Monitoring the regional impact of forest loss and gain on carbon uptake with solar-induced fluorescence measurements from the GOME-2A and TROPOMI sensors

Abstract. Reliable and robust monitoring tools are crucial to assess the effectiveness of land mitigation techniques (LMTs) in enhancing carbon uptake, enabling informed decision-making by policymakers. This study, addressing one of the scientific goals of the EU H2020 LANDMARC project, examines the feasibility of using satellite solar-induced fluorescence (SIF), in combination with other satellite data, as a monitoring proxy to evaluate the effects of LMTs on carbon uptake. Two distinct cases are explored: (1) instantaneous vegetation destruction caused by a 2019 Eucalyptus wildfire in south-east Australia, and, (2) gradual forest gain resulting from reforestation efforts in northern China over 2007–2012. The cases are monitored using TROPOMI and GOME-2A SIF, respectively. Comparing the temporal variability in SIF across the affected and nearby reference areas reveals that vegetation dynamics changed as a consequence of the land use changes in both cases. Specifically, in the Australia case, TROPOMI demonstrated an immediate reduction in SIF signal of 0.6 mW m⁻² sr⁻¹ nm⁻¹ (−72 %) over the Eucalypt Forest right after the fire. Exploiting the strong correspondence between TROPOMI SIF and gross primary productivity (GPP) at the nearby eddy-covariance Tumbarumba site and through the FluxSat product, we estimate that the fire led to a loss in GPP of 130–200 GgC in the first eight months after the fire. Over the northern Chinese provinces of Gansu, Shaanxi, Sichuan, Chongqing and Shanxi, we report an increase in GOME-2A summertime SIF of 0.1–0.2 mW m⁻² sr⁻¹ nm⁻¹ coinciding with reforestation efforts between 2007 and 2012. This increase in SIF signal is likely driven by a combination of increasingly favourable natural conditions and the reforestation effort itself. A multivariate model that takes into account growth factors such as water availability and maximum temperature as well as satellite-derived forest cover data explains the observed variability in GOME-2A SIF in the Chinese case reasonably well (R²=0.72). The model suggests that both increases in forest cover as well as in soil moisture have led, in step, to the observed increase in vegetation activity over northern China. In that region, for every 100 km² of additional forest cover, SIF increases by 0.1 mW m⁻² sr⁻¹ nm⁻¹ between 2007 and 2012. Our study highlights that the combined use of satellite-based SIF, together with supporting in situ, modelled and satellite-data, allows to monitor the impact of LMT implementation on regional carbon uptake as long as the scale of the LMT is of sufficient spatial extent.