1982—2015年中国温带不同草地植被枯黄期对极端气候事件的响应

Diverse responses of end of growing season to extreme climate events in different grasslands in temperate China during 1982—2015

随着极端气候事件频率和强度的增加,植被物候正在发生深刻的变化。然而,植被枯黄期(EGS)对极端气候的响应机制目前尚未厘清,特别是对于干旱半干旱地区的草地而言。因此,聚焦我国温带草地,基于1982—2015年全球监测与模型研究工作组归一化植被指数(GIMMS NDVI3g)长时间序列数据提取草地物候参数,并分析其时空变化规律;运用随机森林模型等方法探究温带草地EGS对极端气候变化的响应特征。结果表明:(1)全区多年平均EGS主要发生于270—290儒略日(DOY),59.8%的区域呈延迟趋势,其中显著延迟(P<0.05)的区域分布在新疆天山、阿尔泰山一带和准噶尔盆地西部、黄土高原北部、呼伦贝尔高原的西部和东北小兴安岭。(2)EGS与极端气温暖极值(日最低气温的最大值、日最高气温的最大值、暖夜日数、暖昼日数)之间均以广泛的正相关关系为主;相比之下,极端降水事件与EGS之间的关系相对比较复杂,这与各草地类型自身的生理策略和所处环境密切相关。(3)整体而言,持续干旱日数、气温日较差和暖夜日数对全域草地EGS动态变化具有极大的重要性。就不同草地类型而言,温带草甸草原主要受到气温日较差的影响,温带山地草甸和温带沼泽化草甸EGS变化主要受日最高气温的最大值的影响,日最低气温的最大值和暖夜日数分别对温带草丛和温带荒漠草原的影响最大;持续湿润日数和持续干旱日数则分别对生长在水分限制地区的温带盐生草甸和温带典型草原的EGS产生强烈作用。

Vegetation phenology is undergoing profound changes as the frequency and intensity of extreme climate events increase.However,the mechanisms of end of growing season(EGS)response to the extreme climate has not yet been elucidated,especially for grasslands in arid and semi⁃arid regions.Therefore,this study focused on the temperate grasslands in China,extracted grassland phenological parameters based on the long⁃term GIMMS NDVI3g data from 1982 to 2015,and analyzed its temporal and spatial variation patterns.Additionally,correlation analysis and random forest models were used to explore the response of EGS in temperate grassland to the extreme climate change.The results showed that:(1)the multi⁃year average EGS in the whole region mainly occurred in 270—290 day of year(DOY),59.8%of the pixels showed a delayed trend,and the pixels with a significant delay(P<0.05)were mainly distributed in Tianshan Mountains,the Altai Mountains in Xinjiang,the western Junggar Basin,the northern Loess Plateau,the western Hulunbuir Plateau,and the Xiaoxing′an Mountains.(2)There was a widespread positive correlation between EGS and extreme temperature events(yearly maximum value of daily minimum temperature(TNx),yearly maximum value of daily maximum temperature(TXx),percentage of days when daily minimum temperature>90th percentile(TN90P),percentage of days when daily maximum temperature>90th percentile(TX90P)).In contrast,the relationship between extreme precipitation events and EGS was relatively complex,which was closely related to the physiological strategies and environment of each grassland type.(3)Overall,maximum length of dry spell(CDD),daily temperature range(DTR),and TN90P were of great importance to the dynamic changes of EGS on the temperate grassland.For different grassland types,the EGS changes in temperate meadow steppe(TMS)were mainly affected by DTR;those changes in temperate mountain meadow(TMM)and temperate swampy meadow(TSM)were affected by TXx.TNx and TN90P had the greatest impacts on the temperate grassland forb(TGF)and temperate desert steppe(TDS),respectively;maximum length of wet spell(CWD)and CDD had the stronger effects on the EGS in temperate halobiotic meadow(THM)and temperate typical steppe(TTS)growing in water⁃limited areas,respectively.