Lin Xiong / 熊林 @ UMD

Current Research Projects

GEDI and ICESat-2 fusion for global forest canopy height and biomass. 2023-present
Mapping global mangrove structure, disturbance, and recovery. 2022-present

1 Global forest height mapping with GEDI and ICESat-2

Earth’s wooded ecosystems are large terrestrial carbon sinks and a major nature-based solution for climate change. Accurate estimation of global forest carbon stocks is essential to support climate mitigation decisions for reducing emissions from deforestation and degradation in developing countries (REDD+). Aboveground woody biomass is determined by allometric models based on canopy structure metrics, such as top height and stem diameter. One of the most effective ways to measure global forest structure is using spaceborne light detection and ranging (lidar) technologies with wall-to-wall coverage. Both GEDI (Global Ecosystem Dynamics Investigation) and ICESat-2 (Ice, Cloud, and Land Elevation Satellite-2) are lidar missions and have been providing valuable canopy structure information since their launch in 2018. GEDI has better performance than ICESat-2 for canopy height retrievals owing to its superior canopy penetration capability in high canopy cover areas. However, GEDI observations span between 51.6º latitude N&S due to the ISS orbit, omitting most boreal forests. Here we present an approach to gridding of GEDI and ICESat-2 canopy height footprint data by combining a measurement error model with GEDI’s hybrid inference approach. Height metrics from GEDI L2A (Version 2) and ICESat-2 ATL08 (Version 6) are firstly compared across various plant functional types (PFTs). Then we generate a gridded canopy height product along with estimates of uncertainty for each 1 km grid cell, accounting for both measurement and sampling variability. The results show 20 m ICESat-2 segments provide more consistent measurements of canopy height compared to GEDI RH98. The accuracy and spatial coverage of gridded canopy height estimates is improved with the combination of GEDI and ICESat-2, compared with previous global canopy height products. The approach presented here provides a framework for combining GEDI and ICESat-2 observations and enables a significant advance in their application to global mapping and monitoring of ecosystem structure, carbon stocks, biodiversity and habitat mapping.

GEDI shots at 1km grid cells

ICESat-2 shots at 1 km grid cells

2 Mapping global mangrove structure, disturbance, and recovery

Global mangrove height from Simard et al., 2019

Mangroves are forested wetlands comprised of trees and shrubs that grow along low-lying coastlines in tropical and subtropical latitudes, and support the natural environment and human communities through a wealth of ecosystem services including carbon storage, sequestration, habitat, water quality, and coastal protection (Walters et al., 2008, Polidoro et al., 2010, Donato et al., 2011, Lee et al., 2014, Friess et al., 2019, Menéndez et al., 2020). Globally, mangrove forests account for 11 % of the terrestrial carbon that is transferred to the ocean although they only comprise 0.1 % of the continental surface, indicating their significant contribution to carbon biogeochemistry in the coastal zone (Twilley et al., 1992, Jennerjahn and Ittekkot, 2002, Bouillon et al., 2008, Alongi, 2020).

My goal is to use ground, airborne, and spaceborne lidar sensors to capture 3D global mangrove structure. New generation of spaceborne lidar systems provide unique opportunity for global mangrove forest height mapping and above ground biomass estimation. The current operating systems in space are Global Ecosystem Dynamics Investigation (GEDI) and Ice Cloud and land Elevation Satellite (ICESat-2). GEDI was launched in December 2018 and installed in the International Space Station. During its two-year mission, GEDI has produced over 10 billion observations, which can be gridded into regular coverage with 1 km resolution between 51.6° N and S latitudes (Dubayah et al., 2020). ICESat-2 was launched in September 2018 and could measure forest heights worldwide (Markus et al., 2017). Footprint sizes of GEDI (~25 m) and ICESat-2 (~13 m) and their accurate geolocation allow fusion with Landsat and Sentinel vegetation maps.

GEDI waveform response and footprint size (gedi.umd.edu)

ICESat-2 beams over forested mountains (svs.gsfc.nasa.gov)

Past projects

• Assessing coastal resilience and vulnerability (202209-202310). Report can be found here.

• Mapping coastal erosion along Texas coastal over the past decades using GNSS, Lidar and UAV. 2015-2022.

• Change detection with airborne lidar over the Slumgullion landslide, Colorado . 2019-2021.

• Past ~300 years’ stress evolution around the Yushu-Xianshuihe fault system using three-dimensional finite element models. 2012-2015.

• Coulomb stress changes caused by the Wenchuan earthquake on main active faults close to the Longmenshan Fault. 2010-2012.