Evaluation of the relationship between nitrogen dry deposition and wildfire in a boreal forest of interior Alaska

Abstract:

In the boreal forests of interior Alaska, wildfires may play an important role in nitrogen dry deposition, which is one of the major input pathways affecting nitrogen availability. In order to examine the relationship between nitrogen dry deposition and wildfire, we applied regression analysis to the annual dry deposition of an Alaska boreal forest and annual burned area in Alaska for the 1999-2013 period. Furthermore, we estimated long-term nitrogen dry deposition for the past 54 years (1950-2013) at the same Alaska boreal forest, using an obtained regression equation and annual burned area. We used the data for annual nitrogen dry deposition (nitric acid, particulate nitrate, and particulate ammonium) measured at the CASTNET station (DEN417) in Denali National Park, available for 1999-2013. The annual burned area data for Alaska were referred to in previous reports (Todd and Jewkes, 2006; AICC, 2013). Nitrogen dry depositions at DEN417 were strongly correlated with the annual burned area in Alaska, where that area explained more than 68 % of the variation in annual nitrogen dry deposition. The background dry depositions defined as intercepts in the regression equations were 4.70, 0.24, and 2.15 mg m^-2 year^-1 for nitric acid, particulate nitrate, and particulate ammonium, respectively. Assuming that the increase in observed dry deposition above the background level (7.09 mg N m^-2 year^-1) was equal to nitrogen dry deposition originated from wildfire, we estimated that wildfire contributions to nitrogen dry deposition were 14 ± 20 % in 1999-2013. The long-term reproduction of nitrogen dry deposition revealed an increase in wildfire enhanced nitrogen dry deposition of up to 20 % for the 2000s, compared to the previous decadal age. The regression analysis and long-term reproduction in this study showed that the increase of wildfires in Alaska since 2000 has significantly enhanced the nitrogen dry deposition at DEN417.