青海湖流域草地植被动态变化趋势下的物候时空特征

Characteristics of spatial and temporal phenology under the dynamic variation of grassland in the Qinghai Lake watershed

植被物候不仅直接受气候变化的影响,还反作用于气候变化。因此,明确植被物候变化的驱动机制对于进一步研究气候变化与物候的相互作用具有重要的意义。选取位于青藏高原东北部的青海湖流域,利用MODIS 16d增强植被指数(EVI)合成数据,来分析草地物候时空格局特征以及不同EVImax变化趋势下草地物候期(返青期、枯黄期及生长季)的变化趋势。研究得到以下结果:(1)在气候变化和人类活动等因素的共同作用下,青海湖流域的EVImax变化呈现多元化趋势,EVImax增加、不变、降低趋势并存;(2)1990—2012年期间,流域内温度上升、降水量增加趋势显著,温度上升速率为0.42—0.83℃/10a,降水量增加速率为43.20—44.68 mm/10a。刚察、天峻气象站草地返青期在2001—2012年期间呈现延迟趋势,枯黄期变化趋势不显著,生长季呈现缩短趋势;(3)流域内草地返青从4月下旬持续到6月上旬,枯黄期从8月中旬持续到10月上旬,青海湖东岸、南岸、布哈河入湖口区域以及流域西部山坡和平坦的谷底地区牧草最早进入返青期,返青空间格局呈现由湖岸向四周高海拔地区延伸趋势,草地枯黄空间格局与返青期相反;(4)不同EVImax变化趋势下,草地返青期、枯黄期、生长季变化趋势表现出差异。草地EVImax降低趋势下,牧草返青期呈现提前趋势,枯黄期延迟,生长季延长;EVImax增加趋势下,牧草返青期延迟,枯黄期变化不明显,生长季缩短;EVImax不变区、农田的返青与枯黄期变化趋势并不明显,但是农田生长季缩短趋势较明显。

Phenology is a critical feature of vegetation, and it could reflect the effects of climate variability and change on vegetation growth. There are complex feedbacks between vegetation phenology and climate change, therefore, researches on the driving mechanism of the change of vegetation phenology would provide a better understanding of the interactions between climate change and phonology. In this study, we selected Qinghai Lake watershed located in the northeastern of the Qinghai-Tibet Plateau as study area and used MODIS 16 days Enhanced Vegetation Index（EVI）composite data to analyze the spatiotemporal pattern and trend of grassland phenology （the start of the growing season, the end of growing season and days of growing season） under the different variations of EVImax of grassland. The results of the study were concluded as： （1） The changes in EVImax of grassland showed a diverse trend under the influence of climate change and human activities, an increasing, unchanged and a decreasing trend of EVImax coexisted at the same time in the whole watershed. （2） Temperature and precipitation increased significantly from 1990 to 2012. Air temperature increased at the rate of 0.42-0.83℃/10a, and precipitation increased at rate of 43.20-44.68 mm/10a. The date of the start of the growing season showed a delay trend around both Gangcha and Tianjun weather station, the date of the end of growing season showed no obvious change but the days of the growing seasons showed a shortening trend between 2001 and 2012. （3） The date of the start of the growing season lasted from April 20 to June 10 and the date of the end of growing season from Mid-August to September 10 in the Qinghai Lake watershed. The places have early dates of the start of the growing season include the eastern and southern lakesides of the Qinghai lake, Buha River estuary, hillslope and flat area of the valley in the western of the watershed. The date of the start of the growing season extended from lakeshore to the high-altitude areas, but vice verse for the date of the end of growing season. （4） Vegetation phenology showed different trend under the different variation of EVImax. When the EVImax decreased, the date of the start of the growing season advanced, the date of the end of growing season delayed and the days of the growing season increased. In contrast, the date of the start of the growing season delayed, the date of the end of growing season showed no obvious change and the days of the growing season shorten when theEVImax increased. The farmland and the places without change in EVImax had no obvious changes in the date of the start of the growing season and of the end of growing season, however, days of the growing season shorten for farmland. （5） The result suggests that the vegetation phenology change was a complex process, probably affected by multiple factors, such as grassland degradation caused by over-grazing, vegetationEVImaxincreased under the climate change, enclosed grassland with unreasonable management, the vegetation ecosystem of succession and so on. Therefore, using of remote sensing data for vegetation phenology studies must distinguish dynamic changes of the vegetation of the different experimental samples firstly. Otherwise, it would affect the phenology study results.