Mountain Pine beetle disturbance effects on Colorado subalpine forest carbon cycling

Background/Question/Methods

An unprecedented mountain pine beetle (Dendroctonus ponderosae) epidemic is ongoing in the high elevation forests of Western North America, reducing net primary productivity and diminishing the potential for these ecosystems to sequester atmospheric carbon. Although the role of disturbance is recognized in moving ecosystems from a carbon sink to a source, insect outbreaks are seldom accounted for in large-scale carbon budget models. Estimates and trajectories of carbon loss and uptake as forests undergo recovery remain uncertain due to lack of knowledge about the impacted carbon cycle processes. We employed a chronosequence approach to explore these uncertainties, using plots of lodgepole pines (Pinus contorta) that experienced beetle kill or were girdled (simulating beetle kill) in the Niwot Ridge AmeriFlux and Fraser Experimental Forest of Colorado. The chronosequence extended from approximately 2002 to 2010. Over the growing season of 2010, we measured soil respiration rates, pools of extractable soil and microbial biomass carbon and nitrogen, and the ¹³C/¹²C of soil respired CO₂.

Results/Conclusions

We found that loss of autotrophic respiration and photosynthetic labile carbon to the soil pool diminishes soil respiration in the first one to four years after disturbance. In older plots, respiration increases five to six years after tree loss likely due to a pulse of increased litter and conditions stimulating microbial activity; however soil respiration never exceeds bounds of observed respiration in plots with live trees. Respiration then diminishes again to a lesser degree in plots seven to eight years after disturbance. We observed similar patterns in soil extractable carbon and nitrogen pools. Our results indicate that subalpine forests impacted by mountain pine beetle may not become immediate carbon sources to the atmosphere to the degree previously predicted. Over almost a decade, we do not see large increases in heterotrophic respiration from needle and root death. Succession and competitive release stimulating gross primary productivity may become key compensatory players as respiration dynamics follow a slow trajectory of change post beetle disturbance.