Mechanisms Of The Variations In Radial Growth And Stable Carbon And Oxygen Isotopes Of Picea Crassifolia Across Different Microenvironments

In recent years,global climate change has intensified and many forest ecosystems around the world have been greatly affected by changes in temperature and precipitation,particularly those in arid and semi-arid regions.The Qilian Mountains are located in the the arid and semi-arid region of northwest China,which serve as an important ecological barrier for ecological security in the northwest region and the country.Many studies have been conducted on the impact of climate change on forests in the Qilian Mountains,which have shown that the climate change has already altered forest distribution,composition,and structure.Building upon these studies,this study will focus on the adaptability of trees in different microenvironments,further investigate the mechanisms about how climate change affects the forests of the Qilian Mountains,and reveal trees’strategies for drought resistance.This study selected Picea crassifolia as the research object.The distribution of this species is governed by the direction of the slope and the differences in solar radiation it causes,and is only found on the shady slopes of the Qilian Mountains or in valley areas with less shady solar radiation.It is one of the dominant species in the forest of the Qilian Mountains and is sensitive to climate change.Therefore,systematically studying the response processes and rules of P.crassifolia to dry-wet changes in different microenvironments would deepen our understanding on the impact mechanisms of dry-wet changes on tree growth.Our study will provide scientific basis for the management of P.crassifolia forests in the Qilian Mountains.In this study,the central part of the Qilian Mountains was selected as the study area,and three sample plots with different soil thickness were selected for the climatology of the tree rotations within a 2 km area:PC1（shallow soil）,PC2（medium soil thickness）and PC3（thick soil）sample plots.The study was also supplemented with measurements of plant water potential and photosynthetic characteristics.In this study,we built chronologies of tree-ring width and tree-ring stable carbon(δ¹³C)and oxygen(δ¹⁸O)isotope sequences.We then evaluated the responses of trees to climate changes,and revealed how the microclimate caused different responses in tree-ring width and tree-ring stable carbon and oxygen isotope across the study sites.In addition,the comparison of tree-ringδ¹⁸O andδ¹³C values between study sites was used to determine the drought tolerance strategy of trees based on the tree-ring information.The main findings of this paper are as follows:（1）The radial growth of P.crassifolia was limited by moisture conditions in the three study sites during 1953-2019.There was a positive correlation between tree-ring widths and sc PDSI（self-calibrated Pamler Drought Severity Index,an indicator of wet and dry conditions in the vicinity of the site）at three sites.The correlation between tree-ring width and sc PDSI was weak in the PC1 sample.Radial growth of trees in the PC3sample was positively correlated with precipitation and temperature in autumn of the previous year;radial growth of trees in the PC2 sample increased with increasing precipitation and temperature in June of that year.（2）Stable isotopes of tree rings differed significantly between study sites in dry and wet years.Stable isotopes of carbon(δ¹³C)and oxygen(δ¹⁸O)in the tree rings at the three study sites showed a decreasing trend from the dry years（1938-1942,1946-1950）to the wet years（2006-2010,2016-2020）.The most pronounced decrease was at sample site PC1,where theδ¹³C value decreased by 0.56‰and theδ¹⁸O value decreased by 2.55‰in the wet years compared with the dry years;theδ¹³C value at sample site PC2 decreased by 0.30‰and theδ¹⁸O value decreased by 2.30‰;theδ¹³C value of tree rings at PC3 decreased by 0.25‰and theδ¹⁸O value of tree rings changed less,decreasing by 0.56‰.The correlation between stable isotopes of tree rings and climatic factors differed among study sites.A negative correlation between tree-ringδ¹³C andδ¹⁸O and sc PDSI was observed in all three sample sites.Trees in the PC1sample,which had the shallowest soil,were most vulnerable to drought,compared to the PC3 sample,which had thicker soil and was less vulnerable to drought.（3）The difference in soil depths may be the main factor causing differences in drought resistance strategies among the three study sites.The P.crassifolia in site PC3,which has thicker soil,showed stronger resistance and recovery elasticity in drought years.In years with severe drought,the tree-ringδ¹⁸O in PC3 with thicker soil was lower than the other two sites,but in wet years,the difference in tree-ringδ¹⁸O among the three sites was not significant,indicating that the trees in PC3 can absorb water from deeper soil layers than the other two sites in response to drought.Although the spruce is generally shallow-rooted,P.crassifolia in PC3 did absorb water from deeper soil layers in drier years.In contrast,in the other two study sites with shallower soil,the tree-ringδ¹⁸O did not change significantly regardless of the severity of drought.The depth of water absorption may not change significantly.Therefore,the trees in these two sites may cope with drought only by regulating stomata conductance.In summary,differences in radial growth,response relationships between tree-ring stable isotopes and climatic factors,and drought tolerance strategies were found in P.crassifolia stands at the three study sites in close proximity due to differences in microenvironment,especially soil thickness.Thicker soils give P.crassifolia greater adaptability to drought,especially in drier years when it can absorb water from deeper soils.Therefore,the influence of the microenvironment needs to be considered in future research work on changes in forest growth dynamics.