Vegetation and Its Reflectance Properties

Analyzing vegetation using remotely sensed data requires knowledge of the structure and function of vegetation and its reflectance properties. This knowledge enables you to link vegetative structures and their condition to their reflectance behavior in an ecological system of interest. Vegetation reflectance properties are used to derive vegetation indices (VIs). The VIs are used to analyze various ecologies. VIs are constructed from reflectance measurements in two or more wavelengths to analyze specific characteristics of vegetation, such as total leaf area and water content.

A VI is a simple measure of some vegetation property calculated from reflected solar radiation measurements made across the optical spectrum. The solar-reflected optical spectrum spans a wavelength range of 400 nm to 3000 nm. Of this range, the 400 nm to 2500 nm region is routinely measured using a variety of optical sensors ranging from multispectral (for example, Landsat TM) to hyperspectral (for example, AVIRIS). Vegetation interacts with solar radiation differently from other natural materials, such as soils and water bodies. The absorption and reflection of solar radiation is the result of many interactions with different plant materials, which varies considerably by wavelength. Water, pigments, nutrients, and carbon are each expressed in the reflected optical spectrum from 400 nm to 2500 nm, with often overlapping, but spectrally distinct, reflectance behaviors. These known signatures allow scientists to combine reflectance measurements at different wavelengths to enhance specific vegetation characteristics by defining VIs.

The optical spectrum is partitioned into four distinct wavelength ranges:

• Visible: 400 nm to 700 nm
• Near-infrared: 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 from near-infrared to SWIR-1 is marked by the 1400 nm atmospheric water absorption region in which satellites and aircraft cannot acquire measurements. Similarly, the SWIR-1 and SWIR-2 transition is marked by the 1900 nm atmospheric water absorption region.

Vegetation is divided into the following general categories:

• Plant Foliage
• Canopies
• Non-Photosynthetic Vegetation

These topics provide background material needed to understand how remote sensing aids vegetation analysis. This information is derived from the following reference (and related sources):

Reference

Asner, G.P., 1998, Biophysical and Biochemical Sources of Variability in Canopy Reflectance, Remote Sensing of Environment, 64:234-253.

See Also

Spectral Indices, Vegetation Indices, Vegetation Analysis Tools