Post-wildfire riparian forest recovery processes along a regulated river corridor

Abstract

Riparian forest recovery was monitored over a 23 km study site that encountered a mixed low to moderate severity wildfire on the floodplain that was followed by a sequence of flows ranging from below bankfull to a 50-year flood on the regulated South Fork Boise River, Idaho, USA. The goals of the study were to examine the dominant mechanisms of post-wildfire riparian forest regeneration along the river corridor in relation to upstream dam operations. Results are put in context with trends occurring in the regulated river system over the past 70 years that exemplify post-impoundment floodplain dynamics. Aerial imagery and field surveys were used to examine trends in geomorphic and forest response related to flow regulation, riparian wildfire, and a 50-year flood. Additionally, a one-dimensional sediment transport model was employed to assess seedling recruitment related to local and reach-scale erosional and depositional processes over a five-year period following the wildfire. A conceptual model is presented to illustrate riparian forest response to the interacting influences of fire severity and flood disturbance to help guide post-wildfire riparian forest management activities.Aerial imagery between 1957 and 2011 shows that flow regulation after dam closure led to channel narrowing of 62% of historic active channel width and an associated increase in floodplain tree density of 59%. This expansion of forest cover on the floodplain was at the expense of seedling recruitment habitat for pioneer species in the Salicaceae family that require bare sediment, light exposure, and access to the shallow groundwater. A 2013 riparian wildfire led to substantial top kill on the floodplain that provided a disturbance for potential seedling recruitment. However, dam releases during average and wet hydrologic conditions led to only 5% of the total observed seedling recruitment in the system occurring on the floodplain due to the limited magnitude of flood disturbances following the burn, highlighting the importance of the sequence of environmental flows immediately following riparian wildfire. Alternatively, 95% of seedling recruitment occurred within the active channel and was directly related to sediment transport processes. Post-wildfire related debris flows were responsible for providing the requisite sediment supply to the channel in an otherwise sediment supply-limited, regulated river system. Resultant sediment deposition was responsible for providing 100% of seedling recruitment habitat in the active channel during dam releases in average hydrologic conditions, whereas deposition accounted for 84% and erosion accounted for 16% of observed recruitment habitat during wet hydrologic conditions.