Evaluation Of Vegetation Restoration In Loess Plateau And Time-lag Effect Of Climate Change And Drought

Vegetation carries more than 50%of the carbon stocks of global terrestrial ecosystems,making it an important carbon reservoir on land and playing an important role in the global terrestrial carbon cycle.However,under the influence of global warming and drying and negative human activities,vegetation is seriously degraded,which in turn restricts the carbon cycling process of terrestrial ecosystems and the supply of ecological service value of vegetation.For a long time,the vegetation in the Loess Plateau has been greatly improved in terms of species composition,cover structure and trait characteristics thanks to the project of returning farmland to forest and grass in China.In recent years,however,with large-scale revegetation,problems such as over-recovered vegetation or vegetation cover close to the upper limit of available water resources have emerged,threatening the sustainability of vegetation.Therefore,a quantitative assessment of the effectiveness of vegetation restoration and its driver response on the Loess Plateau in the context of complex climate change is important for the improvement of the ecological environment of the Loess Plateau and the formulation of future restoration policies.This study takes the vegetation cover of the Loess Plateau as the research object,and uses the time series of the last 16 years from 2000 to 2015 as the research period,and systematically analyses the spatial and temporal dynamics of vegetation distribution and drought on the Loess Plateau from 2000 to 2015 based on trend analysis at annual and seasonal scales.The correlation between NDVI(Normalized Difference Vegetation Index),SPEI(Standardized Precipitation Evapotranspiration Index)and climatic factors at different spatial and temporal scales was calculated.The model was constructed to estimate the potential NDVI of the Loess Plateau using climatic factors as input,and the residual analysis was used to assess the impact of human activities on the vegetation dynamics of the Loess Plateau.The results of the study show that.(1)the NDVI of the Loess Plateau showed an increasing trend from northwest to southeast,with obvious spatial heterogeneity distribution characteristics.16 years NDVI showed a significant increasing trend,with an average increasing rate of 0.0102/a and fluctuation range between 0.54-0.71;during 2000-2015,the overall SPEI of the Loess Plateau showed a significant increasing trend(R~2=0.05,P<0.05),with an average growth rate of 0.128.(2)There is a strong correlation between NDVI trends and precipitation on the Loess Plateau,with a partial correlation coefficient of 0.53.77.54%of the total area of the Loess Plateau is covered by vegetation with a significant increasing trend,and the response of different seasons and regions to precipitation and temperature varies significantly,with precipitation having a greater impact on vegetation than temperature in general;most of the vegetation on the Loess Plateau is affected by drought,mainly in the central and northwestern parts of the Loess Plateau.The correlation between different vegetation types on the Loess Plateau and SPEI showed that the NDVI of agricultural land,mosaic grassland,shrubs,grassland,mosaic woodland,evergreen broadleaf,deciduous broadleaf,evergreen conifer,deciduous conifer and other ten vegetation types on the Loess Plateau were all positively correlated with drought(P<0.05).This indicates that there is a relationship between climatic drought conditions and vegetation cover as reflected by using SPEI.(3)The NDVI of the Loess Plateau showed an overall increasing trend in different seasons,with a significant positive correlation between NDVI and precipitation in spring,and precipitation was the most direct factor determining the change in cover of all vegetation types in spring.The response of NDVI to temperature and precipitation was not significant for all vegetation types in summer,and the lagging effect of NDVI on spring precipitation was more significant in summer(P<0.05).(4)The residual analysis showed that human activities had a greater influence on the fluctuation of NDVI on the Loess Plateau and were an important driver of vegetation cover change on the Loess Plateau.(5)The time lag effect of vegetation cover and precipitation on the Loess Plateau showed the greatest correlation between NDVI and precipitation in the current month in most areas of the Loess Plateau,and did not show any significant lag effect.Temperature,drought and NDVI all have different time lags on different time scales.