AUTHOR=Wigneron Jean-Pierre , Ciais Philippe , Li Xiaojun , Brandt Martin , Canadell Josep G. , Tian Feng , Wang Huan , Bastos Ana , Fan Lei , Gatica Gabriel , Kashyap Rahul , Liu Xiangzhuo , Sitch Stephen , Tao Shengli , Xiao Xiangming , Yang Hui , Espinoza Villar Jhan Carlo , Frappart Frederic , Li Wei , Qin Yuanwei , De Truchis Aurélien , Fensholt Rasmus TITLE=Global carbon balance of the forest: satellite-based L-VOD results over the last decade JOURNAL=Frontiers in Remote Sensing VOLUME=5 YEAR=2024 URL=https://www.frontiersin.org/journals/remote-sensing/articles/10.3389/frsen.2024.1338618 DOI=10.3389/frsen.2024.1338618 ISSN=2673-6187 ABSTRACT=

Monitoring forest carbon (C) stocks is essential to better assess their role in the global carbon balance, and to better model and predict long-term trends and inter-annual variability in atmospheric CO2 concentrations. On a national scale, national forest inventories (NFIs) can provide estimates of forest carbon stocks, but these estimates are only available in certain countries, are limited by time lags due to periodic revisits, and cannot provide spatially continuous mapping of forests. In this context, remote sensing offers many advantages for monitoring above-ground biomass (AGB) on a global scale with good spatial (50–100 m) and temporal (annual) resolutions. Remote sensing has been used for several decades to monitor vegetation. However, traditional methods of monitoring AGB using optical or microwave sensors are affected by saturation effects for moderately or densely vegetated canopies, limiting their performance. Low-frequency passive microwave remote sensing is less affected by these saturation effects: saturation only occurs at AGB levels of around 400 t/ha at L-band (frequency of around 1.4 GHz). Despite its coarse spatial resolution of the order of 25 km × 25 km, this method based on the L-VOD (vegetation optical depth at L-band) index has recently established itself as an essential approach for monitoring annual variations in forest AGB on a continental scale. Thus, L-VOD has been applied to forest monitoring in many continents and biomes: in the tropics (especially in the Amazon and Congo basins), in boreal regions (Siberia, Canada), in Europe, China, Australia, etc. However, no reference study has yet been published to analyze L-VOD in detail in terms of capabilities, validation and results. This paper fills this gap by presenting the physical principles of L-VOD calculation, analyzing the performance of L-VOD for monitoring AGB and reviewing the main applications of L-VOD for tracking the carbon balance of global vegetation over the last decade (2010–2019).