Old Growth, Disturbance, Forest Succession, and Management in the Area of the Northwest Forest

150 Pages·2017·9.38 MB·English

Chapter 3: Old Growth, Disturbance, Forest
Succession, and Management in the Area of the
Northwest Forest Plan
Thomas A. Spies, Paul F. Hessburg, Carl N. Skinner, Klaus and mitigation) are addressed mainly in chapter 2 of this
J. Puettmann, Matthew J. Reilly, Raymond J. Davis, Jane A. report. Although, our effort is primarily based on pub -
Kertis, Jonathan W. Long, and David C. Shaw1 lished literature, we bring in other sources where peer-re -
viewed literature is lacking, and we conduct some limited
Introduction analyses using existing data. We are guided by the NWFP
In this chapter, we examine the scientifc basis of the monitoring questions, those from federal managers and
assumptions, management strategies, and goals of the our reading of the past three decades of science.
Northwest Forest Plan (NWFP, or Plan) relative to Old-growth forests can be viewed through many
the ecology of old-growth forests, forest successional ecological and social lenses (Kimmins 2003, Moore 2007,
dynamics, and disturbance processes. Our emphasis is Spies and Duncan 2009, Spies and Franklin 1996). Socially,
on “coarse-flter” approaches to conservation (i.e., those old growth has powerful spiritual values symbolizing
that are concerned with entire ecosystems, their species wild nature left to its own devices (Kimmins 2003, Moore
and habitats, and the processes that support them) (Hunter 2007), and many people value old growth for its own sake
1990, Noss 1990). The recently published 2012 planning (“intrinsic” values, sensu Moore 2007). Old growth also has
rule has increased emphasis on land management rooted many “instrumental” or useful functions, including habitat
in ecological integrity and ecosystem processes, using for native plants or animals (e.g., the northern spotted owl
coarse-flter approaches to conserve biological diversity [Strix occidentalis caurina]), carbon sequestration (Har-
(Schultz et al. 2013). Fine-flter approaches (e.g., species mon et al. 1990), and other ecosystem services. No single
centric), which are also included in the 2012 planning viewpoint fully captures the nature of the old-growth issue
rule, are discussed in other chapters. We synthesize new as it relates to federal forest management. We focus here
fndings, characterize scientifc disagreements, identify on ecological perspectives (Kimmins 2003, Oliver 2009,
emerging issues (e.g., early-successional habitat and Ruggiero et al. 1991, Spies 2004, Spies and Franklin 1996),
fre suppression effects) and discuss uncertainties and many of which are overlapping conceptually and in com-
research needs. We also discuss the relevance of our mon parlance. Old growth is many things at the same time;
fndings for management. Climate change effects on vege - for example, old growth is:
tation and disturbance and possible responses (adaptation • An ecosystem “distinguished by old trees and
related structural attributes. Old-growth encom-
passes the later stages of stand development that
1 Thomas A. Spies is a senior scientist, U.S. Department of
typically differ from earlier stages in a variety of
Agriculture, Forest Service, Pacifc Northwest Research Station,
3200 SW Jefferson Way, Corvallis, OR 97331; Paul F. Hessburg characteristics including tree size, accumulation
is a research landscape ecologist, U.S. Department of Agricul-
ture, Forest Service, Pacifc Northwest Research Station, 1133 of large dead woody material, number of canopy
N Western Ave., Wenatchee, WA 98801; Carl N. Skinner is a
layers, species composition and ecosystem function”
geographer (retired), U.S. Department of Agriculture, Forest Ser-
vice, Pacifc Southwest Research Station, 3644 Avtech Parkway, (USDA FS 1989).
Redding, CA 96002; Klaus J. Puettmann is a professor, Oregon
State University, Corvallis, OR 97331; Matthew J. Reilly is a • An ecological state resulting from interactions among
postdoctoral researcher, Humboldt State University, Department of
successional, disturbance, and ecosystem processes
Biological Sciences, 1 Harpst Street, Arcata, CA 95521; Raymond
J. Davis is a wildlife biologist and Jane A. Kertis is an ecologist, (e.g., nutrient and carbon cycles, microclimate).
U.S. Department of Agriculture, Forest Service, Pacifc Northwest
Region, 3200 SW Jefferson Way, Corvallis, OR 97331; Jonathan • A biological condition defned in terms of life histo-
W. Long is an ecologist, U.S. Department of Agriculture, Forest
ries and demographics of forest plant species.
Service, Pacifc Northwest Research Station, 1731 Research Park
Dr., Davis, CA 95618; David C. Shaw is a professor, Oregon State • A habitat for particular fauna, fora, and fungi.
University, Corvallis, OR 97331.
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GENERAL TECHNICAL REPORT PNW-GTR-966
We defne old-growth forests based on live and O’Hara et al. 1996, Spies 2006, and Davis et al. 2015 for
dead structure and tree species composition (see below). more discussion of old-growth or old-forest defnitions).
Old-growth forests in the NWFP area differ with age, According to Spies and Franklin (1988), old-growth is part
forest type, environment, and disturbance regime (Reilly of a structural and compositional continuum of successional
and Spies 2015, Spies and Franklin 1991). The variability stages that varies by environment. According to O’Hara et
and complexity of site conditions, forest succession, and al. (1996), speaking of frequently disturbed environments,
disturbance processes make defning old-growth diffcult old forest is a part of the successional continuum that varies
or impossible under a single defnition. Under the U.S. by environment and disturbance processes, which have the
Department of Agriculture (USDA), Forest Service ability to advance or retard succession.
(USDA FS 1989) defnition (above), the only features To operationalize the successional continuum con-
distinguishing old-growth from other forests, across all cept of old-forest development, Davis et al. (2015) created
forest types, are the dominance or codominance of old, an old-growth structure index (OGSI) to characterize the
large, live and dead trees (multiple canopy layers are not degree of old-growth structure (“old-growthiness” cali -
necessarily a defning characteristic). For example, in brated by potential vegetation type) that occurs in a stand
fre-frequent historical forest types, old-growth forests of any age or history, for use in mapping and monitoring
have large old live and dead trees, but amounts of dead - in the Plan area. Two defnitions for late successional/old
wood are low, canopies are generally open, and areas with growth were then created: OGSI 80 (structural conditions
multiple canopy layers are uncommon (Dunbar-Irwin and commonly found in forests that are 80 years and older)
Safford 2016, Safford and Stevens 2016, Youngblood et al. and OGSI 200 (structural conditions that are represen -
2004) (fg. 3-1). tative of forests containing trees that are more than 200
In the NWFP, “older forests” were defned as “late-suc- years of age). These classes roughly correspond to the
cessional/old-growth” based largely on stand developmental defnitions used by FEMAT, the Forest Ecosystem Man -
and successional patterns of Douglas-fr/western hemlock agement Assessment Team (FEMAT 1993), for mature
(Pseudotsuga menziesii/Tsuga heterophylla) forests (Frank- trees (80 to 200 years old) (e.g., “late- successional” in
lin et al. 2002) (fg. 3-2). This multilayered closed-canopy the NWFP) and old growth (>200 years) but have the
old growth (e.g., canopy cover >80 percent) was the focal advantage of being structure based and calibrated to dif -
point of old-growth conservation during the development of ferent potential vegetation types. Also, given that this is a
the NWFP, but as we shall argue, old growth is far more continuous index, other age/development thresholds (e.g.,
diverse than that and functions quite differently across the 120 years) could be used for mapping and monitoring.
range of the northern spotted owl. “Older forests” in the We note that the structure index and defnitions used
original NWFP includes mature forests, 80 to 200 years of in the monitoring program are based on current forest
age—a pre-old-growth stage, known somewhat confusingly conditions from forest inventory plots, which means that in
2
as “late-successional” in the Plan), and old-growth forests. fre-frequent dry zone forests, the structure and composition
Old-growth has been defned in the NWFP and elsewhere as of old growth is a product of 100 years or more of fre
forests containing large and old, live and dead trees, a exclusion and highly altered forest development processes.
variety of sizes of other trees, and vertical and horizontal Inventory defnitions for dry, old forests based on densities
heterogeneity in tree clumps, gaps, and canopy layering (see of large-diameter fre-tolerant trees have been developed for
the eastern Washington Cascade Range (Franklin et
al. 2007a). However, defnitions and indices of dry, fre-
2
Most of the time in this document, we use the term “late suc-
dependent, old-growth forest structure at stand and land-
cessional” to refer to vegetation that is in the later stages of forest
succession where age, height, and biomass are near maximum and
scape scales are still needed for the larger NWFP area (see
shade-tolerant species are the primary understory or overstory tree
species. This broad class would include old growth according to below for further discussion).
classic defnitions in textbooks (Barnes et al. 1998).
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Synthesis of Science to Inform Land Management Within the Northwest Forest Plan Area
Figure 3-1—Open, old-growth ponderosa pine stand maintained by low-severity fre in central Oregon.
Figure 3-2—Multilayered, old-growth Douglas-fr and western hemlock stand in the western Oregon Cascades.
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Tom Iraci Tom Iraci
GENERAL TECHNICAL REPORT PNW-GTR-966
Old growth has been the focal point for forest con- old growth, so the successional dynamics of the entire
servation and restoration on federal lands in the Pacifc landscape ought to be the broader focus of discussions.
Northwest. However, the broad goals of forest biodiversity Consequently, our discussion includes these other stages of
conservation would not be scientifcally viable if they forest succession, in addition to old growth.
focused on only one stage of a dynamic system—all
Guiding Questions
developmental phases and ecological processes must be
considered (Spies 2004), including postdisturbance stages This chapter characterizes the current scientifc understand-
(fg. 3-3), nonforest vegetation, and younger forests that ing of old-growth forest conditions and dynamics and other
constitute the dynamic vegetation mosaics that are driven successional stages in the NWFP area, especially as they
by disturbance and succession. These other stages and apply to conservation and restoration of forest ecosystems
types contribute to biodiversity, and hence, are as important and landscapes. We give special attention to composition
to any discussion of forest conservation or management and structure of trees (live and dead) as dominant compo-
for ecological integrity as is the discussion of old growth. nents of forests but acknowledge that other characteristics
Indeed, these other successional conditions become future are also important, including age (or time since disturbance)
and composition, and structure of shrub, herb, and grass
communities. Our focus is on the broad landscape, which
inherently is a mosaic of vegetation conditions; questions
related to conservation and restoration of animal species in
terrestrial habitats and riparian and aquatic ecosystems and
their habitats are dealt with in other chapters.
We address the following major questions in this
chapter, though not directly given their breadth, complex-
ity, and certain degree of overlap. See the conclusions
section for bullet statements that are explicitly linked to
these questions.
1. What are the structures, dynamics, and ecological
histories of mature and old-growth forests in the
NWFP area, and how do these features differ from
those of other successional stages (e.g., early and
mid successional)?
2. How do these characteristics differ by vegetation
type, environment, physiographic province, and
disturbance regime?
3. What is the scientifc understanding about using
historical ecology (e.g., historical disturbance
regimes and natural range of variation [NRV]) to
inform management, including restoration?
4. What are the principal threats to conserving
and restoring the diversity of old-growth types
and to other important successional stages (e.g.,
diverse early seral), and to processes leading to
Figure 3-3—Early-successional vegetation 8 years after a high- old growth?
severity fre in multilayered old growth in southwestern Oregon.
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Thomas Spies
Synthesis of Science to Inform Land Management Within the Northwest Forest Plan Area
5. What does the competing science say about needs (i.e., biological and physical environments) (Hessburg et
for management, including restoration, especially al. 2007, Kellogg et al. 2007, Wright and Agee 2004,) and
in dry forests, where fre was historically frequent? differences in potential vegetation types or forest compo-
6. How do the ecological effects of treatments to sition do not necessarily mean differences in fre history
restore old-growth composition and structure differ (Taylor and Skinner 1998).
by stand condition, forest age, forest type, distur- The major biophysical driving variables (aka “drivers”)
bance regime, physiographic province, and spatial of structure, composition, and dynamics of old-growth
scale? forests (and forests in general) are climate, topography,
7. What are the roles of successional diversity and soils, succession processes, and disturbance processes
dynamics, including early- and mid-seral vegeta- (Franklin and Dyrness 1973; Gavin et al. 2007; Hessburg et
tion, in forest conservation and restoration in the al. 2000a, 2015; O’Hara et al. 1996; Oliver and Larson 1990;
short and long term? Spies and Franklin 1996). In conjunction with landform and
8. What is the current scientifc understanding con- soil conditions, the geographic and historical variability of
cerning application of reserves in dynamic land- the regional climate set the stage for somewhat predictable
scapes? biotic communities, pathways of forest development, levels
9. How do recent trends of forests in the NWFP of ecosystem productivity, and spatial patterns of distur-
reserve network relate to both original NWFP bance regimes (Agee 1993, Gholz 1982, Hessburg et al.
goals, those of the 2012 planning rule, and climate 2000a, Reilly and Spies 2015, Weisberg and Swanson 2003,
change adaptation needs? Whitlock 1992). Climatic variation over time and space
10. What is the current understanding of postwildfre exerts a strong control over fre frequency (Agee 1993,
management options and their effects? Gavin et al. 2007, Walsh et al. 2015), and forest dynamics
11. What are the key uncertainties associated with is a product of the self-organizing interactions of climate,
vegetation under the NWFP, and how can they be topography, disturbance, and plant communities (Scholl
dealt with? and Taylor 2010). Forest succession is the process of change
in tree, shrub, and herb species composition, and structure
We address these questions using an organization based
(size, density, and age structure) over time. Disturbances
on major forest regions, disturbance regimes, and potential
can advance, arrest, or retard succession either slowly and
and existing forest vegetation types.
imperceptibly, rapidly and abruptly, steadily, or in other
complex and poorly understood ways (O’Hara et al. 1996,
Key Findings
Spies and Franklin 1996). In combination, forest succession
Vegetation Patterns and Classifcation
and disturbance processes can produce a wide range of
Drivers of regional variation in vegetation—
forest conditions within the NWFP area.
Forest ecosystems of the vast NWFP region are ecolog-
ically diverse and complex and do not lend themselves Classifcation of vegetation—
to simple generalizations (fg. 3-4). In this synthesis, we Ecological classifcations of environment and succession
account for some of that diversity by classifying ecosys- are used to promote understanding and implementation of
tems based on potential vegetation types at the zone or management objectives. One way that Oregon and Wash-
series level (Henderson et al. 1989, Lillybridge et al. 1995, ington ecologists account for environmental differences in
Simpson 2007) in a manner similar to Küchler (1964, succession and in old-growth characteristics (Davis et al.
1974). Potential vegetation types and disturbance regimes 2015, Reilly and Spies 2015) is to use potential vegetation
are somewhat correlated, although disturbance regimes type (fg. 3-4).
can differ signifcantly within potential vegetation types Potential vegetation type is named for the native,
late-successional (or “climax”) plant community that would
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GENERAL TECHNICAL REPORT PNW-GTR-966
Figure 3-4—Geographic distribution of potential vegetation zones (aka vegetation types) (Simpson 2013) and physiographic provinces
across the Northwest Forest Plan area.
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Synthesis of Science to Inform Land Management Within the Northwest Forest Plan Area
occur on a site in the absence of disturbances (i.e., wildfre, Moist and dry forests—
bark beetle outbreaks, root disease, weather events), and At a broad scale, forests of the NWFP area can be clas-
refects the biophysical environment (climate, topography, sifed into moist forests (including the western hemlock,
soils, productivity) and composition of overstory and Sitka spruce [Picea sitchensis], coastal redwoods, Pacifc
understory species (Pfster and Arno 1980). Stages along silver fr [Abies amabilis], and mountain hemlock [Tsuga
the continuum within a potential vegetation type may be mertensiana] potential vegetation zones west of the crest
binned or categorized into distinct successional stages, of the Cascade Range in Oregon and Washington), and dry
which are mileposts for visualizing forest development forests (mainly ponderosa pine [Pinus ponderosa], Doug-
subjectively given that no clear thresholds in development las-fr, grand fr [A. grandis], and white fr [A. concolor]
are known (Franklin et al. 2002, Hunter and White 1997, potential vegetation types) east of the Cascade Range and
O’Hara et al. 1996, Oliver and Larson 1990, Reilly and in southwestern Oregon and northern California (Franklin
Spies 2015, Spies and Franklin 1988). This classifcation is and Johnson 2012). We use this moist forest and dry forest
often required to enable large-landscape analyses, which classifcation to frame much of this chapter.
cannot effciently deal with developmental conditions
Disturbance Regimes
treated as continuous variables.
Not all ecologists and managers use potential vegeta- Fire regime classifcation—
tion to stratify or map vegetation for management or For most forest types, fre was and continues to be the major
research purposes. For example, managers in California do landscape disturbance agent that resets succession or shifts
not use potential vegetation but use existing or “actual” its course to a new pathway (Reilly and Spies 2016). Other
vegetation cover type instead to classify their forests for disturbance agents are important as well, including wind
management (CALVEG)3 (http://www.fs.usda.gov/detail/r5/ and biotic agents, but most disturbance regime classifca-
landmanagement/resourcemanagement/?cid=stelprdb tions and maps focus on fre. We characterize the ecology of
5347192.) To help make our discussion more useful to multiple disturbances for moist and dry forests in sections
managers in California, we provide a cross-walk table (app. below. In this section, we focus on approaches to classifying
1) that links the Pacifc Northwest Region (Region 6) historical fre regimes.
potential vegetation types (see chapter 2, fg. 3-1) to Pacifc Most of our current understanding of historical fre
Southwest Region (Region 5) existing vegetation classes. regimes is based on frequency—empirical studies of severity
We also note, where appropriate, what the CALVEG classes proportions and spatial patterns at landscape scales are
might be for a given potential vegetation type. Most of our relatively few (Hessburg et al. 2007, Reilly et al. 2017). Fire
discussions in the text use estimated potential vegetation disturbances occur along a continuum of frequency, severity
types for California and the rest of the Plan area based on a (e.g., tree mortality), seasonality, spatial heterogeneity, and
provisional map prepared by Michael Simpson (ecologist, event sizes. While there is no single classifcation of distur-
Deschutes National Forest) (fg. 3-4). bance regimes, they are often binned into regime types that
are based on fre frequency and severity (Agee 1993, 2003).
Average fre frequency interval classes of frequent (<25 years),
moderately infrequent (25 to 100 years), infrequent (100 to
3
One reason given for doing this is that in California vegetation,
300 years), and very infrequent (>300 years) (Agee 1993) are
historical fre frequencies were quite high and the time since fre
exclusion has been too short (e.g., 100 years) to really know what often used, but other frequency classifcations exist as well:
the capacity (potential future vegetation) would have been in the
absence of disturbance. For purposes of this document, we use e.g., ≤35, 35 to 200, and >200 years (Hann and Bunnell 2001,
potential vegetation types, because we have a classifcation and
Hann et al. 2004, Rollins 2009, Schmidt et al. 2002).
map of these that covers the entire NWFP area (e.g., Simpson
2013), and there is no existing vegetation classifcation and map A widely used classifcation of fre-severity regimes for
for Oregon and Washington. The lack of consistent vegetation data
layers between the two regions makes it challenging to apply the vegetation uses three bins of basal area or canopy mortality:
fndings from one Forest Service region to another.
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GENERAL TECHNICAL REPORT PNW-GTR-966
low (<20 percent), mixed or moderate (20 to 70 percent), developed for the stand or patch scale, but the metric has
4
and high (>70 percent) (Agee 1993, Hessburg et al. 2016, also been applied to larger regional areas (Agee 1993,
Perry et al. 2011) (fg. 3-5). Other classifcations have been Heinselman 1981, Reilly et al. 2017) or entire fre events,
used, often with higher thresholds for canopy cover loss or which can create confusion about the meaning of fre
mortality (e.g., 75 to 95 percent) (Miller et al. 2012, Reilly et severity (Hessburg et al. 2016): Is it a fne-grained mix of
al. 2017). The classifcation of Agee (1993) was initially severities, or coarse-grained mix of high and low severity,
or both? Severity can also be characterized in terms of
fre-induced changes to soils (i.e., soil burn severity);
4
Note that while individual patches can exceed 70 percent however, we focus on vegetative effects in this chapter. Soil
mortality, fres typically have such high levels of mortality in only
burn severity is used in Burned Area Emergency Response
a small fraction of their total area. For example, the high-severity
area of the 1988 Yellowstone fres was 56 percent (Turner et al. analyses and is often confused with burn severity to
1994), and the high-severity percentage of the 2002 Biscuit Fire
in the Klamath of Oregon and California was 14 percent with an vegetation (Safford et al. 2007).
additional 23 percent at moderate severity based on a sample of
inventory plots (Azuma et al. 2004).
Figure 3-5—Conceptual diagram characterizing the proportions of low-, moderate-, and high-severity fres in three major fre regime
classes. Inset panels represent idealized landscape dynamics associated with each regime based on proportions and size class distribu-
tions of patches at each of the three levels of severity. From Reilly et al. 2017, who modifed it slightly from Agee (1993, 1998).
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Synthesis of Science to Inform Land Management Within the Northwest Forest Plan Area
For management applications and regional planning, This classifcation and map are meant to be a rough
broad-scale regime classifcations are typically used (Haugo guide for understanding and visualizing ecological varia-
et al. 2015), but fre history studies indicate that fre regimes tion at regional scales and for framing a discussion about
can be relatively distinctive at topographic and landform forest conservation and restoration science in the NWFP
3
scales (10° to 10 ac) (e.g., Taylor and Skinner 1998, Tepley area (fgs. 3-4 and 3-5). They refect current understanding
3 6
et al. 2013). At landscape scales (ca. 10 to 10 ac), most of fre ecology and geographic variability in the region.
fres occur as a mix of low, moderate, and high severity, This typology is different from that used in the record
driven by variation in topography, land forms, microcli- of decision (USDA and USDI 1994) and FEMAT (1993)
mate, surface and canopy fuels, soils, and vegetation, as we documents, which divided the NWFP region into moist
explore in later sections. and dry physiographic provinces but did not characterize
Combining fre regimes into broad average frequency variability in regimes within them. The physiographic
and severity types is useful for regional planning (e.g., provinces explained much of the variation in the physical
Rollins 2009, USDA and USDI 1994), but it oversimplifes environment, but they contain considerable subregional
variability that exists at fner scales, which is important for variations in vegetation types and fre regimes that are
landscape planning and management. In general, simplify- important to understanding the ecology of the forests
ing fre into a few regime classes can obscure ecological in NWFP area. The potential vegetation types differ in
diversity associated with fre effects (Hutto et al. 2016). distributions of fre regimes that occur within them (fg.
Note that fre-severity proportions for any particular 3-7), and the distribution of potential vegetation types
landscape or landform is often more restricted than implied differs between fre regimes, though the differences are
by the broad ranges used to defne broad regime classes. For relatively small between regimes within the moist or dry
example, for some landscapes in the very high frequency, forests (fg. 3-8). Almost all fres in these regimes have
low-severity regime (see below), the historical range of mixed-severity effects, but they typically differ in the
high-severity fre may be in the low end of the 0 to 20 proportion and distribution of the high-severity effects. The
5
percent range used to defne this class. very frequent low-severity regime, for instance, contains
some area in high-severity fre patches at the scale of acres
A new fre regime classifcation—
to tens of acres. The recognition of a drier, more fre-fre-
For national and regional planning and management pur-
quent mixed-severity zone on the west side of the Cascade
poses, managers often use the LANDFIRE (Rollins 2009)
Range in Oregon (fg. 3-6) is based on a number of studies
fre regime classifcation. Our review of recent science in
(Agee and Edmunds 1992; Dunn 2015; Impara 1997; Reilly
the NWFP region suggests that the national-scale product
and Spies 2016; Tepley et al. 2013; Weisberg 2004, 2009).
oversimplifes the fre history within the NWFP area. Thus,
This regime, which typically burns with mixed severity
we developed a new classifcation and map (table 3-1, fg.
and includes medium to large patches of high-severity fre,
3-6) by synthesizing existing data on climate, lightning, and
was frst identifed by Agee (1993), based in part on the fre
potential vegetation types (see app. 2 for methods) and fre
history work of Morrison and Swanson (1990) from the
history studies (app. 3).
western Cascades in Oregon.
Our classifcation also recognizes that the California
5
Odion et al. (2014) called for restricting defnitions of historical portion of the NWFP area cannot be simply divided into a
low- and mixed-severity fres to regimes where crown fres and
moist (Coastal province) and dry (Klamath and Cascades
active or passive torching are generally absent. However, this
classifcation would not be useful, as crown fres can occur in all
provinces) province for understanding succession and
fre regimes including low-severity regimes (Agee 1993), partic-
ularly when the regimes are intermixed, as they often are, where disturbance regimes. In fact, that area has relatively little
large landscape contain a range of topography, environmental, or
of the “moist” forest that is characterized by historically
vegetation conditions.
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GENERAL TECHNICAL REPORT PNW-GTR-966
Table 3-1—Characteristics of major historical fre regimes used in this report and in fgure 3-6
NWFP
forest
zone Regime and landfre group PVTs and cover types Spatial characteristics
Moist Infrequent (>200-year return PVT: wetter/colder parts of western Area dominated by large to very large
3 6
intervals), stand replacing; hemlock, Pacifc silver fr, patches (10 to 10 ) of high-severity fre;
LANDFIRE group V mountain hemlock low- and moderate-severity fre also
Cover types: Douglas-fr, western occurs. Small- to medium-size patches
hemlock, Pacifc silver fr, noble were most frequent.
fr, mountain hemlock
Moderately frequent to PVTs: drier/warmer parts of Mixed severity in space and time, typically
3 4
somewhat infrequent (50- to western hemlock, Pacifc silver fr including large (10 to 10 ac) patches of
200-year return intervals), and others high-severity fre and areas of low- and
mixed severity; LANDFIRE Cover types: Douglas-fir, western moderate-severity fre. Small patches of
regime group III hemlock, Pacific silver fir, high-severity would be common within
noble fir lower severity areas.
Dry Frequent (15- to 50-year PVTs: Douglas-fr, grand fr, white Mixed-severity fre with medium to large
2 3
return intervals) mixed fr, tanoak (10 - to 10 -ac ) patches of high-severity
severity; LANDFIRE Cover types: Douglas-fr, white fr, fre.
regime group I and III red/noble fr, western white pine
Very frequent (5- to 25- PVTs: ponderosa pine, dry to moist Dominated by low-severity fre with fne-
2
year return intervals) low grand fr, white fr grained pattern (<10° to 10 ac) of high-
severity; LANDFIRE Cover types: ponderosa pine, severity fre effects; large patches of high-
regime group I Douglas-fr, mixed pine, oak severity fre rare in forests except in earlier
seral stages (e.g., shrub felds).
NWFP = Northwest Forest Plan, PVT = potential vegetation type/zone used in the Pacifc Northwest Region. Cover type = current vegetation
classifcation used in the Pacifc Southwest Region. LANDFIRE regime groups follow Rollins (2009).
6
infrequent, high-severity fres. Rather, forests in the 3-9). The relatively high percentage of area burned in the
California Coastal province were dominated by frequent, infrequent fre regime may refect elevated ignitions from
mixed-severity regimes, while the eastern Klamath and Euro-American settlement activities, because lightning
California Cascades were dominated by historical regimes densities in these areas are low (fg. 3-10) and these forests
of very frequent, low-severity fre. are not typically fuel limited (Agee 1993). American
Historical maps of high-severity burned forest patches Indian burning practices would have also been a historical
from Washington and Oregon (data not available from component in some parts of the region, but the importance
California) (Plummer et al. 1902, Thompson and Johnson would have varied considerably among regimes (see
1900) provide an independent source of primary data to chapter 11). For example, several studies (app. 3) have
evaluate the regional regime map. These maps support the
hypothesis that the largest patches and percentage of
forest burned by high-severity fre occurred in the 6 th
These early 20 century maps are our best snapshots of this time
period but do not necessarily represent the range of variability
infrequent high-severity regime; whereas the smallest
in fre sizes that would occur in these regimes over time. This is
patches and lowest area of forest burned by high-severity especially true for the infrequent, high-severity regime where
sample of historical fres is small and extremely large patches of
fre occurred in the very frequent/low-severity regime (fg.
fre may have occurred in past centuries.
104