Fuel treatment effectiveness in California yellow pine and mixed conifer forests

Abstract

We assessed the effectiveness of forest fuel thinning projects that explicitly removed surface and ladder fuels (all but one were combined mechanical and prescribed fire/pile burn prescriptions) in reducing fire severity and tree mortality in 12 forest fires that burned in eastern and southern California between 2005 and 2011. All treatments and fires occurred in yellow pine or mixed conifer forests, in a variety of landscape conditions. Most fires burned under warm, dry conditions, with moderate to high winds. With few exceptions, fire severity measures (bole char height, scorch and torch height, scorch and torch percentage) and tree mortality were much lower in forest stands treated for fuels than in neighboring untreated stands. Fire-tolerant species like Pinus jeffreyi and Pinus ponderosa exhibited much higher postfire survivorship than fire-intolerant species like Abies concolor. Among variables related to fire weather, fuel loading, and treatment age, ten-hour fuel moisture was found to be a better predictor of tree survival in untreated stands than in treated stands, while fuel loading was a better predictor of survival in treated stands. We did not find an effect of treatment age, but our oldest treatments (nine years when burned) were below the mean pre-Euroamerican settlement fire return interval for these forest types. Within treatments, fire severity decreased with distance from the treatment boundary, and canopy fires were almost always reduced to surface fires within 70m of entering the treatment. In California yellow pine and mixed conifer forests, treatment prescriptions should allow for levels of fire-driven canopy tree mortality (c. 5–15%) that better mimic natural fires. Our results add significantly to the growing evidence that fuel treatments that include removal of surface and ladder fuels in these forest types are highly effective management tools for reducing fire severity and canopy tree mortality. In our opinion, quantitative assessments of fuel treatment effects on fire severity in frequent-fire forest types hardly merit further effort. Rather, we suggest that future work focus on documenting and comparing other ecological outcomes of fuel treatments in burned and unburned forest, such as effects on plant and animal diversity, soil conditions, and habitat heterogeneity.