GEDI and Sentinel data integration for quantifying agroforestry tree height and stocks

Agroforestry is a strategic asset for combating climate change and mitigating the environmental impacts of agricultural intensification, offering a nature-based solution for enhancing landscape resilience. In particular, poplar plantations contribute to the development of ecological networks within homogeneous agricultural landscapes, while also producing high-demand plywood and sequestering CO₂ in durable manufactured goods. Monitoring short-rotation poplar plantations requires frequent updates, which are infeasible with conventional National Forest Inventories (NFIs). Remote sensing (RS) has emerged as a highly effective tool for monitoring the structural variables of poplar plantations. This study aims to estimate the carbon stocks of poplar plantations in the Padan Plain, which spans approximately 46,000 km² in northern Italy. To achieve this, we developed a 10m high-resolution canopy height model (CHM) using a deep learning U-Net approach, with Sentinel-1 and Sentinel-2 multi-band imagery as predictors for GEDI waveforms derived tree height. The U-Net CHM for 2021 was evaluated with external validation data from NFI plots, achieving a mean absolute error of 2.6 m. Using annual poplar plantations data in the survey area, along with a poplar-specific yield table derived from terrestrial laser scanning, we applied the U-Net CHM to predict key forestry variables, including diameter at breast height (DBH), growing stock volume (GSV), aboveground biomass (AGB), and carbon stock (CS) in all stands. Results were compared with external validation data from NFI, yielding RMSE values of 30.7 %, 46.2 %, and 63.2 % for DBH, GSV, and AGB, respectively. Meanwhile, independent field surveys produced RMSE values of 19 % and 37.7 % for DBH and GSV, respectively. The average GSV estimated was 70 m³ ha^-1, while total CS were 12 MgC ha^-1. Based on poplar plantation maps for 2021 and 2022, we estimated the total harvested GSV of poplar trees to be 370,000 m³, equal to 66,000 MgC. The corresponding average harvested area was 1.5 ha, with an average yield of 130 m³ per hectare. The integration of multiple RS datasets with advanced machine learning techniques facilitates the effective monitoring of dynamic poplar plantations, for both mapping purposes and quantifying key forest variables relevant to climate change mitigation, such as carbon stocks.