Vegetation Cover Dynamics In The Western Greater Khingan Range Based On Dendrochronology

Global warming has increased the frequency and intensity of extreme weather events,severely affecting terrestrial ecosystem composition and function.Vegetation plays an important role in terrestrial ecosystem function.However,the lack of historical data makes it difficult to study vegetation cover dynamics at long time scales.Since the industrial revolution,the intensity of human activities has been increasing and the impact on terrestrial ecosystems has been deepening.But human activities are difficult to quantify,the quantitative relationship between human activities and vegetation is not fully clear.Therefore,the research of vegetation change patterns over long time series and the relationship between vegetation cover dynamics and climate and human activities can provide scientific reference for sustainable management of vegetation and ecological restoration.Tree-rings are perfect materials for studying climate change and vegetation cover dynamics over long time series.In this research,we established a tree chronology of Pinus sylvestris in western Greater Khingan Range based on tree-rings;the 193-year vegetation cover dynamics of the research area was reproduced by the chronology of Pinus sylvestris,revealing the extreme change pattern of vegetation cover dynamics in the research area;the influence of climate on vegetation cover dynamics was explored by correlation analysis,and the influence of human activities on vegetation cover dynamics was obtained by residual analysis.Finally,the degree of human actions affecting vegetation was ranked by using random forest.The main findings and conclusions of the research are as follows:(1)The reconstruction of 193 years of vegetation cover dynamics revealed that the vegetation cover dynamics showed a fluctuating change pattern.The tree-rings of Pinus sylvestris were significantly correlated with the NDVI of the growing season in the research area,and both of them were controlled by precipitation.So,the research suggested that the changes in the width of tree-ring could represent the changes in vegetation cover dynamics in this area,and thus reconstructed the historical vegetation cover dynamics.The results showed that the overall vegetation cover dynamics had fluctuated and slightly increased in the past 193 years;meanwhile,it was found that 1950 was the key node year of the vegetation cover dynamics sequence,and the vegetation cover dynamics before and after the node showed different fluctuation patterns due to different intensity of human activities.(2)Analysis of the characteristics of extreme changes in vegetation cover dynamics elucidated the differences in the characteristics of changes in different periods.In the research,the characteristics of vegetation cover dynamics were evaluated by the scale and intensity of extreme events over time.It was found that in the past 193 years,there were 32 years of extremely dense vegetation and 37 years of extremely sparse vegetation,and the extreme years occupied 35.8%of the whole period.After the 1950s,the duration,scale,and intensity of extreme changes in vegetation cover dynamics increased greatly,but this interval had a relatively large decrease.(3)The factors affecting vegetation cover dynamics were explored.It was found that climate change was the dominant factor in the change of vegetation cover dynamics in the research area and human activities had an important influence.The correlation analysis of temperature and precipitation with NDVI showed that NDVI was significantly positively correlated with precipitation,weakly negatively correlated with temperature,and positively correlated with PD SI,which indicated that moisture was the main influencing factor governing vegetation cover dynamics in the research area during the growing season.The residual analysis revealed that the influence of human activities on the vegetation cover dynamics in the western Greater Khingan Range showed an "S" shape change,with the influence of human activities showing an upward,then downward and then upward trend after 1950,peaking in 1973 and then declining in 1993,formally turning into a negative influence in 2006,and then bottoming out in 2006.After reaching the bottom,it gradually developed for the better until now.Further analysis of the behavior of human impact on vegetation through random forests revealed that the total number of livestock was the largest factor affecting vegetation,followed by afforestation area,total population and farmland area.The research reconstructed 193 years of historical vegetation cover dynamics in the research area through the chronology of tree-rings of Pinus sylvestris in the western Greater Khingan Range,which solved the problem of insufficient remote sensing data and filled the gap of vegetation research in the region;provided data support for further research and sustainable management of vegetation cover dynamics models in the western Greater Khingan Range in the future;reconstructed vegetation cover dynamics of long time series based on tree chronology and analyzed its influence;the research model of reconstructing vegetation cover dynamics based on tree chronology and analyzing its influence factors can provide a model for other regions.