Spatial distribution and properties of ash and thermally altered soils after high-severity forest fire, southern California

Abstract

After a century of fire suppression, dense forests in California have fueled high-severity fires. We surveyed mixed conifer forest with 995–1178 trees ha−1 (stems > 10 cm diameter at breast height), and nearby pine–oak woodland having 175–230 trees ha−1, 51 days after a severe burn, to contrast the spatial extent and properties of thermally altered soil at sites with different tree densities. Water-repellent soils were more extensive in forest than woodland. Deposits of white ash, composed largely of calicite, covered at most ~25% of the land surface, in places where large fuel items (e.g. logs, branches, exfoliated oak bark) had thoroughly combusted. At least 1690 kg ha−1 of CaCO3 in ash was deposited over the forest, and at least 700 kg ha−1 was added to the woodland. Combustion of logs and large branches also reddened the underlying yellow-brown soil as deep as 60 mm (average 8 mm), and over ~1–12% of the land surface. The reddened soils have magnetic susceptibilities that are three to seven times greater than surrounding unreddened soils within the burn, indicating thermal production of maghemite. Such fire-altered conditions persist over spatial and temporal scales that influence soil genesis in Mediterranean-type climate regions.

Spatial models for monitoring the spatio-temporal evolution of ashes after fire – a case study of a burnt grassland in Lithuania

“…Ash colour is a key variable to understanding fire severity (Smith and Hudak, 2005;Goforth et al, 2005;Úbeda et al, 2009) and is a clear tracer of ash thickness as we observed here, which confirms previous research in the Mediterranean type ecosystems (Pereira et al, 2011). In all studied plots, black ash was thicker than the light grey or white ash due to the lower degree of combustion that leaves a greater amount of organic material in the ash.…”

“…Some studies have reported ash thicknesses up to 70 mm in an oak forest burned by wildland fires (Ulery et al, 1993), 60 mm in a mixed pine forest (Goforth et al, 2005), and 17 mm in a mixed fir and larch forest (Woods and Balfour, 2008). However little information is available about fire severity effects on ash thickness and its temporal evolution, and no studies have been conducted on this topic on boreal grassland ecosystems.…”

“…In the grided area we tested several interpolation methods in order to observe the best method. In addition, ash thickness was calculated according to ash colour, which is frequently used to measure fire severity (Goforth et al, 2005;Pereira et al, 2012a;Smith and Hudak, 2005), in order to determine if the impact of fire in the soil system depended upon fire severity.…”

Effects of high temperature processes on physical properties of silica sand

“…The sand is comprised primarily of silicon dioxide; iron oxides make up a small fraction of its composition. High temperatures may cause additional changes in mineralogy that may be less likely to be detected by visual examination (Goforth et al, 2005;Pomiès et al, 1998). Similar effects may occur within the silicon dioxide, which becomes unstable with high temperatures and forms silica polymorphs such as trydimite or cristobalite (Hand et al, 1998;Wenk and Bulakh, 2004).…”

“…In most cases it changes from yellowish brown to reddish brown. This is due to the oxidation of soil iron content from goethite to maghemite or hematite (Goforth et al, 2005;Ketterings and Bigham, 2000). Decomposition of soil particles, especially clay minerals, starts at temperatures above 550°C (Certini, 2005).…”