Shengrui Zhang

East Asian summer monsoon precipitation variability since the last deglaciation

The lack of a precisely-dated, unequivocal climate proxy from northern China, where precipitation variability is traditionally considered as an East Asian summer monsoon (EASM) indicator, impedes our understanding of the behaviour and dynamics of the EASM. Here we present a well-dated, pollen-based, ~20-yr-resolution quantitative precipitation reconstruction (derived using a transfer function) from an alpine lake in North China, which provides for the first time a direct record of EASM evolution since 14.7 ka (ka = thousands of years before present, where the “present” is defined as the year AD 1950). Our record reveals a gradually intensifying monsoon from 14.7–7.0 ka, a maximum monsoon (30% higher precipitation than present) from ~7.8–5.3 ka, and a rapid decline since ~3.3 ka. These insolation-driven EASM trends were punctuated by two millennial-scale weakening events which occurred synchronously to the cold Younger Dryas and at ~9.5–8.5 ka, and by two centennial-scale intervals of enhanced (weakened) monsoon during the Medieval Warm Period (Little Ice Age). Our precipitation reconstruction, consistent with temperature changes but quite different from the prevailing view of EASM evolution, points to strong internal feedback processes driving the EASM, and may aid our understanding of future monsoon behaviour under ongoing anthropogenic climate change.

Relative pollen productivities of typical steppe species in northern China and their potential in past vegetation reconstruction

The Relative Pollen Productivities (RPPs) of common steppe species are estimated using Extended R-value (ERV) model based on pollen analysis and vegetation survey of 30 surface soil samples from typical steppe area of northern China. Artemisia, Chenopodiaceae, Poaceae, Cyperaceae, and Asteraceae are the dominant pollen types in pollen assemblages, reflecting the typical steppe communities well. The five dominant pollen types and six common types (Thalictrum, Iridaceae, Potentilla, Ephedra, Brassicaceae, and Ulmus) have strong wind transport abilities; the estimated Relevant Source Area of Pollen (RSAP) is ca. 1000 m when the sediment basin radius is set at 0.5 m. Ulmus, Artemisia, Brassicaceae, Chenopodiaceae, and Thalictrum have relative high RPPs; Poaceae, Cyperaceae, Potentilla, and Ephedra pollen have moderate RPPs; Asteraceae and Iridaceae have low RPPs. The reliability test of RPPs revealed that most of the RPPs are reliable in past vegetation reconstruction. However, the RPPs of Asteraceae and Iridaceae are obviously underestimated, and those of Poaceae, Chenopodiaceae, and Ephedra are either slightly underestimated or slightly overestimated, suggesting that those RPPs should be considered with caution. These RPPs were applied to estimating plant abundances for two fossil pollen spectra (from the Lake Bayanchagan and Lake Haoluku) covering the Holocene in typical steppe area, using the “Regional Estimates of Vegetation Abundance from Large Sites” (REVEALS) model. The RPPs-based vegetation reconstruction revealed that meadow-steppe dominated by Poaceae, Cyperaceae, and Artemisia plants flourished in this area before 6500-5600 cal yr BP, and then was replaced by present typical steppe.

Vegetation succession and East Asian Summer Monsoon Changes since the last deglaciation inferred from high-resolution pollen record in Gonghai Lake, Shanxi Province, China:

A cal. 20-year-resolution pollen record from Gonghai Lake presented the detailed process of mountain vegetation succession and East Asian Summer Monsoon (EASM) changes since the last deglaciation in Shanxi Province, North China. Modern vegetation distribution and lake surface pollen assemblages suggested that the fossil pollen mainly came from local and surrounding vegetation in Gonghai Lake, which reflected the elevational changes of plant communities in study area. From 14,700 to 11,100 cal. yr BP, open forests and mountain meadows dominated by shrubs and herbaceous species in surrounding area, suggesting a weak EASM with less precipitation. In the period between 11,100 and 7300 cal. yr BP, bushwoods and grasses were gradually replaced by mixed broadleaf-conifer forest, first developed by pioneer species of Betula and Populus and then replaced by Picea, Pinus, and Quercus, implying an enhanced EASM and increased temperature and precipitation. During the period of 7300–5000 cal. yr BP, warm-fitted trees became expanded and widespread, indicating a climax community of mixed broadleaf-conifer forest and warm and humid climate with higher temperature and sufficient precipitation and the strongest period of EASM. From 5000 to 1600 cal. yr BP, Pinus pollen increased, but Quercus pollen decreased, showing the breakup of the climax community and the recession of the EASM. Since 1600 cal. yr BP, under the threats of land reclamation and deforestation, forest cover sharply decreased, and mountain grass lands were developed. The EASM changes inferred from pollen record of Gonghai Lake were asynchronous to the oxygen isotope records of stalagmites from southern China. We suggest that the existence of remnant Northern Hemisphere ice sheets and relative low sea levels might hampered the northward penetration of the EASM in early Holocene, which caused the maximum monsoon precipitation to reach northern China until mid-Holocene.

Indicator pollen taxa of human-induced and natural vegetation in Northern China

Research on modern pollen assemblages of human-induced vegetation is conducive to extracting human impact information, and provides basis for determining human impact intensity. The use of 189 surface soil pollen samples from human-induced and natural vegetation shows that there were significant discrepancies of indicator pollen taxa and human impact intensity between different vegetation types in Northern China. The results demonstrate that forest and grassland pollen assemblages are dominated by natural vegetation pollen taxa, which show little effect from human impact. Farmlands are dominated by Cereal Poaceae pollen. Cultivation methods, climate conditions and human impact intensity are the main reasons that cause discrepancy in different regions. Uncultivated lands could be effectively distinguished based on common human-companion plant pollen types and certain amount of crop pollen, which display the first step of secondary succession from human-induced to natural vegetation. Indicator species analysis shows that Cereal Poaceae, Trilete spore, Humulus and Brassicaceae indicate farmlands; weeds Poaceae, Chenopodiaceae, Ranunculaceae and Selaginella sinensis indicate uncultivated lands; grasslands have the largest number of indicator pollen taxa, in which Convolvulaceae, Artemisia, Asteraceae, Liliaceae, Polygonaceae, and Nitraria pollen have the highest indicator values; in forests, Betula, Larix and Quercus have the highest indicator values with statistical significance. Meanwhile, Human Influence Index (HII) values can be used to differentiate human-induced and natural vegetation. The calibration model of pollen-HII based on the weighted averaging plus partial least squares (WA-PLS) method exhibits a good statistical performance (R2 = 0.69), and the HII values have the same trend of change with Cereal Poaceae percentage. Our results confirm that pollen from human-induced vegetation can provide reliable estimates of HII, which provides a good reference for restoring human impact intensity in fossil pollen assemblage.

A novel procedure for pollen-based quantitative paleoclimate reconstructions and its application in China

Traditionally, the evaluation of pollen-based quantitative paleoclimate reconstructions focuses on the ability of calibration sets to infer present climatic conditions and/or the similarity between fossil and modern assemblages. Objective criteria for choosing the most appropriate climate parameter(s) to be reconstructed at a specific site are thus lacking. Using a novel approach for testing the statistical significance of a quantitative reconstruction using random environmental data, in combination with the advantageous large environmental gradients, abundant vegetation types and comprehensive modern pollen databases in China, we describe a new procedure for pollen-based quantitative paleoclimatic reconstructions. First, the most significant environmental variable controlling the fossil pollen assemblage changes is identified. Second, a calibration set to infer changes in this targeted variable is built up, by limiting the modern ranges of other environmental variables. Finally, the pollen-based quantitative reconstruction is obtained and its statistical significance assessed. This novel procedure was used to reconstruct the mean annual precipitation (Pann) from Gonghai Lake in the Lvliang Mountains, and Tianchi Lake in the Liupan Mountains, on the eastern and western fringe of the Chinese Loess Plateau, respectively. Both Pann reconstructions are statistically significant (p<0.001), and a sound and stable correlation relationship exists in their common period, showing a rapid precipitation decrease since 3300 cal yr BP. Thus, we propose that this procedure has great potential for reducing the uncertainties associated with pollen-based quantitative paleoclimatic reconstructions in China.

The manifestation of the Younger Dryas event in the East Asian summer monsoon margin: New evidence from carbonate geochemistry of the Dali Lake sediments in northern China:

The processes and mechanisms of the Younger Dryas (YD) event in the modern northern margin of the East Asian summer monsoon (EASM) are still heatedly debated. This study presents new high-resolution (~25 years) records of elements and stable isotopes of <38-µm calcites from a sediment core from Dali Lake in order to investigate the climatic change in the EASM margin at the last glacial–interglacial transition. The <38-µm calcites in the Dali Lake sediments are cubical or rhombohedral, implying that they are predominated by endogenic calcites precipitated within the water body of the lake. High values of Ca and Mg concentrations of the endogenic calcites are interpreted as strong evaporation and low dissolved CO2 concentration of the lake water related to high regional temperature. Concurrent increases in δ13C and δ18O values of the endogenic calcites are interpreted as the result of intensified evaporation associated with high temperature or low precipitation in the region. These data indicate that the climate in the Dali Lake region was relatively warm and wet from 15,500 to 12,800 and from 11,550 to 10,000 cal. yr BP, and cold and dry from 12,800 to 11,550 cal. yr BP, which was generally supported by the evidence from the data of sedimentary organic matter from the same sediment core. In addition, the abruptness of the temperature change in the Dali Lake region from 12,800 to 11,550 cal. yr BP could be corresponded, within age uncertainties, to the YD cold reversal occurring over northern high latitudes. The atmospheric coupling between the North Atlantic region and the EASM margin was proposed as the dominant pattern influencing the climatic change in the Dali Lake region during the YD event.

Holocene environmental changes and human activity at the Jijitan site in the Nihewan Basin, China:

Knowledge of the relationship between human activities and environmental changes during the Holocene is important for understanding the survival and development of prehistoric humans. Using AMS 14C dating and pollen and charcoal analysis, we reconstructed the history of environmental changes and human activity during the Holocene at the Jijitan site in the Nihewan Basin. During 13,000–7500 cal. yr BP, the East Asian Summer Monsoon (EASM) gradually intensified and precipitation increased, the vegetation changed from temperate grassland to wooded grassland and human activity remained at a low intensity. During 7500–5500 cal. yr BP, the EASM reached peak level, and the climate was warm and humid interval, and human activity intensified substantially. Within this interval, from 6600 to 6000 cal. yr BP, the forests were burned to increase the area of farmland. After 5500 cal. yr BP, the forest cover continued to gradually decrease due to the combined influences of the weakening EASM and increasing human activity. Comparison with regional climate records shows that the optimum period of forest development in northern China was approximately 8000–5000 cal. yr BP, indicating that the EASM reached a peak level in the mid-Holocene, which we suggest may have been due to the reduced influence of the high northern-latitude ice sheets and rising global sea level, rather than in the early Holocene.

Spatial patterns of vegetation and climate in the North China Plain during the Last Glacial Maximum and Holocene climatic optimum

Reconstructing the spatial patterns of regional climate and vegetation during specific intervals in the past is important for assessing the possible responses of the ecological environment under future global warming scenarios. In this study, we reconstructed the history of regional vegetation and climate based on six radiocarbon-dated pollen records from the North China Plain. Combining the results with existing pollen records, we reconstruct the paleoenvironment of the North China Plain during the Last Glacial Maximum (LGM) and the Holocene Climatic Optimum (HCO). The results show that changes in the regional vegetation since the LGM were primarily determined by climatic conditions, the geomorphic landscape and by human activity. During the LGM, the climate was cold and dry; mixed broadleaf-coniferous forest and deciduous-evergreen broadleaf forest developed in the southern mountains, and cold-resistant coniferous forest and mixed broadleaf-coniferous forest were present in the northern mountains. The forest cover was relatively low, with mesophytic and hygrophilous meadow occupying the southern part of the plain, and temperate grassland and desert steppe were distributed in the north; Chenopodiaceae-dominated halophytes grew on the exposed continental shelf of the Bohai Sea and Yellow Sea. During the HCO, the climate was warm and wet; deciduous broadleaf forest and deciduous-evergreen broadleaf forest, with subtropical species, developed in the southern mountains, and deciduous broadleaf forest with thermophilic species was present in northern mountains. Although the degree of forest cover was greater than during the LGM, the vegetation of the plain area was still dominated by herbs, while halophytes had migrated inland due to sea level rise. In addition, the expansion of human activities, especially the intensification of cultivation, had a significant influence on the natural vegetation. Our results provide data and a scientific basis for paleoclimate modelling and regional carbon cycle assessment in north China, with implications for predicting changes in the ecological environment under future global warming scenarios.

Phenotypic plasticity of the gastropod Melanoides tuberculata in the Nile Delta: A pollution-induced stabilizing selection

To understand the effect of metal pollution on the speciation process, we conducted comparative analyses of six populations of the gastropod Melanoides tuberculata, which dominated the Manzala lagoon (Nile Delta, Egypt). Geometric morphometric analysis was implemented to quantify the phenotypic plasticity of the species. The results from both Canonical Variate Analysis and Relative Warp indicated an overall decrease in the morphological breadth of M. tuberculata in the polluted sites. The favored phenotypes in the polluted sites have moderate whorl section, moderate ovate aperture, less-prominent radial ornament, and overall moderate-spired shells. Lack of morphological variations and dominance of intermediate phenotypes in the polluted sites indicate that stabilizing selection is driving the morphological pattern of this species. Moreover, analysis by using the partial least square model confirmed that metal pollution is the major predictor of the observed shape variations, whereas other biotic/abiotic traits are a minor predictor.

A Review of Relative Pollen Productivity Estimates From Temperate China for Pollen-Based Quantitative Reconstruction of Past Plant Cover

Model-based quantitative reconstruction of past plant cover in Europe has shown great potential for: (i) testing hypotheses related to Holocene vegetation dynamics, biodiversity, and their relationships with climate and land use; (ii) studying long term interactions between climate and land use. Similar model-based quantitative reconstruction of plant cover in China has been restricted due to the lack of standardized datasets of existing estimates of relative pollen productivity (RPP). This study presents the first synthesis of all RPP values available to date for 39 major plant taxa from temperate China and proposes standardized RPP datasets that can be used for model-based quantitative reconstructions of past plant cover using fossil pollen records for the region. We review 11 RPP studies in temperate China based on modern pollen and related vegetation data around the pollen samples. The study areas include meadow, steppe and desert vegetation, various woodland types, and cultural landscapes. We evaluate the strategies of each study in terms of selection of study areas and distribution of study sites; pollen- and vegetation-data collection in field; vegetation-data collection from satellite images and vegetation maps; and data analysis. We compare all available RPP estimates, select values based on precise rules and calculate mean RPP estimates. We propose two standardized RPP datasets for 31 (Alt1) and 29 (Alt2) plant taxa. The ranking of mean RPPs (Alt-2) relative to Poaceae (= 1) for eight major taxa is: Artemisia (21) > Pinus (18.4) > Betula (12.5) > Castanea (11.5) > Elaeagnaceae (8.8) > Juglans (7.5) > Compositae (4.5) > Amaranthaceae/Chenopodiaceae (4). We conclude that although RPPs are comparable between Europe and China for some genera and families, they can differ very significantly, e.g., Artemisia, Compositae, and Amaranthaceae/Chenopodiaceae. For some taxa, we present the first RPP estimates e.g. Castanea, Elaeagnaceae, and Juglans. The proposed standardized RPP datasets are essential for model-based reconstructions of past plant cover using fossil pollen records from temperate China.