Spatial and temporal variations of carbon in global tropical forests using satellite and ground observations
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Tropical forests play an important role in the global carbon cycle. Covering 7-10% of the Earth land surface, they contribute to more than half of carbon stock in the world’s forests. Spatial and temporal variations of canopy structure and carbon stock are thus key indicators of ecological processes associated with the changing climate. At macroscales, we evaluated the contributions of climate, soil and topography to the structural variations of pan-tropical forests. Using LiDAR observations from satellite, we built spatial regression models between the LiDAR-derived canopy height and abiotic variables. Results show these factors and spatial contextual information can explain more than 60% of the variations in the heights of these forests. Within the tropics, Amazonian forests contain nearly half of the tropical carbon stocks and thus a vital part to the global carbon budget. The impacts of droughts in Amazonia have been recorded as short-term tree mortality and biomass loss from inventory plots. Using interannual satellite LiDAR measurements from 2003 to 2008, we quantitatively assessed carbon lost after the 2005 Amazon drought. Through careful signal filtering and sampling strategies, we found a significant loss of carbon over the Amazon basin, turning the ecosystem to a net source of carbon at 0.63 PgC/yr (0.16-1.10 PgC). And there was no sign of complete recovery 3 years after the drought. Besides natural disturbances such as droughts, human activities vastly alter the carbon footprint in the tropics. Tropical secondary forests (SF), mainly restored from deforestation, are often identified as a major terrestrial carbon sink. We analyzed changes in SF from 2004 to 2014 in the Brazilian Amazon and found SF contribution to regional carbon sink was negligible, due to significant turnover and frequent clearing activities. But it has the capacity of more than 0.2 PgC/yr net sink to compensate for total emissions from deforestation, if policies to restore secondary forests are implemented and enforced. My dissertation studies provide a clearer picture of abiotic controls over the pan-tropical forests and a better understanding of the carbon dynamics in regions of post-drought Amazonia and secondary forests in the Brazilian Amazon.