地表水热要素在青藏高原草地退化中的作用

Role of surface hydrothermal elements in grassland degradation over the Tibetan Plateau

在全球气候变暖和频繁的人类活动影响下,青藏高原草地生态系统发生了生产力下降、生物多样性减少及生态功能退化等一系列现象。与传统观测技术相比,遥感技术具有大范围、快速和连续监测等优点,因此被广泛用于区域尺度的草地植被长时间序列监测。以往对青藏高原草原植被影响因子的研究多集中在气温与降水,而相比较于气温和降水,地表温度和土壤湿度直接作用于植物的根部,对植物种子的萌芽和植株的生长也都有着重要影响,所以地表温度和土壤湿度与植被生长的关系更加紧密。基于遥感技术,利用青藏高原草地区域的MODIS和AVHRR数据,选择草地植被覆盖度作为草地退化的遥感监测指标,建立了青藏高原草地退化遥感监测和评价指标体系,并对青藏高原2001—2017年的草地退化状况进行了遥感监测和评价。同时,利用遥感数据获取青藏高原区域尺度的地表温度和温度植被干旱指数数据,用于指示地表水热状况,最后基于回归方法分析了地表水热要素在青藏高原草地退化中的作用。结果表明:从2001—2017年,青藏高原植被退化程度空间差异明显,柴达木盆地和青海湖附近退化较为严重,喜马拉雅山脉北部、昆仑山脉南部、冈底斯山脉北部交汇的地区退化也较严重。在2001—2017年间,青藏高原草地未退化面积从50.60%上升到59.00%,说明青藏高原草地整体上在朝着改善的方向发展。2001—2017年内,青藏高原草地整体上大部分时间处于轻度退化状态,但是2001年和2015年这两个年份青藏高原草地退化整体上达到中等退化水平。通过回归分析发现,土壤湿度主导的对青藏高原草地的影响面积达到14.04%。地表温度主导的影响面积达到草地总面积的约36.61%。但地表温度与植被之间相互影响,且主要呈现负相关关系。其中,在温性草甸地区,当植被覆盖度较低时,地表温度正向影响植被生长。

Under the influence of global warming and frequent human activities,a series of deterioration phenomena including the decreased productivity,decreased biodiversity,and ecological function degradation have taken place in the grassland ecosystem on the Tibetan Plateau.Compared with the traditional monitoring method,remote sensing technology has the advantages of wide range,rapid and continuous monitoring.Therefore,it is widely used to monitor grassland vegetation at the regional scale.Previous studies on impact factors of grassland vegetation on the Tibetan Plateau mainly focused on the climatic hydrothermal elements(i.e.,temperature and precipitation).However,compared with the temperature and precipitation,the surface temperature and soil moisture have more direct effects on the physical and biochemical processes of vegetation growth.Based on remote sensing technology,MODIS and AVHRR data were collected on the Tibetan Plateau,and fractional vegetation cover was selected as the remote sensing index of grassland degradation to calculate the grassland degradation index(GDI).Then,the surface temperature and soil moisture index at the regional scale of the Tibetan Plateau were obtained to indicate the surface hydrothermal condition.Finally,the influence of surface temperature and soil moisture on vegetation was studied based on regression analysis.The results show that there were obviously spatial differences in vegetation degradation on the Tibetan Plateau from 2001 to 2017.The degradation is more severe near the Qaidam Basin and Qinghai Lake than other regions on the Tibetan plateau.The degradation is also severe in the northern part of the Himalayas,the southern part of the Kunlun Mountains,and the northern part of the Gangdise Mountains.However,the non-degraded grassland area proportion of the Tibetan Plateau increased from 50.60% to 59.00% from 2001 to 2017,which indicated that the vegetation of the Tibetan Plateau was in the direction of improvement overall.From 2001 to 2017,the grassland on the Tibetan Plateau was in mild degradation for most of the time,but in 2001 and 2015,the grassland degradation on the Tibetan Plateau reached a level of moderate degradation.The area dominated influence by soil moisture accounted for 14.04% of the whole grassland over the Tibetan Plateau through regression analysis.And the area dominated influence by land surface temperature accounted for about 36.61%of the whole grassland.Despite this,the relationship between land surface temperature and vegetation coverage affected each other,and the relationship between them was mainly negative.When the NDVI is at a lower level,land surface temperature positively affected the growth of temperate meadow.