| Since the research of vegetation coverage changes and relevant environmental effects is one of the most important approaches of understanding the influence of huge human project on regional environment, study on the space-time evolution pattern, variation process, driving mechanism and effect intensity of vegetation coverage under various scales has great practical and scientific significance in correctly recognizing the mechanism and effect intensity of human's ecological project on environment evolution. Based on the data from field observation and experiment, NOAA/AVHRR GMMIS NDVI analysis of long time series from 1982 to 2003, MODIS analysis of short term series from 2000 to 2005 and many other approaches, the relationship between the space-time evolution characteristic of vegetation coverage changes and the climate factors in Loess Plateau was analyzed through a series of methods such as trend analysis, correlation analysis and difference analysis, also the mechanism and effects of vegetation coverage changes on regional climate environment were revealed since the Converting Farmland into Forest and Grassland Project was carried out in this region and the main results could be concluded as follows:(1) Characteristic of vegetation coverage changes in Loess Plateau from 1982 to 2003The vegetation NDVI in Loess Plateau increased in the year level, which indicated that the vegetation coverage condition was becoming better although negative changes existed in some regions; in the season level, the vegetation NDVI had no evident changes in summer and winter while it increased in spring and autumn especially the former one; in the month level, the vegetation NDVI increased from January to August and decreased in the other time of a year because April to September was the main growing season and July to August was the typical growing season, the NDVI in August can best describe the vegetation coverage condition in Loess Plateau since the vegetation coverage ratio of this time is the highest in the year.(2) Characteristic of vegetation coverage changes in Loess Plateau from 2000 to 2005This time can be defined as the initial stage of converting farmland into forest and grassland, results of the analysis of average NDVI changes in typical growing season showed that the vegetation coverage condition of the whole region was becoming better, which meant that the vegetation was under rehabilitation process.(3) Relationship between vegetation NDVI and climate factors in Yan'an from 1982 to 2003In the year level, climate factors such as annual average temperature, precipitation and relative humidity had positive effects on the dynamic changes of vegetation NDVI, but the relativity was not significant; in the season level, vegetation NDVI was more sensitive with precipitation and relative humidity than temperature, of which the precipitation in spring, relative humidity in summer and temperature in winter had a great influence on vegetation NDVI, and meanwhile, the relative effect of precipitation on vegetation NDVI had a season lag phenomenon.(4) Relationship between vegetation NDVI and climate factors in Yan'an from 2000 to 2005In the typical growing season of vegetation, precipitation and relative humidity had positive effects on the vegetation while the temperature had a negative one. Precipitation and relative humidity in July had positive effects on vegetation conditions in August, which indicated that there was a lag effect between vegetation growing and climate factors not only in Yan'an but also the whole Loess Plateau region.(5) Driving mechanism and regional climate effects of vegetation coverage changes in Loess PlateauThe space-time evolution of vegetation coverage in Loess Plateau was promoted both by climate factors and human factors. In recent years, human factors had great influence on regional climate environment, and the climate effect of vegetation rehabilitation could be shown by the decrease of windy weather, better atmospheric visibility, controlled water and soil erosion, stopping the spread of desertification and the yield increase of alpine meadow.(6) Influence and ecological effects of vegetation rehabilitation on regional climateIn order to research the positive effects of vegetation rehabilitation and reconstruction on regional environment, the method of located observation was adopted to monitor the climate characteristic of three artificial vegetation included mixed forest, shrubbery and artificial grassland compared to bare land, also the effect on regional climate of natural vegetation on abandoned land and artificial vegetation were analyzed compared to farmland, at last the climate effects of artificial shrubbery with canopy density of 75% and 40% was compared and the results could be drawn as follows:(a) Compared with bare land,the daily mean values of light intensity of mixed forest, shrubbery and grass land reduced 3.9×104, 3.4×104, 0.9×104 lx, air temperature dropped1.53℃, 1.32℃, 0.51℃, temperature of surface soil (0 cm) dropped 14.9℃, 16.8℃, 10.7℃, and the air relative humidity increased 8.38 %, 7.34 %, 2.04%;light intensity, air temperature, relative humidity, soil temperature and their changing amplitude followed the order of mixed forest |
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