Impact of frozen soil changes on vegetation phenology in the source region of the Yellow River from 2003 to 2015

Changes in frozen soil caused by global warming are widely expected to have significant effects on ecosystems in cold regions, and the response of the start of the vegetation growing season (SOS) to climate change on the Qinghai-Tibetan Plateau (QTP) has drawn great attention. In this study, we investigated the changes in the freezing/thawing processes and their relationship with the SOS from 2003 to 2015 in the source region of the Yellow River (SRYR) on the northeastern QTP. The results indicate that the soil thaw onset (STO) at a depth of 5 cm advanced significantly in most regions at a rate ranging from 0.09 to 1.47 day center dot year(-1), and the maximum frozen depth decreased in most regions at a rate of 0.007 to 0.031 m center dot year(-1), despite a nonsignificant increase in the spring air temperature. The SOS derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) leaf area index (LAI) data advanced in the regions covered by alpine meadow. The logistic method was shown to be better than the polynomial method in retrieving the SOS using remote sensing indexes. The gray relational analysis suggested that the advance in the STO was the major factor leading to the advance in the SOS of alpine meadow regions. Changes in the LAI during the initial period of the growing season were likely to be primarily influenced by the maximum frozen depth and soil temperature. Furthermore, the substantial effect of frozen soil changes on the SOS in the SRYR necessitates the importance of analyzing frozen soil processes to predict the response of spring vegetation phenology to climate change on the QTP.