Middle to late Holocene plant cover variation in relation to climate, fire, and human activity in the Songnen grasslands of northeastern China.

Introduction: For future vegetation projections and conservation planning in grassland ecosystems, accurate estimates of past plant cover changes in grassland composition and their responses to the various driving factors are essential. This study quantitatively reconstructs the past regional plant cover in the Songnen grasslands (northeastern China) and explores the relative importance of climate, fire, and human activity on vegetation dynamics. Methods: For this purpose, the Regional Estimates of Vegetation Abundance from Large Sites (REVEALS) model is applied to three pollen records from two areas, two in the center of the Songnen grasslands and one located in an area marginal to the grasslands. Results: Results from the most reliable REVEALS scenarios show that from the mid-Holocene, steppe (mean cover 40.6%) and dry steppe (mean cover 54.2%) alternately dominated the central part of the Songnen grasslands while the marginal grasslands were mainly characterized by alternating broadleaved forests (mean cover 26.3%), coniferous forests (mean cover 41.9%) and dry steppes (mean cover 30.1%). Discussion: By comparing the plant cover results with previous published regional climate, fire and human activity records, the results show that long term vegetation dynamics were mainly driven by East Asia Summer Monsoon (EASM) and the related precipitation variations, but was also affected by fire frequency and human activity. Moreover, vegetation evolution was sensitive to abrupt cooling events including the 4.2 ka BP and stacked ice-rafted debris (IRD) events; the change from steppe to dry steppe, for example, was driven by these abrupt climate changes. Fire events can alter the original vegetation stability allowing the vegetation to respond rapidly to climate changes while human activity merely has limited influence on vegetation changes.

Responses of phytoliths in topsoil samples to temperature variation in temperate region.

Modern phytolith research is the premise to accurately reconstruct the regional paleovegetation and paleoclimate. It is thus particularly important to study the spatial distribution characteristics of the phytolith assemblages and the response of the phytolith to environmental factors. In this study, we collected 54 topsoil samples along the isohyet of 600 mm in Northeast China to examine the distribution patterns of phytolith along the gradient of single environmental factor. With this special sampling method, we targeted to reveal the responses of phytolith assemblages in the topsoil to the variation of temperature. The results showed that there were a large number of phytoliths with various morphotypes in the 54 topsoil samples in Northeast China. The phytolith morphotypes were the same, including elongate, lanceolate, blocky, cuneiform, tabular, saddle, rondel, bilobate, trapeziform polylobate, prism, silicified stomata, conduit, papillae, sclereid and carinate. In gene-ral, the contributions of elongate and lanceolate were high; the contributions of short cell phytoliths, blocky, tabular and cuneiform were small relatively; the contributions of the other types of phytoliths were less than 1.0% and appeared sporadically in the topsoil samples. Besides, the phytolith assemblages of the topsoil samples were different among Xiaoxing'an Mountains, Songliao Plain and Liaoxi hills. Combined with the discriminant analysis, the samples from the three regions could be effectively separated based on the phytoliths assemblages, with a total accuracy of 94.4%. Specifi-cally, the average percentage of elongate, lanceolate, prism was the largest in Xiaoxing'an Mountains, and was the smallest in Liaoxi hills; the percentage of saddle, bilobate, papillae was the largest in Liaoxi hills and the smallest in Xiaoxing'an Mountains; the percentage of blocky, tabular, cuneiform was the largest in Songliao Plain. Meanwhile, the percentages of the elongate, lanceolate, prism, saddle, bilobate, papillae, blocky, tabular, cuneiform were significant diffe-rences among the three regions. There were significant negative correlations between the percentages of elongate, lanceolate, prism and the annual average temperature, and positive relationships between the percentages of the blocky, tabular, cuneiform, bilobate, papillae and the annual average temperature. However, the percentage of saddle, rondel and trapeziform polylobate did not change with increasing annual average temperature.

Translocation of Phytoliths Within Natural Soil Profiles in Northeast China.

Phytoliths are a reliable paleovegetation proxy and have made an important contribution to paleoclimatic studies. However, little is known about the depositional processes affecting soil phytoliths, which limits their use for paleoclimate and paleovegetation reconstructions. Here, we present the results of a study of the vertical translocation characteristics of phytoliths in 40 natural soil profiles in Northeast China. The results show that phytolith concentration decreases within the humic horizon of the soil profiles and that ∼22% of the phytoliths are translocated below the surface of the studied soils. In addition, we find that the translocation rate of phytoliths varies markedly with phytolith type and that phytolith size and aspect ratio also have a significant effect. Phytoliths with length >30 µm and with aspect ratio >2 and those with length <20 µm and aspect ratio <2 are preferentially translocated compared to those with length >25 µm and aspect ratio <2. Our results demonstrate that differential translocation of phytoliths within soil profiles should be considered when using soil phytoliths for paleoclimate and paleovegetation reconstruction.