Effects of mountain pine beetle oubreak on biogeochemical cycling in whitebark pine ecosystems

Background/Question/Methods

Ecological disturbances can significantly impact biogeochemical cycles in terrestrial ecosystems, but the effects of the current widespread mountain pine beetle outbreak on ecosystem processes like carbon (C) and nitrogen (N) cycling are poorly understood.  This is especially true in high elevation whitebark pine (WBP) (Pinus albicaulis) ecosystems of western North America.  WBP has been described as a keystone species, providing a critical food source and regulating hydrologic regimes.  However, widespread WBP mortality caused by the mountain pine beetle drives structural and physiological changes in WBP forests, which could result in shifts in pools and fluxes of C and N within these ecosystems.  To assess the biogeochemical consequences of the mountain pine beetle outbreak on whitebark pine ecosystems, we measured above and belowground nitrogen and carbon pools and fluxes around trees at three different times since beetle attack, including unattacked trees.  We hypothesized that the large pulse of needle litter to the forest floor and reduced tree nutrient uptake following beetle attack would result in an increase in inorganic N availability in the soil under attacked trees. We also predicted that beetle-killed trees would have reduced rates of soil CO_sefflux due to the loss of root respiration following mortality

Results/Conclusions

Litterfall inputs under beetle-attacked WBP trees were more than ten times higher than those under unattacked trees, and standing litter mass under beetle-attacked trees was ~45% higher than standing litter mass under unattacked trees. In response, soil NH₄⁺ concentrations in the organic horizons increased from 15 µN/g under unattacked trees to 33 µN/g under attacked trees. However, there were not significant differences in NH₄⁺ in the mineral soil horizons. Overall, soil NO₃^- concentrations were low and highly variable, but generally increased following beetle attack. However, there was no change microbial biomass N in the soil between attacked and unattacked trees, implying that changes in N cycling in response to the initial stages of WBP attack were subtle.  Soil CO₂ efflux rates were generally higher under unattacked trees, but overall, the similarities were more apparent than the differences. Our results indicate that while beetle attack drives a large pulse of C and N canopy to the forest floor after beetle attack, these increases in litterfall do not have immediate and profound effects on soil biogeochemical cycling. However, continuous observation of these important ecosystems will be crucial to determining the long-term biogeochemical effects of the mountain pine beetle outbreak.