Soil Water Repellency and Hydrologic Recovery in Reburned Moist, Mixed-Conifer Forests

Climate-driven increases in western United States wildfire activity can lead to novel post-fire recovery trajectories as ecosystems are challenged by warming, drying conditions and potential deviations from historic wildfire regimes. Several recent studies have documented unprecedented wildfire frequency in forests adapted to longer wildfire return intervals, but the impact of these novel, increasingly prevalent, more frequent wildfires on watershed recovery remains unknown. We compare soil water repellency (SWR) profiles across hillslopes unburned, or once-, twice-, or thrice-burned between 2004 and 2015 to assess the impact of short-interval wildfire (SIF) events on the hydrologic recovery of a moist, mixed-conifer forest over 3-11 years following fire. We find that fire-affected SWR profiles in the top 30 cm deviate from unburned soils for more than a decade, adding nuance to the typical findings in literature that peak SWR (and resulting runoff impacts) recedes within the first two years after wildfire. Notably, measured soil recovery trajectories vary by number of SIFs: once-burned SWR profiles approached unburned conditions (high SWR at the surface, 0-5 cm, and low SWR at depth, >12.5 cm) faster than twice-burned and thrice-burned hillslopes. Further, thrice burned hillslopes exhibited significantly different profiles than all the other natural `treatments', with thrice-burned SWR persistently greatly elevated at depth (12.5-20 cm) and negligible repellency at the surface. We conclude that, although SWR at the surface (as is typically measured) may not persist longer-term on reburned hillslopes, more delayed recovery at depth with increasing numbers of SIFs may greatly impact long-term system hydrologic recovery in reburned moist, mixed-conifer forests. Slower return to pre-fire soil conditions at depth has implications for fluxes such as evapotranspiration and groundwater recharge in reburned forested ecosystems and highlights the need for more long-term and deeper-depth studies of hydrologic recovery at both the hillslope and watershed scales as novel wildfire activity continues to increase in the western United States.