Ken R. Hubbert

Prefire and postfire erosion of soil nutrients within a chaparral watershed

Prescribed burns are an effective and increasingly popular strategy for inhibiting wildfires. The goal of this study was to characterize soil nutrient loss after a prescribed fire within a chaparral watershed in southern California. The study compared hillslope sediments for approximately 1 year before the fire with those during the year after the fire. Samples were collected in 11 sediment traps located randomly along fall line transects throughout a steep, 0.65-ha watershed (∼65% average slope). Concentrations of soluble ammonium nitrogen, nitrate nitrogen, phosphate phosphorus, Ca, Mg, K, and Na, exchangeable ammonium nitrogen, and total C and N were analyzed. Sediment production averaged approximately 1600 g per sediment trap for the entire year before the fire but increased to approximately 10,700 g for the year after the fire. The average sedimentation rates for the less than 2-mm fraction doubled from a prefire rate of 2.7 to 7.5 g d−1 after the fire and increased to 86 g d−1 after the first major rain event after the fire. The greatest concentrations of soluble and exchangeable base cations (Ca, Mg, K, Na) in the eroded sediment were measured at the first sampling date after the fire. Input and microbial mineralization of fresh organic matter approximately 4 months after the fire is believed to be the source of increases in the concentrations of soluble nitrate nitrogen and phosphate phosphorus and exchangeable ammonium nitrogen. Our findings indicate that prescribed fire may cause less loss of N and P than wildfires.

Soil heating during burning of forest slash piles and wood piles

Pile burning of conifer slash is a common fuel reduction practice in forests of the western United States that has a direct, yet poorly quantified effect on soil heating. To address this knowledge gap, we measured the heat pulse beneath hand-built piles ranging widely in fuel composition and pile size in sandy-textured soils of the Lake Tahoe Basin. The soil heat pulse depended primarily on fuel composition, not on pile size. Burn piles dominated by large wood produced extreme temperatures in soil profile, with lethal heating lasting up to 3 days. In contrast, the heat pulse was moderate beneath piles containing a mixture of fuel sizes. Considerable spatial variability was noted, as soil temperatures were generally greatest near pile centres and decline sharply toward the pile edges. Also, saturating pile burns with water 8 h after ignition (‘mopping up’) effectively quenched the soil heat pulse while allowing near-complete fuel consumption. The findings suggest that burning of hand piles will not result in extreme or extensive soil heating except for uncommon conditions when piles are dominated by large wood and occupy a high percentage of the ground surface.

Effects of hydromulch on post-fire erosion and plant recovery in chaparral shrublands of southern California

Following the Cedar Fire (one of seven large wildfires that burned in southern California during the autumn of 2003), aerial hydromulch was applied at 50 and 100% cover to reduce hillslope erosion in chaparral shrublands. Our objectives were to determine the effectiveness of hydromulch in preventing erosion, and to see if plant recovery was hindered by treatment. We installed 54 silt fences to measure sediment production. Five 1-m2 grids were placed behind each fence to measure plant recovery. Hydromulch was effective in reducing erosion immediately after the fire; however, its benefits appeared to be limited to the first 2–4 months following fire, raising doubts as to its overall cost-effectiveness. The rapid breakdown of the hydromulch during the first 6 months after the wildfire provided little hillslope protection during the above-average October 2004 storm events. During the October events, both rainfall amount and storm intensity played a role in the magnitude of sediment production. Hydromulch did not affect post-fire plant recovery, with plant cover measuring >60% at all sites less than 2 years following the wildfire. Accelerated growth of chamise and forbs was likely due to hydromulch prolonging soil moisture retention. Large accumulations of dead litter following die-off of the herbaceous species could increase dry fuels, thus promoting wildfire and therefore shortening the fire return interval.