Using GEDI as training data for an ongoing mapping of landscape-scale dynamics of the plant area index

Abstract

Abstract Leaf or plant area index (LAI, PAI) information is frequently used to describe vegetation structure in environmental science. While field measurements are time-consuming and do not scale to landscapes, model-based air- or space-borne remote-sensing methods have been used for many years for area-wide monitoring. As of 2019, NASA’s Global Ecosystem Dynamics Investigation (GEDI) mission delivers a point-based LAI product with 25 m footprints and periodical repetition. This opens up new possibilities in integrating GEDI as frequently generated training samples with high resolution (spectral) sensors. However, the foreseeable duration of the system installed on the ISS is limited. In this study we want to test the potential of GEDI for regional comprehensive LAI estimations throughout the year with a focus on its usability beyond the lifespan of the GEDI mission. We study the landscape of Hesse, Germany, with its pronounced seasonal changes. Assuming a relationship between GEDI’s PAI and Sentinel-1 and -2 data, we used a Random Forest approach together with spatial variable selection to make predictions for new Sentinel scenes. The model was trained with two years of GEDI PAI data and validated against a third year to provide a robust and temporally independent model validation. This ensures the applicability of the validation for years outside the training period, reaching a total RMSE of 1.12. Predictions for the test year showed the expected seasonal and spatial patterns indicated by RMSE values ranging between 0.75 and 1.44, depending on the land cover class. The overall prediction performance shows good agreement with the test data set of the independent year which supports our assumption that the usage of GEDI’s PAI beyond the mission lifespan is feasible for regional studies.