High levels of soil and litter nitrogen contents after bark beetle-induced lodgepole pine mortality

Background/Question/Methods

As of 2010, over 20 million ha of conifer forest in Western North America have been decimated by an outbreak of bark beetles and their accompanying fungal pathogen that started in the early 21^st century. Beetle disturbance is common in forest ecosystems across the globe, but given the enormity of the current epidemic, little is known about how these forests will respond in the short or long-term with respect to water and biogeochemical cycling, vegetation recovery and productivity, and their role in climate change. The goal of this study was to quantify the short-term impacts on nitrogen (N) cycling between plant litter, soil, and air. We hypothesized that litter N inputs, soil N availability, microbial biomass-N, and nitrous oxide (N₂O) emissions would increase with degree of beetle mortality for at least three years following attack. We tested our hypotheses in a lodgepole pine forest in southeastern Wyoming in which stands of varying age, tree density, and beetle infestation co-occur.  Soil and litter inorganic N, microbial biomass N, dissolved organic N, and N₂O emissions were measured over a three year period.  

Results/Conclusions

Our results suggest that soil and litter mineral, labile organic, and microbial biomass N increased in the stands with greatest beetle-induced mortality three years after the infestation compared to uninfested stands. These observations corresponded with higher soil moisture content in beetle impacted stands. The contribution of litter to soil N was greatest in stands with beetle impact. This finding coincides with past research that has shown beetle-killed needle litter has higher N concentrations compared to living tree needle litter. Our previous study showed higher N₂O emissions from soils in stands that had highest available N and moisture, suggesting that ecological disturbance can contribute to greenhouse warming via its effects on N cycling. These results also suggest that forest mortality is associated with a ‘pulse’ in soil resource availability that has implications for succession. On-going research in our study area is investigating the duration of the resource pulse and subsequent changes in succession that will likely vary when compared to better understood disturbances such as clear-cutting and fire.