The effects of forest thinning practices and altered nutrient supply on soil trace gas fluxes in Colorado

Colorado’s Front Range is one of the most rapidly developing regions of the U.S. As a consequence of both historical fire suppression and expanding populations, wildfire risk has increased. More than 50,000 forested acres have undergone thinning practices to reduce fire proneness. Despite their widespread use, relatively little is known about the ecological impacts of these practices. We conducted a study in Boulder County, Colorado to evaluate the impact of thinning treatments on soil-atmosphere flux rates of carbon dioxide, methane, and nitrous oxide, and to address nutrient interactions with those treatments. We investigated the effects of thinning combined with chipping (TC), and thinning with biomass removed (TR), in addition to a control (unthinned). Methane uptake in the TC treatment was 25% less than that of TR and unthinned controls. Both thinning treatments increased CO₂ flux rates ~20% relative to controls. TC treatments had 35% greater N₂O flux rates than TR and control (each ~16.5 μg N m^-2 h^-1). We superimposed additions of N, P, sucrose (N reduction or -N), and gypsum (P reduction or -P) on the thinning treatments to evaluate nutrient interactions. Of all possible combinations of treatments, we found the greatest CH₄ uptake (-0.03 mg C m^-2 h^-1) in control and –N, the greatest CO₂ fluxes in +N+C (430.94 mg C m^-2 h^-1), and greatest N₂O in +N+P+C (107.75 μg N m^-2 h^-1). Overall, nutrient treatments had a larger effect than thinning treatments, suggesting a minimal effect of forest management practices on flux rates in this system.