United States Department of Agriculture

To Masticate or Not:

Useful Tips for Treating 

Forest, Woodland, and 

Shrubland Vegetation

Theresa Jain Pamela Sikkink

Robert Keefe John Byrne

Forest Service

Rocky Mountain

Research Station

General Technical 

Report

RMRS-GTR-381

November 2018
Jain, Theresa; Sikkink, Pamela; Keefe, Robert; Byrne, John. 2018. To masticate or not: Useful tips for treating 

forest, woodland, and shrubland vegetation. Gen. Tech. Rep. RMRS-GTR-381. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Research Station. 55 p.

Abstract

Forest managers use mastication to grind or shed vegetation to remove competition, prepare a site for natural or artificial 

regeneration, or release sapling-sized trees; or they use mastication to convert ladder fuels to surface fuels and enhance 

decomposition of biomass. However, determining the best mastication configuration within the context of management 

objectives and site limitations is challenging. This report synthesizes our current knowledge on mastication as a forest 

management tool. We found that excavators, skid steers, and tractors can all be carrier machines and different types of 

vertical and horizontal cutting heads exist that can be front-end mounted or boom mounted, each with its own advantages 

and disadvantages. We provide a summary on the ecological effects from mastication. We found that there were several 

studies on plant and soil impacts, but limited information on impacts to wildlife habitat. Although costs of mastication widely 

vary depending on machine size, the physical setting, size and configuration of pre-treatment biomass, and operator skill, 

mastication does have market and non-market benefits. Depending on the management objective, if mastication is an 

option, then a thorough site evaluation should consider slope, nonnative species invasions, vulnerability of soils to erode or 

compact, and treatment costs.

Keywords: fuel treatments, silvicultural methods, vegetation management, forest mulching, site preparation, 

precommercial thinning

Cover: Choosing a mastication method can often 

be a confusing. Managers may be taken down 

several different pathways to make a decision 

depending on their desired outcomes and 

financial limitations. Some of the most common 

pathways to masticate material include (from far 

left and clockwise): (1) a vertical drum on a skid 

steer (photograph by Mike Battaglia USFS); (2) 

a horizontal cutting head (photograph by Mike 

Battaglia USFS; (3) vertical head masticator 

(photograph by Mel Peterson, Director of Marketing 

and Training, Diamond Mowers Inc.); (4) horizontal 

drum cutting head (photograph by Dana Mitchel 

USFS); (5) chipper (photograph courtesy of 

Morbark LLC); and (6) dry mixed conifer forests 

(photograph by Jonathan Sandquist, USFS). Cover 

design by Pam Sikkink and Audrey Peterson, 

RMRS Missoula Fire Sciences Laboratory.

(1) (2

(3) (4)

(6)

(5)
All Rocky Mountain Research Station publications are published by U.S. Forest Service employees and are in the 

public domain and available at no cost. Even though U.S. Forest Service publications are not copyrighted, they 

are formatted according to U.S. Department of Agriculture standards and research findings and formatting cannot 

be altered in reprints. Altering content or formatting, including the cover and title page, is strictly prohibited.

Authors

Theresa Jain, Research Forester, Moscow Forestry Sciences Laboratory, Forest and Woodland Ecosystems Program, 

USDA Forest Service, Rocky Mountain Research Station, Moscow, Idaho.

Pamela Sikkink, Research Biologist, Moscow Forestry Sciences Laboratory, Forest and Woodland Ecosystems Program, 

USDA Forest Service, Rocky Mountain Research Station, Moscow, Idaho.

Robert Keefe, Assistant Professor of Forest Operations, Department of Forest, Rangeland, and Fire Sciences, College of 

Natural Resources, University of Idaho, Moscow, Idaho. 

John Byrne, Forester, Moscow Forestry Sciences Laboratory, Forest and Woodland Ecosystems Program, USDA Forest 

Service, Rocky Mountain Research Station, Moscow, Idaho.

Disclaimer: The use of trade names in this publication is for reader information and does not imply endorsement 

by the U.S. Department of Agriculture of any product or service. All images used with permission..
Contents

Introduction...................................................................................................................................................................1

Methods ........................................................................................................................................................................4

Statistical Analysis.....................................................................................................................................................5

Literature Synthesis........................................................................................................................................................5

Characteristics of Carrier Machines and Cutting Heads.............................................................................................5

Operator and Economic Factors..............................................................................................................................11

Ecosystem Response ...............................................................................................................................................14

Decision Trees for Machine Selection ..........................................................................................................................24

Decision Tree 1.......................................................................................................................................................24

Decision Tree 2.......................................................................................................................................................25

Decision Tree 3.......................................................................................................................................................26

Conclusion and Management Implications..................................................................................................................32

Acknowledgments .......................................................................................................................................................32

Literature Cited............................................................................................................................................................32

Appendix A—Summary of Key Results From Mastication Studies......................................................................................39

Introduction.................................................................................................................................................................39

References...................................................................................................................................................................47

Appendix B—Methods Used for Determining Sample Sites, Collecting Fine Fuels, and Calculating 

Particle Density and Surface Area of Masticated Fuels for the MASTIDON Study ..........................................................50

Site Descriptions..........................................................................................................................................................50

Field Sampling Protocols .............................................................................................................................................50

Laboratory Tasks ..........................................................................................................................................................52

Sorting ...................................................................................................................................................................52

Physical Measurements...........................................................................................................................................52

Particle Density of Individual Particles.....................................................................................................................53

Surface Area of Individual Particles .........................................................................................................................54

Literature Cited............................................................................................................................................................55

ii
USDA Forest Service RMRS-GTR-381. 2018 1

Introduction

Mastication grinds, shreds, or chops noncommercial sized trees or shrubs into 

small chunks or pieces. The method does not reduce biomass; rather, the operator 

creates these small pieces and places them in contact with the soil surface to decompose (McDaniel 2013) (fig. 1). Silviculturists use mastication to eliminate vegetation 

competition (understory vegetation, saplings, and pole-sized trees), to prepare a site for 

natural or artificial regeneration, or to weed or clean sites in noncommercial thins (Jain 

et al. 2012). Fuel managers use mastication to convert ladder fuels to surface fuels, to 

enhance decomposition of dead biomass, to make prescribed fire easier and more controllable, or to slow the rate of fire spread during wildfires to assist suppression efforts 

(McDaniel 2013; Rummer 2010).

Several scientists have investigated the ecological effects from mastication in forest, woodland, and shrubland ecosystems throughout the western United States, Canada, 

and Spain (table 1, Appendix A). Vegetation types where these studies occurred included dry and moist mixed conifer, loblolly pine (Pinus taeda L.), longleaf pine (Pinus 

palustris Mill.), slash pine (Pinus elliottii Engelm.), lodgepole pine (Pinus contorta

Douglas ex Loudon), spruce (Picea species), ponderosa pine (Pinus ponderosa Lawson 

& C. Lawson), pinyon-juniper (Pinus edulis-Juniperus species), oak (Quercus species), 

California chaparral woodlands, and gorse (Ulex species) shrublands. In these studies, 

investigators sometimes mention the machine used to masticate the biomass, but rarely 

do they provide details that guide implementation. Instead, the studies have quantified 

fuel loadings and distribution (e.g., Battaglia et al. 2010), soil chemical and physical 

properties (e.g., Busse et al. 2005), wildlife effects (e.g., Burnett et al. 2014), understory 

vegetation response (e.g., Fernandez et al. 2015), potential fire behavior (e.g., Kreye et 

al. 2014), and fuel bed characteristics (e.g., Keane et al. 2018) (table 1).

Figure 1—Mastication 

machine treating surface 

fuels and providing a 

fire break surrounding 

buildings (photograph 

by Dana Mitchell, USDA 

Forest Service, Southern 

Research Station).
2 USDA Forest Service RMRS-GTR-381. 2018

Table 1—Mastication studies that noted the machine type throughout the United States, Canada, and Spain between 2005 

and 2016.

Vegetation 

typea

Material masticated

Machinec life form Size (inch) Literature source b

- - - - - - - - - - - - - - - - - - - - - - - - - -Study Location: California- - - - - - - - - - - - - - - - - - - - - - - - - - -

Mixed conifer Trees, shrubs Unknown Machine-mounted HS Bradley et al. 2006

Chaparral Shrubs Unknown Various Brennan and Keeley 2015

Mixed conifer Trees, shrubs Unknown Not specified Burnett et al. 2014

Mixed conifer Trees Unknown Excavator w/ HS fixed teeth Hatchett et al. 2006

Mixed conifer Slash Unknown Chipper Johnson et al. 2014

Mixed conifer Trees <9 Not specified Kobziar et al. 2009

PP Conifers, hardwood <1 Excavator boom-mounted VS Reiner et al. 2009

Mixed conifer Trees, hardwoods 1 to 10 Excavator-mounted HS Stephens and Moghaddas 2005

Shrub Slash Unknown Excavator-mounted HS Vitorelo 2011

- - - - - - - - - - - - - - - - - - - - -Study Location: California and Colorado- - - - - - - - - - - - - - - - - - - - - -

P-J, PP Not described Unknown VS knife Hood and Wu 2006

- - - - - - - - - - - - - - - - - - - - -Study Location: California and Oregon- - - - - - - - - - - - - - - - - - - - - - -

Mixed 

hardwood Hardwoods, shrubs Unknown Excavator w/ VS, w/ HS, machine w/ VS knife Kane et al. 2009

- - - - - - - - - - - - - - - - - - - - - - - - - - -Study Location: Colorado- - - - - - - - - - - - - - - - - - - - - - - - - - -

P-J, PP, LP Not described Unknown HS hammers, VS knife Battaglia et al. 2010

P-J Trees, Shrubs Variable Machine mounted w/ 

swinging knives Gottfried and Overby 2011

PP Trees ≤6 Chipper Wolk and Rocca 2009

- - - - - - - - - - - - - - - - - - - - - - - - - - -Study Location: Florida- - - - - - - - - - - - - - - - - - - - - - - - - - - -

Longleaf pine,

slash pine

Shrubs, saw 

palmetto, trees <8 Excavator w/ HS Kreye 2012

- - - - - - - - - - - - - - - - - - - - - - - - - - -Study Location: Georgia- - - - - - - - - - - - - - - - - - - - - - - - - - - -

Loblolly pine, 

hardwoods Trees hardwoods; pines <8 Mulcher Brockway et al. 2009

- - - - - - - - - - - - - - - - - - - - - - - - -Study Location: South Carolina- - - - - - - - - - - - - - - - - - - - - - - - -

Loblolly pine Hardwoods, shrubs Down fuels Chipper Glitzenstein et al. 2016

Loblolly pine Beetle-killed trees Unknown Excavator w/ HS Stottlemyer et al. 2015

- - - - - - - - - - - - - - - - - - - - - - - - - - - -Study Location: Texas- - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Oak, juniper All vegetation <6 HS hammer Reemts and Cimprich 2014

- - - - - - - - - - - - - - - - - - - - - - - - - - - - -Study Location: Utah- - - - - - - - - - - - - - - - - - - - - - - - - - - - 

P-J Trees Unknown HS McIver et al. 2010

P-J Trees Unknown Machine w/ HS (brush cutter) Moss et al. 2012

P-J Unknown Unknown HS fixed teeth Roundy et al. 2014

- - - - - - - - - - - - - - - - - - - - - - - -Study Location: Alberta, Canada- - - - - - - - - - - - - - - - - - - - - - - - -

LP, spruce Understory trees, 

shrubs Unknown HS fixed teeth Schiks et al. 2015

- - - - - - - - - - - - - - - - - - - - - - - - -Study Location: Santander, Spain- - - - - - - - - - - - - - - - - - - - - - - - 

Gorse shrubland All vegetation Unknown Excavator w/ HS fixed teeth Fernandez and Vega 2016

a PP = ponderosa pine (Pinus ponderosa), P-J = pinyon pine (Pinus edulis)/juniper (Juniperus sp.), LP = lodgepole pine (Pinus contorta), 

longleaf pine (Pinus palustris), slash pine (Pinus elliottii), saw palmetto (Serenoa repens), loblolly pine (Pinus taeda), white fir (Abies 

concolor), oak (Quercus sp), spruce (Picea sp.), gorse (Ulex sp.).

b Size is at d.b.h. (diameter at breast height).

c VS = vertical shaft; HS = horizontal shaft.
USDA Forest Service RMRS-GTR-381. 2018 3

Experienced foresters and fire managers recognize that treatment execution influences the desired outcome. For example, fire practitioners can apply prescribed fire in 

an infinite number of ways, which can create a diversity of postfire outcomes. When 

fire practitioners design a fire plan, they identify the time of day to ignite, the range of 

weather conditions they want when they will be able to burn, the ignition pattern they 

will use, and sometimes even specific people to implement the prescribed fire. When a 

fire practitioner combines these factors, they know that their burn prescription will influence the fire behavior, which subsequently creates a desired postfire outcome (Jain et al. 

2012). Silviculturists combine treatment types, such as applying mechanical methods 

and prescribed fire, to create an outcome that favors a desired vegetation response. For 

instance, Jain et al. (2008) combined hand slashing followed by grapple piling followed 

by pile burning that created a mosaic of soil substrates and favored regeneration of multiple species. Similarly, executing mastication by altering the piece size, machine type, 

and cutting head—and even selecting the operator—can alter an outcome; however, 

limited information exists for managers concerning mastication implementation.

The variety of mastication configurations available today provides an opportunity to match the best equipment available with efficiency and safety to meet desired 

management objectives. The studies we found identified several references on machine 

configurations that can guide managers (Bennett and Fitzgerald 2008; Bolding et al. 

2006; Halbrook et al. 2006; Rummer 2010; Vitorelo et al. 2009; Windell and Bradshaw 

2000). For example, there are specific carrier machines, such as tractors or excavators, 

that carry the cutting head or other attachments; and cutting-head configurations that are 

better suited to specific physical settings (Jain et al. 2012). Some machines, such as carrier machines with rubber tracks, can minimize soil compaction (Schafer 2013; Windell 

and Bradshaw 2000). Alternatively, operators can limit the number of passes over any 

given area to limit compaction when they use a boom-mounted cutting head (Rummer 

2010). Boom-mounted cutting heads can reach trees from above, typically working 

downhill on slopes, and avoid the necessity for the machine to drive to each tree because of their reaching capabilities. Alternatively, small carrier machines with front-end 

mounted cutting heads work well in the wildland-urban interface because these machine 

configurations treat younger stands and shrubs around buildings (Coulter et al. 2002). 

This report provides information to land managers on the application and implementation of mastication as a fuels-treatment and a silvicultural method, and is organized 

using the following objectives.

• Objective 1: Synthesize the current literature to describe the characteristics and 

costs associated with mastication machines and cutting heads used to treat forest, 

woodland, or shrubland biomass.

• Objective 2: Summarize the current literature that documents the ecological effects 

from mastication on (a) vegetation, (b) soils, (c) insects, mammals, and birds, and 

(d) surface fuels <3 inches in diameter.

• Objective 3: Conduct a meta-analysis on selected studies that quantify fine-fuel 

amounts (woody material ≤3 inches in diameter) after mastication across a large 

geographic area to determine if post-mastication fine-fuel loads for different 

vegetation types vary.
4 USDA Forest Service RMRS-GTR-381. 2018

• Objective 4: Provide three decision trees and implementation criteria to aid managers 

in determining the type of treatment best suited to the project area and management 

objectives.

Methods

We used the scientific and technical literature to address objectives 1 and 2. To 

fulfill the first objective, we focused on informational sources that discussed different 

machines, cutting heads, and other related equipment. The industry is constantly changing, making it difficult to cite any one publication, manufacturer or source; however, we 

did find a website with a very large database of equipment related to mastication called 

the Forestry Mulcher Guide1 (Catalytic Response, LLC 2017). We found this to be a 

good internet source that summarizes machine attributes by manufacturer. This source 

provided detailed specifications across a wide range of machine sizes and types. We 

explicitly cite the Forestry Mulcher Guide as the source of information on the ranges in 

horsepower, widths, lengths, heights, and other carrier and cutting-head specifications.

To fulfill the second objective, we obtained information on ecological effects 

from studies conducted in mixed-conifer forests, woodlands, and shrublands across the 

United States, Canada, and Europe (table 1). The keywords that guided this literature 

search were mastication, mulching, slash busting, noncommercial fuel treatments, and 

hazardous fuel reduction methods. We also investigated several final reports that summarized the ecological effects from mastication conducted for the Joint Fire Science 

Program (https://www.firescience.gov/). Appendix A summarizes the literature (reference, location, and general results) we used in the synthesis.

The third objective focused on the amount of fine woody fuels ≤3 inches in diameter, classified as 1-hour (0 to 0.25 inch), 10-hour (0.25 to 1.0 inch), and 100-hour (1.0 

to 3.0 inches) produced by masticators. We hypothesized that mastication always results 

in the same distribution and amount of fine fuels regardless of vegetation type. We obtained data from selected papers that studied multiple sites or forest types and reported 

detailed data on fine fuels. Battaglia et al. (2010) and Kane et al. (2009) contained these 

type of data. We also added another data source from Keane et al. (2018). These data, in 

addition to fine-fuel amounts across different sites, also provided data on the variation in 

particle surface area of masticated pieces created by different cutting heads. Appendix 

B provides detailed descriptions of the field and laboratory methods from Keane et al. 

(2018).

To meet the fourth objective, we developed three decision trees. The first decision 

tree identified the critical site characteristic that influences whether to use mechanical 

treatments or prescribed fire at a site. The second decision tree provides parameters 

that guide treatment options, including mastication, prescribed fire, or other mechanical 

treatments. The third decision tree focused only on factors associated with mastication. 

It includes information to help decide the general size and type of machine and cutting 

1 Because providing a specific equipment guide for mastication machines for purchase or evaluation was not the objective of this 

synthesis, we used information from this site to provide the reader with a range and diversity of machines that are currently 

available. As of August 28, 2017, this website and domain no longer exist. However, the author was able to access data listed in 

tables 2, 3, and 4 through the Wayback Machine (https://web.archive.org/web/20161111035457/http://www.forestrymulcherguide.

com), and the lead author has the equipment specifications on file taken from this site. Data used on the regression are on file with 

the author.
USDA Forest Service RMRS-GTR-381. 2018 5

head and discusses the particular types of mastication equipment. We organized the discussion surrounding the three decision trees around questions that managers might ask 

when they want to identify the best method or machine to use.

Statistical Analysis

We used regression analysis to determine diameter-to-cutting head horsepower 

and diameter-to-weight relationships using the regression procedure in SAS software 

(Myers 1990; SAS Institute 2013). We used a subset of machine specifications from 

manufacturers listed on the Forestry Mulcher Guide (Catalyst Response, LLC. 2017). 

In this analysis, we applied a log transformation of diameter to address residual error 

normality and variance homogeneity. We conducted two statistical analyses to address 

objective 3. First, we used a meta-analysis to determine variation in fine fuels across 

vegetation types. The meta-analysis method combines weighted results from various 

comparable studies such as an omnibus test of treatment effects analogous to standard 

Analysis of Variance (ANOVA) methods. For the meta-analysis, we used data from two 

published studies (Battaglia et al. 2010; Kane et al. 2009) and data from the Keane et al. 

(2018) study to determine the variation in fine fuels across sites. We used the R metaphor package (Viechtbauer 2010) to test the null hypothesis that all 3-study effects are 

simultaneously zero. We also used the multcomp package in R to evaluate post-hoc pairwise comparisons of the three studies (Hothorn et al. 2008). The Benjamini-Hochberg 

method controlled the false discovery rate for the pairwise comparisons (Benjamini and 

Hochberg 1995).

For the second analysis, we evaluated differences between cutting heads and chipping on surface area of masticated pieces. We used the nonparametric Mann Whitney 

(Wilcoxon) tests in R to explore relationships between surface area means and two cutting groups on the moist mixed-conifer study sites (Keane et al. 2018; Appendix B). We 

also used a Kruskal-Wallis nonparametric test to explore the relationships between the 

cutting heads or chipping on fine-fuel amounts in the dry mixed conifer study sites. We 

used Q-Q plots and Levine and Bartlett tests to evaluate for normality and homogeneity, 

also using R (R Core Team 2015).

Literature Synthesis

Characteristics of Carrier Machines and Cutting Heads

Mastication requires a carrier machine, a cutting head, and a cutting-head attachment. Attaching the cutting head to the carrier machine can occur in one of two ways: 

either directly to the machine with a front-end attachment or to a swinging boom. 

Equipment configurations that combine these three pieces offer land managers several 

options to match the equipment to the site and management objectives. We provide a 

short summary that illustrates the range of options.

Carrier Machines

Carrier machines can be excavators (fig. 2a), skid steers (fig. 2b), tractors (skidders) (fig. 2c) with hydraulic systems, or tractors with power take-off systems (PTO); 

and each machine type varies in the amount of horsepower (hp) they have, and their 
6 USDA Forest Service RMRS-GTR-381. 2018

length, clearance, and whether they are on tracks (fig. 2a) or wheels (fig. 2c). The 

Forestry Mulcher Guide contained over 50 different carrier machines with tracks that 

range from 100 to over 700 horsepower (Catalytic Response, LLC 2017) (table 2). The 

dimensions of the carrier vehicles with tracks range from at least 170 to over 400 inches 

long and 96 to 135 inches high. Ground clearance also varies from 13 to 26 inches. 

Depending on the size of the machine, small machines can weigh as little as 13,500 

lbs. and the largest machines can weigh up to 62,800 lbs. The Forestry Mulcher Guide

(Catalytic Response, LLC 2017) also summarized 30 carrier machines that have wheels 

and operate at 160 to 500 hp. These carrier machines range from 247 to 358 inches 

long and 112 to 130 inches high. Widths range from 80 to 126 inches, ground clearance 

ranges from 19 to 21 inches, and they weigh between 16,100 to 38,000 lbs.

Dedicated mastication machines (i.e., those that only perform mastication) or machines that can conduct multiple tasks with different attachments including masticating 

head attachments have advantages and disadvantages depending on the site and project 

objectives (Schafer 2013). Both dedicated mastication machines and machines capable 

of running multiple attachments, such as large excavators and harvesters, may be effective in large units and when treating large diameter trees (>8 inches in diameter). Small 

machines, such as skid steers with small cutting-head attachments, may be more effective around homes.

Figure 2—A variety of carrier machines including excavators, 

skid steers, tractors with hydraulic attachments, and 

tractors with power-takeoff attachments can have attached 

mulching heads (photograph 2A provided by Nikia 

Hernandez, USDA Forest Service, Kootenai National 

Forest; photograph 2B provided by Mel Peterson, Director 

of Marketing and Training, Diamond Mowers; photograph 

2C provided by Dana Mitchell, USDA Forest Service, 

Southern Research Station).

A) B)

C)
USDA Forest Service RMRS-GTR-381. 2018 7

Whether machines are wheeled or tracked, machine width directly affects project 

success, because a machine’s wheelbase length or track width determines the minimum 

residual tree spacing allowed, particularly the places where the equipment operates. 

This is simply because of the requirement for the equipment to be able to pass between 

two given trees while working in the stand. For example, in a study on the University 

of Idaho Experimental Forest, a CAT 320B excavator with a Denis brushing head conducted treatments. This machine has a track width of approximately 9.3 ft (112 inches). 

Assuming approximately 2 feet of clearance on either side, the residual spacing for a 

stand treated with this machine might be approximately 14 feet between stems, or 220 

trees per acre, to avoid causing damage. However, if there are more than 220 stems 

acre-1, a machine of this size would most likely cause damage to the residual stand from 

the boom hitting the trees, the machine sliding into trees while maneuvering through the 

stand, or the cab bumping branches. In these situations, the smaller machine the better. 

Trails with spacing of 50 feet or more between trail centers allow operators to maneuver 

equipment. With a boom, they can also reach a considerable distance to remove trees or 

other biomass. Variable tree spacing also adds some options to the treatment because the 

operator can maneuver the machine on a wider trail system or around tree groups and 

minimize residual tree damage.

Table 2—Size variations in carrier machines. The Forestry Mulcher Guide (Catalytic Response, LLC. 2017) identified 79 

different carrier machines that have tracks and 30 carrier machines that have wheels. Units are presented in inches (in), 

pounds (lb) and horsepower (hp).

Specification Minimum Median Mean Maximum

Number of 

observations

- - - - - - - - - -Tracked carrier vehicles with cutting head attached- - - - - - - -

Power (hp) 99 290 326 765 77

Length (in) 166 242 251 410 55

Height (in) 96 113 114 135 56

Width (in) 59 98 95 132 73

Ground clearance (in) 13 18 18 26 43

Weight (lb) 13,500 26,500 30,597 62,800 57

- - - - - - - -Wheeled carrier vehicles with cutting head attached- - - - - - - - -

Power (hp) 160 245 279 500 29

Length (in) 247 294 292 358 9

Height (in) 112 120 121 130 7

Width (in) 80 106 105 126 30

Ground clearance (in) 19 20 20 21 4

Weight (lb) 16,100 26,600 26,861 38,000 11
8 USDA Forest Service RMRS-GTR-381. 2018

Cutting Heads and Cutting-Head Attachments

Either vertical or horizontal shaft cutting heads masticate biomass. Vertical shaft 

cutting heads include a disk with fixed teeth (fig. 3a) or one or more swinging blades 

(fig. 3b) attached to the disk (table 3). These cutting heads work well for masticating shrubs and saplings. When boom mounted, the narrow vertical shaft cutting head 

enables the operator to reach around trees. This type of cutting head can masticate trees 

less than 8 inches in diameter, produces ragged stumps, and either shreds the biomass 

or creates chunks. In contrast, the horizontal shaft consists of a rotor (drum) with fixed 

teeth, swinging hammers, or fixed knives (fig. 4). The wider horizontal shaft cutting 

heads can treat larger diameter trees. Horizontal heads tend to create small pieces and 

chips, and they tend to leave clean-cut stumps.

The Forestry Mulcher Guide (Catalytic Response, LLC 2017) noted 74 different 

manufacturers that produce masticating heads (over 400 modes). There are over 256 

models for excavators, 117 models for skid steers, 65 models for tractors with hydraulic 

systems, and 296 models for tractors with PTO systems. The guide also identified cutting heads that have fixed teeth (189 models) or have swinging hammers or swinging 

knifes (245 models). The cutting-head models range in size and power depending on 

the carrier machine (table 4) (Catalytic Response, LLC 2017). Cutting heads made for 

excavators can have working widths that range from 20 inches to 91 inches and weigh 

from 320 to 6,800 lbs. Skid steers tend to have cutting heads that range from working 

widths of 36 to 83 inches and weigh from 660 to 3,400 lbs. Tractors with hydraulic 

systems can have cutting heads with working widths between 58 to 101 inches and 

weigh from 1,800 to 10,800 lbs. Tractors with PTO have the greatest number of different cutting-head models. They range in size from 39 to 118 inches working width and 

weigh 948 to 11,757 lbs. Each cutting head has its own carrier machine requirements; 

but typically the larger the cutting head, the greater the need for a large engine and more 

hydraulic power from the carrier machine, although some cutting heads have their own 

Manager Question

Are there any figures that show ground pressure? I’ve heard soil compaction (as contributing 

to detrimental soil disturbance) is a concern with mastication, so showing ground pressure 

ranges might be helpful in facilitating a conversation between managers and soil scientists.

Based on the literature, soil texture, soil moisture, how many times a machine drives over the same area, 

and whether the machine drives on top of slash influence soil compaction. Machine size and the presence of 

a rubber or steel track or tires can displace or compact the soil. For example, Han et al. (2006) identified a 

three-way interaction among the number of machine passes, soil moisture, and slash loading on harvested 

sites that used a cut-to-length logging system. Combining all of these factors makes it difficult to identify one 

clear relationship or graph. Mastication studies did not identify any negative soil effects from mastication (see 

section on Soil Impacts in this paper) provided the machine drove on slash. However, we suggest that forest 

managers evaluate compaction potential on their sites using available tools like the Soil Disturbance Field Guide

(Napper et al. 2009) and adjust the implementation protocol to alleviate potential soil susceptibilities, such as 

limiting the number of passes, conducting the treatments when soils are dry, and having the machine walk over 

the slash.
USDA Forest Service RMRS-GTR-381. 2018 9

Figure 3—Vertical shaft cutting heads can either have fixed teeth (a), such as the Diamond Rotary Mower; or swinging knives 

(b), such as the Diamond Forestry Mulcher. These vertical shaft cutting heads come in a variety of sizes (photographs 

provided by Mel Peterson, Director of Marketing and Training, Diamond Mowers). 

A) B)

Table 3—Characteristics of the vertical and horizontal-shaft brush-cutting heads (from Coulter et al. 2002; Forest and 

Rangelands 2015; McKenzie and Makel 1991; Rummer 2009; Vitorelo et al. 2009; Windell and Bradshaw 2000,).

Vertical-shaft Horizontal-shaft

- - - - - - - - - - - - - - - - - - - -Head and cutting attachments- - - - - - - - - - - - - - - - - - - -

Cutting devices are attached to a disk or robust mowers Cutting devices attached to a horizontal shaft or drum

Fixed teeth or blade (mower type) Fixed teeth, swinging hammers, or ax/knife blade

Boom or front end mounted Boom or front end mounted

- - - - - - - - - - - - - - - - - - - - -Vegetation best suited to treat- - - - - - - - - - - - - - - - - - - -

Slash and shrubs

Trees 6 to 8 inch diameter when boom mounted 

Slash, shrubs when front end mounted

Trees up to 30 inches diameter when boom mounted

- - - - - - - - - - - - - - - - - -Piece size and posttreatment condition- - - - - - - - - - - - - - - -

Creates large pieces (chunks or shredded)

Leaves ragged stumps 

Creates small pieces

Leaves clean cut stumps 

- - - - - - - - - - - - - - - - - - - - - - - -Carrier machines- - - - - - - - - - - - - - - - - - - - - - - - - -

Excavator, skid steer, 

tractors (hydraulic and power take-off)

Excavator, skid steer, tractors (hydraulic and power take-off)

- - - - - - - - - - - - - - - - - - - - - - - - -Microtopography- - - - - - - - - - - - - - - - - - - - - - - - -

Broken or dissected topography with a diversity 

of slope angles and aspects

Continuous and similar slope angle and aspect 

power source. Single purpose masticators (i.e., machines only used to masticate) are, in 

general, more powerful than carrier machines that have attachments (Schafer 2013).

Cutting-head attachments can also vary in configuration. Both types of cutting heads 

can be mounted on the front-end of a carrier machine, boom mounted, or pulled. When 

attached to excavators, operators can extend the boom-mounted cutting heads to hard-toreach locations. Extended arms on long-reach excavators can range from around 40 feet to 

over 100 feet (http://www.purchasing.com/construction-equipment/excavators/types-andattachments/). Front-end mounts attach directly to the machine but have limited reach.
10 USDA Forest Service RMRS-GTR-381. 2018

Table 4—Size variations for masticator heads. The Forestry Mulcher Guide (Catalytic Response, LLC. 2017) summarized 

masticator head sizes for each type of carrier machine. The units are inches (in), centimeters (cm), pounds (lb), kilograms 

(Kg), gallons per minute (gpm), liters per minute (lpm), and horsepower (hp), kilowatts (kW). 

Description Minimum Median Mean Maximum

Number of 

models

- - - - - - - - - - - - - - - - - - - -Masticator heads for excavators- - - - - - - - - - - - - - - - - - - -

Total width (in) (cm) 33 (84) 62 (157) 64 (162) 102 (258) 147

Working width (in) (cm) 20 (50) 49 (124) 50 (128) 91 (230) 254

Weight (lb) (kg) 320 (145) 2,080 (945) 2,198 (997) 6,803 (3085) 238

Min hydraulic flow (gpm) 4 (16) 30 (114) 30 (115) 74 (280) 201

Max hydraulic flow (gpm) (lpm) 5 (20) 40 (151) 45 (169) 210 (795) 153

- - - - - - - - - - - - - - - - - - - -Masticator heads for skid steers- - - - - - - - - - - - - - - - - - - -

Overall width (in) 45 (115) 73 (185) 73 (186) 102 (258) 108

Working width (in) (cm) 36 (91) 60 (152) 61 (153) 83 (211) 115

Weight (lb) (kg) 660 (300) 2,382 (1,079) 2,171 (984) 3,400 (1,540) 108

Min hydraulic flow (gpm) (lpm) 12 (45) 26 (100) 26 (97) 40 (150) 101

Max hydraulic flow (gpm) (lpm) 18 (67) 37 (140) 38 (143) 65 (246) 92

- - - - - - - - - - - -Masticator heads for tractors with hydraulic system- - - - - - - - - - - - - -

Overall width (in) (cm) 69 (175) 105 (265) 101 (258) 121 (307) 60

Working width (in) (cm) 58 (147) 89 (2,25) 85 (215) 101 (256) 65

Weight (lb) (Kg) 1,800 (820) 5,400 (2,449) 5,596 (2,538) 10,800 (4,900) 63

Min hydraulic flow (gpm) (lpm) 27 (102) 75 (285) 76 (291) 150 (600) 58

Max hydraulic flow (gpm) (lpm) 45 (170) 120 (454) 122 (463) 210 (795) 56

- - - - - - - -Masticator heads for tractors with power take-off system (PTO)- - - - - - - - -

Overall Width (in) (cm) 51 (129) 93 (236) 92 (233) 138 (350) 270

Working Width (in) (cm) 39 (100) 79 (200) 77 (197) 118 (300) 296

Weight (lb) (Kg) 948 (430) 3,682 (1,670) 4,363 (1,979) 11,757 (5,333) 290

Min PTO power (hp) (kW) 55 (41) 105 (78) 141 (104) 295 (218) 32

Max PTO power (hp) (kW) 90 (67) 17 (127) 221 (161) 450 (336) 32 

Figure 4—Horizontal shaft cutting heads can have knives or teeth designed to work in different settings, such as on rocky soils 

(photographs provided by Fecon Inc.).
USDA Forest Service RMRS-GTR-381. 2018 11

Operator and Economic Factors

Operator Experience Level

Several anecdotal statements from the literature identify the value of operator experience to complete mastication treatments effectively. Coulter et al. (2002), 

Kryzanowski (2007), and Windell and Bradshaw (2000) noted that operator skill can 

highly influence mastication outcomes and the efficiency of operating the machine. 

Mastication is a violent activity that shatters, grinds, or shreds wood, so an inexperienced operator can damage a machine within a few weeks or it may be necessary to 

replace the cutting head more often than repair or replacement costs associated with an 

experienced operator (Kryzanowski 2007). Boom-mounted machines have less operator 

visibility and thus may require a more skilled operator to reposition the carrier machine 

and maneuver the cutting head to the target tree efficiently (Windell and Bradshaw 

2000).

An inexperienced operator may also need a spotter to help guide machine movement, which adds another crewmember and makes the spotter vulnerable to objects 

spraying from the machine (Bolding et al. 2006). An experienced operator knows how 

to maneuver the machine to avoid creating berms or making excessive turns so the overall site impact from tree damage to soil impacts may be less (Vitorelo 2009). The more 

experienced the operator, the more cost efficient the project will be, regardless of the 

configuration of the machine.

Economic Advantages and Disadvantages

For foresters accustomed to using classical forest economics to balance costs of 

timber harvesting operations with anticipated revenues, mastication treatments appear 

as an expensive option. We identified two reasons why mastication is expensive. First, 

mastication does not produce merchantable forest products (e.g., saw logs, ton wood) 

that generate immediate revenue like some treatments (e.g., commercial thinning). 

Second, the stands that benefit most from mastication treatments treat biomass that has 

low commercial value. Mastication treats individual trees and removing each additional 

stem per acre translates into additional dollars spent, so lack of revenue combined with 

high costs lacks appeal from an economic standpoint.

Operator experience:

The more complex the project, the greater the need for an experienced operator. Factors requiring highly skilled 

operators include broken slopes, a high density of residual trees (>100 trees acre-1), and whether or not the 

project requires masticating biomass around houses or maneuvering the machine on steep slopes (35 percent 

to 50 percent slopes).

• An experienced operator can create smaller or larger piece sizes by adjusting the time spent masticating a 

particular piece.

• Skilled operators can efficiently move the machine and cutting head and minimize the number of passes 

over a particular area, thus reducing project costs.

Suggestion: A site evaluation can help determine the operator skill needed to conduct the project successfully and 

efficiently.
12 USDA Forest Service RMRS-GTR-381. 2018

Mastication costs range from $300 to $400 per acre up to $1,500 or more (Coulter 

et al. 2002; Halbrook et al. 2006; Vitorelo et al. 2009). Lyon et al. (in review) reported 

that costs were $3.88 and $4.45 per individual tree when masticated to create chunky 

and fine chips, respectively. At 200 trees per acre masticated, that corresponds to $776 

per acre for the treatment, or 2.5 to 3 times the cost of a hand-thinning crew cutting in 

a similar stand with a lop-and-scatter treatment, if limbs and tops were left to degrade. 

However, if chipping or hand piling and burning followed the hand thinning, then costs 

could start to become more comparable (Halbrook et al. 2006; Jain et al. 2008).

Even though mastication initially appears cost prohibitive, mastication can 

provide economic benefits into the future. For example, Bagdon et al. (2016) describe 

several examples of long-term economic benefits through reduced future costs resulting 

from a variety of methods to treat fuels. These include both market and non-market 

benefits. Market benefits from fuel reduction treatments include harvested saw logs 

(Bagdon et al. 2016; Selig et al. 2010) and positive impacts to local economy (Bagdon 

et al. 2016; Hjerpe and Kim 2008). Fuel treatment implementation also creates jobs 

locally for administrators, foresters, equipment operators, and hand crews. The Four 

Forest Restoration Initiative (4FRI) restoration project required an estimated 491 fulltime equivalent (FTE) positions in the public and private sector (Bagdon et al. 2016; 

Combrink et al. 2012). Moreover, the manager designed fuel treatments to reduce fire 

suppression costs in the future. For example, Buckley et al. (2014) and Bagdon et al. 

(2016) found that the offset of fuel treatments ranged from $158 to $422 per acre when 

compared to fire suppression costs.

In a forest management context, long-term advantages can be gained from 

masticating to prepare a site for planting or natural regeneration, removing unwanted 

advanced regeneration, or conducting pre-commercial thinning. Pre-commercial thinning in dense conditions lead to a favorable growth response from a timber perspective. 

Hand thinning stands that contained 600 to 800 stems per acre of 3- to 4-inch diameter 

trees typically cost $125 to $250 per acre in the Inland Northwest United States based 

on communication with regional contractors (J. Odell, Alpha Services, LLC, Coeur 

d’Alene, Idaho, personal communication, August 2016). However, these treatments also 

tended to generate bulky fine fuels that persisted for several years after treatment. They 

required additional costs for in-woods chipping or hand piling and burning (Jain et al. 

2008) to decrease fire hazard in the short-term. In addition to removing ladder fuels, 

this dense layer of masticated fuels also acts as mulch that cools the soil and suppresses 

subsequent shrub growth and natural regeneration. Alternately, masticated biomass can 

also increase fire hazard; but the piles may not last as long as hand-felled slash because 

this material is in contact with the soil surface and decomposing. Mastication has the 

great benefit of compacting fuels in small or large pieces that are close to the forest floor 

to promote decomposition.

Given the wide range of costs associated with mastication, there are several attributes that affect mastication costs. These include:

• Tree diameter. Mastication costs increase as tree diameter increases (Vitorelo et 

al. 2009). Whether mounted with vertical (disk) or horizontal (drum) type head 

attachments, excavator-based mastication equipment will always require more 
USDA Forest Service RMRS-GTR-381. 2018 13

horsepower to treat larger stems and greater boom length to reach taller trees that 

occur in more mature stands. When the machine size or cutting head and target trees 

are incompatible, productivity decreases leading to higher costs.

• Fuel load. The more biomass that needs treatment, the more expensive the project 

becomes. However, mastication is a mechanical method making this technique a 

better alternative than hand slashing, piling, and burning (Halbrook et al. 2006; Jain 

et al. 2008).

• Site conditions. Costs are commensurate with site conditions, with uniform 

topography providing easier maneuverability than sites with broken topography or 

sites with steep slopes (e.g., 40 percent) (Halbrook et al. 2006; Rummer 2008).

• Particle size requirements. Costs vary by the target piece size desired for the 

mastication treatment. The more time spent to treat the biomass (i.e., the number of 

passes that a machine needs to make to reach target size), the more costs increase 

(Coulter et al. 2002; Halbrook et al. 2006).

Factors Affecting Mastication Costs

Increased or decreased costs may result from:

• The number of stems that are treated; more stems are more expensive.

• The physical setting, such as whether the topography is broken or uniform. Broken topography may 

be more expensive than uniform topography.

• Large machines may cost more than small machines.

• The less time the operator spends masticating one piece, the more efficient and less expensive the 

project.

Decreased costs may result from:

• Mastication can treat numerous stems (>100 stems/acre) with considerably lower labor costs than 

hand slashing, piling, and burning.

• Mastication can accomplish work in one entry. Other treatments can require multiple entries, such 

as slashing (1 entry), piling (2 entries), and burning (3 entries). Prescribed fire requires a specific 

burning window, which may or may not occur in a given time period.

• Mastication does not remove non-merchantable biomass and avoids removal costs.

Manager Question

It is clear that mastication removes competing vegetation, but can it be used for site 

preparation?

Yes, Jain et al. (2008) used mastication for treating advanced regeneration and as a site preparation. As a site 

preparation tool, mastication diminished competing vegetation, which aided in finding planting spots. When 

managers want excessive advanced regeneration removed, mastication becomes a viable option.
14 USDA Forest Service RMRS-GTR-381. 2018

Ecosystem Response

Vegetation

We identified over 22 studies that related vegetation (forbs, grasses, shrubs) response 

to mastication, and results varied across forest types, regions, and time since treatment 

(Appendix B). Nine of the studies noted vegetative benefits associated with mastication. 

Brockway et al. (2009) noted that mastication in eastern pine and hardwoods in Georgia 

led to an increase in woody and herbaceous plants in the first 2 to 3 years after treatment. 

In pinyon-juniper woodlands, mastication contributed toward maintaining a shrubdominated system, and mastication tended to have increased cover and species richness 

when compared to other treatments over the 8 years of measurements after treatment 

(Bybee et al. 2016; Shakespear 2014; Young et al. 2013a). Owen et al. (2009) also found 

that masticated plots 2.5 years after treatment had more plant cover and richness than 

untreated plots. However, Coop et al. (2017) noted an expansion of herb and shrub cover 

11 years after treatment and an increase in nonnative plant species. In dry mixed-conifer 

forests, Burnett et al. (2014) and Collins et al. (2007) noted that mastication reduced shrub 

and herbaceous cover 2 to 6 years after treatment. In contrast, Fornwalt et al. (2017) found 

that masticated sites had higher understory plant richness and cover and had little effect 

on shrubs when they compared their results to a control (no treatment) 6 to 9 years after 

treatment.

In contrast to positive vegetative response, several studies found negative vegetative 

effects after mastication. After chaparral (Calystegia sp.) ecosystems were masticated, 

they tended to have an expansion of nonnative grasses and a reduction in native forbs 3 

to 7 years after treatment (Perchemlides et al. 2008; Potts et al. 2010; Potts and Stephens 

2009). In pinyon-juniper woodlands, Havrilla et al. (2017) and Coop et al. (2017) found an 

increase in nonnative grasses several years after treatment. In gorse shrublands in Spain, 

Fernandez et al. (2013a,b, 2015), and Fernandez and Vega (2016) found that species richness on masticated sites was less than burned areas.

Some studies related vegetation cover and richness to forest floor depth. Fornwalt 

et al. (2017) and Wolk and Rocca (2009) showed a significant negative relation between 

understory plant cover and richness and forest floor depth after mastication. Wolk and 

Rocca (2009) noted that as woodchip and natural litter depth increased on ponderosa pine 

sites in Colorado, plant cover and species richness decreased. They found that when forest 

floor depth was greater than 4 inches (10 cm), species richness and plant cover diminished 

to zero. Fornwalt et al. (2017) noted similar results on masticated studies also conducted 

in Colorado in pinyon-juniper, dry mixed-conifer, and lodgepole pine forests. In their 

study, they found that total plant richness and total plant cover diminished to zero when 

forest floor depth also exceeded 4 inches (10 cm). In both these studies, species richness 

and total plant cover appeared to plateau when the forest floor depth was between 0 and 2 

inches (5 cm) deep.

Tree mortality and regeneration also varied across ecosystems on masticated sites. 

In mixed-conifer forests, there was minimal tree mortality from mastication (Collins et 

al. 2014; Kreye and Kobziar 2015). Moghaddas et al. (2008) noted variation in seedling 

density of tree species as a function of soil substrate created from mastication, prescribed 

fire, and mastication followed by prescribed fire. They showed that black oak (Quercus 

kelloggii) and sugar pine (Pinus lambertiana) seeding density was less in all treatments 
USDA Forest Service RMRS-GTR-381. 2018 15

when compared to the control. However, they found that ponderosa pine and Douglasfir (Pseudotsuga menziesii) seedling density increased in masticated sites followed by 

prescribed fire and on sites that only had prescribed fire. Hamma (2011) identified no 

significant differences between mastication and prescribed fire for stand-level carbon.

Manager Question:

Did these studies look at depth of mastication slash? Are there any guidelines from research 

that could be helpful for managers to know related to masticated material depth that may inhibit or change site characteristics? I know there has been research on chipping but I was not 

sure if there was research related for mastication as a tool to remove densely stocked areas 

where there could be heavier mastication residue/particles left. If depth of mastication residue 

could become an issue, it would be great to know depth ranges so we could put specifications in contracts to help mitigate any potential issues.

We found two studies that explicitly related total plant cover and species richness to forest floor depth 

(including masticated biomass) (Fornwalt et al. 2017; Wolk and Rocca 2009). Vegetation establishment 

diminished to zero when forest floor depth exceeded 4 inches (10 cm); when forest floor depths were 

approximately 2 inches (5 cm) or less, species richness increased. In current research at the University of 

Idaho, Becker and Keefe (in prep.) are using pretreatment stand conditions based on LiDAR and silvicultural 

prescriptions, particularly the trees/acre to be removed during mastication, to characterize expected fuel bed 

depths after treatment over several thousand stands on a portion of the Nez Perce-Clearwater National Forests.

Many factors influence unwanted plant species invasion (Hughes et al. 2007; 

Radosevich et al. 2007). The environment, species traits, growing space, release of 

resources, and how often a site is treated can increase the potential for invasive plants 

(Radosevich et al. 2007). Radosevich et al. (2007) and Coop et al. (2017) identified several 

attributes to consider in treatment planning that will minimize invasive plants:

• Identify the potential nonnative species that surround the treatment area.

• Identify the environment that favors regeneration for the specific species of concern, 

such as the type of soil substrate (mineral, organic, blackened). For example, some 

species (e.g., Ceanothus velutinus) regenerate on burned surfaces easier than soils with 

a deep organic surface layer (Morgan and Neuenschwander 1988).

• Identify the species shade-tolerances because the amount of, or lack of, residual 

overstory may influence establishment (Coop et al. 2017).

• Clean equipment before entering a site to diminish the chance of introducing outside 

seed from nonnative species.

• Avoid scarifying the forest floor.

• Consider increasing forest floor depth to 4 inches or greater to avoid plant 

establishment.
16 USDA Forest Service RMRS-GTR-381. 2018

Soil Impacts

We identified 17 studies that evaluated the effect of mastication on soil compaction, 

erosion, and nutrition (Appendix A). Two studies in mixed-conifer forests in California 

noted that mastication did not increase runoff using rainfall simulators when compared 

to bare mineral soil using a rainfall rate of 2.9 inches hr-1 (73 mm hr-1) (Hatchett et al. 

2006), or when residue covered >25 percent of the study area (Harrison et al. 2016). 

Soil compaction was also insignificant when machines drove over masticated residue 

(Moghaddas and Stephens 2008). Cline et al. (2010) noted that masticated residue in pinyon-juniper sites had higher infiltration rates and lower sediment yield than bare mineral 

soil using a rainfall simulator. Ross et al. (2012) also found that soil aggregate stability in 

pinyon-juniper ecosystems was lower in thin pile-and-burn treatments and masticated sites 

when compared to areas with no treatment.

Soil moisture and temperature did vary in masticated sites when compared to other 

treatments. Gottfried and Overby (2011) noted in pinyon-juniper woodlands that materials 

in masticated sites served as a mulch and the residue prevented the infiltration of rainwater 

leading to a decrease in soil moisture. However, other studies conducted on pinyon-juniper 

sites showed masticated sites had higher soil moistures than untreated sites (Owen et al. 

2009; Rhoades et al. 2012; Young et al. 2013b) and Rhoades et al. (2012) found similar 

results on ponderosa pine forests. These studies also showed that the residue mitigated 

temperature extremes when compared to temperatures measured on untreated sites. 

Microbial activity did not differ among mastication and other treatments on chaparral in 

California or southern pines in South Carolina (Southworth et al. 2011; Stottlemyer et al. 

2013). Most studies did not identify a decrease in C and N; and some studies identified 

an increase in plant available nitrogen on masticated sites when compared to controls 

(Moghaddas and Stephens 2007; Rhoades et al. 2012).

Vegetation response after mastication varies depending on:

• Time since treatment. Most monitoring occurred 2 to 9 years after treatment, but the response varied 

depending on the vegetation type.

• Vulnerability to nonnative plant establishment increases in grasslands or some types of shrublands and 

woodlands.

• Mastication resulted in minimal or no residual tree mortality.

• Prescribed fire after mastication may lead to better seedling regeneration, depending on the vegetation 

type.
USDA Forest Service RMRS-GTR-381. 2018 17

Insects, Mammals, and Birds

Five studies focused on insect or wildlife habitat (Appendix A). In mixed-conifer 

forests in California, Apigian et al. (2006) observed that prescribed fire and mastication 

treatments resulted in overall beetle community composition change, but the communities 

remained diverse and abundant. They further stated that there were minimal differences in 

beetle communities among the treatments but the conditions the treatments created, and 

therefore management objectives, should dictate the preferred treatment method. In similar forest ecosystems, Amacher et al. (2008) examined control, fire only, mastication only, 

and mastication plus fire treatments and found that deer mouse (Peromyscus maniculatus) 

populations diminished on masticated only and control sites but increased in the fire only 

and mastication plus fire treatments. California ground squirrel (Spermophilus beecheyi), 

brush mouse (Permyscus boylii), and long-eared chipmunk (Tamias quadrimaculatus) 

abundance increased between pre- and posttreatment period, but their abundance did not 

vary among the treatments. The authors concluded that the increased abundance of these 

mammals related to sampling years that had higher precipitation, a major cone crop event, 

and a reduction in predator occurrence prior to implementing the treatments. In California 

mixed-conifer forests, Burnett et al. (2014) found that bird community abundance varied 

on the residual forest structure and not necessarily on the type of treatment. They reported 

that changes in avian guilds related to canopy cover, shifts in ground-cover species and 

their abundance, and snag abundance. They found that these changes did not relate to the 

actual treatment, but to conditions created from the treatment. In oak woodlands and chaparral ecosystems, some bird species were favored after mastication, particularly if their 

habitat needs contained open areas (Seavy et al. 2008). In Texas oak shrublands, Reemts 

and Cimprich (2014) did not find any differences in the probability of black-capped vireo 

nests on masticated sites when compared to a control. Overall recommendations included 

Mastication Effects on Soils:

• There were no negative effects to erosion or compaction if the operator drives the carrier machine over 

masticated residue (Cline et al. 2010; Hatchett et al. 2006; Harrison et al. 2016, Appendix A).

• Soil nutrition was not adversely affected by mastication (Moghaddas and Stephens 2007; Rhoades et al. 

2012).

• Masticated materials insulated the soil, created uniform temperatures, and diminished temperature 

extremes (Owen et al. 2009; Rhoades et al. 2012).

• Soil moisture was higher on masticated sites on lodgepole and dry mixed-conifer when compared to the 

controls (Rhoades et al. 2012).

• On pinyon-juniper sites, mastication did prevent rainwater from infiltrating into the soil, which 

decreased soil moisture (Gottfried and Overby 2011); however, Rhoades et al. (2012) did not find any 

decrease in soil moisture on their pinyon-juniper sites.

• Mastication did not diminish microbial activity (Southworth et al. 2011; Stottlemyer et al. 2013).

Suggestion: Although mastication did not adversely affect the soils in these studies, good management 

practices (such as executing mastication on dry soils, driving on slash, deciding whether the machine needs 

to drive to each tree; or if a boom-mounted cutting head is desired) are preferred. All of these factors will help 

diminish soil scarification or compaction.
18 USDA Forest Service RMRS-GTR-381. 2018

(1) that mastication could replace prescribed fire with no effect on vireo nesting (Reemts 

and Cimprich 2014), (2) that size and shape of treated areas could benefit some bird species (Seavy et al. 2008), and (3) that diversifying treatment types in an area could increase 

habitat heterogeneity benefiting multiple leaf litter arthropods including rare species.

Fire Behavior in Masticated Fuelbeds

Kreye et al. (2014) produced a literature review on studies associated with masticated fuel beds and fire behavior. In summary, laboratory experiments showed that fuel 

load, fuel depth, and fuel moisture influenced fire behavior. However, field studies show 

that the fuel variability and recovering vegetation can also influence fire behavior. Heinsch 

et al. (2018) noted that ignition of masticated fuelbeds requires fine litter, such as litter or 

leaves. Once ignited, masticated fuels have short-flaming and long-smoldering combustion and long-duration heating. Smoldering fires in masticated fuelbeds can potentially 

flare up under high wind conditions causing embers to ignite fuels outside of the masticated treatment (Heinsch et al. 2018; Kreye et al. 2014). Bass et al. (2012) suggested that 

managers place buffers that separate masticated fuel treatments from surrounding areas 

that managers can later use as a fireline.

Fine-Fuel Characteristics Created by Mastication

Fine-fuel amounts can vary significantly among sites within specific studies 

(table 5). Battaglia et al. (2010) identified that 1-hour and 10-hour fuels on masticated 

sites were significantly greater than untreated controls from four different cover types in 

Colorado. On pinyon-juniper sites, when compared to untreated sites, there was a 72 percent increase in 1-hour fuels, 90 percent increase in 10-hour fuel, and 75 percent increase 

in 100-hour fuels. Kane et al. (2009) also showed significant differences among sites 

dominated by shrub and hardwood vegetation in California. Using data from Keane et al. 

(2018), there were no significant differences among the forest types in 1-hour and 10-hour 

fuels. The most productive sites (moist mixed conifer) had more fine fuels than the less 

productive sites (dry mixed conifer and ponderosa pine).

Wildlife Effects From Mastication

• Only a few studies exist that quantify the effects of mastication on fauna.

• Posttreatment conditions influence wildlife species selection and not necessarily the treatment itself. 

Different treatments create different compositions and structures, which can favor some species over 

others.

• Treatments did influence beetle community composition but communities remained diverse and 

abundant.

Suggestions:

• In general, the species of concern and their habitat needs will determine if mastication will affect wildlife.

• An option to consider when planning treatment may be to diversify treatment types in a given area, 

which also diversifies the habitat (Apigian et al. 2006). This strategy can address multiple objectives, 

diversify the surface vegetation, create different soil substrates to favor a diversity of species that were 

spatially diverse, and produce a variety of habitat attributes (Jain et al. 2008).
USDA Forest Service RMRS-GTR-381. 2018 19

The meta-analysis from selected studies did identify some differences in fine-fuel 

loads based on where the sites were masticated (Battaglia et al. 2010; Kane et al. 2009; 

Keane et al. 2018). For 1-hour fuels, sites measured by Battaglia et al. (2010) had significantly more 1-hour fuels than the Kane et al. (2009) or Keane et al. (2018) sites (fig. 5). 

Particularly, masticated sites in lodgepole pine forests had a mean of 16.9 mg/ha with a 95 

percent confidence interval (CI) of 5.3 and 28.5 mg/ha; mixed-conifer forests had a mean 

of 23 mg/ha with a CI of 5.7 to 40.2 mg/ha. Among the different studies, there was more 

variation in 10-hour fuels among the sites (fig. 6). For example, Kane et al. (2009) had 

some sites exceed 15 mg/ha. Battaglia et al. (2010) also had high amounts of 10-hour fuels 

(18 to 24 mg/ha). However, when comparing 10-hour fuels across the three studies, our 

meta-analysis did not identify any significant differences. The 100-hour fuel amounts were 

consistent across the sites and studies. More variation tended to occur on the more productive sites in Battaglia et al. (2010) dry mixed conifer sites and the moist mixed-conifer 

sites of Keane et al. (2018) (fig. 7).

Cutting heads did lead to significant differences in surface area among the 1-hour, 

10-hour, and 100-hour fuel loads (fig. 8). In the dry mixed-conifer sites and pinyon-juniper 

woodlands, chipping created the smallest pieces in 1-hour (1 inch2, 7 cm2), 10-hour (3 

Table 5—Time-lag fuel loads (tons/ac) separated by size class and depths of masticated sites from studies in the United 

States. Within columns of a specific study, means that are followed by the same letter are not statistically different.

1-hr 10-hr 100-hr

Vegetation type Mean SE1 Mean SE Mean SE Total

- - - - - - - - - - - - - - - - - - - - - - - - - - -Kane et al. 2008- - - - - - - - - - - - - - - - - - - - - - - - -

Mixed hardwood

(10 locations)

2.5b 0.4 7.5abc 1.3 4.1a 1.0 14.4

0.4de 0.1 6.4bc 6.4 5.8a 0.6 12.6

1.0cde 0.2 2.7d 0.5 2.1a 0.6 5.8

1.9bc 0.2 13.4a 1.7 3.2a 0.8 18.6

0.9de 0.1 3.6cd 0.7 1.4a 0.5 5.9

0.4de 0.1 3.6cd 1.0 3.2a 0.7 7.2

4.4a 0.5 9.5ab 1.5 4.0a 0.8 17.8

2.0bcd 0.2 7.0bc 0.9 3.7a 0.9 12.7

0.7de 0.1 4.5cd 0.6 0.8a 0.2 5.9

1.2cde 0.4 4.5bcd 0.6 3.4a 0.6 10.4

- - - - - - - - - - - - - - - - - - - - - - - - -Battaglia et al. 2010- - - - - - - - - - - - - - - - - - - - - - - - -

Pinyon-juniper 3.5a 1.0 5.4a 1.1 1.9a 0.3 10.7

Ponderosa pine 3.6a 0.8 8.0a 1.5 3.3a 0.4 14.9

Lodgepole pine 7.5a 2.6 8.6a 1.2 2.3a 0.4 18.5

Dry mixed conifer 10.3a 3.9 10.9a 2.5 4.8a 1.8 26.0

- - - - - - - - - - - - - - - - - - - - - - - - - -Keane et al. 2017- - - - - - - - - - - - - - - - - - - - - - - - -

Ponderosa pine 1.4a 0.4 4.5a 0.8 3.5a 1.0 9.4

Dry mixed conifer 1.2a 0.4 4.1a 0.7 3.4a 0.6 8.7

Moist mixed conifer 2.3a 0.4 5.7a 0.7 8.6b 2.3 12.1

1 SE = Standard error.
20 USDA Forest Service RMRS-GTR-381. 2018

Figure 5—One-hour fuels (mg/ha) mean 

differences among studies and sites. The 

forest plot shows the mean (squares), 

95th confidence interval. Numbers on 

the left are the values for the mean and 

confidence intervals. Battaglia et al. 

(2010) had significantly more 1-hour fuels 

than Kane et al. (2009) and our field study 

(Keane et al. 2018).

inches2, 19 cm2), and 100-hour (12 inches2, 77 cm2) fuel classes. The vertical shaft cutting 

heads with fixed teeth tended to create the largest piece sizes within a given fuel time-lag 

including in 1-hour (3.4 inches2, 22 cm2), 10-hour (10 inches2, 65 cm2), and 100-hour (46 

inches2, 300 cm2) fuels. Similar cutting heads consisting of swinging knives also created 

larger pieces in 1-hour (2.5 inches2, 16 cm2), 10-hour (7.4 inches2, 48 cm2), and 100-hour 

(27 inches2, 173 cm2) fuels than machines with fixed teeth cutting heads. The surface area 

of each time-lag fuel created by horizontal shaft cutting heads did not exceed surface area 

of sites masticated with vertical shaft cutting heads. In the moist mixed-conifer forests, 

significant differences in surface area occurred only in the 1-hour size class. Although the 

means appear different, the variation in each fuel class was not significant in the 10-hour 

and 100-hour fuel classes.
USDA Forest Service RMRS-GTR-381. 2018 21

Figure 6—Ten-hour fuels (mg/ha) mean differences 

among studies and sites. The forest plot shows the 

mean (squares), 95th confidence interval (lines). 

Numbers on the left are the values for the mean 

and confidence intervals. Studies include Keane 

et al. (2009), Battaglia et al. (2010), and our field 

study (Keane et al. 2018).

Figure 7—One-hundred hour fuels (mg/h) mean 

differences among studies and sites. The forest 

plot shows the mean (squares), 95th confidence 

interval (lines). Numbers on the left are the values 

for the mean and confidence intervals. The 100-

hour fuel amounts were similar across sites and 

the variation in amounts were more consistent 

across studies and sites. Studies include Keane 

et al. (2009), Battaglia et al. (2010), and our field 

study (Keane et al. 2018).
22 USDA Forest Service RMRS-GTR-381. 2018

Figure 8—The cutting head designs 

can result in different piece sizes. 

Chipping creates the smallest 

pieces and the vertical shaft cutting 

head with fixed teeth or swinging 

knives created the largest pieces 

(data is from our field study (Keane 

et al. 2018) (Appendix A). Different 

letters indicate significant statistical 

differences among cutting heads 

within different piece sizes (time 

lag fuel).
USDA Forest Service RMRS-GTR-381. 2018 23

Fire Behavior

• Needles, leaves, or other 1-hour fuels are required for masticated fuels to ignite.

• Once ignited, masticated fuels smolder for long periods.

• Fine-fuel loads:

• One-hour, 10-hour, and 100-hour fuels always increase after mastication (Kreye et al. 2014).

• 1-hour fuels vary depending on vegetation types and studies.

• 10-hour fuels do not vary among the vegetation types and studies.

• A site’s productivity and amount of biomass influences 100-hour fuel loads.

Suggestions:

• Not all biomass on a site need treatment or mulching into small pieces. Some trees can remain as down 

logs. These additional logs provide wildlife habitat and do not contribute to an increase in the fine fuels. 

Some trees can have tops cut (particularly with a boom-mounted masticator) and left standing for future 

snag recruitment.

• The operator has the ability to adjust the piece size by using a vertical shaft that creates larger pieces 

or by minimizing the amount of time spent chopping each piece, resulting in larger pieces left on the 

ground.

Resistance to Decomposition

• Resistance to decomposition can determine whether masticated fuel will decompose quickly. Species 

resistant to decay may pose a long-term surface fuel problem compared to species that tend to decay 

more rapidly.

• The wood handbook lists domestic and imported woods according to their average heartwood decay 

resistance (Clausen 2010). Species very resistant to decay include Pacific yew (Taxus brevifolia), locust 

(Robinia pseudoacacia), and red mulberry (Morus rubra). Species resistant to decay include the cedars 

(Thuja sp.), old-growth redwoods (Sequoia sempervirens; Sequoiadendron giganteum), junipers, oaks 

(Quercus sp.) and other hardwoods. Moderately resistant species include Douglas-fir, western larch 

(Larix occidentalis), young redwoods, black cherry (Prunus serotina), young bald cypress (Taxodium 

distichum), old longleaf pine, old slash pine, and old eastern white pine (Pinus strobus). Slightly and 

nonresistant to decomposition include the true firs (Abies sp.), aspen and other poplars (Populus sp.), 

hemlock (Tsuga sp.), elms (Ulmus sp.), maples (Acer sp.), sweetgum (Liquidambar styraciflua), spruces 

(Picea sp.), several pines, plus several other species.
24 USDA Forest Service RMRS-GTR-381. 2018

Decision Trees for Machine Selection

The complexity of integrating multiple factors can be overwhelming. Partitioning 

these factors into a set of decision trees can help identify the right method, and subsequent equipment, to address the requirements and limitations needed on any given 

management project. We used a series of questions to aid in deciding what method may 

best fit the project and whether or not to use mastication (Hover 2012; Schafer 2013; 

Windell and Bradshaw 2000). When designing any fuel or silvicultural treatments, a site 

assessment identifies the scope of work, matches the appropriate equipment to the project goals and objectives, and identifies the level of operator experience needed to meet 

desired outcomes (Mitchell 2005; Nyland 2016; Windell and Bradshaw 2000).

Decision Tree 1

How Does Slope Percent Influence Treatment Options?

The slope percent will dictate treatment options (fig. 9). If slopes are greater than 

40 percent, treatment options may only include prescribed fire, hand slashing and piling, or no treatment, unless managers plan to use mechanical equipment specifically 

designed to maneuver on steep slopes. However, if slopes are less than 40 percent, then 

managers can apply prescribed fire, mechanical treatment, or hand thinning. Moreover, 

if the biomass does not create a fire hazard or regeneration success does not require site 

preparation, then the site may not need a treatment.

Ground-based carrier machines work best on slopes less than or equal to 35 percent slope. However, with recent improvements to ground-based machines to work on 

steeper slopes, there are a few more options to treat sites with mastication depending 

on the carrier machine. Visser and Berkett (2015) suggest that rubber-tired skid steer 

and tractors should be operated on slopes less than or equal to 35 percent. Excavators, 

Figure 9—Decision tree 1. There are 

several choices that need consideration 

when determining if mastication is 

the best treatment option. The most 

important component is to determine 

if the slope angle is less than 40 

percent, which would favor the use 

of machinery. Slopes greater than 

40 percent mechanical treatment is 

rarely an option, unless machines are 

tethered so as not to tip over.
USDA Forest Service RMRS-GTR-381. 2018 25

crawler tractors, or similar machines should operate at or below 40 percent, and ground 

equipment specifically designed for operating on steep slopes should not exceed 50 

percent slope.

Decision Tree 2

Does the Site Need Post-Harvest Slashing or Does the Site Have 

Excessive Advanced Regeneration?

For sites less than 40 percent slope, the abundance, distribution and type of unmerchantable material will dictate if the site needs treatment (fig. 10). If the result of a 

commercial harvest created the biomass, or if there is excessive advanced regeneration 

or a shrub-dominated understory, then managers may need to apply some type of mechanical treatment followed by prescribed fire or mastication. If smoke, risk of escape, 

or residual species (not fire resistant) do not limit using prescribed fire, then prescribed 

fire becomes a viable option if managers want a blackened surface to promote certain 

types of regeneration or to meet other management objectives, including decreasing 

treatment costs. If smoke production or residual live biomass is not fire resistant, then 

managers can execute a mechanical treatment, such as mastication or grapple piling. 

Mastication becomes the best option when substantial advanced regeneration (>100 

stems/acre) exists.

Figure 10—Decision tree 2 

provides treatment site 

conditions that may identify 

treatment including no 

treatment option, chipping 

(not included), prescribed fire, 

or grapple pile followed by 

burning. If mastication is the 

preferred option then go to 

decision tree 3 to determine 

what machine combination 

may be the best option.
26 USDA Forest Service RMRS-GTR-381. 2018

Mastication becomes viable when a site has no commercial value but still has 

numerous shrubs or hundreds to thousands of small unmerchantable trees per acre. In 

contrast, if only a few trees or shrubs exist, then more practical options emerge, such as 

hand slashing followed by either prescribed fire or grapple piling. This would remove 

the biomass and create a blackened surface; or with grapple pile and burning, the 

site would have a diversity of soil substrates that may address site preparation objectives. Chipping (not included in the decision tree) also can achieve desired objectives; 

however, it tends to create very small pieces and deep fuel beds (Battaglia et al. 2010). 

Chipping does produce other products such as biochar (Anderson et al. 2016).

Does the Time of Year When Masticators Create Slash Favor Increased 

Mortality From Ips Beetle (Ips sp.)?

Another important consideration when choosing among mastication methods, prescribed burning, or other treatments is the timing of the treatment window and any possible 

adverse effects associated with insects or disease. For example, in treatments that leave 

green slash, Ips beetle can undergo rapid population growth, which could cause subsequent 

residual damage to standing trees. Under these circumstances, masticating the biomass 

removes the bark, which diminishes Ips survival (Conner and Wilkinson 1983).

Decision Tree 3

If a manager decides to use mastication, then decision tree 3 provides the elements 

that will help match the masticator to the site and project objectives (fig. 11). We adapted a 

decision tree from Jain et al. (2012) that provides the flow of elements to guide the choice 

of masticator to use.

Is the Soil Prone to Compaction?

Soil moisture and structure, the machine type and size, whether the machine has 

tracks or tires, the number of passes over a given area, and whether the machine drives 

over slash all influence soil compaction. Dry soils lead to less compaction (Coulter et al. 

2002; Han et al. 2006; Rummer 2010). Driving on slash mats can also decrease compaction 

(Harrod et al. 2009; Moghaddas and Stephens 2008). More efficient and mobile, wheeled 

carriers can create ruts, particularly on wet soils (fig. 12). Tracked machines, in general, 

cause less soil disturbance because the machine weight is spread over a larger area (track) 

and these machines can maneuver through the site without causing ruts like wheeled 

carriers (Sakai et al. 2008; Windell and Bradshaw 2000). Rubber tracks tend to offer the 

lowest pressure and are the best choice on sensitive soil conditions. However, if the ground 

is rocky, then steel tracks are preferred (Schafer 2013). Equipment with boom-mounted 

cutting heads enable the operator to reach over difficult areas or maneuver around tighter 

spaces; therefore, this equipment avoids driving to every tree, which also diminishes soil 

disturbance.

Is the Terrain Uniform or Complex, or Are There Wet Areas Present That 

Need Protection?

Uniform terrain favors a machine-mounted cutting head. A more complex site with 

broken topography favors a boom-mounted cutting head (Bolding et al. 2006). A boommounted cutting machine can maneuver down ridges and reach over wet areas and across 
USDA Forest Service RMRS-GTR-381. 2018 27

draws to reach trees. A machine-mounted cutting head requires driving to each tree (Coulter 

et al. 2002).

What Are the Project Size and Access? Do Homes or Other Buildings Exist 

in the Project Area That Will Require Machine Maneuvering?

Initial plans will need to identify the transportation costs, the ease of machine fueling, and if repair sites are located close by. Walking the machine from site to site can also 

decrease costs. When operating in difficult conditions or long days, these machines can 

experience wear and tear. Replacing broken teeth or knives makes easy road access necessary. Schafer (2013) noted that skid steers and compact track excavators typically have the 

advantage in tight places, such as around homes. However, a larger dedicated masticator fits 

well when sites are inaccessible and sufficient space for maneuvering the machine exists.

Figure 11—Decision tree 3 provides 

the site specific components that 

may aid in identifying the best 

machine and masticator head for 

the project.
28 USDA Forest Service RMRS-GTR-381. 2018

What Is the Target Size, Type, and Density of Biomass?

Vertical shaft masticating heads work best on slash and shrubs and can typically treat 

trees 6 to 8 inches in diameter when they are boom mounted. In contrast, horizontal-shaft 

masticating heads can treat larger trees (some up to 30 inches in diameter) particularly 

when they are mounted on a boom (table 2). However, larger trees need a more powerful 

and larger cutting head and a carrier machine may be required for the job. To illustrate 

this relationship, our analysis indicated a highly significant relationship (P < 0.0001) 

between target tree size, machine power, and cutting head weight. As trees get larger, 

machine power needs to increase, as does the cutting head weight (r2 = 0.60) (fig. 13). For 

example, to masticate a 20-inch diameter tree will require about a 200 horsepower engine 

and a 5,800 lb. cutting head (fig. 13). A complementary machine designed to masticate 

a 10-inch diameter tree would have a 200 horsepower cutting head and weigh 3,000 lbs. 

Figure 12—Examples of soil rutting (a) erosion, 

and compaction (b) (photographs provided 

by Deborah Page-Dumroese, USDA Forest 

Service, Rocky Mountain Research Station).

B)

A)
USDA Forest Service RMRS-GTR-381. 2018 29

However, the head size and horsepower of the machine should be complementary to the 

tree size targeted for mastication and there may be additional parameters to consider when 

identifying the best machine for a particular job. As stem density increases, such as in 

stands dominated by large shrubs, a dedicated masticator is more efficient than a machine 

with a cutting-head attachment (Schafer 2013). Tree size and biomass density ultimately 

influence the cost of implementation. Cost increases as density increases; as mentioned 

previously, every increase in the number of stems above approximately 100 stems ac-1 

results in a cost increase of 1 percent (Fight and Barbour 2005).

What Is the Desired Residual Forest Structure?

The desired conditions, such as residual stand density and its spatial distribution, 

can influence the type of carrier, mounting technique, and cutting head. Widely spaced 

trees (e.g., 22 x 22-foot spacing) favor a drive-to-tree mastication method because it 

takes less time (Bolding et al. 2006). However, if tight spacing between trees exists, 

a carrier machine with a boom-mounted cutting head has greater flexibility because 

this machine allows the operator to move the cutting head into places that might have 

narrow tree spacing. In addition, a skid steer may be preferable to an excavator in tight 

places if trees are small (Schafer 2013). 

For pre-commercial thinning (trees < 2 in d.b.h.), skid steers with a vertical shaft 

masticator armored with blades (similar to large blade mowers) can maneuver easily and 

cut close to residual trees at a 14-ft spacing.

Figure 13—Relation of cutting head weight (a) and 

horsepower (b) to tree size. As the target tree size 

increases, a larger and more powerful cutting 

head and carrier machine is needed (data from 

Catalytic Response 2017, currently on file with 

author Theresa Jain, Rocky Mountain Research 

Station, Moscow, ID). 
30 USDA Forest Service RMRS-GTR-381. 2018

What Is the Preferred Piece Size of Masticated Biomass?

Piece size influences the distribution of fine fuels defined as 1-hour (0 to 0.25 

inch), 10-hour (0.25 to 1 inch), and 100-hour (1 to 3 inches) fuels. The time designation on these fuels indicates a particle size and drying time. The cutting head, how 

much the operator spends time masticating a particular piece, and the size of targeted 

biomass influence the posttreatment piece size.

Characteristics of the treated biomass will differ based on the cutting tool used. 

The vertical shaft cutting head creates larger piece sizes by shredding the biomass 

than the horizontal cutting head (table 3, fig. 4). The vertical shaft also usually creates 

pieces with higher surface areas, particularly in the 10-hour and 100-hour fuel classes, 

compared to the surface area of pieces created from horizontal shaft. If piece size is 

not of concern, then a horizontal drum attached to the machine may be the most efficient method to use, even if it tends to create more 1-hour and 10-hour fuels.

Depending on the operator’s skill level, either a vertical shaft or a horizontal 

cutting head is capable of creating larger or smaller pieces. An operator also can 

influence the piece size that the machine creates using slow, methodical passes or 

multiple passes across an area. Typically, the more time the operator spends grinding, 

the smaller the pieces become; thus, when the operator moves quickly, the piece size 

increases.

Battaglia et al. (2010), Kane et al. (2009), and Keane et al. (2018) showed that 

the amount of 1-hour and 10-hour fuels varied across sites. This variation often results 

from the type and size of pretreatment biomass. For example, if the target biomass 

is all less than 3 inches diameter, such as some shrubs or saplings, then the reconfiguration of the biomass will tend to add more 1-hour and 10-hour fuels because all 

material being masticated is below the 100-hour diameter threshold. However, if the 

target biomass that is treated is larger, such as 5 inches or 7 inches in diameter like 

pole-sized trees, then 100-hour fuels may increase as well as the 1-hour and 10-hour 

fuels.

The productivity of the site may have contributed to fine-fuel abundance. Keane 

et al. (2018) noted sites that were more productive, such as those that occurred in the 

Manager Question

Do the authors have any knowledge of, or literature to reference, mastication of surface fuels 

with lots of large woody debris (such as blowdown, high loads of mortality fuel, etc.)?

Mastication typically treats standing biomass or post-harvest slash. However, there are other circumstances, 

such as wind events, beetle infestations, or wildfires that can create standing dead and down wood. In these 

situations, mastication becomes a viable option. Stottlemyer et al. (2015), working in beetle-killed stands 

in South Carolina, reported treatment effects on surface fuel quantities, determined if these sites required a 

custom fuel model, and conducted a fire behavior simulation. Their management recommendations suggested 

that timing of treatments is important when treating beetle-killed stands. They noted that if prescribed fire 

were the preferred method, burning toward the end of the beetle infestation would provide sufficient surface 

fuels that would carry the fire. Unlike with prescribed fire, masticated sites had no dead standing trees (the 

masticating machine removed all standing trees to reduce occupational hazard and improve stand accessibility) 

and they suggested that mastication might be a better site preparation for artificial regeneration.
USDA Forest Service RMRS-GTR-381. 2018 31

moist mixed conifer (DC1, PRCC1), tended to have more 100-hour fuels than sites 

that were less productive (Amber, BHMix, BHMow) (Appendix B). Similar trends 

also occurred on sites measured by Battaglia et al. (2010) where they showed the dry 

mixed conifer had the highest fine-fuel loads after mastication and pinyon-juniper had 

the lowest fuel loads after mastication. Kane et al. (2009) may have also had similar 

cause for increased fuels on some sites, but they did not mention differences in productivity related to the total amount of fine fuels.

What Are the Operator’s Skill Requirements for the Project?

As previously mentioned, machine operator skill is a very important factor to 

consider in mastication implementation. A person who has more experience (hundreds 

of hours) on masticator equipment can move through the project efficiently and optimize the machine’s assets. In our review of the literature, several anecdotal references 

stated that operator experience could highly influence mastication outcomes and the 

efficiency of operating the machine (Coulter et al. 2002; Kryzanowski 2007; Windell 

and Bradshaw 2000). Just as an inexperienced operator can damage a mastication 

cutting (Kryzanowski 2007), so can an inexperienced operator damage the residual 

stand by bumping, hitting residual stems, or removing bark with flying projectiles of 

masticated particles. An experienced operator may be able to move the cutting head 

to avoid homes or other values at risk. Some operators prefer to work in a downslope 

direction when visibility is impaired in dense stands because of improved visibility 

seeing the tops of stems. An inexperienced operator may need a spotter to help guide 

him, which adds another crewmember, and the spotter may be vulnerable from objects 

coming from the machine (Bolding et al. 2006). The more experienced the operator, 

regardless of the machine, the more cost efficient the project will be and the overall 

site impact from tree damage to soil impacts may be less by minimizing turns and 

limiting the number of passes (Vitorelo et al. 2009). Experienced operators can more 

carefully direct the thrown material away from areas of concern by proper machine 

and cutting-head placement, which increases safety.

Manager Comment

A side note with small tree mastication from issues I have run into with contracts and mastication projects. If there are a high density of small trees in the stand adjacent to the residual 

trees, then the mastication heads cannot get close enough to remove the small seedlings. 

Thus, the contractor always has to have a saw present and manually cut small seedlings 

adjacent to residual leave trees to prevent damage. Does any of the current research provide 

suggestions for types of heads/mastication equipment to help with this issue? Otherwise, 

managers should be aware of this potential limitation and design their project and/or contracts accordingly.

In our literature search, we did not find any reference that specifically addressed this issue; however, in the 

Black Hills Experimental Forest, a small skid steer with a vertical head attachment was able to accomplish 

noncommercial thinning in saplings that were very close together. The smaller machine and cutting head 

appeared to have considerable maneuverability and was able to accomplish the task.
32 USDA Forest Service RMRS-GTR-381. 2018

How About Safety Considerations?

Masticators produce two types of hazardous objects (1) sawteeth or cutters 

and (2) foreign objects (wood) displaced by the cutting tool (Rummer and Klepac 

2011). OSHA and ISO standards require protective cab glazing material to protect 

the operator from broken saw teeth and flying wood. Injuries to foresters and contract 

supervisors overseeing mastication activities can occur because they sometimes get 

too close to the equipment and do not have an enclosed cab to protect them. Even with 

specific standards, safety is a continuous process of hazard recognition, engineering 

analysis, and adoption of improved countermeasures and practices. However, carrier 

machines used in forestry operations need to meet OSHA standards that have rollover 

protective structures (ROPS) and falling object protective structure (FOPS).

Conclusion and Management Implications

Managers can apply a wide range of silvicultural methods that provide many 

options to identify the best method or a combination of methods best suited to meet 

project objectives. Mastication is unique, but fortunately, there is an abundance of literature to inform decisions on whether to use mastication as a treatment option. This 

paper attempts to synthesize this literature to provide information to decisionmakers 

to determine if mastication will achieve their objectives. That said, we still lack literature and research to inform several aspects of mastication. We know that masticator 

machines and implementation strategies will continue to evolve; this report provides 

information on the current knowledge. Ecologically, we know more about plant and 

soil response, but not all ecosystems have mastication studies; therefore, we cannot 

assume consistent results across all vegetation types. We have limited knowledge 

of the effects of mastication on wildlife and wildlife habitat. If a manager selects 

mastication as a treatment option, then treatment implementation becomes paramount. 

A thorough site evaluation should include consideration of factors such as nonnative 

species invasion, the vulnerability of soils to erode or compact, and treatment costs. 

Only after evaluating the site, objectives, and goals can a manager identify the carrier 

machine, cutting head, and mounting system best suited to complete the project safely 

and efficiently.

Acknowledgments

We would like to thank our reviewers: Mike Battaglia, Research Forester; Dana 

Mitchell, Research Engineer; Robert Keane, Research Ecologist; Nikia Hernandez, 

Fire Management Officer; Shelagh Fox, Regional Silviculturist; and Marcus Chin, 

Vegetation Management Specialist. The Joint Fire Science Program and the Rocky 

Mountain Research Station funded this research.

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USDA Forest Service RMRS-GTR-381. 2018 39

Appendix A—Summary of Key Results From Mastication 

Studies

Introduction

Each of the four tables in Appendix A provides a summary of key results from 

studies that evaluated the effects of mastication on the biological, ecological, or physical 

response. Table A.1 summarizes understory and shrub vegetation. Table A.2 summarizes 

the effects of mastication on remaining trees and regeneration. Table A.3 summarizes 

soil erosion, compaction, and nutrition. Table A.4 summarizes the effects of mastication 

on insects, mammals, and birds. Each table has the literature reference, the location 

where the study occurred, the forest or vegetation type, the treatments that were tested, 

and a short summary of the results. Unique to the understory vegetation table is the listing of lifeforms (shrubs, forbs, grass) that were studied. In some cases, multiple studies 

occurred on the same study site; in these situations, we placed all the references relevant 

to the results in the same table row.

Each table has the following headings:

Study: Number of each study in a particular subject area, for tracking purposes. 

Reference: Literature reference. 

Location: State or country where the study occurred.

Ecosystem: Vegetation type of the study site.

Treatments: Treatments the authors evaluated.

Life-form studies: This is unique to table A.1 and lists the vegetation lifeform the 

authors evaluated.

Summary: This provides the primary results from the individual study.
40 USDA Forest Service RMRS-GTR-381. 2018 Table A.1—Effect on understory and shrub vegetation from mastication and associated treatments. Study Reference Location Ecosystem Treatments Life forms studied Summary 1 Brockway et al. 2009 Georgia Pine, hardwoods • Mastication • Mast. plus prescribed fire (PF) • Control tree, shrub, forb, grass • 2-3 yrs posttreatment, • Mastication (Mast.) led to initial increase of woody and herb. plants, • Addition of mast. and prescribed fire was needed to reduce woody regrowth and increase grasses and forbs. 2 Burnett et al. 2014 California Mixed conifer • Mastication • Thinning tree, shrub, forb • 2-6 yrs posttreatment, • Mast. and thinning treatments reduced shrub and herbaceous cover. 3 Busby and Southworth 2014 Oregon Chaparral • Mast. plus PF grass • 7 yrs posttreatment, • Seeding after Mast+PF had only a minimal effect on bunchgrass persistence, native and introduced cover, species richness over control. 4 Bybee et al. 2016; Shakespear 2014; Young et al. 2013a Utah Pinyon/Juniper (P-J) • Mastication • Prescribed fire • Thinning • Control tree, shrub, forb, grass, nonnatives • 1-8 yrs posttreatment, • Mast. and thinning produced similar results, • Mast. maintained a shrub-dominated ecosystem, • Mast. and Mast+PF increased herbaceous plant cover, • Mast. improved growth of cheatgrass and bluebunch wheatgrass. 5 Carvajal-Acosta et al. 2015 Nevada Pine/Fir • Mastication • Control forb • 1-yr posttreatment, • No significant difference in plant community variables (richness, cover, etc.) to mast. 6 Collins et al. 2007 California Mixed conifer • Mastication, • Prescribed fire, • Mast. plus PF • Control tree, shrub, forb, grass, nonnatives • 1-yr post treatment, • Mast. reduced shrub cover relative to PF and Mast+PF, • Mast+PF had greatest species richness and number of nonnative plants, • PF and Mast+PF decreased native species.
USDA Forest Service RMRS-GTR-381. 2018 41 Study Studies Location Ecosystem Treatment Life forms studied Summary

7 Fernandez et al. 

2013a,b, 2015;Fernandez and Vega 2016

Spain Shrubland • Mastication 

• Prescribed fire 

shrub • 1.5-4 yrs. posttreatment,

• Higher species richness in PF than mast.,• More shrub seedlings in burned areas than mast,• Neither mast. or PF hindered shrub recovery.

8 Fornwalt et al. 

2017

Colorado P-J, Mixed 

Conifer

• Mastication• Control

shrub, forb, grass, nonnatives

• 6-9 yrs posttreatment,• Mast. had higher understory plant richness and cover than control,• Mast. had little effect on shrubs.

9 Gottfried and 

Overby 2011

Colorado P-J • Mastication

• Thinning (pile burning)• Control

tree, shrub, forb, grass

• 1-3 yrs posttreatment,• Mast. and thinning-pile burning produced veg. mosaics and adequate tree regeneration• Pile burns resulted in higher occurrence of nonnatives.

10 Kane et al. 2010 California Ponderosa pine • Mastication

• Mast. plus PF• Control

shrub, forb, grass, nonnatives

• 1-4 yrs posttreatment,• Mast. had no effect on understory species richness,•Mast+PF increased both native and nonnative species richness as well as shrub seedling density in comparison to controls.

11 Kreye et al. 2014 Florida Pine forest Mastication tree, shrub • 1-yr posttreatment,

• Shrub layer recovered quickly after mast.

12 Owen et al. 2009 Colorado P-J • Mastication

• Prescribed fire (piles)• Thinning• Control

shrub, forb, grass

• 2.5 yrs post treatment,• Mast. plots had more plant cover and richness than pile burns or control,• Pile burn plots dominated by nonnatives and 6x times fewer native plants than controls.

13 Perchemlides et al. 

2008

Oregon chaparral, oaks • Mastication 

• Control

tree, shrub, forb, grass

• 4-7 yrs posttreatment,• Mast. led to expansion of nonnative grasses, reduction in native perennial cover, dominance of annual forbs.

14 Phillips et al. 2013 South 

Carolina

Pine forest • Mastication

• Prescribed fire• Mast. plus PF• Control

nonnatives • 7 yrs posttreatment,

• Pine stands affected by pine beetle had highest nonnative species after Mast.,• Pine stands not affected by pine beetle had highest nonnative species after Mast+PF.

Table A.1—Continued.
42 USDA Forest Service RMRS-GTR-381. 2018 Study Studies Location Ecosystem Treatment Lifeforms Studied Summary 15 Potts and Stephens 2009; Potts et al. 2010 California Chaparral • Mastication • Prescribed fire shrub, forb, grass • 3 yrs posttreatment, • PF had higher shrub cover than mast., • Mast. had highest numbers of nonnative species, esp. nonnative grasses, • Cool season fire may be best for controlling nonnative grasses. 16 Redmond et al. 2014 Utah P-J • Mastication • Prescribed fire • Thinning (pile burn) • Control shrub, forb, grass • 2 yrs posttreatment, • With seeding, both mast. and PF increased herbaceous cover over pile-burn, • With no seeding, herb. cover only increased with PF, • All treatments increased nonnatives, esp. PF. 17 Ross et al. 2012 Utah P-J • Mastication • Prescribed fire • Thinning • Control shrub, forb, grass • 1-2 yrs posttreatment, • Both mast. and thin-scatter-burn had positive influence on understory plant cover. 18 Sikes and Muir 2009 Oregon Chaparral • Mastication • Thin-pile-burn • Control tree, shrub, forb, grass, nonnatives • 1-2 yrs posttreatment, • More shrub seedlings in both treated areas than in controls, • Thin-pile-burn increase in nonnatives, • Mastication may reduce species diversity. 19 Weekley et al. 2008 Florida Oak scrublands • Mastication • Prescribed fire • Mast. plus PF • Control Shrub, forb, grass • 1-5 yrs posttreatment, • PF and Mast+PF better than Mast. for recruitment of rare herbaceous plants because of increased bare sand & reduced litter and lichen cover. 20 Wolk and Rocca 2009 Colorado Ponderosa pine • Mastication • Thinning • Control tree, shrub, forb, grass, nonnatives • 3-5 yrs posttreatment, •Chip depth negatively- related to understory cover, • Cover of nonnatives higher in thin only as opposed to control or thin plus mast, • Species composition different in thin plus mast as opposed to control or thin only. Table A.1—Continued.
USDA Forest Service RMRS-GTR-381. 2018 43 Table A.2—Effect on residual tree attributes and regeneration from mastication and associated treatments. Study Studies Location Ecosystem Treatment Summary 1 Brockway et al. 2009 Georgia Pine, hardwoods • Mastication • Mast. plus PF • Control • 2-3 yrs posttreatment, • Mast. resulted in reduced stand density and increased mean DBH, • Mast. reduced tree species richness. 2 Collins et al. 2014a California Mixed conifer • Mastication • Prescribed fire • Mast. plus PF • Control • 7 yrs posttreatment, • For Mast, residual tree mortality was consistently lower for all species, • For PF, except Douglas-fir, residual tree mortality was higher, • Mast+PF had high residual tree mortality for white fir and sugar pine. 3 Hamma 2011 California Mixed conifer • Mastication • Prescribed fire • Control • 4-11 yrs posttreatment, • No differences between mast. and PF treatments for stand-level carbon dynamics or air pollution removal. 4 Kreye and Kobziar 2015 Florida Southern pine • Mastication • Prescribe fire • Mast. plus PF • 1-week post-burn, • Minimal tree mortality after PF and Mast+PF. 5 Moghaddas et al. 2008a California Mixed conifer • Mastication • Prescribed fire • Mast. plus PF • Control • 4 yrs posttreatment, • All treatments decreased black oak and sugar pine seedling density, • PF and Mast+PF increased Douglas-fir seedling density, • Mast+PF increased ponderosa pine seedling density. 6 Potts et al. 2010 California Chaparral • Mastication • Prescribed fire • 3-yrs posttreatment, •Treatment did not affect shrub height, species richness, or composition. 7 Reemts and Cimprich 2014 Texas Oak shrubland • Mastication • Control • 1-6 yrs posttreatment, • Mast. treatment produced lower shrub heights for 4 growing seasons. 8 Stephens et al. 2009a California Mixed conifer • Mastication • Prescribed fire • Mast. plus PF • Control • 1-2 yrs posttreatment, • Aboveground live carbon was significantly reduced with Mast. and Mast+PF, and unchanged for PF compared to control, • PF and Mast+PF had significantly greater CO2 emissions than Mast or control. 9 Stephens and Moghaddas 2005a California Mixed conifer • Mastication • Prescribed fire • Mast. plus PF • Control • 1-2 yrs posttreatment, • Treatments did not affect snag volumes for all the decay classes, • Snag density in decay class 1 was significantly greater in PF and Mast+PF compared to Mast and control. a Used the same study site in northern California.
44 USDA Forest Service RMRS-GTR-381. 2018 Appendix A.3—Effect on soil erosion, compaction, and nutrition from mastication and associated treatments. Study area Studies Location Ecosystem Treatment Summary Soil Erosion and Compaction 1 Cline et al. 2010 Utah P-J • Mastication • Control • 1-yr posttreatment, • Tire tracks increased penetration resistance, • Bare plots with mast. residue had higher infiltration rates and lower sediment yields than plots without residue. 2 Harrison et al. 2016 California and Nevada Mixed conifer • Mastication • Prescribed fire • Control • Mast. plots manipulated for residue density; PF plots on as-is burned areas with varying fire severity, • Mast. residue >25% coverage mitigated erosion, • Sites with >35% surface burned resulted in highest sediment yield. 3 Hatchett et al. 2006 California Mixed conifer • Mastication • Mast. in fall; meas. next spring, • Mast. resulted in slight to insignificant erosion as long as a layer of masticated wood chips left on site. 4 Moghaddas and Stephens 2008 California Mixed conifer • Mastication • Mast. plus PF • Control • 1-2 yrs posttreatment, • No significant soil compaction due to mast. as it was buffered by created debris bed, • Greater compaction in skid trails so any reduction in number of skid trails is advantageous to long-term soil health. 5 Ross et al. 2012 Utah P-J • Mastication • Prescribed fire • Thin • Control • 1-2 yrs posttreatment, • Compared to controls, soil aggregate stability was lower in thin-pile-burn and mast. sites. Soil moisture, temperature, and nutrition 1 Busse et al. 2005, 2010 California Oak-Ponderosa pine • Mast. plus PF • Constructed fuel beds, • Soil type did not have an effect on soil heating, • Soil moistures >20% limits lethal soil heating, • Burning fuel depths >7.5 cm in dry soils resulted in soil damage. 2 Godwin 2012 Florida Southern pine • Mastication • Prescribed fire • Control • Soil CO2 efflux rates not altered by mast. or PF, • Forests regularly burned over 60 yrs had lower soil CO2 efflux rates than forests that had never been burned. 3 Gottfried and Overby 2011 Colorado P-J • Mastication • Thin • Control • 1-3 yrs posttreatment, • Soil microbes were not decomposing mast. material, • Mast. decreased soil moisture and temp. extremes.
USDA Forest Service RMRS-GTR-381. 2018 45 Study area Studies Location Ecosystem Treatment Summary 4 Johnson et al. 2014 California Mixed conifer • Mastication • Prescribed fire • Mast. plus PF • Control • 1-yr posttreatment, • Mast+PF resulted in the export of soil nitrogen from the site, • Mast. added potassium to the forest floor. 5 Kobziar and Stephens 2006 California Ponderosa and Jeffrey pine • Mastication • Prescribed fire • Mast. plus PF • Control • 1-2 yrs posttreatment, • Mast. resulted in reductions in soil respiration and soil moisture, • PF raised soil respiration rates, • Mast+PF lowered soil respiration rates. 6 Moghaddas and Stephens 2007 California Mixed conifer • Mastication • Prescribed fire • Mast. plus PF • Control • 1-yr posttreatment, • PF and Mast+PF reduced in C and N of forest floor, • No difference in total C and N in mineral soil compared to control. 7 Owen et al. 2009 Colorado P-J • Mastication • Prescribed fire • Mast. plus PF • Control • 2.5 yrs post treatment, • Pile burns degraded soil properties and mycorrhiza levels, • Mast. had lower soil temps and higher soil moistures. 8 Rhoades et al. 2012 Colorado P-J, mixed conifers • Mastication • Control • 3-5 yrs posttreatment, • Mast. had lower max. summer soil temperatures and increased soil moisture, • Available nitrogen higher in mast. than in control. 9 Ross et al. 2012 Utah P-J • Mastication • Prescribed fire • Thin • Control • 1-2 yrs posttreatment, • Soil aggregate stability and nitrogen fixation potential were lower in the pile and burn and mast. treatments than control. 10 Southworth et al. 2011 California Chaparral • Mastication • Prescribed fire • Mast. plus PF • Control • 5 yrs posttreatment, • Ectomycorrhizal communities did not differ between treatments, • PF and Mast+PF inhibited fruiting of truffles. 11 Stottlemyer et al. 2013 South Carolina Southern pine • Mastication • Prescribed fire • Control • 0-1 yr posttreatment, • Mycorrhizal inoculum did not differ among treatments, but was highly variable within treatments. 12 Young et al. 2013b Utah P-J • Mastication • Control • 2-5 yrs posttreatment, • Mast. sites had more wet degree days than untreated sites. Table A.3—Continued.
46 USDA Forest Service RMRS-GTR-381. 2018 Table A.4—Effect on insects/animals/birds from mastication and associated treatments. Study Studies Location Ecosystem Treatments Summary 1 Amacher et al. 2008 California Mixed conifer • Mastication • Prescribed fire • Control • 1-yr posttreatment, • PF and Mast+PF increased deer mouse populations, • Mastication decreased deer mouse populations, • Brush mice positively influenced by leaving dense, low vegetation, • Other small mammals not affected by treatments. 2 Apigian et al. 2006 California Mixed conifer • Mastication • Prescribed fire • Mast. plus PF • Control • 1-yr posttreatment, • Moderate changes in species richness and abundance of leaf litter arthropods among treatments • Increased habitat heterogeneity by implementing a diversity of treatments may provide habitat for more rare species. 3 Burnett et al. 2014 California Mixed conifer • Mastication • Thin • Control • 2-6 yrs posttreatment, • Bird community composition and abundance only moderately affected by mast. and thinning treatments. 4 Reemts and Cimprich 2014 Texas Oak Shrubland • Mastication • Control • 1-6 yrs posttreatment, • No difference in probability of black-capped vireo nests in mast. sites vs. control, • Mast. can serve as substitute for PF in restoring shrubland structure with no detriment to vireo nesting. 5 Seavy et al. 2008 Oregon Oak woodlands; chaparral • Mastication • Control • 1-5 yrs posttreatment, • Bird species associated with shrub cover more abundant on controls, • Bird species associated with open areas more abundant in treated areas, • Size and shape of treated areas benefit particular species.
USDA Forest Service RMRS-GTR-381. 2018 47

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50 USDA Forest Service RMRS-GTR-381. 2018

Appendix B—Methods Used for Determining Sample Sites, 

Collecting Fine Fuels, and Calculating Particle Density and 

Surface Area of Masticated Fuels for the MASTIDON Study

This appendix is an excerpt from Keane et al. (2018) that describes how the sites 

were chosen, data were collected, and laboratory analysis were used to obtain the fine 

fuel loads and the particle characteristics that we present in this report.

Site Descriptions

Keane et al. (2018) sampled 14 study sites in mixed coniferous forests of the 

Rocky Mountains (table B.1). The sites selected in the study required a variety of 

treatment ages, mastication methods, mixed-conifer stand types, and geographic 

areas throughout the Rocky Mountains. After contacting numerous agencies and fire 

managers, we found only 14 sites that met these criteria and that could be feasibly 

sampled, processed, and analyzed within the 2 years allotted to this study. These sites 

represented seven ages (i.e., years after treatment) of masticated fuel (table B.1) and 

two moisture regimes. Moist mixed-conifer sites were dominated by western hemlock 

(Tsuga heterophylla), western red cedar (Thuja plicata), western white pine (Pinus monticola), and western larch (Larix occidentalis). These sites were located on Priest River 

Experimental Forest (EF) and Deception Creek EF in northern Idaho. Collaboration 

with the University of Idaho provided fuels from an additional moist site that was dominated mainly by ponderosa pine (Pinus ponderosa) within a pine plantation. All other 

sites were dry or xeric mixed-conifer sites dominated by ponderosa pine and Douglas-fir 

(Pseudotsuga menziesii) or juniper (Juniperus sp.). The xeric sites ranged from central 

Idaho to southern New Mexico and east to the Black Hills Experimental Forest in western South Dakota.

Field Sampling Protocols

For the field sampling, a 30 x 50 m macroplot was established that bounded the 

measurement area within the study site. This macroplot broadly represented the general 

conditions of the mastication treatment. The 30 m sides of the rectangular macroplot 

were oriented up the slope and the 50 m sides were established perpendicular to the 

slope (fig. B.1). Latitude and longitude were recorded at each corner of the macroplot, 

and at several points on a grid established within the macroplot, using a global positioning system. Within the macroplot, 20 microplots were established to collect data and 

fuel samples. To establish these microplots, a random number between 6 and 24 was 

selected to delineate a starting point (in meters) (1) along the 10-m line and (2) the 

reverse starting point (30 minus the random number) along the 40-m line. Each of these 

random points marked the intersection of four transects running in the four cardinal 

directions (fig. B.1). Microplots were 1 m × 1 m and established every 5 meters (out to 

20 m) in the four cardinal directions from this initial cross point. The microplots were 

oriented as shown in figure B.2; where, facing uphill, the bottom left corner was placed 

at the appropriate meter mark. We photographed the microplot from below its downhill 
USDA Forest Service RMRS-GTR-381. 2018 51

side. Finally, we established a 0.25-m2 (0.5-m × 0.5-m) quadrat at the lower left corner 

of each microplot from the downhill side to delineate where fuels would be collected.

At each microplot, depths of five masticated fuelbed layers were taken at the 

corners of the microplot and at the corners of the quadrat (fig. B.2). The five layers 

included undecomposed litter, masticated fuels, masticated-duff mixed together, duff 

only, and duff mixed with mineral soil. After all depth measurements were taken, all 

materials within the quadrat were collected down to the mineral soil, then sorted into 

three general fuel types (fresh litter, masticated fuels, duff) and placed in paper bags that 

were labeled as to site, transect, location along transect segment, date, photo number, 

and fuelbed category. The material could not be sorted into the same five layers as the 

depth measurements because it was too difficult to separate these categories in the field 

during the destructive sampling. As a result, we collapsed the masticated-duff, duff, and 

duff-soil layers into just one bag and called it duff. We sorted the duff bag in the laboratory into its proper components. We did not collect live biomass from shrubs, herbs, or 

logs because they were rare and beyond the scope of this study, but general vegetation 

was recorded at each microplot. The collections from the University of Idaho sites (UI, 

Figure B.1—Grid sampling design implemented at each site. The 50 x 30 m macroplot was oriented so that the top was 

uphill. Six lines were located within the macroplot at 0 m, 10 m, 20 m, 30 m, 40 m and 50 m. Two methods were 

used to sample within this macroplot. Sampling method 1 tested the consistency in depth of masticated material 

within the entire macroplot. Depths of each fuelbed layer were measured at 3 m intervals along each of the six lines 

starting at the zero baseline. Samples were labeled as (major line number, meter footage), that is (0, 3). These depth 

plots are represented within this figure by the regularly spaced small blue diamonds within the macroplot. Sampling 

method 2 tested the variation in depths of the masticated materials at 0.5 to 1 m intervals and provided the materials 

used to describe the masticated material within the macroplot. Small-scale sampling using Hood and Wu (2006) 

microplots is located at the closely spaced dots along lines 10 and 40 m that form the tightly arranged squares. The 

randomly located start point for each line of microplots is designated with a large red circle. Samples were labeled 

using the following protocol: major line number, direction, and meter mark with direction varying from northerly (1) 

to westerly (4). These samples were taken along the 10 m and 40 m major lines only.
52 USDA Forest Service RMRS-GTR-381. 2018

table 4 in main text) were done with slightly different methodologies and these are 

documented in Lyon (2015).

Laboratory Tasks

Laboratory tasks consisted of five broad types of activities. These included (1) 

sorting of the field particles, (2) measuring and weighing a subsample of individual 

particles, (3) obtaining particle densities from this subsample, (4) burning fuel to 

compute heat content and lignin and cellulose fractions from this subsample, and (5) 

running chemical tests on particle subsample for percentages of carbon and nitrogen. In 

this Appendix, we will only focus on activities associated with activities one, two, and 

three. Ten of the 20 microplot collections from each study site were randomly selected 

to acquire data for these three activities in the laboratory. A total of 151 microplots were 

analyzed in this way.

Sorting 

The first step in the processing of a masticated sample to create a physical description of its components was to sort collected material into the five masticated fuelbed 

layers. The fresh litter did not require sorting or sizing. Masticated wood was sorted 

into 15 shape categories (not presented in this paper) and three size categories (1-, 10-, 

100-hr fuels). Bark was sorted into large chunks (100-hr) and small pieces (1-, 10-hr). 

We removed all fresh litter, bark, and wood particles from the duff collected in the field, 

which left pieces of debris less than 6 mm (0.25 inch) for the duff category. See Keane 

et al. (2018) for a complete description of the shape categories that were divided and 

analyzed.

Physical Measurements

After sorting, the particles in each size and shape class were oven-dried at 90 oC 

for 2 days, then weighed to compute total load in each shape, size, and layer class. A 

randomly selected subset of particles from each shape and size class was then measured 

for their individual dimensions and to obtain their particle dry weight (PW). A minimum 

Figure B.2—A cross section of the masticated fuelbed illustrating the five fuelbed layers that were characterized in 

this study.
USDA Forest Service RMRS-GTR-381. 2018 53

of one or two particles from each shape, size, and layer class were selected; however, 

in some cases, there were more than 200 particles (e.g., 1-hr parallelograms). In these 

situations, a maximum of 10 particles was selected for each subsample. Length (mm), 

width (mm), and height (mm) of each woody particle was measured to the nearest 0.1 

mm using a caliper connected to a computer to capture measurements. Depending on 

the particle shape and other dimensional measurements needed to compute volume and 

surface area for each particle, more data measures were added. 

Particle Density of Individual Particles

Particle densities (PDs) of individual particles were determined using a two-fluid 

displacement process that has historically been used to determine density in soils or duff 

(Williamson and Wiemann 2010). The method consisted of slowly submerging particles 

in a large cylindrical tube containing a combination of two fluids (fig. B.3). The upper 

fluid was 100 percent kerosene; the lower fluid was a solution of 50 percent glycerin 

and 50 percent water. Both fluids were approximately 20 cm deep to allow enough room 

Figure B.3—Apparatus used to estimate the density and volume of masticated fuel particles. (A) Parts of apparatus 

include a scale (top), a fluid column with two fluids (kerosene at top and a 50:50 mixture of glycerin and 

water at bottom), and a lift (bottom) to raise and lower fluid column. (B) Closeup of a particle in kerosene. 

Particle has weight attached to assure it stays submerged in each fluid.

B)

A)
54 USDA Forest Service RMRS-GTR-381. 2018

for submersion of large particles; the cylinder sat on a lift so that it could be raised and 

lowered, as needed, during the submersion process. The technician attached the particle 

to plastic line that had a large lead weight at the end to keep it submerged. The line 

with the lead weight and the particle was attached to a scale. The lead weight and line 

were tared by submerging them in each fluid without a particle attached and recording a 

weight in each fluid from the scale. Densities for each fluid were taken from the literature for inputs in the formulas that follow.

In the displacement method, the scale was first tared to zero with the particle, 

line, and lead weight all connected but outside of the fluids. Then the technician slowly 

lowered the particle into the kerosene until it was about 1 cm above the kerosene-glycerin boundary, and it remained at that depth for 3 minutes (fig. B.3). After 3 minutes, 

the weight on the scale was recorded. The technician than lowered the particle into the 

glycerin-water layer to within 1.25 cm of the boundary between the two fluids and left 

to equilibrate and displace glycerin. After 3 minutes, the weight of the particle in the 

glycerin-water layer was recorded. 

The PD was computed using the following equation (Sarli et al. 2001): 

PD = PW ((Pk-Pmix))/((Wk-Wmix)) 

where PD is the particle density (g/cm-3); PW is the particle dry weight (g); Pk and 

Pmix are the densities of the kerosene and the glycerin-water mixture, respectively 

(g/cm-3); and Wk and Wmix are the weights (g) of the particle in the kerosene and 

glycerin-water mixture layers, respectively.

Surface Area of Individual Particles

Surface area (SA) was calculated by solving for particle volume (PV) from the PD 

measurement (determined above) and then calculating a new length from generalized 

volume equations for each shape. The new length was put in generalized surface area 

equations. 

First, PV was calculated as follows: PV = PW/PD

where PV is the particle volume (cm3); PW is the particle dry weight (g); and PD is 

the particle density (g/cm-3). Using PV from the particle density, a new length was 

calculated for the shape using the standardized shape-volume equations taken from the 

literature. The new length was applied to standard formulas used to compute surface 

area for the individual particle shapes. Although not a perfect solution, the resulting surface areas were at least semi-adjusted for the departure from a perfect shape in a manner 

similar to the particle densities. The method had problems, especially in the parallelograms, where a total length was measured and a mean length was calculated for the two 

long sides. In this case, only the mean length was adjusted in the surface area formula. 

Calculating new lengths in ellipses also required some assumptions, including that the 

longest axis was length and the shortest axis was height.
USDA Forest Service RMRS-GTR-381. 2018 55

Table B.1—Field locations examined in this study. EF = Experimental forest; NF = National Forest; NP = National 

Park. Tree abbreviations include PP = ponderosa pine, Pinus ponderosa, DF = Douglas-fir, Pseudotsuga 

menziesii, WH = western hemlock, Tsuga heterophylla; WRC = western Redcedar, Thuja plicata, WP = western 

white pine, Pinus monticola, and WL = western larch, Larix occidentalis.

Location Site Name Primary tree species

Treatment 

Date Mastication type

Dry mixed conifer

Boise Basin EF, ID Amber PP 2004 Vertical shaft/teeth

Boise Basin EF, ID Amber

New PP, DF 2010 Horizontal shaft/teeth

Black Hills EF, SD BHMix PP 2012 Vertical shaft/knives

Black Hills EF, SD BHMow PP 2012 Vertical shaft/knives

Manitou EF, CO MEFChip PP, DF 2004 Chipped

Manitou EF, CO MEFWS PP, DF 2005 Horizontal shaft/teeth

San Juan NF, CO Skelton PP, DF 2010-2011 Horizontal shaft/teeth

Santa Fe NF, NM LG PP 2006 Horizontal shaft/teeth

Santa Fe NF, NM PAL PP 2011-2012 Horizontal shaft/teeth

Valles Caldera NP, NM VC1 PP 2007-2008 Horizontal head/teeth

Valles Caldera NP, NM VC2 PP 2012 Horizontal head/teeth

Moist mixed conifer

Deception Creek EF, ID DC1 WH, WWP, WL 2004 Vertical shaft/teeth

Priest River EF, ID PR3 WRC, WH, WWP, 

WL 2011 Horizontal head/teeth

Priest River EF, ID PRCC1 WH, WRC, WWP, 

WL 2007 Vertical shaft/teeth

University of Idaho EF, ID UI PP 2014 Horizontal head/teeth

Literature Cited

Keane, Robert E.; Sikkink, Pamela G; Jain, Theresa B. 2018. Physical and chemical 

characteristics of surface fuels in masticated mixed-conifer stands of the U.S. Rocky 

Mountains. Gen. Tech. Rep. RMRS-GTR-370. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Research Station. 56 p.

Hood, Sharon; Wu, Ros. 2006. Estimating fuel bed loadings in masticated areas. In: Andrews, 

Patricia L.; Butler, Bret W., comps. Fuels management—How to measure success: Conference 

proceedings; 2006 March 28–30; Portland, OR. Proceedings RMRS-P-41. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 333–340.

Lyon, Zachary D. 2015. Fire behavior in masticated fuels: Lab and prescribed burn experiments. 

Thesis. Moscow, ID: University of Idaho.

Sarli, Guillermo O.; Filgueira, Roberto R.; Giménez, Daniel. 2001. Measurement of soil 

aggregate density by volume displacement in two non-mixing liquids. Soil Science Society of 

America Journal. 65(5): 1400–1403.

Williamson, G. Bruce; Wiemann, Michael C. 2010. Measuring wood specific gravity correctly. 

American Journal of Botany. 97(3): 519–524.

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