The Analysis Of Temporal And Spatial Characteristics Of Net Primary Productivity(NPP) In Chongqing Based On Remote Sensing Data

In recent decades, global warming has made the warn of threatening to human existence, to which researchers have drawn attention. The carbon cycle in terrestrial ecosystem that was regarded as source and convergence of CO2was an important part of the global carbon cycle, and played an important role in global climate change. The research of Net Primary Productivity (NPP) was the base for assessing the carbon balance in ecosystem. Owning a typical subtropical evergreen broadleaf forest, Chongqing city was a major component of the carbon storehouse in southwestern China; on the other hand, the storehouse of The Three Gorges Reservoir in Chongqing had a large area and spreads widely, for which the vegetation status in Chongqing especially The Three Gorges Reservoir affected the ecological security of the entire Yangtze River valley. It had a very imporatant significance for scientific research and the decision-makings of government to monitor the status of vegetation productivity and the changes of temporal and spatial in Chongqing.1) NDVI dynamics in Chongqing city and its responses to climateBased on AVHRR and MODIS remote sensing data and combined with the meteorological data, we analyzed the temporal and spatial variation of Normalized difference vegetation index (NDVI) and its responses to two climatic factors in Chongqing during1982-2011. The results showed that:(1) The vegetation NDVI had a decreasing trend. The significantly decreasing area occupied39.08%in the total land in Chongqing, which mainly distributed in the urban areas of south-east, midland and the mountain range of northeast and mainly happened in July and August.(2) During the time of research, the climate in Chongqing presented a process of distinct warming and drying. The relationship between NDVI and temperature was much stronger than that between NDVI and precipitation.(3) The analysis of monthly changes suggested that the heat application in winter was an important factor to affect the growth of vegetation, and the trend of temperature rise in summer inhibited the growth of vegetation. Vegetation variation lagged responding to temperature and precipitation.(4) The correlation between NDVI and hydrothermal conditions of Chongqing had spatial differences:in most areas, the correlation between NDVI and temperature, precipitation were not significant; the correlation between NDVI and temperature was significant negative in the northeast and west of Chongqing, and the correlation between NDVI and precipitation was significant negative in the northeast of Chongqing.2) The research of NPP in Chongqing based on GLO-PEM modelBased on GLO-PEM, meteorological data in the same period, and the usage data of land, we analyzed the vegetation NPP space distribution, changes and influencing factors during1981-2000, in Chongqing. The results demonstrated that:(1) The annual average of vegetation NPP in20years was995.08g C (m2·a)’1. NPP in Chongqing had been increasing, but the78.98%area had no significant change.(2) The climate appeared the warming-drying trend in Chongqing. At yearly sacle, temperature played a decisive role in NPP in Chongqing.(3) NPP observably increased in spring. NPP of four seasons was more sensitive to temperature than to precipitation or solar radiation. The correlations between NPP in every season and climatic factors were different from each other, of which the vegetation NPP in spring was more obviously influenced by temperature.(4) At monthly scale, NPP, temperature and precipitation all did not change obviously, but solar radiation increased evidently in September. It suggested that the changes in vegetation NPP and simultaneous temperature had obvious positive correlation and the lag obviously existed between NPP and precipitation. The correlation of monthly NPP and climate factors demonstrated that heat suppliction in spring was an important factor to affect vegetation NPP. At the same time, the precipitation in spring and summer also obviously affected vegetation NPP.(5) The changes in the usage or cover of land, especially the rapid urbanization, to some extent, promoted the changes of NPP.3) The Temporal-spatial Characteristics of NPP in Chongqing based on the ecological remote sensing coupling modelBased on MOD17A3remote sensing data, we analyzed the spatial distribution and interannual variation during2001-2006, in Chongqing City. The results demonstrated that:(1) The range of annual NPP in the time of reseach was between97.35g C (m2·a)-1and389.67g C (m2·a)-1, and the annual average of NPP was244.02g C (m2·a)-1.(2) The annual NPP of Daba mountain range in the northeast and of the Wuling mountain range in the middle of Chongqing was relatively higher.(3) The vegetation NPP had been obviously decreasing from2001to2006, of which the area accounted for86.73%land in Chongqing. 4) The NPP research in Chongqing based on CASA ModelBased on MODIS remote sensing data, meteorological data in the same period, and the usage data of land, I analyzed the space distributions, changes and influencing factors of the vegetation NDVI and NPP during2001-2011, in Chongqing. The results demontrated that:(1) The annual average of NDVI was0.844and the annual average of NPP was626.51g C (m2·a)-1. The annual averages of NDVI and NPP in Daba mountain range and Wuling mountain range are higher than those in other areas.(2) At yearly scale, NDVI and NPP both increased. In space distribution, the areas in which NDVI and NPP did not change a lot respectively accounted for86.64%and85.85%. At monthly scale, in July, NDVI and NPP both decreased (P<0.05), but NDVI in October, NDVI and NPP in November increased significantly (P<0.05).(3) The relationship of annual average of NDVI and NPP and climate factors was not obvious. The analysis of monthly correlation discovered that the response from vegetation NDVI to the changes of temperature was more sensitive than that to precipitation. Correlation between climate factors and NPP presented a certain regularity:solar radiation>temperature>precipitation. The effect of vegetation NPP that was made by the heat supplicationin the whole year, epecially in spring, and the precipitation in spring and summer.(4) As for the whole region, the impacts in the changes of NPP led by the changes of climate was more dominated than that in the changes of usage of land. However, in the region where the changes happened on usage of land, the impacts of climate change was less dominated. During the period of2001-2009, the land cover had changed dominatedly with cultivated land turning into woodland and grassland, which, to some extent, had made NDVI and NPP keep increasing during the research period.To sum up, through the comparison of three kinds of models, this paper argued that GLO-PEM, being of the priority of temporal accuracy, was able to be applied to reflect the historical NPP in Chongqing city; due to its the problem of accuaracy, we suggested MOD17A not to apply in research and monitor for NPP in Chongqing city. Because of the good accuracy of NPP simulated by space CASA model that is based on the MODIS data and relatively enitre temporal order, even though the MODIS data just can be gotten from2000, We suggest that researchers and government could use the model to monitor current NPP in Chongqing.