Does wildfire ash block soil pores? A micromorphological analysis of burned soils.

Increases in runoff and erosion after forest wildfires are often attributed to the removal of surface cover, the formation of water repellent soils, and sealing of the soil surface by ash. The latter process involves clogging of pores by ash as well as rainsplash induced compaction of the ash layer. However, few studies have directly addressed the hydrologic role of ash and no studies have documented ash sealing in a forest fire environment. In an attempt to determine whether ash contributes to reduced infiltration after fire we conducted a micromorphological analysis of soils collected before and after three controlled pile burns at the Lubrecht Experimental Forest in western Montana. The burns were conducted with a fuel load of 90 Mg ha-1 on sites dominated by Lodgepole pine ( Pinus contorta) with scattered Douglas fir ( Pseudotoga menziesii), sandy loam soils and a mean of 99% ground cover (litter, duff and live vegetation). Soil cores were collected before burning, immediately after burning and after the burned areas had been subjected to simulated rainfall at an intensity of 80 mm hr-1 for 1 hour. The cores were impregnated with resin from which thin sections were made and microscopically analyzed to determine the vertical distribution of organic material, ash, mineral soil and porosity. Burning consumed all of the surface litter and duff and formed a <1cm layer of black and gray ash above the mineral soil, indicating a moderate severity burn. The mean soil temperature in the upper 1 cm of the mineral soil was 70° C, and there was no detectable increase in water repellency. Rainfall simulations conducted before and after the fires indicated that burning reduced the infiltration capacity from a pre-fire mean of 87 mm hr-1 to a post-fire mean of 35 mm hr-1. Prior to burning the upper 1 cm of the soil was comprised of 41% non- ash organic material, 4% clastic material and 55% pore space. After burning the porosity in the upper 1 cm decreased to 36% and the solid component consisted primarily of black and white ash (34% and 7% respectively). The biggest decrease in porosity was in the upper 2 mm of the soil where porosity decreased from 62 to 22% with a corresponding increase in the proportion of ash. Following the rainfall simulations the black ash content of the upper 1 cm decreased to 21%, suggesting that ash particles were removed in the runoff. However the remaining ash particles became noticeably more aligned parallel to the soil surface. Our observations indicate that ash may contribute to reduced infiltration after fire in two ways: 1) by filling pore space, and 2) by orienting parallel to the soil surface, so creating a thin water repellent organic layer in the upper few millimeters of the soil.