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Fire Patterns and Post-Fire Vegetation Response in a Mojave Desert Spring Ecosystem
AdvisorWeisberg, Peter J
Natural Resources and Environmental Science
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Desert spring ecosystems provide critical ecological services and have been prized locations for human settlement in the arid regions of the southwestern United States. Despite the important roles that they play, little is known about many ecological processes operating in desert spring ecosystems. Disturbance processes are likely important in desert springs, but have not been well studied. This study examined the role of fire in the desert spring ecosystems in Ash Meadows National Wildlife Refuge of the northern Mojave Desert, Nevada, United States. In the Ash Meadows spring complex, fire frequency appears to be greater than that reported for typical Mojave Desert landscapes that have not been recently invaded by exotic annual grasses. This study quantified historic fire patterns using a remote sensing analysis, constructed predictive models for fire occurrence and burn severity, and examined the post-fire response of native and exotic plant species.Remote sensing methods are commonly used to gain information about fires that have occurred over recent decades, but these methods have not been tested in desert spring environments. This study compared the results obtained using two techniques for estimating fire perimeters and delineating burn severity mosaics, the differenced normalized burn ratio (dNBR) and differenced linear spectral unmixing (dSMA). The dNBR method outperformed the dSMA method, but required some post-processing manipulation to reduce errors of commission. Burned areas were correctly identified with 86% accuracy, and fire severity was classified with 76% accuracy. Several small (< 5-ha) fires were not detectable using the methods tested, indicating that additional methodological refinement is necessary to better distinguish small burned areas from shifts in background conditions common in desert spring environments.Fire occurrence and burn severity were modeled as selective processes using ecological niche factor analysis and a Bayesian model averaging logistic regression technique. Fuel availability, in terms of pre-fire vegetation abundance and connectivity, played a major role in predicting both fire occurrence suitability and burn severity, while anthropogenic alteration played a more prominent role in predicting burn severity than fire occurrence suitability. Areas in close proximity to anthropogenic disturbance are likely to have more exotic herbaceous plants which increase fine fuels, make an area capable of sustaining a fire, and promote quicker, less destructive burns. Riparian forest and emergent vegetation areas had the greatest proportion of their area burned from 1980 - 2008 and were most likely to experience high-severity fire, although they were among the rarest land cover types found in the study site. These densely vegetated wetlands may facilitate the relatively high rate of fire occurrence and promote high-severity fire in this desert spring ecosystem. Altering the amount of wetland habitat may have substantial consequences by changing fire-induced disturbance regimes.A field study was conducted to examine vegetation response 3 - 8 years post-fire. Repeated burning within a five-year time period favored native herbaceous perennials, capable of rapidly resprouting from the root-crown, and may limit opportunities for establishment and survival of exotic annuals. In contrast with results observed in more xeric areas of the Mojave Desert, most fire conditions reduced relative cover of exotic species in this desert spring ecosystem. However, increased relative cover of exotic annuals was observed in areas that had most recently experienced high-severity fire, and sites that burned more frequently had the highest percent cover of many exotic species. Native tree populations may tolerate fire activity similar to that experienced at Ash Meadows from 2000-2008, as all species studied were likely to resprout following fire. <italic>Prosopis pubescens</italic> (screwbean mesquite) was more likely to be completely killed by fire than either <italic>Prosopis glandulosa</italic> (honey mesquite) or <italic>Fraxinus velutina</italic> (velvet ash), but it also was most commonly observed establishing from seed in burned areas. Active restoration may be needed most in areas with fine-textured, highly saline soils, where damage to native trees is likely to be greatest.