| The global warming has brought great influence on geosphere-biosphere, and it not only attaches closely with ecological environment, but also greatly affects the human society. As one of the main parts of land ecological system, vegetation is the most active factor for global circulation of materials and energy conversion, and as well the indicator of climate and ecological environment changes. For the desert ecosystem has simple structure and tends to a weak resisting disturbance, therefore, it can respond quickly to the changes of outside environments. Hence, there are also some considerable uncertainties in desert vegetation activity and about its response to climate alteration under the background of global warming. Thus, monitoring and evaluation of desert vegetation in arid area will be generally beneficial to realize thoroughly the interaction between vegetation and atmosphere, which reveals that the effects of global warming on land ecological system, and provides scientific basis for adjusting as well as restoring the desert ecological environment in arid regions.In this study, based on"3S" technologies, we adopted and integrated GIMMS NDVI (1982-2006) and MODIS NDVI (2001-2013) remote sensing data to analyze spatial-temporal variations of desert vegetation and its response to climatic changes in the Hexi area, Gansu Province during 1982-2013. In addition, the model of desert vegetation phenology has been constructed and applied based on the MODIS NDVI in the Hexi area since 2000, which includes GD (Greenup Date), SD (Senescence Date) and LG (Length of Growth Season), and the spatial-temporal variations of desert vegetation phenology, furthermore its response to climate changes was also been analyzed. The meteorological data were collected from major weather stations in the Hexi area during 1982-2013, were used to study the effects of climate warming on desert vegetation from three aspects of water (precipitation), heat (temperature,≥0℃ accumulated temperature) and sunshine (solar radioactivity) conditions. The results of this research reveal that:1. The spatial-temporal variations of desert vegetation in the Hexi area from 1982 to 2013The latitudinal and vertical zonality of desert vegetation growth were investigated to be apparent in the Hexi area, the NDVI maximum in the growth season (NDVImax) showed a decreasing tendency of -0.022/1°N from low latitudes to high latitudes, and an increasing tendency of 0.047×10-3/m from low altitude to high altitude region. From 1982 to 2013, the significant improvement conditions take account for 40.51% in the desert vegetation regions, and this value is higher than the marked degrading regions (5.96%). In general, the NDVImax was subjected to a significant increasing trend of 0.65×10-3yr-1 during 1982-2013, thus, the Non-vegetation area tended to decrease. It was obvious that vegetation degradation occurred in the Hexi area from late 1990s to early 2000s, while vegetation improvement had mainly took place in the recent 10 years. Generally speaking, the desert vegetation activities mostly remain stable in the Hexi area from 1982 to 1999 (Phase Ⅰ), which was significantly improved in 52.11% of the area during 2000-2013 (Phase Ⅱ), except for individual areas that was suffered desertification increasing in the Phase Ⅱ.The desert vegetation had been largely stable in different river basins during the Phase Ⅰ, however, the improvement trend occurred in all river basins especially in the Shule River Basin of the western Hexi area during the Phase Ⅱ. The vegetation improvement phenomenon mainly occurred in the middle and lower reaches of the Shule River Basin and the Shiyang River Basin, and the WMC moved in the same direction. During the period of 1982 to 2013, the significant restoration area of typical desert vegetation accounted for 40.7% of the total typical desert vegetation regions, which was higher than that of the mountain desert vegetation (18.45%). The area ratio of improved and degraded of the typical desert vegetation in phase Ⅱ was faster than that of the mountain desert vegetation, and the ratio of the degraded area of the typical desert vegetation was enhanced with greater fluctuation during the Phase Ⅱ than the phase Ⅰ.The average phenology values of GD and SD as well as LG for desert vegetation in the Hexi area during 2000-2013 were 127th,295th and 169 days, respectively. The GD and SD of desert vegetation in the eastern were later than that of the western Hexi area, which demarcated by the Heihe River Basin and characterized by ladder-shaped distribution, and there was a wide fluctuation of phenology in the central Hexi area. Insignificant delaying tendency of the GD was taken place from low latitude to high latitude areas; however, it was on the contrary for the SD. The characteristics of the GD below 1700 m indicated delay from low elevation to high elevation area with great fluctuation, and while there was an advancement trends above 1700 m. As a whole, there were insignificant advancing ratios observed for the GD and the SD, respectively, with -6.2d/10a and -1.8 d/10a during 2000-2013, and insignificant prolonged ratio of the LG was 2.77 d/10a. The GD and the SD have advanced tendency in the majority of the Shiyang River and the Heihe River basin, but the change was different in the Shule River Basin from 2000 to 2013. Besides, the advanced tendency of the GD, the SD, and extension of the LG of the mountains desert vegetation were relatively obvious than that of the typical desert vegetation.2. The impact of climate change on the desert vegetation in the Hexi areaThe annual average temperature in the Hexi area had significant increasing trend of 0.5℃/10a during 1982-2013, The increasing trend in Phase Ⅰ, which was especially significant was 0.73℃/10a, while it showed a moderate increasing trend of 0.17℃ /10a in phase Ⅱ. At the same time, the annual precipitation showed an insignificant increasing trend of 4.37mm/10a during 1982-2013, with the increasing rate of 4.31 mm/10a and 0.43mm/10a in Phase Ⅰ and Phase Ⅱ respectively. In the late 1990s, the temperature was rapidly rising while precipitation was gradually decreasing. In different seasons, the temperature showed significant increasing rate during the Phase Ⅰ, however, it reduced in the Phase Ⅱ, even the negative temperature increased in winter. In spring, the precipitation showed a significant decreasing trend in the Hexi area in both the Phase Ⅰand the Phase Ⅱ, however, it increased continuously in winter. The changing trend of the summer precipitation shifted from decreasing to increasing from Phase Ⅰ to Ⅱ, while the precipitation in autumn showed a contrary tendency. The ≥0℃ accumulated temperature as well as the annual solar radiation had a significant increasing trend in the Hexi area during 1982-2013, the increasing trend of ≥0℃ accumulated temperature was dropped during the Phase Ⅱ. And the change trend of solar radiation was unsignificant in the Phase Ⅱ.The main factors for vegetation growth were precipitation of spring and summer, the vegetation growth was affected by precipitation during March and August that covered about 95% of the total desert vegetated land, and the temperature had little effect on the growth of desert vegetation. Been drought and with little rain, the precipitation was usually utilized timely by the vegetation growing, therefore the growth of the desert vegetation had not lagging effect on inter-annual variations of temperature and precipitation actually. In general, the distribution of the desert vegetation in extremely arid regions was moderate relatively with good water conditions, thus, the precipitation had less effect on the desert vegetation in the Shule River Basin than the other two river basins. In addition, the mountain desert vegetation growth was influenced more significantly by precipitation than the typical desert vegetation, because of lower drought-resistance capacity of plant and poor catchment condition of the mountain areas.The spring precipitation and the winter≥0℃ accumulated temperature were main controlling factors of the GD of desert vegetation in the Hexi area (Phase Ⅱ), its increasing could lead to the GD earlier. The lack of spring precipitation, the increasing of temperature and evaporation could accelerate the reduction rate of soil moisture, which would lead to delay of the GD in the Hexi area. The increasing solar radiation at the beginning of leaf expansion was helpful for the process of photosynthesis, which could accelerate the vegetation growth and slightly advanced the GD, as a matter of fact, the spring temperature and solar radiation had little effect on the GD. In general, the increasing of precipitation and temperature in September were conductive to delay the SD, however, the effect of precipitation was more important. Furthermore, the effect of precipitation on the SD was more significant in the western and the central Hexi area than that of in the eastern area. On the other hand, increasing of solar radiation between September and October may lead to higher plant respiration and make the SD earlier. However, compared with temperature and precipitation, solar radiation had less impact on the desert vegetation phenology. In addition, high temperature in August could accelerate the plant respiration and transpiration, which led to advance of the SD and this phenomenon was relatively significant in the Shule Basin because of the high temperature in vigorous period of the desert vegetation. |
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