RESPONSE OF TREE GROWTH TO ATMOSPHERIC AND SOIL MOISTURE IN THE SOUTHEASTERN TIBETAN PLATEAU
Under the context of increasing carbon dioxide concentration(CO2)and global warming,the influence of atmospheric vapor pressure deficit(VPD)and soil moisture on tree growth has attracted increasing attention.As one of the most sensitive regions to global change,the southeastern Tibetan Plateau(25°～35°N,93°～103° E)has experienced rapid warming and weakening southern Asia summer monsoon,and the impact of dry-wet changes on the regional forest ecosystem has gradually been recognized.However,the response of tree growth to VPD and soil moisture are still unknown.This study investigates the influence of VPD and soil moisture on tree growth on the southeastern Tibetan Plateau,a region particularly sensitive to global climate change.By analyzing tree ring width chronologies from 28 sites,the research aims to elucidate the effects of changing VPD and soil moisture conditions on tree growth over the period from 1960 to 2005.All tree ring width chronologies exceeding 200 years are from different elevations and four species,including spruce,fir,juniper,and hemlock,thus minimizing the"age effect"on growth patterns.We found that tree growth is primarily controlled by VPD in the early growing season,while in the middle and late growing season,it is mainly controlled by soil moisture.There is a seasonal transition in the relationship between VPD and tree growth,with VPD negatively correlated with tree growth in the early growing season and then changing to a positive correlation in the middle growing season.This may be due to higher VPD values in the early growing season leading to stomatal closure and caused tree growth decrease.However,in the middle of the growing season,with high temperature in summer season facilitating the cambium activity,the lower VPD values could favor transpiration and thus promote tree growth.Additionally,the relationships between tree growth and VPD and soil moisture are not stable.The atmospheric and soil drought after 1978 may have weakened the relationship between tree growth and VPD,making tree growth more dependent on soil moisture variations.The study highlights the intricate balance between VPD and soil moisture in regulating tree growth.In the context of ongoing climate change,understanding these dynamics is crucial for predicting the future health and productivity of forest ecosystems on the southeastern Tibetan Plateau.The findings underscore the importance of maintaining high VPD in the early growing season and adequate soil moisture levels in the middle and late growing season to support tree growth for the southeastern Tibetan Plateau.
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