Summary of vegetation spectral features

The relationship between vegetation and solar radiation is different from other substances, such as bare soil and water, such as the "Red Edge" Phenomenon of vegetation, that is, strong absorption near <700nm,> 700nm High Reflection. Many factors affect the absorption and reflection of solar radiation by vegetation, including wavelength, moisture content, pigments, nutrients, and carbon.

The wavelength range of vegetation is generally 400 to 2500 nm, which is also the wavelength range of sensor design. The wavelength range can be in the following four parts:

• Visible Light (visible): 400 nm to 700 Nm

Near Infrared (near-infrared -- NIR): 700 nm to 1300 nm

• Shortwave infrared 1 -- SWIR-1: 1300 nm to 1900 nm
• Shortwave infrared 2--swir-2: 1900 nm to 2500 nm

The transition zone of the NIR and SWIR-1 (near 1400nm) is the strong absorption range of atmospheric water. satellites or aerial sensors generally do not obtain the reflection value in this range. The transition zone of SWIR-1 and SWIR-2 (near 1900nm) is also the strong absorption range of atmospheric water.

Vegetation consists of three parts:

• Plant foliage)
• Plant canopies)
•  Non-photosynthetic vegetation (non-photosynthetic vegetation)

These three parts are the basis of vegetation analysis.

1. Plant foliage)

Plant Leaves include leaves, petioles, and other green substances. Different types of leaves have different shapes and chemicals. The main chemical components that have an important impact on spectral characteristics include pigments, water, carbon, and nitrogen, which are also the basis of Remote Sensing inversion, for example, the vegetation index is used to estimate the chemical composition of leaves.

• Pigment (pigments)

Leaf pigments mainly include chlorophyll, lutein and anthocyanins. These are the health indicators of vegetation. For example, vegetation with high concentrations of chlorophyll is generally healthy. On the contrary, lutein and anthocyanins are often found in healthy vegetation, the dying vegetation is red, yellow, or brown.

Leaf pigments only affect visible part (400nm ~ 700nm), figure 1 shows the relative spectral absorption characteristics of several leaf pigments in the visible light range.

Figure 1 Relative Spectral Absorption Characteristics of leaf pigments

• Water)

The geometric characteristics of leaves, canopy structure, and water requirements affect the moisture content of vegetation. The effect of water on vegetation reflectivity is in the band of NIR and swir (Fig. 2 ). There are absorption troughs near 1400nm and 1900nm, but the sensor generally avoids these two band ranges. There are also strong absorption characteristics near 970nm and 1190nm, which can be used to monitor vegetation moisture.

• Carbon)

Carbon in plants exists in many forms, including sugar, starch, cellulose and lignin. The Absorption Characteristics of cellulose and lignin are shown in the short-wave spectral range (Figure 3 ).

 

Figure 2 relative spectral absorption characteristics of water and carbon (cellulose and lignin) in leaves

• Nitrogen (nitrogen)

   Nitrogen elements in leaves are generally contained in chlorophyll, protein, and other molecules. Vegetation Index (vi) is sensitive to the nitrogen elements contained in chlorophyll (approximately 6% of nitrogen ). The nitrogen contained in protein ranges from 1500nm ~ In the 1720 nm range, the blade spectral characteristics are greatly affected.

As can be seen from the above, the interaction between vegetation and radiation is mainly reflected in the spectral characteristics of the leaves. Therefore, in the visible spectrum, the absorption of solar radiation mainly comes from chlorophyll, lutein and anthocyanins, the formation of the absorption valley near nm and Nm; in the near infrared spectrum, the main absorption of solar radiation from water, the formation of 970nm and 1190nm water absorption band; in the short infrared spectrum, in addition to water, carbon and Nitrogen in various forms also contribute to the absorption of solar radiation, forming the absorption troughs of 1400nm and 1900nm. Figure 3 shows the comparison of leaf reflectivity and transmission spectrum (transmittance spectra). The content of woody vegetation and herbal vegetation in pigment, water, nitrogen, etc. is different, and the relationship between reflectivity and transmission spectrum is also different.

 

Figure 3 leaf reflectivity and transmission spectra of woody plants (A) and herbs (B)

2. plant canopies)

The Reflection Characteristics of a single leaf are important to the spectral characteristics of the vegetation canopy. In addition, the number of leaves and the canopy structure also have an important impact on the scattering and absorption of the vegetation canopy. For example, forests, grassland, or agricultural land use in different ecosystems have different reflection characteristics, although they are similar to individual leaves.

There are many vegetation models used to describe the spectral characteristics of the canopy. The two most important factors are the leaf area index (LAI) and the angle distribution of leaf tilt (LAD ). Lai refers to the area of green leaves on the ground per unit of area, which represents the total number of green vegetation in the canopy. Lad describes the orientation of all types of leaves, and the average leaf angle (MLA) is usually used for approximation. MLA indicates the average value of the difference between the angle of each blade and the horizontal direction in the canopy.

Figure 4 shows the effect of LAI and LAD on the vegetation canopy. MlA is similar to lad. In the near infrared spectrum, the vegetation strongly reflects solar radiation, and the vegetation canopy is strongly absorbed in visible light and SWIR-2. Vegetation indexes using visible light and SWIR-2 are very sensitive to upper canopy.

 

Figure 4Effects of increase and decrease of Lai (A) and MLA (B) on Vegetation Canopy

3. Non-photosynthetic vegetation (non-photosynthetic vegetation)

In nature, there are also aging or dead vegetation that account for half of the global vegetation coverage, known as non-photosynthetic vegetation (NPV ). The Canopy Layer of NPV also has a woody forest structure, such as trunk, stem, and branches.

NPV mainly contains carbon elements and exists in the form of starch, cellulose and lignin. The spectral characteristics of NPV are mainly dominated by these substances. The fluctuation in Short Wave Infrared is relatively large, which is opposite to that in green vegetation, scattering in SWIR-1 and SWIR-2 is dominant. Figure 5 shows the green vegetation and NPV canopy spectral features.

 

Figure 5 Changes in canopy Reflection Characteristics of transmission green vegetation and dry vegetation (400nm ~ 2500nm)