Many low to mid elevation forests in the Sierra Nevada of California, as in much of the southwestern United States, experienced regular understory fire prior to European settlement.  Post-settlement, anthropogenic fire suppression and harvesting practices have caused increases in tree density and surface fuel loads, heightening the risk of severe stand-consuming wildfire.  Changes in temperature and precipitation from climate change may further increase fire frequency and intensity in these forests.  To move toward restoring fire in these systems, and to reduce risk at the wildland-urban interface, forest-thinning treatments have been implemented across several US National Forests in California, involving a combination of mechanical and hand thinning of small trees, and prescribed burning to reduce standing biomass and post-harvest surface fuels.  During 2008, at least 5 wildfires in the Sierra Nevada burned into these fuel treatment zones.  We established 1-3 transects that crossed the treatment boundaries within each of these 5 burned sites, encompassing a range of treatment protocols, forest types, and physical topography.  We measured indices of fire severity along these transects, as well as tree mortality and understory species diversity and abundance.

Results/Conclusions

We show that fuel reduction treatments have strong effects on fire behavior and on ecological dynamics in forest communities following wildfire.  At all 5 sites, treatments significantly reduced the height of trunk char (caused by surface fire) by an average of 6.36 meters, and significantly reduced crown torch (the extent of canopy foliage consumed by crown fire) by an average of 43.6%. Furthermore, at all 5 sites, areas treated for fuel reduction exhibited significant reduction in adult tree mortality in response to fire.  The average canopy mortality was 83.1% in untreated areas, compared to 23.0% in treated areas.  Other responses to fuels treament varied by site, but treated sites generally had increased litter cover the year following the fire, which is important for post-fire soil retention.  At some sites, understory plant diversity was higher in treated areas than in untreated areas after fire. Further, several untreated areas exhibited significantly higher shrub seedling recruitment than treated areas, which could slow tree growth in these areas, and potentially facilitate a transition to shrub-dominated communities with dramatically different fire regimes.