Factors affecting connectivity and sediment yields following wildfire and post‐fire salvage logging in California's Sierra Nevada

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Active wildfire seasons in the western U.S. warrant the evaluation of post‐fire forest management strategies. Ground‐based salvage logging is often used to recover economic loss of burned timber. In unburned forests, ground‐based logging often follows best management practices by leaving undisturbed areas near streams called stream buffers. However, the effectiveness of these buffers has not been tested in a post‐wildfire setting. This experiment tested buffer width effectiveness with a novel field‐simulated rill experiment using sediment‐laden runoff (25 g/L) released over 40 min at evenly timed flow rates (50, 100 and 150 L/min) to measure surface runoff travel length and sediment concentration under unburned and high and low soil burn severity conditions at 2‐, 10‐ and 22‐month post‐fire. High severity areas 2‐month post‐fire had rill lengths of up to 100 m. Rill length significantly decreased over time as vegetation regrowth provided ground cover. Sediment concentration and sediment dropout rate also varied significantly by soil burn severity. Sediment concentrations were 19 g/L for the highest flow 2‐month post‐fire and reduced to 6.9–14 g/L 10‐month post‐fire due to abundant vegetation recovery. The amount of sediment dropping out of the flow consistently increased over the study period with the low burn severity rate of 1.15 g L−1 m−1 approaching the unburned rate of 1.29 g L−1 m−1 by 2‐year post‐fire. These results suggest that an often‐used standard, 15 m buffer, was sufficient to contain surface runoff and reduce sediment concentration on unburned sites, however buffers on high burn severity sites need to be eight times greater (120 m) immediately after wildfire and four times greater (60 m) 1‐year post‐fire. Low burn severity areas 1‐year post‐fire may need to be only twice the width of an unburned buffer (30 m), and 2‐year post‐fire these could return to unburned widths.

Section: Methodssupporting

confidence: 89%

Section: Introductionmentioning

confidence: 99%

Effectiveness of post‐fire salvage logging stream buffer management for hillslope erosion in theU.S.Inland Northwest Mountains

Robichaud

Bone

Lewis

et al. 2020

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“…Soil compaction by salvage logging equipment significantly increased bulk density by 25% and 30% at 0–5 and 5–10 cm soil depths, respectively. Other researchers have reported increases after logging traffic in unburned or burned forests with sandy loam soils of 9%–17% (Olsen et al, 2020), 20%–37% (Croke et al, 2001; Han et al, 2009; Sosa‐Pérez & MacDonald, 2017) and 44%–58% (Sheridan et al, 2008; Wagenbrenner et al, 2016). In these studies, the compaction effect increased with soil moisture, traffic load and traffic levels of logging equipment, especially wheeled skidders.…”

Section: Discussionmentioning

confidence: 98%

“…Beside wildfires, post‐fire management such as the passage of heavy machinery during salvage logging of burned timber can have negative effects. The most important factors related to post‐fire salvage logging are the compaction of soils (Croke et al, 2001; García‐Orenes et al, 2017; Wagenbrenner et al, 2015) and loss of soil surface cover (Cole et al, 2020; García‐Orenes et al, 2017; Lucas‐Borja et al, 2019; Olsen et al, 2020; Wagenbrenner et al, 2015). Post‐fire soil compaction has been associated with an increase in runoff and erosion (Malvar et al, 2017; Wagenbrenner et al, 2015), although opposite results have also been measured (James & Krumland, 2018).…”

Section: Introductionmentioning

confidence: 99%

“…More information is available regarding soil compaction in unburned forests, especially related to unpaved roads, and we can use this body of research to help assess the potential impacts of compaction of burned soils on runoff generation and erosion. Although the network of forest roads-including skid trails, haul roads and landings-can constitute between 10% and 24% of the forest surface (Han et al, 2009;Kolka & Smidt, 2004;Olsen et al, 2020), it contributes to a large portion of basin-runoff and erosion, even in unburned forestland (Arnáez et al, 2004;Jordán et al, 2009;Sheridan et al, 2008). Forest roads with regular traffic and maintenance strongly reduce infiltration on the running surface through compaction, increase erosion by replenishing the supply of sediment, and hamper the establishment of vegetative cover (Foltz et al, 2009).…”

mentioning

confidence: 99%

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Compaction and cover effects on runoff and erosion in post‐fire salvage logged areas in the Valley Fire, California

Prats

Malvar

Wagenbrenner

2020

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Runoff and erosion processes can increase after wildfire and post‐fire salvage logging, but little is known about the specific effects of soil compaction and surface cover after post‐fire salvage logging activities on these processes. We carried out rainfall simulations after a high‐severity wildfire and post‐fire salvage logging to assess the effect of compaction (uncompacted or compacted by skid traffic during post‐fire salvage logging) and surface cover (bare or covered with logging slash). Runoff after 71 mm of rainfall across two 30‐min simulations was similar for the bare plots regardless of the compaction status (mean 33 mm). In comparison, runoff in the slash‐covered plots averaged only 22 mm. Rainsplash in the downslope direction averaged 30 g for the bare plots across compaction levels and decreased significantly by 70% on the slash‐covered plots. Sediment yield totalled 460 and 818 g m−2 for the uncompacted and compacted bare plots, respectively, and slash significantly reduced these amounts by an average rate of 71%. Our results showed that soil erosion was still high two years after the high severity burning and the effect of soil compaction nearly doubled soil erosion via nonsignificant increases in runoff and sediment concentration. Antecedent soil moisture (dry or wet) was the dominant factor controlling runoff, while surface cover was the dominant factor for rainsplash and sediment yield. Saturated hydraulic conductivity and interrill erodibility calculated from these rainfall simulations confirmed previous laboratory research and will support hydrologic and erosion modelling efforts related to wildfire and post‐fire salvage logging. Covering the soil with slash mitigated runoff and significantly reduced soil erosion, demonstrating the potential of this practise to reduce sediment yield and soil degradation from burned and logged areas.

Wildfire and hydrological processes

Boyer

Wagenbrenner

Zhang

2022

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Climate change is a crucial factor in increasing wildfire risks, where warmer and drier conditions, increased drought periods and increased lightning strikes have made many areas more susceptible to burning. This special issue focuses on Wildfire and Hydrological Processes, exploring how wildfire has impacted watersheds and water resources. The manuscripts in this collection underscore how wildfire can change the nature of vegetation, characteristics of soils, hydrological flow paths, and residence times of water in the critical zone, and provide new insights toward predictability of wildfire impacts on watersheds. The studies reveal that wildfire can affect water quantity and quality over varying timescales, from during the active burning to years and decades afterward.