Forest Ecology and Management 285 (2012) 204–212

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Forest Ecology and Management
journal homepage: www.elsevier.com/locate/foreco

Fuel treatment longevity in a Sierra Nevada mixed conifer forest
Scott L. Stephens a,⇑, Brandon M. Collins b, Gary Roller b
a
b

Ecosystem Sciences Division, Department of Environmental Science, Policy, and Management, 130 Mulford Hall, University of California, Berkeley, CA 94720-3114, USA
USDA Forest Service, Pacific Southwest Research Station, 1731 Research Park Drive, Davis, CA 95618, USA

a r t i c l e

i n f o

Article history:
Received 5 July 2012
Received in revised form 7 August 2012
Accepted 17 August 2012

Keywords:
Fire ecology
Fire management
Forest ecology
Restoration
Fire hazards
Fire behavior

a b s t r a c t
Understanding the longevity of fuel treatments in terms of their ability to maintain fire behavior and
effects within a desired range is an important question. The objective of this study was to determine
how fuels, forest structure, and predicted fire behavior changed 7-years after initial treatments. Three different treatments: mechanical only, mechanical plus fire, and prescribed fire only, as well as untreated
control, were each randomly applied to 3 of 12 experimental units. Many aspects of the initial fuel treatments changed in 7 years. The overall hazard of the control units increased significantly indicating continued passive management has further increased already high fire hazards. Mechanical only fire hazard
decreased after 7 years and are now similar to the two fire treatments, which both maintained low hazards throughout the study. Tree density declined significantly 7 years after the initial fire only treatments,
while basal area in both fire treatments was unchanged relative to immediate post-treatment conditions.
Our findings indicating reduced fire hazard over time in mechanical only treatments might provide an
opportunity for a staggered treatment schedule that included prescribed fire which could increase overall
treatment longevity to approximately 20 years. Changes in our mixed conifer forests after fuel treatment
were generally larger than those reported from ponderosa pine forests in the Rocky Mountains.
Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction
It is recognized that past and current management practices
including harvesting, livestock grazing, and fire exclusion have increased fire hazards in western US forests that once burned frequently under low-moderate intensity fire regimes (Fulé et al.,
2012; Stephens et al., 2012). Such forest conditions concern fire
managers because the increased fuel loads and altered forest structure have made many forests vulnerable to fire severities outside of
desired ranges (Miller et al., 2009). Changing climates in the next
several decades will further complicate fire management by
increasing temperatures and fire season length (McKenzie et al.,
2004; Westerling et al., 2006), which further emphasizes the need
to promote resilient forested ecosystems (Millar et al., 2007).
Research has determined that the reduction of surface fuels is
the most important component of reducing forest fire hazards
since this leads to lower fireline intensity and increased ability to
manage fire when needed (Stephens et al., 2009). The second most
important fuel stratum in terms of fire hazard reduction is commonly ladder fuels which can provide vertical continuity to move
fire from the surface to the forest overstory. Retaining and growing
larger trees is also an important aspect of fire hazards reduction
⇑ Corresponding author. Tel.: +1 510 642 7304; fax: +1 510 643 5438.
E-mail addresses: sstephens@berkeley.edu (S.L. Stephens), bmcollins@fs.fed.us
(B.M. Collins), groller@fs.fed.us (G. Roller).
0378-1127/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.foreco.2012.08.030

treatments since these trees have a higher survival probability
because of thicker bark and elevated crowns (Agee and Skinner,
2005; Fulé et al., 2007; Hurteau and North, 2009).
Several papers have analyzed the change in potential fire
behavior resulting from initial fuels treatments using empirical
field studies (Kilgore and Sando, 1975; Covington et al., 1997;
Omi and Martinson, 2004; North et al., 2007; Stephens et al.,
2009; Fiedler et al., 2010) or simulations with or without the aid
of field data (Keane et al., 1990; van Wagtendonk, 1996; Stephens,
1998). Far fewer studies have investigated how forests that have
received fuel treatments change over time (Peterson et al., 1994;
Sackett and Haase, 1998; Fulé et al., 2005, 2007; Fajardo et al.,
2007) and most of these studies have occurred in relatively xeric
Rocky Mountain ponderosa pine (Pinus ponderosa) forests.
Understanding the longevity of fuel treatments in terms of their
ability to maintain fire behavior and effects within a desired range
is an important management question. Initial treatment effects
such as reduced surface and ladder fuels will diminish over time
(van Wagtendonk, 1985; Kiefer et al., 2006) and information from
longer-term studies can assist managers in providing important
information to aid in deciding how to allocate finite resources,
e.g., maintenance of existing fuel treatment versus implementation
of new fuel treatments. The objective of this study was to determine how fuels, forest structure, and predicted fire behavior have
changed 7-years after initial fuel treatments in mixed conifer
forests in the northern Sierra Nevada. The null hypothesis

 S.L. Stephens et al. / Forest Ecology and Management 285 (2012) 204–212

investigated is there will be no differences in forest structure and
predicted fire behavior 1-year post-treatment (Stephens and
Moghaddas, 2005a) as compared to similar measurement and
analysis 7-years post-treatment.

2. Methods
2.1. Study area and treatments
This study was performed at the University of California Blodgett Forest Research Station (Blodgett Forest), approximately 20 km
east of Georgetown, California. Blodgett Forest is located in the
mixed conifer zone of the north-central Sierra Nevada at latitude
38°540 4500 N, longitude 120°390 2700 W, between 1100 and 1410 m
above sea level, and encompasses an area of 1780 ha (Fig. 1). Tree
species in this area include sugar pine (Pinus lambertiana), ponderosa pine, white fir (Abies concolor), incense-cedar (Calocedrus decurrens), Douglas-fir (Pseudotsuga menziesii) Franco, California black
oak (Quercus kelloggii), tanoak (Lithocarpus densiflorus), bush chinkapin (Chrysolepis sempervirens), and Pacific madrone (Arbutus
menziezii).
Soils at Blodgett Forest are well-developed, well-drained Haploxeralfs (Alfisols), derived from either andesitic mudflow or granitic/granodiorite parent materials (Moghaddas and Stephens,
2007). Cohasset, Bighill, Holland, and Musick are common soil series. Soils are deep, weathered, sandy-loams overlain by an organic
forest floor horizon. Common soil depths range from 85–115 cm.
Slopes across Blodgett Forest average less than 30%.
Climate at Blodgett Forest is Mediterranean with a summer
drought period that extends into the fall. Winter and spring receive
the majority of precipitation which averages 160 cm (Stephens and
Collins, 2004). Average temperatures in January range between

California

*

TREATMENT
Control
Thin only
Thin & burn
Burn only
Headquarters
Contour interval (40m)

N

0

1,000
Meters

Fig. 1. Experimental units (three replicates per treatment type) within Blodgett
Forest, California, USA.

205

0 °C and 8 °C. Summer months are mild with average August temperatures between 10 °C and 29 °C, with infrequent summer precipitation from thunderstorms (averaging 4 cm over the summer
months from 1960 to 2000) (Stephens and Collins, 2004).
Fire was a common ecosystem process in the mixed conifer forests of Blodgett Forest before the policy of fire suppression began
early in the 20th century. Between 1750 and 1900, median composite fire intervals at the 9–15 ha spatial scale were 4.7 years with
a fire interval range of 4–28 years (Stephens and Collins, 2004).
Forested areas at Blodgett Forest have been repeatedly harvested
and subjected to fire suppression for the last 100 years reflecting
a management history common to many forests in California and
elsewhere in the Western US (Graham et al., 2004).
2.1.1. Fuel treatments
The primary objective of the treatments was to modify stand
structure such that 80% of the dominant and co-dominant trees
in the post-treatment stand would survive a wildfire modeled under 80th percentile weather conditions (McIver et al., 2009). The
secondary objective was to create a stand structure that maintained or restored several forest attributes and processes including,
but not limited to, snag and coarse woody debris recruitment, floral and faunal species diversity, nutrient cycling, and seedling
establishment. To meet these objectives, three different treatments: mechanical only, mechanical plus fire, and prescribed fire
only, as well as untreated control, were each randomly applied
(complete randomized design) to 3 of 12 experimental units that
varied in size from 14 to 29 ha. Total area for the 12 experimental
units was 225 hectares. To reduce edge effects from adjoining
areas, data collection was restricted to a 10 ha core area in the center of each experimental unit (Stephens and Moghaddas, 2005a).
Control units received no treatment during the study period
(2000–2012). Mechanical only treatment units had a two-stage
prescription; in 2001 stands were crown thinned followed by thinning from below to maximize crown spacing while retaining 28–
34 m2 ha 1 of basal area with the goal to produce an even species
mix of residual conifers (Stephens and Moghaddas, 2005a). Individual trees were cut using a chainsaw and removed with either
a rubber tired or track laying skidder. During harvests, some hardwoods, primarily California black oak, were coppiced to facilitate
their regeneration. All residual trees were well spaced with little
overlap of live crowns in dominant and co-dominant trees. Following the harvest, approximately 90% of understory conifers and
hardwoods up to 25 cm diameter at breast height (DBH) were masticated in place using an excavator mounted rotary masticator.
Mastication shreds and chips standing small diameter live and
dead trees in place and this material was not removed from the
experimental units. The remaining un-masticated understory trees
were left in scattered clumps of 0.04–0.20 ha in size.
Mechanical plus fire experimental units underwent the same
treatment as mechanical only units, but in addition, they were prescribed burned using a backing fire. Fire only units were burned
with no pre-treatment using strip head-fires. All initial prescribed
burning was conducted during a short period (10/23/2002 to 11/6/
2002; the fire only units were burned a 2nd time in fall 2009 but
this study will not include the results of these fires) with the
majority of burning being done at night because relative humidity,
temperature, wind speed, and fuel moistures were within predetermined levels to produce the desired fire effects (Kobziar
et al., 2007). Prescribed fire prescription parameters for temperature, relative humidity, and wind speed were 0–10 °C, >35%, and
0.0–5 km h 1, respectively. Desired ten-hour fuel stick moisture
content was 7–10%.
2.1.2. Vegetation measurements
Overstory and understory vegetation was measured in twenty
0.04 ha circular plots, installed in each of the 12 experimental units

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S.L. Stephens et al. / Forest Ecology and Management 285 (2012) 204–212

(240 plots total) in 2001 (PRE), 2003 (POST-1YR), and 2009 (POST-7YR).
Individual plots were placed on a systematic 60 m grid with a random starting point. Plot centers were permanently marked with a
pipe and by tagging witness trees to facilitate plot relocation after
treatments. Tree species, DBH, total height, height to live crown
base, and crown position (dominant, co-dominant, intermediate,
and suppressed) were recorded for all trees greater than 15 cm
DBH. Canopy cover was measured using a 25 point grid in each
0.04 ha plot with a site tube.

2.1.3. Fuel measurements
Surface and ground fuels were sampled with two random azimuth transects at each of the 240 plots using the line-intercept
method (Brown, 1974) on the same schedule as the vegetation
measurements. A total of 480 fuel transects were installed and
the same azimuths were used here as done in the original measurements in 2001 and 2003 (Stephens and Moghaddas, 2005a).
One-hour (0–0.64 cm) and 10-h (0.64–2.54 cm) fuels were sampled from 0 to 2 m, 100 h (2.54–7.62 cm) fuels from 0 to 3 m,
and 1000 h (>7.62 cm) and larger fuels from 0 to 11.3 m on each
transect. Duff and litter depth in cm were measured at 0.3 and
0.9 m on each transect (same points as done previously). Surface
and ground fuel loads were calculated using appropriate equations
developed for California forests (van Wagtendonk et al., 1996,
1998). Coefficients required to calculate all surface and ground fuel
loads were arithmetically weighted by plot basal area fraction to
produce accurate and precise estimates of ground and surface fuel
loads (Stephens and Moghaddas, 2005a).

2.2. Fire modeling
We modeled potential fire behavior for each inventory plot, at
each time step, with the Fire and Fuels Extension (FFE) to the Forest Vegetation Simulator (Reinhardt and Crookston, 2003). FFE uses
established equations to predict fire behavior and crown fire potential based on user-input tree lists and fire weather (Reinhardt,
2003; Rebain, 2010). We used the conditions during large spread
events in two nearby wildfires (2001 Star Fire, 2008 American River Complex) for the weather inputs (Table 1). By using actual conditions from nearby wildfires that posed substantial fire control
problems we believe predicted fire behavior may better characterize wildfire potential as opposed to using conditions based on fireweather percentile thresholds.
We used surface fuel models for different treatment/time period combinations (Table 2). These fuel model assignments were
based on both measured plot fuel loads for each treatment/time
period and observed fuelbed characteristics in the field. We opted
to use the standard 13 Anderson (1982) surface fuel models to
maintain consistency with previous analyses for the same study
area (Stephens and Moghaddas, 2005a,b) and regionally within
the FFS network (Stephens et al., 2009). We modeled fire behavior
and crown fire potential for each plot/time period combination

Table 1
Weather parameters used in fire modeling in mixed
conifer forests at Blodgett Forest.
Weather parameter

Value

Wind speed (km h 1)

32

Fuel moisture (%)
1h
10 h
100 h
Live herbaceous
Live woody

3
4
5
70
70

Table 2
Plot fuel model assignments based on treatment type and time period. Assignments
were based on both measured plot fuel loads and observed fuelbed characteristics in
the field.
Time period

PRE
POST-1YR
POST-7YR

Anderson (1982) fuel model by treatment
Control

Burn only

Thin only

Thin and burn

10
10
10

10
8
9

10
12
11

10
8
9

(n = 702). Note that a total of six plots were removed from the analysis because we did not have complete records at each time step.
Our analysis focused on two fire behavior outputs from FFE: total flame length and torching probability (P-torch). The calculation
of P-torch first involves randomly populating 0.01 ha sub-plots
from the stand tree list using a Monte Carlo simulation. For each
sub-plot FFE computes the surface fire flame length that would
be required to cause torching. Next the program computes the
height above the ground that the predicted surface fire can ignite
crowns based on discussion in Scott and Reinhardt (2001, p. 13).
The torching probability is based on whether this predicted height
exceeds the flame length needed to ignite tree crowns. Rebain
(2010) explains that torching probability is the proportion of stand
area where crowns of larger trees can be ignited by surface fire or
flames from burning crowns of small trees. Rebain (2010) argues
that this index may better characterize hazard due to torching
compared to the conventionally used torching index (see Scott
and Reinhardt, 2001), due to the lack of dependence on the problematic calculation of canopy base height. We also report canopy
base height and canopy bulk density for each treatment/time period combination, which was derived from FFE.
2.3. Data analysis
Based on the plot-level tree measurements we calculated live
tree basal area, live tree density, and species composition (based
on live tree basal area proportion). We used these calculated metrics along with plot-level estimates of canopy cover, fuel loads in
four classes: duff, litter, fine woody (1–100 h time lag classes),
and coarse woody (1000 h time lag class), FFE-derived canopy base
height, FFE-derived canopy bulk density, predicted flame length,
and torching probability to test for differences among time periods
(PRE, POST-1YR, and POST-7YR) and among treatments (control, fire only,
mechanical only, and mechanical plus fire) with a repeated measures analysis (Proc Mixed – SAS, 2009). We examined diagnostic
plots of the residuals to check compliance with normality and
homogeneity of variance assumptions for all variables. Several
variables were log + 1 transformed and some were arcsine
square-root transformed to meet assumptions. We used an autoregressive covariance structure for all variables. Differences among
time periods and treatments were inferred from Tukey–Kramer adjusted P-values, with a = 0.05. Pairwise comparisons among treatments and time periods were only investigated when either the
time period or treatment fixed effect was significant and the time
period-treatment interaction was significant.
3. Results
Tree density was similar in the control across time periods (PRE,
while both live basal area and canopy cover increased significantly POST-7YR relative to the PRE and POST-1YR
(Fig. 2). Both mechanical treatments significantly reduced tree
density and basal area POST-1YR, relative to PRE levels and POST-1YR control. By POST-7YR tree densities were unchanged in both mechanical
POST-1YR, POST-7YR),

 S.L. Stephens et al. / Forest Ecology and Management 285 (2012) 204–212

207

Fig. 2. Average forest structure attributes by time period and treatment type. These attributes were constructed using only live trees P 15 cm diameter at breast height
(dbh). Error bars represent the standard error for each mean. Letters above bars indicate significantly different time period/treatment estimates based on pairwise
comparisons (n = 66) using Tukey–Kramer adjusted P-values. Comparisons indicated for tree density are for aggregated tree density (all trees > 15 cm dbh).

treatments, relative to POST-1YR, while basal area and canopy cover
increased significantly in the mechanical only, relative to POST-1YR
(Fig. 2). Despite this increase, basal area in the mechanical only
POST-7YR was significantly lower than that in both the mechanical
only PRE and the control POST-7YR. Canopy cover and basal area in
the mechanical plus fire treatment was unchanged POST-7YR relative
to POST-1YR, and was significantly below that for the control POST-7YR
(Fig. 2).Tree density did not change initially in the fire only treatment, but did decline significantly by POST-7YR, relative to POST-1YR
and PRE levels, as well as all three time periods for the control. Basal
area and canopy cover in the fire only treatment did not change
significantly over time (Fig. 2).
Species composition was generally stable across time periods
and treatments, with a few exceptions. Both mechanical treatments in POST-1YR and POST-7YR had a significantly lower proportion
of incense-cedar, relative to PRE levels (Fig. 3, significance not reported). Pine proportion in both mechanical treatments POST-7YR
was significantly greater than PRE levels. In the mechanical plus fire
treatment, pine proportion POST-7YR was significantly higher than all
other treatments POST-7YR, while both hardwood and white fir proportion POST-7YR was significantly lower than PRE mechanical plus
fire only (Fig. 3).

Duff, litter, fine woody, and coarse woody fuel loads were significantly reduced in both burning treatments POST-1YR, relative to
PRE levels, as well as POST-1YR for the control (Fig. 4). These reductions relative to PRE levels held POST-7YR in all fuel classes, but were
not statistically different from POST-7YR control for all fuel classes except duff (Fig. 4). Litter loads increased POST-7YR in both burning
treatments relative to POST-1YR for the same treatments, and were
not different from the other treatments POST-7YR (Fig. 4). Litter and
duff loads in the mechanical only treatment were stable over time
and statistically indistinguishable from the control at all time periods (Fig. 4). Fine woody fuel loads in the mechanical only treatment increased significantly from PRE to POST-1YR, and then
decreased significantly from POST-1YR to POST-7YR (Fig. 4). The only statistically significant difference in fine woody fuel loads among
treatments POST-7YR is between the mechanical only (higher) and
the mechanical plus fire (lower) treatments (Fig. 4). Coarse woody
fuel loads in the mechanical only treatment did not change PRE to
POST-1YR, but decreased significantly from POST-1YR to POST-7YR
(Fig. 4). There were no differences in coarse woody fuel loads
among treatments POST-7YR.
Canopy base height was stable in the control and fire only treatments from PRE to POST-1YR, but decreased for the control and

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S.L. Stephens et al. / Forest Ecology and Management 285 (2012) 204–212

Fig. 3. Average tree species composition, based on live basal area proportion, by time period and treatment type, at Blodgett Forest.

Fig. 4. Average fuel loads by time period and treatment type. Error bars represent the standard error for each mean. Letters above bars indicate significantly different time
period/treatment estimates based on pairwise comparisons (n = 66) using Tukey–Kramer adjusted P-values.

increased in the fire only from POST-1YR to POST-7YR (Fig. 5). Canopy
bulk density was stable across all three time periods in the control,
and from PRE to POST-1YR for the fire only. By POST-7YR in the fire only
treatment, canopy bulk density decreased significantly relative to
PRE and POST-1YR (Fig. 5). Canopy base height increased significantly

and canopy bulk density decreased significantly from PRE to POST-1YR
for both mechanical treatments. These relationships held POST-7YR,
relative to PRE, with the exception of canopy base height for
mechanical only, which decreased significantly and was not statistically different from the PRE level (Fig. 5). Predicted flame length

 S.L. Stephens et al. / Forest Ecology and Management 285 (2012) 204–212

209

Fig. 5. Average canopy structure and predicted fire behavior by time period and treatment type. These values were derived from the Fire and Fuels Extension to the Forest
Vegetation Simulator (Rebain, 2010). Error bars represent the standard error for each mean. Letters above bars indicate significantly different time period/treatment estimates
based on pairwise comparisons (n = 66) using Tukey–Kramer adjusted P-values.

and torching probability was significantly lower in the two burning
treatments POST-1YR and POST-7YR relative to PRE levels and to the control at all time periods (Fig. 5). In the mechanical only treatment,
predicted flame length and torching probability was unchanged
from PRE to POST-1YR, but by POST-7YR both decreased significantly,
and were statistically indistinguishable from POST-7YR in the two
burning treatments (Fig. 5). Predicted flame length was stable in
the control across all time periods, but torching probability
increased significantly POST-7YR, relative to both PRE and POST-1YR
(Fig. 5).

4. Discussion
Many aspects of the 2002 fuel treatments in our Sierra Nevada
mixed conifer forests have changed in 7 years, both when compared to control conditions and to immediate post-treatment conditions. Fire hazard initially following the mechanical only
treatment was unchanged due to the interaction between increased fine woody fuels as a result of activity fuel inputs from
thinning and mastication and the overall reduction in canopy fuels
(increased canopy base height and decreased canopy bulk density)
(Figs. 4 and 5). Seven years after treatment fine fuel loads in these
stands have decreased, likely due to decomposition, while canopy
bulk density has remained the same, and canopy base height has
decreased to what it was before treatment. The net effect is that
fire hazard (indicated by predicted flame length and torching probability) noticeably decreased POST-7YR and is similar to the two fire
treatments (Fig. 5) which was surprising. The increased time after
initial treatment has been very favorable to the mechanical only
treatments at Blodgett Forest from a fire hazard standpoint.

Both fire treatments appear to have limited stand-level growth.
The lack of change in canopy cover and basal area in the fire treatments seven years after initial treatment is in strong contrast to
the mechanical only and control stands, which grew significantly
over the same period (Fig. 2). Apparently the backing fires in the
mechanical plus fire treatment and the strip-head fires in the fire
only treatment used to reduce surface and ladder fuels caused
residual damage to many remaining trees, limiting the ability of
these trees to grow and take advantage of the reductions in stand
density, particularly in the mechanical plus fire treatment. This
damage can be seen today with large fire scars on the lower boles
of many trees. The similarity between the two fire treatments in
limiting stand-level growth despite the different applications of
fire, and resulting fire intensity, are likely due to the additional fuel
inputs from the thinning and mastication operations in the
mechanical plus fire treatment. We hypothesize that these additional fuel inputs increased fire residence time, which resulted in
similar overall damage to the higher intensity fire in the fire only
treatment (Kobziar et al., 2007). How long tree growth will continue to remain relatively stagnant is a question for future research. As an interesting anecdote regarding tree damage in the
mechanical plus fire treatment, forest operators working to remove
recent snags within fall distance to roads for safety reasons reported difficulty in felling due to unnoticed rotten wood under
the bark (K. Sunmmers, personal comm., 2011).
The fire only treatment did not result in changes in tree density
initially, but after 7 years, tree density significantly declined and
was similar to both treatments that included mechanical methods
(Fig. 2). This change in tree density was primarily driven by a decrease in the smallest tree size class (Fig. 2), and is indicative of delayed secondary mortality after fire. This longer-term response

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S.L. Stephens et al. / Forest Ecology and Management 285 (2012) 204–212

apparently did not affect larger trees (>30.4 cm dbh) enough to result in significant mortality. The lack of change in canopy cover and
tree basal area over time provides evidence that although our
strip-headfires, with flame lengths of approximately 1 m (Kobziar
et al., 2007), may have affected growth, the fires did not kill many
overstory trees. Other studies have determined high resistance to
prescribed fire mortality of larger mixed conifer trees in the Sierra
Nevada (Miller and Urban, 2000; Stephens and Finney, 2002). As
tree size increases it becomes more and more difficult to kill them
with a fire only treatment since most prescribed fires are designed
to be of low-moderate intensity.
Coarse wood remains at relatively low levels in the fire only and
mechanical plus fire treatments because of initial high consumption of these fuels (Stephens and Moghaddas, 2005c). The fire only
treatments did result in a significant decrease in crown bulk density and an increase in canopy base height after 7 years (Fig. 5),
coupling this with very low levels of fine woody fuels results in
continued very low torching probabilities. It is clear that both
treatments that included prescribed fire continue to produce
stands that are very resistant to high severity fires. Use of prescribed fire also has the distinct advantage of incorporating the
most fundamental ecosystem process back into these forests which
has been shaping them for millennia (Swetnam, 1993).
Tree species composition remained similar over the course of
the study with a few exceptions. The mechanical plus fire and
mechanical only treatments resulted in a significantly higher proportion of pine (ponderosa and sugar pine) 7-years post-treatment
(Fig. 3) which follows our marking prescription that emphasized
leaving more pines and removing more of the shade tolerant species. The mechanical plus fire treatment also had the lowest proportion of white fir and California black oak 7-years after
treatment. Reduction of California black oak is likely a result of
high heat loads to tree boles from flaming and smoldering combustion that top-killed many oaks; some of these top-killed trees resprouted but this was not universal. Both treatments that included
mechanical methods resulted in a significantly lower proportion of
incense-cedar. This is partially explained because this species has
the highest lumber value in the Sierra Nevada and these treatments did produce positive revenues (Hartsough et al., 2008).
Control plots continued to grow with basal area and canopy
cover significantly increasing over the 7-year period (Fig. 2). In
addition to these changes, canopy base height decreased but crown
bulk density was unchanged. All of these changes were somewhat
expected as Blodgett Forest has productive soils with moderately
high precipitation (Moghaddas and Stephens, 2007). A bit more
surprising is the significant reduction in fine fuels POST-7YR in the
control units from both the PRE and POST-1YR levels (Fig. 4). Reduction
in woody understory fuels in the control units must have been
from decomposition since no other treatments were applied. The
history of all 12 experimental units used in this experiment at
Blodgett Forest may assist in an explanation for this reduction.
All of the experimental units were commercially thinned from below 5–10 years before the initiation of this study in 2000. The past
harvests left all activity fuels on site, a common practice in the
Sierra Nevada and elsewhere in the western US (Graham et al.,
2004). We believe those activity fuels likely reached a decomposition state that moved them from the fine woody category into the
duff layer (duff load did increase POST-7YR versus POST-1YR but not significantly). Despite this reduction in fine woody fuels, the overall
hazard of the control units increased significantly POST-7YR from
both PRE and POST-1YR levels (Fig. 5) indicating that continued passive
management (Agee, 2003) has further increased the already high
fire hazards in these forests.
The duff, litter, and fine fuel loads were similar in the fire only
and mechanical plus fire treatment across the whole study period
(Fig. 4). This is probably explained with surface and ground fuel

loads that were initially high (approximately 120 t/ha), relatively
homogeneous with few discontinuities, and low fuel moisture contents during burning (Stephens and Moghaddas, 2005a). Managers
that desire to retain more of the surface or ground fuels for wildlife
habitat could explore the use of spring prescribed fires (Knapp
et al., 2005; Stephens et al., 2012).
Our findings indicating reduced fire hazard over time in the
mechanical only treatment might provide an opportunity for a
staggered treatment schedule. Initial mechanical treatments did
not reduce the torching probability (this work) but did increase
the probability that larger trees would survive wildfire (Stephens
and Moghaddas, 2005a). This study determined that decomposition was sufficient to significantly reduce fine woody fuel loads
to a point where fire hazards were low 7-years post-treatment.
Canopy base height has declined to a value similar to the PRE condition, whereas it was significantly increased POST-1YR. Implementing a prescribed fire in the next 2–4 years would reduce fine
woody and litter fuels, increase canopy base height (remove tree
saplings and shrubs), and would result in a forest condition that
would be very resistant to wildfire. It could effectively double
the longevity of the overall treatment plus re-introduce the most
fundamental ecosystem process into these forests. Timing the prescribed fire approximately 10-years post-treatment would also be
a benefit to prescribed fire operations because most of the dead
leaves and small woody fuels would be either decomposed or compressed from snow resulting in a less intense prescribed fire. A prescribed fire in these conditions could also be ignited over a wider
range of weather conditions which would be an advantage for fire
managers. Burning 10-years post-treatment may create an effective fuel treatment for up to 20 years in these forests, but further
research would need to be done to verify this estimate.
Under certain initial stand conditions first-entry prescribed fire
treatments can lead to elevated surface fuels as small fire-killed
trees fall to the ground; these conditions likely necessitate reburning to maintain treatment effectiveness. Another response
that can limit treatment longevity following first-entry prescribed
fire is shrub regeneration from soil-stored seed. However, under
higher fuel moisture conditions shrubs could act as a heat sink during burning and lower fire intensity and this has been observed
during some wildfires (e.g., Ritchie et al., 2007).
Anecdotal observations from mixed conifer forests at Blodgett
suggest treatment effectiveness from the initial fire only treatment
may have begun to diminish at approximately 10 years post-treatment. Re-burning these stands will likely increase this effectiveness interval by an additional 15–20 years from the consumption
of residual downed wood from the death of many small trees.
Mechanical followed by fire treatments at Blodgett have a longer
effectiveness interval initially (possibly 15–20 years) because of
low post-prescribed fire tree mortality and high initial surface fuel
consumption. Shrub responses after mechanical plus fire or fire
only treatments may necessitate the application of a patchy, low
intensity prescribed fire to break up the horizontal continuity of
shrub fuels but this will probably not be required in most areas.
After installing initial fire hazard reduction treatments moving
these areas into a wildfire management regime where lightning
fires are used to manage these ecosystems may be desirable from
both an economic and ecological perspective (North et al., 2012).

5. Conclusion
Results from this study on the mid-term effects of fuels treatments agree with some aspects of previous research but the
changes recorded here are larger than those found in most Rocky
Mountain studies. Fulé et al. (2007) working in ponderosa pineGambel oak (Quercus gambelii) forests in the southwest US found

 S.L. Stephens et al. / Forest Ecology and Management 285 (2012) 204–212

changes in forest structure were relatively minor 5-years after
treatment. They found that prescribed fire was an effective treatment in reducing tree density but the combined full restoration
treatment (including thinning and prescribed fire) was the most
effective in promoting pre 1890 conditions. Severe drought during
their study period made causation of tree mortality difficult to
identify, but trees in treated stands significantly increased in
growth rate except those in the burn only treatment. Similarly,
Sackett and Haase (1998), also working in the southwest ponderosa pine forests, found repeated prescribed fires reduced tree density, lowered surface and ground fuels, and increased tree growth,
especially in stands treated every 4–6 years with prescribed fire.
Working in ponderosa pine forests in the Northern Rocky
Mountains, Fajardo et al. (2007) found delayed mortality in ponderosa pine after their mechanical plus fire treatment. They concluded ‘Special consideration needs to be taken on the cut–burn
treatments (relative to cut-only), which appeared to dampen response in terms of growth and vigor, particularly for mature and
young trees.’ Blodgett Forest does not contain a large number of
old-growth trees because of the history of early railroad logging
in this area (Stephens and Collins, 2004), but we did also see a reduced growth response from our prescribed fire treatments. Perhaps increasing the interval between the mechanical treatment
and prescribed fire would have resulted in lower residual tree
damage due to both decomposition of the augmented surface fuel
pool and potential increases in tree vigor from reduced competition. Additionally, increasing the interval between mechanical
treatment and prescribed burning could result in a longer period
for which treated areas have low fire hazards.
While this research investigated the mid-term responses to
some of the most common fuel treatments in mixed conifer forests
in the Sierra Nevada, continued research is needed to quantify the
effects of repeated treatments on the same area. Once areas are
treated to reduce fire hazards, managed wildfire (Collins and Stephens, 2007; Collins et al., 2009; North et al., 2012) would also
be a good option to maintain low fuel hazards and improve ecosystem resiliency, which may be even more important as climate continues to warm (Stephens et al., 2010). The changes we found in
Sierra Nevada mixed conifer forests after fuel treatments were generally larger than those reported from the Rocky Mountains, this is
probably explained by the higher productivity of the forests in the
Sierra Nevada, which would enable change to occur more rapidly.
Acknowledgements
This project was funded by the USDA–USDI Joint Fire Sciences
Program. Special thanks to Blodgett Forest Research Station and
their dedicated staff and to all those who helped implement the
prescribed burns. We also thank all graduate students and summer
field technicians that were integral to this project. We particularly
thank Jason Moghaddas for his earlier work on this project. The paper was improved from comments provided by two referees.
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