Accelerated rate of vegetation green-up related to warming at northern high latitudes.

Mid- to high-latitude vegetation are experiencing changes in their seasonal cycles as a result of climate change. Although the rates of seasonal growth from winter dormancy to summer maturity have accelerated because of changes in environmental conditions, less attention has been paid to the rate of vegetation green-up (RVG) and its dynamics, which could advance vegetation maturity. We analyzed the long-term changes in RVG and the drivers at high northern latitudes for 35 years (1982-2016) using satellite-retrieved leaf area index data based on partial correlation analyses and multivariable linear regression. The rates tended to increase significantly with time, particularly at high latitudes above 60°N in North America (1.8% mon-1 decade-1 , p < .01) and Eurasia (1.0% mon-1 decade-1 , p < .01). The increasing trend in North America was mostly because of increased heat accumulation in spring (1.2% mon-1 decade-1 ), that is, more rapid green-up owing to warming, with an increased carbon dioxide concentration (0.6 mon-1 decade-1 ). The trend in Eurasia, however, was induced by warming, increased carbon dioxide concentration, and stronger radiation, 1.0%, 0.7%, and 0.5% mon-1 decade-1 , respectively, but was partly counteracted by earlier pregreen-up dates of -1.2% mon-1 decade-1 , that is, earlier initiation of growth which counteracted green-up rate acceleration. The results suggested that warming was the predominant factor influencing the accelerated RVG at high latitudes; however, Eurasian vegetation exhibited different green-up dynamics, mitigating the influence of warming with the earlier pregreen-up. Our findings imply that high-latitude warming will drive vegetation seasonality toward rapid green-up and early maturity, leading to the reinforcement of climate-vegetation interactions; however, the consequences will be more distinct in North America owing to the absence of alleviation by earlier pregreen-up.