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Published on: October 7, 2020
Biogeochemistry of beetle-killed forests: explaining a weak nitrate response
Charles C Rhoades1, James H McCutchan, Leigh A Cooper
1US Department of Agriculture Forest Service, Rocky Mountain Research Station, Fort Collins, CO 80523, USA.
Mountain pine beetle outbreaks in Colorado did not increase stream nitrate levels, unlike other forest disturbances. This is due to low nitrogen deposition and unaffected vegetation retaining nitrate.
Area of Science:
- Forest Ecology
- Ecosystem Hydrology
- Insect Outbreak Impacts
Background:
- Mountain pine beetle infestations cause widespread lodgepole pine mortality in Colorado.
- Forest disturbances like logging typically increase stream nitrate concentrations in low-to-moderate nitrogen deposition watersheds.
- Previous studies show a significant nitrate increase (approx. 400%) after forest disturbance.
Purpose of the Study:
- To investigate streamwater nitrate concentration changes following mountain pine beetle infestation in Colorado.
- To understand the factors influencing nitrate retention in beetle-impacted watersheds.
- To contrast beetle-induced tree mortality impacts with other forest disturbances.
Main Methods:
- Modeling nitrate release from Colorado watersheds using field data.
- Calibrating the model with data from beetle-infested and pre-disturbance conditions.
- Analyzing atmospheric nitrogen deposition rates and vegetation uptake capacity.
Main Results:
- No significant increase in streamwater nitrate concentrations was observed after extensive tree mortality from mountain pine beetles.
- A model indicated that stimulated nitrate uptake by unaffected vegetation retained significant nitrate.
- Low atmospheric nitrogen deposition and multi-year mortality spread contributed to nitrate retention.
Conclusions:
- Colorado watersheds infested by mountain pine beetles exhibit significant nitrate retention.
- Undisturbed residual vegetation and soils possess a high compensatory capacity for nitrate uptake.
- Unlike other disturbances, severe pine beetle mortality does not lead to elevated stream nitrate in this ecosystem.
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