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.