Strength and persistence of fire-induced soil hydrophobicity under ponderosa and lodgepole pine, Colorado Front Range

Fire-induced soil hydrophobicity is presumed to be a primary cause of the o bserved post-fire increases in runoff and erosion from forested watersheds in the Colorado Front Range, but the presence and persistence of hydrophobi c conditions has not been rigorously evaluated. Hence the goals of this stu dy were to: (1) assess natural and fire-induced soil hydrophobicity in the Colorado Front Range, and (2) determine the effect of burn severity, soil t exture, vegetation type, soil moisture, and time since burning on soil hydr ophobicity. Five wild and prescribed fires ranging in age from 0 to 22 months were stud ied. Each fire had four study sites in ponderosa pine forest, that had been burned at high, moderate, or low severity, and three sites were in unburne d areas. Additional sites were established in lodgepole pine stands and an area with unusually coarse-textured soils. At each site the soil hydrophobi city was assessed in two pits using the water drop penetration time (WDPT) and the critical surface tension (CST). Measurements were made at the miner al soil surface and depths of 3, 6, 9, 12, 15, and 18 cm. In sites burned at moderate or high severity the soils were often strongly hydrophobic at 0. 3, and 6 cm. Unburned sites or sites burned at low severi ty were typically hydrophobic only at the surface. Although soil hydrophobi city generally strengthened with increasing burn severity, statistically si gnificant differences in soil hydrophobicity were difficult to detect becau se of the high variability within and between sites. Hydrophobicity also in creased with increasing percent sand and was not present when soil moisture s exceeded 12-25%. There were no significant differences in soil hydrophobi city between ponderosa and lodgepole pine stands, regardless of burn severi ty. Repeat measurements on one fire suggest a weakening of fire-induced soil hy drophobicity after 3 months. Comparisons between fires suggest that fire-in duced soil hydrophobicity persists for at least 22 months. Overall, CST val ues were more consistent and more highly correlated with the independent va riables than the WDPT, and the CST is recommended for assessing soil hydrop hobicity rather than the more commonly used WDPT. Copyright (C) 2001 John W iley & Sons, Ltd.