藏东南地区树木生长对大气和土壤湿度的响应差异

Response of tree growth to atmospheric and soil moisture in the southeastern Tibetan Plateau

在大气二氧化碳浓度(CO_(2))升高和全球变暖背景下,大气饱和水汽压差(VPD)和土壤湿度对树木生长的影响得到广泛的关注。青藏高原东南部森林生态系统对全球变化响应敏感,随着温度迅速升高和南亚夏季风降水减少,干湿变化对区域森林生态系统的影响逐渐引起学术界的重视。然而,树木生长对大气和土壤湿度的响应特征依然未知。本研究使用藏东南地区28个样点的树轮宽度序列,分析了1960年以来树木生长与大气和土壤湿度在不同季节和时段的关系。研究发现,生长季早期,树木生长主要受到VPD的限制,而在生长季中晚期,则主要受到土壤湿度的控制。同时,VPD与树木生长的关系存在季节上的转变,即生长季早期,VPD与树木生长呈负相关,而在生长季中期转变为正相关。这种现象可能与生长季早期VPD值较高,气孔关闭导致树木生长下降;而生长季中期,VPD值较低,且不再是主要控制因子,较低的VPD对蒸腾有促进作用,而且温度的升高会有利于形成层分裂分化,从而促进树木生长。从长时间尺度上来看,树木生长与VPD和土壤湿度的关系并不稳定。1978年后的大气和土壤干旱可能造成了树木生长与VPD关系的减弱,使得树木生长与土壤湿度关系增强,树木生长更依赖于土壤湿度的变化。

Under the context of increasing carbon dioxide concentration(CO_(2))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.