水胁迫下典型盐生植被梭梭光谱特征分析

荒漠地区由于气候干燥,降水稀少,水分常成为制约植被生长的因素之一,水分胁迫对植物长势和产量的影响比任何其他胁迫都要大。随着高光谱技术的发展,国内外已有众多学者利用高光谱数据研究植被遭受胁迫作用,然而这些研究对象多集中于甜菜、棉花、玉米、水稻等作物,针对干旱区盐生植被遭受胁迫作用的研究较少。梭梭作为荒漠、半荒漠地区的典型盐生植被之一,具有极高的经济和生态效益。选择梭梭作为研究对象,培育一年生梭梭,并设置三个水分梯度,形成受不同水分量胁迫的梭梭。使用原始光谱、红边位置参数,结合植被指数及二维相关光谱研究其叶片光谱特征,为干旱区利用高光谱遥感监测盐生植被提供借鉴。结果表明:(1)分析梭梭叶片反射光谱曲线发现,在可见光至中红外各波段范围内,受不同水分量胁迫作用的梭梭叶片光谱反射率有显著差异。在可见光(350～610nm)波段,各水分处理的梭梭叶片反射率依次为100mL>500mL>200mL,这是由于100和200mL水分促进梭梭内部叶绿素合成,使该波段反射率降低,而过多的水分(500mL)对梭梭内部的叶绿素合成没有更大的促进作用。在红光区(611～738nm),随着水分量的增多,受不同水分量胁迫的梭梭叶片光谱反射率依次减小。在738～1 181和1 228～1 296nm波段,受不同水分量胁迫作用的梭梭叶片光谱反射率为:200mL>100mL>500mL;在1 182～1 227nm波段,受不同水分量胁迫作用的梭梭叶片光谱反射率为:100mL>200mL>500mL。这是由于植被细胞结构对近红外区域的反射率影响较大,因而受不同水分胁迫作用的梭梭叶片光谱反射率有显著差异。在1 300～1 365和1 392～1 800nm波段,受各水分胁迫作用的梭梭叶片反射率为:100mL>200mL>500mL。这表明在500mL水分胁迫量范围内,水分越多,叶子的细胞液、细胞膜对水分的吸收能力越强,使得反射率下降。通过对原始光谱求取一阶导数并提取红边位置参数发现,各水分处理下的梭梭叶片一阶微分光谱曲线中红边位置未发生移动。这是由于梭梭在长期的干旱环境影响下,形成了特殊的适应机制,水分对其红边位置影响不敏感。(2)选取若干植被指数分析各水分处理下的梭梭光谱指数变化。当水分胁迫量由100mL增至200mL时,WI/NDWI,MSI和NDII指数值变化显著,可用于研究水分胁迫下梭梭的光谱特征。(3)使用二维相关光谱技术分析受各水分胁迫作用的梭梭光谱特征,得出在100mL水分胁迫下,在536,643,1 219和1 653nm波段处,吸收峰对水分的微扰敏感;在200mL水分胁迫下,在846和1 083nm波段处,吸收峰对水分的微扰敏感;在500mL水分胁迫下,在835和1 067nm波段处,吸收峰对水分的微扰敏感。总之,在近红外波段,与100mL水分量相比,梭梭受200和500mL水分量胁迫时,吸收峰对水分的微扰敏感度上升。由100mL水分胁迫下梭梭的二维同步相关谱图可知,1 044和1 665nm,1 072和903nm,903和1 264nm,1 230和1 061nm波段处形成正交叉峰,表明这些波段处光谱强度随水分的干扰同时变化。

Analysis of the Spectral Characteristics of Haloxylon Ammodendron under Water Stress

Due to the dry climate and scarce precipitation in desert areas, water content is one of the factors that restrict the growth of vegetation. The stress factors include ozone stress, salt stress, and water stress involving terrestrial and aquatic plants. Water stress has a greater effect on plant growth and yield than any other stress. Along with the development of hyperspectral remote sensing technology, there have been many scholars at home and abroad who have been using hyperspectral data to study the effects of stress on vegetation. However, these research objects mainly focused on beet, cotton, corn, rice and other crops. There are few studies on the stress of saline vegetation in arid areas. Haloxylon is one of the typical halophytic vegetation in desert and semi-desert area, which is also known as Haloxylon ammodendron. It belongs to Chenopodiaceae, shrub or small tree, widely distributed in the desert and semi-desert regions. The plants’s root is well developed. It has a great effect on breaking wind and fixing sands and has the characteristics of salinity tolerance, drought resistance and so on, which has extremely high ecological value and economic value. In this paper, we selected the Haloxylon ammodendron as the research object. We developed the annual Haloxylon ammodendron, and set three water gradients, forming the plant with different water stress. The spectral characteristics of leaves were studied by using the original spectra, the red-edge position, combined with vegetation index and two-dimensional correlation spectra. This provides reference for using hyperspectral remote sensing to monitor saline vegetation in arid area. The results showed that: (1)By analyzing the leave reflectance of Haloxylon ammodendron under different water treatnment, we have found that: with in the range of visible to mid-infrared bands, Haloxylon ammodendron’s leaf spectral reflectance of different water stress was significantly different. In the visible region (350~610 nm), the leaf reflectance of Haloxylon ammodendron under various water stress was 100 mL>500 mL>200 mL. This was because of the fact that the water content of 100 mL and 200 mL promoted the synthesis of chlorophyll of this plant, which leads to the decrease of reflectance in these wavebands. However, too much water (500 mL) had no greater effects on the chlorophyll synthesis of this plant. In the red light region (611~738 nm), the leaf spectral reflectivity of Haloxylon ammodendron under different water stress decreased in turn as water content increased. In 738~1 181 and 1 228~1 296 nm wavebands, the leaf spectral reflectance of Haloxylon ammodendron under various water stress was 200 mL>100 mL>500 mL. In 1 182~1 227 nm wavebands, the leaf spectral reflectance of Haloxylon ammodendron under various water stress was 100 mL>200 mL>500 mL. This was because of the fact that the leaf spectral reflectance in the near-infrared region is mainly affected by the cell structure of leaf. It leads to the difference of leaf spectral reflectance of Haloxylon ammodendron under different water treatment. In the mid-infrared bands of 1 300~1 365 and 1 392~1 800 nm, the leaf spectral reflectance of Haloxylon ammodendron under various water stress was 100 mL>200 mL>500 mL. This indicates that, within the water content of 500 mL, the more water content was, the stronger the water absorption capacity of cell sap and cell membrane of leaves was. Thus the leaf reflectance decreased. By calculating the first derivative of the original spectrum and extracting the red edge position parameters, it was found that the red-edge position of the plant under different water treatment did not shift. This was because of that fact that Haloxylon ammodendron formed a special environmental adaptation mechanism under the influence of long-term drought stress. Water is insensitive to its red-edge position. (2) We selected several vegetation indices to analyze the changes of Haloxylon ammodendron’s leaf spectral indices under different water treatment and found that: when water content increased from 100 mL to 200 mL, WI/NDWI, MSI and NDII indices changed significantly, which can be used to study the spectral characteristics of Haloxylon ammodendron under the influence of water content. (3) The spectral characteristics of Haloxylon ammodendron treated by different water stress were analyzed by two-dimensional correlation spectra. We concluded that: when water treatment was 100 mL, at the bands of 536, 643, 1 219 and 1 653 nm, the absorption peaks were sensitive to the water perturbation. When water treatment was 200 mL, at the bands of 846 and 1 083 nm, the absorption peaks were sensitive to the water perturbation. When water treatment was 500 mL, in the bands of 835 and 1 067 nm, the absorption peaks were sensitive to the water perturbation. In conclusion, in the near-infrared bands, the sensitivity of the absorption peaks to the water perturbation increased when Haloxylon was stressed by 200 and 500 mL water content compared with 100 mL water content. The two-dimensional synchronous correlation spectra of Haloxylon ammodendron under the water treatment of 100 mL water content revealed that the positive cross-peaks were formed at 1 044 and 1 665 nm bands, 1 072 and 903 nm bands, 903 and 1 264 nm bands, 1 230 and 1 061 nm bands, which indicating that the spectral intensity of these bands changed simultaneously with the disturbance of water.