Fire regimes, past and present

Abstract

Fire has been an important ecosystem process in the Sierra Nevada for thousands of years. Before the area was settled in the 1850s, fires were generally frequent throughout much of the range. The frequency and severity of these fires varied spatially and temporally depending upon climate, elevation, topography, vegetation, edaphic conditions, and human cultural practices. Current management strategies and those of the immediate past have contributed to forest conditions that encourage high-severity fires. The policy of excluding all fires has been successful in generally eliminating fires of low to moderate severity as a significant ecological process. However, current technology is not capable of eliminating the high-severity fires. Thus, the fires that affect significant portions of the landscape, which once varied considerably in severity, are now almost exclusively high-severity, large, stand-replacing fires. The resulting landscape patterns are much coarser in grain.

Full text

1041
Sierra Nevada Ecosystem Project: Final report to Congress, vol. II, Assessments and scientific basis for management options. Davis: University of California, Centers for
Water and Wildland Resources, 1996.
38
Fire Regimes,
Past and Present
CARL N. SKINNER
U.S. Forest Service
Pacific Southwest Research Station
Redding, California
CHI-RU CHANG
Duke University
School of the Environment
West Durham, North Carolina
ABSTRACT
Fire has been an important ecosystem process in the Sierra Nevada
for thousands of years. Before the area was settled in the 1850s,
fires were generally frequent throughout much of the range. The fre-
quency and severity of these fires varied spatially and temporally
depending upon climate, elevation, topography, vegetation, edaphic
conditions, and human cultural practices.
Current management strategies and those of the immediate past
have contributed to forest conditions that encourage high-severity
fires. The policy of excluding all fires has been successful in gener-
ally eliminating fires of low to moderate severity as a significant eco-
logical process. However, current technology is not capable of
eliminating the high-severity fires. Thus, the fires that affect signifi-
cant portions of the landscape, which once varied considerably in
severity, are now almost exclusively high-severity, large, stand-
replacing fires. The resulting landscape patterns are much coarser in
grain.
Many gaps still exist in our knowledge of fire as an ecological pro-
cess in the Sierra Nevada.
INTRODUCTION
Fire has been an important ecological force in Sierra Nevada
ecosystems for thousands of years. In only a few vegetated
areas of the Sierra would fire not be considered an important
element of the ecosystem. The Mediterranean climate, with
cool, wet winters and warm, dry summers, predisposes much
of the Sierra Nevada to conditions that would carry fire an-
nually. As a result, prior to the mid-1800s, many of the plant
communities experienced fire at least once, and often a num-
ber of times, during the life spans of the dominant plant spe-
cies. Appropriately, much of the vegetation of the Sierra Ne-
vada exhibits traits that allow survival and/or reproduction
in this environment of regular fire (e.g., Chang 1996).
The patterns in which fires occur in an area, known as the
fire regime for that area, influence the nature of the vegeta-
tion mosaic to be found within a particular landscape. Know-
ing the history of fire in a landscape is therefore important in
gaining an understanding of the role of fire in ecosystems.
An understanding of fire ecology and fire history may pro-
vide managers, decision makers, and policy makers with in-
formation that will help them avoid being shocked by
unanticipated situations. When one is planning for resource
management, for ecosystem management, and for commu-
nity well-being, a knowledge of fire history and fire ecology
provides a reference for assessing how much deviation is de-
veloping from long-term past patterns and conditions (e.g.,
Swanson et al. 1994; Manley et al. 1995). Evaluating the prob-
ability of success of future long-term alternatives for resources
or communities is problematic without considering the physi-
cal and biological potential for fire and its function in Sierran
environments.
Substantial research and documentation regarding fire ecol-
ogy and regimes have demonstrated the importance of fire in
Sierra Nevada environments. It is not possible to summarize
all of the relevant information here. This chapter provides only
sufficient background to establish for the reader the general
nature of, and thereby the importance of, fire in the ecosys-
tems of the Sierra Nevada. For the reader wishing more on
the topic, the references should serve as a good starting place.
Kilgore 1973, Biswell 1989, and Arno in press provide excel-
lent summaries. Additionally, though primarily focused on
fire ecology in vegetation north of California, Agee 1993 and
Agee 1994 summarize fire ecology information for vegetation
types (i.e., mixed conifer, red fir, subalpine, and east-side veg-
etation types) found in the northern Sierra as well as in the

1042
VOLUME II, CHAPTER 38
extended SNEP study areas of the southern Cascades and
northeastern California.
Unless otherwise indicated, the descriptions of past fire
regimes presented in this chapter pertain to conditions be-
fore 1850. We have done this because many changes in fire
regimes have occurred since the influx of Euro-American cul-
ture during and following the gold rush of the mid-1800s.
THE PALEOECOLOGICAL
RECORD
Paleoecological studies show that Sierra Nevada fire regimes
are dynamic in space and time on many scales. The long-term
importance of fire in Sierran ecosystems is suggested by the
common occurrence of charcoal in the paleoecological record
of the Holocene (e.g., Smith and Anderson 1992; Davis and
Moratto 1988). Analyses of fossil pollen suggest that climate
and vegetation have varied considerably over this period
(Woolfenden 1996). Vegetation and fire appear to have var-
ied, sometimes greatly, in concert with the variation in cli-
mates (Davis et al. 1985). Fire may serve as a catalyst for the
reorganization of vegetation during periods of rapid climate
change (e.g., Whitlock 1992; Wigand et al. 1995). It is note-
worthy that large charcoal peaks from the early Holocene were
followed by vegetation that was considerably different from
that found before this period of heightened fire activity
(Edlund and Byrne 1990). However, the resolution of tempo-
ral data available for the Sierra Nevada is insufficient to de-
fine the role of fire in reorganizing vegetation at various times
in the past.
The evidence from interpretation of long-term trends in
sediment cores has shown that fire has been an important
component of the Sierran environment since before current
vegetation assemblages became established. Charcoal concen-
trations at one site were greatest during the warm period that
followed the end of the Pleistocene, approximately 10,000
years ago (Smith and Anderson 1992). These concentrations
of charcoal appear to coincide with the end of the Pleistocene
vegetation typical of subalpine forests today. The charcoal
concentrations were followed by species assemblages more
similar to the mixed conifer forests found today at middle
and lower elevations (Anderson 1990). Following this warm
period there was a general cooling trend until approximately
3,000 years ago, when a relatively cooler, more moist climate
regime appears to have become established. Charcoal, though
varying over time with changes in climate and vegetation, is
routinely present in sediment core samples (Smith and Ander-
son 1992).
More-detailed reconstructions of climate variations for the
last few millennia have recently been developed using tree-
ring analysis (e.g., Graumlich 1993; Hughes and Brown 1992;
Stine 1994). These variations in temperature and moisture
patterns have been found to correspond well with variations
in the frequency and apparent severity of fires (Swetnam
1993). Swetnam 1993 demonstrated that thirty years was the
longest period without fire in any of five sequoia groves for
more than 2,000 years. These fire scar records also show that
many of these same groves have now experienced more than
100 years without fire under modern management policies
(Swetnam 1993).
THE ETHNOGRAPHIC RECORD
Ethnographic accounts show that Native Californians com-
monly used fire as a management tool in the Sierra Nevada
(Reynolds 1959; Wickstrom 1987; Blackburn and Anderson
1993). Fire was used to provide many important foodstuffs
and materials (Lewis 1993). The spatial extent of the influ-
ence of burning on the landscape is not known and has been
subject to some debate (Barrett 1935; Wickstrom 1987). How-
ever, accounts of the frequency of fire necessary to maintain
specific resources in conditions required by the various cul-
tures suggest that extensive and very intensive burning would
have been common in important vegetation types (Anderson
and Moratto 1996).
Enhancing the production of foodstuffs was one important
reason for burning (Wickstrom 1987). For example, acorns
were a major staple in the diet of the Native Californians, and
burning was reported to enhance the production of acorns.
Acorn crops are described to have been improved by burn-
ing in two important ways: (1) by reducing the losses to in-
sects and (2) by encouraging larger, more productive trees
(Anderson 1993b).
A second important reason for burning was to encourage
the production of basketry materials (Anderson 1993a; Lewis
1993). The better materials for making baskets were young,
straight shoots of many sprouting species. As the shoots ma-
tured, they would become unsuitable due to side branching
and lack of flexibility.
A third reason for burning usually given by Native Cali-
fornians was to reduce the hazard of large, severe fires (Lewis
1993). The native cultures were reliant upon local resources
for their livelihood. A large, severe fire could change the local
plant communities in a way that affected the ability of the
communities to survive in the area. For example, a large, se-
vere fire could top-kill the old oaks that provided acorns, the
main staple. These trees could not produce sufficient supplies
for many years following a fire of this type.

1043
Fire Regimes, Past and Present
FIRE REGIMES
Fire ecologists refer to the general characteristics of fires found
within any specified area of interest as the fire regime. Fire
regimes can vary considerably by vegetation and landscape.
Thus, they offer a convenient way to categorize areas for study
and management purposes. Fire regimes are described by the
following characteristics: frequency, rotation, spatial extent,
magnitude, and seasonality (White and Pickett 1985; Agee
1994). These terms are defined as follows:
Frequency: The frequency describes how often fires oc-
cur within a given time period. This characteristic is of-
ten described in terms of return intervals rather than
frequency. The return interval is the length of time be-
tween fires.
Rotation: The fire rotation is the length of time necessary
to burn an area equal to the area or landscape of inter-
est. For example, if one is working with a landscape of
100,000 acres and it takes fifty years for fires to burn
100,000 acres within that landscape, the fire rotation
would be fifty years. Keep in mind that all 100,000 acres
need not burn if some acres are burned more than once.
The only requirement for this term is a total accumu-
lated burn area equal to the original area of interest.
Spatial extent: The spatial extent refers to the size or area
covered by a fire and the spatial patterns created.
Magnitude: The magnitude of a fire refers to both its in-
tensity and its severity. Intensity is a technical term used
to describe the amount of energy released from a fire.
Intensity may or may not be directly related to fire ef-
fects. Severity is related to the change in the ecosystem
caused by the fire and can be either quantitatively or
qualitatively related to fire effects. Fires that burn only
surface fuels and in which most of the woody vegeta-
tion survives are usually considered low-severity fires.
Fires that kill large trees over more than a few acres
by burning their crowns are usually considered high-
severity fires.
Seasonality: The seasonality, or timing, of a fire is impor-
tant in relation to the moisture content of fuels, the phe-
nology of the vegetation, and the resulting fire effects.
The vegetation found within a particular ecosystem has
adapted over time to the season or seasons in which the
fires generally occur.
Few fire-history studies have attempted to describe all of
the fire-regime characteristics just defined. Most describe the
fire frequencies for points (a single tree) or small sites. These
data are the easiest and least costly to obtain. Some have also
included seasonality as interpreted from the location of the
scar in the rings (i.e., latewood or early wood) of the year of
the fire. Few studies have attempted to describe the rotation,
spatial extent, or magnitude of past fires, because acquiring
these data requires intensive sampling of many sites over
a landscape. These latter studies are quite costly due to the
time and labor involved in field sampling and laboratory
analysis.
Each of the fire-regime characteristics, when used, is usu-
ally described in terms of the mean or median and sometimes
in terms of a measure of variability. The median is used in
this chapter because of the variability in fire-return intervals
associated with vegetation types that do not have very regu-
lar, frequent fires. The median is less affected by erratic ex-
tremes than is the mean (Snedecor and Cochran 1980). The
mean is often interpreted and applied in a way that would
assume that the data come from a normal distribution with
little variation, but fire-return intervals for many sites are of-
ten not represented by a normal distribution. Instead, they
are often multimodal (Johnson and Gutsell 1994) or strongly
skewed, with many shorter intervals and a few longer,
extreme intervals. The pattern of fire-return intervals often
varies from period to period, and a simple mean is not repre-
sentative of longer records (Swetnam 1993).
FIRE HISTORY
Fire history can be reconstructed from a variety of data
sources: written records, historical accounts, dendrochronol-
ogy (tree-ring analysis), and the analysis of charcoal in sedi-
ments (Patterson and Backman 1988). Each of these data
sources has its own limitations regarding spatial and tempo-
ral detail and accuracy. Within forested ecosystems, detailed
reconstruction of fire histories before written records is pos-
sible through fire scar analysis using dendrochronology tech-
niques (Agee 1993; Arno and Sneck 1977). Fire history, in
contrast to human history, is not limited to written records or
accounts.
Fire histories from fire scar analysis generally fall into one
of three categories: (1) single-tree samples; (2) composites of
multiple trees for specified areas (Dieterich 1980a, 1980b); and
(3) composites of multiple sites for landscapes (Taylor 1993a).
The single-tree sample is usually considered the most con-
servative estimate of past fire history for many areas (although
Minnich et al. [in press] have some data to suggest this may
not always be the case). This is because all fires passing a point
may not have been of sufficient intensity to have scarred the
single sample. Composites of multiple trees will usually pro-
vide a more comprehensive record of past fires for the site in
question (Agee 1993). However, describing in spatial and
temporal terms the influence of fire on landscape dynamics
(age-class distributions, species composition patterns, stand
structures, patch patterns, etc.) requires detailed landscape-
level sampling (e.g., Teensma 1987; Morrison and Swanson

1044
VOLUME II, CHAPTER 38
1990; Caprio and Swetnam 1995; Minnich et al. in press; Solem
1995). Because of the time and costs involved in this type of
sampling, few studies of this nature have been undertaken.
Instead, most fire-history studies have been site-specific, fire-
return-interval studies.
In each type of fire history, the fire dates can be determined
either by cross-dating (Fritts and Swetnam 1989) or by esti-
mating correspondence among years (Arno and Sneck 1977).
The cross-dating method is precise and can determine the
calendar year of fires hundreds or thousands of years ago
(Swetnam 1993). The second method is not as accurate in de-
termining the actual calendar year of a fire and often under-
estimates the number of fires within a period of interest.
However, it still provides valuable, though less detailed, fire-
interval information (Madany et al. 1982) that can be useful
in describing the fire regime, especially at the level of detail
required by most natural resource managers.
Fire histories based on tree-ring analysis rely on interpre-
tation of scars that formed in response to fire-caused damage
in the tree ring of the year of the fire. A number of factors can
influence the way in which fires are recorded as scars. Trees
are the best recorders, since they are long-lived and are large
enough to be able to survive fires of low to moderate inten-
sity. Little information is usually left following fires in herba-
ceous or shrub communities because of heavy consumption
and the fact that the parts of the plants that are above ground
are often killed. In addition, the various tree species vary con-
siderably in their susceptibility to damage or mortality by fire.
In areas consisting solely of species of trees that are usually
killed even by low-intensity fires, there may not be a record
of fire prior to the last fire that initiated the current stand.
Fire-severity classes used in this chapter are
• Low severity: light surface fire; some small trees may be
killed.
• Moderate severity: most small trees killed; some subcanopy
trees killed or heavily damaged. Charring on bark of live
trees. Overstory trees may occasionally be killed.
• High severity: small and subcanopy trees killed; many to
most overstory trees killed.
Although none of the fire-history studies described in the
next section meets the exacting standards of the randomized
sampling design described by Johnson and Gutsell (1994),
taken together they provide valuable information about the
past temporal patterns of fires within forest stands on a local-
ized scale. The sampling design suggested by Johnson and
Gutsell was developed in forests characterized by infrequent,
large, stand-replacing fires. These fires result in very coarse-
grained landscape patterns. The objective of fire-history stud-
ies in such forests is primarily to describe when the last fire
occurred in each patch by dating the age of the trees that re-
generated after the burns. The spatial scales on which topog-
raphy and vegetation vary, along with the spatial variability
in fire behavior and the effects from the frequent fires in the
forests of the Sierra Nevada, create very complex, fine-grained
spatial patterns. Attempting to carry out a landscape-scale
study based upon the design suggested by Johnson and
Gutsell under these latter conditions would be very difficult
and costly.
Several fire-history studies have been completed within the
Sierra Nevada and adjacent geographical areas. Most of these
studies have been limited to providing information on fire-
return intervals (FRIs) for a small area. A few have developed
fire history at the landscape scale in the Sierra (e.g., Caprio
and Swetnam 1995; Kilgore and Taylor 1979). For studies of
areas with vegetation similar to that in portions of the Sierra
Nevada, see McNeil and Zobel 1980, Taylor and Halpern 1991,
Taylor 1993a, Solem 1995, and Minnich et al. in press. Each of
these will be discussed in more detail later in relation to ap-
propriate vegetation types.
Table 38.1 summarizes fire-history information from vari-
ous published and unpublished sources for the Sierra Nevada
and other areas that have similar vegetation and climate. The
spatial context of the return intervals is given to facilitate com-
parisons among the areas. There exist a number of other fire-
history studies within the Sierra Nevada (e.g., Rice 1990, 1992).
These were not included in table 38.1 primarily because the
spatial reference for the sampling was not clear and we were
unsure of the spatial comparability of the reported fire fre-
quencies. Other studies were not included because the data
were presented in a fashion that was difficult to present within
the structure of table 38.1 (e.g., Mensing 1988, 1992). These
latter studies are referenced in the text where appropriate.
The FRIs as presented for a small, localized place do not
necessarily provide information on how fire would have in-
fluenced the landscape-scale patterns. Periods of more fre-
quent fires may have many small, low-severity fires scattered
throughout the landscape, while periods of longer FRIs may
be associated with larger, more severe fires (Swetnam 1993).
The spatiotemporal variation in fire frequency and severity
may be important in influencing stand structure and regen-
eration patterns over time (e.g., Minnich et al. in press;
Stephenson et al. 1991), leading to the complex spatial pat-
terns of the vegetation that are so characteristic of the Sierra.
Landscape-scale fire-history studies are especially impor-
tant to our understanding of the role of fire in Sierran ecosys-
tems. The continued lack of such studies for the Sierra Nevada
leaves important questions unresolved concerning the spa-
tial and temporal influences of fire on vegetation dynamics,
aquatic and riparian environments, wildlife habitat, coarse
woody debris accumulations, and so on. Although many fire-
history studies have been conducted in the Sierra, there is
a considerable need to expand the geographical coverage
and to conduct landscape-scale studies of fire history tied to
vegetation-related dynamics.

1045
Fire Regimes, Past and Present
TABLE 38.1
Fire-return intervals (FRIs) from the Sierra Nevada and areas of similar vegetation and climate.
Area and Median Minimum Maximum Years Since Sample Years of
Vegetation FRIs
a
FRI
a
FRI
a
Last Fire Method Area Record
b
Location Source
West-Side Areas
Foothill Zone
Blue oak–gray pine 8 (29) 2 (8) 49 (49) 14–34 Composites 5 ha 78–267 Northern McClaran and
of multiple Sierra Bartolome
trees 1989
Black oak– 8 2 18 82–102 Composites <2 ha Not Central S. Stephens,
ponderosa pine of multiple reported Sierra e-mail
trees communi-
cations with
the author,
21 April and
9 and 30 May
1995
c
6–9
e
2 23 Not Composites 1 ha 175 Southern Kilgore and Taylor
reported of multiple Sierra 1979
trees
Mixed Conifer Zone
Mixed evergreen– 15 (13) 3 (5) 50 (41) 43–71 Composites 5–8 ha 235–245 Klamath Wills and Stuart
tan oak of multiple Mountains 1994
trees
Canyon live oak– 13 (11) 7 (7) 39 (33) 5–75 Composites <1 ha 112–116 Klamath Taylor and
mixed conifer of multiple Mountains Skinner in
trees preparation
c
Ponderosa pine– 8–10
e
3 14 Not Composites 1 ha 175 Southern Kilgore and
mixed conifer reported of multiple Sierra Taylor 1979
trees
5–11
e
Not Not ~3–135 Composites <100 ha ~125–340 Southern Caprio and
reported reported of multiple Sierra Swetnam
trees 1995
d
11 (11) 3 (5) 55 (46) 35–90 Composites <2 ha 151–306 Klamath Skinner in
of multiple Mountains preparation
c
trees
Giant sequoia– Not 1 15 Not Composites <100 ha 1,050 Southern Swetnam et al.
mixed conifer reported reported of multiple Sierra 1991
d
trees
Not 1 30 Not Composites 10–100 ha 1,350 Southern Swetnam 1993
d
reported reported of multiple Sierra
trees
15–18
e
4 35 Not Composites 1 ha 175 Southern Kilgore and
reported of multiple Sierra Taylor 1979
trees
14–32
e
Not Not ~10–195 Composites <100 ha ~195–380 Southern Caprio and
reported reported of multiple Sierra Swetnam
trees 1995
d
Douglas fir– 17 (16) 12 (12) 59 (18) 69 Composites 2 ha 169 Klamath Agee 1991
d
mixed conifer of multiple Mountains
trees
13 (14) 3 (3) 57 (52) 26–93 Composites 2 ha 125–396 Klamath Skinner in
of multiple Mountains preparation
c
trees
12 (15) 3 (3) 59 (59) 5–92 Composites <1 ha 248–379 Klamath Taylor and
of multiple Mountains Skinner in
trees preparation
c
White fir–mixed 12 (11) 3 (3) 33 (29) 72–82 Composites 1 ha 133–154 Southern McNeil and
conifer of multiple Cascades Zobel 1980
trees
10 (13) 3 (5) 24 (24) 36–56 Composites 2 ha 192–289 Southern Skinner
of multiple Cascades unpublished
trees data
9 (10) 3 (4) 26 (26) 34–47 Composites 2 ha 214–268 Southern Skinner
of multiple Cascades unpublished
trees data
8 (13) 3 (3) 35 (35) 38–67 Composites 2 ha 103–252 Northern Skinner
of multiple Sierra unpublished
trees data
a
Values in parentheses are specifically pre-1850 where available. Other values are for the entire period of record.
b
The number of years from the earliest fire scar to the latest fire scar. A range indicates multiple sample sites.
c
Unpublished study.
d
Cross-dated samples.
e
These are means for the sites sampled, rounded to the nearest integer. This study reported only means and did not present data in a way to develop medians or
ranges.
continued

1046
VOLUME II, CHAPTER 38
TABLE 38.1 (continued)
Area and Median Minimum Maximum Years Since Sample Years of
Vegetation FRIs
a
FRI
a
FRI
a
Last Fire Method Area Record
b
Location Source
12 (11) 4 (4) 32 (32) 47–101 Composites 2 ha 53–196 Central Skinner
of multiple Sierra unpublished
trees data
18 (18) 7 (7) 27 (27) 100 Single-tree 113 Central Taylor 1993b
c,d
sample Sierra
11–17
e
2 39 Not Composites 1 ha 175 Southern Kilgore and
reported of multiple Sierra Taylor 1979
trees
Mixed conifer 12 (11) 3 (3) 114 (44) 0–109 Single-tree 97–373 Central Rice 1988
c
(Lake Tahoe sample Sierra
Basin, Emerald
Bay State Park)
White fir 17 (16) 4 (4) 61 (39) 72–128 Composites 1 ha 84–154 Southern McNeil and
of multiple Cascades Zobel 1980
trees
9 (11) 4 (4) 56 (56) 35 Composites 2 ha 214 Southern Skinner
of multiple Cascades unpublished
trees data
Riparian areas 31 (36) 7 (7) 71 (71) 49–102 Composites 1 ha 175–265 Klamath Skinner in
of multiple Mountains preparation
c
trees
Upper Montane Zone
Jeffrey pine 16 (13) 4 (4) 157 (157) 93–139 Single-tree 81–345 Southern A. H. Taylor,
sample Cascades telephone
conversation
with the
author, 17
May 1995
c,d
Jeffrey pine– 12 (12) 4 (4) 96 (96) 42 Composites 2 ha 480 Klamath Skinner in
white fir of multiple Mountains preparation
c
trees
29 (29) 4 (10) 100 (93) 93–156 Single-tree 88–326 Southern A. H. Taylor,
sample Cascades telephone
conversation
with the
author, 17
May 1995
c,d
Red fir–white fir 12 (11) 5 (5) 69 (69) 77–98 Composites <4 ha 97–240 Central Bahro 1993
c
of multiple Sierra
trees
Red fir–white pine 69 (57) 14 (14) 109 (109) 102–211 Single-tree 31–167 Southern A. H. Taylor,
sample Cascades telephone
conversation
with the
author, 17
May 1995
c,d
Red fir 20 (20) 8 (8) 35 (35) 128 Composites 1 ha 63–98 Southern McNeil and Zobel
of multiple Cascades 1980
trees
11 (16) 1 (7) 47 (35) 45–52 Composites 3 ha 141–205 Southern Taylor 1993
d
of multiple Cascades
trees
East-Side Areas
Ponderosa pine 16 8 32 34–75 Composites <10 ha 70–169 Southern Olson 1994
c
of multiple Cascades
trees
8 6 15 51 Single-tree 169 Southern Olson 1994
c
sample Cascades
Mixed conifer 9 (10) 3 (3) 71 (71) 40 Composites <10 ha 143–362 Southern Olson 1994
c
(ponderosa of multiple Cascades
pine–white fir) trees
Red fir 27 (21) 9 (19) 91 (22) 63–90 Composites <10 ha 113–135 Southern Hawkins 1994
c
of multiple Sierra
trees
Red fir–Jeffrey 14 (14) 6 (6) 64 (23) 36–143 Single-tree 135–227 Southern Hawkins 1994
c
pine sample Sierra
17 (15) 5 (7) 56 (31) 41–43 Composites <10 ha 161–243 Southern Hawkins 1994
c
of multiple Sierra
trees
a
Values in parentheses are specifically pre-1850 where available. Other values are for the entire period of record.
b
The number of years from the earliest fire scar to the latest fire scar. A range indicates multiple sample sites.
c
Unpublished study.
d
Cross-dated samples.
e
These are means for the sites sampled, rounded to the nearest integer. This study reported only means and did not present data in a way to develop medians
or ranges.

1047
Fire Regimes, Past and Present
FIRE EFFECTS
There is generally more literature on the effects of fire than
there is concerning fire history. The effects of fire on the mixed
conifer forests and chaparral of the west side of the Sierra
Nevada are the subject of an abundant literature, whereas the
literature on fire effects for the higher elevations and the east-
side forests is rather sparse (Chang 1996).
It would have been rare in most vegetation types under
the Mediterranean climate and pre-1850 fire regimes for indi-
viduals of most woody species to have escaped fire during
their life span. As a result, much of the Sierra Nevada vegeta-
tion exhibits traits that have allowed the various species to
persist with periodic fire. Whether these plant communities
are the result of community adaptations to fire (e.g., Mutch
1970) or are coincidental assemblages of species that individu-
ally developed fire-adaptive traits over long periods (e.g.,
Davis 1986) is subject to debate. Regardless, many of the more
common Sierran vegetative communities are generally con-
sidered adapted to recurring fire (Chang 1996). This section
discusss the general adaptive traits and responses of this veg-
etation to fire. For more detail on fire effects, including ef-
fects on soils and fauna, please refer to Chang 1996.
Conditions for successful reproduction of many plant spe-
cies are most favorable immediately following a fire, owing
to increased fertility, removal of potential allelochemicals
(chemicals released by plants that are toxic to other plants),
reduced competition, and so on (Canham and Marks 1985;
Christensen 1993). Consequently, many plants have evolved
adaptive traits that help them survive or reproduce after fire.
Such traits include the following (Sweeney 1968; Christensen
1985; Agee 1993):
1. fire-stimulated seed germination, as in deer brush (Ceano-
thus integerrimus)
2. rapid growth and development that allows a complete life
cycle between fires, as in many herbaceous species
3. fire-resistant buds and twigs, as in ponderosa pine (Pinus
ponderosa) and Douglas fir (Psuedotsuga menziesii)
4. fire-resistant bark, as in ponderosa pine and Douglas fir
5. adventitious or latent axillary buds, as in oaks (Quercus
spp.)
6. sprouting, as in chamise (Adenostoma fasciculatum)
7. serotinous cones and fire-stimulated seed release, as in
knobcone pine (Pinus attenuata) and giant sequoia (Sequoi-
adendron giganteum)
8.
fire-stimulated flowering, as in soap plant (Chlorogalum pom-
eridianum)
Additionally, these varied vegetative responses are often
influenced by a complex interaction of external factors such
as temperature, moisture conditions, heat duration (e.g.,
Rogers et al. 1989), and season of burn (e.g., Parker and Kelly
1989; Weatherspoon 1988).
Changes in Fuel Structure Related to
Fire-Return Intervals
Because photosynthesis produces organic matter on a regu-
lar basis, vegetative biomass (fuel) accumulates with time and
adds to total fuel accumulations. However, not all biomass is
available fuel at any given moment. Available forest fuel is
organic matter that could burn under the prevailing condi-
tions if ignited. The amount of biomass available as fuel at
any one time depends on factors such as the ratio of dead
plant material to live material and fuel moisture content.
Fire plays an important role in regulating fuel accumula-
tions. Fire also influences the horizontal and vertical conti-
nuities of fuels. The importance of fire in regulating fuel
accumulations is amplified where fire occurs frequently. Un-
der the regimes in the era before fire suppression, frequent
fires would consume surface fuels, maintaining them at mini-
mal levels. Periodic low-to-moderate-intensity fires also main-
tained gaps in vertical fuel continuity, inhibiting fires from
moving into the crowns (e.g., Sudworth 1900; Leiberg 1902).
Fire suppression in forests that previously experienced fre-
quent fires has allowed fuels to build up both vertically and
horizontally, increasing the chance of stand-replacement fires
(Brown 1985; van Wagtendonk 1985; Kilgore 1987; Arno in
press).
Landscape Patterns Resulting from Fire
Landscapes can be viewed as a dynamic mosaic of patches
(White and Pickett 1985). The frequency, intensity, and spa-
tial extent of successive fires, along with the vegetative re-
sponse, have influenced and will continue to influence the
grain and pattern of the landscape mosaic.
Fire regimes help to define the pattern or mosaic of age
classes, successional stages, and vegetation types on the
landscape (Turner et al. 1993). For example, periodic, low-
intensity surface burning has been found to cause develop-
ment of an uneven-aged stand, made up of even-aged groups
of trees of various age classes (e.g., Bonnicksen and Stone 1981;
Weaver 1967). Conversely, infrequent, high-intensity, stand-
replacement fires result in larger patches of stands of more
even age (e.g., Heinselman 1973; Hemstrom and Franklin
1982).
Because the dynamics determining landscape patterns are
affected by fire regimes, characteristics of landscapes also
respond to variations in fire regimes. Skinner (1995a), for ex-
ample, found that forest openings have disappeared or be-
come smaller in a remote area of the Klamath Mountains
during the period of effective fire suppression.
Within a given landscape, fire behavior will vary on a vari-

1048
VOLUME II, CHAPTER 38
ety of spatial scales, influenced by local microclimate, topog-
raphy, and fuel conditions. This varying behavior will inter-
act with the postfire climate to induce ecosystem responses
that result in varying landscape mosaics (Christensen et al.
1989).
Much of the literature that discusses landscape patterns in
relation to fire and changing fire regimes has concentrated
on ecosystems in which the fire regimes are characterized by
infrequent or long-return-interval, severe fires (e.g., Hein-
selman 1973; Hemstrom and Franklin 1982; Romme 1982;
Baker 1989). Because of the differences in scale of the effects
of such fires, most studies that discuss the spatial patterns
associated with fire regimes consisting of frequent, low-to-
moderate-intensity fires have concentrated more on stand-
level patterns than on landscape-level patterns (e.g.,
Bonnicksen and Stone 1982). Generally, the relative extent of
fires within a particular vegetation type appears to increase
as the interval between fires lengthens (Swetnam 1993; Husari
and Hawk 1994).
FIRE REGIMES OF MAJOR
VEGETATION TYPES
Due to the past frequency of fires and their influences on spe-
cies composition, stand structure, and spatiotemporal pat-
terns, fire is generally considered an important ecological
process throughout much of the Sierra Nevada. Some fire-
regime characteristics (e.g., frequency [FRI] and seasonality)
of specific vegetation types are described quite well in the
Sierra Nevada. In the southern Sierra, especially in the na-
tional parks, the FRIs in ponderosa pine and mixed conifer
forests (especially those with giant sequoia) are relatively well
documented. Less-detailed data exist for some upper mon-
tane areas and for the foothill areas of the southern Sierra.
However, for many other fire-regime characteristics, vegeta-
tion types, and geographical portions of the Sierra Nevada
little data exist.
Most of the information about fire regimes in the Sierra
Nevada is from studies that used tree-ring analyses to detect
when and how often past fires occurred in a particular place.
In the following discussions, for cases where no descriptions
of the fire regimes for the Sierra Nevada were found, we have
extrapolated information from studies of similar vegetation
in other geographical areas, used anecdotal information from
historical sources, or made inferences based upon our knowl-
edge of fire behavior and effects. The type of source(s) used
is noted in the discussions.
The northern Sierra Nevada is especially lacking in pub-
lished research designed to describe the long-term fire re-
gimes. Only a few sites, in the works of Show and Kotok (1924)
and Wagener (1961), have been studied. The discussions of
fire regimes for the northern Sierra Nevada therefore rely on
information extrapolated from studies of similar vegetation
types in the southern Cascades, the Klamath Mountains, and
the southern Sierra Nevada.
The FRIs given in the following discussions are for the en-
tire period of record unless otherwise stated. The period of
record for FRIs from fire scars is the time from the earliest
scar recorded for the site to the last scar recorded for the site.
Refer to table 38.1 for more detail concerning the FRIs for the
presettlement and postsettlement period, the length of the
record, and the sizes of the sampling areas represented by
the FRIs.
Foothill Zone
The foothill zone is generally below the main belt of the coni-
fer forests and above the Sacramento and San Joaquin val-
leys. The blue oak–gray pine woodlands, chaparral, mixed
evergreen woodlands, and black oak–ponderosa pine forests
are the more common vegetation types found in the foothills
(Parsons 1981). Little is known about fire history in these veg-
etation types. Woodlands usually promote fast-moving fires,
generally of low severity, in herbaceous fuels that may not
leave a record as scars in trees. Chaparral, on the other hand,
usually supports severe fires that kill the above-ground parts
of the plants. For the latter, only the time elapsed since the
last fire can be reconstructed. The discussions that follow for
these vegetation types should be viewed in this light.
Considerable evidence exists that the Native Californians
burned frequently, usually in the late summer or fall months,
within the foothill areas (Anderson 1993a; Lewis 1993). The
spatial extent of this burning is unknown but appears to have
been substantial, at least near communities, considering the
amount of postfire resources required (Anderson and Moratto
1996). This burning increased the number of fires that would
otherwise have been expected from lightning alone. Gener-
ally, fewer lightning strikes occur, and fewer resulting fires
are ignited, in the lower elevations than farther upslope
(Komarek 1967; van Wagtendonk 1991b). The relative propor-
tion of the area burned by human-caused fires to that burned
by lightning-caused fires could vary considerably under dif-
ferent intensities of management.
Blue Oak–Gray Pine
The blue oak–gray pine woodland is common throughout the
lower elevations of the Sierra Nevada. Common trees are blue
oak (Quercus douglasii), gray pine (Pinus sabiniana), interior
live oak (Q. wislizenii), and California buckeye (Aesculus
californica).
Fire History.
Two fire-history studies are available for blue
oak–gray pine woodlands, only one of which is from the Si-
erra Nevada proper. The one study in the Sierra Nevada is
McClaran and Bartolome 1989, from the University of Cali-
fornia Sierra Foothill Range Field Station east of Marysville.
The median FRIs on two sites were 7 and 9 years. The study

1049
Fire Regimes, Past and Present
found shortened FRIs during and following the settlement
period (post-1848). The pre-1848 FRIs ranged from 8 to 49
years, with a median of 28.5, whereas post-1848 FRIs ranged
from 2 to 17 years with medians of 7 to 8 years.
Mensing (1988) found that fire frequency recorded as fire
scars in blue oaks on three sites in the Tehachapi Mountains
had changed considerably since European settlement. He
found mean FRIs for the presettlement period to range from
9.6 to 13.6 years. During the settlement period (1843–1865),
the mean FRIs were 3.3 to 5.8 years, and post-1865 FRIs ranged
from 13.5 to 20.3. Interestingly, he found a period of more
than 60 years (the 1860s to the 1920s) that lacked any evi-
dence of fire scars. He found this period to coincide with the
introduction of livestock grazing, suggesting a reduction in
available fuels. A similar coincidence of grazing with fire scar
reduction in the Sierra Nevada has been noted by Vankat and
Major (1978).
Fire Effects.
The blue oak–gray pine woodlands are well
adapted to frequent, quick-moving, low-intensity surface fires
(Arno in press). Fuels are usually light, and the primary car-
rier of fire is the surface herbaceous vegetation. Notably, the
surface vegetation has changed from consisting largely of
perennials in presettlement times to being dominated by in-
troduced annuals (Heady 1977). Historically, the perennials
may have limited the season of burning. Annuals, on the other
hand, may promote an earlier onset to the burning season
because they dry and cure earlier than the perennials.
The trees usually survive these surface fires except where
increased fire intensity is created by fallen, dead trees or an
increased density of understory shrubs. The oaks and buck-
eyes are strong sprouters when occasional fires do kill the
above-ground portions of the plants. Most of the understory
shrubs are scattered individuals or groups of species usually
associated with chaparral. The frequency of fire in this veg-
etation type usually keeps the shrub cover limited.
Shrubs
Most shrub communities in the Sierra are considered chapar-
ral. Chaparral is a term applied to communities of predomi-
nantly evergreen shrubs adapted to hot, dry summers and
periodic fire typical of Mediterranean climates (Hanes 1977;
Kilgore 1981; Barro and Conard 1991). Chaparral is common
in the lower elevations of the Sierra Nevada, usually between
the oak woodlands and the conifer forests and on steep, often
rocky, south-facing slopes of canyons. Species composition
varies considerably both locally and regionally in the chapar-
ral. Some important common species are chamise, scrub oak
(Quercus dumosa), interior live oak, manzanitas (Arctostaphy-
los spp.), ceanothus (Ceanothus spp.), toyon (Heteromeles
arbutifolia), yerba santa (Eriodictyon californicus), and Califor-
nia buckeye. Locally important nonchaparral shrub commu-
nities are found where Brewer’s oak (Q. garryanna var. brewerii)
is an important component.
The severe nature of the fires and the intermingling of many
rural communities with areas of chaparral present a consid-
erable challenge to natural resource managers regarding the
need to ensure public safety as well as to manage wildlife
habitat and watersheds (Sparks and Oechel 1984).
Fire History.
Despite the common occurrence of chaparral
and its importance to management, fire history for chaparral
in the Sierra Nevada is lacking. Historical information is lim-
ited to fire records from this century and previous anecdotal
accounts (Parsons 1981). Fire-history information about chap-
arral in California is generally confined to studies in the Coast
and Transverse Ranges. These longer-term studies are from
charcoal in oceanic sediment deposits (Byrne et al. 1977;
Mensing 1993). Because of differences in lightning frequency
and burning conditions, these studies may present conserva-
tive estimates of fire frequency for inland areas (Keeley 1982).
FRIs in chaparral appear to be quite variable, depending
upon local site conditions, proximity to areas of aboriginal
human use, and elevation. Chaparral FRIs generally have been
estimated to be twenty to fifty years with ranges of approxi-
mately ten to more than a hundred years (Keeley 1982; Kilgore
1987; Barro and Conard 1991). FRIs in chaparral types have
been limited to estimates because the severe nature of the fires
in chaparral renders the areas unsuited to the reconstruction
of fire history from dendrochronological techniques (Minnich
and Howard 1984). However, Johnson and Gutsell (1994) sug-
gest that FRIs in chaparral may be estimated by using a ran-
domized spatial sampling design similar to that used in boreal
forest types to determine the years since the last fire for vari-
ous portions of the landscape. It is unlikely that this approach
will work well in landscapes that have been affected by the
extremely large fires of the last few decades, because the large
fires will probably have destroyed the previous age-class pat-
terns.
Fire Effects.
Due to the dense growing habit of shrubland
vegetation and the long dry season, fires in this type of com-
munity are usually severe and kill most above-ground por-
tions of the vegetation (Christensen 1985; Barro and Conard
1991). Many of the shrubs found in foothills respond to fire
by resprouting, germinating from seeds stored in soil seed
banks, or both (Sweeney 1956; Keeley 1977; Parker and Kelly
1989). There is often a flush of herbaceous growth in the first
few years following a fire that diminishes as the shrubs re-
gain dominance (Sampson 1944; Sweeney 1956). Variations
in FRIs differentially favor the various species, depending
upon their method of response to fire. Variations in FRIs and
species responses over time can lead to diverse patterns of
vegetative communities, whereas short FRIs with little varia-
tion may lead to a reduction in vegetative diversity (Keeley
1991).
Closed-Cone Conifers
The closed-cone conifers are pines and cypresses that have
adapted to fire by storing seeds in cones on the trees for many

1050
VOLUME II, CHAPTER 38
years. The resin melts from the heat of a crown fire and re-
leases the seeds into a prepared seedbed. These trees are not
as common in the Sierra Nevada as in the Klamath Moun-
tains or the Coast Ranges, but they do occur in widely scat-
tered areas, usually associated with chaparral, mostly in the
central and northern Sierra (Griffin and Critchfield 1976). The
more common species are knobcone pine and McNab cypress
(Cupressus macnabiana). Knobcone pine appears to do well on
poor soils in a fire regime similar to that of chaparral and is
often found growing within and among chaparral stands (Vogl
et al. 1977). The McNab cypress is more limited in distribu-
tion and is generally confined to areas in which the soils are
derived from ultrabasics (Griffin and Stone 1967), where fu-
els are often limited. This may provide a longer minimum
FRI than in the surrounding vegetation, due to the slower
buildup of fuels on these sites (e.g., Vogl et al. 1977).
Fire History.
We were unable to find any fire-history work
related to the closed-cone conifers of the Sierra. FRIs for stands
of closed-cone conifers are probably similar to, if not slightly
longer than, those of the surrounding chaparral stands
(Minnich and Howard 1984).
Fire Effects.
As we stated earlier, the heat from fire opens
the cones of closed-cone conifers and allows the seeds to dis-
perse. Knobcone pine is a short-lived tree. It is found on sites
where severe, stand-replacement fires usually occur within
the life span of the tree. These species often regenerate dense
stands following stand-replacement fires. The young trees can
begin to produce cones by ten years of age. A loss of these
species could be the result in areas of successful fire suppres-
sion. Once the trees die and fall over, a subsequent fire will
either kill the seeds through the intense heat in the heavy fuel
or consume the cones outright (Vogl et al. 1977; Howard 1992;
Esser 1994).
Black Oak–Ponderosa Pine
The black oak–ponderosa pine forests and woodlands burned
quite frequently with fires generally of low to moderate se-
verity. Two factors contributed to this general fire regime: the
ease of ignition and fire spread due to the relatively loose
fuel beds of long needles and oak leaves (e.g., Rothermel 1983)
and the regular use of fire to manage this forest type by the
native tribes of the Sierra Nevada (Lewis 1993).
Fire History.
Historical FRIs in these forests were generally
from two to twenty-three years (Kilgore and Taylor 1979;
S. Stephens, U.S. Forest Service, Pacific Southwest Research
Station, e-mail communication with the author, April 29 and
May 9 and 30, 1995), commonly being less than ten years
(Swetnam et al. 1991; Swetnam 1993). Once it has been scarred,
ponderosa pine usually becomes a good recorder of fire. The
open wounds often do not heal rapidly and are easily scarred
subsequently by even light fires (McBride 1983). This charac-
teristic of ponderosa pine may allow for the development of
more comprehensive fire histories than in areas where the
species is absent or sparse.
Fire Effects.
The primary carrier of fire in black oak–pon-
derosa pine communities historically was probably grass and
herbaceous vegetation with some needle and leaf litter. How-
ever, as was discussed previously for the blue oak–gray pine
woodlands, the surface vegetation has changed from consist-
ing largely of native perennials in presettlement times to be-
ing made up primarily of introduced annuals (Heady 1977).
Again, historically the predominance of perennials may have
narrowed the season of burning, whereas the annuals may
promote an earlier onset to the burning season because they
dry and cure earlier than the perennials.
Landscape Patterns
Little data exist to describe the historical landscape patterns
of the foothill zone. Much of the information in this regard is
from anecdotal accounts from the early to mid-1800s. The
patterns were probably spatially complex in some areas and
more simple in others, depending upon topography, soils, and
past fire history. Areas dominated by grasses and herbaceous
vegetation, with or without a tree component, would likely
have supported frequent, low-severity fires. These areas could
have remained for long periods as grasslands, savannas, or
open woodlands.
Areas dominated by shrubs would probably show greater
temporal variation due to the nature of the severe burns. These
burns would then be followed by various stages of succes-
sion until a subsequent fire. The rates of fuel accumulation
could vary both temporally and spatially over the landscape
and could potentially lead to diverse patterns of age classes
(Minnich 1983) and species composition (Keeley 1991).
Mixed Conifer Zone
The mixed conifer zone is the main middle-elevation zone of
Sierran forest. The mixed conifer type varies from potentially
being dominated by ponderosa pine to consisting largely of
white fir (Abies concolor), with sugar pine (Pinus lambertiana)
being an important component in many areas. This variation
is generally associated with elevation, site quality, and topo-
graphic moisture effects (Rundel et al. 1977). Other tree spe-
cies of importance in this zone are incense cedar (Calocedrus
decurrens), black oak, and Douglas fir. A variety of hardwoods,
shrubs, and herbaceous plants are also associated with the
mixed conifer forests.
The lower portion of this zone, where ponderosa pine is
often dominant, is commonly used to describe the character-
istic fire regime of the Sierra Nevada. Generally, it is associ-
ated with frequent fires of low to moderate severity (Kilgore
1973). However, the fire regime can vary considerably in both
frequency and pattern of severity by topographic position,
site quality, vegetation, and other local factors.

1051
Fire Regimes, Past and Present
Most published fire-history information for the Sierra
comes from the southern Sierra. The fire histories are gener-
ally associated with the Yosemite, Sequoia, and Kings Can-
yon National Parks, with limited data from elsewhere. Often
the fire histories were developed using giant sequoia samples
because the long-lived trees have distinct rings that are easily
cross-dated, have clear scars, and preserve a long record of
fires and climate variation in the tree rings.
In contrast to the southern Sierra, the northern Sierra has
very little published fire-history information available. The
climate, while still Mediterranean, is more mesic than that of
the southern Sierra. The northern Sierra Nevada receives pre-
cipitation in greater and more consistent amounts than the
southern Sierra at equivalent elevations (Major 1977). Veg-
etation changes along this moisture gradient. Vegetation as-
semblages in the northern Sierra are often similar in species
composition to those found in the southern Cascades and the
Klamath Mountains. However, many of these species are miss-
ing or rare in the southern Sierra (Rundel et al. 1977). Due to
these differences, the discussions of fire regimes in the north-
ern Sierra Nevada draw on data available for similar vegeta-
tion from the southern Cascades and the Klamath Mountains.
Most of the divisions of the mixed conifer zone into veg-
etation types in the discussion that follows are based on Fites
1993 or on discussions with J. A. Fites at various SNEP Sci-
ence Team meetings during the winter and spring of 1995.
Mixed Conifer–Ponderosa Pine
Ponderosa pine is found throughout the mixed conifer belt of
the Sierra Nevada. The characteristic fire regime of much of
the Sierra Nevada (frequent fires of low to moderate sever-
ity) favored the development of ponderosa pine–dominated
forests on many different types of sites where the species is
seral to other conifers (Wright 1978; Agee 1993; Arno in press).
Ponderosa pine, being a shade-intolerant species (Oliver and
Ryker 1990), is rarely a late-successional dominant. Excep-
tions exist where the sites are continually disturbed (usually
by fire) or are warmer, are dryer, or have limited soil devel-
opment compared with other mixed conifer sites (Agee 1993;
Fites 1993).
Some of the earlier attempts to reconstruct FRIs from fire
scars in the Sierra Nevada were in mixed conifer forests domi-
nated by ponderosa pine (Show and Kotok 1924; Wagener
1961). Wagener found median FRIs of five to seven years (with
a range of two to thirty years) for five sites ranging in size
from 15 to 35 ha (37 to 86 acres). However, the fire scar por-
tion of these studies essentially ignored west-side mixed co-
nifer in the northern Sierra Nevada (Wagener 1961), as have
more recent studies. Except in areas where fuel accumulates
slowly due to local site conditions (e.g., Arno in press), there
is no reason to believe that FRIs for ponderosa pine–
dominated sites in the northern Sierra Nevada would be
greatly different from those in other parts of its range (e.g.,
the southern Sierra Nevada, the southern Cascades, or the
Klamath Mountains).
Median FRIs for seven sites from the Klamath Mountains,
where the mixed conifer–ponderosa pine forest types are simi-
lar to those in the northern Sierra, were seven to fifteen years
(with a range of three to fifty-five years) (Skinner in prepara-
tion).
Mixed Conifer–Canyon Live Oak
Canyon live oak (Quercus chrysolepis) is a widespread species
that is found from the upper foothills into the mixed conifer
belt (Myatt 1980). Where canyon live oak is an important com-
ponent of the mixed conifer forests, it is typically associated
with steep slopes and shallow, often rocky soils. Canyon live
oak is often found with ponderosa pine on the harsher sites
and with Douglas fir and/or sugar pine on the more mesic
sites (Fites 1993).
Fire History.
There appear to be no published fire histories
of mixed conifer forests dominated by canyon live oak in the
Sierra Nevada. The fire frequencies were probably similar to,
if not longer and more variable than, those of other mixed
conifer areas, due to lower fuel accumulations and less-con-
tinuous fuels because of site conditions (e.g., Minnich 1980;
Fites 1993; Skinner 1995b). However, this is not always the
case. Where conifers are well represented in the stands, a more
consistent fuel bed can accumulate (e.g., Skinner 1978).
The characteristic fire regime of this type of forest was prob-
ably one of relatively frequent, spatially variable fires of low
to moderate severity. In a fire-history study from the Klamath
Mountains, median FRIs of eleven years (with a range of three
to fifty-five years) were found on three sites by Taylor and
Skinner (in preparation).
Fire Effects.
Following the 1987 wildfires, Weatherspoon
and Skinner (1995) found only small, widely dispersed patches
of apparently stand-replacing fire effects in a large, roadless
area near Hayfork in the Klamath Mountains. Much of the
fire had apparently been a surface fire of low to moderate
severity. The study assessed only sites considered commer-
cial forestlands. However, much of the area is marginal or
noncommercial forestland with a major component of can-
yon live oak. The large proportion of canyon live oak, often
associated with generally sparse, discontinuous surface fu-
els, along with the strong atmospheric temperature inversions
that are characteristic of the region, may have contributed to
the minimal damage observed in the intermixed commercial
forestlands.
Mixed Conifer–White Fir
Within the mixed conifer zone are broad areas where white
fir is considered a major climax component and, depending
upon the disturbance history of the sites, can make up a con-
siderable portion of the stand, especially on more mesic sites
(Laacke 1990). Many of these sites have supported frequent
fires in the past (table 38.1), as evidenced by the occurrence
of ponderosa and sugar pine (e.g., Agee 1993). It is in the up-

1052
VOLUME II, CHAPTER 38
per elevations of the mixed conifer zone, where white fir of-
ten makes up a large component of the stands, that the great-
est density of lightning fires has been found (van Wagtendonk
1986).
Fire History.
In the southern Sierra, the fire regime was gen-
erally one of frequent fires of mostly low to moderate sever-
ity, with occasional, typically small, patches of high-severity
fires (Kilgore 1973). The local FRIs vary in a pattern similar to
the variation in potential species mixtures over the landscape
(Kilgore and Taylor 1979; Caprio and Swetnam 1995). FRIs
generally increase with increasing elevations (McNeil and
Zobel 1980; Caprio and Swetnam 1995). In areas where white
fir is well represented by large, old trees, the FRIs were likely
to have been longer and more variable than those in areas
where larger, older white fir are found only occasionally (Agee
1993).
The median FRIs for mixed conifer–white fir forests ap-
pear to have ranged from approximately seven to twenty years
(with a range of three to forty years) for the southern Sierra
Nevada (Kilgore and Taylor 1979; Caprio and Swetnam 1995).
In the northern Sierra and the southern Cascades, the me-
dian FRIs ranged from eight to twelve years (with a range of
three to thirty-five years) (table 38.1).
Fire Effects.
The variation in actual species dominance is
probably related to the local consistency of past fires. Those
areas where fires were frequent, with little variation in the
frequency, would tend to favor ponderosa pine. In areas where
the fires were somewhat less frequent, especially where there
was more variation in the frequency and severity, more sugar
pine, Douglas fir, and white fir would tend to be found (e.g.,
Agee 1994).
Mixed Conifer–Giant Sequoia
The fire history of giant sequoia groves has been studied more
than that of any other forest type of the Sierra Nevada. The
giant sequoias are particularly interesting for studies involv-
ing tree-ring analysis because of the longevity of the species.
Fire History.
In a study of five sequoia groves along a north-
south 160 km (93 mi) transect, Swetnam (1993) found that
during the last 1,500 years or so the longest fire-free period in
any grove was thirty years before the 1860s. Generally, prior
to 1860, the maximum FRIs were less than fifteen years, with
mean FRIs of approximately three to eight years. Importantly,
most of these fire scars (63%–92%) were found in latewood or
between rings, suggesting that the fires occurred in either late
summer or fall (Swetnam et al. 1992).
Fire Effects.
The fire regime of mixed conifer forests domi-
nated by giant sequoia has been described as being charac-
terized by frequent fires of low to moderate severity (e.g.,
Kilgore and Taylor 1979; Kilgore 1981; Swetnam et al. 1991;
Swetnam 1993; Caprio and Swetnam 1995), with occasional
areas of locally high-severity fires, where small patches of
reproduction (young trees) and individuals occasionally burn
more intensely (Stephenson et al. 1991).
Giant sequoias have closed cones (Harvey et al. 1980) that
release seeds following relatively intense fires and regener-
ate best where seeds are scattered onto bare soil in open con-
ditions (Stephenson et al. 1991). Fire appears to be a necessary
ecological process to provide for adequate long-term repro-
duction in giant sequoia forests (Stephenson 1994).
Mixed Conifer–Douglas Fir
Douglas fir can be an important component of the mixed co-
nifer forest in the northern Sierra. These areas are usually as-
sociated with more mesic conditions than those where
ponderosa pine is more important (Fites 1993). Because of the
moister, cooler conditions of these areas and the relatively
compact fuel beds of short needles, these sites probably
burned somewhat less frequently and less regularly than ar-
eas where the longer-needled pines are more dominant. Since
we know of no published fire-history data from the Sierra
Nevada for this forest type, we must rely on data from simi-
lar forests found in the Klamath Mountains and the southern
Cascades.
Fire History.
Presettlement median FRIs for areas of mixed
conifers dominated by Douglas fir in the Klamath Mountains
were found by Agee (1991) to have been sixteen years (with a
range of twelve to fifty-nine years), by Skinner (in prepara-
tion) to have been ten to nineteen years (with a range of three
to fifty-seven years) for seven sites, and by Taylor and Skin-
ner (in preparation) to have been eleven to eighteen years
(with a range of three to fifty-nine years) for six sites. The
sites represented by these studies are geographically distrib-
uted from near Oregon Caves National Monument (southern
Oregon) to near Castle Crags State Park (northern Califor-
nia). They cover a variety of elevations and topographic po-
sitions. Since they are so consistent for these geographically
dispersed sites, we suggest that they may be representative
of the type in the northern Sierra Nevada. Confirmation of
this hypothesis will require fire-history studies in the Sierran
mixed conifer–Douglas fir forests.
Fire Effects.
It is important to note the range of intervals in
the FRIs in this type. Most sites show infrequent longer peri-
ods (more than twenty-five years) without fire scars. Since
the median intervals are not greatly different from those for
the ponderosa pine–dominated areas, an important difference
between these vegetation types may be the range of variabil-
ity. This variability would periodically allow young trees to
survive to reach a size and condition to become resistant to
low-severity fire. Once Douglas fir is established on a site,
the compact litter bed composed of short needles would also
help reduce the intensity of subsequent surface fires (e.g.,
Rothermel 1983). As a mature tree, Douglas fir is quite resis-
tant to fires of low to moderate intensity. The nonresinous,

1053
Fire Regimes, Past and Present
thick bark of Douglas fir does not appear to allow the tree to
scar as readily as other species (e.g., ponderosa pine, incense
cedar, and sugar pine), and it may heal more rapidly (McBride
1983; Skinner and Taylor in preparation). Due to the suscep-
tibility of Douglas fir to fire damage as a seedling or sapling,
it may be better suited to fire regimes where generally fre-
quent (ten to twenty years) but variable FRIs allow the occa-
sional survival of younger trees (e.g., Agee 1994).
Mixed Conifer–Tan Oak
The mixed conifer–tan oak (Lithocarpus densiflorus) forests of
the northern Sierra are similar to those referred to as mixed
evergreen forests in the Klamath and north Coast Ranges
(Gudmunds and Barbour 1987). These forests are generally
found within the mixed conifer zone at lower elevations as-
sociated with relatively high annual precipitation (Fites 1993).
Fire History.
Fire histories for tan oak–dominated forests
are from the Klamath Mountains of northwestern California
and southwestern Oregon. Wills and Stuart (1994), in a study
conducted near the Forks of the Salmon, found median FRIs
to have been fifteen years (with a range of three to fifty years)
on three sites. Agee (1993) reports a mean return interval of
eighteen years for the type near Oregon Caves National Monu-
ment. The fire regime in mixed evergreen forests generally
consisted of frequent fires of low to moderate severity, with
occasional fires of locally high severity (Agee 1993). The FRIs
were more variable than those of the mixed conifer–ponde-
rosa pine forests.
Fire Effects.
The tan oak, madrone (Arbutus menziesii), and
other hardwoods of these forests are easily top-killed by fires
of moderate or high intensity, but sprout vigorously follow-
ing fire. The Douglas fir often associated with these forests
can survive moderate fires when mature but may be killed in
severe fires. Consequently, following a severe fire, sites can
be dominated for extended periods by tan oak and other hard-
woods, since recurring fires often kill the Douglas fir seed-
lings while the hardwoods continually resprout (e.g., Agee
1993).
Montane Chaparral
Throughout the mixed conifer zone and the upper montane
are tracts dominated by shrubs often called montane chapar-
ral. Some common species associated with these shrub fields
are greenleaf manzanita (Arctostaphylos patula), deerbrush,
snowbrush (Ceanothus velutinus), mountain whitethorn (C.
cordulatus), bitter cherry (Prunus emarginata), and bush
chinkapin (Castanopsis sempervirens) (Sampson and Jespersen
1963). Huckleberry oak (Quercus vaccinifolia) can be impor-
tant on relatively poor sites (Rundel et al. 1977; Fites 1993).
Fire History.
We were unable to find any fire-history stud-
ies that would shed light on FRIs for these vegetation types.
The FRIs were probably quite variable due to the influence of
poor growing conditions. The FRIs would often likely have
been longer and more variable than those for the adjacent for-
est types within the mixed conifer zone. These areas some-
times have widely scattered individuals or clumps of old trees
associated with the shrubs (Fites 1993). The large, old trees in
these latter areas could potentially be used to determine the
fire history of various sites of interest.
Fire Effects.
These montane shrub fields can be rather stable
communities on soils associated with poor growing condi-
tions (Bolsinger 1989; Sampson and Jespersen 1963). How-
ever, many shrub fields of montane chaparral are the result
of secondary succession initiated by stand-replacing fires, log-
ging, or other disturbance (e.g., Leiberg 1902; Bock and Bock
1977; Bolsinger 1989). Most montane chaparral shrub species
are disturbance adapted and can resprout or germinate from
seeds stored in the soil following a fire (Kauffman 1990). Once
a shrub field is established, the shrubs can maintain domi-
nance for long periods. Fires that recur during the life of the
shrubs and prior to the establishment of the succeeding for-
est will tend to maintain the shrub fields (Wilken 1967).
Landscape Patterns
For the mixed conifer zone there exist only anecdotal accounts
of landscape patterns for most of the area prior to the 1900s.
It is likely that by this time much of the mixed conifer had
been affected by various activities associated with the settle-
ment period (Cermak and Lague 1993). Many of the written
accounts from the beginning of the twentieth century do not
clearly indicate whether they describe presettlement condi-
tions or conditions that reflect the effects of the settlement
period (e.g., Sudworth 1900; Leiberg 1902).
Due to the physical structure of the landscapes (e.g., to-
pography, geomorphology, etc.), it is likely that the landscapes
of the mixed conifer zone varied considerably in their spa-
tiotemporal patterns of species composition, age classes, and
stand densities at a variety of scales. Cermak and Lague (1993)
relate numerous accounts of vegetation that describe every-
thing from open, parklike stands of large trees to thick stands
of trees to dense stands of shrubs. However, there are little
data to describe the extent of these conditions or to quantify
what is meant by open, thick, dense, or other such descriptive
terms. Recognizing the lack of such data, the California
Spotted Owl EIS Team made a concerted effort to attempt
landscape-scale characterizations for the Sierra, using the
knowledge of specialists from a variety of disciplines (Toth et
al. 1994).
It is impossible at this time, due to the lack of data, to con-
clusively describe the pre-1850s landscape characteristics and
how they changed prior to the 1900s. However, based on avail-
able knowledge of fire history, fire effects, fire behavior, and
the accounts noted previously, we surmise that the landscape
patterns in the mixed conifer zone were of a relatively fine
scale (e.g., Bonnicksen and Stone 1982; Stephenson et al. 1991).
Large, old trees appear to have been characteristic of many

1054
VOLUME II, CHAPTER 38
forested areas. However, this certainly does not imply that
varying sized patches of shrubs or younger trees were not
present in the landscape. Variation in tree size and species
composition was likely to be greater horizontally (across the
landscape) than vertically (within a single stand). It appears
that many forested areas were generally more open than they
are today, due mostly to the frequency of fires. This may have
promoted more grasses and herbs than are associated with
most forest stands today. It is likely that riparian areas often
served as barriers to low-intensity and some moderate-inten-
sity fire movement, thus contributing to landscape diversity
(see the discussion of riparian areas later in this chapter). The
northerly aspects most likely had different species composi-
tions from and greater densities of trees than the southerly
aspects, as well as different scales of group, aggregation, or
stand patterns. Fires were probably more variable in their
severity and frequency on moist sites than on dryer sites. See
Toth et al. 1994 for more detail concerning characteristics of
landscape patterns.
In addition to site characteristics and landscape structure,
past patterns of fire occurrence are likely to have influenced
the patterns of species composition and dominance within
the mixed conifer zone. The differences among the various
species in traits that affect their survival of low-to-moderate-
intensity fire (e.g., bark thickness, longevity, susceptibility to
rots, etc.) may help suggest the characteristics of past fire re-
gimes. An example would be bark thickness. The five widely
spread conifers (ponderosa pine, sugar pine, Douglas fir, in-
cense cedar, and white fir) develop thick bark when mature
and are generally resistant to low-intensity fires (e.g., Starker
1934; Minore 1979; Wright and Bailey 1982). However, they
vary in the relative ages at which they develop bark thick
enough to withstand low-intensity fires. Thick bark gener-
ally develops rapidly in ponderosa pine and more slowly in
white fir and incense cedar (Weaver 1974), with sugar pine
and Douglas fir developing it at an intermediate rate. These
differences among the species suggest that the spatiotempo-
ral variability of fires may differentially influence the survival
of young trees. More regular FRIs were likely to have been
found in areas dominated originally by ponderosa pine.
Where sugar pine and Douglas fir were a significant portion
of the dominant trees, the frequency and intensity of fire may
have been more variable, though fires were still generally fre-
quent. Greater variability in both frequency and intensity
would probably have occurred where white fir constituted a
major proportion of the dominant trees. Thus, the variation
in spatial and temporal fire patterns is likely to influence the
variation in species composition of the mixed conifer zone
over the landscape.
Upper Montane Zone
The upper montane zone includes the red fir (Abies magnifica),
Jeffrey pine (Pinus jeffreyi), lodgepole pine (P. contorta var.
latifolia), western white pine (P. monticola), aspen (Populus
tremuloides), and vegetation found in the higher elevations of
white fir forests and woodlands (Rundel et al. 1977; Potter
1994). These forest types are found at altitudes and in topo-
graphic areas of high lightning frequency when compared
with the mixed conifer or foothill zones (van Wagtendonk
1991a). The upper montane zone can be found on both the
west and east sides of the Sierra Nevada. However, most re-
search on fire history is from the west side, except in the south-
ern Cascades.
In Sequoia National Park, Vankat (1983) found that the
upper montane conifer types accounted for approximately
53% of the area of the park and 76% of the lightning igni-
tions. Although fires occur frequently in the upper montane
zone, they are not likely to spread readily over the landscape
except under unusual conditions. This is due to the shortness
of the fire season, the compactness of the fuel beds, and the
relatively common natural fuel breaks (meadows, rock out-
crops, etc.).
The fire regimes in these upper montane areas are likely to
be more variable in frequency and in severity than are those
from the lower elevations (Agee 1993). The number of fires
for a 1 ha (2.5 acre) upper montane Jeffrey pine–white fir site
in the Klamath Mountains was found to vary considerably
from one century to the next (Skinner in preparation). Six fires
occurred in the 1500s, one in the 1600s, nine in the 1700s, four
in the 1800s, and three between 1900 and 1944 (the year of the
last fire). Mixed conifer sites at lower elevations in the same
watershed were found to have had less temporal variation in
the numbers of fires recorded in fire scars. Similar variation
has been found on Jeffrey pine–white fir and red fir–white
pine sites in upper montane areas of Lassen Volcanic National
Park (A. H. Taylor, Department of Geography, The Pennsyl-
vania State University, telephone conversation with the au-
thor, May 17, 1995).
Table 38.1 summarizes the high degree of variation in FRIs
in upper montane forest types. Local variation in fuel conti-
nuity may contribute considerably to the variability in the
FRIs from site to site.
Red Fir
The fire regimes of red fir forests appear to vary considerably
from landscape to landscape. The surface litter is often sparse
and compact (Parker 1984) and is usually not conducive to
rapid fire spread. In landscapes broken up by many rock out-
crops and meadow systems, such as those characteristic of
the central and southern Sierra (e.g., Vale 1987), the fire re-
gimes are characterized by longer FRIs (Pitcher 1987). Longer
FRIs in landscapes of Lassen Volcanic National Park have been
found where fuel accumulations are low and the fuel bed is
broken by outcrops of volcanic rock (A. H. Taylor, telephone
conversation with the author, May 17, 1995). However, in ar-
eas of more continuous litter beds, as are often found in the
northern Sierra and the southern Cascades, the FRIs appear
to have been shorter (Taylor 1993a). Fires in these areas, al-
though patchy, appear to spread more easily over larger ar-

1055
Fire Regimes, Past and Present
eas. Pre-suppression-period fire rotations in red fir landscapes
within the Caribou Wilderness in the southern Cascades were
found to have been approximately seventy years (Taylor
1995a).
Stand-replacing fires appear to have occurred infrequently
(Taylor and Halpern 1991; Agee 1993). Leiberg (1902) reported
sizable areas in the red fir zone of the Feather River where
brush fields appeared to be the result of severe burns.
Some fires in red fir forests have been observed to spread
primarily through branch wood and large woody debris, since
the compact needle beds do not readily spread fire (Toth et
al. 1994). This pattern of spotty fire spread helps contribute
to the patchy nature of the burns. These patterns of fire occu-
pance in red fir appear to hold for both the west side and the
east side of the range (e.g., Hawkins 1994).
In the lower elevations of the upper montane, white fir can
be a major component (Potter 1994), often mixing with red
fir. Taylor (1993a) and Taylor and Halpern (1991) have re-
ported on disturbance regimes and stand dynamics in forests
of mixed red fir and white fir in the southern Cascades of
northern California. They found that fire and wind had been
major disturbance factors contributing to spatial patterns of
age and tree sizes. White fir, generally more resistant to dam-
age at a younger age than red fir (C. P. Weatherspoon, U.S.
Forest Service, Pacific Southwest Research Station, personal
conversation with the author, May 18, 1995), may occupy ar-
eas where FRIs are more regular than those where red fir is
found without white fir.
Jeffrey Pine
The Jeffrey pine forests of the upper montane appear to have
had more variable FRIs than the lower-elevation pine forests.
Jeffrey pine is found from the upper mixed conifer zone
through the upper montane on the west slope and extends
onto the east slope of the Sierra. In the upper montane zone,
Jeffrey pine is often associated with white fir, red fir, white
pine, lodgepole pine, and other species (Rundel et al. 1977).
Jeffrey pine is similar to ponderosa pine in its fire-associated
characteristics.
Jeffrey pine is often found on more extreme sites (sites that
are colder, drier, or more nutrient deficient) than many of its
associates (Jenkinson 1990). Because of this, fire histories of
Jeffrey pine forests may show much greater variability than
those of its close relative, ponderosa pine. The increased vari-
ability is related to a limited burn season, slow fuel accumu-
lations, and, often, landscapes broken up by rock outcrops.
The fire regimes for Jeffrey pine forests in the southern Cas-
cades (A. H. Taylor, telephone conversation with the author,
May 17, 1995) and in the Sierra San Pedro Martir (Minnich et
al. in press) appear to have followed the pattern of less fre-
quent and more variable FRIs than would be expected of
lower-elevation ponderosa pine forests of the Sierra Nevada.
Mean fire frequency prior to this century in forests domi-
nated by Jeffrey pine and ponderosa pine in the Caribou Wil-
derness in the southern Cascade Range was found to be 18.8
years (the FRIs ranged from 5 to 39 years) for an area of 127
ha (314 acres). Fire rotation was determined to be 70 years
(Taylor 1995a).
Aspen
We are aware of no published studies concerning aspen fire
regimes in the Sierra Nevada. Elsewhere throughout the west-
ern United States it is recognized that stand-replacement fire
has often played a major, yet infrequent, role in the develop-
ment and maintenance of aspen stands (Kilgore 1981; Jones
and DeByle 1985). However, because of the types of sites that
aspen generally occupies in the central and southern Sierra
(e.g., around moist meadows, near rock piles at the base of
cliffs, etc.), many of these stands may be relatively stable and
unrelated to fire (Rundel et al. 1977). In the northern Sierra
Nevada and the southern Cascades, aspen may often be suc-
cessional to more tolerant conifers such as white fir or red fir
(Potter 1994). FRIs in these locations are likely to be quite vari-
able and long. Fires have been shown to kill competing coni-
fers and regenerate otherwise declining aspen stands (Brown
and DeByle 1989; Bartos et al. 1991). Where aspen has become
established, it may be able to survive more frequent fires in a
shrub state similar to that described by Leiberg (1902).
Lodgepole Pine
Fire has long been recognized as an important ecological pro-
cess in lodgepole pine forests (e.g., Clements 1910). Lodge-
pole pine is commonly thought of as a closed-cone conifer
requiring the heat of fires to open the cones. However, this
feature is absent from the species in the Sierra Nevada (Lotan
and Critchfield 1990). The spatial patterns of age classes within
stands of lodgepole pine in the central Sierra have been re-
ported to be of a fine grain usually associated with small gaps
rather than the large gaps created by the extensive crown fires
characteristic of the type in the Rocky Mountains. Mature
stands are often open and have sparse surface fuels (Parker
1986). These conditions do not easily promote ignition and
fire spread (van Wagtendonk 1991b).
Fire History.
No published information exists concerning
fire history in the lodgepole pine forests of the upper mon-
tane in the Sierra. The type may have a fire regime that is
intermediate between the red fir–white fir or Jeffrey pine–red
fir forests and the subalpine areas.
Data from landscape-level studies in Lassen Volcanic Na-
tional Park (A. H. Taylor, telephone conversation with the
author, May 17, 1995) and the adjoining Caribou Wilderness
area (Solem 1995; Taylor 1995a) in the southern Cascades sug-
gest a disturbance regime similar to that reported by Stuart
et al. (1989) for south-central Oregon. The primary difference
was that there were more frequent fires in the Caribou Wil-
derness (Solem 1995). The mean FRI for nine-point samples
of trees with multiple scars in stands dominated by lodge-
pole pine was 34.5 yrs (with a range of 28 to 41 years). The
fire rotation prior to this century in two lodgepole pine–domi-

1056
VOLUME II, CHAPTER 38
nated areas of the Caribou Wilderness was calculated to be
57 years (162 ha [400 acres]) and 104 years (92 ha [228 acres])
(Taylor 1995a).
Fire Effects.
The lodgepole pine–dominated forests of the
Caribou Wilderness are multiaged but show the influence of
larger-scale episodic events through the dominance of age
classes by one or a few even-aged cohorts. These even-aged
cohorts appear to be related to past fire events (Solem 1995).
Landscape Patterns
A greater variability in the spatial and temporal pattern may
have developed in the upper montane zone than in the lower
mixed conifer zone. The variability in landscape patterns is
likely a result of a number of factors, such as heavy snow
packs that can linger late into the year, influencing fire prob-
ability; patterns of soils and exposed rock; and compact litter
beds, as well as other factors discussed in more detail previ-
ously.
Variation in the spatial extent and severity of individual
fires helps lead to a dynamic, complex pattern of dominance
by age classes and species over the landscape. Fire sizes esti-
mated from fire scars, age classes, and other patterns of tree-
ring variation in the Caribou Wilderness ranged from 22 ha
to 1,067 ha (55 acres to 2,635 acres), with a median size of 101
ha (250 acres). Small fires appear to have been mostly of low
severity, whereas larger burns had considerable portions af-
fected by moderate-to-high-severity fire (Taylor 1995a).
Subalpine Zone
Subalpine areas of the Sierra Nevada are characterized by
forests of widely spaced trees of short stature that often
straddle the crest of the range (Rundel et al. 1977). These types
generally have limited, usually discontinuous fuel accumu-
lations (Kilgore and Briggs 1973; USFS 1983). Characteristic
trees are mountain hemlock (Tsuga mertensiana), white bark
pine (Pinus albicaulis), western white pine (P. monticola), fox-
tail pine (P. balfouriana), limber pine (P. flexilis), and western
juniper (Juniperus occidentalis), with lodgepole pine in the
lower portions. Subalpine areas receive a greater proportion
of lightning strikes than do lower-elevation forests (van
Wagtendonk 1991a). However, the number of ignitions is dis-
proportionately low (Vankat 1983; van Wagtendonk 1991b).
Overall, fires are infrequent and of low severity within the
subalpine types (Kilgore 1981). Only occasionally, and usu-
ally on relatively small areas, do the fires become more se-
vere. Because of the nature of fire in the upper montane and
subalpine forests, the National Park Service initiated a pre-
scribed natural fire program in these areas more than two
decades ago (van Wagtendonk 1986).
Keifer (1991) reports a study in the subalpine zone of Se-
quoia–Kings Canyon National Parks, where lodgepole pine
and foxtail pine are found. She found that monospecific lodge-
pole stands always had evidence of past fires, whereas evi-
dence of past fires was found only occasionally in the foxtail
pine stands. The areas where the two species intermingled
were intermediate in evidence of past fire. She noted that
lodgepole pine recruitment appears to be pulsed with the age
classes associated with past fires. Conversely, where foxtail
pine stands showed evidence of fires the recruitment appeared
to be more sporadic and not necessarily associated with fires.
She suggests that the response to fire of lodgepole pine (a
thin-barked tree) is regeneration in gaps created when the
thin-barked trees are killed by fire. Foxtail pine, on the other
hand, exhibits thicker bark, which may better protect the trees
from the low-intensity fires characteristic of the zone. Climate
variation and factors other than fire influencing mortality may
account for foxtail pine recruitment patterns.
East-Side Ecosystems
We were unable to find published information concerning fire
for the east side of the Sierra Nevada or the east side of the
southern Cascades in California. Limited data from the south-
ern Cascades in Lassen National Forest were supplied by
Olson (1994) for three east-side mixed conifer stands domi-
nated by ponderosa and Jeffrey pine with white fir and in-
cense cedar present. He found median FRIs of eight to sixteen
years (with a range of six to thirty-two years).
Agee (1993, 1994), individually and as part of the Eastside
Forest Ecosystem Health Assessment, recently published re-
views of the fire regimes for most of the vegetation types that
would be found on much of the east-side SNEP assessment
area. Finding only limited fire-history information specific to
the SNEP assessment area, we refer the reader to these recent
summaries as well as to Chang 1996.
Riparian Areas
Riparian areas are generally zones of transition from the ter-
restrial uplands to aquatic habitats. Riparian zones can be
identified by vegetation that requires large amounts of free
or unbound soil water. Because of available water and many
other vegetative characteristics associated with riparian ar-
eas, these zones are disproportionately more important to
many wildlife species than their limited extent on the land-
scape would indicate (Thomas et al. 1979).
We are not aware of any published fire-history studies that
would shed light specifically on riparian fire regimes in the
Sierra Nevada, southern Cascades, Klamath Mountains, or
the east side of the Sierra-Cascade crest. Agee (1993) has con-
ceptually described the probable relationships of fire with ri-
parian areas of various forms. Agee suggests that narrower
riparian zones will be more likely to have been more fre-
quently disturbed by fire than will wider riparian zones, and
that riparian zones in dryer areas will probably burn more
frequently than those in wetter areas.
Skinner (in preparation) gathered fire-history data from
four riparian areas along the east side of the Shasta-Trinity

1057
Fire Regimes, Past and Present
Divide in the Klamath Mountains within the Sacramento River
watershed north of Lake Shasta. These data were gathered
from within the riparian zone and will be summarized here
because of the lack of other data.
The forest type adjacent to all four sites would generally
be described as the Klamath enriched mixed conifer type (e.g.,
Sawyer and Thornburgh 1977). Species common to all four
sites were willows (Salix spp.), western azalea (Rhododendron
occidentale), Port Orford cedar (Cupressus lawsoniana), and
various grasses, sedges, and forbs associated with wet
meadow systems. Other common species on these sites were
spiraea (Spiraea douglasii), Sierra laurel (Leucothoe davisiae),
thimbleberry (Rubus parviflorus), mountain alder (Alnus incana
sp. tenuifolia), and California pitcher plant (Darlingtonia
californica). Elevations ranged from 1,400 to 1,900 m (4,600 to
6,300 ft). Two sites were on north-trending, gently sloped
swales, and two were on steeper south-facing slopes. The sites
were all less than 1 ha (2.5 acres) each.
Fire History
The fire histories, which dated from the mid-1600s, suggest
that riparian areas generally have longer and more variable
FRIs than nearby upland sites. The pre-1850 median FRIs for
north-facing swales were 31 and 36 years (with a range of 9 to
71 years). The time since the last fire scar formed on these
sites was 49 and 95 years. The pre-1850 median FRIs for the
south-facing slopes were 26 and 52 years (with a range of 7 to
65 years). No fires had been recorded in the stumps for 58
and 102 years. Nearby (less than 500 m [1,650 ft] away) up-
land sites had pre-1850 median FRIs of 12 and 15 years (with
a range of 6 to 44 years).
On the west side of the Shasta-Trinity Divide, in the water-
shed of the East Fork of the Trinity River, data were collected
for two 1 ha (2.5 acre) sites that were separated by a small
creek with a well-developed riparian zone. The forest types
were Klamath enriched mixed conifer with riparian species
similar to those described previously. These sites were in the
middle third of a long north-facing slope at an elevation of
approximately 1,450 m (4,750 ft).
This fire history, dated from the mid-1500s, suggests that a
narrow riparian zone only a few meters wide may have longer
and more variable FRIs than adjacent upland sites. The pre-
1850 median FRIs were 13 and 14 years (with a range of 5 to
47 years). Sixty-one years had passed since the last fire scar
was formed. A total of nineteen fires was recorded for the
period. Of these nineteen fires, only ten (53%) were recorded
on both sides of the riparian zone. The median FRI for the
fires recorded on both sides of the riparian zone was 29 years
(with a range of 7 to 47 years).
Fire Effects
Many species associated with riparian areas are angiosperms
that often can respond to fire by sprouting. Although the avail-
able moisture on these sites produces vegetation (potential
fuel) readily, FRIs may be longer than in the surrounding
stands. Consequently, fires may tend to burn more severely,
at least locally, when they do occur. However, the severity of
fires may often be restrained by higher fuel moistures associ-
ated with riparian zones.
Localized severe burns in riparian areas may not greatly
affect aquatic habitat at the landscape scale, depending upon
the proportion of the riparian habitat that is burned severely
(Amaranthus et al. 1989).
Landscape Patterns
Riparian areas may serve as effective barriers to many low-
intensity and some moderate-intensity fires and thus influ-
ence landscape patterns beyond their immediate vicinity.
TWENTIETH-CENTURY
FIRE REGIMES
The twentieth-century fire regimes of the Sierra Nevada are
generally quite different from those prior to Euro-American
settlement. Many factors occurring in the 1800s and early
1900s combined to induce drastic changes in fire regimes.
Additionally, these factors have contributed to landscape pat-
terns that are still evident today. We will first describe the
major factors that contributed to the change in the fire regimes
and then will discuss the nature of current Sierra Nevada fire
regimes.
Factors in the Nineteenth Century That Helped
Influence Changes in Fire Regimes
The following factors, mostly occurring in the 1800s, have
combined to dramatically alter fire regimes in the Sierra.
However, the effects of these factors were not spatially or tem-
porally universal.
Population Decline among the Native Peoples
The populations of native peoples were declining through-
out the nineteenth century. Initially the decline was due to
diseases introduced by Europeans, and later it was augmented
by systematic extermination and forced relocation (Beesley
1996; Cook 1971). These events caused considerable disrup-
tion of traditional land-use patterns and cultural practices
(Moratto et al. 1988), probably including a reduction in the
use of fire. Reductions in fire frequencies in the early 1800s
have been noted on some higher-elevation mixed conifer sites.
It has been hypothesized that these reductions may have been
related to the decline in the native populations (Caprio and
Swetnam 1995). Kilgore and Taylor (1979) suggest that the
decrease in fires by the late 1800s for their study area may
have been related to the decline in burning by natives.

1058
VOLUME II, CHAPTER 38
Influx of Miners
Fire was used to aid in general land clearing during the settle-
ment period and was associated with vegetation type con-
versions in some areas (Barrett 1935). There was a great influx
of miners into the Sierra Nevada following the discovery of
gold in 1848 (Beesley 1996). Leiberg (1902) noted that many
areas that appeared to have been forested at one time had
been converted to brush fields by severe fires, many of which
appeared to be related to the mining locations.
Extraction of Wood Material during the
Settlement Period
Extensive logging to provide materials to support mining and
other settlement activities took place in many locations, for
example, Nevada City, Placerville, and Lake Tahoe Basin
(Beesley 1996; McKelvey and Johnston 1992). Shakes from
sugar pines were more valuable than lumber and could often
be produced economically where general logging did not take
place because of distance to markets and lack of economical
transportation (McKelvey and Johnston 1992). The extraction
of shakes, lumber, and firewood left great quantities of resi-
dues behind, since the portions of the trees with limbs attached
were often not used (Beesley 1996). The residues then fueled
subsequent high-severity wildfires that would kill extensive
tracts of residual and second-growth trees. These fires were
notably more severe than fires that burned in forested areas
that had not been subjected to the extraction of the wood
materials (Leiberg 1902).
Sheepherding
Heavy grazing in the late 1800s appears to have reduced the
landscape effects of fires in many areas. This alteration of fire
regimes appears to have been effected in two basic ways by
the large-scale sheepherding of the late 1800s and early 1900s.
First, sheepherders burned extensive areas in higher eleva-
tions and more mesic sites to promote more forage for their
herds (Barrett 1935; McKelvey and Johnston 1992). This burn-
ing was aimed at reducing the number of downed logs and
patches of seedlings and saplings (Sudworth 1900). Second,
the intensive grazing was also associated with nearly barren
or very lightly covered ground (Sudworth 1900; Leiberg 1902).
The combined effect of these practices appears to have been a
significant reduction in fuel continuity that limited the actual
spread of the fires by the late 1800s, so that many areas show
an actual decrease in fire frequency as recorded in fire scars
at that time (Vankat and Major 1978). This reduction in fire
frequency associated with periods of heavy livestock grazing
has been recorded in other areas of the western United States
as well (e.g., Savage and Swetnam 1990; Mensing 1992).
Fire-Exclusion Policy
The move toward fire exclusion began early in California. The
first law against starting fires was issued under Spanish rule
in 1793 (Barrett 1935). This was aimed at halting Indian burn-
ing of grasslands, because it deprived the Spanish-owned
horses and other livestock of forage. In the late 1800s, forest-
ers were making strong arguments to persuade the public to
support fire exclusion so that wood production would be
higher in the future (California State Board of Forestry 1888).
By the turn of the century, fire exclusion was becoming a gen-
eral policy among government agencies, although it was not
yet fully accepted by the public (Husari and McKelvey 1996;
Office of the State Forester 1912).
Twentieth-Century Fire Regime Changes
Due to the initiation of fire-exclusion policies in the late nine-
teenth and early twentieth centuries, as well as to the suite of
factors just discussed, the characteristic fire regimes of many
forests of the Sierra Nevada appear to have changed dramati-
cally since the mid-1800s. Before the nineteenth century, the
characteristic fires affecting large portions of the landscape
would most likely have been of low or low to moderate se-
verity, with patches of higher severity. By the late twentieth
century, the characteristic fire was generally of high severity,
with only small portions of low to moderate severity. Those
forests that have experienced the greatest changes are most
likely those on productive sites where fires were more fre-
quent in the past (Weatherspoon et al. 1992), for example,
ponderosa pine, black oak, and mixed conifer stands.
The justification for eliminating fires was based primarily
on the perceived damage done to the forests. Damage in this
sense was related to two factors. First, the surface fires would
often cause fire wounds to the bases of trees that reduced the
value of these trees and, usually over many centuries, would
contribute to the demise of the individual trees. Second, the
frequent surface fires were noted to maintain the stands in
open conditions by killing most seedlings and saplings in the
understories and leaving the forests with low stocking levels
(Sudworth 1900; Leiberg 1902; Show and Kotok 1924).
Despite the initial reluctance of local human populations
to accept fire exclusion, there appeared to be the beginnings
of successful reduction of fires by the end of the first decade
of the twentieth century (Office of the State Forester 1912).
Nationally, the disastrous fires of 1910 in the northern Rockies
helped coalesce political support for exclusion of fire (Agee
1993). Data from the records of the national forests show a
steady decline, especially in numbers and acres of human-
caused fires, over most of this century (McKelvey and Busse
1996; Weatherspoon and Skinner 1996). However, numbers
and acres of fires do not tell the whole story of the change in
fire regimes. The major changes in fire regimes are related to
the type of fire behavior and the spatial patterns associated
with the fires. Fire behavior associated with most forest types
in California has changed considerably over this century.
Sudworth (1900), Leiberg (1902), and Show and Kotok
(1924) all remark that crown fires and extensive areas of mor-
tality (except in previously logged areas) were unusual at the
time of their studies. Show and Kotok (1929) describe the char-
acteristic fires associated with major vegetation types in the

1059
Fire Regimes, Past and Present
late 1920s. Only chaparral and brush types were generally
associated with crown fires. Forests composed of ponderosa
pine or mixed conifers were associated with surface, litter fires,
with crown fires being uncommon. They note that places
where crown fires occurred were associated with logging slash
or dense, young, second-growth stands. Fires in the upper
montane forests were generally described as ground fires that
moved primarily through duff. This is a very different pic-
ture from that of today, where most wildfires, if not immedi-
ately suppressed, quickly become at least severe surface fires
capable of killing very large trees. A few examples of recent
large, stand-replacement fires in the SNEP study area are the
Scarface (1977), Indian (1987), Stanislaus Complex (1987),
Stormy (1990), Cleveland (1992), Fountain (1992), and Cot-
tonwood (1994) fires.
Early in the fire-exclusion era, Benedict (1930) indicated
that the fire hazard was increasing even as the policy was
leading to the achievement of the goal of increasing regen-
eration survival and ensuring greater stocking levels of trees.
He noted that fire-suppression costs were increasing dramati-
cally with the change in stand structures and fuel conditions.
Please see Arno (in press) and Weatherspoon and Skinner 1996
for further discussion of fuels and fuel buildup during the
fire-suppression era.
As most fires are now suppressed when they are quite
small, the frequency of the fires that affect the landscape now
appears to be related primarily to the occurrence of burning
conditions that are outside the range that modern fire-
fighting technology can deal with. Warm, dry summers guar-
antee that severe burning conditions will occur each year at
some point. These conditions occur most often at the lower
elevations and are less frequent as elevation increases. The
significant fires now are more likely to occur during severe
burning conditions of the inevitable drier years (McKelvey
and Busse 1996).
Before the fire-exclusion policy, many fires probably burned
for weeks or months. Those ignited in midsummer would
have been able to burn until the fall rains or snows extin-
guished them (e.g., Agee 1993). These fires would have burned
under a variety of weather conditions, ranging from hot, dry,
and windy to relatively benign. This temporal variation in
weather would influence fire behavior such that the result-
ing spatial patterns could be quite variable over the landscape.
Even today, when a fire has burned for an extended period
the result has been considerable spatial variation in the fire’s
severity. Recent examples are prescribed natural fires in the
national parks of the Sierra (e.g., Kilgore and Briggs 1973);
the 1987 wildfires in the Klamath Mountains (e.g., Weather-
spoon and Skinner 1995); and the 1994 Dillon lightning fires
(USFS 1995), also in the Klamath Mountains.
A characteristic of twentieth-century fire regimes that is
different from those prior to the fire-exclusion policy is the
spatial extent and pattern of severe burns. Most fires today,
including the large, severe fires, usually burn for only a few
days. These fires generally burn under severe conditions that
exceed the capabilities of suppression forces. When burning
conditions moderate, the fires are quickly contained. The re-
sult is a more uniform spatial pattern within the burned area
and a more coarse grain to the landscape mosaic as a whole.
HISTORICAL RANGE OF
VARIABILITY
The historical range of variability (also called the reference
variability, the natural range of variability, etc.) has recently
been recognized as an important consideration in natural re-
sources management (Swanson et al. 1994; Manley et al. 1995).
Ecosystems are dynamic and constantly change in response
to various environmental factors (Sprugel 1991; Johnson et
al. 1994). Within the appropriate spatial and temporal con-
text, the historical range of variability can provide a refer-
ence for assessing the status and possible trends of ecosystems
(Laudenslayer and Skinner 1995).
The review of fire regimes given in this chapter, especially
regarding the accumulating evidence from fire-history stud-
ies, reveals that many Sierran fire regimes (and associated
vegetative characteristics) today may be outside their histori-
cal range of variability. The attendees at the paleoecology
workshop held by SNEP in October of 1994 arrived at the
same consensus: Sierran forest ecosystems, viewed at the scale
of the Sierra Nevada, are outside the historical range of vari-
ability as to fire frequency and severity and associated stand
structures and landscape mosaics.
The magnitude of the deviation from the historical range
depends upon the spatial and temporal scale one considers.
A small, localized area of less than a few hundred acres may
not be outside conditions that existed sometime in the past.
However, as we look at larger and larger areas, the condi-
tions today are less and less likely to have existed during the
last few hundreds of years. Large landscape patterns of rela-
tively homogeneous multilayered forest stands, generally
broken only by large changes in site conditions (rocky out-
crops, thin soils, etc.) were probably uncommon before the
twentieth century.
Many historical factors have contributed to the change in
fire regimes. Yet it should be noted that only one of these fac-
tors, the implementation of a fire-exclusion policy, has been
applied universally in the Sierran landscape. The effects of
Euro-American settlement on the native populations and cul-
tures were certainly pervasive. Nevertheless, we lack knowl-
edge of the spatial extent of the native cultural influence on
the fire regime (Anderson and Moratto 1996). However, we
do know that the application of the fire-exclusion policy has
been universal in the Sierra for much of this century (though
it has recently been modified in the national parks).
Table 38.1 shows that the time since the last fire is gener-
ally equal to or greater than the longest FRI recorded for most

1060
VOLUME II, CHAPTER 38
of the sites. These fire-history studies suggest that the fire-
exclusion strategy has been very successful in eliminating low-
severity fires and most moderate-severity fires (those
characteristic of the pre-1850s) from the Sierra. However, the
attempt to exclude all fires from the environment has been
only partially successful. Since it is not within current tech-
nological capabilities to suppress many fires burning under
extreme conditions, the current management strategy has
shifted the characteristic fire regime to one of infrequent, se-
vere, large fires. This shift means that severe fires, rather than
being rare events, have become the rule.
Both Leiberg (1902) and Sudworth (1900) comment on how
open the forests in their areas of examination were. They each
described the landscapes as characterized by large trees (ex-
cept around previously logged and mined areas) that were
widely spaced with sparse undergrowth. Of course, there were
exceptions in local areas, such as the South Fork of the Feather
River (Leiberg 1902) and others (Cermak 1988), but accord-
ing to most accounts these latter areas were the exception.
Both Sudworth (1900) and Leiberg (1902) also indicate that
the fires in their time generally stayed on the surface. Only in
unusual cases were extensive areas of larger trees killed, ex-
cept where there had been considerable logging slash. These
descriptions of landscapes and fire behavior are quite differ-
ent from today’s typical escaped fires and resulting landscape
patterns, where the patches with high proportions of tree
mortality are much larger (it is not unusual for a fire to kill
thousands of acres of trees) and are continuous over the land-
scape. What was described as typical fire behavior in the for-
ests (mostly low-severity, surface fire) is now atypical.
These human-induced changes in the characteristics of Si-
erran fire regimes have taken place during a climatic period
that appears to have been unusually warm and moist when
compared to previous centuries (Stine 1996; Woolfenden 1996;
Hughes and Brown 1992; Graumlich 1993). A warmer and
moister climate would likely have induced a variety of com-
plex responses in Sierran ecosystems. For example, there is
evidence that subalpine tree ecosystems have responded to
this climatic variation through expansion near the upper tree
line (Taylor 1995b). We think it is likely that these climatic
variations may have affected the fire regimes even in the ab-
sence of the modern human influences described earlier. How-
ever, at this time we can only speculate on the direction and
magnitude of change in the responses of the fire regimes to
the anomalous warm, moist period. Nevertheless, the Medi-
terranean climate of warm, dry summers and cool, wet win-
ters has remained a dominant feature.
We suggest that it is improbable that the overall effects of
the recent variation in climate on the FRIs and fire regimes
would have approached the direction and magnitude of the
changes brought about by the various modern human poli-
cies and activities. The warm, dry summers would likely have
continued to support fires in most years at all but the highest
elevations (the historical fire records certainly support this).
Fire frequencies would likely have varied somewhat from past
patterns, but we reason that fires would have remained fre-
quent due to the warm, dry summers. Swetnam (1993), in
describing long-term trends in fire frequency and associated
climate patterns, indicates that fire frequency appears to be
more strongly related to temperature than to moisture trends.
Based on Swetnam 1993, Stine (1996) reasons that fire fre-
quency, in the absence of modern fire-exclusion policies,
would feasibly have increased over the past century in re-
sponse to the increase in temperatures. Of course, the higher
elevations, where snowpacks would remain for extended
periods, and the less-exposed aspects would likely have ex-
perienced greater variation in FRIs due to increased mois-
ture.
The rapidity of the changes in fire regimes over the last
century appears to be remarkably unprecedented, especially
considering the current climatic regime and the vegetation
assemblages that can easily support frequent fire. Thus, if
current management strategies are continued indefinitely, it
is difficult to predict where this extraordinary, rapid change
in fire regimes will ultimately lead, especially with the po-
tential of future warmer and drier climate patterns. However,
if warm, dry years become more common, as many suggest
is likely, we would expect the recent paradigm of large, se-
vere fires to continue.
RESEARCH NEEDS
There is much we do not know about the ecological role of
fire in Sierra Nevada ecosystems. We have some idea of the
progression of change in Sierran ecosystems since Euro-
American settlement and the direction of change that would
be necessary to develop vegetation conditions to restore more
fire-resilient landscapes. However, we have only limited
knowledge of fire as a continuing, ecological process. Much
of our knowledge of how fire influences ecosystems is only
in regards to fire as a single event (usually following many
years of fire exclusion), not as a continuing, ongoing process.
The resolution, or at least the partial resolution, of a number
of poorly understood subjects would help greatly to formu-
late appropriate management goals and strategies and to
determine appropriate methods for fire and ecosystem man-
agement. The following are some of the research areas that
we believe are important.
Spatial-Temporal Dynamics
The spatial and temporal interactions of fire and Sierran eco-
systems have not been extensively researched. Two general
categories of research in this regard need to be addressed.
First, there is need for information regarding the effects of
frequent fires of low to moderate severity. This will be dis-
cussed in more detail later. Second, fire-history studies de-

1061
Fire Regimes, Past and Present
signed to describe patterns of fire and landscape dynamics
over time are needed in order to resolve many management-
oriented questions. These questions concern the interactions
of fire and spatiotemporal patterns of landscapes. The ability
to model these interactions (e.g., Miller 1994) will be extremely
important as managers attempt to project the potential effects
of various alternative management strategies.
There are two areas of need regarding spatiotemporal dy-
namics. First, fire-history information is lacking for the north-
ern Sierra Nevada. The southern Sierra Nevada (primarily
the national parks) is the source for most published fire-
history studies. There are sufficient differences in moisture
regimes and vegetation between the northern and southern
Sierra to suggest that fire-history studies for the northern Si-
erra would be very valuable for long-term management. It
should be emphasized that the fire-history record is being lost.
Much of the logging over the past few decades has removed
many of the old trees that contained the fire scar record. As
time progresses, less and less of this record will be recover-
able due to decomposition of the remaining material (e.g.,
stumps, logs, snags, etc.).
Second, as has been recounted throughout this chapter, very
little published research has been designed to describe the
influence of fire on spatial and temporal patterns of land-
scapes. The long-term spatiotemporal dynamics of landscapes
appear to be related to climate variations, but the relation-
ships are poorly understood. Until studies are undertaken
specifically to address the role of fire in landscape dynamics,
many questions will remain unresolved, for example, (1) how
to evaluate appropriate long-term fire-management strategies,
(2) how to project the effects of the spatiotemporal patterns
of fire on wildlife habitat (e.g., food and cover patterns), and
(3) how to model fire in landscapes as an ecosystem process.
Influence of Frequent Fire on Accumulations
of Coarse Woody Debris
Setting appropriate standards and guidelines for coarse
woody debris (CWD) in forests having fire regimes of fre-
quent, low-to-moderate-severity fires requires research to
specifically address the relationship between CWD and fire
frequency. Available information describing the accumula-
tions and function of CWD is generally not based on work
within forests of functioning frequent, low-to-moderate-
severity fire regimes. Much of the information is from eco-
systems where fire was much more infrequent than that
originally found in the Sierra Nevada (e.g., Maser and Trappe
1984; Harmon et al. 1986; Harmon et al. 1987). Associated in-
formation from the Sierra Nevada represents ecosystems af-
fected by years of fire suppression. Continual suppression of
the fires in many of these forests has probably increased CWD
accumulations above that in pre-suppression-era forests.
Characterization of Old-Growth Forests under
the Influence of Frequent Fires
Developing appropriate descriptions and guidelines for old-
growth forests characterized by frequent fires of low to mod-
erate severity will remain problematic until research designed
to address the relationship between old-growth characteris-
tics and fire is undertaken. The definition of old-growth for-
ests has become standardized based upon work done in
climates and forests of the Pacific Northwest that are quite
different from those found in the Sierra Nevada (e.g., Franklin
et al. 1981; Franklin and Spies 1991a, 1991b). In addition, defi-
nitions of old-growth forests in the Sierra Nevada were based
upon describing sites that met the Pacific Northwest defini-
tions and that had not been significantly disturbed by fire in
recent years (Fites et al. 1992). These descriptions, while rep-
resenting current conditions of old growth, are not necessar-
ily representative of stands dominated by large, old trees that
existed under a functioning presettlement fire regime. It is
likely that stand structure, species composition, and under-
story conditions were very different in many presettlement
old-growth stands from those in the old-growth stands found
today. The conditions found in many old-growth stands to-
day are at least in part the result of years of fire suppres-
sion and may not represent conditions characteristic of
presettlement old growth. Research that includes landscape-
level fire-history studies and large-scale prescribed fire pro-
grams is necessary to adequately develop Sierra Nevada
descriptions of sustainable old-growth forests.
Smoke as an Ecosystem Process
The role that smoke management ultimately plays in achiev-
ing air-quality objectives may be a determining factor in the
amount of prescribed fire eventually used for ecosystem
management (e.g., Sandberg 1987). An estimate of smoke pro-
duction from pre-suppression-era fire regimes based upon our
understanding of those fire regimes would help build a
baseline description of long-term patterns of air quality.
Fahnestock and Agee (1983) have done something similar in
regards to the Olympic Mountains of Washington. An assess-
ment of the background levels of smoke that were character-
istic of the pre-suppression-era fire regimes will provide policy
makers and managers with information that will help them
make more-informed choices concerning the long-term pro-
grammatic use of fire. Such information will help minimize
the imposition of unnecessary restrictions on the use of pre-
scribed fire (e.g., Cahill et al. 1996).
Fire Effects
Although much has been written about the effects of fire,
much of the existing information is based upon describing
the effects of unplanned wildfires. Few studies exist on the
effects of recurring, low-to-moderate-severity fires that would

1062
VOLUME II, CHAPTER 38
have been more characteristic of the pre-suppression-era fire
regimes. There are also few studies that display the effects of
fire suppression on potential ecosystem responses to fire (e.g.,
Swezy and Agee 1991).
Interactions among Disturbance Agents
The interactions among various disturbance agents are poorly
understood. Ferrell (1996) suggests that many of the factors
that contribute to hazardous fire conditions in forests also
increase the vulnerability of the forest to large-scale distur-
bance from insects and pathogens. Interdisciplinary studies
designed to describe the complex interactions among the
multiple agents of change (e.g., Gara et al. 1985; USFS 1994)
will be necessary to gain a more comprehensive understand-
ing of and to project the potential results of various manage-
ment strategies.
MANAGEMENT IMPLICATIONS
AND CONCLUSION
Fire was an important, regular ecological process in most veg-
etative communities of the Sierra Nevada for thousands of
years before the last century. Euro-American settlement and
management activities in Sierran ecosystems over the last 150
years or so have caused many changes in Sierran fire regimes
and in the vegetation associated with those regimes. These
changes include the significant reduction of fire occurrence,
accompanied by a general increase in the density of woody
vegetation and an accumulation of associated fuels over broad
landscapes. Consequently, although most fires are kept quite
small through fire-suppression activities, escaped fires often
become large, severely burned patches.
For most of this century, fire has been regarded as a nui-
sance, as a destructive agent, or occasionally as a tool. In spite
of the fact that a number of works on fire in natural resources
management have been available for decades (e.g., Weaver
1943; Sampson 1944; Shantz 1947; Biswell et al. 1952), the eco-
logical function of fire has been ignored, denied, or treated as
an interesting but inconsequential, academic curiosity by most
managers and policy makers (Mount 1969). Only recently, in
response to attempts to define and carry out more compre-
hensive ecosystem management, has the ecological role of fire
been generally acknowledged (e.g., Agee 1974; Williams 1993;
Manley et al. 1995).
It is often said that an important first step to resolving a
problem is to recognize or admit that the problem exists. Ap-
parently, society in general is beginning to recognize that the
failure to appreciate the role of fire in western North Ameri-
can ecosystems has contributed greatly to what has been char-
acterized as a general forest health problem (e.g., Knudson
1994; Sampson and Adams 1994; Phillips 1995).
It is unlikely that fire will ever be as unrestrained as it was
in past eras. Fire suppression will always play an important
role in managing the Sierra Nevada. There are too many cul-
tural values at risk to disallow it (Agee 1994). However, we
know that fires are inevitable given modern climate and veg-
etation. Developing forest structures and landscape patterns
that are comparable to those that developed under the more
frequent fire regimes of the past will plausibly help amelio-
rate the ecosystem disruptions caused by the severe fires that
are beyond fire-suppression capabilities. The chapters that
follow in this section address various ways of analyzing and
approaching strategies to deal with the ecological and cul-
tural problems associated with the current condition of Si-
erra Nevada ecosystems.
ACKNOWLEDGMENTS
We would like to thank the following individuals for gra-
ciously supplying unpublished data and/or reports: B. Bahro,
J. Fites, S. Gethen, R. Hawkins, R. Olson, S. Stephens,
T. Swetnam, and A. Taylor. We would also like to thank the
following people for valuable comments on an earlier ver-
sion of the manuscript: M. Barbour, D. Erman, J. Fites, G.
Greenwood, D. Leisz, K. McKelvey, C. Millar, D. Parsons, P.
Weatherspoon, and four anonymous reviewers.
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