Tracking Forest Height Growth Over Time with ICESat-2 ATL08

Abstract

Accurate quantification of forest structure is necessary for forest inventory, growth prediction, and carbon stock estimates. Since late 2018, the ICESat-2 mission has employed a photon-counting LiDAR system to estimate along-track ground and canopy heights above the WGS 84 ellipsoid. This mission provides repeat coverage in 100 m data segments across the globe. In this research, we used multiple years of ICESat-2 ATL08 data to identify canopy height growth over time across the forest cover types and disturbance histories in a coastal region of North Carolina, a site selected for its homogenous topography and high industrial forest activity. ATL08 canopy height estimations demonstrate strong alignment (R2 = 0.88, RMSE = 2.64 meters) with coincident airborne laser scanning. Because ICESat-2 covers different locations each year, equivalence tests were used to ensure year-to-year samples were spatially comparable. Equivalence tests show that locations sampled by ICESat-2 are equivalent within a margin of 2 meters of canopy height. Reference data from U.S. Forest Service plots within our study area provide an expected growth rate of 0.34 meters per year, and a net growth of 1.68 meters over a five-year timeframe. Multiple statistical approaches reveal that canopy height growth is detectable within five years of ATL08 data. However, stratifying growth trends by forest cover type and disturbance history yields nuanced results, as these factors are likely to influence each other. Ultimately, this research can serve as a proof-of-concept for using multiple years of spaceborne LiDAR data to detect canopy height growth. Future research should explore the detection of growth using spaceborne LiDAR data in other regions of the globe, as solid results could demonstrate the potential of the ATL08 product in global forest structural monitoring in the face of a changing climate.