A 700-Year Record Of Vegetation And Rocky Desertification Evolution Based On Palynological Data Of The Karst Depression In Zhongliang Mountain,Chongqing City,China

Rocky desertification is a widespread ecological and geological environmental problem in the karst areas of southwest C hina.It has become the third largest ecological environment after desertification in the Northwest and soil erosion in the Loess Plateau,which has severely restricted the local economic and social development of karst areas in Southwest China.Vegetation degradation is an important cause and the symbol of the occurrence and evolution of rocky desertification land scape.During the evolution of rocky desertification,the composition of vegetation species gradually degenerated from tall woody plants to typical low shrubs.As the dryness of the natural environment intensified,it succeeded in the direction of xerobiochemical.The research on rocky desertification control in karst areas of Southwest China began in the 1980s.As one of the main content of rocky desertification research,the evolution process of rocky desertification not only reflects the changes in the regional ecological environment,but also affects the region's socio-economic development.Up to now,the research on the reconstruction of the paleo-ecological environment in Chongqing was mainly focused on the changes in the climate itself.However,there has been rare research involving the change of regionally ecological environment,especially in the past 1000 years,especially,the research on the pattern of ecological environmental evolution is rarely reported.Depressions are unique geomorphic units in karst areas and are ideal sites for paleoenvironmental reconstruction.Their sediments truly record detailed processes including vegetation succession,ecological evolution of karst depression,and human activities.In addition,its sedimentary profile is continuous with high resolution,which provides valuable information for the reconstruction of vegetation succession and rocky desertification evolution in this area.Therefore,we selected Zhongliang Mountain karst depression in C hongqing as the study site,and analyzed the pollen and charcoal indicators on 62 samples of depression sediment based on ¹³⁷Cs,²¹⁰Pb and AMS ¹⁴C dating to obtain the evolution process of vegetation succession,climate change,human activities and rocky desertification in the study area in the past 700 years.On this basis,a comparative analysis with regional related records was carried out,and the process of vegetation,climate change and the impact of human activities on the natural environment in the karst area of Chongqing in the past 700years have been discussed.The main conclusions are as follows:(1)The palynological data revealed the vegetation evolution process in the Zhongliang Mountain karst area of C hongqing in the past 700 years,which can be divided into the following stages.During 1274-1754 cal a AD,the vegetation types were evergreen coniferous forests dominated by Pinus,C upressaceae/Taxodiaceae,mixed with a few broad-leaved trees,and a large number of herbaceous plants dominated by Poaceae,Asteraceae and Dicranopteris grew on the forest edge or barren slope,and the climate was rather dry.The area of coniferous forest decreased and herbaceous plants began to increase in 1530-1640 cal a AD.During 1754-1885 cal a AD,the area of coniferous forests dominated by Pinus,Cupressaceae/Taxodiaceae was greatly reduced,while the area of broad-leaved trees such as Quercus-evergreen,Quercus-deciduous and Pterocarya expanded,showing arbor or shrub shape.The herbaceous plants(Poaceae,Artemisia,Asteraceae)have increased greatly,and the climate has been moist compared to the previous stage.After 1885 cal a AD,Pine forests have been recovered,while Quercus-evergreen,Quercus-deciduous and Pterocarya have been greatly reduced,and the herbaceous plants are flourishing.(2)The palynological data in the karst depression of Zhongliang Mountain in Chongqing indicated that the vegetation in the study area of 1274-1530 cal a AD was dominated by coniferous trees,a small amount of pollen from anthropogenic plants appeared,the influence of human activities was weak,the ecological environment was good,and rocky desertification had not yet occurred.During 1530-1754 cal a AD,the pollen content of Pinus decreased,and the pollen content of anthropogenic plants increased,indicating that vegetation coverage has decreased,human activities have increased,and soil erosion has intensified.During 1754-1820 cal a AD,the Pinus forest area has been greatly reduced,and the pollen content of anthropogenic plants Poaceae,Asteraceae,Artemisia,Brassicaceae,and Zea mays has increased significantly.It indicated that the scope of agricultural activities had expanded on a large scale,human activities had strengthened,the phenomenon of rocky desertification appeard.During1820-1885 cal a AD,Pinus pollen content was the lowest value of the entire core,Cupressaceae/Taxodiaceae pollen appeared sporadically,Zea mays pollen content slightly increased,and the content of anthropogenic plants such as Artemisia,Humulus,Brassicaceae increased significantly,which indicated that the scope of farming activities was further expanded,and human activities have continued to strengthen.The intensification of human activity led to the deforestation of Pinus and Cupressus funebris,and the phenomenon of rocky desertification had further deteriorated.After1885 cal a AD,the pollen content of Zea mays increased significantly,indicating that human activities became more intense and the phenomenon of rocky desertification intensified.The increasing human activities in the past 700 years are the main reason for the occurrence and development of local rocky desertification.(3)The vegetation evolution process reconstructed based on the palynological data was in good agreement with the deforestation caused by human activities recorded in historical documents.Among them,the three forest reduction stages of 1530-1640 cal a AD,1754-1885 cal a AD,and 1950-1970 cal a AD revealed by the pollen records corresponded to the Ming Dynasty(the first"Huguang fills Sichuan")and Q ing Dynasty(the second time"Huguang fills Sichuan")and"Great Leap Forward and Cultural Revolution".Although climate change has had certain impacts on local vegetation in the past 700 years,the continuous increase in population,rapid development of agriculture and implementation of national policies since the historical period are the main reasons for the reduction of local forest vegetation and the degradation of the ecological environment.During 1274-1530 cal a AD and 1640-1720 cal a AD the two forest restoration stages correspond to the frequent wars,the massive loss of populatio n and the desolation of agriculture in the late Yuan and early Ming,as well as the late Ming and early Q ing dynasties,respectively.The results of sporopollen reconstruction and historical documents are mutually verified and supplemented,and provide good support for fully understanding the dynamic process of vegetation and the evolution process of rocky desertification in the karst area of Zhongliang Mountain in Chongqing since the historical period.