Simulating coupled carbon and nitrogen dynamics following mountain pine beetle outbreaks in the western United States
- Univ. of Idaho, Moscow, ID (United States)
- National Center for Atmospheric Research, Boulder, CO (United States)
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Insect outbreaks are major ecosystem disturbances, affecting a similar area as forest fires annually across North America. Tree mortality caused by epidemics of bark beetles alters carbon cycling in the first several years following the disturbance by reducing stand-level primary production and increasing decomposition rates. The few studies of biogeochemical cycling following outbreaks have shown a range of impacts from small responses of net carbon fluxes in the first several years after a severe outbreak to large forest areas that are sources of carbon to the atmosphere for decades. To gain more understanding about causes of this range of responses, we used an ecosystem model to assess impacts of different bark beetle outbreak conditions on coupled carbon and nitrogen cycling. We modified the Community Land Model with prognostic carbon and nitrogen to include prescribed bark beetle outbreaks. We then compared control simulations (without a bark beetle outbreak) to simulations with various mortality severity, durations of outbreak, and snagfall dynamics to quantify the range of carbon flux responses and recovery rates of net ecosystem exchange to a range of realistic outbreak conditions. Prescribed mortality by beetles reduced leaf area and thus productivity. Gross primary productivity decreased by as much as 80% for a severe outbreak (95% mortality) and by 10% for less severe outbreaks (25% mortality). Soil mineral nitrogen dynamics (immobilization and plant uptake) were important in governing post-outbreak productivity, and were strongly modulated by carbon inputs to the soil from killed trees. Initial increases in heterotrophic respiration caused by a pulse of labile carbon from roots were followed by a slight reduction (from pre-snagfall reduced inputs), then a secondary increase (from inputs due to snagfall). Secondary increases in heterotrophic respiration were largest for simulated windthrow of snags after a prescribed snagfall delay period. Net ecosystem productivity recovered within 40 years for all simulations, with the largest increases in the first 10 years. Our simulations illustrate that, given the large variability in bark beetle outbreak conditions, a wide range of responses in carbon and nitrogen dynamics can occur. The fraction of trees killed, timing of snagfall, snagfall rate, and management decisions as to whether or not to remove snags for harvesting or for fire prevention will have a major impact on post-outbreak carbon fluxes up to 100 years following an outbreak.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- AC05-00OR22725
- OSTI ID:
- 1254079
- Journal Information:
- Journal of Geophysical Research, Vol. 116, Issue G4; ISSN 0148-0227
- Publisher:
- American Geophysical Union
- Country of Publication:
- United States
- Language:
- English
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