The Response Of Spring Phenology To Seasonal Snow Cover In Tibetan Plateau

The Tibetan Plateau alpine ecosystem is one of the most climate change-sensitive ecosystems globally,with one of the largest snows covers and the widest snow distribution in the Northern Hemisphere.Snow cover,a critical component of the alpine ecosystem,has an important effect on the spring vegetation growth in the alpine grasslands,which is crucial for the global carbon cycle.For example,snow cover can protect vegetation from frostbite in spring.Meanwhile,snowmelt supplies the essential water required for vegetation growth.Therefore,with the combined spring soil temperature and moisture,studying the effects of the seasonal snow cover on the start of season（SOS）in the alpine grasslands can help to deeply understand the complex vegetation’s dynamic response in snow cover,soil temperature and moisture in the"Third Pole"ecosystem.In this study,we not only evaluated the temporal and spatial SOS trends,snow cover,and spring soil temperature and moisture,but also analyzed how the snow cover affects SOS via spring soil temperature and moisture,using the structural equation modeling（SEM）analysis of the Tibetan Plateau from 2000 to 2020.The results are summarized:（1）In the Tibetan Plateau,from southeast to northwest,the SOS was delayed.SOS was from the 89^th day to the 192^nd day in the Julian year with an average of 126.90^th.From 2000 to 2020,the average SOS advanced by 0.20 d/year.We found that 65.38%of the regions which showed an earlier SOS trend were distributed in the eastern and central parts of the Tibetan Plateau;while a delayed SOS trend was accounted by the remaining 34.6%distributed in the western parts of the Tibetan Plateau.（2）In the Tibetan Plateau,snow started accumulating in September,with the largest snow cover being in February,and then the snow melted from March to June,and finally led to the smallest snow cover being in July to August in the mountainous areas.There are noticeable altitudinal differentiating characteristics for the spatial distribution pattern of snow cover in the region.Compared to low altitude regions,we found that the snow onset date（SOD）came earlier,while the snow end date（SED）arrived later.The snow duration days（SDD）were longer,whereas the snow cover days（SCD）were greater in the high mountainous areas.The SOD averaged on November17;the SED averaged on March 27;the average SDD lasted 130.71 days,while the SCD averaged 64.63 days in the Tibetan Plateau.From 2000 to 2020,the average SOD and SED were delayed by 0.13 d/year and 0.12 d/year,respectively;whereas the average SDD and SCD were prolonged by 0.07 d/year and 0.22 d/year,respectively.In the mountainous areas,the SOD was delayed;the SED had advanced,and the SDD were shortened,while the SCD were reduced.However,we found the opposite trends in the lower altitude areas.（3）The spring daytime and nighttime land surface temperatures averaged between287.63 K and 262.64 K,and increased by 0.06 K/year and 0.13 K/year from 2000 to2020,respectively.The spring daytime land surface temperatures showed an increasing trend in high mountainous area,while the spring nighttime land surface temperatures showed a noticeable distribution pattern of increasing in the eastern and decreasing in the western Tibetan Plateau.The average spring soil moisture was 0.07 m~3/m~3,and it significantly increased from 2003 to 2018（P<0.05）.We observed an increasing spring soil moisture trend in 90.4%of the Tibetan Plateau and this accounted for 58.6%for the significantly increasing trend（P<0.05）.The decreasing trend for 9.3%of the region was mainly located around the watershed of the study region,in the Arjinshan and the Qaidam Basin.（4）Snow cover affected the spring soil temperature and moisture,which then indirectly affected the alpine grassland SOS.The delayed snow melt and the shortened SDD increased the spring soil temperature and moisture,resulting in the early alpine grassland SOS.The earlier SED and extended SDD were the main reasons for the delayed SOS.Our study results showed that snow cover was an indirect factor influencing the SOS change.