Temporal Trend And Influence Factors Of Tree Growth Resilience Indices Under Drought Stress

In recent years,with the intensification of global climate change,its impact is mainly presented as the increasing of temperature and changes in precipitation patterns and resulted in the frequently occurred severe drought events which lead to common problems for global terrestrial ecosystems.It is predicted that the severity of drought stress would continue to increase,which would greatly affect the global carbon/water cycling and proceed to affect the human society.Forests are the main component of terrestrial ecosystems and can play an important role in buffering and regulating the shifts in climate.Therefore,understanding how forests respond to drought is crucial for facing the climate changes in the future.However,our understanding of the quantification criteria,influence factors and sensitivities to environmental drivers for growth resilience is still limited.We employed the soil drought data with a spatial resolution of 0.5×0.5 degrees for latitude and longitude in Global SPEI database,the atmospheric drought data（vapor pressure deficit）calculated from temperature and actual vapor pressure of CRU database,to filter out the soil,atmospheric and compound drought events.Calculate the growth resilience indices using tree-ring data from the International Tree-ring Data Bank（ITRDB）.Reveal the temporal trends and response patterns of tree growth resilience during the 20^th century by fitting the growth resilience indices as a function of climatic factors,individual factors and drought factors.Using the tree-ring data obtained from the field to explore the characteristics of tree growth resilience on regional scale and different site level.The main results are:1.Eight pairs of growth resilience indices were evaluated based on accessibility,objectivity,and mathematical independence.The results show that the Lloret,Li,Isbell,and Orwin&Wardle indices exhibit autocorrelation features inherited from their calculation methods,making it impossible to disentangling the true ecological relationship between resistance and resilience from the false correlation in mathematics.The indices used by Sousa and Tilman exhibit covariation between resistance and resilience.The O’Neill index can only measure the magnitude of interference and cannot reflect the direction of the effect.The Mac Gillivray index,which was obtained by dividing the actual value by the predicted value（the predicted values can be obtained by the autoregressive integrated moving average model,ARIMA）,can effectively avoid the defects above and is suitable for this study.2.The overall severity of soil drought did not change significantly during the 20^th century(with a mean annual shift of SPEI by-0.000335±0.0155 a^-1),which may be related to a significant increase in precipitation.The intensity of atmospheric drought has increased significantly due to the rising temperatures(with a mean annual shift of VPD by 0.00159±0.0262 a^-1).In the three types of drought events,the resistance of angiosperms always increases with time（the overall linear regression slope distribution is 0.000042～0.000414,P<0.05）,while the resilience of gymnosperms always decreases with time（the overall linear regression slope distribution is-0.000201～-0.000144,P<0.05）.As to the fixed factors effect,all factors have varying degrees of influence on the resistance and resilience of the three drought events.Among them,SPEI_d and VPD_pre have a relatively greater impact on the resistance,while annual average temperature,annual precipitation,SPEI_pre and VPD_d have a relatively smaller impact.All climatic and drought factors have a significant impact on resilience of the three types of droughts.The interaction between year and tree species taxonomy generally has a negative effect on the resilience of gymnosperms in three types of droughts,but has a positive effect on the resistance of soil and compound drought.The resistance of angiosperms in the three types of droughts always perform better than that of gymnosperms（a significant difference in the two taxonomies during atmospheric droughts were found,P<0.05）.The decrease in resilience of gymnosperms may be due to a combination of passive response and active adaptation to drought,with angiosperms exhibiting a distinct response pattern.3.The sensitivity of the resistance and resilience of gymnosperms to SPEI and VPD before,during,and after the drought has undergone significant changes in the 20th century,with the direction of this changes often changed in the second half of the 20^th century.These changes in sensitivity have a positive effect on the decreasing trend of resilience,which could alleviate the growth loss caused by drought to a certain extent,and this effect is more significant in atmospheric and compound drought events than in soil drought events.Although changes in sensitivity have had a positive effect on tree growth,the resilience of the three drought types showed an extremely significant downward trend throughout the 20th century（P<0.0001）.4.A study was conducted on the resistance and resilience of natural birch forests on seven site types in the three latitude regions of Northeast China during the past three soil and atmospheric drought events.The results showed that during soil drought events,the resistance of Betula platyphylla in the valley of Xiaoxing’an Mountain was significantly higher than that in the Mao’er Mountain by39.88%（P<0.05）,and the resistance of Betula platyphylla in the upper level of the shady slope of Xiaoxing’an Mountain was significantly higher than that in the Mao’er Mountain and Changbai Mountain by 33.76%to 38.70%（P<0.05）.The resilience of Betula platyphylla in the Changbai Mountain decreased significantly over time during soil drought,with the first time being significantly higher than that in the following two times by 44.72%to 55.62%（P<0.0001）.Overall,the resistance of Betula platyphylla in the Xiaoxing’an Mountain is significantly higher than that in Mao’er Mountain by 17.75%（P<0.05）.During atmospheric drought events,the resilience of Betula platyphylla in the valley of the three regions were significantly higher than those of the middle and lower level of the shady slope by 19.32%to 27.81%（P<0.05）.No significant difference in resistance and resilience of the same site type in different latitude regions were found（P>0.05）.The resilience of Betula platyphylla in the Changbai Mountain decreased significantly over time during atmospheric drought,with the first time being significantly higher than those of the following two times by 39.77%to 45.45%（P<0.01）.Among the three regions,the resistance and resilience of Betula platyphylla in the Mao’er Mountain were the lowest during drought.The larger plot basal area and higher competitive pressure in the region might account for this.The Xiaoxing’an Mountain performed the best in terms of soil drought resistance,while Changbai Mountains performed slightly better in terms of soil drought resilience and atmospheric drought resistance compared to the other two regions.Our research reveals the vulnerability and the temporal trend of Betulla platyphylla to soil and atmospheric droughts on different site types in different latitude regions of Northeast China.Our conclusion can provide scientific basis for the management and protection strategies for natural Betulla platyphylla forests in Northeast China in facing extreme climate conditions in the future.