Repeated wildfires in a Sierra Nevada watershed cause limited post-fire erosion

Abstract:

Many wildfires in mountain regions have been shown to cause damaging post-fire erosion, primarily due to the removal of protective vegetation. This erosion can destabilize hillslopes, reduce groundwater infiltration, alter stream channels, and impair water quality. However, most studies of post-fire erosion occur in areas that have burned at high severity after a long fire-free period in which fuels were able to accumulate to historically high levels. In our study, we aim to explore the post-fire erosion and vegetation regrowth dynamics in a unique watershed which has experienced a near-natural wildfire regime for the past 46 years. The Illilouette Creek Basin in Yosemite National Park has experienced dozens of wildfires since 1974 - most recently in 2017 - thanks to Park managers’ decision to allow lightning-ignited wildfires to burn through this area as they did prior to human suppression efforts. Using aerial Lidar elevation data from multiple flights before and after the 2017 fire, we have identified erosion features (e.g., gullies) that formed following that fire, as well as areas of sediment deposition. We were able to validate and improve our mapping via ground-truth data collected during multiple site visits. Time lapse cameras also captured the development of one gully, as well as post-fire vegetation regrowth. We present an analysis of post-fire erosion in the Illilouette Creek Basin compared to what might be expected in fire-suppressed watersheds with higher fuel loads. These results will be discussed in the context of long-term hydrology and forestry studies that have been conducted in this watershed. This case study will help to demonstrate whether allowing frequent, lightning-ignited fires to burn more often in fire-adapted ecosystems can help increase watershed resilience.