Investigation Of Spatial-Temporal Variation Of Vegetation Net Primary Productivity And Its Influencing Factors In Poyang Lake

Poyang Lake,the largest water-carrying lake that connects to the Yangtze River,has remarkable seasonal variation in water surface.In the flood season,the lake surface can exceed 4000 km²,while the water surface shrinks to less than 1000 km² in the dry season.Large fluctuations in lake water level provide an ephemeral region of some3000 km² and specific hydrological conditions for the growth of wetland plants.Since the beginning of the 21st century,affected by regional climate change and human activities,the pattern of interactions between the rivers and lakes in the middle and lower reaches of the Yangtze River has been changed significantly.The water regime in the Poyang Lake has undergone significant changes with increasing extreme droughts occurred in the lake region,resulting in water supply and irrigation problems.Changes in the lake's hydrological regime may also lead to substantial impacts on the surrounding wetland vegetation and ecosystem health more generally.As the basis of matter conversion and energy transmission in wetland ecosystems,wetland net primary productivity?NPP?is an important index to represent vegetation activity and productivity.NPP reflects the production capacity of wetland vegetation communities and the quality of wetland ecosystems under natural environment.It is also a key factor in determining the carbon source/sink of wetland ecosystems,which plays an important role in global change and carbon balance.Different from ordinary river wetlands and marsh wetlands,the Poyang Lake wetland that connects to the Yangtze River,shows typical features of floodplain wetland due to the dynamics of the wetland are strongly controlled by the shrinking and expansion of the lake water surface,.How to effectively evaluate the spatial-temporal evolution patterns of wetland vegetation NPP is of great scientific significance to the objective understanding of the mechanism of hydroecological changes in those the open lake systems under the changing environment.In this study,the high spatio-temporal resolution?30 m and 16 days?of NDVI images of the Poyang Lake wetland during 2000-2015 from the combined Landsat and MODIS data was successfully reconstructed by using a spatio-temporal fusion model?ESTARFM?.On this basis,the CASA?Carnegie-Ames-Stanford-Approach?model was constructed to evaluate vegetation NPP in the lake floodplain wetland and optimized according to some measured biomasses data.Finally,the short and long term characteristics of vegetation NPP and its driving mechanism were investigated in the Poyang Lake floodplain wetland.The main conclusions are summarized as follows:?1?Intra-annual variation of NPP of vegetation is influenced by the combined effects of vegetation growth rhythm and the exposure of lake floodplain.NPP of main vegetation in Poyang Lake show two peaks in a year.In terms of space,the NPP at different elevations in the lake area is quite different,and the spatial patent of annual and seasonal NPP show an increasing trend from the middle to the edge of the lake.?2?From 2000 to 2015,the average annual NPP range of the Poyang Lake region ranged from 360.09 to 735.94 gC?m^-2,with an average of 528.76 gC?m^-2.The annual average NPP showed an increasing trend,with an average annual growth rate of approximately 10.03 gC?m^-2.In terms of seasonal growth rate,NPP presented autumn>spring>winter>summer,among which the autumn NPP growth rate reached a significant level.Spatially,the area with an significant increase in average annual NPP accounts for 31.56%of the total area of the lake,and is mainly distributed in the center of the lake area of 9¹3 m.The area of NPP increased in autumn accounted for 30.68%of the total area of the lake,and the main distribution area coincided with the annual NPP increase area.Among all the vegetation types,the NPP of the Phragmites and Triarrhena and the Artemisia showed a downward trend,while other vegetation types showed an increasing trend.The NPP of Carex and sparse grass has generally increased in the autumn,and the average annual growth rate is relatively high,while the NPP of the Phragmites and Triarrhena and the Artemisia barley shows a downward trend in spring,summer,and winter.?3?Change in water regime determine the change and distribution of NPP in Poyang Lake.The significant changes of the water regime have had a significant impact on the biological rhythm and distribution of the wetland vegetation,resulting the change in NPP.Because the water regime dominate the spatial distribution of NPP in the lake,the impact of meteorological factors is relatively weakened or overshadowed.In recent years,under the combined effects of changes in the pattern of rivers and lakes,human activities such as sand mining,and climatic changes,the inter-annual water level changes in the Poyang Lake have shown a downward trend,which is the main reason for the increase in annual NPP.The decrease of water level during the dry season and the advance and prolong of duration the dry season in Poyang Lake have led to corresponding changes in NPP in different seasons.In particular,NPP of Carex and sparse grass in lower elevations of the lake areas and have a significant increase in autumn.