Global fire activities and their impacts on the terrestrial carbon budget in context of multiple environmental changes

Abstract

As one major disturbance in the earth system, fires modified vegetation structure and functioning, atmospheric composition, and climate conditions at both regional and global scales. Under the impacts of substantial changes in environmental factors (such as human activities, climate warming, and CO2 concentration), global fire regimes present significant changes, and generate strong feedbacks to ecosystem and climate system. Our knowledge regarding fire disturbances have been substantially improved under the help of satellite observations. However, long-term fire history and fire impacts on ecosystem carbon budget were rarely investigated prior to satellite era due to the lack of available spatially-explicit, large-scale datasets. In this dissertation, multiple study approaches were used to investigate the changes in burned area, fire emissions, and fire impacts on the terrestrial carbon budget and storage in the Conterminous United States (CONUS), boreal North America (BONA), African continent, and the entire globe. Historical burned area datasets were collected from satellite observations, official fire records, and reconstructed based on fire model simulations. Burned area data were applied to drive the Dynamic Land Ecosystem Model (DLEM) to investigate fire emission and fire impacts on ecosystems. As indicated by the analysis, average area burned in the CONUS was 14,430.51 km2 year-1 and fire-induced carbon emissions were ~17.65 Tg C year-1 during 1984-2012. Inter-annual variations of burned area and carbon consumption presented significant increasing trends in the CONUS. In the BONA, average fire-induced carbon emissions were 50.95 Tg C year-1 during 1960-2010, with a significant increasing trend at the rate of 1.08 Tg C year-1. Over the period of 1901-2010, African burned area presented a significant declining trend at the rate of -0.77×104 km2 year-2 due to the intensified human activities across the African continent and climate changes in the South Hemisphere of the African continent (SHAF). In Africa, pyrogenic carbon emissions decreased at the rate of -1.6 Tg C year-2 during 1901-2010, while the decline trend was not significant in the last five decade. During 1901-2007, average global burned area was ~442×104 km2 year-1. Our results suggest a notable declining rate of burned area globally (1.28×104 km2 year-2). From 1901 to 2010, global pyrogenic carbon emissions were estimated to be 2.43 Pg C year-1, and global average combustion rate was 537.85 g C m-2 burned area. Due to fire impacts, global terrestrial net primary productivity and net carbon sink were reduced by 4.14 Pg C year-1 and 0.57 Pg C year-1, respectively. Results of this study suggest that, in the future, the more intensive human activities would continue to suppress fire activities in the tropics and subtropics, while burned area and fire-induced carbon emissions would increase substantially in the boreal regions under the impact of climate warming and more frequent droughts. In order to mitigate global fire emissions, special attention should be paid to fire activities in the peatlands and tropical rainforest ecosystems.