10 key questions and answers
Soil scientists Wally Miller and Dale Johnson, professors emeriti in the College of Agriculture, Biotechnology and Natural Resources at the University of Nevada, Reno, have studied the impact of wildfires in the Tahoe Basin for many years. Their study of the 2002 Gondola wildfire near the Nevada-California border at the south shore of Lake Tahoe has led to new soils management strategies in the basin.
With more than six decades of combined experience and extensive work in forestry, biogeochemical cycling and fire ecology, Miller and Johnson shared their expertise to help answer 10 of the most common questions about the role fire and forest management play in the runoff of nutrients into the upper watershed that feeds into Lake Tahoe, ultimately affecting water quality.
- How has the upper watershed been affected by historic fire management strategies?
Prior to the 20th century, wildfire was a natural part of Sierran ecosystems. It created forests that were less dense, structurally more diverse, had larger trees with less ladder fuels, sparse undergrowth and greatly reduced dead and decaying downed timber. These conditions have been altered by fire suppression over the last 100 years.
- What has changed in today’s forest landscape in the upper watershed?
The buildup of fuels from falling litter and plant life growing beneath the forest canopy has increased the potential for stand-replacing wildfires and resulted in the accumulation of much thicker, nutrient-rich organic residues that can release more available nutrients into runoff and percolating solutions that can reach Lake Tahoe.
- What are the potential effects of wildfire on nutrients in the forest floor?
Wildfire was found to induce an immediate mobilization of inorganic nitrogen and phosphorus, both of which can affect water quality. Although this effect seems to diminish rapidly over time, a wildfire followed by a high water year within the first season after the fire would likely have a much greater impact on water quality than a wildfire followed by a low runoff water year.
- What other fire management strategies are available?The most commonly applied alternative to total fire suppression is fuel reduction by either mechanical thinning, prescribed burning or a combination of both.
- What are the long-term forest health consequences of controlling post-wildfire brush? Eliminating post-wildfire brush may preclude nitrogen fixation, the natural process by which nitrogen is converted to forms plants can use, and thus not allow replenishment of nitrogen lost during the wildfire. However, leaving brush may preclude the re-establishment of forest vegetation for many decades, creating quite a dilemma.
- What are the long-term water quality consequences of controlling post-wildfire brush?
Thus far, we see no negative water quality effects of the most common nitrogen-fixing species, Ceanothus velutinus, a species of shrub with the common names of snowbrush, redroot and tobacco brush, native to western North America. Water quality effects should be minimal.
- What are the long-term forest health consequences of repeated prescribed burning as opposed to mechanical treatment?
Repeated burning is likely to cause greater nitrogen removal than mechanical thinning, perhaps causing reductions in regenerative growth.
- What are the long-term water quality consequences of repeated prescribed burning as opposed to mechanical treatment?
Repeated burning is likely to cause greater nitrogen removal than mechanical thinning, which may help improve water quality.
- What are the effects of combining mechanical thinning and prescribed burning on water quality?
Prescribed burning in conjunction with mechanical harvest may potentially improve runoff water quality by reducing nitrogen and phosphorus litter mass pools and improving the overall health of forest ecosystems without the danger of a stand-replacing wildfire.
- Is climate change important?Absolutely. The fate of these forest ecosystems in a changing climate will have a direct impact on forest health, fire hazard, biomass mitigation strategies and water quality.