Monitoring forest canopy structure dynamics from space using GEDI and Sentinel-1

The distribution of leaves, branches and trunks in the canopy is critical for water, carbon and energy cycles in forests. Light ranging and detection (lidar) and synthetic aperture radar (SAR) are two active remote sensing methods which show the potential to detect forest structure dynamics at different height. Lidar can penetrate the canopy gap and record a reflectivity profile. The vertical distribution of structure metrics, for example, canopy cover, can then be estimated based on this profile. For example, in Amazonian tropical forests, the leaf area index derived using ground- and space-borne lidar at canopy layer and understory layer shows converse behaviour over seasons. For SAR systems, wavelength determines the penetration depth of microwave signal and polarisation reflects the scattering mechanisms between signal and objects. SAR backscatter has been used for monitoring forest phenology and forest classification. However, the potential of using lidar and SAR to monitor canopy structure changes in temperate forests has not been analysed. The relationships between SAR backscatter with the lidar-derived structure metrics at height levels are also not clear.

In this study, we attempt to investigate the influence of forest structure at height levels on SAR backscatter in two study sites in Germany: one deciduous forest in Hainich and one coniferous forest in Tharandt. Level 2B product of the Global Ecosystem Dynamics Investigation (GEDI) and Sentinel-1 ground range detected (GRD) backscatter are used in this study. There are dense multi-temporal GEDI observations around 51ºN, which makes it possible to continuously monitor the forest structure at same area. Auxiliary data including digital terrain model, slope, forest type and forest phenology data are used to extract GEDI footprints which cover similar forests. We only focus on GEDI footprints with forest height between 25 m and 30 m since most trees in study sites are distributed in this range and they have distinguishable canopies. Sentinel-1 data with fixed relative orbit is used for mitigating the influence of incidence angle. SAR backscatter at filtered GEDI locations are extracted for comparison.

We analyse the correlation between the time series of GEDI-derived total structure metrics (e.g., cover), structure metric profiles and Sentinel-1 backscatter metrics (e.g., VH, VV, VV/VH ratio). For deciduous forest, the 15-20 m layer and 20-25 m layer have stronger correlation to Sentinel-1 VH backscatter and VV/VH ratio than other layers as well as the total structure metric. No significant correlation is found between structure metrics and Sentinel-1 backscatter in coniferous forest. We propose to further develop approaches to investigate the joint potential of space-borne lidar and SAR observations to monitor changes in forest canopy structure.