Changes In The Transition Zone Between Betula Ermanii And Coniferous Forest On The Northern Slopes Of Changbai Mountain In The Past 30 Years

The global climate is currently warming at an unprecedented rate,changing the structure and composition of forest ecosystems on a global scale,especially the distribution range of species.The increase in temperature means that the low temperature restriction for plants will gradually ease,leading to a gradual expansion of species on the upper edge of their range.It has been found that plants in mountain ecosystems are moving uphill.However,due to the impact of climate change,the position and rate of vegetation transition zone movement and how the internal forest community structure changes are still unclear,which is the focus of the research on the response of forest vegetation to global change.In this study,multi-temporal and multi-source remote sensing image data were collected,and combined with ground sample observation data,the position changes and internal structure changes of the transition zone between Betula ermanii and coniferous forest on the north slope of Changbai Mountain were analyzed.The best time to identify the transition zone of Betula ermanii and coniferous forest is determined by extracting the phenological curves of Betula ermanii and coniferous forest from Sentinel-2 images during the growing season.The transition zone of Betula ermanii and coniferous forest is extracted from the northern slope of Changbai Mountain based on the texture of Betula ermanii and coniferous forest from high spatial resolution remote sensing images.Mann-Kendall trend analysis method was used to conduct pixel scale analysis of Landsat images in the last 30 years,to obtain pixel by pixel vegetation greenness change data,and UAV images were used to verify the results.By establishing a fixed sample plot along the elevation of the transition zone,each tree was measured and core samples of different tree species were collected in the sample plot,and the changes of forest community structure inside the transition zone were explored combined with the image analysis results.The research results and conclusions mainly include the following aspects:(1)The phenological curve was extracted by calculating the NDVI values of Betula ermanii forest and coniferous forest.The results showed that at the end of the growing season,from September 23 to September 30,the NDVI value of Betula ermanii forest decreased rapidly,and at this time,Betula ermanii forest and coniferous forest showed the maximum travel value on the phenological curve.Therefore,this time range was the best period to identify the transition zone of Betula ermanii and coniferous forest.(2)In the past 30 years,the transition zone of Betula ermanii and coniferous forest on the north slope of Changbai Mountain did not show significant displacement,but the optimal distribution elevation of the transition zone showed a trend of migration to the high altitude area.(3)From the perspective of time change,NDVI of Betula ermanii and coniferous forest transition zone showed an overall increasing trend from 1988 to 2020.From the perspective of spatial distribution,the variation trend of NDVI of Betula ermanii and coniferous forest transition zone showed a significant positive correlation with altitude.In terms of the forest structure in the transition zone of Betula ermanii and coniferous forest,the number of young trees of Betula ermanii gradually decreased with the increase of altitude,while the number of young trees of Picea jezoensis and Abies nephrolepis gradually increased,and the increase of Picea jezoensis and Abies nephrolepis led to the gradual increase of NDVI in the transition zone with the increase of altitude.In conclusion,in the past 30 years,the transition zone of Betula ermanii and coniferous forest on the north slope of Changbai Mountain has not moved obviously,but the optimal distribution altitude has migrated to the high altitude area.In terms of internal structure,the number of young trees of Picea jezoensis and Abies nephrolepis increased with the increase of altitude.This indicates that the coniferous forest on the north slope of Changbai Mountain is encroaching into Betula ermanii forest belt with the gradual acceleration of climate warming.This study has important practical significance to further deepen the understanding of Changbai Mountain vegetation transition zone change and enrich the related research of vegetation transition zone change.