediment magnetic analyses and LOI properties, we investigated whether fires were controlled directly by climate or indirectly by fuel production. 3 Sediment magnetic and LOI data suggest four broad climatic periods occurring c . 11 350‐8250 BP (cool/humid), c . 8250‐4250 BP (warm/dry), c . 4250‐2450 BP (warm/ humid), and c . 2450‐0 BP (cool/humid), indicating that, since the mid-Holocene, climate has shifted towards wetter conditions favouring greater in-lake production and fuel production on the landscape. 4 The area surrounding both lakes was characterized by boreal forest c . 12 500‐ 10 000 BP, changing to an Ulmus-Ostrya forest c . 10 000‐9000 BP, changing to a community dominated by prairie ( Poaceae-Ambrosia-Artemisia ) and deciduous forest taxa c . 8000‐4250 BP, and finally shifting to a Quercus -dominated woodland/savanna beginning c . 4250‐3000 BP. 5 Charcoal influx increased from an average of 0.11‐0.62 mm 2 cm − 2 year − 1 during the early Holocene forest period ( c . 11 350‐8250 BP) to 1.71‐3.36 mm 2 cm − 2 year − 1 during the period of prairie expansion ( c . 8250‐4250 BP) and again increased to 4.18‐4.90 mm 2 cm − 2 year − 1 at the start of the woodland/savanna period ( c . 4250 BP). 6 As a result of the influence of climate on community composition and fuel productivity, changes in fire severity may be the result and not the cause of shifts in vegetation.

We propose a model that explains variations in magnetic parameters of lake sediments as a record of Holocene climate change. Our model is based on records from four lakes and incorporates the effects of erosion, dust deposition, and the authigenesis and diagenesis of the magnetic component of the sediment. Once checked against high resolution multi proxy climate records, which are currently being established for some of our study sites, it will allow us to use magnetic proxies to establish high-resolution climate reconstructions on a regional scale. Our model utilizes a combination of concentration-dependent parameters (magnetic susceptibility, IRM) and grainsize-dependent parameters (ARM/IRM, hysteresis parameters). Magnetic mineralogy is characterized by a combination of low-temperature measurements and S-ratios, and our magnetic measurements are complemented by XRD, LOI and smearslide analyses. During periods of forest growth within the watershed, deposition of terrigenous material is low and the sediment magnetic properties are characterized by low concentrations of mainly authigenic minerals (low values of IRM, high ratios of ARM/IRM). During the early to mid-Holocene dry period, deposition of terrigenous material increased due to intensified dust deposition and the erosion of lake margins caused by lowered water levels. Concentration of magnetic minerals increases (high IRM, χ) and so does the grain-size of the magnetic fraction (low ARM/IRM). During the late Holocene, sediment magnetic properties depend on the varied position of the site with respect to the prairie–forest ecotone.

Study of microscopic charcoal from lake sediments has led to a greater understanding of past veg etation, climate and fire ecology. We investigated the potential of charcoal morphology as an indicator of vegetation type. Grasses, leaves and wood were burned under controlled conditions in the laboratory, and we used a dissecting scope, video camera, and image-capture software to image-sieved (125-μm screen) micro-scopic charcoal. Charcoal from grasses was significantly longer (562 μm) and had a greater length:width ratio (3.62) than charcoal derived from leaves (380 μm; 1.91) or wood (348 μm; 2.13). Length:width ratios of mixtures of grass and leaf charcoal were intermediate (50:50 mixture; 2.36) between ratios for grass or leaf charcoal alone, and charcoal yield (on a weight basis) declined as a function of combustion temperature. While a number of issues may complicate the application of these results to the field, the results do suggest that length:width ratios can be used as an indicator of vegetation type.

The influence of productivity gradients on plant succession, richness, and community composition is the focus of much interest. Earthen mounds and other soil disturbances are a conspicuous feature of many temperate and semi-tem- perate grasslands. The revegetation of artificial earthen mounds located at different positions along a topographic-productivity gradient was studied in a North American northern mixed- grass prairie over a five year period. Relative stem density increased the most on mounds in low prairie (81% of off- mound density in 1992), followed by mounds in middle (64%) and high prairie (38%). Species richness was significantly different between prairie types by the fifth year of the experi- ment, but there was not a strong correlation between species richness and stem density within prairie-types. Composition on mounds reflected species composition in the surrounding off-mound areas and only three species were sampled regu- larly in all three prairie types. Results of this experiment suggest that models and experiments attempting to relate productivity to species richness and composition should in- clude a spatial component.

The importance of concurrent disturbances to the maintenance of plant species diversity in grasslands has been emphasized by several authors, but there are few studies comparing different disturbances at the same site. In this study the size, abundance, vegetation, and environment of thatching ant mounds, earthen mammal mounds, bison wallows, and openings in dry marshes are compared in a northern mixed prairie in north central South Dakota. Patch size and abundance varied considerably between patch types. Ant and earthen mammal mounds were the smallest but most abundant patch types. Bison wallows and dry marshes were larger and occurred less frequently than ant and earthen mounds. Different patch types occupied a similar amount of area. Ordination of patch vegetation revealed marked between-type compositional differences; ant and earthen mounds were dominated by vegetatively reproducing perennials, while annuals occurred more abundantly in bison wallows and dry marshes. Many species were restricted to either wallows or marshes. Levels of nitrate, phosphorus, soil organic matter, and other environmental factors varied significantly between patch types. Different patch types supported different groups of species, and patch vegetation was a product of mechanism of patch creation, patch environment, and location. Key words: ant mounds, disturbance, earthen mounds, northern mixed prairie, patches, potholes, wallows.

Patches of bare soil are thought to be important to the diversity and structure of North American grasslands. In 1987, 45 7.5-dm diameter artificial earthen mounds were built in low-, mid-, and high-prairie types to experimentally study the effects of mound location on patterns of mound revegetation. Stem densities and species abundance data were recorded every other week during the summers of 1987 and 1988. Few stems were recorded in 1987, and stem density more than quadrupled in 1988, but less than 1 % of all stems were seedlings. Grass stem densities were significantly higher on low-prairie mounds than mid- or high-prairie mounds and were concentrated in the outer perimeter of mounds. Forb stem densities did not vary significantly between prairie types or on-mound position. Mound species composition closely resembled surrounding vegetation, reflecting between and within prairie-type compositional variation. Key words: colonization, disturbance, earthen mounds, northern mixed prairie, patch, revegetation.

The fire history of the northern Great Plains has been largely reconstructed from the historical record. To clarify this history, segments of 1-m cores from four lakes in North and South Dakota and northeastern Montana were examined for charcoal fragments. The cores spanned a time interval of 172 to 380 yr. Samples integrating 5-10 yr of deposition were taken from 2-cm sections along each core and sieved. Charcoal concentrations ranged from 6-14,799 fragments/gram of sediment, and mean charcoal abundance was negatively correlated with longitude. Results suggest periods of increased fire from A.D. 1700-1740 and A.D. 1850-1900. These peaks were broadly synchronous across the region and ranged in duration from 20-40 yr. Postsettlement patterns of charcoal deposition were highly variable but generally much lower than presettlement intervals.

Numerous prairie gradient studies have documented a significant correlation between soil moisture and compositional variation. The Wisconsin prairie gradient studies of J. T. Curtis and the Wisconsin Plant Ecology Laboratory (PEL) are among these and are widely cited. Data for 247 of the original Wisconsin PEL prairies were reordinated with Detrended Correspondence Analysis (DECORANA) ordination. The first axis of the ordination was highly correlated with the original Wisconsin prairie continuum. Sand prairies and wet-mesic prairies from extreme southeastern Wisconsin separated on the second axis. Measures of soil moisture were the highest correlate of the first axis. Latitude and longitude, pH, calcium, phosphorus, potassium, nitrate and ammonium nitrogen were also significant correlates of first or second axes. Discussions of pattern in prairie vegetation should look beyond simple correlations with soil moisture, and careful experimental and descriptive studies of prairie vegetation are still needed.