Forest Aboveground Biomass in the Annapurna Region, Himalaya, Derived From Ground-Based and Spaceborne Laser Scanning

Mountain forests encompass heterogeneous environmental conditions and are potentially highly dynamic because of climatic and socioeconomic changes. Although mountain forests are often difficult to access, spaceborne monitoring approaches offer new opportunities for assessing key forest characteristics at high spatial and temporal resolution. The objectives of our study were to compare relative canopy heights (eg 60th and 98th relative height percentiles) and aboveground biomasses derived from ground-based versus spaceborne methods in Nepal's Annapurna range. The region includes broadleaf trees in the windward region, which has high rainfall, and needleleaf trees in the leeward region, which experiences low rainfall. Ground-based data were collected using mobile laser scanning, and spaceborne data were derived from Global Ecosystem Dynamics Investigation (GEDI), a light detection and ranging (LiDAR)–based instrument on the International Space Station. Relative canopy height estimated from both methods was moderately correlated (eg r = 0.42 for the 98th percentile relative canopy height; n = 68), with on average higher values from the GEDI approach. Aboveground biomass derived from ground-based laser scanning ranged from 28 to 260 mg ha–1, whereas GEDI-derived estimates were on average 65% higher and moderately correlated with ground-based values (r = 0.51). We observed that the plant functional types assigned in the standard GEDI biomass estimation approach often did not match our field observations, resulting in an average 18% overestimation in the computed biomass. In studies from other mountain regions, slope explained much of the difference in height or biomass estimates between ground-based and spaceborne methods. In contrast, our study found only a slight influence of slope on relative canopy height, with an improvement in relationship observed at higher elevations. Our study highlights the great potential of spaceborne LiDAR for forest monitoring in remote mountain regions and the need for studies that further refine the approach to using spaceborne laser data in highly heterogeneous and sloped terrain.