Evolution Process Of Wetlands And Its Driving Factors In Mid-High Latitude,Northeast China

Mid-high latitude region is sensitive to global climate change,the Heilongjiang River Basin is located at the north margin of East Asian summer monsoon and is main distribution area of peatlands in mid-high latitude region in northeast China,however,the variation of vegetation and climate in this region is still unclear.In this study,we collected surface soil pollen samples and sediment cores along the Heilongjiang River Basin in northeast China,based on the chronology and palynological analysis,reconstructed the paleovegetation,paleoclimate and historical human activity intensity,revealed the long-term evolution characteristics of wetland ecosystem,and then explored the impacts of human activities and climate change on wetlands evolution.1)The relationships among modern pollen,vegetation,climate and human activities can help improving the reliability of reconstruction of past vegetation,regional climate and human activities based on fossil pollen records.We used a dataset of 114 surface soil pollen samples from natural vegetation(wetlands,forests and grasslands)and human-induced vegetation(farmlands and residences)along the Heilongjiang River basin in northeast China to explore the relationships among modern pollen,vegetation,climate and human activities.The results indicated that surface pollen assemblages differentiated modern vegetation well in natural and human-induced vegetation types.The wetlands were mainly composed of Cyperaceae,along with Artemisia,natural Poaceae(φ < 35 μm)and Sanguisorba.The forests were predominated by Pinus and Betula.Artemisia,natural Poaceae(φ < 35 μm)and Chenopodiaceae were the most important pollen taxa in grasslands.The farmlands were characterized by Artemisia,Aster,Chenopodiaceae,cultivated Poaceae(φ > 35 μm)and Taraxacum.The pollen assemblages of residences were composed of natural Poaceae(φ < 35 μm),Chenopodiaceae and Salix.Ordination analyses based on main pollen taxa and climatic variables and human influence index(HII)were used to determine the relationships between pollen and climate and human activity,suggesting the surface pollen assemblages were primarily influenced by the mean annual temperature(Tann)in northeast China.The statistical performance of transfer function between pollen and climatic variables and human influence index were well indicating the modern pollen assemblages could be reliably used in reconstruction of paleoclimate and historical human influence intensity in our study area.Furthermore,human-induced vegetation had high frequencies of human-companion pollen taxa,such as Chenopodiaceae,Aster,Taraxacum and cultivated Poaceae(φ > 35 μm).Pollen concentrations of human-induced vegetation were lower than natural vegetation types,which could be used as an indicator of human influence intensity.2)Pollen record in sediments could reflect vegetation variation which responded to climate change.To better understand the historical variation of vegetation and climate along the Heilongjiang River Basin during the late Holocene,based on the AMS 14 C dating,we analyzed the pollen assemblages to reconstruct vegetation history in Tuqiang(TQ)and Youhao(YH)wetland,we also used the principal component analysis(PCA)and weighted averaging partial least squares(WA-PLS)to reconstruct the temperature and precipitation and compared with other palaeoclimate records.The results show that: the climate was moderately warm and humid during the 3300 ~ 1150 cal yr BP,the vegetation was mainly Cyperaceae,with coniferous and broad-leaved mixed forests surrounding the mountains.The temperature was higher in the Greater and Lesser Khingan Mountains at 1150 ~ 1000 cal yr BP and 1000 ~ 900 cal yr BP,respectively.This period was corresponded to Medieval Warm Period,the content of broad-leaved trees increased.After that,the temperature declined,the climate was cool and humid.During the period of 600 ~ 100 cal yr BP,the climate was cold and dry,corresponding to the Little Ice Age.And the vegetation types also obviously changed that the conifers expanded,and the contents of herbaceous Cyperaceae decreased.Since about 100 cal yr BP,the climate tended to be warm which belonged to Current Warm Period,and the main vegetation type was the secondary forests.The climate along the Heilongjiang River Basin showed strong similarities with other palaeoclimatic proxy records in the East Asian summer monsoon region,and the climate was mainly controlled by the East Asian Monsoon which related to ocean-atmosphere interactions in the tropical Pacific.The spread of the Han farming culture accompanied by the territorial expansion of the Tang Dynasty to the Sanjiang Plain at around 1300 cal yr BP caused human disturbance occurred earlier in the Sanjiang Plain than the Greater and Lesser Khingan Mountains.Additionally,the anthropogenic activities obviously intensified during the past hundred years along the Heilongjiang River Basin.3)Human activities and climate are two crucial drivers that varied terrestrial ecosystem,but the interaction(or relative contribution)of the two factors is debated.To better understand anthropogenic and climatic impacts on wetland vegetation and environment,we assessed the impact of human influence on modern pollen assemblages from 74 surface soil samples(different land-use types)in northeastern China by using detrended correspondence analysis(DCA)and redundancy analysis(RDA).Based on palynological analysis,210 Pb age-depth model and weighted averaging partial least squares(WA-PLS),we reconstructed HII values of Tuqiang peatland(TQ-1)in Greater Khingan Mountains during the last 150 years.The transfer function of pollen-HII exhibited a good statistical performance(r~2 = 0.71).The reconstructed HII values demonstrated that the intensity of human activities was relatively low before 1900 AD,the population of Heilongjiang Province was less and the native inhabitants remained primitive hunt and gather activities.During the period of 1900-1950 AD,human influence intensity increased sharply and reached peak values.Wars and placer gold mining caused large numbers of people immigrated north to Heilongjiang Province,rapid population growth strengthened the human impact intensity.In addition,invaders exploited the forest resources without limit.Widescale deforestation and land reclamation destroyed vegetation landscape and reduced forest coverage seriously,which led to soil erosion and land degradation.With the foundation of new China,the implementation of forest protection policies clearly reduced the human disturbance intensity.However,due to the needs of economic development,human influence intensity increased since 1980 AD.