Effects Of Interspecific Competition On The Chlorophyll Inversion Of Phragmites Australis And Spartina Alterniflora In The Coastal Wetland Of The Yangtze River Estuary

The Yangtze River Estuary coastal wetland,a typical tidal flat wetland resource in China,is considered with important ecological value.Its major native species,Phragmites australis(PA),is under the threaten with degradation in both area and function,due to invasive alien species like Spartina alterniflora.The mixed growth area of Phragmites australis and Spartina alterniflora(SA)is key to the protection of native species.Both ecologist and administrator,therefore,need to understand the condition of the two pieces in such area for scientifically assessing the level of invasion of Spartina alterniflora with appropriate measures for protection.It is difficult to monitor the condition of wetland area because it is generally inaccessible.Besides,considering the complex dynamic changes of the mixed growth area,it could hardly be monitored through a fixed-point way either.Remote sensing,a means of large-scale ground observation,has been applied to dynamic monitoring of wetland vegetation.Meanwhile,using remote sensing spectrum to conduct quantitative retrieval research on chlorophyll content of the plant could quantitatively reflect its growth status.However,such studies,at present,mainly focus on qualitative research such as species classification.The research of quantitative remote sensing inversion on the biochemical parameters of wetland vegetation is relatively lacking,and even on the interspecific competition caused by the mixed growth of various vegetation.This study,therefore,taking Phragmites australis and the invasive species Spartina alterniflora as the research objects,selected the Yangtze Estuary coastal wetland of Chongming Island in Shanghai as the research area.Three years of ground control experiments and field measurements were conducted to explore the spectral characteristics under different conditions(Interspecific competition,Nitrogen levels)at leaf and canopy scales,and quantitative inversion based on which on chlorophyll content was conducted then.The major contents and results of this paper are as follows:(1)Measured spectra of the leaf and canopy of Phragmites australis and Spartina alterniflora under different mixed proportion(simulating interspecific competition in mixed growth area)were compared,as well as under different nitrogen levels(simulating river eutrophication)and different water levels(simulating tidal level).The results showed that three experiments all have impacts on the spectra of the leaf and canopy,and the characteristics are various in different months due to the different phenological periods of the two species.(2)Pure spectral indices were developed through comparing the main spectral indices sensitive to chlorophyll content.Pure frequency indices were also developed through Harmonic Analysis(HA)and Hung Hilbert Transform(HHT),as well as a mixed index combining both.The results showed that although the accuracy of the three models was lower than that of the single species under the inversion of chlorophyll content with mixed species,in general,the mixed indices performed better in quantitative inversion of chlorophyll content of the mixed growth zone with better accuracy.(3)Quantitative inversion of chlorophyll content of Phragmites australis and Spartina alterniflora were conducted using hyperspectral image of unmanned aerial vehicle(UAV).The results showed that the inversion accuracy of mixed indices in pure PA zone,pure SA zone and the mixed zone is all better than that of pure spectral indices and pure frequency indices.This indicated that the mixed indices are more suitable for field experiment.The highlights of this paper are as follows:(1)The mixed growth of Phragmites australis and Spartina alterniflora,based on the experiment,has a significant effect on the inversion of chlorophyll content,which is one of the factors leading to the uncertainty of the retrieval of chlorophyll content in the mixed growth zone of the two species.(2)Experimental results illustrated the importance of frequency domain index in the inversion of chlorophyll content of Phragmites australis and Spartina alterniflora,and the interference,especially in its mixed zone,is less than that of spectral index.(3)It is found that the chlorophyll inversion model established by fusing the spectral index and the frequency domain index is less disturbed by the mixture of Phragmites australis and Spartina alterniflora.The applicability of the model is verified by the model simulation,control experiment and UAV field data...