Effects of Climate and Fuels Management on Wildfire Occurrence, Size, Severity and Emissions in the Sierra Nevada

Abstract:

Annual wildfire burned area in federally managed Sierra Nevada forests has increased by more than 10,000 ha per decade since the early 1970s. At the same time, recent years have seen some extremely large fires compared to the historical record, with significant areas of moderate to high severity fire (e.g., McNally 2002, Rim 2013, King 2014 fires).

Changes to fuels and fire regimes due to fire suppression and land use, as well as warming temperatures and the occurrence of drought, are thought to be significant factors contributing to increased risks of large, severe fires in Sierra Nevada forests.

Over 70% of the vegetated area in federally managed forests in the Sierra Nevada is classified as having altered fuels and fire regimes, while average annual temperature in the Sierra Nevada has been above the long term mean for all but four years in the past two decades.

As climate is expected to continue warming for decades to come, we explored fuels management scenarios as the primary tools available to modify risks of large, severe wildfires.

We developed experimental statistical models of fire occurrence, fire size, and high severity burned area, to explore the interaction between climate and altered fuels conditions. These models were applied to historical climate conditions, a sample of future climate projections, and to both current fuels conditions and a range of scenarios for fuels treatments. Emissions from wildfires were estimated using the Fire Inventory from the National Center for Atmospheric Research.

Our models project that average annual burned area in the Sierra Nevada will more than double by mid-century. Similarly, particulate and other pollution emissions from Sierra Nevada wildfires are projected to more than double, even if future fire severity does not change. Fuels treatment scenarios significantly reduced simulated future burned area and emissions below untreated projections.

High severity burned area responded to both climate and fuels treatments. A sensitivity analysis indicated that in areas where the fraction of highly altered fuels is high, successfully restoring fuels to prehistoric conditions could more than compensate for expected climate change effects on fire severity by mid-century.