Pollen Assemblages Research Articles

Pollen-based climate changes since the middle MIS 3 in Jilantai Salt Lake in the marginal region of the Asian summer monsoon domain, Inner Mongolia, China

• The pollen record since mid-MIS 3 was provided for a current-hyperarid region of China. • The climate was subhumid during the mid-late MIS 3 and hyperarid during the Holocene . • Insolation and ice volume were the forcing mechanisms for climate changes in the Jilantai Basin. The climate conditions of the marginal region of the Asian summer monsoon (ASM) domain during Marine isotope stage (MIS) 3 (60–27 cal kyr BP) are poorly understood and require further study. Here, we collected fossil pollen samples from a 24.0-m-long drill core from Jilantai Salt Lake (Inner Mongolia, China) to reconstruct the regional climate changes since mid-MIS 3. During 47.0–43.0 cal kyr BP, the frequencies of the occurrence of aquatics and Pediastrum indicate relatively humid conditions. At approximately 43.0 cal kyr BP, the climate became dry, as revealed by the increase in Ephedra and the reduced aquatics and Pediastrum concentrations. The climate during 43.0–27.1 cal kyr BP was drier than that in the previous interval. Aquatics, ferns, and algae were barely detectable, indicating harsh climate conditions during 27.1–12.2 cal kyr BP. During the Holocene, aquatics and algae vanished, and the observed pollen assemblages were dominated by herb taxa; however, a substantial loss of taxa numbers occurred, suggesting that the climate was hyperarid. In general, the climate was relatively humid during the mid-late MIS 3, arid during MIS 2 (27.0–11.7 cal kyr BP), and hyperarid during the Holocene. The evaporation capacity connected with the Northern Hemisphere summer insolation and the Northern Hemisphere ice volume may have played a vital role in vegetation and climate changes in the Jilantai Basin since mid-MIS 3.

Agricultural practices in India during the Holocene: A pollen view point and a critical appraisal

Pollen assemblages provide valuable insights into the beginning of cereal-based agricultural practices and the transition from a hunting and gathering to a sedentary and food-producing way of life. Anthropogenic pollen indicators (APIs) and their precise identification, with respect to taxonomic resolution, can help to document the history of agricultural development, pastoral activities and human-induced land-use changes, particularly for the Holocene Epoch. Moreover, careful selection of pollen types, and/or indices, established for a particular region, are useful for obtaining meaningful reconstructions of anthropogenic activities through time. Specific pollen-markers have been used to deduce the inception of agriculture and the impact of anthropogenic activities on the landscape, from the different regions of the world. For India, Cerealia, and other cultural plant pollen taxa, such as Amaranthaceae, Caryophyllaceae, Brassicaceae, Polygonaceae, Artemisia, Cannabis sativa, Alternanthera, Urtica, Rumex and Borreria have been used as marker pollen types for indicating agricultural practices and other anthropogenic activities. In this communication, we have made an attempt to trace the advent and intensification of agricultural practices in the varied physiographic regions of the Indian sub-continent, on the basis of the available palynological records. We have further critically analysed the plausibility of agricultural practices and other human activities during the Late Pleistocene/Pre and Early Holocene with respect to the Indian scenario.

Holocene history of the landscape at the biogeographical and cultural crossroads between Central and Eastern Europe (Western Podillia, Ukraine)

Continentality is a globally significant gradient influencing broad-scale biogeographical patterns. An excellent example is the transition from the European temperate forest biome to the continental steppe and forest-steppe of Eurasia. One of the biogeographic crossroads where the two biomes meet is the Western Podillia in the western part of Ukraine. It is known for its rich biodiversity with a mixture of steppe, forest and montane species, despite a relatively humid climate suitable for closed-canopy forests. Biologists have postulated the refugial character of the local steppes, but a modern paleoecological reconstruction of the environmental history of the region has been lacking. We fill this gap here with a multi-proxy study (pollen, plant and mollusc macrofossils, microcharcoal, geochemistry) of two profiles sampled in calcareous fens adjacent to species-rich steppe grasslands. To link the reconstructed environmental history with the history of human settlement, we compiled available archaeological records from the studied region. Together, the analyzed profiles cover the entire Holocene and the end of the Last Glacial period, as shown by high-quality age-depth models. All studied proxies support the hypothesis that an open or semi-open landscape existed in Western Podillia during the Holocene. The complete absence of wood remains in Holocene sediments, and the persistence of fen specialists showed the exceptional long-term stability of open wetlands. The continuous presence of pollen of light-demanding plants, low abundance of closed-canopy trees compared to open-canopy trees, and stable concentrations of geochemical indicators of erosion suggest a semi-open landscape with a mosaic of forests, steppe grasslands, and other open habitats. Multivariate analysis revealed the similarity of pollen assemblages with sites in the forest-steppe zone at the interface between the Pannonian Basin and the Western Carpathians. The continuous presence of non-woody microcharcoal in high abundance suggests a role of fire in maintaining open habitats, and the archaeological record provides evidence of human activity throughout the Holocene near the study sites. Comparison with modern pollen spectra suggests that the landscape was probably somewhat more forested in the past than today, but rather by open-canopy trees. Our results indicate that Western Podillia has become a biogeographic crossroads not only because of its location on the border between Central and Eastern Europe, but also because of the unusual combination of relatively humid climate and continuity of open or semi-open landscapes since the Last Glacial.

Modern Pollen Assemblages From Lake Sediments and Soil in East Siberia and Relative Pollen Productivity Estimates for Major Taxa

Modern pollen–vegetation–climate relationships underpin palaeovegetation and palaeoclimate reconstructions from fossil pollen records. East Siberia is an ideal area for investigating the relationships between modern pollen assemblages and near natural vegetation under cold continental climate conditions. Reliable pollen-based quantitative vegetation and climate reconstructions are still scarce due to the limited number of modern pollen datasets. Furthermore, differences in pollen representation of samples from lake sediments and soils are not well understood. Here, we present a new pollen dataset of 48 moss/soil and 24 lake surface-sediment samples collected in Chukotka and central Yakutia in East Siberia. The pollen–vegetation–climate relationships were investigated by ordination analyses. Generally, tundra and taiga vegetation types can be well distinguished in the surface pollen assemblages. Moss/soil and lake samples contain generally similar pollen assemblages as revealed by a Procrustes comparison with some exceptions. Overall, modern pollen assemblages reflect the temperature and precipitation gradients in the study areas as revealed by constrained ordination analysis. We estimate the relative pollen productivity (RPP) of major taxa and the relevant source area of pollen (RSAP) for moss/soil samples from Chukotka and central Yakutia using Extended R-Value (ERV) analysis. The RSAP of the tundra-forest transition area in Chukotka and taiga area in central Yakutia are ca. 1300 and 360 m, respectively. For Chukotka, RPPs relative to both Poaceae and Ericaceae were estimated while RPPs for central Yakutia were relative only to Ericaceae. Relative to Ericaceae (reference taxon, RPP = 1), Larix, Betula, Picea, and Pinus are overrepresented while Alnus, Cyperaceae, Poaceae, and Salix are underrepresented in the pollen spectra. Our estimates are in general agreement with previously published values and provide the basis for reliable quantitative reconstructions of East Siberian vegetation.

Last interglacial lake sediments preserved beneath Laurentide and Greenland Ice sheets provide insights into Arctic climate amplification and constrain 130 ka of ice‐sheet history

AbstractSediment cores from 13 lakes in a 1500 km transect along the eastern North American Arctic contain up to four superposed stratified interglacial units. All 13 lakes contain one unit with sediment similar in character and mass to Holocene gyttja, with 14C ages >40 ka, luminescence ages 90 to 120 ka, and pollen assemblages that require nearly complete Laurentide deglaciation, supporting a Last Interglacial (LIG; MIS 5e) age. Two lakes preserve an older interglacial, with luminescence ages suggesting an MIS 7 age. Four adjacent lakes record a thin, stratified organic unit between the LIG and Holocene units with 14C ages >50 ka, that is probably from late in MIS 5. Temperature estimates from biotic proxies suggest LIG summer temperatures 4–6°C above mid‐20th century values; pollen, chironomids and DNA document a poleward expansion of woody plants and invertebrate species during the LIG, supporting arguments that positive feedbacks native to the Arctic amplified insolation‐driven summer temperature increases. The stratigraphic succession implies the Laurentide Ice Sheet remained intact with sea level below ‐40 m from ~115 ka to ~11 ka, and places new constraints on the interpretation of cosmogenic radionuclide inventories in erratic boulders older than the Holocene throughout this region.

Pollen-Based Holocene Thawing-History of Permafrost in Northern Asia and Its Potential Impacts on Climate Change

As the recent permafrost thawing of northern Asia proceeds due to anthropogenic climate change, precise and detailed palaeoecological records from past warm periods are essential to anticipate the extent of future permafrost variations. Here, based on the modern relationship between permafrost and vegetation (represented by pollen assemblages), we trained a Random Forest model using pollen and permafrost data and verified its reliability to reconstruct the history of permafrost in northern Asia during the Holocene. An early Holocene (12–8 cal ka BP) strong thawing trend, a middle-to-late Holocene (8–2 cal ka BP) relatively slow thawing trend, and a late Holocene freezing trend of permafrost in northern Asia are consistent with climatic proxies such as summer solar radiation and Northern Hemisphere temperature. The extensive distribution of permafrost in northern Asia inhibited the spread of evergreen coniferous trees during the early Holocene warming and might have decelerated the enhancement of the East Asian summer monsoon (EASM) by altering hydrological processes and albedo. Based on these findings, we suggest that studies of the EASM should consider more the state of permafrost and vegetation in northern Asia, which are often overlooked and may have a profound impact on climate change in this region.

Modern pollen–vegetation relationships along an altitudinal transect in the Western Higher Himalaya, India: Palaeoclimatic and anthropogenic implications

Palynology is one of the most reliable tools for the reconstruction of past vegetation and climate and modern pollen analogues are important for the calibration of fossil pollen assemblages. The present study analyses the pollen–vegetation relationships along a steep altitudinal gradient (2700–3680 m), in the western Higher Himalayan region. On the basis of altitude, three vegetation zones were demarcated: Zone I (2700–3100 m) is composed of mixed-temperate forest vegetation, dominated by Quercus semecarpifolia and Rhododendron arboreum; Zone II (3100–3250 m) is marked by sub-alpine forest vegetation, characterised by R. campanulatum and R. barbatum, along with Abies spectabilis and Q. semecarpifolia; Zone III (3250–3680 m) is above the tree-line (3250 m) and represented by alpine-scrub and meadows. Thirty-five surface soil samples (twenty, seven and eight from each zone, respectively) were analysed along the altitudinal transect to decode the representation of the extant vegetation in the pollen-rain. The pollen–vegetation relationship is non-linear due to the over-representation of extra-local Pinus pollen in each zone. Nonetheless, the modern pollen assemblages show a general correlation with the local broad-leaved taxa and the herbaceous elements; with the exception of Rhododendron pollen, which is under-represented. Among the non-pollen palynomorphs (NPPs), the presence of coprophilous fungal spores is compatible with the grazing activities in the area. Multivariate statistical analyses performed on the surface pollen data indicate that the dataset can efficiently distinguish the different vegetation zones across the altitudinal gradient. This work provides the modern analogues for pollen-based palaeoclimatic reconstructions for the Western-Higher Himalayan region, and would also help to decipher the inception and intensification of anthropogenic activities in the region.

Paleovegetation and paleotemperature in North China during the mid-Holocene based on sedimentological and palynological evidence from Lake Baiyangdian

The North China Plain is climatically sensitive and is also noted as one of the principal regions in East Asia experiencing pronounced climatic warming. We reconstructed the paleoclimate and paleoenvironment of a lake site in North China Plain during the mid-Holocene warm period, in order to provide reference data for assessing possible futures changes in the ecological environment in the context of ongoing climate change. The reconstruction is based on the chronology, sedimentology and pollen assemblages of two sedimentary sequences from the Lake Baiyangdian area, which are used to determine the regional vegetation composition and paleotemperature of the hinterland of the North China Plain during the mid-Holocene. The results show that the extent and distribution of Lake Baiyangdian varied due to river channel changes, which also affected the sedimentary facies and the patterns of erosion and accumulation. The pollen assemblages from the lacustrine deposits are derived from the entire catchment of Lake Baiyangdian and they reflect regional patterns of climatic and environmental change. During the mid-Holocene, mixed broadleaf-coniferous forest dominated the western mountains and hills, intrazonal grassland developed on the alluvial fans, and lake-swamp-floodplain environments developed in the hinterland of the North China Plain. Statistical analysis of the modern distribution and climate thresholds of Ceratopteris were used to estimate the mean annual temperature (MAT) and mean January temperature (MJaT) of the Lake Baiyangdian area during the mid-Holocene, which were respectively 3.5 °C and 7.7 °C higher than today. Our findings provide a scientific basis for landscape reconstruction and paleoclimate modelling in North China. • Characteristics of sedimentary strata in Lake Baiyangdian area during the Holocene • Environmental implications of sedimentary pollen assemblages from Lake Baiyangdian • Regional vegetation composition of the Lake Baiyangdian area during the mid-Holocene • Estimates of the mid-Holocene paleotemperature in the hinterland of North China Plain

Human activities have reduced plant diversity in eastern China over the last two millennia.

Understanding the history and regional singularities of human impact on vegetation is key to developing strategies for sustainable ecosystem management. In this study, fossil and modern pollen datasets from China are employed to investigate temporal changes in pollen composition, analogue quality, and pollen diversity during the Holocene. Anthropogenic disturbance and vegetation's responses are also assessed. Results reveal that pollen assemblages from non-forest communities fail to provide evidence of human impact for the western part of China (annual precipitation less than 400 mm and/or elevation more than 3000 m.a.s.l.), as inferred from the stable quality of modern analogues, principal components, and diversity of species and communities throughout the Holocene. For the eastern part of China, the proportion of fossil pollen spectra with good modern analogues increases from ca. 50% to ca. 80% during the last 2millennia, indicating an enhanced intensity of anthropogenic disturbance on vegetation. This disturbance has caused the pollen spectra to become taxonomically less diverse over space (reduced abundances of arboreal taxa and increased abundances of herbaceous taxa), highlighting a reduced south-north differentiation and divergence from past vegetation between regions in the eastern part of China. We recommend that care is taken in eastern China when basing the development of ecosystem management strategies on vegetation changes in the region during the last 2000 years, since humans have significantly disturbed the vegetation during this period.

Reconstructing burnt area during the Holocene: an Iberian case study

Abstract. Charcoal accumulated in lake, bog or other anoxic sediments through time has been used to document the geographical patterns in changes in fire regimes. Such reconstructions are useful to explore the impact of climate and vegetation changes on fire during periods when human influence was less prevalent than today. However, charcoal records only provide semi-quantitative estimates of change in biomass burning. Here we derive quantitative estimates of burnt area from vegetation data in two stages. First, we relate the modern charcoal abundance to burnt area using a conversion factor derived from a generalised linear model of burnt area probability based on eight environmental predictors. Then, we establish the relationship between fossil pollen assemblages and burnt area using tolerance-weighted weighted averaging partial least-squares regression with a sampling frequency correction (fxTWA-PLS). We test this approach using the Iberian Peninsula as a case study because it is a fire-prone region with abundant pollen and charcoal records covering the Holocene. We derive the vegetation–burnt area relationship using the 31 records that have both modern and fossil charcoal and pollen data and then reconstruct palaeoburnt area for the 113 records with Holocene pollen records. The pollen data predict charcoal-derived burnt area relatively well (R2 = 0.44), and the changes in reconstructed burnt area are synchronous with known climate changes through the Holocene. This new method opens up the possibility of reconstructing changes in fire regimes quantitatively from pollen records, after regional calibration of the vegetation–burnt area relationship, in regions where pollen records are more abundant than charcoal records.

Pollen–vegetation relationship based on lake surface sediments of arid and semi-arid northern China; considerations and limitations

In this paper, we investigated pollen assemblages of 92 lake center surface sediments of arid and semi-arid northern China to statistically disclose the pollen–vegetation relationships at the regional scale and explore considerations and limitations for lacustrine pollen-based reconstructions. As a result, the Principal component analysis (PCA), the Indicator species analysis, the Discriminate analysis (DA), the Random forest (RF), and the Supportive vector machine (SVM) generally reveal positive correlations between pollen assemblages and vegetation types or ecotones and suggests capabilities of pollen to represent regional vegetation and climate. However, the DA, RF, and SVM reveal that the samples of desert steppe, typical steppe, and meadow steppe cannot be differentiated by pollen assemblages, suggesting weaknesses to represent regional vegetation and climate. Mental test analyses of pollen assemblages and azonal vegetation of the dune desert and the Gobi desert yield better correlations than that of regional vegetation, suggesting that the pollen of azonal vegetation distort the regional pollen–vegetation relationships and thus weaken the capabilities to represent regional vegetation and climate. Our results present considerations and limitations for pollen-based past vegetation and climate reconstructions and provide references for interpretations of lacustrine fossil pollen records in arid and semi-arid northern China.

Late Quaternary fire and vegetation history inferred from the Xifeng loess-paleosol sequence of the Chinese Loess Plateau

The focus of this paper is to reconstruct late Quaternary wildfire and vegetation history in Chinese Loess Plateau (CLP) region, and to determine the interrelationships between wildfire, climate, and vegetation. To achieve this aim, we analyzed the black carbon concentration and pollen assemblages within an interval correlated with Marine Isotope Stage (MIS) 5 (equivalent to paleosol S1) to MIS 4–2 (equivalent to Loess L1) in the Xifeng section of the CLP. Findings show that during the MIS 5–2 intervals, changes in precipitation and temperature were essentially synchronous on the CLP, and changes in biomass reflected by total organic carbon content and pollen assemblages were determined mainly by the climatic conditions. Wildfire was controlled mainly by fuel humidity and vegetation type, which influenced by precipitation; however, there was no significant relationship between wildfire occurrence and temperature or the amount of combustible plant material. During the MIS 5 interval, the climate was warmer and wetter than today, while during the MIS 3 interval (Loess L1SL1) it was similar to today; however, during the MIS 2 (L1LL1) and MIS 4 (L1LL2) the climate was cooler and drier. Fires were less frequent during the MIS 5 interval compared to the MIS 3, and fires during the MIS 2 and MIS 4 were more frequent than the MIS 3. Results suggested that if the climate on the CLP became warmer and wetter in the future, natural fires would not necessarily occur more frequently and intensively. However, if climate evolved towards cooler and drier conditions, natural fires may be more likely to occur.

Palynostratigraphy of the Oligocene and Neogene-Quaternary deposits of the Ishim and Irtysh Interfluve (by example of boreholes 3 and 6)

A biostratigraphic subdivision based on palynological data of Oligocene and Neogene-Quaternary deposits recovered by wells 3 and 6 in the south of Western Siberia on the border of the Tyumen and Omsk regions is presented. Eight spore and pollen assemblages are identified and compared with zonal assemblages of the West Siberia regional scale. Layers with dinocysts are identified in the Turtas Formation. To include them in the stratigraphic scheme of the new generation, it is proposed to consider the territory between the villages of Katai and Listvyagi as a stratotype area of this biostraton.

Angiosperm pollen assemblages from the Lower Cretaceous (Barremian–lower Aptian) of offshore Saudi Arabia and their implications for early patterns of angiosperm radiation

Early flowering plants constituted a minor component of the Early Cretaceous conifer- and fern- dominated flora. However, their pollen provides a rich, albeit incomplete, record of the group's early evolutionary history. Palynological analysis of Barremian–lower Aptian successions from the Biyadh and Shu'aiba formations in the Arabian Gulf reveals remarkably high angiosperm diversity. While Chloranthaceae, magnoliids, monocots, eudicots, and other “basal” groups represent the bulk of flowering plants, pollen with uncertain affinities is also a significant component of the angiosperm assemblage. Analysis of the palaeobiogeographic distribution of angiosperm pollen during the late Barremian–early Aptian allowed us to discern three groups of angiosperms. The first group, including Clavatipollenites, produced pollen with a near-cosmopolitan distribution. The second group, including Stellatopollis, had a somewhat restricted distribution in northern Gondwana and western Laurasia. The pollen distribution of the third group, which includes Dichastopollenites, is restricted to central and/or northeastern Gondwana. These varied patterns of palaeobiogeographical spread of early angiosperms reflect different migration and colonisation abilities. When compared with coeval palynological successions from other Gondwanan localities, the composition and diversity of the Arabian angiosperm assemblages support early hypotheses on the radiation of angiosperms that identify northeastern Gondwana as one of the most important centres of angiosperm diversification.The following new taxa are described: Splitipollis gen. nov., Splitipollis reticulatus sp. nov., Tucanopollis doylei sp. nov., Pennipollis spinosus sp. nov., Clavamonocolpites woodii sp. nov., and Retimonoporites arabiensis sp. nov.

A new indicator approach to reconstruct agricultural land use in Europe from sedimentary pollen assemblages

The reconstruction of human impact is pivotal in palaeoecological studies, as humans are among the most important drivers of Holocene vegetation and ecosystem change. Nevertheless, separating the anthropogenic footprint on vegetation dynamics from the impact of climate and other environmental factors (disturbances such as fire, erosion, floods, landslides, avalanches, volcanic eruptions) is a challenging and still largely open issue. For this purpose, palynologists mostly rely on cultural indicator pollen types and related indices that consist of sums or ratios of these pollen types. However, the high environmental and biogeographical specificity of cultural indicator plants hinders the application of the currently available indices to wide geographical settings. Furthermore, the achievable taxonomic resolution of cultural indicator pollen types may hamper their indicative capacity. In this study, we propose the agricultural land use probability (LUP) index, a novel approach to quantify human impact intensity on European ecosystems based on cultural indicator pollen types. From the ‘classic’ cultural indicators, we construct the LUP index by selecting those with the best indicator capacity based on bioindication criteria. We first train the LUP index using twenty palynological sequences along a broad environmental gradient, spanning from treeless alpine to subtropical mediterranean evergreen plant communities. We then validate the LUP index using independent pollen datasets and archaeological proxies. Finally, we discuss the suitability of the selected pollen types and the potential of the LUP index for quantifying Holocene human impact in Europe, concluding that careful application of the LUP index may significantly contribute to refining pollen-based land-use reconstructions.

Modern analogs for understanding pollen-vegetation dynamics in a Mediterranean mosaic landscape (Balearic Islands, Western Mediterranean)

This paper presents the study of modern pollen analogs from the Balearic Islands. While similar studies have been largely applied to mainland areas, research focused on modern vegetation dynamics on Mediterranean islands remains very rare. In this research, we combine vegetation surveys, pollen analysis and multivariate statistics to understand landscape composition. The main objectives of are: (1) to examine pollen-vegetation relationships in relation to environmental and land-use variables; (2) to understand modern pollen representation in a mosaic landscape structure; and (3) to propose pollen indicators that characterize the primary vegetation types from the Balearic Islands to better interpret past pollen records in Mediterranean island environments. Pollen results and Redundancy Analysis (RDA) distinguish three major groups: (a) Holm oak and box formations; (b) maquis and garrigues; and (c) anthropogenic and open habitats. Landscape form, mean decadal rainfall, mean decadal temperature, fire activity, trampling, slope percentage, wet/flooded soil, saline soil, distance to agropastoral cells, gHM index, domestic herbivory presence, agropastoral use, and soil type are the major variables explaining modern pollen assemblage variation in our research. Poaceae undiff., Plantago sp., Apiaceae undiff., Cerealia-t, and Cichorieae are highly correlated to human activities but should be interpreted cautiously when occurring in low values. Quercus ilex-t, Hypericum, and Buxus are correlated to humid locations while Pistacia, Pinus, Juniperus-t, and Olea to high mean decadal temperatures. Our study indicates how pollen analysis and multivariate analysis are powerful tools for characterizing the mosaic landscape, with special focus on the main vegetation types of the Balearic Islands.

Paleoclimate Variations Spanning the Past 29–4 Ma Inferred From Lipid Biomarkers and Carbon Isotopes in the Linxia Basin, Northeast Tibetan Plateau

Thick and continuous deposits in the Linxia Basin, located in the landing area of westerlies and monsoons, offer a good opportunity for understanding the climatic evolution and tectonic activities. However, detailed paleoclimate reconstruction based on lipid biomarkers was rare, which limited our further knowledge, even though there were some relevant reports regarding pollen assemblages, microbial communities. In the present study, we conducted systematic analyses on the lipid biomarkers and carbon isotope values of the sediments, in an effort to reconstruct the evolution history of paleoclimate and figure out the potential driving mechanism. Our results showed that the organic matter was from mixed sources including lower organisms and terrestrial higher plants. The organic matter sources varied in response to the change of paleoclimate conditions as revealed by the lipid-derived proxies and organic carbon isotopes. Significant climatic events like late Oligocene Warming, Mid-Miocene Climatic Optimum and aridification at ∼8.5 Ma were observed throughout the sequence. Our results further indicated that the paleoclimate conditions in the study area primarily followed the pace of long-term global cooling, and the aridification at ∼8.5 Ma was associated with the uplift of the Tibetan Plateau.

Validation of relative pollen productivities for major tropical plant taxa: An empirical test using the REVEALS model in Hainan, China

The reliability of estimates of tropical lowland vegetation cover based on lacustrine pollen records has long been debated. Here, we assess the reliability of relative pollen productivity estimates (RPPs) obtained in tropical Hainan island using surface pollen assemblages from six reservoirs and the Regional Estimates of Vegetation Abundance from Large Sites (REVEALS) model. The resulting REVEALS estimates are compared to observations of vegetation coverin the studied region and pollen percentages. The REVEALS estimates for most tropical taxa including Poaceae, Moraceae, Euphorbiaceae, Mallotus/Macaranga, Castanopsis/Lithocarpus, were more similar to the observed vegetation cover than estimates based on simple pollen percentages, suggesting that the previously reported RPPs are reliable. The REVEALS estimates for some taxa (i.e. Rubiaceae and Anacardiaceae) are comparable to those based on pollen percentages, but RPPs for these taxa are difficult to evaluate. However, the pollen percentages for some taxa including Asteraceae and Papilionaceae agree better with the observed vegetation cover than the REVEALS corrections, implying either that there are problems with the corresponding RPPs or that there is bias in the observed vegetation data used for validation. We suggest that the reliability of the RPPs for plant taxa in tropical areas depends strongly on their abundance in the pollen and vegetation data: high abundance implies high reliability and vice versa. REVEALS cover estimates for groups of taxa consistently agreed well with observed vegetation cover, suggesting that the REVEALS model can be applied even in tropical regions if individual species are grouped, which may be useful when reconstructing historical landscape changes in tropical Asia.

A modern pollen data set for the forest–meadow–steppe ecotone from the Tibetan Plateau and its potential use in past vegetation reconstruction

The relationship between modern pollen and vegetation provides the basis for the interpretation of stratigraphic pollen assemblages and the quantitative reconstruction of past vegetation. We selected 168 topsoil samples from four different vegetation types on the south‐eastern Tibetan Plateau to explore the relationships between pollen assemblages, vegetation and climate. The results reveal that pollen assemblages discriminate the vegetation types well; the subalpine coniferous and evergreen broad‐leaved forest is characterized by a high proportion of arboreal taxa (e.g. Pinus, Picea, Betula); the alpine shrub and meadow and alpine steppe vegetation types are dominated by Cyperaceae, followed by Artemisia and Brassicaceae; and the alpine steppe‐shrub is characterized by a high percentage of Artemisia, with Cyperaceae, Asteraceae and Brassicaceae as common taxa. Redundancy analysis shows that mean temperature of the coldest month (Mtco) is the main climatic factor that influences pollen distribution. Pollen diversity indices (including richness and evenness) gradually decrease from SE to NW. The random forest classification has good performance in distinguishing vegetation types. Our study supplies a comparatively detailed description of the relationship between the pollen assemblage and vegetation in the forest–meadow–steppe ecotone on the south‐eastern Tibetan Plateau. In addition, the random forest model has potential application for reconstructing the past vegetation type of the fossil pollen spectra on the south‐eastern Tibetan Plateau.

Holocene hydroclimate changes revealed by multiple proxies from an alpine lake in the central Tianshan Mountains, Northwest China

Alpine wetland ecosystems are of importance in water conservation and animal husbandry in the arid central Asian. However, there is limited research on their formation and relationship with climate. In this study, we evaluated the long-term interactions of vegetation, hydrology, and climate based on multiple proxies (lithology, grain size, loss-on-ignition (LOI) at 550 °C, and pollen) obtained from a sediment core (BY10A) from Lake Swan, an alpine lake in the central Tianshan Mountains of northwest China (2541 m a.s.l.). Our records show that Holocene sediment facies change considerably within core BY10A, with the succession from bottom to top comprising fluvial, peat, and lake sediments. The fluvial deposits were characterized by coarser sediment, which was probably reworked from extensive sand dunes developed over the basin during the early Holocene (12.0–7.5 cal kyr BP). Regional biological productivity was low at this time as indicated by the low LOI value, and the remains of vegetation was degraded under the dry climate. A warm and wet climate prevailed from 7.5 to 2.0 cal kyr BP, resulting in the establishment of peat. The ability of the peat surface to efficiently trap fine particles was enhanced when regional sand dune bodies (coarser part) became largely stabilized by vegetation growth in response to increases in moisture availability. After 2.0 cal kyr BP, a rise in water level, leading to the formation of Lake Swan, indicated a further shift to a wetter climate. During this interval, there was an increasing influence of lacustrine settling and/or wave action on the grain size distribution and the pollen assemblage. This study indicates a stepwise wetting trend from the early to late Holocene. Findings show that changes in moisture, regulated by the prevailing westerlies, exerted a significant influence on the evolution of vegetation communities and lake level in our study area.