Quantifying photosynthetic carbon uptake following land cover changes using TROPOMI and GOME-2 Solar-Induced Fluorescence (SIF) data 

Abstract

<p>The implementation of land management is widely included in national climate mitigation strategies as negative carbon technology. The effectiveness of these land mitigation techniques to extract atmospheric carbon is however highly uncertain. The H2020 LANDMARC, Land Use Based Mitigation for Resilient Climate Pathways, project monitors actual land mitigation sites to improve the understanding of their impact on the carbon cycle and focuses on the development of accurate and cost-effective monitoring techniques. Here we aim to assess the ability of satellite-based solar-induced fluorescence (SIF) observations to quantify the impact of land cover changes on the terrestrial gross primary production (GPP) – the carbon fixated during photosynthesis.</p><p>We use SIF measurements from the European TROPOMI and GOME-2A sensors to monitor the GPP dynamics following land cover change. We evaluate the impact of changed land cover on GPP for two distinct case studies with (1) an increasing trend in GPP (negative carbon emission) and (2) a decreasing trend in GPP (positive carbon emission) by examining the time-series of SIF signal over both cases. The positive carbon emission case concerns a massive wildfire in South-East Australia in which 220 km<sup>2</sup> of Eucalypt Forest burned down from January to February 2019. The negative emission case examines China’s large scale afforestation project, the Three-North Shelterbelt Program (TNSP), which started in the 1980’s to combat desertification.</p><p>We analysed the TROPOMI SIF signal over burned and surrounding unburned area to elucidate the reduction in GPP following the destruction of vegetation in the positive carbon emission case. We detected a strong reduction in SIF (70%) immediately after the fire and smaller reductions in SIF (22%) over the winter period, June–July, when vegetation is mostly dormant. The reduction in SIF signal was scaled to loss in GPP via an obtained empirical linear SIF—GPP relation. Namely, positive agreement (R<sup>2</sup>=0.89) was discerned between TROPOMI SIF and GPP from a neighbouring flux site (Tumbarumba), located in a similar ecosystem. Overall, we identified a GPP deficit of ~9.05 kgCm<sup>-2</sup>, or 2TgC, for the first 10 months after the fire. This deficit is 1-2 magnitudes larger than the anomalies linked to intense summer droughts, indicating the significant long-term effects of local wildfires on the carbon cycle.</p><p>For the negative carbon emission case, we analyse long timeseries of GOME-2A SIF (2007—2020) over the TNSP region. We use statistical data on local afforestation in synergy with the SIF observations and compare yearly and seasonal trends for different sub-regions in the area in order to reveal the impact of the implementations on the regional carbon sink. Large scale monitoring of different land management strategies, especially in difficult dryland areas such as the TNSP region, and their success rate is an important step to support policy makers in designing and upscaling of land mitigation techniques.</p>