Graduate Thesis Or Dissertation
 

The Impact of Overlapping Forest Disturbance Across a Stream Network in Western Oregon: Harvesting, Wildfire and Post-Fire Management Effects on Stream Water Nitrate and Riparian Soil Nutrients

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3f462f254

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  • Understanding the magnitude and longevity of wildfire effects in forested watersheds and subsequent impacts to downstream aquatic ecosystems and communities requires congruent investigation into post-fire aquatic and terrestrial processes. However, results from wildfire studies are often confounded by pre- and post-fire land management actions and based on either hydrologic or soil observations, alone. Here, we leverage 10 years of pre-fire stream water quality data to understand overlapping disturbance impacts (i.e., forest harvesting, wildfire, and post-fire management) across a stream network in the Hinkle Creek Experimental Watershed (1,941 ha) located on an intensively managed Douglas-fir plantation (Pseudotsuga menziesii) in Western Oregon, USA. Approximately 98 % of the watershed burned in 2020 during the Archie Creek Fire and partial areas within the burn perimeter were subsequently managed (e.g., salvage logging, herbicide application) post-fire. In 2021, we began collecting weekly stream water grab samples to analyze changes in NO3--N concentrations at six of the original study sites across two catchments (North and South Fork Hinkle). Additionally, we added 12 riparian mineral soil (0–15 cm) sites co-located with our water quality sites. We employed a chronosequence study design to investigate the effects of pre-fire stand age (1–3, 4–6, 11–19 years-old) and time since fire (15 months, 25 months, and 32 months) on concentrations of NO3--N, NH4+-N, 14-day potentially mineralizable N (14-d PMN), and active C (POXC) in mineral soil. Interestingly, pre-fire stand age had no effect on soil NO3--N or POXC and a minimal effect on NH4+-N and 14-d PMN. Time since fire had no effect on soil NO3--N, a negative effect on POXC and NH4+-N, and a positive effect on 14-d PMN. In our study streams, we did not observe any change in NO3--N concentrations after forest harvesting. In contrast, after wildfire and post-fire management activities South Fork Hinkle, we observed a 3-fold increase in stream water NO3--N compared to the pre-harvest and post-harvest study periods. Comparatively, in North Fork Hinkle, we observed a 22-fold increase compared to pre-harvest period and a 43-fold increase compared to the post-harvest period. The variability in stream water NO3--N between our study catchments after wildfire and post-fire management activities may be related to differences in the magnitude and spatial variability of pre- and post-fire land management actions, driving differences in post-fire vegetation communities and/or differences in hillslope runoff processes. To better understand the magnitude and legacy of chemical water quality response after wildfire, future research should aim to incorporate process-based investigations within the aquatic and terrestrial environments over multiple disturbance phases.
  • Keywords: salvage logging, watershed processes, water quality, forest harvesting, forested ecosystems, nitrate, wildfire, soil, post-fire management
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