Bark Beetle Epidemic in Rocky Mountains Lodgepole Pine Forests: Effects On Soil Carbon, Nitrogen and Greenhouse Gas Emissions.

Unprecedented outbreak of pine beetle (PB) and associated blue stain fungi epidemic have decimated millions of hectares of forests in the western United States and Canada. Consequently, numerous ecosystem services have ceased or significantly declined. Not only the timber extraction and recreational forest values have been affected by PB, but the disturbance of such magnitude is likely to have a critical impact on terrestrial ecosystem carbon (C) and nitrogen (N) storage and cycling as well as the fluxes of greenhouse gases (GHG) that are critical players in the radiative forcing and climate. However, we do not clearly understand the magnitude of these changes.  Therefore, understanding GHG emissions and the ecosystem properties and drivers of biogeochemical processes that trigger GHG emissions following PB infestations is lacking.  This important problem needs to be addressed in order to project large scale, regional-level ecosystem GHG budgets. The overall objective of this research was to assess ecosystem storage, soil processes and GHG production after PB infestations on multiple sites.  We hypothesize that PB-induced tree mortality significantly changes chemistry of plant inputs to the soil and soil C and N content, thereby altering soil long-term storage, labile C and N cycling and GHG emissions. The study monitored the impact of a massive PB outbreak on C, water and N cycling and regeneration of lodgepole pine (Pinus contorta) forests in the Medicine Bow National Forest in Wyoming that have been undergoing a gradual infestation for the last 3 years. Initial results suggest that lodgepole pines experienced over 50% reduction in transpiration rates per tree within the first month following the infection before the needles turn red and the sapwood is stained blue. Subsequently, there was a notable increase in soil moisture content.  Periodical soil GHG measurements performed during the snow-free season suggest that soils beneath the mature stand infested in year 2007 respired on average, 50% less CO₂, assimilated 63% less CH₄ , and produced 133% more N₂O when compared to a regenerating stand at the sapling phase. These ecosystem responses indicate decoupling of tight terrestrial C and N cycling as early as two years following the infection. Prolonged local increases in greenhouse gas emissions are highly likely to transform these forest ecosystems from effective sinks to sources of atmospheric C and N greenhouse gas compounds, which, on a scale of massive regional PB outbreak will have a substantial impact on modulating atmospheric composition.