Phytolith Assemblages as a Promising Tool for Quantitative Canopy Coverage Reconstruction in Subtropical Forests, China.

This study investigates the reliability of phytolith assemblage analysis for characterizing subtropical vegetation and explores the potential for using these modern phytolith-vegetation relationships for paleoenvironmental interpretation in southeastern China. The samples were collected from five common subtropical vegetation communities in the Daiyun Mountains, southeastern China, with the above-ground vegetation recorded at each plot. Constrained ordination analysis was used to determine the most important factor governing the variations in phytolith assemblages that could be quantitatively reconstructed with weighted averaging partial least squares regression (WAPLS). The relationship between modern phytolith assemblages and the parent vegetation, as well as production, dispersal, and taphonomic processes, was discussed. Results demonstrated that the main subtropical biomes in southeastern China could be well distinguished by soil phytolith assemblages. In particular, the overall amount of tree coverage was well represented by topsoil phytolith assemblages. Grass silica short cell phytoliths (GSSCP) tended to occur in higher proportions in open habitats (shrub-meadow) at higher elevations, whereas non-grass phytolith morphotypes attained higher frequencies under mixed and broadleaf forests at lower elevations. Human-induced deforestation might increase the frequency of GSSCP within the bulk phytolith assemblage. Our results constitute the primary phytolith reference data for the subtropical zone in southeastern Asia where vegetation change during the Holocene period, particularly forest shifts, anthropogenic deforestation, and early agriculture are poorly documented.

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.

Diatom evidence for mid-Holocene peatland water-table variations and their possible link to solar forcing.

Peatlands located at the northern edge of the East Asian monsoon (EAM) are well placed to provide a terrestrial record of past climate and hydrological changes for this globally sensitive region. Here we present a middle to late Holocene, diatom-derived water-table records from a peatland in the Greater Hinggan Mountains, northeastern China. An age-depth model was achieved through AMS14C dating and Bayesian piece-wise linear accumulation modelling. The diatom-based water-table reconstructions show that the peatland water-table rose from 5100 to 3500 cal. yr BP, but fell approximately 3500 cal. yr BP. From about 2800 to 1500 cal. yr BP, the peatland water-table stabilized. After about 1500 cal. yr BP, several rapid hydrological shifts, which correspond with global climate anomalies such as ice-rafted debris (IRD) events, were registered in the reconstructed water-tables. Compared with other paleoclimate records in East Asia, the general trend of peatland water-table fluctuations follows the variations in the East Asian summer monsoon (EASM) intensity. Spectrum analysis of the water-table profile yielded a statistically significant periodicity of 470-year that may be related to the "~500-year" inherent solar irradiation cycles. In addition, positive correlation between the peatland water-table levels and cosmic-isotope-reconstructed sunspot numbers underscores the role of the sun in regulating hydrological processes in the EASM margin area. The data suggest that the regional climate and hydrological variations at the EASM margin were first triggered by changes in solar output, but may have been amplified by interactions with oceanic and atmospheric circulations.