- The extent and severity of wildfires in forested regions are increasing throughout many regions on the planet, including western North America. High-severity wildfires directly affect soils and vegetation by altering soil hydraulic properties, reducing soil organic matter, exporting carbon and nitrogen, and killing trees and understory vegetation. These impacts can increase runoff, erosion, and sediment delivery to streams, producing a range of impacts on terrestrial and aquatic ecosystems and downstream source water quantity and quality. Due to the broad range of post-fire threats, land managers often undertake active post-fire land management (e.g., salvage logging, subsoiling, revegetation) to promote forest regeneration and maintain forest and aquatic ecosystem functions. However, the magnitude and longevity of effects from wildfire and the various post-fire land management approaches remains uncertain.
Here, I present results from a study, which quantified and compared sediment yields eroded from (a) burned, (b) burned and salvage logged, (c) burned, salvage logged, and subsoiled, (d) burned, salvage logged, and pre-emergent herbicide, (e) burned, salvage logged, and foliar herbicide, and (f) burned, salvage logged, subsoiled, and pre-emergent herbicide plots in the northern California Coast Range. We constructed 25 sediment fences on four hillslopes that burned at high severity in the 2015 Valley Fire. We quantified sediment yields from the upslope contributing area to each silt fence (~75 m2). We also quantified ground cover,
precipitation, soil properties, and canopy cover from areas representative of each plot. In the second year after the fire, sediment yields were ~4- to 10-times greater in the burned plots compared to the salvage logged or subsoiled plots. By the third and fourth years after fire, there was no statistical evidence for a difference in sediment yields among the six site types. In general, we observed higher sediment yields across all site types following precipitation events with greater amounts or intensities. Our results suggested sediment yields were greater from sites with greater canopy closure and more exposed bare soil. Salvage logging operations initially increased soil bulk density, but by the fourth year, bulk density was similar among all site types. Sediment yields in all site types decreased over the course of the study and stabilized by the fourth post-fire year, which may be indicative of site recovery or exhaustion of easily mobile sediment.
We also present results from a related study, in which we quantified differences in soil carbon, nitrogen, carbon/nitrogen (C:N) ratios, and vegetation recovery from three of the post-fire management strategies, including: (a) burned, (b) burned and salvage logged, and (c) burned, salvage logged, and subsoiled sites. We collected 180 soil samples from two depths (0‒5 cm and 5‒10 cm) and 27 vegetation samples across sites burned at high severity representative of the three post-fire management strategies. We also collected samples to quantify soil carbon, nitrogen, and C:N ratios in eroded sediment from hillslope sediment fences. The eroded sediment and soils in the burned sites had C:N ratios ~9–22 % lower than in the salvage logged or subsoiled sites. Interestingly, both carbon and nitrogen concentrations were greater in the more heavily disturbed sites. In other words, C and N concentrations in soils were generally ranked as: burned, salvage logged, and subsoiled > burned and salvage logged > burned. Vegetation biomass in burned sites was ~2.7-times greater than salvage logged and ~6-times greater than
subsoiled sites. Coarse wood cover in our erosion plots was positively correlated with C:N ratios from our eroded sediment, but vegetation cover was not correlated with carbon, nitrogen, or C:N ratios. These results suggest that salvage logging may increase soil C:N ratios by increasing the coarse wood input to the forest floor, which may negatively impact vegetation recovery three years after fire by suppressing plant available nitrogen. Post-fire forest regeneration is critically important to increase carbon storage and clean water supply in disturbed forest ecosystems, so forest management practices should not exacerbate sediment transport or limit vegetation recovery.