| Vegetation is a core component of terrestrial ecosystems,connecting the atmosphere,hydrosphere,biosphere,pedosphere,and lithosphere.The start of the growing season（SOS）is an important indicator used to describe the beginning of vegetation growth on the land surface and is sensitive to climate factors such as temperature and precipitation changes.New research suggests that the accumulation of spring heat during vegetation growth may potentially affect the SOS,making it crucial to understand the impact of climate change on vegetation growth and SOS.In this study,based on MOD13C1 data,three methods including the Logistic curve curvature extreme value method,the dynamic threshold method,and the double logistic four-parameter method were used to extract the SOS.Based on the SOS and meteorological data,the growing degree-days（GDD）were calculated,and the heat requirements for spring vegetation greening on the Mongolian Plateau and its main influencing factors（temperature and precipitation）were discussed by analyzing the changes in GDD.Research indicates:（1）From 2001 to 2020,the distribution patterns of growing degree days（GDD）in the Mongolian Plateau were generally similar using either the 0℃or 5℃threshold method.The regions with the minimum heat requirement were mainly located in the sparsely vegetated areas of the southwestern Mongolian Plateau and the extremely small area of the Kent Mountains in the north,with a GDD requirement of approximately 50-150℃for the onset of greenness.Conversely,the northern section of the Altai Mountains,the Hangai Mountains,and the western side of the Daxing’anling Mountains had higher GDD requirements of approximately350-450℃.The GDD requirements in other regions were mainly concentrated in the range of 200-300℃.In addition,the overall trends in GDD change were consistent among the three methods,showing an increasing trend.Approximately 40%of the pixels showed a significant increasing trend（P<0.05）,and the differences in significant areas were small.Under the 0℃threshold,the multi-year variation range of GDD was 5.09℃·a-1,with an average value of 172.9℃.Under the 5℃threshold,the multi-year variation range was 4.89℃·a-1,with an average value of 149.9℃.There was no significant increasing trend in GDD under either threshold during the study period.（2）At the threshold of 0℃,the relationship between the length of the growing season and the number of days with low temperatures is negatively correlated in approximately 99.9%of the area,meaning that an increase in the number of days with low temperatures leads to a decrease in the length of the growing season.The correlation between the length of the growing season and spring precipitation is roughly equal in area,with a negative correlation accounting for 54.6%.The area with a negative correlation between the length of the growing season and the winter precipitation of the previous year accounts for 69.5%of the total pixels.In other words,as the winter precipitation of the previous year increases,the amount of heat required for vegetation to turn green（GDD）decreases.Compared to the threshold of 5℃,the relationship between the length of the growing season and the number of days with low temperatures is negatively correlated in approximately 99.3%of the area,with a primarily negative correlation with spring precipitation accounting for about 53.6%.The winter precipitation of the previous year is negatively correlated in the majority of the area,accounting for 65.3%of the total pixels.（3）The temperature sensitivity of the Mongolian Plateau is mainly negative,accounting for about 62.6%of the total area,mainly distributed in the central and western regions of the plateau.Among them,the significantly negative correlation accounts for 1.5%（P<0.05）,indicating that an increase in accumulated temperature（GDD）in a given year will advance the vegetation green-up period,and vice versa.The sensitivity of growing degree days to changes in low-temperature days is mainly negative,accounting for about 98.8%of the total area,with significantly negative sensitivity occupying less than 1%of the area.The sensitivity of growing degree days to low-temperature days varies with different patterns depending on changes in low-temperature days and altitude.The number of low-temperature days gradually increases from low to high latitudes,and the sensitivity（change amplitude）gradually decreases with increasing latitude.The altitude of the study area gradually increases from east to west,and the sensitivity（i.e.the magnitude of the numerical change）also decreases in order. |
