Time-varying trends of vegetation change and their driving forces during 1981-2016 along the silk road economic belt

Understanding the trend in vegetation evolution and its underlying causes is critical to revealing changes in ecosystem's structure and function. However, less is known about the time-varying trends in vegetation and their main driving forces along the Silk Road Economic Belt (SREB). Using the two-band enhanced vegetation index (EVI2) as the proxy for vegetation growth, we investigated the time-varying trends in vegetation and their possible responses to the various driving factors during 1981-2016 using the ensemble empirical mode decomposition (EEMD) method. Results indicated that the vegetation exhibited significant trend shifts, which mainly occurred after 2000. Moreover, the greening to browning shifts occupied much larger areas than the browning to greening shifts (23.23% and 7.44%, respectively). The browning trends were largely expanded and enhanced after the turning points, which were primarily located in Eastern Europe and Central Asia. The vegetation greenness increased faster south of 50 degrees N and east of 60 degrees E, at elevations of 1000-3000 m, and in evergreen needle-leaf forests and sparsely vegetated areas, respectively. The shifted trends in the vegetation changed earlier and more quickly than the monotonic trends, which mainly occurred at lower elevations, in croplands and evergreen needle-leaf forests and near 40 degrees E, 70 degrees E, and north of 45 degrees N. The vegetation experiencing non-significant changes was water-limited in most of the study area, especially in Central Asia. Warmer temperatures and droughts were likely the main causes of the greening to browning shifts. Time-varying trend analysis can correctly reveal the actual trend in vegetation and its responses to climate change, which provides scientific reference for ecological protection along the SREB.