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植被物候被认为是全球变暖的一个敏感指标,特别是在青藏高原。然而,自2000年以来,对气候变暖是否能够促进草地物候期的提前存在争议。升温以及生长季节早期降水如何与之交互对半干旱草原物候产生影响鲜为人知。本研究中,我们提取藏北当雄草地2000–2014年生长季平均NDVI(GNDVI)和草地物候变化信息,分析了NDVI、物候变化和气候变化的关系。结果表明天然草地GNDVI呈下降趋势。GNDVI的年际变化主要受5月下旬至7月的气温和4月至8月的降水影响。由于生长季起始期不断延迟,生长季结束期基本不变,天然草地生长季长度大大缩短,这大部分是由于春季变暖和降水减少造成。水分有效性是研究区草地生长的主要决定因素,当草地生长的温度限制被打破后,升温将增加对水的需求。近10年来,由于蒸散和水分限制不断增强,降水减少进一步加剧了气候变暖对植被物候变化的效应。全球变暖和降水减少的综合影响可能会延迟半干旱高寒草地的物候响应。
Vegetation phenology is considered a sensitive indicator of global warming, especially in the Tibetan Plateau. However, since 2000, there has been controversy over whether warming could advance the phenophase of grasslands. How warming interacts with early precipitation during the growing season is poorly understood. In this study, we extracted information on average NDVI (GNDVI) and grassland phenology during the growing season in Dangong grassland in northern Tibet from 2000 to 2014, and analyzed the relationship between NDVI, phenological changes and climate change. The results showed that the natural grassland GNDVI showed a downward trend. The interannual variation of GNDVI is mainly affected by the temperature from late May to July and precipitation from April to August. Due to the continuous delay in the beginning of the growing season and the end of the growing season, the length of the natural grassland growing season has been largely shortened, mostly due to the warming of the spring and the reduction of precipitation. Water availability is the major determinant of grassland growth in the study area and warming will increase the demand for water when temperature limits for grassland growth are broken. In the past 10 years, the declining precipitation has further aggravated the effect of climate warming on the change of vegetation phenology due to the continuous increase of evapotranspiration and water limitation. The combined effects of global warming and reduced precipitation may delay the phenological response in semiarid and alpine grasslands.