Graduate Thesis Or Dissertation
 

Reburn Severity and Vegetation Recovery in Southwestern Oregon, USA

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/k930c4873

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  • The Klamath-Siskiyou Ecoregion of southwest Oregon and northern California is greatly departed from its historic, mixed-severity fire regime. This departure manifests in larger wildfires, greater proportions of high burn severity within wildfire perimeters, and decreased diversity of post-fire vegetation successional stages and trajectories across the landscape compared to historical norms. Reburn events (i.e. when wildfires burn through areas that have recently burned) can exacerbate this departure by facilitating a conversion from mixed-conifer forests to an alternative stable state dominated by pyrogenic shrubs. However, reburns also offer an opportunity to restore typical fire activity for a mixed-severity fire regime and encourage greater post-fire successional diversity. Considering that reburn events are increasingly likely under climate change conditions, proactive management efforts aimed at restoring a mixed-severity fire regime and maintaining resilient forest ecosystems will benefit from greater insight into the predictors of reburn severity, and into the distribution of post-fire vegetation successional trajectories during a reburn interval. Previous research concerning drivers of reburn severity has considered the severity of the initial fire, topography, and fuel and weather conditions immediately preceding the reburning fire but has not evaluated the potential effects of vegetation dynamics occurring between fire events on reburn severity, which could serve as an early indicator for developing pre-fire vegetation conditions. Understanding whether the rate of vegetation accumulation following a fire is a predictor for future reburn severity would allow managers to identify relatively soon after a fire event which areas should be prioritized for proactive management treatments to mitigate the risk of high reburn severity versus areas where projected low to moderate burn severity would facilitate managing wildfire using non-suppression tactics to restore historical proportions of lower severity fire. The first manuscript in this thesis investigates whether rates of vegetation recovery following the initial fire, measured in change in Normalized Burn Ratio (NBR) over 2, 5, and 10 years, are important predictors of reburn severity, once other known drivers of fire severity have been accounted for. Rates of vegetation recovery were assessed for points that reburned across the Rogue River-Siskiyou National Forest (RRSNF) utilizing fitted annual NBR values calculated by LandTrendr from a time series of Landsat imagery. Random forest analysis was used to rank the predictors of reburn severity based on permutation importance. Though other variables (notably pre-reburn vegetation cover, average fire season PDSI, and initial fire severity) were more important predictors of reburn severity, vegetation recovery rates are moderately important predictors of reburn severity, with comparable importance to fire weather variables and higher importance than some topographical variables. Short-term (2-year and 5-year) vegetation responses to wildfire, representing both delayed mortality and elevated vegetation recovery, were associated with higher reburn severity, as were higher long-term (10-year) rates of annual vegetation accumulation. These results indicate that along with known drivers of reburn severity, remotely sensed metrics of vegetation recovery relatively soon after a fire provide important insight that can support proactive management efforts. In areas experiencing delayed mortality, elevated vegetation recovery, or higher rates of annual vegetation accumulation, fuels reduction and prescribed burning treatments could be used to mitigate the risk of high reburn severity, and in areas with lower rates of vegetation recovery, reburning wildfires could be managed to restore historic levels of low and moderate severity fire. Proactive management is also necessary to maintain resilient mixed-conifer forests, featuring a diversity of seral stages and successional trajectories. Visualizing vegetation dynamics during the reburn interval between fire events can help managers understand the distribution of varying seral stages and successional trajectories across the landscape and the influence of fire activity on this distribution. The second manuscript in this thesis explores whether the LandTrendr segmented trendline fit to a time series of annual NBR reflectance during the interval between two wildfires during the past 30 years in the RRSNF differs due to varied previous fire activity. Generalized linear models were used to assess differences in the 1) complexity of post-fire vegetation trajectories (number of segments and average segment duration during the reburn interval); 2) immediate vegetation response (first segment duration and rate of change); and, 3) overall vegetation accumulation (overall magnitude of change and average segment rate of change during the reburn interval) as a function of initial burn severity and the number of previous fire events. The results indicate that initial burn severity affected the complexity of the post-fire vegetation response by contributing to an increase in the number of segments in the trendline corresponding to more distinct periods of change in NBR signal associated with vegetation growth, and affected initial vegetation response by increasing the odds of a longer first segment and increasing the first segment rate of vegetation change, patterns which reflect a return to an early seral state. Lower initial burn severity resulted in less complex trendlines with fewer segments representing distinct periods of change in NBR and associated vegetation growth, as well as a muted initial vegetation response, likely representing gradual recovery to a mid- or late-seral state. Experiencing more previous fires resulted in decreased complexity of post-fire vegetation response with a decrease in the number of segments in the trendline and slightly decreased first and average rates of vegetation accumulation, perhaps due to the decreased biomass present on a site. The distinct trendlines fit by LandTrendr corresponding to different vegetation successional trajectories could potentially be utilized by managers to identify areas in need of restoration to achieve historical proportions of successional diversity in the RRSNF, but calibration of the remotely sensed data with field observations is necessary to understand how periods of change described by LandTrendr related to changes in vegetation composition, structure, and percent cover.
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