THE EFFECT OF LITTER AND DUFF CONSUMPTION AND SURFACE SEALING BY ASH ON POST-FIRE RUNOFF AND EROSION

Increases in runoff and erosion after forest fires have been attributed to: 1) the development of water repellent soils; 2) consumption of the litter and duff layers; and, 3) sealing of the soil surface by ash. While there has been considerable work on the influence of water repellent soils on post-fire runoff, less is known about the hydrologic significance of litter and duff consumption and surface sealing by ash. In an effort to better understand the effect of these two processes on post-fire runoff we conducted three controlled pile burns in each of two forest habitat types in western Montana – a wetter site dominated by Lodgepole pine (Pinus contorta) with scattered Douglas fir (Pseudotoga menziesii), sandy loam soils and a mean of 71% ground cover, and a drier site dominated by Ponderosa pine (Pinus Ponderosa) with silt loam soils and a mean vegetative cover of just 13% – and used a rainfall simulator to measure pre- and post-burn infiltration and runoff in replicated 0.5 m² plots within each burned area. We also conducted rainfall simulations on adjacent unburned plots before and after mechanical removal of the litter and duff layers.

The controlled burns, which were conducted with a fuel load of 90 Mg ha^-1, heated the uppermost 1 cm of the soil in the plots to a mean of just 70°C, and there was no detectable increase in water repellency. All of the surface litter and the uppermost duff layer in the plots were consumed, leaving a <1cm layer of ash and char on the surface. Burning reduced the infiltration capacity of the P. contorta sites by 61%, from a pre-fire mean of 77 mm.hr^-1 to a post-fire mean of 30 mm.hr^-1. In contrast, the pre- and post-burn mean infiltration capacities on the P. ponderosa sites (25 mm.hr^-1 and 22 mm.hr^-1 respectively) were not significantly different. Mechanical removal of the litter and duff layers reduced the infiltration capacity in both the P. contorta and P. Ponderosa sites by a similar amount to that caused by burning. We attribute the larger reduction in infiltration on the P. contorta sites to the greater loss of ground cover, and clogging of the larger pores in the sandy loam soils by ash.