1982—2015年青藏高原植被变化的主导环境因子

The dominated environmental factors of vegetation change on the Qinghai-Tibet Plateau from 1982 to 2015

近几十年中随着全球气候环境变化,青藏高原处于变暖变湿过程之中,植被生长发生了显著变化。基于卫星遥感归一化植被指数(NDVI),采用增强回归树模型(BRT)定量分析了1982—2015年影响青藏高原植被生长变化的主要环境因子的相对重要性。结果表明:(1)整个青藏高原生长季(6—9月份)空间平均NDVI和降水呈上升趋势(1.265×10^(-4)a^(-1)和0.746 mm/a,P>0.05);温度和土壤湿度呈现显著增加趋势(0.048℃/a和3.954×10^(-4)a^(-1),P<0.01);向下短波辐射显著减小(-0.070 W m^(-2)a^(-1),P<0.01)。(2)青藏高原34.0%地区NDVI表现出显著增加趋势,主要分布在青藏高原北部大部分地区和西部部分区域;9.2%地区NDVI呈显著减小趋势,主要位于青藏高原东部地区。(3)土壤湿度、年均温、年降水和向下短波辐射分别解释了生长季NDVI变化的42%,19%,10%和9%。(4)土壤湿度、年均温、年降水和向下短波辐射对青藏高原植被生长动态影响的相对大小具有明显的空间差异特征。温度上升是青藏高原东北部和中部部分地区NDVI变化的首要因素(相对贡献率>40%),而土壤湿度增加是青藏高原西南部及东南部分地区NDVI变化的主要原因(相对贡献率>50%)。总体上,温度上升及由此所导致的冰冻圈消融引起的土壤含水量上升是近几十年青藏高原植被生长动态快速变化的主要原因。

With the global climate change in recent decades,the Qinghai-Tibet Plateau is in the process of warming and wetting,and the vegetation on the Qinghai-Tibet Plateau has undergone significant changes.Based on the normalized vegetation index(NDVI)from satellite remote sensing observations,we used the boosted regression tree approach(i.e.,a machine learning algorithm)to quantitatively examine the relative importance of the environmental factors including temperature,precipitation,soil moisture,and solar downward shortwave radiation on influencing the vegetation dynamics in the growing seasons on the Qinghai-Tibet Plateau from 1982 to 2015.The results showed that:(1)the spatially averaged NDVI and precipitation on the Qinghai-Tibet Plateau in the growing seasons(from June to September)increased during 1982—2015,with the trend values of 1.265×10^(-4)a^(-1)and 0.746 mm/a,respectively,but the trends were not significant(P>0.05);both the spatially averaged temperature and the spatially averaged soil moisture in the growing seasons significantly increased during 1982—2015(P<0.01),with the trend values of 0.048℃/a and 3.954×10^(-4)a^(-1),respectively;while the spatially averaged solar downward shortwave radiation significantly decreased at a rate-0.070 W m^(-2)a^(-1)(P<0.01).(2)The NDVI in 34.0%area of the Qinghai-Tibet Plateau showed a significantly increasing trend,which mainly distributed in most of the northern and western Qinghai-Tibet Plateau;in 9.2%area of the Qinghai-Tibet Plateau,the NDVI showed a significantly decreasing trend,which mainly located in the eastern Qinghai-Tibet Plateau.(3)The variations in mean soil moisture,mean temperature,cumulative precipitation,and mean solar downward shortwave radiation in the growing seasons during 1982—2015 explained 42%,19%,10%and 9%of the NDVI variation,respectively.(4)The relative magnitudes of the effects of mean soil moisture,mean temperature,cumulative precipitation,and mean solar downward shortwave radiation showed obvious spatial differences on the dynamics of vegetation growth in the growing seasons on the Qinghai-Tibet Plateau.The increasing temperature is the primary factor for the NDVI changes in the northeastern and central Qinghai-Tibet Plateau(with relative contribution rate above 40%),however,the increases in soil moisture are the main reason for the NDVI changes in the southwestern and southeastern Qinghai-Tibet Plateau(with relative contribution rate above 50%).In general,the rising temperature and its resulting in the increased soil water content through melting cryosphere(snow,ice,glaciers,and frozen soil)are main reasons for the rapid changes in vegetation dynamics on the Qinghai-Tibet Plateau in recent decades.