Pollen-climate Transfer Functions Research Articles

A novel approach for quantitatively distinguishing between anthropogenic and natural effects on paleovegetation.

How to distinguish and quantify past human impacts on vegetation is a significant challenge in paleoecology. Here, we propose a novel method, the error inflection point-discriminant technique. It finds out the inflection points (IPs) of the regression errors of pollen-climate transfer functions using modern pollen spectra from vegetation with different values of the Human Influence Index (HII), which represent the HII threshold values of native/secondary and secondary/artificial vegetation systems. Our results show that the HII value at the native/secondary vegetation IPs is approximately 22 and globally uniform, whereas it varies regionally for the secondary/artificial vegetation IPs. In a case study of the Liangzhu archaeological site in the lower Yangtze River, discriminant functions for pollen spectra from three vegetation types and pollen-climate transfer functions of the native vegetation were established to reconstruct paleovegetation and paleoclimate over the past 6,600 years. Our study demonstrates this method's feasibility for quantitatively distinguishing human impacts on paleovegetation and assessing quantitative paleoclimate reconstructions using pollen data.

Controls of precipitation and vegetation variability on the NE Tibetan Plateau during the late Pliocene warmth (~3.5–3.0 Ma)

To better understand precipitation variability in a warmer-than-present world with elevated atmospheric CO2 concentrations, this study presents high-resolution palynological and element geochemical analyses of a late Pliocene drill core (3.5–3.0 Ma) from the today hyperarid Qaidam Basin on the north-eastern Tibetan Plateau. Quantitative rainfall estimates based on modern pollen-climate transfer functions indicate a 10-fold higher annual rainfall during the mid-Piacenzian Warm Period (mPWP; 3.264–3.025 Ma) than today, with values of 300–400 mm/a. Throughout the late Pliocene, the Qaidam Basin was covered with a temperate semi-desert shrubland. Varying percentages of the local lakeshore vegetation and long-distance arboreal pollen, as well as fluctuations in carbonate-silicate geochemistry, indicate a highly variable rainfall. Our study suggests that precipitation in the northern Qaidam Basin was primarily controlled by an East Asian Monsoon system (EAMS) that was located further north than today. Spectral analysis of the Artemisia/Chenopodiaceae (A/C) pollen ratio indicates an orbitally controlled cyclicity with a strong link between moisture availability and insolation forcing. A decline in precipitation before the end of the mPWP at ca. 3.15 Ma is likely to be linked to cooling in the North Atlantic and initial Northern Hemisphere ice-sheet build-up. Our study identifies NH insolation and ice-sheet advances as major controls of the late Pliocene variability and strength of the EAMS in semi-arid NW-China.

Evidence for climate instability during the Younger Dryas interval in the Hengduan Mountains, Yunnan, southwestern China

The Younger Dryas (YD) is the most recent abrupt climatic event recognized during the transition from the last glaciation to the Holocene. Growing evidence from the middle and high latitudes of Europe and North America shows that the climate within the YD event was highly unstable. Until recently, however, climate instability during this event has received little attention elsewhere in the world. Here we present evidence of a climatically unstable YD based on pollen-climate transfer function and REDFIT spectral analysis of pollen data from Haligu in the Hengduan Mountains of southwestern China, a low latitude temperate biodiversity hotspot. Our data show that the climate fluctuated significantly during the YD, with three warm peaks recognized at 11.5, 11.7 and 12.3 cal kyr BP. In particular, we estimate a gradual rise in mean annual temperature by 3.7 °C between the end of the YD and the early Holocene (11.2–10.9 cal kyr BP). In response to this, the subalpine conifer forest dominated by Picea and Abies may have migrated about 610 m upwards. We also detected that the climate fluctuations during the YD-Holocene transition show significant centennial periodicities of 76 yr, 84 yr, 91 yr, 95 yr, and 638 yr. The former four cycles are possibly related to the Gleissberg (around 80 yr) solar cycle, which was also detected in the Hongyuan peat, the PC-6 core of the East China Sea and the GISP2 ice core. These new findings enhance our understanding of millennial-scale abrupt climate events which may be of great significance in predicting the impact of future climate change on mountainous ecosystems in southwestern China. The warming at the end of the YD may provide a potential analog for global warming in the near future, during which the upward migration of vegetation zones will likely lead to the loss of habitat for threatened species. We therefore should take action to restore habitats and provide protected areas for species at risk.

Modern pollen and its relationship with vegetation and climate in the Mu Us Desert and surrounding area, northern China: Implications of palaeoclimatic and palaeocological reconstruction

This study establishes the spatial and quantitative relationship between modern pollen, vegetation and climate based on 84 surface pollen samples from a nature reserve and ecological restoration area in the Mu Us Desert and surrounding areas, including seven natural vegetation types. Pollen spectra, discriminant analysis and spatial distribution patterns of the main pollen types are used to explore the relationship between pollen assemblages and vegetation types. Results show that each vegetation type can be characterized by a distinct modern pollen assemblage as follows: (1) coniferous forest: Pinus Diploxylon-type and Picea; (2) deciduous forest: Pinus Diploxylon-type, Quercus, Artemisia and Poaceae; (3) deciduous shrub: Pinus Diploxylon-type, Artemisia and Poaceae; (4) grass meadow: Poaceae and Cyperaceae; (5) grass steppe: Poaceae and Artemisia; (6) desert steppe: Chenopodiaceae, Artemisia and Poaceae; and (7) desert: Chenopodiaceae, Nitraria and Artemisia. The higher percentage and lower concentration of Pinus pollen in the Mu Us Desert can be explained by effective long-distance transport from the mountain forest, while Picea has a limited pollen dispersal ability via wind-borne dispersal. Poaceae pollen is well represented and has a higher percentage in several different types of ecological conditions. Modern pollen–climate relationships are assessed using redundancy analysis and weighted averaging partial least squares regression. Significant relationships exist between pollen assemblages and mean annual temperature (R2 = 0.67; RMSEP 1.7 °C; max. bias 2.5 °C), July precipitation (R2 = 0.58; RMSEP 17.8 mm; max. bias 26.0 mm), and January precipitation (R2 = 0.82; RMSEP 0.9 mm; max. bias 1.8 mm). Furthermore, the climate variables obtained by spatial interpolation of data from meteorological stations are used to cross-check the data reconstructed by pollen-climate transfer functions. Our work provides a more comprehensive understanding of the qualitative and quantitative relationships between modern pollen and vegetation/climate and serves as a basis for the paleovegetation and paleoclimate reconstruction in arid and semiarid regions of northern China and similar areas.

Modern pollen–climate relationships in north Xinjiang, northwestern China: Implications for pollen-based reconstruction of Holocene climate

Fossil pollen records are widely used to reconstruct past climate. Such reconstructions require that the relationships between pollen assemblages, vegetation, and climate are well understood. These can be studied in present circumstances given we assume that modern vegetation and climate are analogous to past ones. In this study, we analyze pollen–vegetation–climate relationships in the Jungar desert and Altay Mountains, northwestern China, a region for which careful reconstruction of past climate is needed to answer unsolved questions on past climate in an area located at the boundary between two different climate regimes (westerlies and monsoon). We use a dataset of 66 surface pollen samples from forest, meadow, steppe, and desert vegetation and six related climate variables, Tann, TJan, TJul, Pann, PJan, and PJul. Principal components analysis, redundancy analysis, Monte Carlo permutation tests, and variation partitioning are applied to quantify these relationships. We also assess pollen ratios as indices of aridity. We find that (1) Pannis the major climatic factor influencing pollen assemblages, followed by PJul, (2) the two variables are not correlated, and (3) the shared effect of (1) PJanand PJul, (2) PJanand Pann, (3) PJuland Tann, and (4) Tann, TJan, and TJulexplains a larger portion of the variation in pollen data than the individual effect of each variable. Therefore, robust pollen–climate transfer functions can be developed for Pannand PJul, and several climate variables treated in combination. Artemisia/Chenopodiaceae is a strong index of aridity and Artemisia/Gramineae might be a useful index of Pannand PJul.

Pollen‐based reconstructions of Late Holocene climate on the southern slopes of the central Tianshan Mountains, Xinjiang,NWChina

ABSTRACTTo quantitatively describe the Late Holocene palaeoclimatic history of the southern slopes of the Tianshan Mountains in Xinjiang, China, we collected 74 surface pollen samples (from −154 to 4285 m), together with 81 fossil samples from Swan Lake. Based on modern pollen assemblage and redundancy analysis (RDA) of surface pollen data, five pollen zones were divided (alpine cushion vegetation, alpine meadow, subalpine meadow steppe, montane desert, and typical desert zones). Modern pollen–climate transfer functions were developed to quantitatively reconstruct palaeoclimate of the area using the weighted averaging partial least squares regression method. Reconstructed mean annual precipitation and mean annual temperature results suggest that a dry/warm or humid/cold climate pattern prevailed over the past 4200 aBP. Notably, three cold and humid stages were detected at 4200–4000 cal. aBP, 3700–2000 cal. aBP, and 1170–930 cal. aBP. Moreover, a humid ‘Medieval Warm Period’ (MWP) occurred not only on the northern slopes but also on the southern slopes of the Tianshan Mountains. However, based on palaeo‐environmental records and on archaeological documents, it would appear that a dryMWPoccurred in southern parts of south Xinjiang, with this being different from theMWPon the Tianshan Mountains. Interactions between monsoon and the westerly airflows might account for this difference. The data will improve understanding of past atmospheric circulation systems in the different parts of Xinjiang and will help to predict the future climate in the world.

Vegetation, climate and lake changes over the last 7000 years at the boreal treeline in north-central Siberia

Abstract Palaeoecological investigations in the larch forest-tundra ecotone in northern Siberia have the potential to reveal Holocene environmental variations, which likely have consequences for global climate change because of the strong high-latitude feedback mechanisms. A sediment core, collected from a small lake (radius ∼100 m), was used to reconstruct the development of the lake and its catchment as well as vegetation and summer temperatures over the last 7100 calibrated years. A multi-proxy approach was taken including pollen and sedimentological analyses. Our data indicate a gradual replacement of open larch forests by tundra with scattered single trees as found today in the vicinity of the lake. An overall trend of cooling summer temperature from a ∼2 °C warmer-than-present mid-Holocene summer temperatures until the establishment of modern conditions around 3000 years ago is reconstructed based on a regional pollen-climate transfer function. The inference of regional vegetation changes was compared to local changes in the lake's catchment. An initial small water depression occurred from 7100 to 6500 cal years BP. Afterwards, a small lake formed and deepened, probably due to thermokarst processes. Although the general trends of local and regional environmental change match, the lake catchment changes show higher variability. Furthermore, changes in the lake catchment slightly precede those in the regional vegetation. Both proxies highlight that marked environmental changes occurred in the Siberian forest-tundra ecotone over the course of the Holocene.

Holocene climate events inferred from modern and fossil pollen records in Butuo Lake, Eastern Qinghai–Tibetan Plateau

Expansion of the spatial coverage of pollen data is essential to improve understanding of Holocene climate variation. To address this, we collected 25 surface and 72 fossil samples from Butuo Lake in the eastern Qinghai–Tibetan Plateau, China. We reconstructed the vegetation history of the region through detrended correspondence analysis (DCA) of the pollen data. Based on the results of this analysis, we divided the samples into five pollen zones (alpine sparse cushion vegetation, alpine scrub, alpine meadow, montane scrub meadow, and montane coniferous forest) corresponding to the major vegetation types. The observed temporal changes in vegetation (as indicated by the DCA of surface and fossil pollen spectra results, ratios of Artemisia to Cyperaceae (A/Cy), the sum of percentages of dryness indicators (SDI), and percentages of main pollen types) and modern pollen–climate transfer function (developed using the weighted averaging partial least squares regression method (WAPLS)) yield a sensitive record of Holocene monsoonal climate change in the area. During 11,140–8700, 8000–6000, and 5600–1780 cal. yr BP, the climate was wet and the vegetation was dominated by alpine meadow, indicating the occurrence of a strong southwest Asian monsoon that spanned almost the entire Holocene. Notably, two major cold and drought episodes are detected at 8700–8000 and 6000–5600 cal. yr BP, with vegetation dominated by alpine steppe, suggesting that the southwest Asian monsoon was extremely weak during these periods. The data will improve understanding of long-term variations of the southwest Asian monsoon in the region.

A MIS 3 charcoal and pollen record and quantitative precipitation inferences from the Jingerwa section of the Nihewan Basin, north-central China

Pollen and charcoal data from the Jingerwa section of the Nihewan Basin, north-central China, were used to reconstruct vegetation and climate changes during Marine Isotope Stage (MIS) 3. Mean annual precipitation changes were quantified by applying pollen-climate transfer functions. Sparse vegetation cover dominated by herbs indicates relatively dry climate between 51 and 43 cal ka BP. Between 43 and 35 cal ka BP, a Pinus-dominated forest reached its maximum extent, implying that climate was wetter than today. Severe fire episodes during that period suggest warm temperatures. Between 35 and 32 cal ka BP, forest retreat and the expansion of dryland vegetation are indicative of drier climates. Slightly wetter conditions prevailed between 32 and 29 cal ka BP, as indicated by the expansion of ferns. Our results suggest that the climate conditions in the Nihewan Basin during middle and late MIS 3 were probably wetter than today. This may have been caused by intensification of Asian monsoon circulation in response to greater insolation in June at 30°N. However, increases in summer temperatures and evaporation, triggered by peak summer insolation levels, may have led to a reduction in humidity around 35 cal ka BP.

A pollen-climate transfer function from the tundra and taiga vegetation in Arctic Siberia and its applicability to a Holocene record

This study aims to establish, evaluate, and apply a modern pollen-climate transfer function from the transition zone between arctic tundra and light-needled taiga in Arctic Siberia. Lacustrine samples (n=96) from the northern Siberian lowlands of Yakutia were collected along four north-to-south transects crossing the arctic forest line. Samples span a broad temperature and precipitation gradient (mean July temperature, TJuly: 7.5–18.7°C; mean annual precipitation, Pann: 114–315mm/yr). Redundancy analyses are used to examine the relationship between the modern pollen signal and corresponding vegetation types and climate. Performance of transfer functions for TJuly and Pann were cross-validated and tested for spatial autocorrelation effects. The root mean square errors of prediction are 1.67°C for TJuly and 40mm/yr for Pann. A climate reconstruction based on fossil pollen spectra from a Siberian Arctic lake sediment core spanning the Holocene yielded cold conditions for the Late Glacial (1–2°C below present TJuly). Warm and moist conditions were reconstructed for the early to mid Holocene (2°C higher TJuly than present), and climate conditions similar to modern ones were reconstructed for the last 4000years. In conclusion, our modern pollen data set fills the gap of existing regional calibration sets with regard to the underrepresented Siberian tundra–taiga transition zone. The Holocene climate reconstruction indicates that the temperature deviation from modern values was only moderate despite the assumed Arctic sensitivity to present climate change.

Pollen–climate transfer functions intended for temperate eastern Asia

Pollen data of 646 surface samples from northern China and Mongolia and climatic data from the relevant meteorological stations were collected in this study to develop more reliable pollen–climate transfer functions for temperate eastern Asia. Canonical correspondence analysis (CCA) was used to examine the pollen–climate relationships, and mean summer precipitation (MSP) and mean January temperature (MJaT) were inferred to be the first and second important factors controlling the spatial distribution of the surface pollen in the study area. The original dataset was screened with CCA for MSP and MJaT separately to detect anomalous samples that show the extreme values. The first screened dataset was established after excluding those anomalous samples, and the initial transfer function was generated using the weighted averaging partial least squares (WAPLS) method. The jackknife test was then applied to the WAPLS model for determining the optimum transfer function and for detecting large-residual samples, and the final transfer function was generated after removing the large-residual samples from the first screened dataset. The final dataset for MSP and MJaT consists of 428 and 419 samples, respectively. The root mean square errors of prediction for both WAPLS models are 34 mm and 2.7 °C, and the coefficients of determination are 0.85 and 0.73. This study suggests that the values of climatic parameters could be better estimated and the reliability of pollen–climate transfer functions would be significantly improved through removing anomalous and large-residual samples from the original dataset with mathematical methods.

Reconstruction of palaeoprecipitation based on pollen transfer functions – the record of the last 16 ka from Laguna Potrok Aike, southern Patagonia

Based on modern pollen samples from different vegetation units in southern Patagonia, showing a close relation to yearly amounts of precipitation and mean annual temperatures, different pollen–climate transfer functions are developed and tested. Comparing the performance of MAT (Modern Analogue Techniques), WA (Weighted Average), as well as WAPLS (Weighted Average Partial Least Square) statistical techniques, it is possible to determine the statistically most robust model (WAPLS for precipitation). This transfer function is then used to estimate palaeoprecipitation amounts based on Laguna Potrok Aike pollen results for the last 16,000 years. Generally, the results of the precipitation model indicate less precipitation during the Lateglacial and alternating wet and dry periods during the Holocene. The Holocene started with higher amounts of precipitation until about 8 ka cal. BP, followed by a period with lower amounts between 8 and 2.5 ka cal. BP, while the Late Holocene shows a general increase in precipitation. Comparisons with former shoreline reconstructions and carbonate concentrations in the sediments of Laguna Potrok Aike not always show similarities due to the complex environmental factors recorded by these proxies. Moreover, changes in the moisture availability due to the interplay of precipitation and temperature, cannot be reconstructed directly. Nevertheless, the general long-term trend of palaeoprecipitation is in accordance with the absolute moisture content in the air, which is determined mainly by temperature: during cold periods with less absolute moisture, the model shows less precipitation. Moreover, the model also points to a relation with the position and strength of the Southern Hemisphere Westerlies.

A modern pollen–temperature calibration data set from Korea and quantitative temperature reconstructions for the Holocene

Modern surface pollen samples from the mountains along the east coast of Korea were used to derive pollen–temperature transfer functions. Detrended correspondence analysis (DCA) and detrended canonical correspondence analysis (DCCA) were performed to test the robustness of the relationship between the modern pollen assemblages and temperatures. The relationship exhibited a high correlation (DCA, r = −0.887; DCCA, r = −0.908). The performance of the best weighted averaging partial least squares (WA-PLS) transfer function was statistically good ([Formula: see text] = 0.74; RMSEP = 1.79°C). In order to quantitatively reconstruct the Holocene temperature changes, the best model was applied to five fossil pollen records produced from four coastal lagoons of the east coast and one high-altitude peat bog. Anomalies calculated from reconstructed paleotemperature data were combined to generate a synthesis temperature reconstruction for the east coast of Korea, in which the ‘Medieval Warm Period’, ‘Little Ice Age’, and ‘Migration Period’ were clearly shown. This study demonstrated the validity of the quantitative reconstruction of paleotemperature using the pollen–climate transfer function, even in heavily human-impacted areas such as the Korean Peninsula.

The Holocene thermal maximum and late-Holocene cooling in the tundra of NE European Russia

To investigate the Holocene climate and treeline dynamics in the European Russian Arctic, we analysed sediment pollen, conifer stomata, and plant macrofossils from Lake Kharinei, a tundra lake near the treeline in the Pechora area. We present quantitative summer temperature reconstructions from Lake Kharinei and Lake Tumbulovaty, a previously studied lake in the same region, using a pollen–climate transfer function based on a new calibration set from northern European Russia. Our records suggest that the early-Holocene summer temperatures from 11,500 cal yr BP onwards were already slightly higher than at present, followed by a stable Holocene Thermal Maximum (HTM) at 8000–3500 cal yr BP when summer temperatures in the tundra were ca. 3°C above present-day values. A Picea forest surrounded Lake Kharinei during the HTM, reaching 150 km north of the present taiga limit. The HTM ended with a temperature drop at 3500–2500 cal yr BP associated with permafrost initiation in the region. Mixed spruce forest began to disappear around Lake Kharinei at ca. 3500 cal yr BP, with the last tree macrofossils recorded at ca. 2500 cal yr BP, suggesting that the present wide tundra zone in the Pechora region formed during the last ca. 3500 yr.

Pollen distribution along climatic and biogeographic gradients in northern Central America

Environmental reconstructions based on fossil pollen rely on the understanding of modern pollen distribution along climatic and biogeographic gradients. This study analyses the modern pollen spectra of Central America using three basic approaches: (1) the evaluation of using modern pollen spectra to differentiate the main vegetation types of the region, (2) the usage of non-linear regression to predict individual pollen abundances as a function of climate, and (3) the construction of pollen—climate transfer functions. Standard pollen analysis was carried out on mud—water interface samples from 81 lakes in the Yucatan Peninsula and adjacent mountains of Guatemala and Mexico. Detrended correspondence and cluster analyses were used to evaluate the biogeographic patterns revealed by this modern pollen data set. Non-parametric locally weighted scatterplot smoothing (LOESS) regression was used to construct pollen—climate functional relationships. Five modern vegetation types were clearly identifiable through their associated pollen spectra: Pinus forest, Quercus forest, mountain mesophyllous forest, tropical rainforest, and tropical seasonal forest. The last group includes three subcategories (evergreen seasonal, tropical semi-deciduous, and tropical deciduous forests), which were not separable via this analysis. Precipitation and temperature trends were consistent and robust for at least 28 and 30 taxa, respectively, in the LOESS regression. While floristic patterns driven by temperature were clearly reflected by the pollen spectra, those driven by precipitation were less sharply defined. Nevertheless, pollen data from the study area offered good resolution to identify broad biogeographic patterns. Furthermore, individual taxa showed high predictability along precipitation and temperature gradients, allowing the theoretical construction of pollen—climate transfer functions. This study provided valuable tools for the interpretation of fossil pollen sequences from Central America.

Early Holocene drought in the Laurentian Great Lakes basin caused hydrologic closure of Georgian Bay

Multiple proxies record aridity in the northern Great Lakes basin ~8,800–8,000 cal (8,000–7,200) BP when water levels fell below outlets in the Michigan, Huron and Georgian Bay basins. Pollen-climate transfer function calculations on radiocarbon-dated pollen profiles from small lakes from Minnesota to eastern Ontario show that a drier climate was sufficient to lower the Great Lakes, in particular Georgian Bay, to closed basins. The best modern climate analog for the early Holocene late Lake Hough stage in the Georgian Bay basin is Black Bass Lake near Brainerd MN. Modern annual precipitation at Brainerd is ~35% lower than at Huntsville ON, in the Georgian Bay catchment; warmer summers and colder, less snowy winters make Brainerd drier than the Georgian Bay snow belt. These values parallel transfer function reconstructions for the early Holocene from pollen records at five small lakes in the Georgian Bay drainage basin. Higher evaporation and evapotranspiration due to greater seasonality during the early Holocene produced a deficit in effective moisture in Georgian Bay that is recorded by the jack/red pine pollen zone that spanned ~8,800–8,200 cal (8,000–7,500) BP. This deficit drove late Lake Hough ~5 m below Lake Stanley in the Huron basin, following diversion of Laurentide Ice sheet meltwater from the Great Lakes basin. The level of Georgian Bay largely depends not on fluvial input from its own drainage basin, but rather from Lake Superior, where the early Holocene moisture deficit was greater. Reconstruction of paleoclimates in Minnesota, northwestern Ontario and Wisconsin produced a closed lake in the Superior basin, which removed the main water input to Georgian Bay. Once the inflow through the St. Marys River was reduced and inflow from other tributary streams was adjusted for isostatic and climatic differences, input was <5% of modern values. Consequent high evaporation rates produced a significant fall in lake level in the Georgian Bay basin and a negative water budget. This reduction in basin supply, together with the high conductivity of stagnant water in late Lake Hough inferred from microfossils in lowstand sediments, peaked at the end of the jack/red pine zone, ~8,300–8,200 (7,450 ± 90) BP. These major hydrologic changes resulting from climate change in the recent geologic past draw attention to possible declines of the Great Lakes under future climates.

Quantitative summer-temperature reconstructions for the last 2000 years based on pollen-stratigraphical data from northern Fennoscandia

Quantitative reconstructions of mean July temperatures (T jul) based on new and previously published pollen-stratigraphical data covering the last 2000 years from 11 lakes in northern Fennoscandia and the Kola Peninsula are presented. T jul values are based on a previously published pollen-climate transfer function for the region with a root-mean-square error of prediction (RMSEP) of 0.99°C. The most obvious trend in the inferred temperatures from all sites is the general decrease in T jul during the last 2000 years. Pollen-inferred T jul values on average 0.18 ± 0.56°C (n = 91) higher than present (where “present” refers to the last three decades based on pollen-inferred T jul in core-top samples) are indicated between 0 and 1100 AD (2000–850 cal year BP), and temperatures −0.2 ± 0.47°C (n = 78) below present are inferred between 1100 and 1900 AD (850–50 cal year BP). No consistent temperature peak is observed during the ‘Medieval Warm Period’, ca. 900–1200 AD (1100–750 cal year BP), but the cooler period between 1100 and 1900 AD (850–50 cal year BP) corresponds in general with the ‘Little Ice Age’ (LIA). Consistently with independent stable isotopic data, the composite pollen-based record suggests that the coldest periods of the LIA date to 1500–1600 AD (450–350 cal year BP) and 1800–1850 AD (150–100 cal year BP). An abrupt warming occurred at about 1900 AD and the twentieth century is the warmest century since about 1000 AD (950 cal year BP).

A pre‐European settlement pollen–climate calibration set for Minnesota, USA: developing tools for palaeoclimatic reconstructions

AbstractAim The bias in modern North American pollen assemblages by landscape disturbance from Euro‐American settlement has long been overlooked in the construction of pollen–climate transfer functions. Our aim is to examine this problem and to develop an unbiased pre‐settlement pollen–climate transfer function, and to test its performance and inference power in comparison with commonly used techniques.Location Minnesota, USA, is of palaeoclimatic interest because within the state are located two continental‐scale ecotones, controlled by temperature and available moisture. Shifts of these ecotones can be tracked using palaeoecological techniques.Methods Using a data set of pre‐settlement pollen assemblages from 133 lakes, which were coupled to climate data from the earliest instrumental records (i.e. 1895–1924), a pre‐settlement pollen–climate data set was developed that lacked the influence of anthropogenic landscape disturbance. A corresponding modern pollen data set (from lake sediment core tops) and a modern climate (i.e. 1961–90) data set were also developed. The two pollen sets were compared to demonstrate the effects of landscape disturbance from human activities. Ordination (redundancy analysis with Monte Carlo permutation tests) and regression techniques (generalized linear modelling) were used to establish the relationships between the early instrumental climate variables and pre‐settlement pollen assemblages and individual taxa, respectively. Transfer functions for the most suitable climate variables (i.e. those forming a minimal set of non‐collinear climate variables that explained the greatest amount of pollen variance) were developed from the pre‐settlement data set using bootstrapping.Results Comparison of pre‐settlement pollen and modern pollen showed an over‐representation of Ambrosia, Chenopodiaceae and Poaceae, and an under‐representation of arboreal taxa (e.g. Pinus, Quercus, Ostrya) in the modern assemblages. Not surprisingly, ordination and regression techniques showed a strong relationship between the early instrumental climate variables and pre‐settlement pollen assemblages and taxa. Transfer functions were developed for May and February mean temperature and available moisture. Pre‐settlement transfer functions substantially improved the root mean squared error by 37–72% in comparison with modern transfer functions inferring pre‐settlement conditions, suggesting that the modern transfer functions have poorer predictive abilities.Main conclusions For climatic reconstructions, there can be a serious distortion of inferences based solely on modern pollen–climate data sets in regions where anthropogenic landscape disturbance has occurred. By using historical climate data, coupled with pre‐disturbance pollen assemblages, robust transfer functions for temperature and effective moisture were developed.

Development and testing of transfer functions for generating quantitative climatic estimates from Australian pollen data

AbstractWe review attempts to derive quantitative climatic estimates from Australian pollen data, including the climatic envelope, climatic indicator and modern analogue approaches, and outline the need to pursue alternatives for use as input to, or validation of, simulations by models of past, present and future climate patterns. To this end, we have constructed and tested modern pollen–climate transfer functions for mainland southeastern Australia and Tasmania using the existing southeastern Australian pollen database and for northern Australia using a new pollen database we are developing. After testing for statistical significance, 11 parameters were selected for mainland southeastern Australia, seven for Tasmania and six for northern Australia. The functions are based on weighted‐averaging partial least squares regression and their predictive ability evaluated against modern observational climate data using leave‐one‐out cross‐validation. Functions for summer, annual and winter rainfall and temperatures are most robust for southeastern Australia, while in Tasmania functions for minimum temperature of the coldest period, mean winter and mean annual temperature are the most reliable. In northern Australia, annual and summer rainfall and annual and summer moisture indexes are the strongest. The validation of all functions means all can be applied to Quaternary pollen records from these three areas with confidence. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Quantitative relationships between modern pollen rain and climate in the Tibetan Plateau

Quantitative relationships between modern pollen rain and climate are poorly studied in China, partly due to the extensive human impact on the modern vegetation. A dataset consisting of 227 modern pollen samples from forests, shrublands, meadows, steppes, and deserts in the Tibetan Plateau, the least anthropologically-disturbed region in China, provides a unique opportunity to study the quantitative relationships between modern pollen rain and climate. Pollen percentage data were calculated on a sum of 20 pollen taxa. Climatic data for each site, including mean annual precipitation (MAP), mean annual temperature (MAT), July temperature ( T july), and January temperature ( T jan), were derived from 214 meteorological stations in the Tibetan Plateau and adjacent areas using natural neighbor interpolation and linear interpolation methods. Canonical correspondence analysis (CCA) was used to reveal the climatic parameters that best reflect the main patterns of variation in the modern pollen rain, and to detect anomalous observations. Results of CCA indicate that MAP and T july are two climatic parameters controlling the variation of modern pollen rain in the Tibetan Plateau. Pollen–climate transfer functions for MAP and T july were then developed using the inverse linear regression and weighted-averaging partial least squares regression models. The functions derived from these two models are statistically significant at the 0.0000 level. A case study, in which these functions were applied to a fossil pollen record from an alpine lake in the eastern Tibetan Plateau, was conducted to show the feasibility of these functions in paleoclimate reconstruction. The results demonstrated the applicability of these pollen–climate transfer functions to fossil pollen data.