青藏高原沱沱河源多年冻土区NDVI时空变化及影响因素分析

Analysis of spatiotemporal changes and influencing factors of NDVI in the permafrost area of the Tuotuo River source area on the Qinghai-Xizang Plateau

为了更精细地刻画青藏高原多年冻土区气候变化对植被生长的影响,选取多年冻土分布较为连续的沱沱河源为研究区,基于Landsat NDVI数据,使用趋势分析、偏相关分析和地理探测器等方法,分析了2000—2021年研究区植被生长季(5—9月)NDVI对气候变化及不同地形和植被类型的响应特征。结果表明:(1)研究区NDVI整体呈波动上升趋势,增长速率为0.013 (10a)^(-1);NDVI增长区域的面积约占总面积的84.1%,其中,显著增长区域约占45.0%,主要分布于海拔较低的河流两侧及冰川区附近;而16.0%地区的植被NDVI呈降低趋势,主要分布于研究区北部的高寒荒漠草原区。不同植被类型及不同地形区的NDVI变化速率存在差异。(2)研究期间生长季降水量、气温、太阳辐射和土壤水分含量均呈波动上升趋势,生长季NDVI与降水量和土壤水分含量在年际变化中整体呈正相关关系,与土壤水分含量的相关性更强;生长季NDVI变化与气温的相关性在空间上差异较大,其中55.4%的区域呈正相关关系;生长季NDVI变化与太阳辐射整体呈负相关关系。气温升高对草甸区植被生长的促进作用更强,对草原区植被生长的抑制作用更强。(3)研究区NDVI的空间分布主要受土壤水分含量、气候因子和坡度的影响,其中,土壤水分和坡度共同作用时影响最大。本文利用多种方法量化了研究区气候因子及其交互作用对植被变化的影响,进一步揭示了区域气候环境要素对NDVI变化的影响规律,可为青藏高原多年冻土区的气候环境变化研究、生态环境保护等提供新的依据。

As a sensitive region to global climate change,the climate of the Tibetan Plateau has gradually changed to a warm and humid state in recent decades,and this change and the resulting permafrost changes have directly affected the hydrothermal conditions for vegetation growth.The normalized difference vegetation index(NDVI)is an important variable to characterize the growth of vegetation,and its time-series variation can well reflect the response of vegetation to natural factors.In order to more accurately characterize the impact of climate change on vegetation growth in the permafrost areas of the Qinghai-Tibet Plateau,this study selected the source of the Tuotuo River,where permafrost is relatively continuous,as the study area.Based on Landsat NDVI data,trend analysis,partial correlation analysis,and geographic detectors were used to analyze the response characteristics of NDVI in the vegetation growing season(May-September)in the study area from 2000—2021 to climate change and different terrains and vegetation types.The results show that:(1)NDVI in the growing season in the source area of the Tuotuo River showed an overall fluctuating growth trend during 2000—2021,with a growth rate of 0.013(10a)^(-1),of which the NDVI growth rate was faster during the period of 2000—2010[0.019(10a)^(-1)],and then slowed down to half of the growth rate of NDVI during the period of 2011—2021[0.011(10a)^(-1)].The interannual changes of NDVI in different vegetation types were analyzed,and it w as found that the better the vegetation development,the faster the growth rate of NDVI,among which the fastest growth rate was found in alpine swamp meadow[0.018(10a)^(-1)]and alpine meadow[0.017(10a)^(-1)],followed by alpine grassland[0.015(10a)^(-1)],and the slowest growth rate of NDVI was found in alpine desert grassland[0.009(10a)^(-1)].The analysis of the inter-annual changes of NDVI in different terrain areas showed that the growth rate of NDVI increased and then decreased with the elevation,and the growth rate was faster between 4700~5300 m[>0.02(10a)^(-1)];The growth rate of NDVI showed a trend of increasing first and then decreasing with the change of slope,and the fastest growth rate[0.023(10a)^(-1)]was found in the gentle slope area with a slope of 2°~6°;the growth rate of NDVI decreased with the increase of the thickness of the active layer of permafrost,and the fastest growth rate[0.029(10a)^(-1)]was found when the thickness of the active layer was less than 150 centimeters.(2)On the spatial scale,the areas with an increasing trend of NDVI in the growing season in the source area of the Tuotuo River during the period of 2000—2021 accounted for a total of 82.1%of the total area of the study area,and the area with a highly significant increasing trend accounted for 27.0%,which was mainly distributed on both sides of the river at lower elevations and in the areas near the glacier;and the areas with decreasing trend of NDVI accounted for 16.0%,which was mainly distributed in the northern part of the study area in the alpine desert grassland areas in the northern part of the study area.(3)From 2000—2021,the source area of the Tuotuo River showed an overall trend of warming and humidification,and precipitation,air temperature,solar radiation and soil water content all showed fluctuating upward trends in the growing season.2000—2021 NDVI changes in the source area of the Tuotuo River in the growing season were most significantly affected by soil water content,and warmer temperatures would prompt the melting of the cryosphere and increase soil water content,which would improve the growth status of the vegetation;NDVI changes in the growing season showed the same trend as precipitation;NDVI changes in the growing season showed the same trend as precipitation.NDVI changes in the growing season were positively correlated with precipitation;the correlation between NDVI changes in the growing season and air temperature varied widely in space,with 55.4%of the regions showing a positive correlation and 43.8%showing a negative correlation;and NDVI changes in the growing season were negatively correlated with the solar radiation as a whole.Increased precipitation in the growing season in the source area of the Tuotuo River promoted vegetation growth in the meadow area more than in the grassland area;changes in air temperature promoted vegetation growth in the meadow area more than in the grassland area,and inhibited vegetation growth in the grassland area more than in the grassland area.(4)The spatial differentiation of NDVI in the study area is mainly influenced by soil water content,climate factors and slope,and the influence is greatest when soil water content and slope interact.This paper quantifies the effects of climatic factors and their interactions on vegetation changes using various methods through the study of NDVI changes in the source area of the Yangtze River,which further reveals the influence mechanism of regional climatic and environmental factors on NDVI changes,and provides a new basis for the study of vegetation changes and ecological environmental protection in the Tibetan Plateau region.