<b>QUANTIFYING WILDFIRE IMPACTS ON BOREAL FOREST ECOSYSTEM CARBON DYNAMICS USING PROCESS-BASED BIOGEOCHEMISTRY AND ATMOSPHERIC TRANSPORT AND INVERSION MODELS</b>

<p dir="ltr">Circumpolar regions contain large amounts of carbon that is vulnerable to climate change and wildfires. Impacted net ecosystem productions (NEP) can significantly change the carbon fluxes in this region, exerting positive feedback on climate system. To better understand the role of circumpolar regions in carbon cycling, I employ both top-down (atmospheric inversion) and bottom-up (process-based modeling) approaches to simulate boreal forest ecosystem dynamics in the circumpolar region, with a focus on soil physical properties, vegetation distribution, wildfire, and carbon fluxes.</p><p dir="ltr">For bottom-up method, I use a revised LPJ-STM model to simulate snow and canopy’s impact on soil thermal regime and carbon dynamics for both historical time and the warming future. Results show that snow has a warming effect on soil temperature in winter while canopy has a cooling effect on soil temperature in summer. With the affected vegetation distribution and ecosystem production, I estimate snow’s warming can release 1.54 Pg C year-1 to the atmosphere in circumpolar region while canopy’s cooling can get more carbon sequestered into ecosystem at 1.00 Pg C year-1 for present day. These changes continue to impact the caron dynamics in the warming future.</p><p dir="ltr">Afterwards, I use a revised TEM model to simulate wildfire’s impact on soil thermal, soil hydrological and carbon fluxes in circumpolar boreal forests. With the derived increasing burn severities from satellite data, we find that fire can release large amounts of carbon through combustion emission (67.7 Tg C year-1 for North America and 95.9 for Eurasia Tg C year-1) and decrease post-fire NEP, shifting the forests from a net carbon sink to a source. During this process, fire-induced canopy change can decrease NEP by inhibited nitrogen processes from canopy’s cooling effect. Also, post-fire soil water content and run-off can be increased due to the reduced evapotranspiration process from decreased vegetation coverage, potentially impacting the river discharge.</p><p dir="ltr">For top-down method, I use Local Ensemble Transform Kalman Filter (LETKF) coupled with GEOS-Chem model to infer the land carbon fluxes in circumpolar regions based on satellite observations. With the prior fluxes simulated from my bottom-up method and other studies, I estimate the terrestrial ecosystem in circumpolar regions acts as a net carbon sink of about 0.63-1.04 Pg C year-1. My inverted atmospheric CO2 fields show improved agreement with GLOBALVIEW observations, and the inferred fluxes are generally consistent with previous inversion studies. However, some discrepancies remain over the North American boreal forests, which can be attributed to the relatively sparse satellite observational coverage in this region.</p><p><br></p>