Effectiveness and longevity of fuel treatments in coniferous forests across California

Authors

Nicole M. Vaillant, USDA Forest ServiceFollow
Erin Noonan-Wright, USDA Forest Service
Scott Dailey, USDA Forest Service
Carol Ewell, USDA Forest Service
Alicia Reiner, USDA Forest Service

Date of this Version

2013

Document Type

Article

Citation

Final Report, Project ID: 09-1-01-1

Comments

US government work.

Abstract

Longevity of fuel treatment effectiveness to alter potential fire behavior is a critical question for managers preparing plans for fuel hazard reduction, prescribed burning, fire management, forest thinning, and other land management activities. Results from this study will help to reduce uncertainty associated with plan prioritization and maintenance activities. From 2001 to 2006, permanent plots were established in areas planned for hazardous fuel reduction treatments across 14 National Forests in California. Treatments included prescribed fire and mechanical methods (i.e., thinning of various sizes and intensities followed by a surface fuel treatment). After treatment, plots were re-measured at various intervals up to 10 years post-treatment. Very few empirically based studies exist with data beyond the first couple of years past treatment, and none span the breadth of California’s coniferous forests. With the data gathered, this research aimed to meet three main objectives: Objective 1) Determine the length of time that fuel treatments are effective at maintaining goals of reduced fire behavior, by a) measuring effects of treatments on canopy characteristics and surface fuel loads over time, and b) modeling potential fire behavior with custom fuel models. Objective 2) Quantify the uncertainty associated with the use of standard and custom fuel models. Objective 3) Assess prescribed fire effects on carbon stocks and validate modeled outputs. Results have shown initial reductions in surface fuels from prescribed fire treatments recover to pre-treatment levels by 10 yr post-treatment. Mechanical treatments continue to have variable effects on surface fuels. With the exception of mechanical treatments in red fir, both treatment types resulted in increased live understory vegetation by 8 yr post-treatment relative to pre-treatment. Mechanical treatment effects on stand structure remains fairly consistent through 8 yr post-treatment. Fire-induced delayed mortality contributes to slight decreases in canopy cover and canopy bulk density over time. For both treatment types, overall canopy base height decreases in later years due to in-growth of smaller trees, but it remains higher than pre-treatment. The changes in fuel loads and stand structure are reflected in fire behavior simulations via custom fuel modeling. Surface fire flame lengths were initially reduced as a result of prescribed fire, but by 10 yr post-treatment they exceeded the pre-treatment lengths. Though a low proportion of type of fire, initial reductions in potential crown fire returned to pre-treatment levels by 8 yr post-treatment; passive crown fire remained reduced relative to pre-treatment for the duration. Mechanical treatments showed variable and minimal effects on surface fire flame length over time; however the incidence of active crown fire was nearly halved from this treatment for the duration. The Fire and Fuels Extension to the Forest Vegetation Simulator (FFE-FVS) was used to model potential fire behavior for plots treated with prescribed fire to determine the differences in modeled fire behavior using standard and custom fuel models. In general predicted fire behavior from custom versus standard fuel models were similar with mean surface fire flame lengths slightly higher using standard fuel models for all time steps until the 8 yr post treatment. Similarly, custom fuel models predicted a higher instance of surface fire than standard fuel models with the exception of 8 yr post-treatment. To better understand the impact of prescribed fire on carbon stocks, we estimated aboveground and belowground (roots) carbon stocks using field measurements in FFE-FVS, and simulated wildfire emissions, before treatment and up to 8 yr post-prescribed fire. Prescribed fire treatments reduced total carbon by 13%, with the largest reduction in the forest floor (litter and duff) pool and the smallest the live tree pool. Combined carbon recovery and reduced wildfire emissions allowed the initial carbon source from wildfire and treatment to become a sink by 8 yr post-treatment relative to pre-treatment if both were to burn in a wildfire. In a comparison of field-derived versus FFE-FVS simulated carbon stocks, we found the total, tree, and belowground live carbon pools to be highly correlated. However, the variability within the other carbon pools compared was high (up to 212%).