Soil erosion increases in response to fire mitigation in a piñon-juniper woodlad

Background/Question/Methods - Federally-funded fuels reduction programs are removing woody species from millions of acres of western forests and woodlands. While benefits for reducing catastrophic wildland fire risk are well understood, the impacts on soil erosion are virtually unknown, particularly for less resilient, semi-arid ecosystems. In this study we examined soil erosion responses to a range of fuels reduction treatments in a piñon-juniper woodland ecosystem in Southeastern Utah. We quantified wind and water erosion with wind aspirated dust samplers (BSNEs) and 1 m silt fences in 1 common mechanical and two prescribed fire fuels reduction treatments. In the mastication treatment, a mechanical chopper and wood mulcher were used to thin the tree canopy. In the hand thin + pile burn treatments, piñon-juniper overstory was thinned and slash was placed in piles across the landscape. Piles were left to dry and subsequently burned. A third treatment, lop, scatter and burn, trees were cut and slash was scattered across the landscape to provide a continuous fuel layer for a broadcast burn.

Results/Conclusions -Prescribed fire was shown to result in 40-100 fold increases in wind borne sediment transport as compared to the untreated control and masticated sites 6 months after treatment. After one year, wind borne material was still elevated in the prescribe fire sites but had declined to nearly pre-fire levels. A similar pattern was observed in soils captured by silt fences. The prescribed fire treatments resulted in 10-40X higher sediment accumulation in silt fences relative to the control and masticated treatments within the first six months after treatment. Soil accumulation within the silt fences appeared to decline as vegetation cover increased during the spring and summer months and increased again during the fall (October). Our results suggest that the use of prescribed fire may result in significant soil loss in the year immediately following treatment, whereas mastication strategies that provide a protective mulch layer and less overall soil surface disturbance had similar soil losses to the untreated control site. As land managers embark on fuels reduction project on thousands of acres of public lands, there is a clear need to evaluate the potentially negative effects of these actions on ecological processes and seek to mitigate these effects. Taken together, our results suggest that mastication should be the preferred fuels reduction method when there are concerns related to soil and site stability and the use of prescribed fire should be implemented with caution.