Causes and consequences of bark beetle-induced mortality on water, carbon, and nitrogen cycling

Background/Question/Methods

The recent epidemic of bark beetles across western North America has impacted conifers from low to high elevations from New Mexico to Yukon. While the mechanism of mortality from bark beetles may vary from phloem girdling to xylem blocking by blue stain fungi, mountain pine beetle and spruce beetle both appear to kill through xylem occlusion. Understanding the mechanism of mortality is required to predict subsequent impacts at the stand and larger spatial scales. We hypothesized that 1) both lodgepole pine and Engelmann spruce died from hydraulic failure due to blue stain fungal occlusion of xylem, 2) soil water and nitrogen increase while soil respiration decreases due to diminished root resource uptake during the growing season, 3) surviving vegetation will benefit and the rate of succession compared to clearcutting or fires will increase and 4) the spatial patchiness of mortality and rapid succession will decrease the watershed to landscape level impacts of mortality. We tested these hypotheses with data from mid-elevation lodgepole pine and higher elevation spruce and fir forests using Bayesian model-data fusion.

Results/Conclusions

The combination of sap flux, needle gas exchange and xylem vulnerability to cavitation all supported a hydraulic failure mechanism of tree mortality. Transpiration in both conifer species declined rapidly after infestation but gas exchange estimates of maximum photosynthesis, day respiration and quantum yield were unchanged. The Terrestrial Regional Ecosystem Exchange Simulator (TREES) model, which combines tree hydraulics, needle gas exchange, respiration, carbon allocation and nitrogen uptake, predicted a declining critical value for hydraulic failure that matched empirical measurements. The model also predicted the contrasting dynamic in nonstructural carbohydrates; living trees increased while dying trees decreased over the growing season. Ecosystem respiration decreased while soil moisture and nitrogen increased. After five years some of these dynamics are becoming similar to pre-epidemic stand conditions. The respiration dynamic was not predictable from standard ecosystem respiration drivers so the model was modified to include tissue nitrogen and heterotrophic respiration kinetics. The final model was thus able to capture the dynamics in both evapotranspiration and net ecosystem exchange of CO₂. Our results show that the incorporation of tree mortality and ecosystem response mechanisms are required to capture the impact of bark beetle induced mortality on ecosystem cycles of carbon, water and nitrogen.