Grassland Vegetation Change And Its Driving Force In The Loess Plateau Of China

The structure and function of grassland ecosystem response sensitively to climate change and human disturbance.Under the background of global warming and intensified human activities,clarifying the law of grassland vegetation change,quantifying the impact of environmental factors on the change of grassland vegetation and identifying the driving factors of the change of grassland vegetation are of a significant importance to understanding the change mechanism of the structure and function of grassland ecosystem and developing effective policies for ecological management and regulation.As an important part of the ecological security barrier in China,the ecological environment of the Loess Plateau is vulnerable.In the past two decades,the ecological environment of the Loess Plateau has improved significantly.However,it remains unclear that what specific factors mainly drive the dynamics of vegetation.Moreover,there is still a lack of understanding the contribution and driving mechanism of key environmental factors to the grassland vegetation activity.In order to clarify the response mechanism of grassland vegetation activities to the changes of environmental factors on the Loess Plateau,in this study,the multi-source vegetation index was used to analyze the characteristics of grassland vegetation changes in the Loess Plateau,the contribution of human activities and climate change to grassland vegetation changes was quantitatively evaluated,the specific factors driving the grassland vegetation changes in the Loess Plateau were identified by building a structural equation model,and the sensitivity of grassland vegetation response to climate change was simulated and analyzed based on the model of grassland aboveground biomass response to climate factor change on the Loess Plateau.Finally,aboveground biomass,coverage,species richness and height were used to characterize grassland vegetation growth activity,and the environmental factors affecting natural?undisturbed?grassland activity were comprehensively attributed by structural equation model on the Loess Plateau.The main results are as follows:?1?The grassland vegetation increased significantly at spatiotemporal scales on the Loess Plateau.Significant correlations?P<0.001?for SPOT-VEGETATION NDVI with the measured coverage,and MOD17A3 NPP with the measured aboveground biomass were detected.It indicated that the above two vegetation indexes were reliable for monitoring the dynamic change of grassland vegetation on the Loess Plateau.The grassland NDVI increased by an average of 0.0076?P<0.001?a year from 1998 to 2018 in the Loess Plateau.In terms of spatial distribution,78%of grassland NDVI increased significantly?P<0.05?,and the areas with higher increase rate were mainly distributed in the middle of the Loess Plateau.The grassland NPP increased by an average of 3.92 g C m^-2?P<0.001?a year from 2000 to 2015.In terms of spatial distribution,94%of grassland NPP increased significantly?P<0.05?,and the spatial distribution characteristics of annual NPP(g C m^-2)were similar to those of NDVI.The annual increase rate of total NPP of grassland,farmland and woodland are shown below followed a descending order:grassland>farmland>woodland,and the total NPP of grassland was almost equal with that of farmland in the Loess Plateau.?2?The actual grassland aboveground biomass(AGB_actual)showed a significant?P<0.001?increase trend in the Loess Plateau from 1998 to 2018,and the abrupt change of the whole trend occurred in 2007.The area of grassland AGB_actual(g m^-2)which showed a significant increase trend was 8%after 2007,more than the area in the year before 2007.During the same period,the potential grassland aboveground biomass(AGB_potential)driven by climate factors also increased significantly?P<0.05?.The positive effect contributed by human activities on the grassland AGB_actual was increasing,while the negative effect was decreasing.It indicated that the contribution of human activities to promoting the growth of grassland vegetation was increasing on the Loess Plateau.It was found that human activities contributed 47.8%of the effect,while the contribution of climate change was 52.1%in 1998-2018 by evaluating the contribution of human activities and climate change to the grassland AGB_actual change.Comparing the contribution of climate change and human activities in period of1998-2007 and 2007-2018,the contribution of climate change increased from 51%in the previous stage to 59%in the later stage,however,the contribution of human activities decreased from 49%to 41%.It indicated that the impact of human activities on the growth of grassland vegetation was gradually decreasing compared with the impact of climate change.?3?The migration of rural population caused by urbanization was the dominant factor driving the total grassland vegetation?total biomass?10⁴ t?and area?increase on the Loess Plateau.The population change and urbanization,temperature and humidity,and agriculture and economy accounted for 62.5%,31.2%,and 7.7%,respectively,of the overall impact directly affecting total grassland vegetation dynamics.The higher the urbanization rate promoted the increase of the total grassland above-ground biomass,and the decrease of agricultural population would promote the increase of grassland vegetation area on the Loess Plateau.The higher the urbanization rate means the higher the number of residents migrates from rural to urban areas.Following this argument,the disturbances of human activities to grassland vegetation were expected to gradually decrease in rural areas,where the vast majority of the Loess Plateau is located.In this case,the total amount of grassland vegetation will increase.Meanwhile,the impact of climate factors increased in recent years.?4?The response of grassland above-ground biomass to precipitation was more sensitive than that to temperature on the Loess Plateau.The sensitivity of grassland aboveground biomass to precipitation and temperature decreased with the increase of precipitation and temperature.Under the simulating scenario of future climate change,the increase of temperature and precipitation both promoted the increase of grassland above-ground biomass,and the increase of above-ground biomass caused by the increase of precipitation was more than 2 times of the increase of temperature.The grassland above-ground biomass will decrease with the decrease of temperature and precipitation,and the decrease of grassland above-ground biomass caused by the decrease of precipitation was about 2 times of the decrease of temperature.?5?The water condition was the dominant factors controlling and regulating the growth of natural grassland vegetation,and was not limited by geographical location on the Loess Plateau.On the entire region,the increase of precipitation and humidity?water conditions?directly intensified the grassland vegetation activity,and its direct effect on the grassland growth is more than twice that of light and temperature conditions.The influence of light and temperature on the growth of grassland showed obvious geographical differences.The rise of temperature will stimulate the grassland vegetation activities in high altitude and high latitude areas,and the increase of light and range of temperature drove the increase of grassland vegetation activities in the eastern?higher longitude?area of the Loess Plateau.