| Located at the northern edge of East Asian monsoon area,Northeastern China is very sensitive to past global changes.Thus,reconstructing the paleo-vegetation in NE China is essential for our understanding of vegetation dynamics in East Asia and its response to climate change.In this paper,we applied the phytolith analysis as the main analytical method.Firstly,by analyzing 135 plant species,189 typical plant community samples and 358 topsoil samples in Northeast China,we systematically explore the differences in source and preservation degree of different phytolith types among these typical plant communities,and build a quantitative relationship between the phytolith assemblages in topsoil and numerical characteristics of the corresponding plant communities.In addition,we establish the topsoil-phytolith standard that can identify typical plant communities and construct the empirical function that suitable for vegetation identification in this area.Then,based on this phytolith standard,the paleovegetation evolution history of three sections(LDBS,SY,HN,510 samples)in Northeast China was quantitatively reconstructed,and then the spatial and temporal evolution of the paleovegetation pattern in the study area was reconstructed quantitatively.The main conclusions are as follows:(1)In Northeast China,the phytolith production and morphological characteristics of common herbaceous plants and arboreal plants are significantly different.The production of herb phytolith is generally higher than arboreal phytolith,and the characteristic value of 21 herb phytolith types also have better classify function than that of arboreal plants(14 types).The typical plant communities in the region also present different production and assemblage characteristics of the community phytolith.Therefore,three types of grassland communities(Stipa steppe,L.chinensis meadow,weedy meadow)and five types of forest communities(warm temperate deciduous broad-leaved forest,P.koraiensis mixed broadleaf-conifer forest,broad-leaved mixed forests,P.koraiensis-Picea-Abies forest,Mountainous poplar-birch forest)can be identified by community phytolith assemblages.Meanwhile,the common phytolith climatic index(Ic)and tree cover index(W/G)can be used to semi-quantitatively indicate changes in vegetation types of the region.In contrast,the dry and wet index(Iph)is completely unsuitable in Northeast China.The difference of phytolith assemblages of typical plant communities and their index adaptation in Northeast China provide an important basis for further evaluation of topsoil phytolith-based paleovegetation reconstruction.(2)The topsoil phytolith assemblage characteristics are various in different typical plant communities in Northeast China.Based on the phytolith assemblages and indexes,the modern topsoil phytolith-standards for distinguishing different plant communities are established.The discriminant function constructed by using the topsoil phytolith assemblages can accurately identify the grassland community,forest steppe and forest community.As for the grassland community,the Stipa steppe and L.chinensis meadow can be well distinguished by the discriminant function.However,the discriminative accuracy rate is relatively low in terms of the two types of meadow.As for the forest communities,this discriminant function can be used to distinguish all the forest communities in a relatively high accuracy except for the confusion of P.koraiensis mixed broadleaf-conifer forest and broad-leaved mixed forests.In addition,it is possible to semi-quantitatively distinguish the types of plant communities by determining the value ranges of the three topsoil phytolith-index types in different plant communities.The forest and grass index(W/G)is bounded by 0.23 and 0.40,which can indicate grassland communities,forest grasslands,and forest communities respectively.The herb index(Ig)can be used to distinguish Stipa steppe and L.chinensis meadow with 0.60 as the boundary.The type of community is more likely to be identified as non-grass meadow when the Ig index is less than 0.35.As for the Iw°index,it can be used to distinguish five types of forest communities,namely warm temperate deciduous broad-leaved forest(<0.38),P.koraiensis mixed broadleafconifer forest(0.38-0.58),broad-leaved mixed forests or P.koraiensis-Picea-Abies forest(0.58-0.85),Mountainous poplar-birch forest(>0.85).The exploration of the topsoil phytolith assemblages characteristics and their identification criteria in the typical plant communities can provide modern phytolith reference standards for the accurately reconstruction of paleovegetation types in the study area.(3)There is a certain correspondence and significant quantitative deviations between topsoil phytoliths and corresponding plant communities in their morphotypes,concentrations and contents in Northeast China.Based on this relevance,this paper quantitatively defines the representativeness of topsoil phytolith types in grassland and forest areas,respectively.According to the degree of representativeness,the phytolith types can be divided into four categories: associated types,over-represented types,under-represented types,and special types.Most of the same morphotype have different degree of representativeness in forest and grassland areas.Moreover,this paper proposes a correction coefficient R-value for effectively correcting the representative deviations of the topsoil phytoliths.This paper also established the quantitative relationship between topsoil phytoliths with the numerical characteristics of the plant communities in the grassland and forest areas of the Northeast China.In the local region,the regression functions are constructed based on the ratio of topsoil phytoliths can accurately quantify the quantitative changes of coverage and diversity of plant communities.We also find that the forest biomass ratio in forest communities also has a very significant linear regression relationship with the topsoil phytolith index.According to this relationship,the appearance characteristics of forest communities,such as community structure and forest canopy closure can be speculated.At the regional level,there is a significant nonlinear fitting relationship between the corrected tree cover index and the regional vegetation cover index(NDVI).Therefore,the tree cover index can be used to quantitatively reconstruct the vegetation coverage within the region.The results of this study can greatly improve the accuracy of the phytolith indicators,and also provide important modern phytolith standards for comprehensive and fine restoration of paleo-vegetation patterns.(4)Based on the standard of the modern phytoliths,this paper quantitatively reconstructs the evolutionary history of paleo-vegetation of the three regions(LDBS,SY,HN)since the late glacial period.The vegetation types in the LDBS area since the Holocene were mainly grassland communities.The vegetation had experienced various stages: the alternatively stage of the sparse Stipa steppe with richness species and weedy meadow--denser Stipa steppe with less species--denser weedy meadow with richer species--denser L.chinensis meadow with richness of C3 species.As for the SY area,the vegetation types had been dominated by forest grasslands,and had been converted to forest types in some periods since the Middle Holocene.The dynamic succession process of paleo-vegetation was: sparser forest communities with richer species--dense forest grasslands with rich species--dense forest grasslands with singular species--sparse forest communities with richer species.The HN was dominated by the forest communities recorded by the phytolith indicators since the late glacial period.The dynamic evolution process of forest communities was: sparse Mountainous poplar-birch forest with singular species--sparser P.koraiensis mixed broadleaf-conifer forest with more diverse species of larch--dense P.koraiensis mixed forest and broad-leaved mixed forests with rich species--dense P.koraiensis mixed broadleaf-conifer forest with diverse species.The quantitative reconstruction of paleo-vegetation patterns of the three regions based on the phytoliths were consistent with other paleo-vegetation results in the region,and the vegetation patterns were sensitively respond to the climate change.The results verify the accuracy of using modern phytolith standard in paleo-vegetation reconstruction,and open up the quantification process of phytolith-paleovegetation reconstruction and also provide basic data for the study of evolutionary paleo-vegetation pattern in the region.(5)Combined with the above results of paleo-vegetation reconstruction,the spatial and temporal patterns of paleo-vegetation in Northeast China were further reconstructed.During the Holocene,the vegetation evolution in Northeast China has obvious periodic variation characteristics.The significant periodicities of vegetation evolution in different regions has similarity with the vegetation cycle of 1500 a,500a and 150 a,and the relative weak vegetation cycle is different in different regions due to the influence of natural geographical factors.According to the above vegetation cycle,the vegetation in northeastern region is at a stage where the relative amount of forest and grass is relatively low.In the horizontal space,the vegetation distribution pattern and the forest-grassland transitional boundary in Northeast China have obvious fluctuations since 5.0 ka BP.The migration process of the forest-grassland transitional zone is that the transitional zone first migrated to the east in a small range(4.0 ka BP),then the west boundary of the forest-grassland moved westward obviously(3.0 ka BP).After that,the eastern boundary of the forest-grassland moved eastward obviously and the transition zone reached the largest range(2.0 ka BP),then the western boundary of grassland migrated to the east(1.0 ka BP).Until now,the western boundary of the forest-grassland has been characterized by migration to the west.Since the Middle Holocene,the forest-grassland has reached the westernmost position at 3.0 ka BP.The maximum migration distance of forestgrassland is about 150-200 km.The migration distance of forest-grassland to grassland area is obviously higher than that of forest area.The vegetation evolutionary periodicities and distribution pattern in Northeast China are highly consistent with global climate change.The dynamics of vegetation evolution in this region is very sensitive to climate change,and the dynamic causes of climate change such as solar radiation and North Atlantic circulation are also the driving factors for vegetation evolution in Northeast China. |
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