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Master of Science (MS) in Biology
Climate change and fire suppression have altered historic fire regimes, creating conditions for larger, more intense fires. Intense burns can alter watershed hydrology, increasing the potential for harmful channel incision, which impairs riparian ecosystem function by lowering the water table, disconnecting floodplains from aquatic environments. However, wetlands and functioning riparian zones can reduce burn intensity. Beaver, with their unique ability to build dams, can restore incised and degraded streams, store water, and expand wetland environments, potentially decreasing wildfire intensity, fire spread and create fire breaks across the landscape. My objective was to test the hypothesis that beaver impoundments increase landscape resistance to wildfire by increasing soil and fuelmoisture in riparian zones and surrounding uplands, hindering fuel ignition and fire rate of spread. To test this hypothesis, beaver impounded sites and paired undammed reference reaches were compared using field moisture surveys, GIS analysis of burn severity, and remote sensing of plant water stress via drone. Soil and fuel moisture samples were collected at repeated times throughout the fire season, above, within and below ten beaver impounded streams and ten non-impounded reference reaches within the Methow Watershed, WA, USA. The six of these paired sites that had recently burned, and an additional eight paired burned reaches (n=14 pairs) were selected for GIS analysis comparing post fire burn severity, measured as dNBR, within valley bottoms and upland zones. Potential plant water stress of riparian grasses, shrubs, and upland conifers was compared at one site and a paired reference reach at the beginning, middle and end of the fire season using NDVI analysis of drone survey imagery. GIS analysis of historically burned beaver sites showed that beaver dams, slope, and solar radiation interacted to affect fire intensity in beaver impounded riparian zones, but not their adjacent uplands. Soil and fuel moisture sampling showed that beaver impounded sites had higher average soil and dead fuel moistures than nonimpounded sites during the driest times of the year, however beaver dam presence was not associated with increased live fuel moisture content. NDVI analysis revealed increased fluorescence in riparian grasses and upland conifers in beaver impounded riparian zones throughout the fire season. These results support my hypotheses that beaver impounded riparia have a higher resistance to burn events compared to non-impounded riparian zones, indicating that beaver can play a key role protecting river networks from burn events, increasing landscape resistance to wildfire.
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Weirich III, Joseph John, "Beaver moderated fire resistance in the North Cascades and potential for climate change adaptation" (2021). EWU Masters Thesis Collection. 660.