| With the global climate change,the intensity and frequency of drought in the world are continuously strengthened.Drought can directly or indirectly affect the photosynthesis of vegetation and have an important impact on terrestrial ecosystems.In recent decades,frequent droughts in Shandong Province have had a great impact on the dynamic changes of vegetation.Studying the impact of drought in Shandong Province on the dynamic changes of vegetation can reveal the impact of regional drought on surface vegetation and ecosystems,which is of great significance to the construction of regional ecological civilization and the sustainable use of water resources.In this paper,Shandong Province is taken as the research object,combined with ArcGIS and Matlab software,based on the NPP and NDVI data sets with a spatial resolution of 1km from 2000 to 2015,the temporal and spatial variation characteristics of vegetation in Shandong Province are analyzed.Based on topographic data,the relationship between NPP of vegetation and topographic factors in Yihe Basin,a typical rocky mountain area in Shandong province,is analyzed.At the same time,using the vegetation NPP data,the coefficient of variation and Hurst Index of NPP in Shandong Province were calculated to detect the stability and sustainability of vegetation change.Based on the Standardized Precipitation Evaporation Index?SPEI?,Sen Trend Analysis Method and Mann-Kendall Trend Test Method are used to analyze the spatio-temporal variation characteristics of drought on annual,seasonal and monthly scales in Shandong Province from 1970 to 2017.The effects of drought on vegetation NPP and NDVI are also studied.The main results are as follows:The overall spatial pattern of annual NPP average of vegetation in Shandong Province from 2000 to 2015 is characterized by high in the east and low in the west and high in the south and low in the north.The high-value areas of multi-year average NPP are mainly distributed in coastal Weihai,Yantai and Qingdao cities,while the low-value areas are mainly distributed in the north and middle of the study area.From2000 to 2015,the NPP in Shandong Province showed a fluctuating upward trend.The annual NPP maximum value appeared in 2004,with 414.3 g C·m-2·a-1,while the minimum value appeared in 2000,with only 299.4 g C·m-2·a-1.In terms of spatial change of vegetation NPP,with 92.13%in the growth area and 7.87%in the decrease area.The area with extremely significant increase accounted for 12.66%of the study area,and the area with significant increase accounted for 17.6%.However,the extremely significantly decreased area accounts for only 0.63%of the study area,and the significantly decreased area accounts for 0.72%,indicating that the vegetation growth in Shandong Province has been improved on the whole.The coefficient of variation of NPP in Shandong Province has the spatial distribution characteristics of southeast low and northwest high,and the coefficient of variation of NPP in most areas of Shandong Province is in a low range,which indicates that the change of vegetation NPP in Shandong Province is relatively stable and the ecosystem is in a virtuous circle.The overall NPP in Shandong Province is dominated by weak persistence sequence.Human activities are frequent in Shandong region,the weak persistence of vegetation growth indicates that vegetation improvement in Shandong region is the result of the comprehensive effect of human activities and natural factors.The distribution pattern of different land use types in the Yihe Basin has obvious topographic gradient characteristics.The distribution index of cultivated land,construction land and water areas that are greatly affected by human activities occupies a dominant position at low topographic level,while forest land and grassland present a dominant distribution at high topographic level.The maximum value of NPP is distributed in the shady slope or plain area.The area with the most obvious upward trend of NPP is the hilly area with the shady slope and little surface fluctuation,and the area with the most stable vegetation is located in the plain area.The seasonal and annual NDVI in Shandong Province have obvious spatial differentiation,but the low-value areas are relatively consistent and are all distributed in wetlands such as the Yellow River Delta,Laizhou Bay,Nansi Lake,as well as in urban areas.From 2000 to 2015,the seasonal and annual NDVI values of vegetation in Shandong Province showed a fluctuating upward trend,with autumn NDVI contributing the most to the annual NDVI growth,followed by spring and winter.At the same time,the vegetation NDVI and NPP in Shandong Province have a very significant positive correlation,indicating that NDVI and NPP have great similarity in expressing regional vegetation conditions,but there are also differences.The multi-time scale SPEI can effectively identify and reflect the year and degree of drought in Shandong.SPEI values at different time scales in Shandong province showed a fluctuating downward trend from 1970 to 2017,indicating that Shandong province showed a trend of aridity from 1970 to 2017,and with the increase of time scales,the frequency of droughts decreased and the duration increased.Among them,annual scale and autumn aridity are the most obvious,followed by spring,and winter is the weakest.In terms of spatial pattern of SPEI,almost all areas in autumn show a drought trend.In terms of spring,summer and annual scales,the eastern coastal areas show a drought trend,while the western inland areas show a weak wetting trend,which is opposite in winter.The intensity of drought occurrence in Shandong Province is stable in the range of 15.50%-24.85%,and there are obvious spatial differences.Generally,the intensity of drought occurrence is stronger in the east and northeast,and weaker in the northwest,southeast and southwest.In terms of drought frequency,the monthly drought frequency is the highest,followed by autumn and summer,and the lowest is winter,with significant spatial difference.NPP and NDVI are positively correlated with SPEI in most areas of Shandong.The correlation between SPEI and NPP is stronger than that between SPEI and NDVI.The correlation degree between SPEI and grassland is the highest,and the correlation degree with water area is the weakest.Human factors have an important influence on NDVI changes of vegetation in Shandong Province,and the destructive effect of negative human factors on vegetation is gradually increasing. |
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