Influence Of The Morphological Structure Of Major Wetland Plants In The Yangtze River Estuary On The Canopy Spectra

The Yangtze River estuary wetland is a typical coastal tidal flat wetland,which is an important wetland resource in China.Optical remote sensing has been widely applied in wetland plants monitoring by the advantages of obtaining dynamic information rapidly on a large scale,and can provide important basic data for wetland biodiversity conservation.Since the Yangtze River Estuary wetland is deeply affected by tidal activities,invasion of alien species and high-intensity human activities,which lead to the complex environment of underlying surface and the significant difference between the growth status of plants.These ultimately result in the common phenomenon of "same object with different spectra",which brings great difficulties to remote sensing classification and quantitative inversion research.Compared with forests and farmland,there are few studies on the radiative transfer process of wetland plants.Exploring the mechanism of how the morphological structure of major wetland plants in the Yangtze River Estuary influence the canopy spectra will be helpful to improving the research level of wetland vegetation monitoring by remote sensing and quantitative study in this area.Aiming at native species Phragmites australis and alien species Spartina alterniflora,the typical plant species in the Yangtze River Estuary wetland,this paper based on field investigation and UAV hyperspectral aerial photography and used the canopy radiative transfer model PROSAIL,which based on the assumption of turbid and homogeneous medium,and the three-dimensional radiative transfer model DART,which based on the real scene,to quantitatively analyze the sensitivity of canopy structure parameters such as leaf area index(LAI)to canopy spectra.By simulating the radiative transfer processes of the canopy of two species with different plant densities,leaf morphology and other scene structures,this paper analyzes the sensitivity and anti-saturation of the vegetation indexes to the changes of LAI,thus,reveal the mechanism of how three-dimensional structures influence canopy spectra for P.australis and S.alterniflora,and provides theoretical support for improving the problems of classification and inversion quantitatively which caused by structural diversity.The main research contents and conclusions are as follows:(1)The local and global sensitivity of the morphological and structural parameters of P.australis and S.alterniflora communities to the canopy spectra were quantitatively analyzed by using the One Factor At a Time method(OFAT)and the extended Fourier Amplitude Sensitivity Test(EFAST).The result showed that the spectra were highly sensitive to LAI and average leaf inclination angle(ALA)in the visible-near-infrared band,especially in the near-infrared band(750nm-1000nm).When the interaction between factors was considered,the total sensitivity index in 400nm-1000 nm was similar to the first-order index in shape and increased in value.Due to the limited number of parameters characterizing the morphological structure in the PROSAIL model,which are all statistical parameters,the results of sensitivity analysis for the two species were similar and the difference was not obvious.(2)Based on the three-dimensional radiative transfer model DART,the community scenes of P.australis and S.alterniflora which reflected the detailed information of plant and canopy morphological structure were constructed,and the sensitivity and anti-saturation ability of 15 commonly used narrow-band vegetation indices to LAI changes were analyzed.The results revealed the differences in the sensitivity and anti-saturation ability between different vegetation indices to the changes of LAI,and this difference also existed between P.australis and S.alterniflora.When the LAI is the same,there are still differences in the vegetation indices corresponding to P.australis and S.alterniflora communities,which was caused by the different morphological structure between two species and difficult to reflect in the PROSAIL model.(3)The effects of canopy morphological structures,such as plant density,leaf size and quantity and leaf area density distribution,on the radiative transfer process of P.australis and S.alterniflora canopies were further investigated by DART model.The results showed that the interception,scatter and absorb process of canopy were affected by the above morphological and structural parameters to a certain extent,which also revealed the limitations of the PROSAIL model in describing the canopy structure.