Radial Growth Of Key Tree Species In Response To Warming And Monsoon Fluctuations In Boreal Coniferous Forests Of China

The global climate is currently undergoing dramatic changes,such as rapid temperature increases,uneven precipitation distribution and the frequency of extreme weather events.In the context of climate change,it is worthwhile and urgent to study how the radial growth and forest dynamics of major tree species in cold-temperate coniferous forests in the monsoon margins respond to climate change,whether the response of pioneer and established species to climate is consistent,and the environmental and forest dynamics changes based on the radial growth characteristics of trees.In response to the above problems,a series of experiments were conducted on a regional scale using the tree-ring climatology,the tree ring ecology and natural test methods in the cold-temperate coniferous forests,where warming is significant,with larch(Larix gmelinii),a dominant species,and white birch(Betula platyphylla),an associated tree species,as the research objects.A total of 62 sample sites were selected,including 687 trees with 1333 cores from 31 sample sites of larch and 741 trees with 1480 cores from 31 sample sites of birch.The effects of warming and monsoon fluctuations on tree growth were investigated from the perspectives of radial growth-climate correlations and their spatial and temporal variations,the response of the intrinsic xylem structure to warming,and the tolerance of trees to drought events.The main results are as follows:(1)Radial growth of larch and birch increased significantly with age in most of the sample sites(87%)in the first 50 years of the tree’s life(p<0.05).Larch and birch in the same region had similar fluctuations in the Basal area increment(Basal area increment,BAI).There was also consistency in BAI variation between larch and birch at the regional scale.After 1970,both larch and birch showed a rapid upward trend with 1.1 cm~2/10year for larch and 0.39 cm~2/10year for birch.In the same year,BAI was higher in larch than in birch.(2)The radial growth of larch and birch was basically the same in response to each meteorological factor,and both were limited by moisture in the growing season.The average temperature in the pre-growing season(February-April)and the growing season(June-August)had a negative effect on the growth of larch and birch in most areas,and the average temperature in May mainly had a positive effect on the growth of both;the precipitation in December-February and June-August had a positive effect on the growth of larch and birch;the Palmer drought severity index(PDSI)in June-August was significantly positively correlated with the growth of both.In addition,the correlation between larch radial growth and the average temperature in June-August of the growing season showed significant spatial characteristics,and the effect of the average temperature in June-August on larch growth was significantly enhanced with increasing annual precipitation(p<0.05).(3)Structural changes in larch xylem were related to age and temperature.There was a significant low-frequency trend change in the proportions of latewood and earlywood(Proportions of latewood and earlywood,PLE)larch wood in the first 100 years of growth,which was mainly influenced by the age of the tree(p<0.05);After more than 100 years of age,the proportions of latewood and earlywood showed high frequency fluctuations(no low frequency trend)with significant temperature effects.The proportion of larch latewood increased significantly(p<0.05)as the mean annual temperature decreased.Temperature plays a decisive role in changes in the proportions of latewood and earlywood,with the lower the temperature,the higher the percentage of latewood in the year.This suggests that tree support structures,and intrinsic physiological processes associated with the allocation of early and late timber change with continued global warming.(4)There were interspecific differences in the resilience of larch and birch in response to extreme drought,with larch having higher resistance than birch.However,they had the same resistance and recovery in response to moderate and severe drought.There was a spatial and temporal trade-off mechanism between resistance and recovery in larch,with a decreasing trend in resilience when resistance tended to increase along the moisture gradient and vice versa,regardless of whether the drought occurred early in the growing season or during the growing season;white birch had the lowest resistance and highest recovery to extreme drought and increased resistance and decreased resilience as the drought level decreased.(5)Tree growth in the study area is influenced by the monsoon,and there is a remote correlation between it and SST variation.The tree-ring width variation of larch and birch was positively correlated with sea level temperature in the Okhotsk Sea and the equatorial western Pacific Ocean in June-August.Wavelet analysis and cross-wavelet transform revealed a 2-4 years common cycle of larch and birch growth with SST,and the results demonstrate that tree growth in the study area is influenced by large-scale climate circulation.In summary,Larch and birch growth in the study area have similar climatic response patterns;the proportion of latewood of larch decreases with increasing temperature;and the resistance of larch to extreme drought is higher than that of birch.The structure of the larch tree-ring changes under future warming,and in the event of extreme drought events,white birch may experience growth decline before larch,and the forest species composition remains dominated by larch.This study provides a scientific basis for understanding the internal structure of the tree-ring and changes in forest species composition in warming.