How to interpret vegetation indicators?

How to interpret vegetation indicators?

Vegetation indices (VI) based on remote sensing images are fairly simple and effective algorithms for quantitative and qualitative assessments of plant cover, vigor and growth dynamics. The indicators are determined based on the value of electromagnetic wave reflection from the surface of plants/grasses and recorded by multispectral cameras in specific spectral channels. The spectrum, i.e. the reflection coefficient of electromagnetic waves from plants, changes depending on the type of plant, water, content in tissues and the amount of biomass or chlorophyll content in plants. Appropriate interpretation of the values of vegetation indicators allows you to determine the condition of plants, nutrient deficiencies, diseases and other needs.

NDVI (Normalized Difference Vegetation Index)
NDVI is the most commonly used vegetation index. The core of the received information is the ratio between the red (R) and near-infrared (NIR) bands. NDVI allows you to determine the state of plant development and vegetation.
A healthy plant should have high spectrum values in the infrared channel (NIR) and low spectrum values in the red channel (R). For grasses, it can be assumed that a healthy plant has NDVI values above 0.70 (optimally around 0.80). However, in the case of grass, values below 0.50 indicate very poor vegetation, potential diseases or damage to the grass.
Higher reflection in the infrared channel (NIR) indicates an increased content of organic matter in the plant, while higher reflection in the red channel (R) indicates leaf color, which may be a symptom of diseases, water stress or drying of plants.

NDRE (Normalized Difference Red Edge Index)
NDRE is an indicator similar to NDVI, which uses the Red Edge channel instead of the near-infrared (NIR) channel. The Red Edge channel is sensitive to the chlorophyll content in plant leaves (the higher the content, the higher the value of the spectrum in the Red Edge channel).
Due to the Red Edge channel properties, the NDRE index, unlike the NDVI index, is less correlated with the amount of biomass and grass species. Based on the NDRE index, the content of chlorophyll and nitrogen in plants can be determined. It is assumed that healthy grass plants have index values above 0.35.

PVR ( Photosynthetic vigour ratio)
The PVR indicator uses the ratio between the spectrum values in the green (G) and red (R) channels. In other words, it is based on the information relating to the color of plants, which in turn correlates with information about the intensity of the photosynthesis process.
The assumption for the PVR indicator is that a healthy plant in an intensive period of growth is characterized by increased reflection in the green channel (G), which indicates the intensity of the photosynthesis process. The value of the indicator depends on the season and correlates with the grass growth dynamics. In the case of healthy grass, the indicator ranges from 0.25 to 0.55.

GLI (Green leaf index)
The GLI index uses the ratio between the spectrum values in the green (G), red (R) and blue (B) channels. The use of additional information in the blue channel allows you to determine the influence of soil on the obtained indicator values. Thus, the indicator is intended to determine the turf density, i.e. the ratio of the amount of biomass to the surface area.
In the case of grass, areas with full turf should have an index value above 0.4.