Response Of Vegetation Net Primary Productivity To Composite Meteorological Factors On The Loess Plateau

Global warming is becoming more severe with the massive use of fossil fuels.The climate has changed the redistribution of water resources,making extreme weather and droughts and floods frequent.Net Primary Productivity(NPP)of vegetation is an important component of the terrestrial carbon cycle,and its evolutionary pattern is inextricably linked to climate change.The Loess Plateau is the most ecologically fragile region in China and is an area sensitive to the response to global climate change.Water scarcity and soil erosion are the major problems facing the region,and these problems deepen the dependence of the region on surface vegetation.It becomes important to reveal the relationship of vegetation net primary productivity in response to meteorological factors.This study takes the composite meteorological factors of the Loess Plateau as the research object,takes the temperature vegetation drought index(TVDI)and standardized precipitation evapotranspiration index(SPEI)as composite meteorological indicators,constructs a month-bymonth remote sensing dataset of vegetation net primary productivity(NPP)based on the ESTARFM model and CASA model,and based on the results of this calculation,analyzes the spatial and temporal patterns of NPP and composite meteorological factors in the Loess Plateau region from 1982-2018 The spatial and temporal patterns of NPP and composite meteorological factors in the Loess Plateau region from 1982 to 2018 were analyzed,the response of vegetation productivity to composite meteorological factors at different lag times were analyzed,and the driving force of single meteorological factors and composite meteorological factors on vegetation NPP was explored.The main conclusions are as follows:(1)The ESTARFM model was validated by MODIS data,and the results showed that the ESTARFM NDVI fitted well with MODIS NDVI,indicating that the dataset could fully reflect the pattern of vegetation growth changes on the Loess Plateau in the past 20 years.(2)In the horizontal space,the multi-year mean NPP of Loess Plateau showed a trend of increasing from north to south;for different land types,the NPP of forest land was the largest and that of wasteland was the smallest.In vertical space,with the increasing elevation,the multiyear mean of NPP of Loess Plateau experienced a decreasing and then increasing process;the multi-year mean of NPP was the smallest between 1233～1630m and the largest between 2181～3013m.In terms of interannual variation,the Loess Plateau NPP showed a general trend of gradual improvement,with slight and moderate improvement.Within the growing season,it showed a fluctuating increasing trend,with the largest NPP in August and the smallest NPP in October;September was the month with the largest percentage of improved status,and August was the month with the largest percentage of deteriorated NPP status.(3)On the annual scale,the TVDI showed a characteristic change of rising and then declining,with a predominantly downward trend;on the monthly scale,the TVDI fluctuated and declined at an average rate of 0.0002/mon.During the growing season,the average TVDI index was 0.60.TVDI indicated that medium drought was the most likely type of drought in the study area,with June being the month when drought was extremely likely to occur each year and November to February having the least probability of drought occurring each year.In the study of the spatial characteristics of TVDI,the TVDI of Loess Plateau showed an overall spatial trend of high in the south and low in the west,and the TVDI was dominated by the trend of gradual improvement,with an average TVDIslope of-0.0006/a.The spatial pattern of TVDI in the growing season was similar to the spatial pattern of multi-year average TVDI.Among the different land cover types,the TVDI of construction land was the largest,the TVDI of forest land was the smallest,the TVDIslope of construction land was the largest,and the TVDIslope of forest land was the smallest.(4)At the annual scale,the multi-year average of SPEI of the Loess Plateau shows a fluctuating upward trend.On the monthly scale,the SPEI of the Loess Plateau increased slightly at a rate of 0.014/mon.The average SPEI was-0.004 during the growing season.During the growing season,the average SPEI was-0.004.The SPEI indicates that light drought is the most likely type of drought in the study area,and August is the month in which drought is extremely likely to occur each year.In the study of the spatial distribution characteristics of SPEI,the overall SPEI of Loess Plateau showed a gradual increasing trend from south to north,with a slight deteriorating trend,with an average SPEI of-0.408 and an average SPEIslope of-0.0019;within the growing season,the SPEI showed a high trend in the south and a low trend in the north,with a multi-year average SPEI of-0.54 in the growing season.different Among the land use types,the SPEI of wasteland was the largest,the SPEI of cropland was the smallest,the SPEIslope of wasteland was the largest,and the SPEIslope of construction land was the smallest.Among different vegetation types,grass SPEI was the largest,desert SPEI was the smallest,alpine vegetation SPEIslope was the largest,and bog SPEIslope was the smallest.(5)Spatially,the correlation between NPP and TVDI in the study area showed a spatial pattern of high in the west and low in the middle,and the correlation between NPP and TVDI was mainly positive.In terms of land type,the correlation between NPP on forest land and TVDI was the most significant,and the response of NPP on TVDI was weakest in wasteland.For vegetation type,coniferous forest NPP had the strongest positive correlation with TVDI,and alpine NPP had the strongest negative correlation with TVDI.For the response characteristics of NPP to TVDI,spatially,the response of NPP to TVDI was dominated by a 2-month lag.In time,annual scale NPP showed significant negative correlations to current month TVDI,lagged January TVDI,and lagged February TVDI.Within the growing season,the response of NPP to TVDI showed a lag of 1 month.Spatially,the correlation between NPP and TVDI in the study area showed a spatial pattern of high in the north and low in the south,and the correlation between NPP and SPEI was mainly positive.In terms of land type,the response of grassland to SPEI was the most significant,and the response of construction land to SPEI was the weakest.For vegetation types,grassland NPP had the strongest correlation with SPEI,and alpine NPP had the weakest correlation with SPEI.For the response characteristics of NPP to SPEI,spatially,the response of NPP to SPEI was dominated by a 2-month lag in space.In time,the correlation of NPP to SPEI gradually increased with the accumulation of SPEI on the time scale.Within the growing season,the response of NPP to SPEI was dominated by a 2-month lag.(6)For the study on the driving force of single and composite meteorological factors on vegetation NPP,for risk detection,precipitation showed a positive correlation with NPP in spatial distribution,while SPEI showed a negative correlation with NPP in spatial distribution,and vegetation in the suitable zone of TVDI and SPEI had higher NPP than the original single meteorological factor.For factor detection,SPEI has the strongest influence on NPP,and temperature has the weakest influence on NPP.For the interaction detection,the influence of precipitation ∩ PET was the strongest and the influence of SPEI ∩ TVDI was the second strongest.Compared with single factors,the driving force of composite factors on NPP is significantly stronger,and the NPP of Loess Plateau is most prominently limited by local precipitation and potential evapotranspiration.