Fire exclusion practices during the last century increased fuel and fire hazard in the western U.S., where conditions have also become drier and warmer in recent decades. As a result, fire frequency and extent have increased significantly. Wildfires and forest management alter soil carbon and nitrogen availability and the physical environment. These factors are primary controls on greenhouse gas (carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)) flux rates. The two-way interaction between forest wildfires/management and flux rates may be significant considering the positive feedback loop that could lead to further climate warming. I explored these relationships in a series of field studies in which I measured soil trace gas exchange rates in ponderosa pine forests of the Colorado Front Range that had recently experienced a wildfire or forest thinning. I also used the ecological simulation model, Daycent, to simulate the effects of long term climate variability, varied fire frequency and fire suppression in order to estimate the changes in CH4, N2O, NO (nitric oxide) fluxes and gross nitrification rates at four sites in the Colorado Front Range.
My findings suggest that soil CO2 fluxes increase in the years after a wildfire, and that local scale variables such as soil moisture, temperature, and fire severity are important controlling factors for these trace gas fluxes. Forest thinning practices increased substrate availability in some cases such that CO2 and N2O fluxes increased, but only when soil moisture was high, during the sampling season. Using Daycent, I found CH 4 uptake was consistent among sites with different landscape characteristics, and showed minimal changes in response to fire. Daycent simulations estimate a 13–37 % decrease in N2O and NO fluxes, and gross nitrification rates during the fire suppression era relative to before the suppression era.
Overall, my research revealed that wildfire and forest management do alter the exchange rates of CO2 and N2O primarily by increasing substrate availability and environmental variability. Therefore, as wildfire activity and forest management are anticipated to increase in both frequency and extent, my research suggests that CO2 and N 2O source strength may increase from Colorado ponderosa pine ecosystems.
Keywords: carbon dioxide, methane, nitrous oxide, trace gas, greenhouse gases, fire, soil, ponderosa pine, Colorado Front Range, wildfire, Daycent, forest management