Vegetation And Environment Changes Of The Typical Wetland Since 4700 Yr BP In The Northern Part Of Xinjiang

The Holocene period is called the"Anthropocene"because of more human activities during the period,which makes it an important period to study the impact of human activities on the ecological environment.Wetland,known as the"kidney of the earth",is an important biological habitat on the earth.However,due to the intensification of global climate change and human activities,many wetlands in northern Xinjiang have undergone significant degradation and even leading to the endangerment or extinction of many biological species.For example,Betula halophila,which once grew in the Salt Lake area of Alahak,has been extinct for many reasons,such as habitat changes.In this paper,northern Xinjiang was selected as the research area.Atmospheric pollen samples for three consecutive years?September 2012-September 2015?were collected in the Ebinur Lake Wetland National Nature Reserve,in order to explore the relationship between atmospheric pollen and meteorological factors.This provides strong evidences for reconstructing paleovegetation and paleoclimate during the historical period by using fossil sporopollen.In addition,the representative profile I drilled at the shore of Alahak Salt Lake,60 kilometers west of Altay City,was tested for sporopollen,organic carbon,inorganic carbon,electrical conductivity and AMS¹⁴C dating.Based on the sporopollen data with higher temporal resolution and other environmental data obtained in the same period,the sedimentary environment and paleoclimate change of the Alahak Salt Lake since 4730 cal.yr BP were reconstructed by using the weighted averaging partial least squares?WAPLS?model.The results showed that the relationship between atmospheric pollen concentration and climatic factors varied with season in Ebihu National Nature Reserve.Temperature mainly affected pollen concentration in spring and autumn,precipitation mainly affected pollen concentration in summer and autumn,and the peak period of pollen had strong regional characteristics.The flowering period of arbor plants is mostly in spring,while that of herbs is usually in summer or early autumn.Among the three-year atmospheric pollen samples collected,the concentration of atmospheric pollen was the lowest in the first year,the highest in the second year and the middle in the third year,which is related to the fact that the precipitation in the second year is much lower than that in the other two years.When the precipitation is too high or continuous,the transmission distance of sporopollen is limited because of the hydration of sporopollen.When the maximum wind speed in the second year was obviously higher than that in the other two years,the pollen which fell into the surface soil earlier could be blown to the atmosphere again under the condition of high wind speed,thus increasing the concentration of atmospheric pollen.In the study area,the flowering period of Betula mostly concentrated from mid-April to mid-May.The optimum flowering temperature was 18?,and the precipitation was positively correlated with the pollen concentration of Betula.Wind speed did not have a significant impact on birch pollen collection,possibly because the installed atmospheric pollen collector is close to the existing birch stand.The peak period of spruce pollen appeared from mid-May to mid-June,and the change of peak period was closely related to the temperature rise before germination and precipitation during its growth.At the same time,spruce pollen is more affected by wind speed than other pollen taxa.It is noteworthy that the spores of Thelypteris are easier to deposit in the original dispersed land because they are close to the ground,and are less likely to be affected by wind,which makes it difficult to spread in the air over a long distance.It is inferred that once the spores of Thelypteris are accurately identified in the profile in the wetland environment,they will grow in the area at that time.In arid and semi-arid areas,Chenopodiaceae is the main component of desert vegetation,and pollen yield is high and easy to preserve,so it is the main reason for the increase of airborn pollen concentration in winter.There is a good correspondence between the pollen assemblages of plants in the surface soil of Alahak Salt Lake and the species of local living plants.The pollen assemblages of plants in the surface soil are mainly xerophytic and halophytic herbaceous Chenopodiaceae plants.Pine and birch pollen are occasionally found.Based on the characteristics of sporopollen assemblages and paleoclimate restoration in the Alahak Salt Lake profile,it is concluded that there are four stages of environmental change in the past4730 years.In the first stage,from 4,730 to 3,600 cal.yr BP,the vegetation was dominated by Compositae-Chenopodiaceae steppe and sporadic forest?composed mainly of Pinus,Picea,and Betula?in the surrounding area.Aquatic plants?Typha and Sparganium?grew at the same time.This period also had a relatively high percentage of trees and high diversity of vegetation types.The sediment in the lake indicated that surrounding soils were lightly salinized.From 4730 to 4030 cal.yr BP?2780 BC-2080BC?,the area experienced a significant cooling event for 700 years,and the climate changed from warm-dry to cold-wet.Since 4030 cal.yr BP,the temperature gradually rose and the precipitation gradually decreased,but there were still short-term climate fluctuations during the period.In the second stage,between 3,600 and 790 cal.yr BP,the number of trees growing in this area decreased.Vegetation was dominated by grass during that time.The number of Chenopodiaceae plants increased significantly,even surpassed Compositae plants,and became the main plant type.The abundance of aquatic plants decreased significantly,indicating that the water area decreased relative to the previous period.The climate was still warm and dry in general,but there were two short-term cooling phenomena from 1550 BC to 1360 BC and 1120 BC to 1010 BC,which coincided with the historical records of the cooling in Xia,Shang and Zhou Dynasties.In the third stage,from 1,660 to 790 cal.yr BP,the abundance of Abies and Sparganium peaked during this time.The climate became colder and wetter compared with the second stage,with occasional occurrences of Abies.In the fourth stage,after 790 cal.yr BP,the region was primarily vegetated by Chenopodiaceae—the typical vegetation type of desert grassland.Meanwhile,the salinity of the soil sharply increased.It is worth mentioning that there was a short-term cooling phenomenon during 1560-1630 AD?390-320 cal.yr BP?,which is very similar to the beginning time of the Little Ice Age.Up to 180 cal.yr BP,arboreal plants have decreased or even disappeared.The landscape of Alahak Salt Lake is similar to that of modern landscape,and has become a desert landscape with a large amount of saline and alkali substances on the soil surface.During the sporopollen statistics of the profile,the number of charcoal pieces in each sample was also counted.It was found that the highest charcoal abundance occurred in zone IV,which may indicate strong human activities after 790 cal.yr BP.Because of the influence of soil salinity and human activities,the plant species in Alahak Salt Lake area are reduced.It is unreliable to use A/C ratio to indicate the change of humidity and dryness in the study area.However,using broad-leaved forest/coniferous forest?Br/Co?as temperature index,SFI=?Artemisia+Chenopodiaceae+Ephedra+Caryophyllaceae?/Arbor as drought index is credible.