| Society and economy as well as the ecological environment we depend on have been critically influenced by global warming in recent decades.As the major part of the terrestrial ecosystem,vegetation is an indicator of global climate and terrestrial ecological changes with functions such as regulating global carbon balance,conserving water,maintaining soil and water,and regulating climate.Vegetation cover in China has already changed significantly and complexity under the combined influence of continuing climate change and increasing human activities.Therefore,it is highly important to study the process of vegetation change in China and its response to external disturbances such as climate change and human activities.However,previous studies are still insufficient or absent in terms of gaining insight into the evolution of vegetation cover and its attribution analysis,and assessing the sensitivity of vegetation response to short-term climate variability in China.In view of this,based on the long-term multi-source datasets including Normalized Difference Vegetation Index(NDVI),climate,land cover/use and DEM and various methods of trend analysis,BFAST algorithm,calculation model of Vegetation Sensitivity Index(VSI),moving window technology etc.,this study explored the changes of climate and vegetation cover in China from 1982 to 2015 at varied spatiotemporal scales,extracted and discussed the abrupt changes in vegetation change trends,revealed the sensitivity of vegetation to monthly-scale climate change and its interannual variation pattern,quantitatively analyzed the climate drivers of vegetation sensitivity changes and their evolutionary characteristics in order to reveal the influence mechanisms of vegetation cover changes from a dynamic perspective,and qualitatively explored the role played by human activities in these.The main conclusions of this study are as follows:(1)Climate warming in China was significant during 1982-2015,and the average increasing trend of annual mean temperature was about 0.291?/10a(p<0.01).Precipitation had large interannual fluctuations but the overall trend was not significant.Solar radiation showed a phase change of decreasing first and then increasing,which is consistent with the"global dimming"and"global brightening"phenomena.Among them,the temperature and radiation fluctuated most strongly between 1990 and 2000.There are clear regional differences in climate change in China.The largest warming occurred in the arid zone,while the smallest was in the humid region.Precipitation showed an increasing trend only in arid areas,which radiation exhibited significant phase changes in all climatic zones.Spatially,the temperature showed a latitudinal distribution of hot in the southeast and cold in the northwest,while the rate of warming increased gradually from southeast to northwest with the greatest increase in the northwest.The distribution of precipitation is generally characterized by high in the southeast and low in the northwest,but the trend was not significant.Radiation has a distribution pattern of high in the west and low in the east,and increased significantly in most areas of China,especially in the Qinghai-Tibet Plateau.(2)The consistency analysis with the SPOT-VGT NDVI dataset showed that the GIMMS NDVI3g data is applicable in monitoring and analyzing the long-term evolution of vegetation cover in China.Based on the annual mean NDVI trend generated from the GIMMS NDVI3g data,we found that terrestrial vegetation cover showed a continuous increasing trend in China from 1982 to 2015,with an increase rate of 0.7×10-3/a,while the change rate varied among different vegetation types.Overall,the vegetation NDVI showed a spatial distribution pattern of high in the southeast and low in the northwest.About 67.9%vegetated areas of the country showed an increasing trend,of which 37.0%(27.1%of highly significant and 9.9%of significant)showed a significant increase,and was mainly distributed in the Loess Plateau area and most of the south-central region.NDVI decreased in 32.1%of the vegetation areas,and was mostly concentrated in areas such as the high latitude mountainous regions in the north and the northeastern edge of the Qinghai-Tibet Plateau.Among the vegetation types,the increasing trend of NDVI was most prominent for crops,followed by grasslands,shrubs and coniferous forests,while in each climate zone,the ranking of NDVI increase was:semi-arid zone>humid zone>arid zone>sub-humid zone,indicating that the change of vegetation cover was closely related to the vegetation type and regional climatic conditions.(3)Study based on the BFAST algorithm confirmed the existence of abrupt changes in the NDVI time series of Chinese vegetation,but the occurrence of mutations in NDVI varied between regions.A large area(78.7%of the total vegetated areas)had experienced abrupt changes,which mainly distributed in Northwest China,especially in the Loess Plateau,followed by some parts of North China,the western Qinghai-Tibet Plateau and the Yellow River Basin.In contrast,the highest stability in the NDVI occurred in South China.The abrupt changes detected in the NDVI trends increased sharply in 1999–2000 and were mainly due to increases in vegetation dynamic in North,Southwest,and Northwest China,which coincides with the period and scope of the implementation of large-scale ecological projects in China.Overall,increasing(greening)with setback(reversal:increase to decrease)and monotonic increases with a negative break(interruption:increase with negative break)were the most commonly observed types of breakpoints,and were generally concentrated on the western and eastern sides of the Hu Huanyong line.The proportion of NDVI with positive trend shift decreased from 66.5%to 53.5%between 2000 and 2012.Grassland and cropland had the higher likelihood of breakpoints occurring,while forests had a higher tolerance to external disturbances.The subject(i.e.,vegetation type)in which the vegetation change points occurred varied in space.The breakpoints in the East China mainly occurred in cropland,while in North,Northwest,and Southwest China,changes in vegetation dynamics were detected in grassland.The continuous greening(monotonic increase)trends mainly occurred in crops,but about39.4%of the grassland cover exhibited greening to browning.Generally,trends in unstable vegetation had a higher probability of changing than those in stable vegetation.However,the percentage of breakpoints detected in stable vegetation has been continuously increasing after 2000,while in unstable areas,the percentage remained relatively steady or even declined.(4)The sensitivity of vegetation to short-term climate variability in China exhibited significant spatial heterogeneity.Higher VSI mainly appeared in southwestern and central China,and in some mountainous areas.In contrast,most of the northwest region,the central part of North China,and the western part of the Southwest region had lower VSI.Generally,vegetation in drier regions was less sensitive to climate variability,and vice versa.In addition,the vegetation sensitivity within each climate zone showed differences depending on the vegetation type.Mean VSI temporal trends were explored by applying a 15-yr moving window,and results showed that apparent phase changes appeared in vegetation sensitivity.In both parts,annual mean VSI both showed a declining trend,with a slope of-0.92(R~2=0.59,p>0.05)in the former,and-0.1685(R~2=0.66,p<0.01)in the latter period,respectively.Spatially,the areas with increased vegetation sensitivity(approximately 76%of vegetated areas)were significantly higher than areas with decreased(23.6%of vegetated areas).The evolutionary pattern of vegetation sensitivity is also influenced by vegetation type and climatic zone in which the vegetation is planted.(5)Solar radiation determined VSI in the majority region of China,accounting for 48.93%of the whole vegetated area,while air temperature and precipitation accounted for 30.67%and 20.40%,respectively.Solar radiation made the largest contribution to VSI in each climate zone,especially in the humid regions(accounted for 54.89%of vegetated areas of this zone).Temperature and precipitation had the greatest influence on VSI in the sub-humid regions(38.56%)and arid regions(32.93%),respectively.The main climatic drivers of vegetation sensitivity and its relative contribution varied across time.Notably,the proportion of regions where precipitation controlled VSI increased rapidly,indicating that precipitation is gradually replacing solar radiation and temperature as the main climatic factors affecting vegetation change in China.In summary,vegetation cover in China showed an overall increasing trend from1982 to 2015,but there was a more obvious spatial heterogeneity.Under the combined influence of frequent climate extremes and intensified human activities,the vegetation of northern and northwestern China and other regions,especially the Loess Plateau,has undergone more significant trend shifts at different times.The sequential implementation of ecological projects(e.g.,the Three-North Shelter Forest Program,the Grain for Green Program,etc.)appeared to be the most direct driving factor.The sensitivity of vegetation to short-term climate variability also exhibited significant spatial heterogeneity.Besides,influenced by changes in climatic conditions,especially in the amount of solar radiation,vegetation sensitivity changed over time with varying degrees and spatial.This suggests that as the intensity of external disturbance changes,the response or resistance of vegetation to climate change will also change accordingly.In particular,there is a concentrated decrease in vegetation sensitivity in those key areas where large-scale ecological projects were implemented,especially in the overlapping areas of multiple projects,which indirectly indicates that the vegetation construction activities carried out in China in the past have played an effective buffering role in the response of vegetation to climate change.The deeper understanding of the evolutionary trends of vegetation cover and the relationships between vegetation,climate,and human activities in this study could provide a reasonable scientific basis for promoting the implementation of ecological projects and supporting sustainable development of ecosystems in China. |
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