Mid-late Holocene Vegetation Spacial Variations In Songnen Grasslands And Their Response To Climate Changes

The Songnen Grassland is located in the Northeast China and also at the easternmost edge of the Eurasian Steppe.The ecological environment in Songnen grasslands is fragile and it is very sensitive to climate change.Reconstructing the evolution characteristics of past vegetation since the Holocene in this area is very important for understanding the vegetation dynamics on the eastern edge of the Eurasian Steppe and their response to climate change.In the past few decades,some scholars have used the pollen and phytolith records extracted from the aeolian palaeosoil profile to conduct studies on the evolution of paleovegetation.However,due to the lack of high-resolution sedimentary carriers,and the palaeovegetation reconstruction results varies when using different indicators leading to the fact that the paleovegetation patterns in this area since the Holocene are still unclear and controversial.In this paper,high-resolution samples were collected from both Dabusu peatlands and Qianliusicun aeolian paleosol profile in the central part of the Songnen Grassland.Pollen,TOC,carbonate contents,humification degree,grain size,and diatoms are used for palaeovegetation and palaeoclimate reconstruction.By integrating previous climate reconstruction research results,we systematically reconstructed the distribution characteristics and evolution patterns of palaeovegetation in the Songnen Grassland since the mid-Holocene,and then discussed its response to regional climate change.The following main conclusions were obtained:（1）For the first time,this study provides the high-resolution pollen datasets from both peatlands（with an average resolution of 24 years）and aeolian sand-palaeosol profiles（with an average resolution of 180 years）in the Songnen grasslands to reconstruct the palaeovegetation dynamics.For pollen datasets extracted from both Dabusu peatlands and Qian64cun aeolian paleosol profile,the proportions of herb pollen were 78.8%and 83.3%,respectively.In addition,the pollen of Artemisia and Chenopodiaceae accounted for a high proportion.There are also differences for the above two pollen datasets.The pollen concentration in the Dabusu peat core is relatively high,and aquatic plant pollen and fern spores are also very common.Among the herbaceous pollen,Artemisia percentages in the Dabusu peat core is slightly higher than the Chenopodiaceae proportions.For pollen data from Qian64cun section,the pollen content of Chenopodiaceae was slightly higher than that of Artemisia.Moreover,Poaceae and Cyperaceae pollen percentages are high in Dabusu peat core while Asteraceae,Ephedra and Tribulus pollen are more abundant in Qian64cun section.For the arboreal pollen,Pinus occupies an absolute advantage in both the Dabusu peat core and the Qian64cun section.Quercus pollen is abundant in the Dabusu core and Salix pollen is more abundant in the Qian64cun section.（2）This study,for the first time,explores the spatial differentiation of palaeovegetation in the different geographical units of the Songnen grassland since the mid-Holocene by integrating the previous palaeovegetation research results and deepens the academic understanding of the paleovegetation features of the Songnen grassland.The study found that since the mid-Holocene,the palaeovegetation in the Songnen grassland has been distributed in vertical space（on the top of the sand dunes and on the inter-dune lowlands）and there are significant differences due to the dynamic differences in water balance between different micro-geomorphological units.Between 9000–6000 cal yr BP,Chenopodiaceae and Artemisia mainly grew on the top of and dunes,while on the inter-dune lowlands,aquatic and hygrophytes such as Typha and Cyperaceae had an absolute quantitative advantage.During 6000–3000 cal yr BP,the number of Asteraceae increased significantly and became the dominant plant types in the sand dunes,while the number of aquatic plants decreased significantly in the inter-dune lowlands accompanied by the increase of the hygrophytes such as Cyperaceae and ferns.After3000 cal yr BP to the present,Chenopodiaceae,Ephedra and Ulmus plants expanded on the top of the sand dunes,while on the inter-dune lowlands,the numbers of Chenopodiaceae and Poaceae plants increased to varying degrees along with the decrease of wet and aquatic plants.To sum up,compared with the top of the sand dunes,the vegetation types in the inter-dune lowlands of the Songnen grasslands are more abundant,and the coverage and diversity are also relatively high.Meanwhile,aquatic and hygrophytes were growing on the inter-dune lowlands while the drought-tolerant plants mostly distributed on the top of the sand dunes.（3）In this study,the REVEALS model was firstly utilized for quantitatively palaeovegetation reconstruction of central and eastern marginal Songnen grasslands by using three pollen datasets after verifying the applicability of the REVEALS model to the study area.The research results show that since the mid-Holocene,the palaeovegetation in the central area of the Songnen grassland were dominated by steppes and dry steppes while the coverage of broad-leaved forests and coniferous forests is very low.While in the eastern marginal region of the Songnen grasslands,broad-leaved forests and dry steppes were alternatively dominated the area in different time periods.Between 7200 and 4800 cal yr BP,steppes were widely distributed in the central area of the Songnen grasslands while broad-leaved forests were dominated in the eastern marginal region of the Songnen grasslands.During 4800–2700 cal yr BP,the central area of the Songnen grassland was occupied by the rapidly expanding dry steppes,while the coniferous forest cover was relatively high in the eastern marginal area of the Songnen grassland.Between 2700–900 cal yr BP,the dry steppe expanded simultaneously in the central and eastern marginal area of the Songnen grasslands becoming the main vegetation type in the two areas.After 900 cal yr BP,the steppe and broad-leaved forest re-dominated the central and eastern marginal area of the Songnen grasslands.（4）This study firstly reconstructed the high-resolution Dabusu peatland water level fluctuation process by using multi-proxy analysis including diatoms,grain size and Ca CO₃contents to indicate the paleoclimate changes in the Songnen grassland region since the mid-Holocene.And we further explored he sensitivity of paleovegetation dynamics to the climate changes.The research results show that the water level of the Dabusu peatland was at the highest level in the early mid-Holocene indicating that the East Asian summer monsoon was strong,and the regional climate was warm and humid.Then it gradually decreased and remained at a low level indicating that the East Asian summer monsoon gradually declined,and the climate tended to be cold and dry.In the late Holocene,the water level of Dabusu peatlands rose again slightly indicating that the East Asian summer monsoon began to strengthen,and the climate became warm and humid.By comparing our palaeovegetation and palaeoclimate reconstruction results with regional climate data,it can be seen that the vegetation evolution of the Songnen grassland is mainly driven by the East Asian summer monsoon,and it can also sensitively response to the short-term climate events such as the 4.2 ka BP events and Ice Rafted Debris cooling events.