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TECHNICAL REPORTS

Surface Water Quality

Effects of Mechanical Harvest plus Chipping and Prescribed Fire on Sierran Runoff Water Quality

^a recent graduates from the MS program in Hydrologic Sci
^b recent graduate from the MS program in Nat. Resources and Environ. Sci
^c faculty in the Dep. of Nat. Resources and Environ. Sci., College of Agric., Biotechnology, and Nat. Resources, Univ. of Nevada, Reno, 1000 Valley Rd., Reno, NV. 89512. A contribution of the Dep. of Nat. Resources and Environ. Sci., Research supported in part by Nevada Agric. Experiment Station

^* Corresponding author (wilymalr{at}cabnr.unr.edu).

Received for publication September 14, 2007. Fire suppression in Sierran ecosystems creates a substantial wildfire hazard and may exacerbate nutrient inputs into Lake Tahoe by allowing the buildup of O horizon material, which serves as a source for high N and P concentrations in runoff water. The purpose of this study was to evaluate the effects of biomass reduction using cut-to-length mechanical harvest followed by chipping and controlled burning on surface runoff volume and water quality. Based on previous findings regarding N and P leaching flux and soil solution concentrations, we hypothesized that controlled burning and/or mechanical harvest with residue chipping does not increase inorganic N, P, and S concentrations in overland flow. Runoff, snowmelt, and rainfall were collected, volume measurements were taken, and samples were analyzed for NO₃–N, NH₄–N, PO₄–P, and SO₄. Runoff volume, season, and year were identified as important parameters influencing overland flow nutrient concentrations and loads. Higher nutrient concentrations were commonly associated with summer rather than winter runoff, but the opposite was true for nutrient loads due to the higher runoff volumes. Treatment (unharvested, harvested, unburned, burned) effect was a strong predictor for discharge loads of NO₃–N and SO₄ but was a weak predictor for PO₄–P. Discharge loads of NO₃–N and SO₄ were greater for the unburned harvested and the burned unharvested treatments than for the unburned, unharvested control sites or the burned and harvested combined treatment. Although mechanical harvest and/or controlled burning had a small initial impact on increased nutrient loading, the effects were minimal compared with background levels. Hence, these management practices may have the potential to improve forest health without the danger of large-magnitude nutrient mobilization and degradation of runoff water quality found with wildfire.

Abbreviations: AICc, Akaike Information Criterion • B, burned • H, harvested • O, organic • RV, runoff volume • S, season • T, treatment type • UB, unburned • UBUH, unburned unharvested • UH, unharvested • Y, year