1. Key Laboratory of Smart City and Environmental Modeling of Higher Education Institute, College of Resources and Environment Sciences, Xinjiang University, Urumqi 830046, China
2. Key Laboratory of Oasis Ecology, Xinjiang University, Urumqi 830046, China
3. Engineering Research Center of Central Asia Geoinformation Development and Utilization, National Administration of Surveying, Mapping and Geoinformation, Urumqi 830002, China
Abstract：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.
Key words：Haloxylon ammodendron; Water stress; Red edge position; Two-dimensional correlation spectra; Spectral index
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