Brazilian ecosystems are facing major threats to their conservation and functioning from human induced disturbances in combination with climate change. The two largest Brazilian ecosystems, the Amazon Forest and Cerrado, lost together approximately 15,690 km2 year-1 of their intact vegetation cover between 2010 and 2020 (equivalent to 12% year-1 of England area), with an intensification of vegetation loss from 2019.
Brazil is responsible for the largest carbon dioxide emissions from land use and land cover changes (LULCC). Still, uncertainties arise in global carbon budget assessments on the magnitude and trends of these disturbance fluxes and its impacts on the carbon balance. In this thesis, I investigate the components of human disturbance and how they impact the contemporary carbon cycle of Brazilian ecosystems, such as LULCC, as well the relationship between fire occurrence and landscape fragmentation. The thesis ends with a synthesis of the contemporary net land carbon balance of the Amazon using a set of state-of-the-art estimates from different methodological approaches.
In chapter 2 I attempt to reconcile estimates of LULCC emissions for Brazil. I first evaluated new global LULCC maps used in global bookkeeping and Dynamic Global Vegetation Models (DGVMs) from the Global Carbon Budget (GCB) assessments for Brazil. These new maps were based on Food Agriculture Organization (FAO) statistics combined with multi-year land cover data from the European Space Agency. I found good spatial agreement between the global LULCC maps with MapBiomas, the Brazilian remote sensing based LULCC maps. However, the global LULCC dataset still failed to capture the magnitude and recent increase in deforestation in Brazil. This is due to the method used to infer deforestation from agricultural area change reported to FAO in addition the use of a simple linear trend to extrapolate beyond the final census year. The findings of this study have been welcomed by the global land modelling community and led to the subsequent adoption of MapBiomas in the Global Carbon Budget assessment for 2022 (GCB2022).
Conversion of intact vegetation cover to other land uses induce landscape fragmentation and fire spread. In chapter 3, I test the relationship between fire occurrence within intact vegetation fragments and the degree of landscape fragmentation in the two major and contrasting Brazilian ecosystems (Amazon and Cerrado). I found that there is an ecosystem-specific relationship between burned area fraction within intact vegetation fragments and landscape fragmentation induced by human activity. I find that fire occurrence within intact vegetation fragments associated to landscape fragmentation has an opposite relationship when comparing Amazon and Cerrado. These human-induced disturbances cause large losses of carbon to the atmosphere, thus impacting the carbon balance of tropical areas. Building on all the findings of chapters 2 and 3, in chapter 4, I use the latest data from a set of regional and global models to attribute and synthesise the contemporary net land carbon balance of Amazon. The different approaches agree that the south-eastern Amazonia is a net carbon source to the atmosphere. Bottom-up approaches suggest that Brazilian Amazonia was a net land carbon source in 2020 due to an increase in deforestation and fire emissions. Given the large uncertainties from the models and remaining knowledge gaps, there is insufficient data to reject the hypothesis that the Amazon was not neutral between 2010 and 2018.
In conclusion, this thesis reconciles diverging estimates of land use and land cover emissions of Brazil in global carbon budget assessments. Moreover, this thesis brings together state-of-the art estimates of the Amazon carbon budget using bottom-up and top-down models and the knowledge gaps to reconcile them. Finally, I highlight the importance of the use of remote sensing data to constrain global estimates of land use and land cover change datasets and emissions. This thesis also demonstrates divergent fragmentation-fire relationships in the two major and contrasting Brazilian biomes. The results present in this thesis have important implications for the mitigation potential of Brazilian ecosystems within the goals of Paris Agreement
