Ancient wetlands can be damaged by extreme fires, but post-fire in-channel treatments may conserve them

Background/Question/Methods

Understanding the impacts of extreme wildfires on valuable headwater wetlands is critical to designing appropriate management strategies to prevent or repair damage. Studies of channel dynamics over decades are especially important in parts of the Southwest that have been prone to extreme post-fire erosion, because previous research has questioned whether post-fire mitigation treatments in channels are effective or necessary. We measured and documented erosion at Turkey Spring, a spring-fed wetland in the Mogollon Highlands of east-central Arizona, for 11 years after its 71 ha watershed burned at high severity in the Rodeo-Chediski wildfire in 2002. The site was located on tribal lands within the Fort Apache Indian Reservation, and through a collaborative project, tribal high school and college students remeasured cross-section and longitudinal surveys over time. We obtained radiocarbon dates for charcoal buried in three debris flows that were exposed in the channel walls by post-fire erosion. We also monitored erosion with cross-sections and vegetative recovery in plots at a second wet meadow site, Swamp Spring, that the Rodeo-Chediski wildfire had also severely burned, but where large rock riffle formations were placed in the channel to conserve wetlands two years after the fire.

Results/Conclusions

Erosion at the untreated Turkey Spring site following the wildfire amounted to 13.5 m² per meter of stream channel along the 550 m study reach (7400 m³ total). As gully headcuts progressed upstream, erosion continued at an increasing rate until 2010. In contrast, at the Swamp Spring site following treatment in 2004, channel incision stopped and wetland vegetation quickly reestablished. Charcoal-laden debris flows at Turkey Spring suggests that wildfires were an important formative process at this site over thousands of years. A radiocarbon date from the base of the eroded channel wall indicated that the fire induced erosion of soils that were over 8200 years old. Several factors may account for rapid degradation and loss of wetland soils at both Turkey Spring and Swamp Spring, including high burn severity across their watersheds and construction of roads during the past century. Sites within the region that are similar to the two in this study may be vulnerable to extreme incision events following wildfires with very large high severity patches. In such cases, both preventive treatments to reduce road impacts and fire severity, and post-wildfire channel treatments may help conserve wetland soils and associated values that have established over thousands of years.