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Riparian post-fire response: factors influencing vegetation recovery and channel stability
AuthorDencker, Camie M.
AdvisorSwanson, Sherman R.
Natural Resources and Environmental Science
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The BLM Emergency Stability and Rehabilitation Handbook suggests a rest from grazing following wildfire for two years or until objectives are met for the recovery of vegetation and key processes. However, both land users and managers dispute this policy because of economic, ecological and social implications and little supporting scientific evidence. Riparian areas are of particular concern because of concentrated grazing-use and importance for wildlife, humans, livestock production, and hydrologic functions. This research sought to quantify rates of change and variation post-fire in riparian condition and response across channel and watershed attributes, fire severity, and pre- and post-fire grazing-use. To quantify stream recovery, we used Multiple Indicator Monitoring (MIM) of Stream Channels and Streamside Vegetation (Burton et al. 2011) because it is becoming a standard method for quantifying if riparian objectives are met. We monitored 23 streams burned in 2012 wildfires on public lands in Nevada, focusing on reaches of greatest management concern, such as those classified as functional at-risk, or with threatened species habitat or aspen stands. Watershed and stream channel characteristics were quantified in ArcGIS with the exception of stream gradient, which was measured at site. We used MIM variables that had been measured over two years as indicators of riparian condition: greenline-to-greenline width, greenline plant composition, woody species cover and height, and streambank stability and vegetation cover. Winward greenline stability and wetland indicator rating were calculated from greenline plant composition and used as metrics of ecosystem functionality. Riparian species composition was most related to variables associated with watershed position, such as substrate size, gradient, and elevation. Wetland obligate species were found at sites with high sinuosity or bank stability and within watersheds characterized by high percentage volcanic bed material. Bank cover was associated with higher position in the watershed, Winward greenline stability rating, and streambank stability. Banks were more stable with increased bank cover and decreased percent fine substrate, stream gradient, and post-fire grazing-use. Over the two-year study, bank stability decreased from 2014 to 2015 with increased post-fire grazing duration at sites with higher percent fine substrates. Bank stability, species richness, and woody species cover and height class increased with duration of recovery periods and decreased with continuous, hot season grazing-use (July-September) prior to the fires, from 2006-2012. Woody species height increased with riparian width and recovery after grazing during the growing season and decreased with stream gradient and high burn severity. Sites lower in the watershed were grazed for longer duration with shorter recovery periods during the growing season and fewer years of rest. Lower-position sites also had the greatest percent fine substrate and lowest bank cover, making them more unstable. Two-year grazing deferment may not be adequate for recovery of riparian functionality at lower watershed sites if streambank cover and stability are compromised. Continued monitoring is necessary to ascertain the required bank cover and time for recovery for these lower reaches to be resilient to the pressures of post-fire livestock grazing.