Study On Vegetation Dynamic Based On MODIS-NDVI In Typical Grasshopper Plague Area And Climatological Cause Of Grasshoppers Occurrence In Xinjing

Grasshopper plague has been one of the major natural disasters in Xinjiang, and causes extensive damage to grasslands every year.Global warming,abnormal local climate and the complex habitats in the grasshopper plague areas are helpful to the recent increase in frequency and severity of grasshopper plagues since 1980s . Therefore,to master the relationship between environmental factors and grasshopper occurrence would be great import and valuable to sustainable development of grasslands and grasshopper control. The project focuses on the relationship between grasshopper occurrence and ecological factors such as vegetation and climate.In this paper,several typical grasshopper plague areas where grasshoppers break out frequently have been chosed as the study sites.The main results are as follows:1.Using MODIS-NDVI data, vegetation dynamic in the grasshopper plague area in Jimunai County from 2000 to 2006 are studied and relationship between environmental factors, the area of grasshopper occurrence and NDVI is analyzed as well. The results show that the NDVI of the region has cerntain spatio-temporal patterns.The average monthly NDVI is varied from 0.2 to 0.5 from April to October.The monthly variations of the NDVI are very remarkable.Vegetation growth is best in July except cerntain years.The average vegetation coverage has increasing trend in the recent seven years.The temperature plays an important role in the monthly changes of vegetation.The monthly NDVI shows a significant negative correlation with the area of grasshopper occurrence.2.Relationships between altitude,vegetation,soil pH value, soil salinity, soil organic matter and grasshopper occurrence are analyzed in the grasshopper plague area at Whitestone Town,Hami Area.The results indicate: As altitude from 2000 m to 2400 m increasing, the grasshopper densities reduce. Grasshoppers are mainly distributed in mountain desert and mountain grasslands. Vegetation coverage for high-density grasshoppers is between 35%~55%. Alkaline conditions help to the survival of grasshopper.Salinity in a certain range has no obvious effects on the grasshopper density. Soil organic matter directly affects the growth of ground vegetation, thus affecting the survival of grasshoppers.3.The correlation between the area of grasshopper occurrence in Xinjiang from the year of 1952 to 2005 and the seventy-four character indexes of the atmospheric circulation is studied in this paper.The result shows that there is close relationship between the grasshopper occurrence in Xinjiang and atmospheric circulation. Index of the area, index of the strength of the North African subtro pical high, index of the northern extend of the North African subtropical high,index of the area of the northern hemisphere polar vortex, zonal index over Asia, index of the strength of the India-Burma trough and index of the frequency of cold-air intrusion into China are closely related to the area of grasshopper occurrence in Xinjiang.Among these factors the effect of the North African subtropical high on the grasshopper occurrence is mainly caused by ranifall from April to June which influence the development of grasshopper eggs and nymphs and grasshopper occurrence.Thirty-nine atmospheric circulation character factors which correlated with the area of grasshopper occurrence in Xinjiang are chosed as prediction variables which construct four models to forecast grasshopper occurrence .4.Tacheng Area,Hami Area,Yili Area are chosed as study sites.Effects of temperature and rainfall on grasshopper infestation in the north of Tianshan Mountain in Xinjiang are examined by methods of composition and anomaly analysis during 1990~2001. The results indicate that temperature and rainfall have significant differences between years of infestation and no-infestation, and higher temperature during oviposition is helpful to grasshopper occurrence. Also, higher temperature in winter and higher temperature with less rainfall from May to June contribute to grasshopper outbreaks in Tacheng Area and Hami Area, whereas temperature is lower in infestation years during hatch period and in winter in Yili Area.