Dynamic Change Of Grassland In Northern Tibetan Plateau Based On Remote Sensing And IBIS Model

Grassland ecosystem,ranked as the first largest terrestrial ecosystems in Tibetan Plateau,plays an important role in regulating global carbon cycle and climate.Northern Tibetan Plateau forms the component of the Tibetan Plateau,where grassland ecosystem is related to the ecological and environmental conditions of its surrounding areas,as well as the national and regional ecological safety.In recent years,with the increase of interference and destruction,grassland degradation has accelerated and this may become a threat to regional environment and sustainable development.Therefore,on the basis of MODIS-NDVI remote sensing data and meteorological data during 2000-2015,we analyze spatial distribution and changes in the NDVI and the relationships between NDVI and climate factors by using methods of multiple linear regression and correlation analysis.By the means of IBIS model,the inter-annual variation of grassland NPP is explored in order to identify the applicability of this model in Northern Tibetan Plateau,by which is explored to predict the dynamics of NPP in the future climate background.The results are shown as follows:(1)The spatial-temporal dynamics of climate in Northern Tibetan Plateau.During the period 2000-2015,the tendency of an increase in temperature is significant,so is precipitation.In a word,climate is warming and wetting in Northern Tibetan Plateau.However,the climate changes are found with difference in terms of grassland types.For example,on alpine meadow area,its precipitation shows a downward trend,but the temperature increase is slightly higher than that of alpine grassland.On alpine grassland area,the climate is towards to warming and wetting,with the significant increase of temperature and precipitation.In general,the climate shows a warming and wetting trend in Northern Tibetan Plateau and alpine grassland area,while the climate of alpine meadow area is warming and drying.(2)The spatiotemporal dynamics of grassland NDVI in Northern Tibetan Plateau.The spatial distribution of mean NDVI shows a continuous band characteristic,which is high in the southeast and low in the northwest.It decreases gradually from southeast to northwest.On alpine meadow area,located in the semi-humid climate zone of the sub-frigid zone,with the sufficient water,NDVI is relatively higher.Alpine meadow is composed of crops and alpine shrub meadows.On the contrary,on alpine grassland area,with the harsh environment dominated by the high-cold climate,NDVI is lower.Alpine grassland is composed of alpine steppe and alpine desert steppe.As a whole,the grassland in Northern Tibetan Plateau shows a slight degradation trend during 2000-2015,mainly emerged in the southeast part.Respectively,the alpine grassland is relatively stable and alpine meadow is slightly decreased.Generally,the degradation trend is obvious in good vegetation coverage on the southeast part,while the improvement in poor vegetation coverage on the west part.(3)The response of vegetation to climate.The NDVI in Northern Tibetan Plateau is closely related to the temperature and precipitation.On alpine meadow area,there is a significant positive correlation between NDVI and precipitation,so is as the spring precipitation.The cause of degradation may be the result of warming and an arid climate.On the background of warm-dry climate,with the decrease of rainfall and an increase of temperature,the soil moisture content reduces,the evaporation enhances,the soil nitrogen mineralization rate decreases and inorganic nitrogen content declines.Degradation is prominent with the decreasing growth rate.On the contrary,there is a significant negative correlation between NDVI and precipitation on alpine grassland area.There is an increasing trend of temperature and precipitation,but a fluctuating and decreasing tend of NDVI.The reason may be that the increase of spring temperature would cause the soil frozen depth become thinner,shorten the frozen period,strengthen the evaporation of soil surface,cause severe water dispersion,and reduce nutrients.Furthermore,it limits the amount of water needed of vegetation growth and may not conducive to the growth of alpine grassland.In addition,we can conclude that there is a significant spatial heterogeneity in response of vegetation to climate in different eco-geographical regions.(4)The applicability of IBIS model: Comparison between simulation of NPP in 2000-2015 and observation of NDVI indicates that the approach of IBIS was feasible,with the reason of that spatiotemporal dynamics of grassland are consistent.Meanwhile,it could evaluate change of net primary productivity under future climate change conditions.