Fuel accumulation and forest structure change following hazardous fuel reduction treatments throughout California

Abstract

Altered fuel conditions coupled with changing climate have disrupted fire regimes of forests historically characterised by high-frequency and low-to-moderate-severity fire. Managers use fuel treatments to abate undesirable fire behaviour and effects. Short-term effectiveness of fuel treatments to alter fire behaviour and effects is well documented; however, long-term effectiveness is not well known. We evaluated surface fuel load, vegetation cover and forest structure before and after mechanical and fire-only treatments over 8 years across 11 National Forests in California. Eight years post treatment, total surface fuel load returned to 67 to 79% and 55 to 103% of pretreatment levels following fire-only and mechanical treatments respectively. Herbaceous or shrub cover exceeded pretreatment levels two-thirds of the time 8 years after treatment. Fire-only treatments warranted re-entry at 8 years post treatment owing to the accumulation of live and dead fuels and minimal impact on canopy bulk density. In general, mechanical treatments were more effective at reducing canopy bulk density and initially increasing canopy base height than prescribed fire. However, elevated surface fuel loads, canopy base height reductions in later years and lack of restoration of fire as an ecological process suggest that including prescribed fire would be beneficial.

Long-Term Effects of Burn Season and Frequency on Ponderosa Pine Forest Fuels and Seedlings

“…But the differences we did document for all but 1000 hr fuels following reburn one had all disappeared following reburn three. It appears that, for the control treatments, fuel loads are being reduced through decay, which is similar to results from other studies with thinning-only treatments (Youngblood et al 2008, Vaillant et al 2015, while fuel loads are stable (recruitment is equal to consumption) in the burn treatments, which resulted in the lack of treatment differences by the end of the study period.…”

“…Interestingly, our spring 5 yr treatment ended up with the lowest woody fuels load across all four fuel sizes, and the fall 5 yr treatment tended to have higher fuel load (except for 100 hr fuels), although these differences were not always significant. This result is somewhat surprising given that others report initial entry fall burns consuming more fuel than spring burns (Knapp et al 2005, Fettig et al 2010, and the general assumption that shorter between-fire intervals prevent fuel buildup (Vaillant et al 2015). The few differences we document for woody fuels in 2014 may be partly due to the highly variable nature of woody fuels within the stands, plus the difficulty and expense in adequately replicating a long-term field study such as this.…”

“…Fuel reduction is a key objective driving the use of thinning and prescribed fire in western US ponderosa pine forest restoration, yet there are few studies documenting post-treatment trends in forest floor and surface fuel loads (Keifer et al 2006, Chiono et al 2012, Vaillant et al 2015, and even fewer examining effects after repeated burns (Webster andHalpern 2010, Higgins et al 2015).…”

“…Limited information about treatment longevity in ponderosa pine forests suggests that fuels can accumulate to near pre-treatment levels within a few years (Keifer et al 2006, Chiono et al 2012, and treatments can promote abundant conifer regeneration and survival (Bonnet et al 2005, Zald et al 2008. Thus periodic reburning through prescribed fire or managed wildfire is necessary to retain resilient conditions in which wildfire can occur without severely damaging consequences (Agee 1996a, Mc-Candliss 2002, Reinhardt et al 2008, Vaillant et al 2015.…”

“…The overall impact on fuel loads of varying fire return frequency is not well documented. More frequent burning is often assumed to reduce fuel loads more effectively; however, longer fire return intervals allow more fuel buildup (Keifer et al 2006, Chiono et al 2012, Vaillant et al 2015, likely resulting in greater consumption when burned. Mean fire return intervals (MFRI) for ponderosa pine forests in the western US vary from 3 yr to 43 yr, depending on climate, vegetation type, and topographic and geologic features that either restrict or facilitate fire spread (Stephens and Moghaddas 2005).…”