Abstract
Prescribed fire is an important tool for fuel reduction, the control of competing vegetation, and forest restoration. The accumulated fuels associated with historical fire exclusion can cause undesirably high tree mortality rates following prescribed fires and wildfires. This is especially true for sugar pine (Pinus lambertiana Douglas), which is already negatively affected by the introduced pathogen white pine blister rust (Cronartium ribicola J.C. Fisch. ex Rabenh). We tested the efficacy of raking away fuels around the base of sugar pine to reduce mortality following prescribed fire in Sequoia and Kings Canyon national parks, California, USA. This study was conducted in three prescribed fires and included 457 trees, half of which had the fuels around their bases raked away to mineral soil to 0.5 m away from the stem. Fire effects were assessed and tree mortality was recorded for three years after prescribed fires. Overall, raking had no detectable effect on mortality: raked trees averaged 30% mortality compared to 36% for unraked trees. There was a significant effect, however, between the interaction of raking and average pre-treatment forest floor fuel depth: the predicted probability of survival of a 50 cm dbh tree was 0.94 vs. 0.96 when average pre-treatment fuel depth was 0 cm for a raked and unraked tree, respectively. When average pre-treatment forest floor fuel depth was 30 cm, the predicted probability of survival for a raked 50 cm dbh tree was 0.60 compared to only 0.07 for an unraked tree. Raking did not affect mortality when fire intensity, measured as percent crown volume scorched, was very low (0% scorch) or very high (>80% scorch), but the raking treatment significantly increased the proportion of trees that survived by 9.6% for trees that burned under moderate fire intensity (1% to 80% scorch). Raking significantly reduced the likelihood of bole charring and bark beetle activity three years post fire. Fuel depth and anticipated fire intensity need to be accounted for to maximize the effectiveness of the treatments. Raking is an important management option to reduce tree mortality from prescribed fire, but is most effective under specific fuel and burning conditions.
Highlights
A century of fire exclusion has dramatically shifted forest structure and fuel availability so that many western US forests are more prone to severe fires than in the past (Keeley and Stephenson 2000, Agee and Skinner 2005, Westerling et al 2006, Littell et al 2009)
A common concern with prescribed fire, especially after long periods of fire exclusion, is that fire behavior will be outside the historical range due to high fuel loads and changes to forest structure that have occurred during the previous period of fire exclusion (McHugh and Kolb 2003, Varner et al 2005, Kolb et al 2007)
Among western US conifers, sugar pine (Pinus lambertiana Douglas) is a species that has experienced an apparent shift in health due to the change in forest structure and dynamics relating to fire suppression and the invasive pathogen, white pine blister rust
Summary
The effecTs of Raking on sugaR Pine MoRTaliTy following PRescRibed fiRe in sequoia and kings canyon naTional PaRks, califoRnia, usa Jonathan C.B. Raking was first mentioned as a way to counter undesirably high rates of mortality that occurred following prescribed fires in ponderosa pine forests, especially for large trees (Thomas and Agee 1986, Sackett et al 1996). In this study we tested whether raking away fuels from the base of sugar pine trees could be an effective management approach for reducing mortality following prescribed fire and, if so, under what conditions raking was most effective. This study examined over 450 trees across three sites within Sequoia and Kings Canyon national parks, California, USA These parks are ideal locations in which to examine these questions because of their long history of prescribed burning, and because the sites contain large numbers of sugar pine across a wide range of sizes and ages, i.e., old-growth forest conditions
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