Spatial And Seasonal Patterns Of Aboveground Net Primary Productivity And Their Responses To Environmental Factors In The Drawdown Zone Of The Three Gorges Reservoir,China

This thesis targeted on a typical drawdown zone(DDZ)area with gentle slope in the Three Gorges Reservoir(TGR).It is named Baijiaxi drawdown zone and located in Kaizhou county of Chongqing.By using remote sensing technique and the field survey data,aboveground biomass(AGB)in different growing stages during waterlogging period were estimated.In this processing,the potential capabilities of the domestic satellite images in monitoring ecosystem of the DDZ in the TGR were explored.Based on the remote sensing estimated AGB data,we then obtained the aboveground net primary productivity(ANPP)in different growing stages with the help mathematic relationship between AGB and ANPP.Further,the spatial and seasonal patterns of the ANPP and their responses to the main environment factors were analyzed.The main conclusions were gained as follows.(1)Domestic high spatial resolution satellite images performed well for monitoring vegetation in the DDZ.Specifically,those images could be efficiently applied for mapping vegetation community and estimating leaf area index(LAI)and AGB by using random forest modelling technique.The predicted(for vegetation community)and estimated(for LAI and AGB)results were assessed by the true observed values and found the differences between them(predicted/estimated and true values)were small(i.e.,predicted/estimated results have high accuracies).It suggested the estimated LAI and AGB data and the predicted vegetation community map can meet the follow-up analysis.(2)Generally,the ANPP in different growing stages showed different spatial distribution patterns along the altitudinal.The main causing factors including: inherent physiological characteristics of different plant community types(such as reproductive modes,resistance,and phenology),differences of water-level-fluctuation(WLF)induced vegetation growth phonological and physiological stress,and the utilization rate of light energy(indicated by averaged daily leaf area factor).In addition,grazing disturbance had some influence on the spatial variation of ANPP.In general,the ANPP in grazing disturbance area was smaller than that in the non-grazing disturbance area.The effects of photosynthetically active radiation(PAR),soil moisture,soil pH,and soil SOM have relatively less effect on the spatial variability of ANPP.(3)The ANPP in the DDZ showed significant seasonal variation.Overall,ANPP decreased from May to June,then increased from June to July,and at last decreased again from July to September before water level arising.Among this pattern,the maximum ANPP appeared the period from June to July.Despite this type of overall seasonal pattern,ANPP in different elevation belts showed their own distinct seasonal patterns,which caused by different types of effects from various environment factors,such as temperature(TEM),precipitation(PRE),PAR,and WLF.The TEM,PRE and WLF were the main factors that affect the season pattern of ANPP in low elevation zones.In the middle and high elevation zones,however,WLF has little influence on those zones,whereas the PAR,TEM and PRE were suggested to be the main factors that drove the season pattern of ANPP in those zones.It shows the detected main impacting factors of ANPP's season variations among different elevation belts were inconsistency.Such inconsistency may be casued by the different times of plant's phenology and the different physiological characteristics of plants,both of which are often jointly induced by the WLF and vegetation community types.(4)The DDZ was found having great capability of carbon sequestration which was often considered in wetlands.Therefore,it is possible to make full use of this characteristic to carry out the scientific utilization of grassland in DDZ.Among all types of vegetation communities in the DDZ,the bermuda grass(i.e.,Cynodon dactylon(L.)Pers.)community has the largest ANPP.It is suggested can served as a basic community for the ecological restoration and management of the DDZ.In view of the essential role of the water level fluctuation on controlling the growth process of plants(especially in the early and mid growth season),it is suggested to be served as a tool for some specific management objects on the DDZ.For example,it is possible to control capacity of carbon sequestration and biodiversity in the DDZ by applying different intensities of water level fluctuation.ANPP is often considered as the main material and energy source of an ecosystem,and a key indicator of ecosystem functions.The results of this study could provide basis theoretical support for the on-going assessments of ecosystem function changing and management of DDZ under the current water-level-operation mode.It is also suggested significant for maintaining ecological stability of DDZ and the ecological security of the TGR.