Impacts Of Climate Change And Human Activities On Vegetation Cover In The Zuli River Basin

Chinese Loess Plateau is one of the most erosive regions in the world,and its ecological environment is fragile,and vegetation coverage is vulnerable to external factors such as climate change and human activities.In recent years,researches have shown that vegetation coverage of the Loess Plateau has been changed greatly.However,the Zuli River Basin,as one of the most serious soil erosion areas in the Loess Plateau,which is rarely studied on its change of vegetation coverage and mechanism.Detailed analysis of its vegetation coverage change and mechanism can provide scientific and accurate research data for the Zuli River Basin,and then provide a theoretical basis for future vegetation protection and policy promotion.In this study,spatio-temporal variations in vegetation coverage in Chinese Zuli River Basin during 1999–2016 were investigated using normalized difference vegetation index?NDVI?extracted from Landsat Thematic Mapper?TM?/Enhanced Thematic Mapper?ETM+?/Operational Land Imager?OLI?and Earth Observation System?EOS?moderate-resolution imaging spectroradiometer?Modis?data.The slopes were divided into five classes:0–6°,7–15°,16–25°,26–40°and slope>40°,according to the policy of farmland to grassland in china.To determine whether the slope is the main factor affecting NDVI change in the Zuli River Basin,these five slope classes were analyzed in detail.We used supervised classification in Erdas to obtain the land-use maps of the study area in 2001 and 2011 from the TM images.The land-use was classified as two types?remained the same or changed?where the NDVI changes were calculated and analyzed separately.Thus,the impact of human factors in NDVI changes can be quantitative analyzed.To further analyze the impacts of climate change in NDVI change in the basin,the relationships between NDVI and climate factors including SPEI,precipitation,temperature in different land-use types were also calculated and analyzed.Grassland ecosystem is fragile and sensitive.Human disturbance and climate change will have a greater impact on the vegetation coverage of grassland.However,studies on quantitatively investigating the climate change and human activities on vegetation coverage change were rare.In this study,we explored the relationship between grassland and natural and human factors in the Zuli River Basin,through analyzing the relationship between NDVI of grassland and climate change and social economy.A new model suitable for grassland in the Zuli River Basin was improved from four classic Net Primary Productivity?NPP?models,which were adjusted by actual biomass.Then the NDVI affected by climate changes(NDVI_climate)and human activities(NDVI_human)was quantitatively and spatially distinguished by the relationship between NPP and NDVI.Based on satellite images,the NDVI of Zuli River Basin in the second stage?2011-2016?is 43.04%higher than that in the first stage?1999-2002?.And NDVI shows a significant linear relationship with the interannual variation.Compared with the first stage?1999–2002?,NDVI of overall slope was higher in the second stage.However,the variation in NDVI?NDVI in 2011–2016 minus that in 1999–2002?decreased with the increasing in slope in the range of 0°–70°.The NDVI of7-15°contributed the most?47.61%?to the increase in NDVI in the Zuli River Basin due to its largest area?5037km²?,though the NDVI of 0-6°increased the most?0.061?.From 2001 to 2011,the NDVI of grassland increased the most?0.067?in the land-use that remained the same,while the NDVI of abandoned farmland transforming to forest increased the most?0.087?in the land-use that changed.To sum up,in 2011 the land-use of farmland?converted from abandoned land,agricultural land,forest and farmland?changed greatest in NDVI?0.066?,but the increase of NDVI in grassland contributed the most?51.39%?in the Zuli River Basin.There was no significant relationship between NDVI of the whole basin and climatic factors including SPEI,precipitation and temperature in the Zuli River Basin.But in some parts of the basin,such as land-use types that remained the same from 2001 to 2011,significant relationships were found between NDVI and SPEI,precipitation in abandoned farmland and grassland affected by both climate and policy.There was no significant relationship between NDVI changes and climatic factors in other land-use types.NDVI of grassland has an obvious relation with climatic factors?e.g.SPEI and precipitation?and social economy data?e.g.nighttime light data and demographic data?in the Zuli River Basin.The NCEAS model,selected from four NPP models was the most suitable for the grassland in the Zuli River Basin.The human activities and climate impacts calculated based on TM data showed that human activities would cause the decline of NDVI.But the trend of declining NDVI affected by human activities is reducing gradually,and then the NDVI affected by human activities i.e.NDVI_human would gradually increase during the study period.Results based on Modis data showed that the human activities reduce the annual NDVI,spring NDVI,summer NDVI and autumn NDVI,with the greatest effect in summer and the smallest effect in autumn.Overall NDVI_human increased gradually during the study period.During2000-2016,human activities were the main factors contributing to the increase of NDVI in annual,in spring,in summer and in autumn.In summary,the main factors of NDVI increase are the protection of grassland and the increase of farmland vegetation coverage brought by the policy of the"Grain for Green"program and the development of agricultural technology,but slope and climate change are not the main factors affecting NDVI change in the Zuli River Basin.The impacts of human activities on grassland are mainly destructive.However,these destructive activities are reducing and the protection behaviors of human promote the increase of NDVI in grassland in the Zuli River Basin.