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Spatial-temporal Dynamics Of Vegetation Coverage And Its Response To Extreme Temperature And Precipitation Processes Over The Arid Region Of Northwest China

Posted on:2016-05-04Degree:MasterType:Thesis
Country:ChinaCandidate:W ZhuFull Text:PDF
GTID:2180330470477004Subject:Physical geography
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The report of IPCC show that the global temperature has been rising since the industrial revolution. In addition, it has the trend of speeding up. Because of increase in frequency and extent of extreme climatic events such as extreme temperature events, heat waves, heavy precipitation events, the global extreme climate change problem has received extensive attention all over the world. Global climate warming has led to significant vegetation changes in the past half century. Meanwhile the research concerning response of vegetation to extreme climate is becoming the important research aspect of science. Vegetation changes of the arid region of northwest China is sensitive to temperature and precipitation, especially to extreme temperature and precipitation processes. It is necessary to investigate the response of vegetation changes to the mean temperature/precipitation process and extreme temperature/precipitation processes for a better understanding of the accumulated consequence of temperature/precipitation change. Vegetation coverage, as an important indicator for evaluating vegetation ecosystem condition, is used to monitor vegetation change. SPOT-NDVI from 1998 to 2012 were adopted and integrated in this study to extract the time series of vegetation coverage, and to analyze its spatial pattern and changes. The response of NDVI to mean temperature/precipitation process and extreme temperature/precipitation processes at annual and monthly time scales was analyzed using temperature/precipitation time series at meteorological stations of the arid region of northwest China from 1998 to 2012.1998~2012, the annual mean temperature(TT), the average daily maximum temperature(TMAX), the average daily minimum temperature(TMIN) of the arid region of northwest China increased with an annual rate of 0.008℃/a、0.022 ℃/a、0.045℃/a respectively; meanwhile the annual mean precipitation(TP), maximum with 5-day precipitation(RX5day) increased with an annual rate of 0.68 mm/a, 0.15mm/a respectively. Climate warming and wetting increased significantly.During 1998~2012, the mean NDVI of the arid region of northwest China was 0.21 but exhibited apparent spatial heterogeneity being highest(0.43) in coniferous forest and lowest(0.086) in alpine vegetation areas. There was a slight increase of the NDVI over the study period with an annual rate of 0.0016%. The increase inagricultural areas was as high as 0.0051% per year, while in alpine vegetation areas, the NDVI increase was the lowest(being 0.0004% per year).At the annual time scale, there were no significant correlations between NDVI and TP(R = 0.11,P > 0.05). However there were significant positive(negative) correlations between NDVI of 6.5%(0.23%) of the total study period and RX5 day. Areas of which were mainly distributed in the North Slope of Kunlun Mountains, the Yili River Valley, the eastern Tianshan Mountains, the Ertix River Valley, the Hexi Corridor and Northwest Gansu, the Badan Jilin desert(the core area of Kunlun Mountains), etc. In general, there were no significant correlations between NDVI and TT、TMAX、TMIN(R =-0.3,-0.32,-0.18;P > 0.05), however, there significant positive(negative) correlations between NDVI of 0.35%、0.21%、0.27%(12.05%、10.49%、2.1%)of the total study period and TT, TMAX, TMIN respectively(P < 0.01)。Areas of which were mainly distributed in very few parts of Kunlun Mountains, Altun Mountain, near Urumqi(to TT、TMAX, were mainly distributed in the North Slope of Kunlun Mountains, the South Slope of Tianshan Mountains, the Kumtag Desert, Badan Jilin desert, the northern Junggar Basin,areas was larger;to TMIN, were mainly distributed in the South Slope of Tianshan Mountains,areas was smaller)。At the monthly time-scale, there were significant positive correlation coefficients between NDVI of 48.8%(53.67%) of the total study period with both TP and RX5 day, respectively; meanwhile 52.13% / 53.86% / 53.36% of the total study period with TT/TMAX/TMIN respectively, indicating that the NDVI is mainly affected by seasonal fluctuations of the the mean temperature/precipitation process and extreme temperature/precipitation processes. NDVI was affected positively(negatively) by the TP /RX5day/TT /TMAX/TMIN of the preceding month(the current month and three preceding months), showing obvious hysteresis.Additionally, there is obvious time lag-effect between vegetation growth with both mean temperature/precipitation process and extreme temperature/precipitation processes for each vegetation type: the highest correlations are observed between the NDVI of agricultural vegetation(grasslands and sparse forest grassland, coniferous forest) and TP /RX5day/TT /TMAX/TMIN of the the current month; however,the highest correlations are observed between the NDVI of shrub and coppice forests(meadow and marsh vegetation, desert vegetation, broad leaved forest) and TP /RX5day/TT /TMAX/TMIN of the the preceding month.
Keywords/Search Tags:NDVI, mean temperature/precipitation process, extreme temperature/precipitation process, spatial-temporal dynamics, correlation, time lag effect
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