Climate Variability In North China As Inferred From The Frequency Of Intra-Annual Density Fluctuations In Chinese Pine

In the context of drastic climate changes,it is extremely important to explore the long-term evolution and driving mechanisms of climate using paleoclimate proxy data and to predict future climate changes accordingly.However,recent studies have found that large discrepancies between paleoclimate reconstructions and model simulations in the Holocene climate change may be primarily attributed to the seasonal bias of proxy indicators.This finding emphasizes the necessity of considering seasonal variations more carefully in paleoclimate research.Therefore,finding quantitative indicators sensitive to seasonal climate and further clarifying the climate change characteristics and driving mechanisms in different seasons becomes the key to accurately understanding past climate environmental changes.Intra-annual density fluctuations（IADFs）can provide sub-seasonal to seasonal scale environmental change information,recording climate signals different from traditional tree-ring indicators,and hold great potential in paleoclimatology research.The North China is characterized by a sensitive climate,fragile ecology,and frequent droughts.In the context of global warming,drought events in this area exhibit an increasing,consecutive,severe,and concurrent trend,resulting in multi-scale drought disasters spanning months,seasons,and even years.Moreover,drought variations between different seasons likely correspond to significantly different driving mechanisms.However,regional tree-ring research is mainly based on tree-ring width index,thus focusing more on longer-term droughts and lacking research on shorter-term droughts.Conducting research based on IADFs is expected to enhance the understanding of short-scale drought changes in the North China.In light of this,we collected Pinus tabuliformis Carr.samples from 5 sites in the North China,and established the IADFs frequency index chronology for the first time in this region,including 5 single-site IADFs chronologies and 1 regional IADFs chronology.Moreover,we conducted a paleoclimate reconstruction using the IADFs chronologies for the first time and explored the relationship between IADFs variations and monsoon anomalies.The main conclusions of this paper are as follows:（1）We analyzed the variation characteristics and climate response patterns of tree-ring width and IADFs frequency.The results showed that the radial growth of trees was mainly affected by the humidity conditions in the autumn of the previous year and the spring and summer of the current year,while IADFs were primarily triggered by short-term drought in June and July,which recorded significantly different climate signals.Severe drought in early summer and favorable conditions after the arrival of the rainy season led to the occurrence of IADFs,which also increased the proportion of latewood.Consequently,the latewood chronology（LW）,the adjusted latewood chronology(LW_adj),and the moisture conditions in July are significantly positively correlated,and the IADF frequency is also significantly positively correlated with LW_adj.This anatomical feature variation and the response of the chronology reflect the rapid trade-off that trees make between xylem hydraulic safety and water conduction rate in order to adapt to seasonal fluctuations in moisture availability.（2）We constructed a regional IADF frequency index chronology using IADF data from four sampling sites in southeastern Inner Mongolia.Based on this chronology,we reconstructed the single-month scale SPEI in June from 1848 to 2017,and identified two persistent dry periods（1880-1895,1993-2005）and four persistent wet periods（1869-1879,1948-1959,1969-1980,2006-2017）.Spatial correlation analysis and comparison with historical records show that the reconstructed series can well represent the dry-wet changes in the Inner Mongolia region both spatially and temporally.However,there is no significant correlation in the low-frequency changes on a decadal scale between our reconstructed series and the hydro-climate reconstructions for the spring,summer,and spring-summer-autumn seasons based on tree-ring width in the nearby area.Also,the distinct drought trend shown by other series since the 1950s is not evident in our reconstructed series.Our study found that the early summer drought in Inner Mongolia mainly originates from the latitudinal Eurasian teleconnection type stimulated by the abnormal warming of the North Atlantic,which strengthens the Eurasian zonal circulation.The anomalously strong zonal circulation in the study area hinders the exchange of cold and warm air from north to south,subjecting the region to the influence of continental warm high pressure,resulting in intense hot weather and reduced moisture input.In conclusion,the June SPEI reconstructed using IADFs shows certain differences in terms of change characteristics and driving mechanisms compared to the hydroclimate of other seasons reconstructed based on tree-ring width.（3）Based on the response patterns of IADFs chronology to climate change on the eastern edge of the Loess Plateau,we explored the impact of the East Asian summer monsoon on IADF frequency by calculating the start time of the rainy season at the Zuoquan station.Correlation analysis and the generalized additive model indicate that the frequent occurrence of IADFs may be related to the drought in June and July caused by the later onset of the monsoon rainy season.This means that we have found a new indicator in tree-ring records that can capture monsoon anomalies,providing a data basis for further understanding the changes of the East Asian summer monsoon on a centennial scale.