Duration of fuels reduction following prescribed fire in coniferous forests of U.S. national parks in California and the Colorado Plateau

Abstract Prescribed fireis a widely used forest management tool, yet the long-term effectiveness of prescribed firein reducing fuels and firehazards in many vegetation types is not well documented. We assessed the magnitude and duration of reductions in surface fuels and modeled firehazards in coniferous forests across nine U.S. national parks in California and the Colorado Plateau. We used observations from a prescribed fireeffects monitoring program that feature standard forest and surface fuels inventories conducted pre- fire, immediately following an initial (first-entry) prescribed fireand at varying intervals up to >20 years post- fire. A subset of these plots was subjected to prescribed fireagain (second-entry) with continued monitoring. Prescribed fireeffects were highly variable among plots, but we found on average first-entry firesresulted in a significant post- firereduction in surface fuels, with litter and duff fuels not returning to pre- firelevels over the length of our observations. Fine and coarse woody fuels often took a decade or longer to return to pre- firelevels. For second-entry fireswe found continued fuels reductions, without strong evidence of fuel loads returning to levels observed immediately prior to second-entry fire. Following both first- and second-entry firethere were increases in estimated canopy base heights, along with reductions in estimated canopy bulk density and modeled flame lengths. We did not find evidence of return to pre- fireconditions during our observation intervals for these measures of firehazard. Our results show that prescribed firecan be a valuable tool to reduce firehazards and, depending on forest conditions and the measurement used, reductions in firehazard can last for decades. Second-entry prescribed fireappeared to reinforce the reduction in fuels and firehazard from first-entry fires.