| The vegetation is one of the important elements to maintain the stability of terrestrial ecosystem. The Net Primary Productivity could reflect the capacity of organic production and self-consumption of organic for plants. The plants could absorb CO2during their growth processes, which is important to reduce greenhouse gases and regulate climate, then the NPP could reflect carbon sequestration in plants. The productivity of plants is impacted by their own characteristic and the surrounding environment. The Shule River Basin is far from the sea, with water vapor being separated by two mountains, resulting in rare precipitation and fragile ecological environment. Coupled with the irrational human activities, the ecological environment is severely damaged, with a series of environmental problems like desertification, drought, and dust storms disasters being arising, which seriously affect the sustainable development of the study area.In this study, MODIS products, Land-cover and Meteorological materials are taken as data resource in the CASA model. The purpose of the paper is to estimate the monthly NPP in the Shule River Basin, respectively in2001,2005,2009and2012. The spatial-temporal variation of NPP is analyzed on these bases. It is helpful to understand vegetation growth conditions, the ecological status and the carrying capacity in the basin. Meanwhile, it is indicative for the regional sustainable development. The conclusions are as follows:(1) The amount of Absorbed Photosynthetic Active Radiation (APAR) in the study area was range from0to2373MJ/m2/a, and the majority was60-500MJ/m2/a.The APAR was generally high in the upstream areas, whereas it was low except the farmland in the mid/downstream areas. The average of Light Use Efficiency (LUE) was ranged from0to0.27gC/MJ/Mon.It showed a gradual decreasing trend from upstream to downstream and from east to west. The above two parameters were both maximum in summer and minimum in winter.(2) The NPP value was varied from the different kinds of land over. There was about72.19%of the whole area covered without or a small amount of vegetation. The grasses in the Qilian Mountain grew well, and most natural vegetation was semi-shrub;the farmland was mainly located around the residents, additionally, there are several rivers, the four major reservoirs and modern glaciers in the basin. The sequencing for was Woodland> Farmland> Grassland> Shrub> Construction Land> Water> Unused Land> Permanent Snow and Ice. Under the correlation analysis between the results and the MODIS-NPP products, the coefficients were all above0.88. Compared with results estimated by other models, it proved a good applicability.(3)For the spatial distribution, NPP was gradually reduced from upstream to downstream, and the farther away from river bank, the lower value of NPP. In addition, artificial vegetation grew better with higher amount of organic than natural vegetation. The high NPP was mainly located in the origin of Shule River, Zhaobi Mountain, and the shady of other hills. The value of NPP was150-350gC·m-2·a-1in the southeast of Yanchiwan natural reserve and so low in other reserves. It was high in the cultivated area of the middle and lower reaches and below100gC·m-2·a-1and50gC·m-2·a-1for the semi-shrub around of residents and Gobi respectively.(4) Under the analysis of changes in the total NPP, it was only6.85×1012gC·a-1in2001, then it increased to2005and decreased slightly to2009, the total NPP reached10.24×1012gC·a-1with a significant increasing rate in2012, which showed that the total NPP was under a increasing trend in the Shule River Basin. For the monthly change of NPP, it increased greatly in May and June, and reached the peak in July with the average of21.72gC·m-2·Mon-1, then it decreased rapidly after September, the lowest value was1.25-2.37gC·m-2·Mon-1. Overall, the growth period of vegetation was short that NPP was mainly accumulated from May to September and at a low level in other periods. With statistics of seasonal accumulation contributed to the total NPP per year, it was up to62.60%in summer, followed by the autumn and spring, whose contribution rate of17.17%and16.33%and only3.91%in winter.(5) It was an integrity change for the NPP in the study area and it was rare for the situation that a dramatic change occurred in a certain region. The NPP increased in the study period and varied form different areas. It increased greatly in the Qilian Mountain and around of residents and slightly increased in the Gobi at the feet of Qilian, while it decreased slightly in the west of Gobi. It was most steady in the modern glaciers and bare land of the western Dunhuang. |
PDF Full Text Request