Change in the elevational pattern of vegetation greenup date across the Tianshan Mountains in Central Asia during 2001–2020

Abstract

Variations in land surface phenology (LSP) along elevation gradients strongly impact human life and wildlife species distribution across the Tianshan Mountains (TM) in arid and semiarid Central Asia. However, changes in the elevational patterns of LSP in recent decades have not been well understood for the TM. Here, we characterized changes in vegetation greenup date (GUD) and its elevational pattern in the TM and five subregions during 2001–2020, with Moderate Resolution Imaging Spectroradiometer (MODIS) time series of the enhanced vegetation index (EVI). Impacts of land surface temperature (LST) and precipitation on GUD changes were also examined. The results show that GUD changes across the TM were mostly nonsignificant (P > 0.05). Approximately 13.4% of the region experienced significant advance in GUD. Furthermore, GUD at low and middle elevations (approximately 1000–2500 m) showed greater proportions of significantly earlier trends. This elevation dependence of GUD changes led to altered elevational patterns of GUD. First, most GUD isolines showed shifts toward higher elevations, and the GUD isolines of day of year (DOY) 110 and 120 located at low and middle elevations exhibited greater mean elevational shifts than those of others for most subregions. Specifically, the mean elevation of the DOY 110 isolines moved from approximately 1325 m to 2126 m in a subregion. Second, increased GUD elevation gradients were observed in several subregions. The spatial pattern of GUD trends may be primarily caused by the increased LST in April, particularly nighttime LST. The results provide information for rangeland management in the context of rangeland degradation across the TM.