Radar remote sensing can enable us to see and construct a full 3D view of forest structure and trees. In a joint research study conducted last year, NASA and DLR proved this concept in airborne flights over a test region in southeastern Bavaria, Germany, where both agencies flew their own airborne radar sensors over a period of a few days. NASA flew its well-known L-band UAVSAR sensor, while DLR flew its F-SAR system. The F-SAR system is unique as it does coincident radar imaging at L-, C-, and X- bands. Radar remote sensing analysts know well that lower frequencies like L-band can penetrate right down to the forest floor, C-band frequencies penetrate the canopy to some extent, while X-band frequencies are reflected from the top of the tree canopy. Utilizing these three frequencies simultaneously for forest imaging allows full 3D mapping of the forest, from the upper section of the forest crown, canopy, branches, down to the under-canopy vegetation and forest floor.
See the DLR official press release for more info.
Many other research groups are also pursuing similar goals to measure forests in 3D using SAR remote sensing. One such technique which can be applied to both airborne and spaceborne SAR sensors is POLinSAR (Polarimetric Interferometric SAR).
The Finnish Geodetic Institute is leading a research effort to measure 3D forest structure using a multiple active sensors, including SAR imagery from Sentinel-1, TerraSAR-X / TanDEM-X, and ALOS-2 PALSAR, along with optical satellite stereo imagery, and Airborne Laser Scanning (ALS). Learn more about their research here and here.