Spatial And Temporal Change Of Urban Vegetation And Surface Temperature And The Mechanism Of Their Feedback Effects

Global warming and rapid urbanization are important contributing factors to the urban heat island effect and urban vegetation change.While urban vegetation mitigates the urban heat island effect,its phenology is also significantly affected by the heat island effect.Studying the spatial and temporal changes in urban vegetation cover and surface temperature and their feedbacks effect can provide scientific guidance for mitigating the urban heat island effect,managing urban vegetation,and exploring the impact of climate change on the environment.Previous studies have focused on the spatial and temporal variation of urban temperature and vegetation cover in different periods and their unidirectional regulation,but there is a lack of systematic investigation of the spatial and temporal variation of both and their mutual feedbacks effect in the same period.Therefore,this study took 311 cities in China as the research objects and used remote sensing products,meteorological data,and a sample survey to analyze: 1)the spatial and temporal change of urban heat island effect and vegetation cover and their influencing factors;2)the influence of urban vegetation cover change on urban surface temperature and its mechanism;3)the response of urban vegetation spring phenology to urban warming and its mechanism.The main research findings are as follows.(1)Differences in background climate and vegetation conditions lead to different rates of urban warming between cities,which in turn leads to spatial and temporal differences in urban heat island intensity.Most cities in China have different degrees of heat island effect.The urban heat island intensity is greatest during the daytime in summer,and 80.67% of Chinese cities have an increasing trend in heat island intensity,with the pattern mainly being faster urban warming than rural warming.Climatic factors(background temperature,precipitation)and biological factors(vegetation changes)contributed13.79% and 33.04% to urban warming,respectively.(2)The increase in the urban fringe vegetation improves the overall urban vegetation cover.Urban vegetation changes are spatially heterogeneous.Cities in warm and humid regions generally have higher vegetation cover.57.24% of urban sites had an increasing trend in vegetation cover from2001 to 2017.Number of Vegetation patches and areas increased in urban fringe areas is due to the conversion of agricultural land in the urban fringe areas into urban green space,thus increasing the overall urban vegetation cover.(3)The degree of vegetation cooling varies between urban sites depending on the background climate and vegetation condition.The effect of vegetation cover on urban surface temperature varies between cities due to differences in background climate and vegetation status.The average vegetation cooling efficiency was-1.66 ℃.Compared with 2001,the increase in urban vegetation cover significantly reduced the local surface temperature in 2017,but not the whole old urban area;the decrease in urban vegetation cover increased the local surface temperature in 2017 but also raised the surface temperature of the whole old urban area.(4)Precipitation mainly regulates the response of vegetation spring phenology to warmingUrban warming had a significant effect on urban vegetation spring phenology.Spring phenology onset,advance rate(-0.52 ± 0.86 days/year),and temperature sensitivity(-2.86 ± 3.57 days/℃)were significantly higher in urban vegetation than in rural areas.The magnitude of temperature sensitivity was influenced by a combination of environmental and biological factors,but was mainly regulated by pre-season cumulative precipitation,which contributed 45% and 63% of the variation in temperature sensitivity in urban and rural areas,respectively.