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Joint Fire Science / Red Mountain Mastication Study

Project Overview

The Greenhorn Ranger District of the Sequoia National Forest and AMSET collaborated to assess effectiveness of mastication and mastication/prescribed burn fuel treatments. The purpose of this study was to provide land managers with information on fuels, potential fire behavior and tree mortality associated with mastication and mastication/prescribed burn treatments.

Four treatments were compared: mastication alone, mastication followed by controlled burn, mastication followed by pulling material away from tree boles, then followed by controlled burn, and controls in which no action is taken. A formal experimental design was applied with replicated random blocks. Fuel conditions were measured before and after mastication and burning. Masticated fuel was quantified by weighing dried fuel samples. During prescribed burning, fire behavior was measured including flamelength, fire type, and surface and subsurface soil temperatures.

Mastication treatment alone showed increases in most surface fuel loadings and decreases in canopy fuel loads. Masticated treatment in conjunction with prescribed burning reduced both surface and canopy fuel loads. Rates of spread and flame lengths as predicted with FMAPlus using 90th and 97th percentile weather and post-treatment fuel conditions for masticated plots were higher than masticated/burned plots. Torching and crowning indices from FMAPlus indicated that higher winds would be necessary to promote torching in plots treated with prescribed fire and the probability of active crown fire was reduced slightly for all treated plots. Tree mortality, as measured in the field at the end of the first growing season post-treatment was 38% for masticated/burn plots and 28% for masticated/pull-back/burn treatments.

Timeline

First Year
(Summer/Fall)

  1. Set up experimental plots.
  2. Collected pre-treatment data.
  3. Implemented mastication treatments on experimental plots.

Second Year
(Summer/Fall 2006-Winter 2006/2007)

  1. Collected post-mastication data on masticated plots.
  2. Gathered fuel samples to aid in masticated fuel model definition.

Third Year
(Fall 2007)

  1. Completed prescribed fire treatments.
  2. Gathered fire behavior data.

Fourth Year
(Summer 2008-Winter 2008/2009)

  1. Collected final post-treatment data on all plots.
  2. Currently analyzing data and preparing reports and presentations

Results

Surface and ground fuel loads

Depth-to-weight relationships for masticated fuel, litter and duff created from on-site samples were fairly strong and were used in calculating fuel loadings from depths (R2 = 0.78). A linear regression through the origin was used to represent the fuel depth to weight relationship. Masticated fuels made up a large proportion of the surface fuel loads in treated plots post-mastication. Mean masticated fuel loads decreased from 25.9 and 35.0 to 5.3 Mg ha-1 and 2.6 Mg ha-1 in masticated plus fire and masticated/pull-back/fire plots, respectively after the burn. When fuel bed bulk densities were computed with all surface fuels other than duff, bulk densities increased after mastication and decreased from post-mastication (2006) to post-burn (2008) for masticated/pull-back/burned plots only. Non-masticated, downed woody fuel showed few significant changes before and after mastication and burn treatments. Litter fuel loads increased slightly in masticated and control plots and decreased slightly in units treated with fire.

Canopy fuel loads

Mean tree height generally increased with treatment whereas trees per hectare, and basal area decreased with treatment. Basal area decreased after treatments incorporating burning, but not mastication-only plots. Canopy base height increased and canopy bulk density decreased with treatment.

Prescribed fire behavior observations

Higher temperatures were found at the top of the soil surface and fuel surface compared to temperatures 4 cm below the soil surface.

Potential wildfire behavior

Summaries of fire behavior output from FMAPlus for 90th and 97th percentile weather conditions (approximating wildfire conditions) given post-treatment fuels showed mean predicted rate of spread was higher in the masticated treatment than control or treatments incorporating prescribed burning. Predicted flame length was higher in masticated plots than other treatments or control plots. Torching and crowning indices, defined as the 20-ft windspeed at which torching is expected to initiate, or active crown fire is possible, showed some differences between treatments. Plots which received fire treatments had much higher torching indices than masticated or control plots, indicating that higher winds would be necessary to produce torching given post-treatment fuel conditions. The crowning index was slightly higher for treated than untreated plots, meaning that the ability of post-treatment stands to sustain active crown fire is faintly lower than untreated plots.

Tree mortality

First year post-burn treatment mean percent scorch and torch as measured in the field after the prescribed fire treatments were analyzed for the two treatments involving prescribed fire. Percent scorch means for plots treated with prescribed fire were all greater than 50%. Mortality, as measured in the field the first year after burn treatment by the presence or absence of green needles or buds, was 38% in masticated/burned areas and 28% in masticated/burned plots where mastication was scraped back from boles before burning.

Table 1

Fuel characteristics incorporated into custom fuel models representing low, medium and high levels of masticated fuels.

Table 1
Low Medium High
1 Hour Loading (t/a) 0.22 0.45 0.67
10 Hour Loading (t/a) 5.8 10.1 19.7
100 Hour Loading (t/a) 8.3 15.0 30.5
Fuelbed Bulk Depth (cm) 15 18 24
Moisture of Extinction (%) 20 25 25