An increasing delay in vegetation spring phenology over northern snow-covered landmass driven by temperature and snowmelt

The start of the growing season (SOS) has received much attention for its key role in the interactions between terrestrial ecosystems and the atmosphere. Recent studies have shown that the warming-induced SOS advancing trend in the Northern Hemisphere has slowed since 2000. This phenomenon is currently attributed mainly to a slowdown of the increase in preseason temperature (PT). However, snow cover dynamics, including the changes in maximum snow water equivalent (SWE_max) and snow cover end date (SCE), also have a significant impact on SOS over the northern snow-covered landmass, but their influence on the slowdown in the SOS advancing trend has not been thoroughly investigated. In this research, we first employed four commonly utilized methods for phenological extraction to extract SOS and its associated trends. We then evaluated the response of SOS to PT, SWE_max and SCE, and determined the relative importance of these three influencing factors. Our results showed that in the northern non-mountainous snowy areas: (1) The slowdown in the overall SOS advancing trend after 2000 can be attributed to the increasing delay in the SOS between 60°N and 70°N. (2) The impact of SWE_max on SOS is negligible in most regions, except for some regions in the northern Chukotka Peninsula and southern Eurasia. In temperate zones, SOS is primarily driven by PT, while in Arctic-boreal zones, it is mainly influenced by both PT and SCE. (3) The expansion of the decreasing trend in PT and the delayed trend in SCE between 60°N and 70°N after 2000 were the primary cause of the increasing delay in SOS. Our study confirms, for the first time, that the delay in SCE is an important but neglected factor contributing to the slowdown of the SOS advancing trend in the Northern Hemisphere after 2000.