Pure Pedantry has a fascinating take on this wrinkle in monitoring: … They use a new technology developed at the Carnegie Institution for Science at Stanford University. The technology -- called the Carnegie Airborne Observatory -- uses an aircraft to sweep over the areas of forest to be surveyed. Then using a combination of light measurements reflected off the forest and radar measurements to determine the relative contributions of different levels of canopy (and really, really accurate measurements of where the aircraft is in the air), the researchers can calculate the different types of plant life in the forest to a reasonably high level of resolution:“We deployed an airborne remote sensing system designed to measure the composition, physiology, and structure of ecosystems. The Carnegie Airborne Observatory (CAO) integrates high-fidelity imaging spectrometers (HiFIS) with light detection and ranging (LiDAR) sensors for regional-scale ecological research. The HiFIS subsystem provides detailed canopy spectroscopic reflectance signatures that express plant chemistry, physiological status, and taxonomic composition. The LiDAR subsystem provides 3D structural information on canopies, underlying vegetation, and the terrain below. The CAO HiFIS and LiDAR are physically and digitally coaligned and packaged with a high performance inertial navigation system that provides highly accurate determinations of aircraft position and the location of ground targets in three dimensions. (Citations removed.)”
The theory behind this technique -- as I understand it -- is that different plants because of different relative concentrations of chromophores in their leaves reflect light differently. This different in light reflectance can be used to establish the identity of the plant.
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