Stand Structure And The Response Of Tree Growth To Climate Change Of Picea Crassifolia In The Qilian Mountains

Climate change has a profound impact on the structure and function of forest ecosystems.The response of forest ecosystem to climate change are complex and regional.Qilian Mountain has an outstanding ecological status,and its forest ecosystem play a great role in maintaining the ecological security and regional stable development in northwest China.Picea crassifolia is the main construction species of coniferous forest in Qilian Mountains,which is widely distributed and sensitive to climate change.The effects of climate change on tree growth and forest structure cannot be ignored and are related to the changes of regional ecological processes.Therefore,to clarify the stand structure,tree growth and its response to climate change in different regions and elevations of Picea crassifolia in Qilian Mountains is the basic for formulating regional ecological environmental protection measures and realizing carbon neutrality.In this study,we carried out plot survey and tree ring sampling in the middle and east Qilian Mountains.Twenty-one tree-ring chronologies were established and analyzed the response of tree radial growth to climate change in the middle and eastern Qilian Mountains.The effects of drought on tree growth were evaluated and the ecological resilience of trees to extreme drought was quantified.The forest structure of Picea crassifolia was analyzed,which would be help us to understand the community structurein the study area.The result of this study are helpful to comprehensively understand the trend of forest dynamics in the study area under the background of global warming,and provide scientific basis for the sustainable development and management of forest.The main conclusions are as follows.(1)The response of tree growth to climate change is different at different regions and elevations in Qilian Mountains.In the middle Qilian Mountains,the tree growth was strongly restricted by the water deficit from June to July of the current year,while in the eastern,tree growth was mainly restricted by the precipitation in September of the previous year.Tree radial growth in central and eastern Qilian mountains changed from drought restriction at low altitude to temperature restriction at high altitude.(2)The response of tree radial growth of Picea crassifolia to summer drought was different in time and space.Spatially,the middle region and low elevations(below 3000m)are more sensitive to drought.Before 1980,the response of tree radial growth to drought is relatively insensitive with low regional consistency in the middle region,and almost unrestricted by the summer drought in the eastern region temporally.During1980-2000,the response of tree radial growth to summer drought was enchanced in the middle and eastern Qilian mountains,and the consistency is also high.After 2000,the response of middle region to summer drought was weakened,while the response of eastern region was strengthened.During 1980-2000,the study area experienced rapid warming,the drought stress lead to a decline for tree growth,and resulted in different degree and duration of growth decline in the middle region and low altitude trees.(3)During the extreme drought,the trees growth in the study area generally decreased,and it recovered after 3 years,but in nearly half of the Picea crassifolia could not recover to the growth state before the extreme drought.The resistance of middle region to extreme drought was higher than eastern region.There was no significant difference in the recovery of Picea crassifolia growth after extreme drought at different regions.In 1966 and 1973,there was a significant difference in the resilience between middle and eastern region,but there was no significant difference in 1981 and 1992.Besides,trees in the high elevation had more resistance and less recovery to extreme drought.(4)The change of stand structure was as follows: from the middle to the east Qilian mountains,with the improvement of water condition,the mean diameter at breast height(DBH),mean tree height and crown width of Picea crassifolia increased,the stand density decreased and the ratio of height to diameter remained stable at about 0.75.In the altitude gradient,the ratio of height to diameter,stand density and competition index increased first and then decreased in the middle Qilian Mountains,and the crown width decreased first and then increased,while the eastern part did not change significantly with the elevation gradient.The diameter class structure in the middle region showed an inverted “J” shape,which was a typical different-age structure,and it was a growth population.In the eastern moutains,there were few individuals in small size class,and the ratio structure of DBH was unbalanced,and the regeneration ability of population was poor.There is no significant difference in the spatial distribution pattern in the middle and east moutains,which is mainly manifested as aggregation distribution at the small scale of 0-2m,and with the increase of spatial scale,the distribution is mainly random.The competition index was consistent with the change of stand density along an elevation gradient.The competition index decreased with the increase of DBH,and the tree that the DBH below 12 cm had greater competition pressure.However,when DBH exceed 20 cm,the competition index tended to be stable.The stand density is closely related to DBH,crown width,tree height and the ratio of height to diameter.The higher the stand density,the smaller the mean DBH,crown height and crown width,and the sensitivity of trees to drought.Therefore,the proper reduction of stand density is beneficial to the growth of Picea crassifolia.In conclusion,the response of tree-ring to climate change,stand structure and tree growth trend of Picea crassifolia in Qilian Mountains changed in different regions and at different altitudes.Therefore,the influence of regional climate conditions,altitude and stand structure should be considered in the research.