Tree Growth Of Qinghai Spruce And Its Responses To Climate Change In The Qilian Mountains

Climate change is the core issue of global change,and it has aroused widespread international attention.The response of the ecosystem to climate change has been becoming a hotspot in global change studies.In the semi-arid regions,forest growth is very sensitive to climate change,and further profoundly affects the regional and global carbon cycle,hydrological cycle,and energy flow.Since the 1980 s,global warming has significantly changed the tree growth in the semi-arid region,and the response of tree growth to warming show great differences in different regions,different altitudes,and different components within a stand.Meanwhile,warming hiatus also significantly changed the tree growth,and lead the response of tree growth to climate changemore complex.However,those were ignored bymost studies.Under the background of warming and warming hiatus,exploring the changes of tree growth and their internalmechanisms in different regions,different altitudes,and different components within a stand play significant roles in the field of climate change and sustainable ecosystemmanagement.The Qilian Mountains,located in the semi-arid area of norther China,are covered with a large number of semi-arid forests.The area is sensitive to climate change and regional heterogeneity,making this region ideal to study forest growth and its influencing factors.In this study,we collected 1006 tree-ring samples from 20 forest-inventory plots of Qinghai spruce(Picea crassifolia Kom.)in east,middle,and east of Qilian Mountain.The biomass formulas and the annual diameter of individual trees,which obtained by tree-ring width and the investigation data,were used to estimate the biomass.On this basis,the study assessed trends of tree growth under the conditions of warming and warming habitus,and further explore the relationship between tree growth and climate factors in a different regions,different altitudes,and different components within a stand.Through analyzing the trends of tree growth in different scales,we found that tree growth of Qinghai sprucein all scale significantly decreased during the warming period,while the decreasing trends altered to increasing trends among all scale when warming hiatus.Although the trends of tree growth in different regions,different altitudes,and different components within a stand were consistent,there are great differences in the growth rates,as well as responding to climate change.For different regions,the western and central of Qilian Mountain are relatively dry compared to the eastern of Qilian Mountain,and therefore the resistance of tree growth to drought was weaker in the western and central of Qilian Mountain compared to that in the eastern of Qilian Mountain.Climate change dominant the trends of tree growth in the western and central of Qilian Mountain,with the interpretation rates,are greater than 50%,however,the interpretation rate of climate change to tree growth in eastern of Qilian Mountain is relatively weaker,the explanation rate was only 33.5%.For different periods,the dominant climatic factors are different: during the period of rapid warming,temperature dominant the decreasing tree growth trend in the eastern and central of Qilian Mountain,and that was controlled by Standardized Precipitation Evapotranspiration Index(SPEI)in the eastern of Qilian Mountain.During the warming hiatus period,SPEI dominant the increasing trend of tree growth,and its contribution gradually increases from the east to the west.At the same time,we found that the impacts of climate change on tree growth in stand-level are similar to that in the individual levels.For different altitude gradients,resistance and resilience of tree growth at high altitudes(3200m–3300m)are stronger than those atmedium-low altitudes(2600m–3100m).Tree growth ofmedium-low altitudes was strongly affected by climate change,with explanation rates ranges between 48.9% and 76.9%,while the explanation rates were relatively lower in high altitudes,with values 35.9%–45.0%.For different periods,the dominant climatic factors are different: during the period of rapid warming,temperature dominant the decreasing tree growth trend,especially in themedium-low altitudes;during the warming hiatus period,increasing trend of tree growth inmedium-low altitudes were dominated by SPEI,and that was dominated by temperature in high altitudes.At the same time,we also found that the impacts of climate change on tree growth in stand-level are similar to that in the individual levels.At canopy scale,dominant canopy position and codominant canopy position have lower resistance to drought than the intermediate canopy position and suppressed canopy position;after the drought stress was alleviated,the resilience of dominant canopy layer and codominant canopy position show greater resilience than intermediate canopy position and suppressed canopy position.Keymeteorological factors have a higher interpretation rate for dominant canopy position and codominant canopy position(49.3 %–59.4%),while a lower interpretation rate for the growth of trees in intermediate canopy position and suppressed canopy position(10%–46.9%).During the rapid warming period,the temperature is themain contributor to the tree growth decline,and dominant canopy position and codominant canopy position aremore affected by temperature than intermediate canopy position and suppressed canopy position;During the period of warming hiatus,SPEI dominant the recovery of tree growth,and the dominant canopy position and codominant canopy position aremore affected by SPEI than intermediate canopy position and suppressed canopy position.For different tree ages,older trees have lower resistance and resilience than younger trees andmiddle trees.The contribution of keymeteorological factors to the tree growth of different age groups is almost similar,with values ranges from 62.0% to 87.2%.Temperature is themain contributor to the decline of tree growth in all age groups during the rapid warming period,explaining about 49% of the growth of trees of different forest ages;SPEI is themain contributor to the increasing trend of tree growth in the period of warming hiatus,the contribution to young,middle and old trees is between 75.6% and 86.6%.The responsemodel of Qinghai spruce tree growth in Qilian Mountains to climate change was constructed,which comprehensively considered the nonlinear relationship between tree growth and temperature,precipitation,and SPEI.Based on thismodel,the growth trend of trees under future climate scenarios was analyzed,and it was found that with increasing temperature,tree growth showed amore significant downward trend,and the decline rate of tree growth was faster in relatively arid areas.Through this study,the tree growth of different regions,different altitudes,and different components within a stand in semi-arid ecosystem have been informed under the condition of rapid warming and warming hiatus.The research explains the resistance and resilience of tree growth to drought and lays the foundation for the studies of the effect of climate change on semiarid forest ecosystems,and it also helps to explore the potential community succession of forests under the prospective climatic scenarios.