Analysis On The Changes And Influencing Factors Of Start Of Growing Season For Vegetation In Khangai Mountain Of Mongolia

Vegetation phenology is an important indicator of the response of terrestrial ecosystems to climate change.Due to the fragility of vegetation ecosystems and hydrological systems,mountainous areas are considered one of the most vulnerable areas to the impact of global warming.The start of growing season is considered one of the simplest and most effective indicators of climate change,as it has a strong impact on the balance of terrestrial ecosystems and is an independent measure of ecosystem response to climate change.Khangai Mountain is located in the central part of Mongolia,in an arid and semi-arid region,with a northwest southeast trend.It is the largest mountain range in Mongolia and also the watershed between the Arctic Ocean basin and the inland river basin.Its ecological environment is fragile and sensitive,and vegetation is easily affected by climate change and extreme climate.Therefore,studying the vegetation phenology of Khangai Mountain has profound significance in revealing the response mechanism of mountain vegetation ecosystems to climate change.This article takes Khangai Mountain as the research area and uses MODIS NDVI data from 2000 to2021.The start of growing season(SOS)of vegetation in the research area is obtained through the dynamic threshold method of S-G filtering.Based on data such as terrain,temperature,precipitation,snow cover,and soil moisture,the changes and main influencing factors of start of growing season in Khangai Mountain in the past 22 years are analyzed.The following conclusion is drawn:(1)The start of growing season in Khangai Mountain is mainly concentrated from the 110-130 d(late April to early May),and spatially,it shows a delayed trend from the surrounding to the middle,which is consistent with the distribution characteristics of mountain altitude and water and heat.In the past 22 years,the vegetation SOS in Khangai Mountain has shown a weak trend of advance,with an advance amplitude of 0.07 d/a(P=0.78).The advance area accounts for 58.11% of the total area,mainly distributed in the higher altitude areas in the southern part of the Khangai Mountains;The SOS in 41.89% of the study area showed a delayed trend,mainly appearing in the lower altitude areas in the north.(2)For different vegetation types,the SOS of other vegetation types outside the weeding meadow grassland has an advance trend,among which the advance of SOS of alpine grassland is the most obvious,with an advance range of 0.21 d/a,and that of coniferous forest is the smallest,with an advance range of 0.006 d/a.As the altitude increases,the SOS of different vegetation shows a delay phenomenon,but the delay amplitude varies among different vegetation.The delay in SOS in desert grasslands is the largest,with vegetation delaying SOS by about 7.2 days for every 500 meters of elevation increase.The delay in SOS in alpine grasslands is the smallest,with vegetation delaying SOS by about 0.5 days for every 500 meters of elevation increase.The SOS of desert grassland vegetation in the same vertical zone is the earliest,followed by typical grassland,meadow grassland and coniferous forest,while the SOS of alpine grassland is later than other vegetation in the same vertical zone.(3)From the perspective of terrain characteristics,as the altitude increases,the vegetation SOS of Khangai Mountain shows a delayed trend,and a significant mutation occurs around 1330 m;Below 1330 m,the vegetation SOS is delayed by about 10 days for every 100 m increase in altitude,while above 1330 m,the vegetation SOS is delayed by about 1 day for every 100 m increase in altitude.The SOS of vegetation in Khangai Mountain shows a significant delay trend with the increase of slope.Among them,when the slope is less than 12 °,the SOS of vegetation is significantly delayed with the increase of slope.After the slope is greater than 12 °,the SOS of vegetation shows a weak delay phenomenon with the increase of slope.On different slopes,the SOS of vegetation on the sunny slope is about4 days earlier than that on the shaded slope.(4)By studying the correlation between influencing factors and vegetation SOS in Hang ’ai Mountain,it is found that SOS in the study area is mainly negatively correlated with temperature,precipitation,snow depth and snow coverage,while has positive correlation with soil moisture.Which indicates that vegetation SOS is advanced with the increase of temperature,precipitation,snow depth and snow coverage,among which temperature in January,precipitation in December,snow depth and snow coverage in May have the greatest influence on vegetation SOS.Finally,the contribution rate of each environmental factor is calculated by multiple linear regression analysis,and it is found that the snow coverage rate is the main influencing factor of the change of vegetation in Khangai Mountain during the start of growing season.