Soil Nitrous Oxide and Methane Fluxes during First Five Years of Mountain Pine Beetle Infestation in Lodgepole Pine Forests.

Forest ecosystems are important sinks for global atmospheric nitrous oxide (N₂O) and methane (CH₄), yet little is known about the consequences of the unprecedented scale of recent mountain pine beetle (Dendroctonus ponderosae, MPB) infestation in lodgepole pine (Pinus contorta var.contorta) forests of western North America on this function. The main objective was to assess N₂O and CH₄ fluxes and changes to forest biotic and abiotic conditions during the first five years after MPB infestation and to extrapolate N₂O fluxes to regional-scale estimates of N₂O emissions during the recent massive scale 2007-2012 MPB outbreak. Four-year (2009-2012) study was established in four lodgepole pine stands. Each stand was infested by MPB in a different year  starting in 2007.  Results showed decreases in live tree basal area (1 to 3 years), increases in litter-fall (1-3 years), increases in understory vegetation (4 to 5 years) and declines in litter-fall (4-5 years). Following, soil and GHG results showed increases in soil water content (4 years, summer), declines in CH₄ assimilation (3 to 5 years) and increases in N₂O emissions (4 to 5 years).  Our results were indicative of on-going disequilibrium in forest ecosystem processes and temporary declines in important terrestrial ecosystem services of modulating soil-atmosphere CH₄ and N₂O exchanges for the period of at least five years after MPB infestation. Based on this study, MPB negatively affects terrestrial ecosystem and atmosphere C and N exchanges, jeopardizing forest capacity to effectively attenuate ecosystem N and to act as a sink for global GHG emissions.