长江口潮滩地带典型盐沼植被光谱特征分析

Spectral analysis of an intertidal saltmarsh in the Yangtze Estuary

以长江口典型盐沼植被为研究对象,考虑不同盐沼植被类型、盖度、土壤类型、土壤湿度以及高中低潮滩地形分布因素,分别在崇明东滩国家级自然保护区、崇明北湖边滩、南汇边滩设置光谱测量样带和样方,使用ASD光谱仪进行现场光谱测量,分析不同植被盖度、土壤下垫面、潮位条件下典型盐沼植被的光谱特征反射曲线。研究结果表明:同一类型盐沼植被的光谱反射率差异是由植被盖度和下垫面共同导致的。在相同土壤背景下,光谱反射率与植被盖度在可见光波段相关系数接近-1.00,呈负相关,在近红外波段相关系数接近0.99,呈正相关。不同潮滩土壤背景,盐沼植被光谱反射率有明显差异。在可见光波段,土壤下垫面对植被光谱反射率影响较大,而在近红外波段的影响则较小。植被盖度越小,其反射光谱受土壤下垫面影响越大。盐沼植被与潮位的相关系数最大可达0.97,低潮位时植被反射光谱高于无潮汐时的光谱,且呈现随潮位升高反射率增大的趋势;当潮位上涨到一定高度,植被反射光谱低于无潮汐时的光谱,且呈现随潮位的升高而下降的趋势。

To consider different vegetation types,coverage,soil types,soil moisture,and gradient elevation of a tidal flat so as to determine the effect of the tide,three typical saltmarsh vegetation types in the Yangtze Estuary were chosen as research objects.Vegetation transects and samples(1 m×1 m) were selected for the above factors,and the growth environment of the saltmarsh was simulated.An ASD Fieldspec HandHeld 2 was used to measure the spectrum of the saltmarsh vegetation canopy;then the spectral reflectance and the first derivative (350-950 nm) were calculated,and correlations between coverage/soil/tidal level and spectral reflectance were analyzed.Results showed that vegetation coverage and the underlying surface greatly affected spectral reflectance of the saltmarsh.For the same soil conditions,spectral reflectance of different saltmarsh vegetation coverages in the visible light band was negatively correlated (r≈-1.00) with coverage and in the near infrared positively correlated (r≈0.99).For both low coverages and high coverages,spectral reflectance of different soil types in the visible light band was positively correlated (r≈0.97) but in the near infrared was less correlated.The saltmarsh and tidal level were significantly correlated (r≈0.97).Also spectral reflectance of the saltmarsh at high tide was higher than at low tide,and the reflectance increased as the tide rose.When the tide level rose to a certain height,spectral reflectance of the saltmarsh was lower than without a tide,and this generally decreased as the tide level fell.The effect of vetegation coverage,underlying(soil types and soil moisture) should not be ignored when monitoring saltmarsh vegetation.[Ch,10 fig.3 tab.27 ref.].