Stand Structure And Response Of Growth To Climate Of Mongolian Pine Natural Forest

Since 20th century,the global climate change has had a profound impact on the global forest ecosystem.The forest-grassland transition zone in arid and semi-arid regions is considered to be sensitive to global climate change.The natural forest of Mongolian pine is an important vegetation type in the Hulunbeir forest grassland zone of Inner Mongolia,which plays an important role in ecological protection.Therefore,under the background of global warming,it is important to study the response of natural forest of Mongolian pine to climate change for understanding the feedback mechanism of vegetation to climate change.Climate is the main factor to regulate tree growth.Competition is a long-term stress factor in tree growth process.Because individual size and age of trees determine their different utilization of light,water and nutrients,trees are subject to different competitive pressures.Therefore,without external interference,how does competition regulate the growth of Mongolian pine and impact on its climate sensitivity?Based on thisquestion,10 plots were established in typical distribution area(A)and marginal distribution area(B)of Mongolian pine natural forest distribution area in Hulunbuir sandy land.Using field survey data and tree core samples,based on single-tree competition model and spatial point pattern method,Mongolian pine in different structural states was analyzed from two levels of individual tree and stand.Based on tree-ring chronology,the response of radial growth of Mongolian pine to annual and monthly climate factors and the relationship between competition and tree radial growth and its climate sensitivity were analyzed.The main results are as follows:(1)Stand structure:Mongolian pine natural forest two areas are medium and young forest,for the competitive intensity of trees in different structural states,dominant tree are the strongest,followed by suppressed tree and young tree.In addition,the logarithmic function relationship between stand density and stand characteristics is Y=a ln(x)+b(Y is stand characteristics,x is stand density,a and b are model parameters).With the increase of stand density,the average DBH,crown width,crown length and crown length ratio of Mongolian pine decreased significantly,while the tree height,height-diameter ratio and under-branch height increased significantly.(2)Spatial pattern:the dominant trees are uniformly distributed on the whole,while the suppressed trees and saplings are aggregated on a small scale to resist the influence of external disturbances;the dominant trees and the suppressed trees,the dominant trees and saplings are independent on the whole,while the relationship between "shelter" and"sheltered" of suppressed trees and saplings are positively correlated on a small scale,the positive correlation tends to be independent with the increase of scale.(3)The relationship of competition and stand structure,the relationship between competition intensity and tree size obeys power function better.With the increase of competition index,namely,the decrease of competition intensity,the DBH,crown width,crown length and crown length ratio of Mongolian pine decrease significantly,while the tree height,height-diameter ratio and under-branch height increase significantly.Generally,dominant trees are located in the upper layer of canopy,compressed trees are located in the middle layer of canopy,young trees are located in the lower layer of canopy,and their pruning ability increases with the decrease of competition intensity.In addition,in the limited resource environment,in the process of competition,the competition intensity of dead trees in suppressed trees and young trees is significantly lower than that of living trees,because the trees with weak viability can not obtain the resources and space needed for their survival.(4)The relationship of competition and tree radial growth:Based on stand level,tree radial growth decreases significantly with the increase of stand density;and based on individual tree level,tree radial growth decreases significantly with the increase of individual tree competition index.(5)Tree growth response to climate:The growth of Mongolian pine was negatively correlated with the monthly mean temperature that from August to September of the previous year and from July to August of current year.Negatively correlated with the extreme minimum temperature that from August to September of the previous year.Positively correlated with the precipitation in September and the relative humidity that from July to August of current year.Positively correlated with PDSI that from July to September of current year.Positively correlated with the sunshine hours in March of current year,and negatively correlated with the sunshine hours in July of current year.Radial growth of Mongolian pine was negatively correlated with temperature,but positively correlated with water content.(6)The relationship of competition and climate sensitivity:With the increase of stand density,the response of tree growth to average monthly temperature,PDSI,precipitation and sunshine hours increased gradually in area A,but not significant.The response of tree growth to precipitation,relative humidity,sunshine hours and extreme minimum temperature increased gradually in area B,mostly reaching significant level.Based on individual tree level,trees with weak competitiveness in area A were usually more sensitive to monthly mean temperature,precipitation and PDSI,while trees with strong competitiveness in Area B were more sensitive to monthly mean temperature and sunshine hours.The relationship of competition and tree resilience:With the increase of stand density,the resilience and resistance of trees in typical distribution area(A)to extreme drought increases gradually,while recovery decreases;the resistance,recovery and resilience of trees in marginal distribution area(B)all show an increasing trend.With the weakening of the competitiveness of trees,the resistance was weaked,but the recovery was strengthed,while resilience of trees are gradually weakened,but not significant.The resilience of trees in area B is gradually increasing.This study systematically analyzed the relationship between competition and stand structure,tree growth and climate response which is helpful to deeply understand the impact of climate change and competition on Mongolian pine natural forest under the background of global climate change,and to provide scientific basis for the protection and management of natural forest of Mongolian pine.