Linking bark beetle caused hydraulic failure to declining ecosystem fluxes in a high elevation Rocky Mountain (Wyoming, USA) forest

Background/Question/Methods

Bark beetle epidemics have caused major disturbances in the forests of western North America where significant tree mortality alters ecosystem photosynthesis, carbon balance, and water exchange.  In these ecosystems the primary mechanism of tree mortality is beetle associated blue-stain fungus which clogs xylem and reduces hydraulic conductivity.  The goal of this study was to link this hydraulic failure to declining ecosystem productivity using canopy conductance (g_c).  We quantified this link by evaluating the growing-season light-response of net ecosystem exchange of carbon dioxide (NEE) and evapotranspiration (ET) in a high elevation Rocky Mountain forest over the three years preceding and three years following a bark beetle outbreak.  Work was done at the GLEES AmeriFlux site (southeastern Wyoming, USA), which is located in a high elevation subalpine forest dominated by Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) and recently experienced an epidemic of spruce beetle (Dendroctonus rufipennis).  The beetle outbreak was described as three phases: endemic (some background beetle-caused tree mortality), epidemic I (immediately after the peak beetle outbreak when impacted trees experience hydraulic failure), and epidemic II (impacted trees ultimately drop their green needles).  Analysis was conducted on daytime growing season 30-min eddy-covariance flux data.

Results/Conclusions

Ecosystem g_c was reduced during epidemic phase I which is consistent with beetle associated blue-stain fungus reducing tree hydraulic conductivity.  As a consequence, both NEE and ET responses to light were reduced from 10 to 25%.  During epidemic phase II there was a fundamental biological shift in the ecosystem causing a change in the relationship between g_c and NEE which coincided with the impacted trees dropping much of their green foliage.  This resulted in a 40 to 55% reduction of the NEE response to light while no further changes in ET occurred.  The bark beetle epidemic impacted the ecosystem carbon and water cycles, as the growing season cumulative NEE was reduced by half (from 185 gC m^-2 to 80 gC m^-2) during epidemic phase I and then to near zero, or carbon neutral, during epidemic phase II.  Likewise, the average cumulative summer ET decreased from 26.0 cm to 21.5 cm during the epidemic.  These results scale knowledge of the beetle disturbance and its resulting tree mortality from the plant level up to the ecosystem scale.