Tree-ring Climate Response And Climate Reconstruction In Fujian And Surrounding Areas

Forests are an important part of terrestrial ecosystems and play an important role in protecting biodiversity,maintaining ecological balance and achieving"carbon neutrality".The frequent occurrence of extreme climatic events under the background of climate warming exerts a profound influence on the composition,structure and function of forest ecosystem,forest productivity,carbon cycle process and succession process,etc.Forest ecosystem in southeast China is a natural barrier of ecological environment security.It is of great significance to water conservation,soil conservation,biodiversity protection,ecological security balance and social and economic development of surrounding areas.Therefore,it is important to understand the response mode and adaptation strategy of radial growth of typical tree species to extreme climate in southeast China,and to reveal the climate signals recorded by forest dynamics over long periods of time.Tree rings provide reliable data for the study of long-term dynamic responses of forest ecosystems to climate change and are important tools for the reconstruction of regional long-term climate change.This study reveals the adaptation strategies of tree growth to climate change and the different climate factors that can be retrieved,basing on tree rings in Fujian area.In this study,the tree-ring data network of typical conifers study area was established,and a variety of ecological analysis methods were combined to study the response of regional forest to extreme climate.The ecological resilience and memory difference of tree radial response to extreme drought were analyzed.Using tree ring carbon isotope,wood anatomical index and VS physiological model,the relationship between tree decay and death and climate and environment change was analyzed and discussed.Through tree ring width and stable oxygen isotope,various climate signals in different regions were revealed in order to deeply understand the response model of regional tree growth to climate change and the future development trend of tree population under the background of climate warming and drying.The main research conclusions are as follows:（1）Quantifying the relationship between tree radial growth responses to extreme drought by using ecological resilience and ecological memory indicators.It was found that tree species and tree age were important factors affecting ecological resilience and memory in southeast China.Endangered tree species such as T.longibracteata and Fokienia hodginsii had more decline period and higher decline rate,and their recovery ability was weak.The pioneer species Pinus massoniana had strong resistance to stress.The resilience and ecological resilience of old trees of the same species are weak.（2）Different from other samples in southeast China,Pinus taiwanensis in Daiyun Mountain area of central Fujian Province did not show the characteristics of drought restriction,but still experienced a wide range of growth decline in recent years.By comparing ring width,anatomic indices,and stable carbon isotopes(δ¹³C)of declining and healthy trees,the VS physiological model was applied to try to explain the mechanism leading to growth decline.The results showed that the recent high temperature event was the main cause of the decline and death of Pinus taiwanensis in Daiyun Mountain,central Fujian Province..（3）The 241-year tree ring chronology of Cryptomeria fortunei in Liancheng area of western Fujian Province was established,which is the longest width chronology in Fujian Province.Tree growth was significantly positively correlated with precipitation and SPEI index in November-December of the previous year,and temperature in winter and spring（December of the previous year to April of the current year）.The results showed that summer drought and drought stress before the growing season were the main factors limiting the growth of Cryptomeria fortunei in western Fujian.Since the 1920s,tree growth has accelerated on an interdecadal scale,corresponding to the wetter climate characteristic of the region.There is uncertainty in the association between tree rings and the Pacific Decadal Oscillation（PDO）,showing different correlations at different periods,which is related to the changes of PDO at different periods,possibly due to the modulation effect of the East Asian summer monsoon.（4）Dendroring oxygen isotope(δ¹⁸O)was used to reconstruct cloud cover from June to October in the Wuyi Mountain area,spanning from 1700 to 2017.During 1951-2017,50.7%of the observed variance can be explained,and the reconstructed series is reliable,which has a good agreement with the reconstructed precipitation and relative humidity records in the southeast region.Reconstructed decadal variation of cloud cover began in the 1920s with a rapid upward phase shift.The second EOF mode indicates that there is a dipole pattern of cloud cover variation between southeast and southwest China.Spatial correlation analysis shows that the reconstructed sequence is highly correlated with SST anomalies in the tropical East Pacific,with a pattern similar to the El Ni（?）o-Southern Oscillation（ENSO）.（5）The standard chronological sequence of tree ring width for 193 years of Pinus taiwanensis in southeast region was established by using the ring core of four samples from Jiangxi and Zhejiang regions.The diurnal temperature range（DTR）sequence from April to July in the growing season was reconstructed and passed the sectional test.The reconstructed results have a strong correlation with the variation of daily range in most southeastern regions of China,and the reconstructed sequence can represent the variation trend of daily range in the study area.Since the 1920s,the daily range from April to July had a slow rising trend,increasing variability and extreme value.The correlation results of sea temperature show that ENSO may be a key factor in the change of DTR trend in the study area,which leads to a slow rising trend in the growing season of DTR in the study area by influencing precipitation,cloud cover and temperature.In conclusion,the tree growth in Fujian and its surrounding areas is limited by different extreme climate factors.In the future,extreme drought will pose more serious threats to endangered tree species and old natural forests at the late stage of succession,thus speeding up the community succession change.The increase of extreme high temperature will seriously threaten the growth of Huangshan pine in high altitude areas.In the context of global warming,rapid warming since the 1920s led to a common climate change feature in this region,which was specifically shown in the following aspects:the overall wetting since the 1920s,increased cloud cover in summer and autumn,increased diurnal temperature range variability and increased extreme values in spring.This study provides further theoretical support for the reconstruction of longer time scale climate signals and the understanding of regional climate response models.