Vegetation Dynamics And Its Response To Climate Change In The Qilian Mountains In Recent 20 Years

Vegetation plays an indispensable role in providing organic matter for terrestrial organisms,regulating the carbon cycle and promoting energy exchange for the ecosystem.The Qilian Mountains are important ecological security barrier and an important source of water in western China,and the changes of its ecological environment have a profound impact on the local and even the construction of ecological security in western China.In recent years,under the combined influence of climate warming and human activities,the vegetation in the Qilian Mountains has changed significantly.Especially in the high,cold and dry environment,the meadow ecosystem is more sensitive and more susceptible to external environmental changes,thus affecting the stability of the plateau ecosystem.However,current research rarely systematically analyzes the time-lag and time-accumulation effects of climate variation on vegetation dynamics,especially in arid and semi-arid mountainous terrain.Therefore,understanding the trend of vegetation change and its reaction to climate variation is important for revealing the mechanism of ecosystem behavior.This study takes the typical mountain,the Qilian Mountains,as the research object,and explores the temporal and spatial variation of the normalized vegetation index(NDVI)during the growing season and its distribution with terrain(altitude,slope and aspect).Then,we analyzed the time-lag and time-accumulation effects of the response of NDVI to climate factors(precipitation,temperature),and performed multiple linear regression model predictions based on the determined combination of time effects of climate factors.After that,the partial correlation analysis method was used to identify and quantify the main controlling factors affecting vegetation growth and their response degrees.Finally,the sustainable development plan and suggestions are put forward according to the problems of vegetation growth in the Qilian Mountains.The results show that in terms of the temporal and spatial dynamics of vegetation,the NDVI in the Qilian Mountains is generally low in the northwest and high in the southeast during the growing season,and increases from west to east.From 2000 to 2019,the NDVI in the growing season in the Qilian Mountains showed an overall upward trend.Due to the intensified development of climate warming and humidification,the NDVI increased particularly significantly since 2014.The vegetation in the Qilian Mountains shows an obvious greening trend as a whole.The improved vegetation area accounts for about 92% of the total area.The vegetation restoration in the northwest is particularly obvious,and some degraded patches are concentrated in the meadow vegetation.In terms of terrain distribution,NDVI in the Qilian Mountains first increases and then decreases with the increase of altitude/slope.NDVI increases at lower altitudes(DEM < 3000 m)than at high altitudes(DEM >4000 m).Due to the difference in soil nutrient content and humidity,the NDVI on the shady slopes(northeast,east,north,and southeast)is generally higher than that on the sunny slopes(south,southwest,west,and northwest)in the Qilian Mountains.In terms of the response of vegetation to climate change,vegetation growth in the Qilian Mountains is greatly affected by early climatic conditions.The time effects of vegetation on climate vary with vegetation types and climate factors.The timeaccumulation effect of precipitation has an obvious effect on the vegetation trend,especially the desert vegetation and meadow which in a relatively arid environment.The accumulation of temperature helps to store the energy required by plants during the growth period to support their own growth and development in a relatively cold ecosystem in the Qilian Mountains.Therefore,the time-lag and time-accumulation effects of temperature work together on vegetation growth.Compared with not considering the time effects,when the optimal time effect is considered,the interpretation degree of climate factors on vegetation changes is increased by 18.5%,indicating that considering the time effect plays a pivotal role in the climatevegetation response mechanism.In addition,when considering the optimal time effect,temperature has a more significant effect on vegetation than precipitation.The inhibitory effect of precipitation on NDVI is mainly reflected in meadow and alpine vegetation in high altitude areas.The above results indicate that when the existing climate models study vegetation-climate interactions,considering the time effects of vegetation response to climate is of great significance for accurately monitoring vegetation dynamics under environmental changes.The main problems of vegetation ecology in the Qilian Mountains are the decrease of livestock carrying capacity of some grasslands,the decrease of waterconserving vegetation and the limitation of vegetation growth by topographic conditions.It is suggested that the management of grassland should be aimed at achieving the balance between grass and livestock,and measures such as tillage reseeding and biological invasion prevention should be taken.In the process of forestland protection,measures should be taken according to local conditions,ecological compensation measures should be promoted,and the awareness of forestland protection should be actively publicized to the society.In the management of vegetation,especially alpine meadow,the implementation period of specific vegetation protection strategy should be considered,and the block management should be carried out according to different terrain and vegetation characteristics.