The carbon sink in recovering degraded and secondary tropical forests

Tropical forests play a key role in climate change mitigation. Recovering, degraded and secondary forests are becoming more dominant in tropical landscapes and yet large uncertainties exist regarding their carbon sink and storage. Remote sensing data enables a spatially and temporally explicit assessment of forests that can improve the accuracy of forest carbon flux estimates. This thesis uses a variety of satellite datasets to improve the spatial representation of the carbon sink in recovering forests. By combining satellite-based datasets of secondary forest age and aboveground carbon, the carbon accumulation was modelled across the Brazilian Amazon. Environmental variables and disturbances were found to drive spatially distinct regrowth patterns, with repeated anthropogenic disturbances reducing regrowth by up to 55%. Carbon recovery in degraded and secondary forests was then modelled across the Amazon, Central Africa, and Borneo. Between 1984 and 2018 recovering forests offset a quarter of carbon emissions from tropical forest loss, indicating the mitigation potential of protecting them, alongside old-growth forest conservation. Uncertainties in estimating the net global anthropogenic greenhouse gas flux from the entire land-use sector are reflected in the difference of 4-7GtCO2eyr-1 between methodological approaches, that has largely been attributed to managed forest fluxes in the tropics. This 'gap' represents a challenge within the Global Stocktake when emissions-reductions progress will be evaluated. This thesis demonstrates how to adjust remote sensing-based estimates of forest carbon fluxes to align them with national greenhouse gas inventory approaches, making them useful tools for Monitoring Reporting and Verification. Knowing how secondary forests are identified, and how associated carbon fluxes are incorporated into respective approaches are key to reconciling differences. Overall, this research uses remote sensing datasets to develop new methodologies, providing new quantitative insights into the carbon sink of tropical recovering, degraded and secondary forests with implications for the scientific and policy communities.