Lethal soil temperatures during burning of masticated forest residues

Abstract

Mastication of woody shrubs is used increasingly as a management option to reduce fire risk at the wildland–urban interface. Whether the resulting mulch layer leads to extreme soil heating, if burned, is unknown. We measured temperature profiles in a clay loam soil during burning of Arctostaphylos residues. Four mulch depths were burned (0, 2.5, 7.5 and 12.5 cm), spanning typical conditions at forested sites in northern California with dense pre-mastication shrub cover. Two soil moisture contents were compared at each fuel depth to simulate spring prescribed burning (moist soil) and late-season wildfire (dry soil). Maximum temperatures reached 600°C on the surface of dry soils and were 100–200°C lower for moist soil. Heating was extensive in dry soil for the two deepest mulch depths, exceeding the lethal threshold for plants (60°C) for a minimum of 7 h throughout the 10-cm soil profile. Minimal heat pulse was found with less mulch. Moist soil also dampened heat penetration; peak temperatures exceeded 60°C only to 2.5 cm in the soil profile for all but the deepest mulch layer. No adverse effects of burning on water repellency were found in dry or moist soil. The potential for biological damage from soil heating during fire exists following mastication, particularly in dry soil with a mulch depth of 7.5 cm or greater. Field projections indicate that up to one-fourth of treated areas with dense pre-mastication vegetation would surpass lethal soil temperatures during a surface wildfire.

Comparison of Heat Transfer and Soil Impacts of Air Curtain Burner Burning and Slash Pile Burning

“…Soil temperatures reaching 100 • C can be lethal to the soil microbes [22] and temperatures ranging 200-500 • C cause reductions of soil carbon (C) and nitrogen (N), aggregate stability, and thermal conductivity [20]. While numerous studies have documented soil temperature flux during slash pile burning (e.g., [23,24]), we could find no information on soil heating while using ACB. We hypothesize that temperatures could be much higher since larger volumes of wood can be burned at once and the high turbulence associated with air movement across the burning wood can increase the chamber temperature to ≥980 • C. However, although the impacts of burning on belowground processes are highly variable [25], lack of in situ heat transfer measurements hinders the evaluation of heating damage from ACB and, perhaps, an increased use in areas with excess woody residues.…”

“…Busse et al [24] mimicked broadcast burning after mastication and reported the maximum soil surface temperatures reached 500-600 • C (dry soil) and 400-500 • C (wet soil), and observed peak temperatures at 10 cm ranged 40-105 • C. Neary et al [22] also observed severe soil heating; 700 • C at surface over 250 • C at 10 cm depth, and greater than 100 • C at 22 cm depth. In the extreme, soil heating has been observed 1.36 m deep under heavy slash pile [23].…”

“…For soil temperature data, peak temperature and lethal temperature duration were tested as the response variables. Lethal temperature duration was calculated through the summation of minutes over 60 • C [22,24] during the 240 min of burning. Peak temperature was log-transformed to satisfy the assumptions of model's error structure.…”

Burning Fire-Prone Mediterranean Shrublands: Immediate Changes in Soil Microbial Community Structure and Ecosystem Functions

“…Higher temperatures were likely prevented both by the presence of a mulching agent (i.e. the surface cover of litter, stones and mosses) and the high soil humidity [8]. However, high moisture conditions can be particularly lethal for soil microbes due to faster heat penetration and a smaller fraction of dormant organisms [11,27,42].…”

Fuel manipulation with herbicide treatments to reduce fire hazard in young pine (Pinus elliottii × P. caribaea) plantations in south-east Queensland, Australia

“…Litter depths were rarely great enough (i.e. .7.5 cm) to allow smouldering fires, which can maintain lethal temperatures for over an hour and result in root death (Beadle 1940;Busse et al 2005;Monsanto and Agee 2008;Varner et al 2009). …”