Analysis Of Tree Competition On Growth And Climate Sensitivity Of Natural Mongolian Pine Forest In Hulunbuir Sandy Land

Pinus sylvestris var.mongolica natural forest mainly distributes in the transition zone from Daxing’an Mountains to Hulunbuir Sandy Land.It has very important function of wind-proof and sand-fixing,and plays an important role in maintaining ecological security and ecological stability in the region.Since the 20 th century,due to global climate change,extreme weather events have gradually increased.The growth of Pinus sylvestris var.mongolica in sandy land shows a declining trend,and even a large number of trees have died,which poses a serious threat to the production,life and ecological security of the region.In addition,tree competition as a long-term "stress factor" in the growth process of trees,the different intensity of competition may affect the response of trees to climate.Therefore,under the background of global climate change,it is urgent to study the climate sensitivity of natural forest of Pinus sylvestris var.mongolica.In this study,the natural Mongolian pine forest in southern margin of Hulunbuir sandy land was studied.Based on the field observation data of natural Mongolian pine forest,with the statistical methods of correlation analysis,regression analysis,principal component analysis and linear mixed effect model,the population structure and competition characteristics of Mongolian oine were preliminarily analyzed,and a standardized tree ring chronology at sample plot and individual tree scale was established,the effects of climate and competition on tree radial growth and climate sensitivity were analyzed.And the relationship between normalized vegetation difference index(NDVI)and tree radial growth and climate factors was analyzed based on MODIS remote sensing image data.The main conclusions are as follows:Population structure and competition of Mongolian pine: The population of natural Mongolian pine forest in the southernmost margin of geographical distribution is mainly small and medium-sized trees,belonging to an increasing population.8 m is the optimum radius range which can better reflect the real situation of intraspecific competition of Mongolian pine.For the relationship between competition and population structure,the results are consistent at stand and individual tree scales.The greater the stand density,the greater the competition intensity between trees.Secondly,the influence of stand density on DBH,crown radius,under-branch height,crown length ratio and height to diameter ratio is significant,but the influence on tree height is not significant.There is a significant negative correlation between stand density and DBH,tree height,crown radius and crown length ratio.The higher the stand density,the smaller the DBH,crown radius and crown length ratio,the higher the under-branch height,and the stronger the natural pruning of trees.Climate sensitivity of Mongolian pine: The tree ring width of Mongolian pine andmonthly temperature,diurnal temperature range and net radiation were negatively correlated,especially in the growing season(May-September),which reached significant or very significant levels.It was positively correlated with precipitation,relative humidity and PDSI during the peak growth period of the current year,especially in summer(June-August)and growing season(MaySeptember),which mostly reached significant or very significant levels.Secondly,the growth of trees in low density stands is mainly affected by soil moisture content,while the growth of trees in medium and high density stands is mainly affected by precipitation.The relationship between competition and tree growth and climate sensitivity: At stand scale,with the increase of stand density,tree radial growth increased first and then decreased,but did not reach a significant level;at single tree scale,with the increase of competition index,tree radial growth decreased significantly.For tree climate sensitivity,at stand scale,there is no significant relationship between stand density and tree growth response to climate,that is,with the increase of stand density,tree growth response to climate has no significant change trend,at individual tree scale,trees with weak competitiveness respond more strongly to climate in growing season,while trees with strong competitiveness respond more sensitively to climate in winter and spring.The relationship between competition and resilience of trees to cope with extreme drought: The best combination of PDSI months with the greatest impact on tree growth is January to December of current year.Secondly,five drought events were determined based on the criteria of drought events,the drought degree in 2007 was the highest,with an average annual PDSI of-3.02.At stand scale,with the increase of stand density,the resistance,recovery and resilience of Mongolian pine to extreme drought increased gradually;at individual tree scale,with the increase of competition index,the resistance of trees to extreme drought decreased slightly,while the recovery and resilience increased gradually.The relationship between vegetation NDVI and tree growth and climate: NDVI value increased slowly during 2000-2016,vegetation growth mainly concentrated in June-August in summer,and was in the initial stage of vegetation growth in March,vegetation growth in September had basically stopped.In addition,due to the impact of extreme drought,the NDVI value in 2007 was significantly lower than that in other years.NDVI in April of growing season was positively correlated with temperature,in growing season was positively correlated with PDSI,and in September of growing season was positively correlated with precipitation.The radial growth of Mongolian pine in 10 plots was in good agreement with NDVI changes in this area,and tree ring width was correlated with NDVI in April-October,with the greatest correlation in June and August,but with the worst correlation in July.This study will help deepen the understanding of how forest competition regulates tree growth and its climate response,reveal the regulatory mechanism of forest competition on tree growth-climate response,predict the ability of Mongolian pine to cope with climate change under future climate change scenarios,and provide scientific guidance for the management of natural Mongolian pine forest under the background of climate change.