2004
Historical Variability of
Natural Disturbances in
British Columbia
F O R R E X S E R I E S 12
A Literature Review
vii
Historical Variability of
Natural Disturbances
in British Columbia
A Literature Review
Carmen Wong, Holger Sandmann, and
Brigitte Dorner
i
Library and Archives Canada Cataloguing in Publication
Wong, Carmen, 1973Historical variability of natural disturbances in British Columbia [electronic
resource] : a literature review / Carmen Wong, Holger Sandmann, Brigitte Dorner.
(FORREX series ; 12)
ISBN 1-894822-13-7
1. Forest health—British Columbia—History. 2. Ecological disturbances— British
Columbia—History. 3. Forest ecology—British Columbia. 4. Biotic communities—
British Columbia. I. Sandmann, Holger II. Dorner, Brigitte, 1967- III. FORREX IV. Title.
V. Series: FORREX series (Online) ; 12.
SD146.B7W65 2004
634.9’6’09711
C2004-902098-6
© 2004 FORREX–Forest Research Extension Partnership
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ii
ABSTRACT
This document aims to provide a comprehensive review of the scientific literature about the historical
dynamics of natural disturbances—such as wildfires, windthrow, or insect and disease outbreaks—
in forests in British Columbia. Our discussion of the literature focuses on characterizing disturbance
frequency and patch size. We summarize information about important disturbance agents by biogeoclimatic
zone, and by subzone and variant where applicable. Where possible, we point out the assumptions or
limitations behind such information.
We reviewed material from 257 sources, including peer-reviewed journal articles, theses, and published
and unpublished reports. We focused on research conducted in British Columbia, but occasionally we
cite research about similar ecosystems in the United States and Alberta because British Columbia-based
research remains limited for several biogeoclimatic zones.
Our synthesis shows that natural disturbance regimes in British Columbia are more complex than the
broad natural disturbance types typically used in the province to set objectives for maintaining biodiversity
(e.g., allocating old-growth reserves or maintaining certain patch-size distributions). These natural
disturbance types were based primarily on the frequency of stand-replacing disturbances. Historically,
disturbance severity was also key in creating certain disturbance patterns on the landscape. Many
biogeoclimatic subzones were influenced by a mixed-severity regime characterized by various combinations
of disturbances of different severity, or by gap-causing disturbances occurring during the long interval
between stand-replacing disturbances. Our review provides a starting point for people interested in:
1) designing strategic and operational plans based on natural disturbance patterns in a specific area,
2) revising and refining the natural disturbance type classification, and 3) identifying areas for future
research.
KEYWORDS: forest management, natural resource management, natural disturbances, disturbance
frequency, patch size, fire, wildfire, windthrow, insects, disease, British Columbia, Canada.
Introducing text-to-table hyperlinking
To enhance the usefulness of this FORREX Series in its on-line format, we’ve created a system of internal
hyperlinks. These links let you navigate between in-text references and the sections of Table 2 in which
those authors’ findings are summarized. Clicking on the authors’ names in Table 2 will bring you back to
the section of text in which the study is first mentioned. We welcome readers’ feedback on the value of
this feature.
Citation —
Wong, C., H. Sandmann, and B. Dorner. 2003. Historical variability of natural disturbances in
British Columbia: A literature review. FORREX–Forest Research Extension Partnership, Kamloops, B.C.
FORREX Series 12. URL: www.forrex.org/publications/forrexseries/fs12.pdf
iii
ACKNOWLEDGEMENTS
We extend sincere thanks to the many people who shared information and directed us to published and
unpublished information. We incorporated useful comments from Andy MacKinnon, Tory Stevens, and
Don Gayton on earlier versions of this paper. Our work was funded by Forest Renewal BC, the British
Columbia Ministry of Sustainable Resource Management, and the British Columbia Ministry of Forests.
Publishing of this document was partially funded by the Province of British Columbia through Forestry
Innovation Investment Ltd. and the Forest Investment Account, Forest Science Program.
iv
CONTENTS
Abstract
.............................................................................................................................................
Acknowledgements
Introduction
iii
.............................................................................................................................
iv
......................................................................................................................................
1
Sources and Presentation of Information
............................................................................................
How Measures of Fire Frequency Depend on Methods
.........................................................................
Literature Review: Disturbance Attributes by Biogeoclimatic Zone
3
..................................................
3
..................................................................................................................
3
......................................................................................................................
5
Alpine Tundra Zone (AT)
Bunchgrass Zone (BG)
1
Boreal White and Black Spruce Zone (BWBS)
Coastal Douglas-fir Zone (CDF)
.......................................................................................
5
.........................................................................................................
7
Coastal Western Hemlock Zone (CWH)
..............................................................................................
Engelmann Spruce–Subalpine Fir Zone (ESSF)
8
....................................................................................
10
..................................................................................................
13
Interior Douglas-fir Zone (IDF)
.........................................................................................................
14
Mountain Hemlock Zone (MH)
.........................................................................................................
16
...............................................................................................................
17
.................................................................................................................
19
Interior Cedar–Hemlock Zone (ICH)
Montane Spruce Zone (MS)
Ponderosa Pine Zone (PP)
Sub-Boreal Pine–Spruce Zone (SBPS)
Sub-Boreal Spruce Zone (SBS)
.................................................................................................
20
...........................................................................................................
21
Spruce–Willow–Birch Zone (SWB)
Assessment of Knowledge Gaps
.....................................................................................................
23
.........................................................................................................
24
Closing the Knowledge Gaps: Recommendations
Literature Cited
................................................................................
24
..................................................................................................................................
49
FIGURES
1
Distribution of 38 studies of disturbance intervals across British Columbia’s biogeoclimatic zones
2
Distribution of 41 studies in British Columbia that have determined return intervals for different
levels of disturbance severity and the size of disturbance patches and remnants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
............
25
TABLES
1 Definitions and uses of various measures of disturbance frequency
.........................................................
3
2 Database of research on disturbance frequency and patch characteristics of various disturbance agents in
biogeoclimatic zones/subzones in British Columbia
2A Alpine Tundra and Bunchgrass Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2B Boreal White and Black Spruce and Coastal Douglas-fir Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2C Coastal Western Hemlock Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
v
2D Engelmann Spruce–Subalpine Fir Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2E Interior Cedar–Hemlock Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2F Interior Douglas-fir Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2G Mountain Hemlock Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2H Montane Spruce Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2I Ponderosa Pine Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2J Sub-Boreal Pine–Spruce Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2K Sub-Boreal Spruce Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2L Spruce–Willow–Birch Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Abstract
Acknowledgements
Introduction
Sources and Presentation of Information
TABLE 1 DEFINITIONS AND USES OF VARIOUS MEASURES OF DISTURBANCE FREQUENCY
Literature Review: Disturbance Attributes by Biogeoclimatic Zone
Alpine Tundra Zone (AT)
How Measures of Fire Frequency Depend on Methods
Bunchgrass Zone (BG)
Boreal White and Black Spruce Zone (BWBS)
Coastal Douglas-fir Zone (CDF)
Coastal Western Hemlock Zone (CWH)
Engelmann Spruce–Subalpine Fir Zone (ESSF)
Interior Cedar–Hemlock Zone (ICH)
Interior Douglas-fir Zone (IDF)
Mountain Hemlock Zone (MH)
Montane Spruce Zone (MS)
Ponderosa Pine Zone (PP)
Sub-Boreal Pine–Spruce Zone (SBPS)
Sub-Boreal Spruce Zone (SBS)
Spruce–Willow–Birch Zone (SWB)
Assessment of Knowledge Gaps
Closing the Knowledge Gaps: Recommendations
Table 2 Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones in British Columbia
Table 2A Alpine Tundra and Bunchgrass Zones
Table 2B Boreal White and Black Spruce and Coastal Douglas-fir Zones
Table 2C Coastal Western Hemlock Zone
Table 2D Engelmann Spruce–Subalpine Fir Zone
Table 2E Interior Cedar–Hemlock Zone
Table 2F Interior Douglas-fir Zone
Table 2G Mountain Hemlock Zone
Table 2H Montane Spruce Zone
Table 2I Ponderosa Pine Zone
Table 2J Sub-Boreal Pine–Spruce Zone
Table 2K Sub-Boreal Spruce Zone
Table 2L Spruce–Willow–Birch Zone
Literature Cited
32
35
38
41
42
43
44
46
48
INTRODUCTION
Natural disturbances—such as wildfires, windthrow, and insect and disease outbreaks—are important
drivers of forest structure at various temporal and spatial scales (Turner et al. 1993; Lertzman and Fall
1998; Swetnam and Betancourt 1998). Thus, knowledge about the historic variability of natural disturbances is an important prerequisite for conducting effective ecosystem-based forest management,
ecological restoration, and conservation activities (Swanson et al. 1993; Cissel et al. 1999). Although the
Forest Practices Code of British Columbia’s Biodiversity Guidebook (B.C. Ministry of Forests and B.C.
Ministry of Environment 1995), and the subsequent ministerial Order Establishing Provincial NonSpatial Old Growth Objectives,1 briefly summarize natural disturbance dynamics in British Columbia’s
forested ecosystems, no comprehensive review has been published. The authors of the Biodiversity
Guidebook used expert opinion to broadly classify biogeoclimatic subzones into five natural disturbance
types; however, the guidebook does not list any of its sources. This document aims to provide a comprehensive review of the scientific literature about the historical dynamics of natural disturbances in British
Columbia.2 Our discussion of the literature focuses on characterizing disturbance frequency and patch
size, and is organized by biogeoclimatic zone. Managers and planners will find our review to be a helpful
reference when developing forest-management plans that include emulations of natural disturbances.
Researchers will find it helpful too, particularly for designing research projects that will target knowledge
gaps. This review can also provide a starting point for incorporating the concept of range of natural
variability into natural resource management guidelines. However, the information presented here is
insufficient, on its own, to replace any existing guidelines.
Sources and Presentation of Information
We reviewed material from many different sources, including peer-reviewed journal articles, theses,
published reports, and unpublished reports. We focused on research conducted in British Columbia, but
occasionally we cite research about similar ecosystems in the United States and Alberta because British
Columbia research remains limited for several biogeoclimatic zones.
We summarize information about important disturbance agents by biogeoclimatic zone, and by
subzone and variant where applicable. Where possible, we point out the assumptions or limitations
behind such information. In Table 2, we present a database of available estimates of disturbance frequency and patch size by biogeoclimatic subzone/variant, although we realize for some disturbance
agents, such as geomorphic disturbances, a physiographic or topographic unit would be more appropriate. The last section of this paper lists recommendations for addressing knowledge gaps that became
obvious during the review process.
It is important for readers to understand that measures of disturbance frequency are usually specific
to a certain location, sample size, size of study area, time period, and method (see Table 1 and inset box,
p. 3). Therefore, we caution readers against using the summarized information without referring directly
to the original studies, particularly in the case of the disturbance intervals reported in Table 2.
1
2
Proposed Order Establishing Non-Spatial Old Growth Objectives: For Public Review and Comment, November 2003; Resource
Management Division, B.C. Ministry of Sustainable Resource Management, Victoria, B.C.; http://srmwww.gov.bc.ca/rmd/oldgrowth/
nonspatial-old-growth.htm
This paper is an updated and modified version of a literature review prepared for the Research Branch of the British Columbia Ministry of
Forests and the British Columbia Ministry of Sustainable Resource Management (Wong et al. 2003).
1
TABLE 1 Definitions and uses of various measures of disturbance frequencya
Measure
Definition
Typical use
Link to other measures
Disturbance cycle b
(Van Wagner 1978)
Time to disturb an area
equivalent to the studied
landscape. (In this time,
disturbances may occur
in some areas more than
once and in others not
at all.)
To estimate the frequency
of high-severity
disturbances from age
structure across the
landscape.
Disturbance cycle equals
the point mean fire
interval if fires start at
random locations and
burn equally across all
areas of a landscape.
Disturbance cycle equals
inverse of annual
disturbance rate.
Independent of the size
of the study area.
Also known as natural
fire rotation (Heinselman
1973).
Mean return interval
(point: e.g., Heyerdahl
1997)
(area: Grissino-Mayer
1995)
Average expected time
between disturbances at
a given point in the
landscape (point
interval). This term is
also commonly used to
describe the expected
average time between
disturbances occurring
anywhere in the study
area (area interval).
To estimate the frequency
of stand-maintaining and
mixed-severity
disturbance regimes (e.g.,
in studies based on
charcoal deposition in
lake sediments, fire scars,
or growth reductions in
tree rings).
Area interval depends on
the size of the study area
and is therefore difficult
to transfer to areas of a
different size.
For example, a point
mean fire interval of
100 years corresponds to
an average of 100 years
between fires at any point
on the landscape and to
an annual disturbance
rate of 1%.
Annual disturbance rate Proportion of the
(e.g., Johnson and Gutsell landscape disturbed
1994; Fall 1998)
annually.
To estimate the frequency Inverse of disturbance
of high-severity
cycle.
disturbances from age
structure across the
landscape.
Turnover time
(e.g., Lertzman et al.
1996; Ott and Juday
2002)
To estimate the average
time between gap
disturbances in a stand.
a
b
2
Disturbance cycle equals
the point mean fire
interval if fires start at
random locations and
burn equally across all
areas of a landscape.
Time to fill in disturbed
areas with mature or old
forest.
See Fall 1998 for more details.
In the literature, the disturbance cycle is commonly synonymous with the fire cycle.
Same as disturbance cycle
if disturbances occur at
random.
How Measures of Fire Frequency Depend on Methods
The methods used to obtain fire frequency estimates affect not only the reliability of these
estimates, they may also affect how the estimates should be interpreted.
For example, if the mean fire interval for a study area is determined by pooling fire dates
(obtained from fire scars) across the study area, the resulting measure of fire frequency describes
the average interval between two fires burning anywhere (not necessarily at the same location)
within the study area. Because an increase in the size of area sampled typically leads to the
discovery of previously unrecorded fires, the mean fire interval obtained by this method depends on the size of the study area (Baker and Ehle 2001).
Alternatively, fire intervals may be computed separately for each plot or point and then
averaged (or otherwise aggregated) across the entire study area. In this case, the resulting
(point) mean fire interval describes the average time between two consecutive fires burning at
the same location. Because an increase in sampled area changes only the point mean fire interval
if the fire regime is substantially different in the new area, this measure of fire frequency is less
dependent on the size of the study area.
Also, because evidence of past fires is more difficult to detect for periods further back in time,
the temporal record should be censored to a “period of reliability” for which the sample size is
large enough to ensure confidence in fire dates (e.g., Grissino-Mayer 1995). Patterns in tree
rings should also be matched or crossdated with those in an established chronology to account
for missing or false tree rings (Stokes and Smiley 1968). Furthermore, when estimating point
mean fire intervals it is desirable to sample multiple trees per plot because not every fire passing
through a plot leaves a scar on each tree (Fall 1998).
Fire cycles or disturbance rates determined from time-since-fire maps (for which the standing age distribution is often used as a proxy) provide a measure of frequency for stand-replacing
disturbances that is also independent of study area size and can be interpreted like point mean
fire intervals (Wong et al. 2003). Studies based on time-since-fire maps are also limited in
temporal depth to the lifespan of the post-fire cohorts, and they are sensitive to erasure of
evidence by harvesting, to temporal and spatial variability in disturbance rates, and to assumptions made about the relative susceptibility of particular types of stands or age groups (e.g.,
Finney 1995; DeLong 1998; Armstrong 1999; Wong et al. 2003).
LITERATURE REVIEW: DISTURBANCE ATTRIBUTES BY BIOGEOCLIMATIC ZONE
Alpine Tundra Zone (AT)
The Alpine Tundra biogeoclimatic zone occupies high mountain areas throughout British Columbia.
In terms of elevation, it is the highest of all the zones and occurs above the three subalpine zones
(Mountain Hemlock, Engelmann Spruce–Subalpine fir, and Spruce–Willow–Birch). The Alpine Tundra
has the harshest climate of all the biogeoclimatic zones, with much wind and snow and very short frostfree periods. Mean annual precipitation is estimated to range from 700 to 3000 mm, most of which
(70–80%) is snow (Pojar and Stewart 1991a).
3
The interactions of wind, snow, and topography result in a mosaic of grasslands, dwarf-shrublands
(alpine heaths), and patches of bare soil or rock. While the Alpine Tundra generally occurs above the
treeline, with the treeline usually forming the boundary, widely scattered individual trees or patches of
trees can be found in sheltered sites at lower elevations within the Alpine Tundra. Typically, these conifers grow in stunted or krummholz form, with their height largely determined by the depth of the
snowpack (Douglas and Bliss 1977).
Most disturbance agents in the Alpine Tundra are directly associated with the harsh climate and local
relief. Abiotic disturbances include wind and frost damage, snow creep, avalanches, and fire (Brink
1964). Main biotic agents of disturbance to trees are defoliators, such as western hemlock looper
(Lambdina fiscellaria lugubrosa) and two-year-cycle spruce budworm (Choristoneura biennis), white pine
blister rust (Cronartium ribicola), as well as vertebrates (through herbivory, rubbing, and trampling of
grass, shrubs, and small trees). The Biodiversity Guidebook designates Alpine Tundra as Natural Disturbance Type 5 (NDT 5), a type that also includes all parkland subzones of the Engelmann Spruce–Subalpine
Fir and Mountain Hemlock zones (B.C. Ministry of Forests and B.C. Ministry of Environment 1995).
Little is known about vegetation dynamics of Alpine Tundra plant communities in British Columbia
(Hamilton 1983, cited in Pojar and Stewart 1991), particularly regarding the spatial and temporal
attributes of natural disturbances. Douglas and Bliss (1977) briefly summarize research that was conducted in British Columbia and Washington State up until the mid 1970s. Most of the disturbance
agents active in the subalpine biogeoclimatic zones likely affect the lower elevations of the Alpine
Tundra to some degree as well. For example, Parfett et al.’s (1995) study of western hemlock looper
indicates that, between 1911 and 1994, a proportion of defoliation (14.1% of the total defoliated area)
occurred in the Alpine Tundra. In the Mackenzie and Fort St. James Forest Districts, between 1985 and
1997, 4% of the total area defoliated by two-year-cycle spruce budworm occurred in the Alpine Tundra
(Shand et al. 1999).
The frequency of fires in subalpine forests ranges from 100 to 300+ years in the interior of the province (Fryer and Johnson 1988; Agee 1993) and from several hundred years to millennia on the coast
(Lertzman et al. 2002). It is unclear, however, whether estimates of the frequency of fires in the subalpine
zones apply to the Alpine Tundra as well.
Whitebark pine (Pinus albicaulis) is a high-elevation tree species of particular concern because it is
declining over much of its range in North America. Several studies of the subalpine zones have linked its
threatened survival to the combined effects of white pine blister rust, pine beetle epidemics, fire suppression, and climate change.3
A fairly extensive body of published studies has examined plant dynamics in subalpine and alpine
regions of the Cascade and Rocky Mountains in the United States (e.g., Woodward et al. 1995; Rochefort
and Peterson 1996). However, only a few papers directly address natural disturbances, such as frost
heaving and snow creeping (Edwards 1980; Evans and Fonda 1990), desiccation-induced loss of foliage
(Cairns 2001), and the effects of wildfire (Kuramoto and Bliss 1970; Douglas and Ballard 1971; Bollinger
1973; Agee and Smith 1984; Little et al. 1994). With the exception of Cairns, who observed an average of
9% of subalpine fir (Abies lasiocarpa) krummholz canopy mortality during one winter, none of the
studies provide quantitative estimates of disturbance dynamics.
3
4
See also the discussion of ESSF (page 10), as well as Kendall and Arno (1990), Antos (1998), Campbell and Antos (2000), and Zeglen (2002).
Bunchgrass Zone (BG)
The Bunchgrass biogeoclimatic zone contains the grasslands in the southern interior of British
Columbia, from valley bottoms up to 700–1000 m elevation (Nicholson et al. 1991). It is the driest zone
as the mean annual precipitation ranges from only 200 mm to 335 mm (Pojar and Meidinger 1991).
Summers are hot and winters are moderately cold. Frequent, low-severity fires are the predominant
disturbance regime although periods of drought are also influential. At one site in the Fraser variant of
the very dry hot subzone (BGxh3) in the Churn Creek Protected Area, the frequency of low-severity fires
averaged 19 years, as recorded on fire-scarred trees (Blackwell et al. 2001). Fires could be even more
frequent than this estimate because many fires were likely not intense enough to scar the few ponderosa
pine (Pinus ponderosa) or Douglas-fir (Pseudotsuga menziesii) trees.
The coupling of low-severity fires with a dry and warm climate in maintaining grasslands is suggested
by patterns in the paleoecological records of Artemisia spp. (sagebrush and pasture sage genus) pollen
and by observations of recent tree encroachment into grasslands. Paleoecological records of Artemisia
spp. pollen in lake sediment indicate that the Bunchgrass zone was at its greatest extent in British
Columbia in the early Holocene (10 000 BP to 8 000 BP), a period considered to be dry and warm with a
high fire frequency (Hebda 1995). More recently, tree encroachment into grasslands in the Cariboo and
Nelson Forest Regions’ Interior Douglas-fir zone—as determined by comparing historic airphotos with
1990s photos and examining patterns of tree establishment—coincides with modern fire-exclusion and
grazing practices (Strang and Parminter 1980; Gayton 1997; Ross 1997, 2000) but has also been influenced by slope and aspect (Bai et al. in press). In the upper Skagit Valley in northwestern Washington
State, relatively recent encroachment into a meadow containing a disjunct ponderosa pine ecosystem
was linked to the virtually discontinued use of fire by First Nations peoples since the turn of the century,
as well as to shallow spring snow packs since the 1970s (Lepofsky et al. 2000). In the Bunchgrass and
Ponderosa Pine subzones in the southern portion of the Okanagan valley and in the Lower Similkameen
valley, invasion and ingrowth of conifers into grasslands or open forest increased significantly in
unburned sites between airphoto pairs of 1938 versus 1985 and 1985 versus 1996 (Turner and Krannitz
2001). Sites that did burn decreased in conifer density, although harvesting was not mapped.
Boreal White and Black Spruce Zone (BWBS)
The Boreal White and Black Spruce biogeoclimatic zone is found at low elevations in the northeastern
part of British Columbia and in major valleys west of the northern Rocky Mountains (DeLong et al.
1991). Winters are long and cold with mean temperatures below 0°C for 5 to 7 months and an average
annual precipitation of 330–570 mm. The ground is frozen for a large portion of the year, and the
growing season is short.
Fires drive disturbance dynamics in boreal forests but they can be highly variable in their frequency,
size, and severity, depending on how they respond to landscape patterns and climate (Cumming 2000).
Black spruce (Picea mariana) dominates on wet sites and lodgepole pine (Pinus contorta) dominates on
dry sites. Both forest types conform to a relatively simple model of stand development after the occurrence of stand-replacing fires (Andison and Kimmins 1999). However, on mesic sites, successional
pathways are much less predictable, therefore the timing, severity, and size of disturbances are particularly important for determining stand development in these cases (Parminter 1983a; Parminter 1983b;
Andison and Kimmins 1999).
In a study of stand establishment after fire, regeneration by early seral species such as trembling aspen
(Populus tremuloides), paper birch (Betula papyrifera), and lodgepole pine occurred over a period of one
to five decades (Hawkes 1982). White spruce (Picea glauca), black spruce, and subalpine fir (Abies
lasiocarpa) tend to dominate the stands later in succession (Parminter 1983a, 1983b). However, recent
5
studies provide evidence that not all parts of the boreal forest are driven by frequent stand-replacing
fires and that gap dynamics, even in aspen stands, can maintain relatively old forests on the boreal
landscape (e.g., boreal mixedwood in Alberta [Cumming et al. 2000] and southern boreal forest in
Minnesota [Frelich and Reich 1998]).
In the Prince George Forest Region, fire cycles of stand-replacing events in the Stikine variant of the
dry cool subzone (BWBSdk1) are estimated to range from 263 to 455 years (mean 333 years) (DeLong
1998). Fire sizes span a wide range: between 1910 and 1930, 23% of the disturbed area consisted of
patches greater than 1000 ha and 19% of patches less than 100 ha (DeLong 1998).
In the MacKenzie Timber Supply Area, fire regimes in watersheds containing BWBSdk were associated
with elevation, aspect, and valley orientation (Rogeau 2001). Airphoto interpretation (photo dates
1942–1952) showed spatial subdivisions of several different fire regimes associated with broad terrain units
and varying levels of vegetation complexity; these subdivisions are considered to be indicative of the
varying burn intensity and degree of overlap in wildfires (Rogeau 2001). Rogeau (2001) estimated historic fire
cycles by using the landscape disturbance model STANDOR. Of the fires observable on the 1950s airphotos,
approximately 60% were less than 1000 ha, but these were responsible for only 9% of the area burned.
Many studies of fire regimes in the Canadian boreal forest have been conducted, but few have been
done in British Columbia. Whether fire cycles determined for boreal forests outside of British Columbia
are similar to those in the Boreal White and Black Spruce zone is unknown, particularly given the
evidence that burn rates vary with the stand’s leading species (Cumming 2001). In addition, comparisons between different studies—even for the same area—are often difficult because of differences in
study methods and data analysis.
Mean fire intervals ranged from 133 to 234 years in the boreal forest in Kluane National Park, Yukon
(Hawkes 1982), as averaged for each climatic region (25–30 plots per region). These intervals were
determined from fire scar information and stand origin dates. Average fire size, mapped from aerial
photographs for the approximate period of 1850 to 1940, ranged from 140 to 1600 ha.
In the 48 000-ha Shakwak Trench close to Kluane National Park, fire cycles ranged from 300 to 350
years for different classes of slope, elevation, and aspect (Francis 1996). Old forest (> 300 years old)
occurred on steep north and east aspects, while frequently burned areas were associated with south and
west aspects. Reconstruction of individual fires showed that the total area burned since 1800 amounts to
78% of the forested area. Some areas burned up to five or six times, and others did not burn at all.
Cumming (1997, cited in Cumming 2000) determined a comparatively long fire cycle of 244 years
(mean annual burn rate of 0.41%) for the boreal mixedwood in northeastern Alberta. Based on a 54year record of mapped fires, mean annual burn rates varied between stand types: 0.27% for aspen
leading, 0.67% for white spruce leading, 0.60% for black spruce leading, and 0.31% for mixed deciduous
and coniferous stands (Cumming 2001). Other studies have estimated burn rates to be more frequent
for aspen stands (Larsen 1997). However, evidence of uneven-aged aspen stands and gap dynamics in
boreal mixedwood stands supports the hypothesis of a longer fire cycle (Cumming et al. 2000).
For boreal forests with a substantial deciduous component, the paleoecological record of charcoal in
lake sediment in northern Alberta suggests that the mean fire interval was ~70 years with no obvious
changes in fire frequency during the past 840 years (Larsen and MacDonald 1998). However, a decline in
fire frequency during the Medieval Warm Period (AD 750–1250) and a coinciding increase in aspen
pollen was found in lake sediment in Elk Island National Park, Alberta (Campbell and Campbell 2000).
Differences of an order of magnitude in fire-cycle estimates between Cumming (1997, cited in
Cumming et al. 2000) and some of the other studies in the boreal mixedwood (Van Wagner 1978; Larsen
and MacDonald 1998; Weir et al. 2000) could be due to inappropriate analysis methods, too short a time
period used by Cumming, or inherent differences in study locations. However, the longer estimates determined
6
by Cumming are similar to those determined for the Foothills Model Forest in Alberta (Andison 2000), the
Yukon (Hawkes 1982), and northeastern British Columbia (DeLong 1998; Rogeau 2001).
Between 1970 and 1983, fires that burned in boreal forests of northern Alberta ranged in size from 21
to 17 770 ha (n = 69 fires) (Eberhardt and Woodard 1987). The median size of remnant patches (or
islands) within these fires ranged from 2.3 to 9.4 ha, and increased with fire size. In contrast, preliminary
data from the Foothills Model Forest show that the sizes of remnant patches do not increase with fire
size; instead, sizes of remnant patches are highly variable among fires and they correlate with fire intensity (Cumming 2000). Both studies found that the density of islands increased with fire size.
Several studies have modelled the effects of various climate change scenarios on fires in boreal ecosystems (e.g., Li et al. 2000). In a scenario of doubled carbon dioxide concentrations in the atmosphere,
lightning ignitions are predicted to increase, and fire season is predicted to lengthen by 22% on average
in Canada (Flannigan et al. 2001). However, significant regional variability and uncertainty exist in
model predictions regarding the effect of climate change on the Canadian Forest Fire Weather Index.
The index is predicted to increase over much of the boreal forest, whereas in eastern Canada, the northern interior of British Columbia, and the Yukon it is predicted to decrease (Flannigan et al. 2001).
Besides moderate-to-high-severity fires, other influential disturbance agents in the Boreal White and
Black Spruce zone include wind and ice storms, spruce and western balsam bark beetle (Dryocoetes
confusus), insect defoliators, tomentosus root rot, and pine stem rusts. In British Columbia, eastern spruce
budworm (Choristoneura fuminifera) is associated primarily with the boreal forest. Dendrochronological
evidence in the Liard River area suggests defoliation has occurred every 14 to 28 years since 1869 (Shore
and Alfaro 1986). A more extensive study in the same area revealed a similar average cycle of outbreaks
(26 years), but it also showed a strong south-to-north gradient of increasing susceptibility to spruce
budworm outbreaks in Boreal White and Black Spruce subzones in this region (Burleigh et al. 2002).
Depending on the degree of defoliation and length of outbreak, this defoliator can cause average mortality
of 1.5 to 6% per year of white spruce (Alfaro et al. 2001). Climate change is also projected to influence
population dynamics of the eastern spruce budworm and forest tent caterpillar (Malacosoma disstria)
(Percy et al. 2002).4
Coastal Douglas-fir Zone (CDF)
The Coastal Douglas-fir biogeoclimatic zone is limited to the areas on the southern Mainland, on a few
islands in the Georgia Strait, and on southeastern Vancouver Island that are below 150 m elevation. The
zone lies in the rain shadow of Vancouver Island and the Olympic Mountains. Mean monthly temperatures are around 10°C and rarely fall below 0°C (Nuszdorfer et al. 1991). Mean annual precipitation
ranges from 647 to 1263 mm; summers are dry and winters are mild and wet. Most of the forest was
harvested at the turn of the century (Nuszdorfer et al. 1991). The Biodiversity Guidebook describes
natural disturbances in the Coastal Douglas-fir zone as NDT 2, with stand-replacing disturbances occurring at mean intervals of 200 years. Low-severity fires did occur, but they are thought to have been
ignited primarily by First Nations peoples.5 Garry oak (Quercus garryana) meadows in the Coastal
Douglas-fir were likely maintained by similar low-severity fires. Fire sizes are thought to have been
relatively small, ranging from 1 to 500 ha.6 Disturbance dynamics likely resemble those in the adjacent
eastern variant of the very dry maritime subzone of the Coastal Western Hemlock zone (CWHxm1).
4
5
6
The influence could be positive or negative, depending on geographic location.
J. Parminter, Research Ecologist, Forest Dynamics, Research Branch, B. C. Ministry of Forests, Victoria, B.C.; personal communication,
March 2002.
J. Parminter, 1992; Typical historical patterns of wildfire disturbance by biogeoclimatic zone: table adapted from old growth forests;
Research Branch, B.C. Ministry of Forests, Victoria, B.C.; unpublished table provided by Parminter to Wong in 2002.
7
Agee and Dunwiddie (1984) examined forest development and fire history on a small island (4.5 ha) in
the Puget Sound of Washington State. Based on regeneration pulses of Douglas-fir (Pseudotsuga menziesii),
bark char, and eyewitness accounts, they inferred a patchy, low-severity to moderate-severity fire regime of
approximately an 80-year rotation. Many of these fires appear to have been ignited by humans.
Coastal Western Hemlock Zone (CWH)
The Coastal Western Hemlock biogeoclimatic zone occurs at low to mid elevations on Vancouver
Island, on the Queen Charlotte Islands, and in the Coast Mountains up to southeastern Alaska (Pojar
et al. 1991a). The Coastal Western Hemlock is the wettest zone in British Columbia—mean annual
precipitation ranges from 1000 to 4400 mm—with cool summers and mild winters. The moist climate
and frequent cyclonic storms strongly influence forest dynamics on the coast. Small canopy gaps of
10 trees or less, caused primarily by wind, in association with pathogens such as rot and hemlock
dwarf mistletoe (Arceuthobium tsugense), are the primary mode of disturbance over the majority of
the zone (Alaback 1991; Hennon 1995; Lertzman et al. 1996; Dorner and Wong 2002). Geomorphic
disturbances can play an important role in susceptible parts of the Coastal Western Hemlock and are
the most frequent stand-replacing events, although wind, flooding, and fire occasionally create larger
canopy openings (Banner et al. 1983; Septer and Schwab 1995; Pearson 2000; Pearson 2003). Other
disturbance agents, such as ungulates, porcupines, weevils, and insect defoliators play a minor role
(Dorner and Wong 2002), but in managed stands these agents can cause significant change to forest
structure and composition (e.g., Sullivan et al. 1986).
Processes that create, expand, and fill canopy gaps vary across coastal temperate rainforests due to
differences in topography and vegetation patterns (Ott and Juday 2002). In the Tofino Creek area on the
west coast of Vancouver Island, an average of 16% of the sampled mature and old-growth forest in the
submontane and montane variants of the very wet maritime subzone (CWHvm1 and CWHvm2) was in
canopy gaps and 14% was in edaphic gaps such as those caused by rocky outcrops (Lertzman et al.
1996). In the Lower Mainland and on Vancouver Island, at three old-growth sites in the Coastal Western
Hemlock zone, an average of 39% of the forested area was in canopy gaps (Arsenault 1995). While
canopy gaps have not been characterized along the central and north coast of British Columbia, canopy
gaps in southeast Alaskan forests similar to those in the cooler variants of the Coastal Western Hemlock
zone occupied on average 9% of the forested area (Ott and Juday 2002). The influence of a canopy gap
extends beyond the actual physical opening: 27% (Ott and Juday 2002) to 38% (Arsenault 1995) to 56%
(Lertzman et al. 1996) of forested area is in so-called “extended gaps,” in which the forest is to some
degree influenced by adjacent gaps.
In the absence of large-scale disturbances, turnover rates of gaps in CWHvm1 and CWHvm2
forests—that is, the average time span between successive gap disturbances in a stand—range from
313 to 1379 years on western Vancouver Island, depending on assumptions made about how long
trees take to grow into a gap (Lertzman et al. 1996). Estimates in southeast Alaska are quite similar,
ranging from 230 to 920 years (Ott and Juday 2002).
Records of stand-replacing wind events that caused partial to complete blowdown of up to several
hundred hectares on Vancouver Island, on the north coast of British Columbia, and in southeastern
Alaska (Keenan 1993; Mitchell 1998; Nowacki and Kramer 1998; Kramer et al. 2001) indicate these
events occur but tend to be episodic. Historic airphotos and field data from Vancouver Island and the
central coast suggest that severe blowdown is fairly rare in those areas (Lertzman et al. 1996; Pearson
2000; Pearson 2003). On the central coast (in the very wet hypermaritime subzone, CWHvh), Pearson
(2003) estimated, through airphoto interpretation and GIS analyses, that stand-replacing wind disturbed
0.001% of the forested area annually. A re-analysis of Mitchell’s (1998) blowdown data from the
8
North Coast Forest District indicates approximately 0.03% of the operable area was disturbed annually
by wind between 1960 and 1996 (Dorner and Wong 2002). If calculated over the entire North Coast
Forest District, this corresponds to a return interval of approximately 3000 years. If blowdown occurred
only in wind-exposed portions of the landscape, return intervals would be shorter, for example 300 years
if only 10% of the landscape is considered susceptible. Although Mitchell (1998) found no evidence that
blowdown was associated with particular areas or exposures, studies from southeast Alaska suggest that
stand-replacing wind disturbance is concentrated in topographically exposed areas and is frequent
enough in exposed forests (intervals of 50–300 years) to prevent all-aged stands from developing
(Nowacki and Kramer 1998; Kramer et al. 2001).
In the Coastal Western Hemlock zone, the most frequent stand-replacing events are geomorphic
disturbances, such as rock slides, debris flows, and avalanches (Banner et al. 1989; Pearson 2003). These
disturbances are confined to specific locations and small areas. In the Clayoquot Valley on western
Vancouver Island, landslides (mostly rock slides) and avalanches each occupied 2.4% of the total area
(Pearson 2000). On the central coast in the CWHvm, geomorphic disturbances disturbed 0.02% of the
area annually but they disturbed much less in the CWHvh (Pearson 2003). Debris flow frequency varies
between unharvested watersheds, from 0.0037 slides/km2 per year in Clayoquot Valley (Jakob 2000) to
0.065 slides/km2 per year on the Queen Charlotte Islands (Schwab 1998).7 The frequency of debris flows
in harvested terrain was approximately nine times higher than the natural rate in Clayoquot Valley
(Jakob 2000) and 15 times higher on the Queen Charlotte Islands (Schwab 1998).
While fires do occur in forests of the Coastal Western Hemlock zone, they are typically rare in the
moist and wetter subzones. According to trends in charcoal accumulation in the sediment of five lakes,
the frequency of fire appears to have varied across Vancouver Island during the mid to late Holocene;
fire-free intervals of ~3000 years were observed in moister (western) sites, whereas charcoal was continuously deposited over time in lakes on the drier (eastern) side of the island (Brown and Hebda 1998).
Many existing stands in the Coastal Western Hemlock zone on Vancouver Island and on the southern
coast are thought to have originated after moderate- to high-severity fires. This hypothesis is supported
by widespread evidence of cohorts of old Douglas-fir, which is a long-lived seral species that establishes
almost exclusively following fire. The date of establishment of old Douglas-fir cohorts on northern
Vancouver Island and in Lower Mainland watersheds suggests fires occurred somewhere in the region
nearly every century for the last 1000 years (Parminter 1990; Green et al. 1999). However, Douglas-fir
can also regenerate in gaps of old stands—the simple presence of Douglas-fir in a stand may not indicate
a fire event (Franklin et al. 2002).
In the CWHvm1 in the Capilano Watershed on the Lower Mainland, the establishment dates of
Douglas-fir cohorts in 17 sites suggest that the time since the last fire ranges from 33 to 1200 years
(Green et al. 1999). It is possible that unsampled late-successional western hemlock (Tsuga heterophylla)
and western redcedar (Thuja plicata) stands also originated from fire (Daniels et al. 1995), but that early
seral Douglas-fir cohorts have disappeared; if so, it would increase the range of time-since-fire. In seven
sites, multiple cohorts were found, which suggests that partial-replacement fires occurred at return
intervals of 200 to 450 years (mean = 345 years). The area of the watershed influenced by these fire
episodes included fragments of forest ranging from 10 to 1048 ha. Other evidence of partial-replacement
fires has been found in Douglas-fir and coastal western hemlock forests in Oregon (Impara 1997).
To reconstruct the fire history in the CWHvm1 in Clayoquot Valley, one study (Gavin 2000; Gavin et
al. 2003) determined point estimates of time-since-fire from tree ages, soil charcoal, and return intervals
of peaks of charcoal in lake sediments. Time-since-fire in 83 sites along a 200-m grid ranged from 64 to
7
Whether or not these are all natural slides is not known.
9
12 220 years BP.8 Fire extent rarely exceeded 250 m in diameter in this study, although sampling may
have been too concentrated at lower elevations to detect fires that may have spread upslope. Area return
intervals of fires occurring within 250 m of Clayoquot Lake increased from 50 years (AD 200–900) to
350 years (AD 1100–present) (Gavin 2000; Gavin et al. 2003). The shift from frequent to infrequent fires
coincides with the onset of the Little Ice Age.
Fire-return intervals in the CWHvm1 can vary greatly with topography: median time-since-fire was
six times greater on terraces (median ~4500 years) than on hillslopes (median ~750 years) in Clayoquot
Valley (Gavin 2000; Gavin et al. 2003). Approximately 20% of the sampling sites were on landforms with
low susceptibility to fire (northerly aspects and low terraces) and had not burned in over 6000 years. All
the sites on highly susceptible landforms, such as south aspects, had burned within the last 800 years.
The occurrence of isolated patches of Douglas-fir on specific topographic sites (Green et al. 1999;
Gavin 2000) and the requirement of a local seed source for Douglas-fir regeneration, suggests that
portions of the Coastal Western Hemlock zone must burn at intervals less than the 700–800-year
lifespan of Douglas-fir in order to maintain the species on the landscape. The association between timesince-fire and topography suggests that certain sites are repeatedly burned by fire, while other sites have
an extremely low fire frequency that permits late-successional forest to develop.
Engelmann Spruce–Subalpine Fir Zone (ESSF)
The Engelmann Spruce–Subalpine Fir zone biogeoclimatic zone occupies the highest forested elevations
in most of British Columbia’s mountain ranges. It lies below the Alpine Tundra zone, typically at elevations of between 1200 m and 2300 m (Coupé et al. 1991). Given their large geographic range,
Engelmann Spruce–Subalpine Fir ecosystems experience a variety of climatic conditions. Generally, the
climate is cold and moist with long snowy winters, but annual precipitation ranges from 400 mm to
2000 mm depending on the site (Coupé et al. 1991). Forests in the Engelmann Spruce–Subalpine Fir
zone span the gradient from continuous cover to subalpine parkland, and 15 of its 16 subzones are
subdivided into forested and parkland types. This zone’s 50 variants attest to its ecological diversity
(B.C. Ministry of Forests 2001).
Wildfires in the Engelmann Spruce–Subalpine Fir zone are considered to occur relatively infrequently.
They tend to be stand replacing due to the high susceptibility of some tree species to fire-caused mortality
(Fischer and Bradley 1987). However, fire-initiated stands are not necessarily even-aged because postdisturbance recruitment can take many decades (Jull 1990; Parish et al. 1999). Two other important abiotic
disturbance agents in the Engelmann Spruce–Subalpine Fir zone are windthrow and avalanches (Veblen et
al. 1991; Ferguson and Pope 2001; Sagar and Jull 2001). Of the many biotic agents that weaken or kill trees
in this zone, bark beetles, two-year-cycle budworm, western hemlock looper, decay fungi, and root rot are
considered to be the most important (Lindgren and Lewis 1997). In the southeastern subzones of the
Engelmann Spruce–Subalpine Fir zone, whitebark pine historically formed an important overstorey
component but today is threatened due to a combination of fire exclusion, blister rust, and mountain pine
beetle (Dendroctonus ponderosae) (Campbell and Antos 2000; Stoffels 2000; Zeglen 2002).
For several Engelmann Spruce–Subalpine Fir subzones in central and northern British Columbia,
attributes of stand-replacing disturbances have been estimated by using forest inventory data as timesince-disturbance maps (Hawkes et al. 1997; Steventon 1997; DeLong 1998; Steventon 2001). Mean
disturbance return intervals determined in those four studies range from 219 to 794 years. General
conclusions from these studies are that:
8
10
See also discussion in Lertzman et al. 2002.
1. disturbance rates are correlated with climatic conditions, with wetter and more snow-prone
subzones exhibiting longer return intervals (e.g., Steventon 1997);
2. the majority of landscapes in the Engelmann Spruce–Subalpine Fir zone in north-central
British Columbia are dominated by older forests;
3. disturbance rates have changed over time; and
4. patch size distributions vary considerably among study areas (e.g., Hawkes et al. 1997 vs. DeLong
1998), although all examined subzones were strongly influenced by large disturbance events
(> 1000 ha).
For the southern Engelmann Spruce–Subalpine Fir subzones, fire history studies have been undertaken in several national parks in the Rocky Mountains (Masters 1990; Tymstra 1991; Rogeau 1994; Van
Wagner 1995; Weir et al. 1995; Rogeau 1996, 2000). Again, time-since-fire distributions were derived
from stand origin maps, although in these cases maps were developed specifically for these studies and
were supported by extensive fieldwork. Fires appear to have been much more common in the drier
southern Rocky Mountains (e.g., in the dry cool subzone, ESSFdk) than in the central and northern
portions of the Engelmann Spruce–Subalpine Fir zone, with fire cycles ranging from 45 to 266 years
(Van Wagner 1995; Rogeau 2000). To some extent, these relatively short return intervals are likely
artifacts of pooling data from drier valley-bottom ecosystem types with wetter subalpine forests. In the
Engelmann Spruce–Subalpine Fir zone, frequent, mixed-severity fire regimes may characterize stands on
south aspects adjacent to the Interior Douglas-fir zone (Gray et al. 2002). In the moist warm subzone
(ESSFmw), preliminary data from fire-scarred whitebark pine near Pemberton indicate a fire interval of
only 21 years during the period 1780–1930.9 The author suggests that First Nations peoples probably
ignited many of the fires.
Regarding the question of whether fire intervals in subalpine forests have increased and what the
driving factors might have been, the debate appears unresolved (e.g., Johnson and Larsen 1991 vs.
Weir et al. 1995). This highlights the strong influence of methodological assumptions as well as the
uncertainties associated with spatial and temporal extrapolation from a limited set of data. Dorner
(2001) used stand-age maps of the Arrow Innovative Forest Practices Agreement (IFPA) area to estimate pre-European settlement disturbance return intervals. For the Columbia variant of the wet cool
subzone (ESSFwc1), intervals range from 90 to 807 years, and for the Selkirk variant (ESSFwc4) they
range from 105 to 508 years, depending on the assumptions made in the analysis. Dorner’s results also
showed that disturbance rates were not stable during the past, and that the early 20th century was a
period of relatively high disturbance rates, followed by a decline during the second half.
Stand-replacing fires are often heterogeneous with respect to burn severity and spatial coverage. As part
of a larger study, Stuart-Smith and Hendry (1998) mapped residual trees and patches in seven fires in the
ESSFdk and ESSFmw subzones of the Nelson Forest Region. They found that residual patches varied greatly
in size (1–100 ha) and that patch size did not correlate with fire size. Residual patches tended to occur near
streams and were more likely to contain specific tree species, particularly Douglas-fir or western larch
(Larix occidentalis). Using similar methods, Brochez et al. (2001) mapped seven wildfires and their residual
patches in the Lakes and Morice Timber Supply Areas in west-central British Columbia. Burns with ESSF
components ranged from 53 to greater than 7618 ha, and residual patches ranged from ~1 to 245 ha.
A number of recently published field studies have characterized the structure and development of
unmanaged Engelmann Spruce–Subalpine Fir stands. Most of the examined stands show evidence of
initiation by fire (Antos and Parish 2002a) and a subsequent transition to fine-scale, patchy mortality
and regeneration caused by senescence or less severe disturbances such as budworm, bark beetles, or
windthrow (Varga and Klinka 2001; Antos and Parish 2002a, 2002b; Parish and Antos 2002). Maximum
9
R.W. Gray, Fire Ecologist, R.W. Gray Consulting Ltd., Chilliwack, British Columbia; personal communication, February 2002.
11
stand ages (i.e., fire-free periods) ranged from ~300 to greater than 460 years. However, despite similarities in stand structure and species composition, the age class distributions of the study sites show
large differences, which suggests a variety of site-specific developmental pathways may exist (Parish
2001). It is important to recognize that most studies of stand structure in the Engelmann Spruce–
Subalpine Fir zone in British Columbia have focused on southern sites dominated by subalpine fir, and
together cover only 6 of the 16 subzones.
In parkland variants, tree regeneration after fire is likely to be largely unpredictable and only partially
correlated with climate or with time since disturbance (Agee and Smith 1984). Instead, facilitation—
positive interactions of neighbouring plants—might be of particular importance for successional
dynamics in areas bordering the alpine (Callaway 1998).
Very little is known about the frequency and severity of windthrow disturbances in Engelmann
Spruce–Subalpine Fir forests. Huggard et al. (1999, 2001) compared harvesting units in the Northern
Monashee variant of the wet cold subzone (ESSFwc2) near Sicamous Creek against uncut controls. Their
data suggest that wind and snow damage are continuing causes of tree death, and damage is highly
variable in annual rates and type (e.g., snapped vs. uprooted stems). During the 5-year period of analysis, annual loss of basal area averaged 3.2% in subalpine fir and less than 0.5% in Engelmann spruce
(Picea engelmannii) (Huggard et al. 2001). The authors conclude that the greater windfirmness of spruce
helps to maintain the species in Engelmann Spruce–Subalpine Fir forests. The extent to which periodic
severe wind storms are stand-replacing is unknown (Sagar and Jull 2001).
Studies of insect disturbance in the Engelmann Spruce–Subalpine Fir zone have focused on two-yearcycle spruce budworm (Shand et al. 1999; Zhang et al. 2001; Parish and Antos 2002; Zhang and Alfaro
2002) and western hemlock looper (Parfett et al. 1995). Dendrochronological data from a single stand10
showed several growth losses, 30 to 45 years apart, induced by two-year-cycle spruce budworm (Parish
and Antos 2002). Using ring-width analyses, Zhang et al. (2001) estimated a 24–39 year frequency in the
Fort Saint James and Mackenzie Forest Districts during the past century. Their data also showed that
outbreaks were synchronized across north-central British Columbia, probably due to a combination of
stand susceptibility and weather conditions (Zhang and Alfaro 2002). A GIS-based analysis of defoliated
stands in the same region showed that biogeoclimatic subzone was a good indicator of susceptibility to
defoliation and that stands in the moist cold subzone (ESSFmc) were more prone to defoliation than stands in
the Omineca or Graham variants of the moist very cold subzone (ESSFmv3 or ESSFmv4) (Shand et al.
1999). Over the province as a whole, outbreaks of western hemlock looper appear to be cyclical, and occur on
average every 9 years (Alfaro et al. 1999; Parfett et al. 1995). Between 1911 and 1994, the very wet cold subzone (ESSFvc) and the Cariboo variant of the wet cool subzone (ESSFwk1) have been particularly affected
(Parfett et al. 1995). Defoliator-induced growth reduction of affected trees can be significant and sustained; mortality is a function of the degree of defoliation (Alfaro et al. 1999).
A preliminary study of pathogens in the ESSFwc2 at Sicamous Creek found that root rot and decay
fungi are important endemic pathogens that influence the ratio of fir and spruce overstorey trees
(Merler 1996).11 In addition, Merler (1996) observed that small rodents had damaged the root systems
of 89% of the subalpine fir sampled, but they had damaged almost none of the spruce root systems.
We are not aware of any study that has attempted to quantify interactions between disturbance agents
active in the Engelmann Spruce–Subalpine Fir zone. However, Lindgren and Lewis (1997) provide a
conceptual model of disturbance ecology for the Engelmann Spruce–Subalpine Fir and Sub-Boreal
Spruce zones in the McGregor Model Forest.
10
11
12
The stand may have been in either the Okanagan variant of the dry cold subzone (ESSFdc1), or in the very dry cold subzone (ESSFxc); the
study results did not specify.
See also Lindgren and Lewis 1997.
Interior Cedar–Hemlock Zone (ICH)
The Interior Cedar–Hemlock biogeoclimatic zone is found below the Engelmann Spruce–Subalpine Fir
zone at low to mid elevations (400–500 m) in southeastern British Columbia and just east of the Coast
Mountains in west-central British Columbia (Ketcheson et al. 1991). The zone extends south into
eastern Washington, northern Idaho, and western Montana. The Interior Cedar–Hemlock zone has cool,
wet winters (mean annual precipitation is 500–1200 mm) and warm, dry summers (temperature averages below 0°C for 2–5 months).
Disturbance regimes in the Interior Cedar–Hemlock zone are complex. Important disturbance agents
are fires, root rot, bark beetles, and defoliators (Wong 2001). Pollack et al. (1997) estimated that return
intervals of stand-replacing disturbances range from 101 years in the dry warm subzone (ICHdw), to
129 years in the Thompson variant of the moist cool subzone (ICHmw2), to 226 years in the Wells Gray
variant of the wet cool subzone (ICHwk1). In the Arrow Forest District, disturbance rates were highly
variable in the past, and were lower during the pre-European settlement period than in the early part of
the 20th century (Dorner et al. 2003). Depending on assumptions about pre-harvesting ages of stands
and predominant disturbance characteristics, return intervals of disturbances during the pre-European
settlement period were estimated to be 65–449 years for the ICHdw and 97–458 years for the ICHmw2.
A preliminary analysis of charcoal deposits in lake sediment in the ICHmw2 and ESSFwc4 in the east
Kootenays indicates that area fire-return intervals ranged from 200 to 800 years.12 These intervals are
longer than fire cycles estimated for the ICHwk1 and Engelmann Spruce–Subalpine Fir zone in Glacier
National Park—average fire cycles for the park area are reported as 80 years between 1519 and 1760, and
110 years after 1760 (Johnson et al. 1990). In the wet cool subzone (ICHwk) around Mount Revelstoke
National Park, the average fire cycle was estimated to be 181 years (Rogeau 2000).13
Evidence from the age structure of three stands in the ICHdw suggests a mixed-severity disturbance
regime; frequent, low-severity fires maintained ponderosa pine on dry sites, and higher-severity fires
with longer return intervals permitted grand fir (Abies grandis) to establish on moister sites (Quesnel
and Pinnell 2000). Mean return intervals of low-severity fires in two ICHdw stands ranged from 11 to
24 years between 1762 and the 1890s. It is not known how extensive these stand types are across the
ICHdw landscape, although such dynamics are found in northern Idaho, western Montana, and northeastern Washington (Schellhaas et al. 2000).14 Preliminary data about the age structure of stands in the
Interior Cedar–Hemlock subzones in Wells Gray Provincial Park, in the Upper Adams Valley, and in the
Nelson Forest Region may support the notion that this complexity in disturbance regimes exists in other
subzones of the Interior Cedar–Hemlock (Arsenault 1997; Holt et al. 1999).
In the Interior Cedar–Hemlock and Engelmann Spruce–Subalpine Fir zones in the Arrow Timber
Supply Area, disturbance severity varies both within and among the areas incurring recent mountain
pine beetle outbreaks and fires (McIntire 2001). Fires resulted in two to three times higher median
mortality than mountain pine beetle (n = 14 outbreaks and 4 fires) and the proportion of basal area
killed by both agents increased along gradients from the edge of the disturbance perimeter. The size
distribution of snags appears to vary substantially, although very few large snags remain after mountain
pine beetle outbreaks. Stuart-Smith and Hendry (1998) and Arsenault (1997) also found residual islands
within surveyed fire perimeters in the Interior Cedar–Hemlock zone.
12
13
14
D. Gavin; postdoctoral scientist, Department of Plant Biology, University of Illinois, Urbana, Illinois; preliminary data, provided by e-mail
January 2002.
See caution in Table 2E, page 38.
See review in Pollack et al. 1997.
13
Western hemlock looper is also influential in the ICH. Between 1911 and 1994, large areas of four
Interior Cedar–Hemlock variants (ICHmw3, wk1, vk1, and vk2) were defoliated by this insect (Parfett et al.
1995). An outbreak in the Prince George Region that began in 1992 led to high levels of tree mortality—
ranging from 51% to 96% of all trees greater than 7.5 cm DBH—four years later (Alfaro et al. 1999). A
study of 13 stands that experienced at least 40% mortality from a western hemlock looper outbreak in the
1990s in Robson Valley found that the looper was not size or species specific; rather, it preferred trees with
small live crown ratios (Hoggett 2000). Following looper outbreaks, a shift in canopy dominance from
western hemlock to western redcedar is likely (Hoggett 2000). The probability of outbreaks occurring in
the Interior Cedar–Hemlock and Engelmann Spruce–Subalpine Fir subzones in the Robson Valley, and the
spread and severity of outbreaks, are described in Sutherland (2001).
Gap dynamics caused by the interaction between root rot and windthrow are key components of
forest dynamics in the Interior Cedar–Hemlock zone at the stand level (Coates and Burton 1997) but are
not well characterized. Gaps may be created by root rot and/or windthrow and, near the northern extent
of the Interior Cedar–Hemlock zone, can make up 6.6% of the area in stands in the moist cold subzone
(ICHmc) (Coates and Burton 1997). Most gaps were less than 150 m2, but could reach up to 600 m2.
Interior Douglas-fir Zone (IDF)
The Interior Douglas-fir biogeoclimatic zone occurs at elevations between ~500 and 1500 m in southcentral interior British Columbia. The climate is continental with a long, warm growing season
characterized by substantial moisture deficits throughout the summer (Hope et al. 1991a). In the Interior
Douglas-fir zone, fire, bark beetles, defoliators (e.g., western spruce budworm [Choristoneura occidentalis]),
Douglas-fir tussock moth (Orgyia pseudotsugata), and root rot are important natural agents of disturbance.
In mountainous areas of the Interior Douglas-fir zone, topography strongly influences local moisture
regimes and the spread of disturbances, leading to structurally complex forest landscapes composed of
multi-aged patches with ill-defined stand boundaries (Sandmann and Lertzman 2003). The wetter and
cooler plateau landscapes in the northern section of the Interior Douglas-fir zone often consist of varying
sizes of even-aged patches of lodgepole pine with scattered large veteran trees (Dawson 1998; Iverson et al.
2002). The Biodiversity Guidebook gives 250 years as the mean interval between stand-replacing disturbances for NDT4 ecosystems (including all Interior Douglas-fir subzones) (B.C. Ministry of Forests and
B.C. Ministry of Environment 1995). The validity of this approach as well as the placement of all Interior
Douglas-fir subzones into this natural disturbance type have been questioned (Pollack et al. 1997; Lloyd
1999; Gayton 2001).
Many stands in the Interior Douglas-fir zone contain fire-scarred trees, which, because the fire intensity
was not enough to kill the entire tree, indicates the importance of low-severity and mixed-severity fires.
The pre-European-settlement mean fire interval ranged from 5 to approximately 50 years in British Columbia. For the Thompson variant of the very hot dry subzone (IDFxh2), Riccius (1998) estimated the
range of point mean fire intervals on six adjacent study sites in the lower Stein Valley to be 9 to 28 years.
In the Kamloops and Lillooet Forest Districts, mean fire intervals ranged from 7 to 39 years in elevational
transects containing the following variants and subzones: the Thompson variants of the very hot dry
subzones of the Ponderosa Pine zone (PPxh2) and of the Interior Douglas-fir zone (IDFxh2), the dry cool
subzone of the Interior Douglas-fir zone (IDFdk1), and the very dry cool subzone of the Montane Spruce
zone (MSxk) (Gray et al. 2002). Mean fire interval was 14 years for one site in the very dry mild subzone
(IDFxm) at the Churn Creek Protected Area (Blackwell et al. 2001). For the Kootenay variant of the dry
mild subzone (IDFdm2) in the Rocky Mountain Trench, point mean fire intervals of 14 and 19 years were
estimated for two sites (Gray et al. in press). In two elevational transects containing IDFdm2 and MSdk
(dry cool variant) on south aspects, mean fire intervals were 14 years in the Cranbrook Forest District and
18 years in the Invermere Forest District (Gray et al. 2002). Point mean fire intervals over 38 plots are
estimated to range between 15 and 47 years for sites in the Cascade variant of the dry cool subzone
14
(IDFdk2) in the mid Stein valley.15 Low-severity fires occurred on average every 13 years in stands in the
IDFdk1 near Merritt (Gray and Riccius 1999). Median of point mean fire intervals ranged from 5 to
49 years in Douglas-fir and lodgepole pine stands in the Fraser variant of the dry cool subzone (IDFdk3)
in the Cariboo (44 stands, Iverson et al. 2002; 9 stands, Daniels and Watson 2003). Although not from
crossdated tree ring data, mean fire intervals fell in a similar range—that is, every 5–44 years in IDFdk4
(Chilcotin variant) lodgepole pine stands of the central Chilcotin (Douglas 2001). For the forests in the
wet warm subzone (IDFww) near Pemberton, a range of point mean fire intervals between 6 and 23 years
was found (Gray and Riccius 2000). Point mean fire intervals were approximately 50 years in forests
similar to the IDFww in Desolation Peak, Washington which borders Skagit Valley Provincial Park (Agee et
al. 1990). Daigle (1996) also provides a table with fire intervals and dates of the most recent fires for seven
sites throughout the dry interior forests; unfortunately, no further details are provided.
Higher-severity disturbances are also hypothesized to have been historically important in parts of the
Interior Douglas-fir zone and to have been influenced by vegetation type and local topography. Fire
history studies that analyzed both fire scars and tree-establishment dates indicate that mixed- to highseverity fires, or other stand-replacing disturbances, have occurred as well. In the Stein Valley, spatial
co-occurrence of disturbances of varying severities has been observed in very small and topographically
distinct areas of less than 50 ha in the IDFxh2 (Riccius 1998; Wong 1999) as well as along landscape-scale
gradients of several thousand hectares in the IDFdk2.16 In the IDFdk3 on the Cariboo Plateau, stands of
pure Douglas-fir, mixed Douglas-fir–lodgepole pine, and pure lodgepole pine experienced the same
frequency of low-severity fires, and, like in the Stein Valley, were influenced primarily by low-severity fires
(Iverson et al. 2002). However, in those stands with lodgepole pine, evidence of patches of even-aged
stand structure indicated these stands did experience higher-severity events and thus are characterized by
mixed-severity fire regimes. Local changes in slope appeared to influence historic fire intensity and stand
densities in the elevational transects sampled by Gray et al. (2002) in the Interior Douglas-fir zone, but
this was not tested statistically. The geographical extent and scale of mixed-severity disturbance regimes
in the Interior Douglas-fir zone in British Columbia is unknown and is the subject of current debate.
Inferences about disturbance severity in the Interior Douglas-fir zone in the Kamloops Forest Region have
been made based on old forest surveys (1910–1930s) as well as fire, lightning, and fire weather index data
since the 1950s (Klenner et al. 2001).
Pollack et al. (1997) applied statistical methods to the Forest Inventory Planning (FIP) “forest cover”
database in order to derive “natural age class” distributions and stand-replacement intervals. For the
IDFdm variants occurring in the Nelson Forest Region, they concluded that the return interval stated in
the Biodiversity Guidebook (250 years) appears to overestimate stand-replacement intervals by a factor of
two (Pollack et al. 1997). A caveat regarding these methods is that they assume even-age stand dynamics.
It is questionable whether this approach gives meaningful results for multi-cohort or all-aged stands
typical of the drier subzones of the Interior Douglas-fir.
The spatial extent of historical low-severity or mixed-severity fires is difficult to determine because
patches are often not distinct enough to determine from airphotos, and analyses of fire scars and age
cohorts face logistical limits. The few studies that estimated fire size distributions in Interior Douglasfir forest types found that most fires tended to be relatively small (< 400 ha in Heyerdahl et al. 2001,
< 100 ha in Lertzman 2001). Low-severity fires in the lower Stein Valley seemed to be much smaller
(2–4 ha), and are thought to be highly constrained by topographic fire breaks (Riccius 1998; Jordan 2002).
In areas with few topographic breaks, such as on the Cariboo Plateau, fires had the potential to be quite
15
16
E. Heyerdahl, Research Forester, Rocky Mountain Research Station, U.S. Department of Agriculture Forest Service, Missoula, Mont.;
preliminary data provided by e-mail, January 2002.
See Footnote 14, re: Heyerdahl.
15
large (Iverson et al. 2002). The largest lightning-caused fire recorded during 1950–1998 in the Interior
Douglas-fir zone in the Cariboo was 1434 ha, and was likely suppressed (unpublished B.C. Ministry of
Forests’ fire database). In all studies, at least one fire year burned the entire study area, which suggests that
episodic, severe, weather-related fire events occurred annually.17
A dramatic decrease in fire frequency over the last century in the dry forest types, observed in all of
the studies cited above, has led to an ongoing debate about causes and effects (e.g., Johnson and
Wowchuk 1993; Gayton 1996; Arsenault and Klenner 2001). Decades-long fire-free periods prior to
European settlement have been observed in most fire history studies but these tended to be local. They
were generally not synchronized across regions (e.g., Gray et al. 2002) and may have been the result of the
stochastic nature of fire ignition and spread (Lertzman et al. 1998). Simulations of a pre-European fire
regime on a pure Douglas-fir IDFdk3 landscape found most of the landscape burned repeatedly, but a small
proportion burned infrequently such that the time since the last fire was up to 250 years (Cumming and
Wong 2002). However, in the Cariboo IDFdk3, most current fire-free intervals exceed the historical maximum in sampled stands (Iverson et al. 2002). Several hypotheses have been proposed to explain the
contemporary lack of fire—climate change, effective fire suppression, livestock grazing, and cessation of
burning by First Nations—none of which can be rejected based on the current level of knowledge.18
Bark beetles, insect defoliators, and root diseases have significant effects on stand and landscape
dynamics in the Interior Douglas-fir zone (Alfaro et al. 1984; Alfaro 1986; Alfaro et al. 1987; Heath and
Alfaro 1990; van der Kamp 1991; Maclauchlan and Brooks 1994; Sturrock and Garbutt 1994; Davis and
Machmer 1998; Miller and Maclauchlan 1998). Currently, timber losses attributed to these disturbances
are much higher than those due to wildfire (Parminter 1998). Most research projects are concerned with
predicting future outbreaks and the effects of management actions on these outbreaks (Alfaro and
Maclauchlan 1992; Shepherd 1994; Negron 1998). Erickson (1987) and Wood and Unger (1996) display
the spatial records of various insects in the Interior Douglas-fir zone since the early 1900s, and Harris
et al. (1985a, 1985b) give descriptive accounts of historical outbreaks of Douglas-fir tussock moth and
western false hemlock looper (Nepytia freemani). However, the authors do not quantify the spatial
historical data used to describe the historical frequency and severity of various insect outbreaks.
Mountain Hemlock Zone (MH)
The Mountain Hemlock biogeoclimatic zone is the subalpine zone above the Coastal Western Hemlock
zone in the mountains along the coast of British Columbia. It is found at elevations of 900–1800 m in
the south, and at 400–1000 m in the north (Pojar et al. 1991b). The Mountain Hemlock zone is characterized by short cool summers and long, wet, cool winters; mean monthly temperatures are above 10°C
for only 1–3 months and mean annual precipitation can be 1700–5000 mm. Disturbances are primarily
fires, landslides, avalanches, wildlife browsing, snow, and wind. The vegetation and environmental
relationships in the Mountain Hemlock zone have been described in Brooke et al. (1970) and Klinka and
Chourmouzis (2001).
Forest fires in the Mountain Hemlock zone occur infrequently—intervals between stand-replacing
events can be centuries to millennia (Lertzman 1992; Hallet 2001; Lertzman et al. 2002). This variability
is thought to be linked to climatic patterns. An 11 000-year fire history, reconstructed from charcoal in
soil and lake sediment at three sites in the subalpine in the Fraser Valley region, found periods of relatively frequent fire during the early Holocene and other periods when the climate was warm and dry
17
18
16
See Agee 1997.
For discussions about the consequences of fire exclusion for British Columbia’s dry interior forests, see Kremsater et al. 1994, Daigle 1996,
Gayton 1996, Hanel 2000, and Turner and Krannitz 2001.
(Hallett et al. 2003). Median fire interval was 1200 years as determined from dated charcoal samples that
were greater than 300 years apart (Lertzman et al. 2002).
Because of the long return intervals, stands tend to be multi-aged and tree age is not a reliable indicator of stand age. Fine-scale processes that cause the death of one to a few trees and create gaps in the
canopy tend to dominate forest dynamics in the Mountain Hemlock zone in ways similar to those in the
Coastal Western Hemlock zone. In stands in the Mountain Hemlock zone on the Lower Mainland, 18%
of the forested area consisted of developmental canopy gaps (Lertzman and Krebs 1991). Stem snap of
canopy dominants infected with stem rot was the principal mode of gap formation (Lertzman and Krebs
1991; Fall and Fall 1996). Gaps, as measured to the boles of trees forming the canopy gap boundaries,
ranged in size from 0.0025 to 0.11 ha (Lertzman and Krebs 1991). For gaps in Mountain Hemlock
forests on the Lower Mainland, turnover time ranged from 556 to 1111 years depending on assumptions
made about the time required to fill a gap (Lertzman and Krebs 1991). Regeneration in the Mountain
Hemlock parklands close to the Alpine Tundra zone is influenced more by tree islands (elevated microsites in association with snowpack duration and depth) than by gap dynamics (Brett and Klinka 1998;
Klinka and Chourmouzis 2001).
Montane Spruce Zone (MS)
The Montane Spruce biogeoclimatic zone occupies mid-elevation slopes and plateaus in the mountains
of the dry, cool, continental southern interior of British Columbia. While the Montane Spruce zone has
strong floristic affiliations with both the adjacent Engelmann Spruce–Subalpine Fir and the Interior
Douglas-fir zones, it is distinguished from these by its extensive seral stands of lodgepole pine (Hope et
al. 1991c). Stand-replacing wildfires and severe mountain pine beetle outbreaks are the two main agents
that maintain forests of lodgepole pine in various successional stages across landscapes in the Montane
Spruce zone. Partial disturbances by low-severity fires, bark beetles, fungal pathogens, and dwarf mistletoes open the canopy and allow the establishment of shade-tolerant spruce and fir (Lloyd et al. 1990).
The Biodiversity Guidebook designates all Montane Spruce subzones as NDT3, with frequent standinitiating events at approximately 150-year intervals (B.C. Ministry of Forests and B.C. Ministry of
Environment 1995).
Probably due to the limited geographical extent of the Montane Spruce zone in British Columbia, very few
projects have established research sites exclusively within this zone. Masters (1990) used stand-age maps to
infer-fire frequency and its variability over time for the dry cool subzones of the Montane Spruce and
Engelmann Spruce Subalpine Fir zones (MSdk and ESSFdk) in Kootenay National Park. The resulting timesince-fire distribution suggested three periods of different fire frequency, reported as fire cycles for the entire
park area: 1508–1788 (60-year cycle), 1888–1928 (130-year cycle), and 1928–1988 (> 2700-year cycle).
An analysis of charcoal and pollen in sediments of Dog Lake in Kootenay National Park provided a
10 000-year record in changes of vegetation types and fire frequency in the lake’s vicinity (Hallett and
Walker 2000). Hallett and Walker concluded that the long-term range of natural variability for climate
and fire in the Kootenay Valley was very broad, with lake-basin forest types ranging from dry, open
Interior Douglas-fir to closed, wet Engelmann Spruce–Subalpine Fir. Current conditions are intermediate in this range similar to those in the MSdk. Inferred fire frequency varied continuously throughout
the Holocene, and for the last ~500 years frequency ranged from 200 to 250 years. This number is much
larger than the 60–130-year fire cycle for Kootenay National Park reported in Masters (1990) and may be
a result of the limited temporal resolution of charcoal records and (or) methodological differences.
Disturbance frequencies based on stand-origin and charcoal data assume that fire is the only agent
responsible for stand-replacing events. In areas susceptible to severe insect outbreaks, such as lodgepole
pine-dominated stands in the Montane Spruce zone, this assumption may not hold. Consequently,
17
stand-replacement intervals based on stand-origin maps are not necessarily fire intervals and charcoal
records may underestimate overall disturbance frequencies because they are unable to detect insect
outbreaks (e.g., Masters 1990 vs. Hallett and Walker 2000).
Reed (1994) used the Montane Spruce zone (likely the MSdk) in the Kootenay Timber Supply Area
as an example for determining the historical probability of stand-replacing fires from the age-class
distribution. The result, based on the maximum likelihood estimator, was 51 years with a 95% confidence interval of 35 to 101 years (Reed 1994). Pollack et al. (1997) used forest inventory age data to
estimate stand-replacement intervals for a number of biogeoclimatic variants in the Nelson Forest
Region. For the MSdk, they estimate replacement intervals of 108–124 years and for the Okanagan
variant of the dry mild subzone (MSdm1) 107–123 years.
Aspect and neighbouring fire regimes appear to be influential in the severity of the dominant fire regime
in the MS zone. South aspects were characterized by frequent, mixed-severity fire regimes of point mean
fire intervals of 14–39 years in elevational transects in the following variants and subzones: the IDFdm2
and MSdk in the Cranbrook Forest District; the MSdm1 in the Invermere Forest District; and the IDFxh2,
the IDFdk1, and the very dry cool subzone of the Montane spruce (MSxk) near Ashcroft in the Kamloops
Forest District (Gray et al. 2002). In similar transects on north aspects, evidence of fire-scarred trees was
scant and most stands had multi-cohorts or were even aged, which indicates the fire regime is mixed to
high in severity.
The influence of site on disturbance regime is supported by Holt et al. (2001) who sampled 38 MSdk
stands in the Nelson Forest Region to develop old-growth definitions based on structural attributes.
While the study did not directly address the role of natural disturbances, it provided qualitative evidence
for differences in disturbance regimes within this subzone. Wet stands developed large-sized trees and
snags with increasing age in a relatively predictable manner, which suggests few post-establishment
disturbances. In contrast, dry stands exhibited a wide variety of structural attributes independent of age,
suggesting a strong influence of mixed-severity disturbances. In the western larch–lodgepole pine forests
of the west side of Montana’s Glacier National Park, Barrett et al. (1991) also found a mixed-severity fire
regime closely correlated with local climate and topography.
Johnson and Fryer (1989) examined the population dynamics of 13 lodgepole pine–Engelmann spruce
stands in the Montane Spruce-like forests of the Kananaskis Valley, Alberta. Their data provided evidence
for long-term co-existence of both species across the landscape, even though lodgepole pine is considered
early successional. The authors suggest that co-existence is facilitated because the average fire-return interval is shorter than the lifespan of pine; less than 2% of the stands in Kananaskis survive to 300 years of age.
The severity of fires can vary within a single event or between events. Stuart-Smith and Hendry
(1998) examined residual trees and patches in five fires that had burned partially or entirely within the
MSdk subzone in the Nelson Forest Region. They found that residual patches varied greatly in size (2–
55 ha) and most survivors were either Douglas-fir or western larch.
Empirical data concerning the frequency and extent of outbreaks of mountain pine beetle or other
insect pathogens are available in the form of annual overflight maps (e.g., Erickson 1987), but remain to
be analyzed for the Montane Spruce zone. Wood and Unger (1996) summarized some of these maps by
Timber Supply Area, but did not attempt to subdivide their analyses by biogeoclimatic zone. Periods
between outbreak peaks ranged between 13 and 60 years, while individual epidemics extended over 5–
8 years (Wood and Unger 1996). Recent literature reviews concerning the dynamics of mountain pine
beetle with application to British Columbia were undertaken by Fuchs (1999) and Drever and Hughes
(2001). Interactions between fire history, mountain pine beetle epidemics, and regeneration in Oregon
were examined by Stuart et al. (1989). The multi-aged structure of the forest was attributed to episodic
moderate-severity disturbances and subsequent gap-phase pine regeneration pulses. Kipfmueller and
18
Baker (1998) studied the degree of infection of lodgepole pine by lodgepole pine dwarf mistletoe
(Arceuthobium americanum) in relation to stand-replacing fire in 34 subalpine stands in southeastern
Wyoming. Mistletoe infection increased with increasing time-since-fire but was highly variable.
Ponderosa Pine Zone (PP)
The Ponderosa Pine biogeoclimatic zone stretches along dry valleys in the southern interior plateau. It
is the driest (mean annual precipitation is 280–500 mm), and during summer it is the warmest forested
zone in British Columbia (Hope et al. 1991c). The Ponderosa Pine zone is characterized by primarily
low-severity fire regimes. Other disturbance agents important in influencing stand structure include
drought, mountain pine beetle, western spruce budworm, Douglas-fir beetle (Dendroctonus pseudotsugae),
Douglas-fir tussock moth, root rots (Armillaria, tomentosus [Inonotus tomentosus], and laminated root
rot [Phellinus weirii]), dwarf mistletoe, stem rusts, Elytroderma needle cast (Elytroderma deformans),
and pine needle blight (Lophodermella sp.) (Lundquist and Negron 2000).
Point mean fire intervals of low-severity fires ranged from 9 to 28 years in a small study area of 28 ha
that bridges the very dry hot subzone of the Ponderosa Pine and Interior Douglas-fir zones (PPxh and
IDFxh) near Lytton (mean = 17 years; Riccius 1998). The areas affected by these fires were estimated to
be quite small, mostly 2–4 ha, because the study site contains a series of river terraces and is highly
constrained by topography. However, low-severity fire regimes were not predominant on every terrace—
evidence of higher-severity disturbances, derived from the patterns of tree establishment, was found on
some of the terraces (Wong 1999). Point mean fire interval of an elevational transect in the PPxh near
Spences Bridge was 7 years (Gray et al. 2002). Other fire history studies in the Ponderosa Pine zone each
found mean fire intervals of less than 10 years (e.g., Dorey 1979; Cartwright 1983 cited in Low 1988;
Parminter 1990). These studies were less rigorous and likely underestimated fire intervals because they
did not follow standard dendrochronological methods (such as crossdating) when dating fire events.
The role of higher-severity fires in ponderosa pine forests is unclear and is subject to much debate in
British Columbia and elsewhere (Shinneman and Baker 1997; Baker and Ehle 2001; Klenner et al. 2001).
Some inferences about disturbance severity in the Ponderosa Pine zone in the Kamloops Region are based
on old forest surveys (e.g., 1910–1930s), and on fire, lightning, and fire weather index records (Klenner et
al. 2001). Analyses of these records indicate that harvesting peaked in the 1960s, that fires have prevailed,
and that insect defoliators appear to have been increasing since the 1950s (Klenner et al. 2001). Little is
known about the distribution of dead and remnant living trees following fires. Two fires in 1940 and 1960,
in the dry hot subzone (PPdh) in the East Kootenays, left veteran Douglas-fir and ponderosa pine as single
trees, clumps, and islands (Stuart-Smith and Hendry 1998).
Substantial research has characterized historic fire regimes in ponderosa pine forests in the United
States.19 Mean return intervals, as determined from fire scars incurred prior to European settlement,
ranged from 2 to 107 years. Fires in these forests were patchy; 10–58% of the area within a fire perimeter
was unburned (Baker and Ehle 2001).
Evidence of invasion and ingrowth of conifers into grasslands or open forest in several Bunchgrass and
Ponderosa Pine locations in the province is attributed to some combination of fire exclusion, grazing, and
(or) climate since the 1940s (Turner and Krannitz 2001).20 Based on a comparison of airphotos from
different periods, Gayton (1997) concluded that, within the NDT4 zone in the Rocky Mountain Trench, grasslands and open forested areas have decreased by 0.76% per year since 1952. Taylor and Baxter (1998)
19
20
See review of 36 studies in Baker and Ehle 2001.
See also the section in this paper about the Bunchgrass zone.
19
simulated fire regimes for two interior British Columbia landscapes (PPdh2 and IDFdm2; PPxh1 and
IDFxh1) and projected changes in classes of forest crown closure and open grasslands to 2032. Preliminary
projections found the area of grasslands decreased in both landscapes but the amount of open forest
increased in one and decreased in the other.
Sub-Boreal Pine–Spruce Zone (SBPS)
The Sub-Boreal Pine–Spruce zone occurs south and west of the Sub-Boreal Spruce zone in the central
interior of British Columbia, in the shadow of the Coast Mountains (Steen and Demarchi 1991). The
Sub-Boreal Pine–Spruce zone is found at elevations above the Interior Douglas-fir zone and below the
Montane Spruce, Sub-Boreal Spruce, and Engelmann Spruce–Subalpine Fir zones. The climate of the
Sub-Boreal Pine–Spruce zone is continental and, compared to the Sub-Boreal Spruce and Boreal White
and Black Spruce, is drier (mean annual precipitation is 335–580 mm) with a shorter growing season.
The Sub-Boreal Pine–Spruce zone is thought to be dominated by mixed-severity and high-severity
disturbances, primarily fires, which create patches of even-aged forest that are further modified over
time by stand-maintaining disturbances. However, increasing evidence of fire-scarred lodgepole pine
(Vera 2000; Francis et al. 2002) suggests lower-severity fires may have been more important in some
subzones than previously thought. Mountain pine beetle and spruce beetle (Dendroctonus rufipennis)
are also capable of causing mortality of host species at landscape levels. Mortality within stands can be
caused by Douglas-fir bark beetle (Dendroctonus pseudotsugae) and root rots. Disturbance agents that
primarily reduce tree growth include lodgepole pine dwarf mistletoe, forest tent caterpillar, and stem
rusts (Wong 2001).
The very dry cold and dry cold subzones (SBPSxc and SBPSdc) on the Chilcotin Plateau are dominated by a mixed-severity fire regime; many frequent, small-to-medium fires are punctuated every 40–
100 years by extremely large fires (Francis et al. 2002). The fires during the years 1869 and 1922 were
extremely large. These were also very dry years in both the Sub-Boreal Pine–Spruce zone and the
IDFdk3 in the Cariboo Forest Region, which indicates regional climate can drive episodic fire events
across large areas (possibly up to 1 million ha) in this landscape (Francis et al. 2002; Iverson et al. 2002).
Large fires like these violate the assumptions of traditional analyses based on time-since-fire distributions (Johnson and Gutsell 1994). However, cycles of mixed-severity to high-severity fires—estimated
from individual fires reconstructed with field data and airphoto interpretation—were similar to mean
fire-return intervals determined from fire scars. For the period 1831 to present, these intervals were
determined to be 45 ± 26 years for the SBPSxc and 64 ± 40 years for the SBPSdc (mean ± 1 SD) (Francis
et al. 2002).21 Annual disturbance rates were approximately 2.0% per year at various times during the
period 1869–1961, and rates decreased to 0.01% per year between 1961 and 2001.
In the wetter subzones, fires appear to be less frequent. In the Cariboo Forest Region, Francis et al.
(2002) found a fire cycle of 170 years between 1922 and 1961 in an area containing the following: the
moist cool subzone of the Sub-Boreal Pine–Spruce zone (SBPSmk), and the Horsefly and Blackwater
variants of the dry warm subzone of the Sub-Boreal zone (SBSdw1 and SBSdw2). The disturbance cycle
of stand-replacing disturbances in the moist cold subzone (SBPSmc) in the Lakes and Morice Timber
Supply Areas was estimated as 91 years for the period 1800–1970 (Steventon 2001).
In the Sub-Boreal Pine–Spruce zone on the Chilcotin Plateau, moderate- to high-severity fires span a
range of sizes, most of which are small (SBPSxc mean = 1557 ha, median = 141 ha; SBPSdc mean = 1369 ha,
median = 33 ha); however, much of the landscape was influenced by very large events (~70 000 ha) at
21
20
See also uncrossdated estimate for SBPSxc in Douglas 2001.
some point in time (Francis et al. 2002). The drier subzone (xc) appeared to have more frequent fires, and
larger fires on average; also, more area incurred overlapping fires (Francis et al. 2002). In the Lakes and
Morice Timber Supply Areas, more than 40% of the current landscape in the Sub-Boreal Pine–Spruce
zone is in patches greater than 500 ha (Steventon 2001).
The mixed-severity fire regime in the SBPSxc and SBPSdc on the Chilcotin Plateau is composed of highseverity fires (within 45% of the 20 000 ha burned by four fires since 1905, < 15% of the original stand
survived) and lower-severity fires (within 20% of the area burned, 16–40% was remnant forest; within 32%
of the area, > 41% was remnant forest) (Francis et al. 2002). Severity varies across subzone—more area in
the SBPSdc was influenced by higher-severity fires than in the SBPSxc. Typically, fires do burn through these
remnants, but at a lower intensity (Vera 2000). Evidence of this is found in multiple fire-scarred trees
in remnant patches of nine fires that burned in the early 1900s in the SBPSxc (Vera 2000). The sizes of
remnant patches are small (average 20–34 ha) but these patches form 3–11% of the total burned area
(Francis et al. 2002).
Mixed-severity disturbance regimes likely also result from interactions between fires and mountain pine
beetle in the extensive lodgepole pine stands in the Sub-Boreal Pine–Spruce zone. Mountain pine beetle
outbreaks increase fire hazard because fallen dead trees create ladders of fuels to the canopy (Lotan et al.
1985). Where beetle-related mortality is low to moderate, the hazard of high-intensity crown fires may
actually be reduced because the continuity between canopy fuels is less (Turner et al. 1999). Records of
multiple fires and mountain pine beetle attacks since 1850 were found in four SBPSxc stands in the
Chilcotin using dendrochronological analyses of fire-scarred and beetle-scarred trees. Most stands experienced two low-severity fires followed by one or two mountain pine beetle attacks. The mountain pine
beetle outbreak in the Chilcotin in the 1980s caused up to 31% mortality of lodgepole pine trees.22 Provincial records of mountain pine beetle outbreaks extend back to 1910 but generally do not span sufficient
numbers of outbreaks to determine the variability in return intervals. However, many forest regions have
experienced two or three outbreaks since 1910 (Wood and Unger 1996), which suggests that mountain
pine beetle outbreaks can occur on a landscape at less than 100-year intervals. It is unknown how often
spruce beetle outbreaks occur in the Sub-Boreal Pine–Spruce zone, but the frequency is likely similar to
that of the SBS.
Wind disturbance in the Sub-Boreal Pine–Spruce zone has not been characterized. However, critical
wind speeds causing endemic and stand-replacing windthrow, while highly specific to landscape position and stand and soil characteristics, are thought to be at least greater than 50 km/h (Sinton et al.
2000). Return intervals for 110-km/h winds, as projected from weather station records, are approximately 26–40 years near Williams Lake and Quesnel (Murphy and Jackson 1997).
Sub-Boreal Spruce Zone (SBS)
The Sub-Boreal Spruce biogeoclimatic zone is found in the central interior of British Columbia at
1100–300 m in elevation (Meidinger et al. 1991). The Sub-Boreal Spruce zone adjoins the Boreal White
and Black Spruce zone in the north, the Interior Cedar–Hemlock zone in the wet areas to the northeast
and east, the Interior Douglas-fir zone and Sub-Boreal Pine–Spruce zone in the south, and the
Engelmann Spruce–Subalpine Fir zone at higher elevations. The climate is continental; average temperatures are below 0°C for 4–5 months of the year and mean annual precipitation is between 440 and
900 mm, of which up to 50% can be snow. Fires of moderate to high severity dominate the natural
disturbance regimes of most forests in the Sub-Boreal Spruce zone (Parminter 1992). Widespread and
22
T. Shore, B. Hawkes, and S. Taylor, Pacific Forestry Centre, Canadian Forest Service, Victoria, British Columbia; unpublished data as of
spring 2002.
21
high-severity mountain pine and spruce bark beetle outbreaks are also influential. Even-aged patches are
modified by stand-maintaining disturbances in the absence of further stand-replacing disturbances (Lewis
and Lindgren 2000). As trees mature, cankers, stem rusts, and lodgepole pine dwarf mistletoe typically
reduce growth in groups of trees but rarely cause widespread mortality. Patchy mortality within mature
stands can be caused by root rots, Douglas-fir bark beetle attacking Douglas-fir on dry sites, and western
balsam bark beetle attacking subalpine fir at mid elevations. Defoliation by forest pests, such as the twoyear-cycle budworm, western blackheaded budworm (Acleris gloverana), western hemlock looper, and
forest tent caterpillar occurs frequently and generally kills less than 20% of the host trees (Wong 2001).
Stand-replacing disturbances (assumed to be primarily fire) appear to vary across the Sub-Boreal
Spruce subzones, following a gradient in precipitation (DeLong 1998). Mean annual precipitation
during spring to fall accounted for 91% of the variation in the mean fire cycle (DeLong 1998). In the
montane ecosystems, the fire cycle during the period 1910–1950 ranged from 244 years in the moist
cool subzone (SBSmk1), to 500 years in the willow variant of the wet cool subzone (SBSwk1), to 1666
years in the very wet cool subzones (SBSvk). In Sub-Boreal Spruce subzones on plateaus, the fire cycle
was lowest for those with intermediate levels of precipitation (SBSmk1 and SBSmc3), ranging from 104
to 270 years (DeLong 1998). For approximately the same time period (1922–1961), Francis et al. (2002)
found a fire cycle of 170 years in an area in the Cariboo Forest Region containing SBSdw1, SBSdw2, and
SBPSmk. These disturbance intervals may not reflect historic intervals, but they do lie within the range
of values reported by other studies for longer time periods.
Other estimates of fire cycles are 100 years for the dry cool subzone (SBSdk), 125 years for the moist
cold subzone (SBSmc) in the Lakes and Morice Timber Supply Areas (Steventon 2001), and from 80 to
100 years for the entire SBSmk1 (Andison 1996). In the very wet cool subzone (SBSvk) on the McGregor
plateau, mean return intervals are estimated to be much longer, ranging from 1205 to 6250 years (Hawkes
et al. 1997). From 1850 to 1950, fire disturbed 0.35–1.2% of the SBSmk1 landscape annually (Andison
1996; DeLong and Tanner 1996). These estimates do not take into account the additional amount reburned by subsequent fires. Andison (1996) estimates that accounting for re-burns would increase the
estimate to approximately 2%. From 1970 to 1990, fire disturbed 0.15% of the SBSmk1 landscape, whereas
harvesting disturbed 12%—harvesting is replacing fire as the dominant disturbance process in these
forests (DeLong and Tanner 1996).
Regional precipitation patterns and topography influence disturbance size; in montane topography,
average patch size decreased along a moisture gradient across Sub-Boreal Spruce subzones, and patch
size distributions were significantly different between the mk1 and the wk1 and vk (DeLong 1998). On
plateaus, the largest patches (mean, maximum, and standard deviation) were found in the areas with
intermediate levels of precipitation (mk and mc). Although large disturbances dominate the sub-boreal
landscape, most patches are small. In the SBSvk on the McGregor Plateau, 53% of the examined patches
were less than 10 ha and less than 1% were greater than 1000 ha (Hawkes et al. 1997). Patch sizes for the
SBSdw1 and SBSdw2 in the Cariboo Forest Region were estimated by Francis et al. (2002) to average
~300 ± ~1000 ha (1 SD), but patch sizes were not adjusted for subsequent overlapping fires and are
likely underestimated.
Fires in the Sub-Boreal Spruce zone can be uneven in severity. Fires in the 1930s to 1950s left 173 ha of
remnant forest in 3–15% of the burned area (SBSmk1) (DeLong and Tanner 1996). Seven selected fires,
which burned in the SBSmc and ESSFmc or dk in the Lakes and Morice Timber Supply Areas, left 1–245 ha
of remnant forest, as determined from airphotos (Cliff Manning Forestry Services 2001). Unlike in the
Boreal White and Black Spruce, the size of remnants in the SBSmk1 appears to increase with the size of the
fire (DeLong and Tanner 1996), although this was not statistically tested. The structure of these fire refugia
differs from the adjacent mature forest by the presence of larger trees, snags, and uneven-aged lodgepole
pine regeneration not typically found in the Sub-Boreal Spruce zone (DeLong and Kessler 2000).
22
Dendrochronological analysis of growth-release patterns of subalpine fir in the Sub-Boreal Spruce
zone suggests that high-severity outbreaks of spruce beetle occurred every 40 to 100 years in the
McGregor Model Forest (Zhang et al. 1999). The patterns of release were found over 10 sites within
165 000 ha, which suggests that widespread, high-severity outbreaks occurred infrequently. Between
1963 and 1986, the size of spruce beetle outbreaks ranged from less than 100 ha to 26 000 ha in the
Cariboo Forest Region (Andrews 1986). Other high-severity disturbances documented in the SubBoreal Spruce zone include wind and mountain pine beetle. As with the Sub-Boreal Pine–Spruce zone
their disturbance regimes have not been quantified for the Sub-Boreal Spruce zone.
In the absence of stand-replacing disturbances, gap disturbances drive stand dynamics in the
Sub-Boreal Spruce zone (Kneeshaw and Burton 1997). Reconstruction of stand development from field
evidence in 14 old stands in the Babine variant of the moist cold subzone (SBSmc2) suggested several
possible trajectories after stand-replacing fires. In some stands, all species were in the stand initiation
stage—a period which can last from 50 to over 100 years; in other stands, subalpine fir was the only
late-successional species. Spruce was recruited early in stand development and maintained itself by reestablishing in gaps of single-tree disturbances created by bark beetles, root rot, and windthrow.
Other, more widespread, low- to moderate-severity disturbances include flooding and defoliators.
Along the Morice River, 31 flood events were recorded over 105 years by scars in trees (Gottesfeld and
Johnson-Gottesfeld 1990). This suggests that flooding of sufficient severity to scar trees—and, presumably,
to cause some mortality—can occur relatively frequently in the Sub-Boreal Spruce zone. Outbreaks of
two-year-cycle budworm in the Fort St. James and Mackenzie Forest Districts occurred approximately
every 30 years over the past 100 years, with each event lasting about 10 years (Zhang and Alfaro 2002).
Outbreaks can cause 16–20% reduction in radial growth each year of the outbreak; outbreak intensity
appears greatest in moist/wet and cool/cold variants of the Sub-Boreal Spruce and Engelmann Spruce–
Subalpine Fir subzones (Shand et al. 1999). Western hemlock looper will defoliate white spruce,
particularly in the wet subzones (SBSmc, mm, vk, wk1). Between 1911 and 1994, western hemlock looper
defoliated 17 500 ha in the Sub-Boreal Spruce zone (Parfett et al. 1995). Outbreaks of forest tent caterpillar
can range in size from a group of aspen to the 190 000 ha defoliated in 1990 in the Prince George Forest
Region (Ferris 1994).
Spruce–Willow–Birch Zone (SWB)
The Spruce–Willow–Birch biogeoclimatic zone is the most northerly subalpine zone in British
Columbia, and it is found at elevations above the Boreal White and Black Spruce zone (Pojar and Stewart
1991b). Winters are long and cold while summers are short and cool; temperatures average more
than 10°C for only one month, and mean annual precipitation is 460–700 mm. Lower elevations of the
Spruce–Willow–Birch zone are generally forested with white spruce and trembling aspen. At higher
elevations, subalpine fir dominates but gives way to shrubs where the growing season becomes too short.
Seral stands of lodgepole pine are uncommon. It appears that fires are less frequent than in the adjacent
Boreal White and Black Spruce (Pojar and Stewart 1991b). The Biodiversity Guidebook describes the
disturbance regime in all four subzones as NDT2 with a mean return interval of 200 years for standreplacing events.
Spatially explicit modelling of fires, which accounted for overlapping fires, was used to estimate historical
fire cycles in the MacKenzie Timber Supply Area (Rogeau 2001).23 While the Spruce–Willow–Birch zone was
not analyzed separately, most fires were found to be less than 1000 ha, although these accounted for only 5–
18% of the area burned, as determined from the BCMOF’s fire database for 1942–1991 (Rogeau 2001).
23
See also discussion in the Boreal White and Black Spruce (page 5) and Table 2.
23
ASSESSMENT OF KNOWLEDGE GAPS
The preceding review of the literature regarding natural disturbances in British Columbia may give the
impression that this field of research is well developed. However, our Table 2, which lists available quantitative
information by biogeoclimatic variant, leads us to a less-encouraging conclusion. Disturbance intervals have
been characterized for only 45% of the 91 biogeoclimatic subzones (excluding parkland and undifferentiated
subzones in the Engelmann Spruce–Subalpine Fir) (Figure 1). For the majority of British Columbia’s 190
biogeoclimatic variants, very little or no empirical data exist for disturbance attributes. The Alpine Tundra
and Coastal Douglas-fir zones have received no formal study, perhaps due to particular methodological
challenges or to the small geographical extent of the Coastal Douglas-fir in British Columbia.
Several of the zones shown in Figure 1—particularly the Engelmann Spruce–Subalpine Fir, SubBoreal Pine–Spruce, and Sub-Boreal Spruce zones—have been fairly extensively researched. However,
most subzones in these zones are described by only one study, which is likely insufficient to represent
the entire range of environmental conditions, particularly for the subzones that are divided into
variants. In some cases, such as in the ICHmw, IDFdk, and CWHvm, more than three studies have been
conducted in different geographic locations within the same subzone. However, within each of these
subzones at least one variant has received no study.
It is important to remember that the studies vary with respect to methods and the quality of data;
consequently, they also vary with respect to confidence in estimates of disturbance attributes. For example,
all four subzones in the Sub-Boreal Pine–Spruce zone appear to be well researched, but two of the studies
used unconventional methods.24 In addition, each study used a different set of assumptions that led to
greater variation in the estimates of disturbance attributes within a subzone than between subzones.
Most of the studies focused on stand-replacing fires and the majority of these focused on fire frequency
only; few disturbance agents other than fire were studied (Figure 2). Figure 2 illustrates how poorly attributes of disturbances other than frequency have been described in British Columbia. For example, only a
quarter of the 91 subzones have descriptions of the size of disturbance patches and only six subzones have
adequate descriptions of the distribution of the size of remnants within disturbance perimeters. Where
disturbance attributes have been quantified, they tend to lack spatial coverage and temporal depth.
Closing the Knowledge Gaps: Recommendations
Quantitative knowledge about the historic variability of natural disturbance dynamics in British
Columbia is incomplete. Below, we make recommendations to address what we believe to be the most
crucial knowledge gaps.
Gaps in Geographical Coverage of Disturbance Research
Spatial “resolution” of the historical variability in natural disturbances is likely smaller than the biogeoclimatic variant and often associated with site-specific topographic conditions. Yet, 45% of all biogeoclimatic
subzones (n = 91) have not been studied and in many subzones only one study on disturbance dynamics has been undertaken in one variant.
Recommendations:
1. Focus future research on biogeoclimatic subzones/variants and topographic positions where natural
disturbances have received little or no study. Also, research priorities should be based on the extent
of the ecosystem in British Columbia, the variability within that ecosystem, the “threat” to historical
24
24
See also discussion of Sub-Boreal Pine–Spruce zone (page 20).
FIGURE 1 Distribution of 38 studies of disturbance intervals across British Columbia’s biogeoclimatic zones.25
Dark bars indicate proportion of province found in a particular biogeoclimatic zone.
FIGURE 2 Distribution of 41 studies in British Columbia that have determined return intervals for different
levels of disturbance severity and the size of disturbance patches and remnants. Most of the studies of
stand-replacing events assumed the agent to be fire. Geomorphic disturbances are not included.
25
Biogeoclimatic ecosystem classification data from M. Eng, Research Branch, B.C. Ministry of Forests, 2001.
25
evidence in that ecosystem, the degree of hypothesized departure from the range of natural variability, the temporal and spatial depth, the quality of existing research, and socio-economic values. For
example, recently created maps of historical fire regimes and the degree of departure hypothesized
for south-central British Columbia could be useful starting points.26
2. Conduct surveys or regional workshops to develop a list of “equivalent ecosystems” for western
North America. This information should be added to an existing project that has extended biogeoclimatic mapping to states that border British Columbia.27 Substantial research on natural
disturbances conducted in other Canadian provinces or the United States has the potential to
provide proximal data for British Columbia subzones/variants (and vice versa) or to allow for
comparison of results and methods. Unfortunately, ecosystem classifications in most other jurisdictions, unlike the biogeoclimatic classification, do not incorporate regional climate; given that climate
influences disturbance patterns (DeLong 1998; Swetnam and Betancourt 1998), these classifications
may not be adequate for describing similar regimes of natural disturbances.
Gaps in Research on Disturbance Agents Other Than Fire, on Attributes Other Than
Frequency, and on Specific Interactions
Recommendations:
3. Prioritize which disturbance agents and attributes within which biogeoclimatic zones/subzones or
other units of classification should receive future research. For example, mixed-severity fire regimes
and the influence of topography are poorly described in British Columbia.
Gaps in Systematic Records of Natural Disturbances and Historic Conditions of Stands
The Canadian Forest Service has recently co-ordinated the British Columbia Natural Disturbance
Database of digitized historical and contemporary maps of fires and insects outbreaks (Natural Resources Canada; Taylor and Thandi 2002). While the historical records of provincial and federal agencies
have proven useful in characterizing disturbances in British Columbia, more comprehensive record
keeping is needed. In particular, no systematic record currently exists for all influential types of disturbances, such as windthrow or mass wasting (Mitchell and Belik 2001). Also, information about the
attributes of forest stands before harvesting or before incurring other disturbances is extremely important
for accurate reconstruction of historical disturbance dynamics, yet this information is not maintained in
the Forest Inventory Planning (FIP) database nor in the Vegetation Resource Inventory.
Recommendations:
4. Maintain the British Columbia Natural Disturbance Database and initiate the systematic
documentation of the location, extent, and severity of windthrow and other currently undocumented disturbance events in British Columbia.
5. Retain complete records of pre-harvesting attributes and consistently fill in all fields in the inventory
database, particularly those on disturbance history. Fill gaps through archival information or field
surveys wherever possible.
6. Develop standards and initiate a database for storing multi-temporal airphoto-based forest cover
maps. This could include re-interpreting old photography in order to extend quantitative disturbance analyses further back in time and to estimate attributes relevant to the analysis of natural
disturbances (e.g., crown damage, snags, canopy gaps, and remnant patches).
26
27
B. Hawkes, Research Scientist, Canadian Forest Service, Victoria, B.C.; personal communication, 2003.
A. MacKinnon, Manager of Ecosystem Conservation, B.C. Ministry of Sustainable Resource Management, Victoria, B.C.; personal
communication, 2003.
26
Gaps in Applying Methods and Communicating Results Effectively
While we did not attempt to systematically or critically review the methods presented in the reports and
papers available to us, we frequently found it difficult to evaluate studies, because they often lacked
transparency or crucial information about methods and assumptions. We believe that this is attributable
to the fact that many projects and reports are not formally peer-reviewed. In addition, most research on
natural disturbances invests little effort in interpreting how the uncertainty of data quality and study
assumptions influence the results.
Recommendations:
7. Develop consistent research strategies that include projects based on both empirical data gathered in
the field and on simulation modelling. These projects should use accepted methods,28 involve a
sensitivity analysis to estimate the effect of uncertainty and specific assumptions made in the analysis, and undergo a formal peer review. The research focus of these strategies should be guided by the
priority-setting exercises we recommend above.
8. Devote energy to developing methods for addressing the challenges of describing the historic variability in disturbances in British Columbia. For example, few studies were able to adequately address
the influence of human activities on the landscape or to obtain sufficient temporal and spatial depth
in field sampling. Methods are lacking for estimating or modelling the characteristics of standmaintaining or mixed-severity disturbance regimes from readily available inventory data.
We certainly do not want to discredit the many past and present research projects concerning the
natural dynamics of British Columbia’s ecosystems; in fact, we were positively surprised by the number
of published and unpublished reports available. Nevertheless, the substantial lack of quantitative information made obvious in this review leads us to conclude that the state of knowledge about natural
disturbances in British Columbia remains quite limited. We believe that these gaps in knowledge are a
function of the incredible diversity of British Columbia’s ecosystems and the limited resources directed
towards disturbance research.
Under ecosystem-based management, knowledge of natural disturbances can guide all scales of
management from silvicultural prescriptions (e.g., characteristics and distribution of retained forest
structure in harvested stands) to strategic forest and land-use planning (e.g., future seral stage distribution). Our synthesis shows that disturbance regimes in British Columbia are more complex than the
broad natural disturbance types described in the Biodiversity Guidebook and in the subsequent ministerial Order Establishing Provincial Non-Spatial Old Growth Objectives,29 where classification is based
primarily on the frequency of stand-replacing disturbances. Many biogeoclimatic subzones were historically influenced by a mixed-severity regime characterized by various combinations of disturbances of
different severity or gap-causing disturbances occurring during the long intervals between standreplacing disturbances. Our review provides a starting point for people interested in: (1) designing
management plans based on natural disturbance patterns in a specific area, (2) revising and refining the
natural disturbance type classification, and (3) identifying areas for future research.
Work towards filling gaps in our knowledge about natural disturbances is important for conducting
effective ecosystem-based forest management, ecological restoration, and conservation activities in
British Columbia. Therefore, we suggest that this review and its accompanying database (Table 2) be
maintained and updated as new information becomes available.
28
29
See review in Wong et al. 2003.
See Footnote 1.
27
TABLE 2A Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones
in British Columbia: Alpine Tundra (AT); Bunchgrass (BG)
Zone &
subzone
Guidebook
mean
return
interval a
(yr)
Author
Location
Method
Churn Ck.
Protected
Area (xh3)
Dendrochronological
dating of fire scars
(n = one 10-ha site,
7 trees).
Period
of record
Disturbance
agent
1706–2000
Low-severity
fires
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
3–81
19
Time-since-fire
Range
Mean
Patch size
Alpine Tundra (AT)
NDT 5
None known
Bunchgrass (BG)
a
xh
NDT 4
Blackwell et
al. 2001
xw
NDT 4
None known
For the AT and BG zones, the Biodiversity Guidebook (B.C. Ministry of Forests and B.C. Ministry of Environment 1995) does not state return intervals for Natural Disturbance Type 4 and Type 5.
28
Remnant
size
Comments
TABLE 2B Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones
in British Columbia: Boreal White and Black Spruce (BWBS); Coastal Douglas-fir (CDF)
Zone &
subzone
Guidebook
mean
return
interval
Author
(yr)
Location
Method
Period
of record
Disturbance
agent
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Boreal White and Black Spruce (BWBS)
?
100–125
Shore and
Alfaro 1986
Near Liard
Dendrochronological
dating of ring patterns in
host and non-host
species.
1869–1980s
Eastern spruce
budworm
?
100–125
Burleigh et
al. 2002
Ft. Nelson
Forest
District
Dendrochronological
dating of ring patterns in
host and non-host
species.
1817–1992
Eastern spruce
budworm
dk
100–125
Rogeau 2001
MacKenzie
TSA,
873 112–
1 549 306 ha
(4 different
terrain units)
< 1860
Spatially explicit
modelling of historic fires
in different terrain units
(see comments). Also,
interpretation of 1950s
airphotos, and analyses of
MOF fire database.
dk
100–125
DeLong 1998
Analysis of forest
633 381 ha,
inventory using Age
near Prince
George (dk1) Classes 3 & 4.
mw
100–125
None known,
but see
comment.
un
1911–1950
14–28
26
Stand-replacing
events
0.31
–1.49
0.32–1.33
60–330
Stand-replacing
events
0.22
–0.38
0.3
333
75 (± 8, 1 SD)
to
303 (± 18)
Outbreaks in northern
sites began about 10 yr
earlier and lasted longer
than in southern sites.
Length of defoliation
periods 6–17 yr, based on
two outbreaks (1950s and
1980s).
1950s airphotos:
1880 ± 2717 ha
4423 ± 7799
(avg. ± 1 SD),
depending on
region
Details on verification and
calibration of the STANDOR
model are not provided;
results should be interpreted
with care.
Four units had some
proportion of BWBS, SWB,
and SBS.
Historic regimes simulated
using probability of
lightning ignitions based on
MOF database > 1950.
1910–1930: patches >
1000 ha = 23% of disturbed
area; patches < 100 ha =
19% of disturbed area
Used Age Classes 3 & 4 to
avoid effects of fire suppression, harvesting,
reburns.
Alfaro et al. 2001 (mw2)
found 1.5–6%/y of spruce
mortality during eastern
spruce budworm oubreak.
None known
vk
100–125
None known
wk
100–125
None known
Coastal Douglas-fir (CDF)
mm
200
None known
29
TABLE 2C Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones
in British Columbia: Coastal Western Hemlock (CWH)
Zone &
subzone
Guidebook
mean
return
interval
Author
(yr)
Location
Method
Disturbance rate
Period
of record
Disturbance
agent
1960–1996
Wind
0.03%/yr
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Coastal Western Hemlock (CWH)
dm
200
None known
ds
200
None known,
but see Gray
et al. 2002
and IDFww
mm
200
None known
ms
200
None known
vh
250
Re-analysis of
Mitchell
(1998) in
Dorner &
Wong 2002
(s.a. Pearson
2003)
North Coast
TSA
(55% vh2,
18% vm)
Airphoto interpretation
of 62 blowdown patches
in operable area
vh
250
Jakob 2000
Clayquot
Sound
62% vh,
23% vm
1:20 000 airphoto
interpretation
1940–
1960+
Debris flows
and rockslides
0.0037–
0.0055
slides/km2/yr
vh
250
Pearson 2003
Central
Coast
55 000
forested ha
Airphoto interpretation
~1860–
2002
Geomorphic
disturbances,
stand-replacing
wind
0.0006±
0.0005%/yr,
0.001%/yr
(wind)
156 000 yr
(mean
rotation
period),
100 000 yr
(mean rotation
period for
standreplacing
wind)
vh
250
Pearson 2003
Central
Coast
117 550
forested ha
Airphoto interpretation
~1860–
2002
Geomorphic
disturbances
0.02±0.01%
/yr
3500 (mean
rotation
period)
un
30
None known
3000
(if landscape
assumed
equally
susceptible,
see text)
1–118 ha,
avg. 11 ha
Notes large episodic event
150 yr ago.
1.67±1.86 ha
(avg. ±1 SD)
Rates vary depending on
assumptions of landslide
age.
Compares to rate of debris
flows in harvested areas.
Geomorphic disturbances
limited to certain topographic
positions
0.2–29 ha
Geomorphic disturbances
limited to certain topographic
positions.
No evidence of standreplacing wind.
TABLE 2C (Continued)
Guidebook
mean
Zone & return
subzone interval
Author
(yr)
Location
Method
Period
of record
Disturbance
agent
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Coastal Western Hemlock (CWH), continued
vm
250
Gavin 2000,
s.a. Lertzman
et al. 2002
700 ha,
Clayoquot
Valley (vm1)
Method 1: Point
estimates from tree
establishment dates and
radiocarbon dates on soil
charcoal (83 sites along
grid)
Tree dates
1550–1886.
Radiocarbon
dates 280–
10 330 BP
Fire
vm
250
Gavin 2000
110 ha
around lake
in Clayoquot
Valley (vm1)
Method 2: Charcoal
concentration in
sediment
200–
present
Fire
797–1964
64–
12 000
yr BP
Slopes = Rarely extended
beyond 250 m in
median
diameter
750 yr
Terraces
= median
4500 yr
Time-since-fire varied
significantly with
topography.
Did not sample higher
elevations where fires
could spread.
S.a. Pearson 2000 & Jakob
2000 for geomorphic
disturbance frequency.
Spatial pattern of fire
around lake not random.
50
(AD 200–900)
350
(AD 1100–present)
Fire
200–450*
345
3–1200
yr BP
*From dated multi-cohorts
in 7 stands.
Primarily focused on areas
with fire evidence (e.g., Fd
cohort, charcoal). 49% of
area did not have this
evidence and could have
burned in the past with
evidence later erased.
vm
250
Green et al.
1999
Capilano
Watershed
(vm1, vm2)
Dating establishment of
old cohorts of Fd along
transects in 17 selected
sites, n = 306 trees
(crossdated)
vm
250
Lertzman et
al. 1996
Tofino Ck.,
Clayoquot
(vm1)
Measured canopy gap in
43 plots along transects
in stands in 3
developmental stages
Gap processes
345–
1379*
5.0±2.9
gapmakers/gap
(mean±SD)
*Depending on time to
fill gap.
vm
250
Lertzman et
al. 1996
Tofino Ck.,
Clayoquot
(vm2)
Measured canopy gap in
plots in 7 plots
Gap processes
313–
1250*
3.7±2.2
gapmakers/gap
(mean±SD)
*Depending on time to
fill gap.
wm
250
None known
ws
200
None known
xm
200
None known
31
TABLE 2D Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones
in British Columbia: Engelmann Spruce–Subalpine Fir (ESSF)
Guidebook
mean
Zone & return
subzone interval
Author
(yr)
Location
Method
Period
of record
Disturbance
agent
1690–1990
2-yr-cycle
budworm
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
30–46
45
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Engelmann Spruce–Subalpine Fir (ESSF)
xc
&/or
dc
150
Parish &
Antos 2002
Damfino
Ck.,
Okanagan
Highlands
Dendrochronological
analysis of growth
suppression relative to
non-host species
xv
200
None known
dc
150
dk
Pollack et al.
1997
Nelson
Forest
Region (dc1)
Fitted age classes;
negative exponential
Stand-replacing
events
149
150
van Wagner
1995, based
on Masters
1990
Kootenay
National
Park
Regression and graphical
analysis (negative
exponential) of age class
distribution
Stand-replacing
fire
110
dk
150
Pollack et al.
1997
Nelson
Forest
Region
Fitted age classes;
negative exponential
Stand-replacing
events
138
dk
150
Rogeau 1996
Mt.
Assiniboine
Provincial
Park
Time-since-fire
distribution based on
stand-origin map
Stand-replacing
fire
220
dk
150
Masters 1990
Kootenay
National
Park
Time-since-fire
distribution based on
stand-origin map
1508–1928
Stand-replacing
fire
dk
150
Stuart-Smith
& Hendry
1998
Invermere
Forest
District
Airphoto interpretation
of post-fire trees and
patches
1938–1967
Mixed-severity
fires
87–1728 ha
2–26 ha
6 fires (s.a. ESSFmw
and MSdk).
Invermere
Forest
District
Airphoto interpretation
of post-fire trees and
patches
1934
Mixed-severity
fires
2909 ha
4–100 ha
1 fire only (s.a. ESSFdk
and MSdk).
dv
150
None known
mw
200
See Gray et
al. 2002 &
IDFww
mw
200
Stuart-Smith
& Hendry
1998
mm
200
None known
mk
200
Steventon
Steventon
2001
2001
32
Lakes/Morice Forest inventory, negative
TSA, Prince exponential, truncated
Rupert Forest
Region
1800–1970
Stand-replacing 0.01–0.07/10 yr
events
(80% CI)
4 plots in one stand.
Length of reduced-growth
periods was 2–22 yr.
Method considered
inappropriate because
spatial homogeneity
assumption violated.
Study area combines
MSdk and ESSFdk.
60
(1508–1788)
130
(1788–1928)
689
TABLE 2D (Continued)
Guidebook
mean
Zone & return
Author
subzone interval
(yr)
Location
Method
Period
of record
Disturbance
agent
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
24–39
32
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Engelmann Spruce-Subalpine Fir (ESSF), continued
Dendrochronological
analysis of growth
suppression relative to
non-host species
mc
200
Zhang &
Alfaro 2002;
Zhang et al.
2001
(includes mv)
Prince
George,
Robson
Valley,
MacKenzie,
& Fort St.
James forest
districts
mc
200
Steventon
2001
Lakes/Morice Forest inventory, negative
TSA, Prince exponential, truncated
Rupert Forest
Region
mv
200
DeLong 1998
280 416 ha
near Prince
George
(mv1)
Forest inventory, used
only Age Classes 3 & 4
wm
350
Pollack et al.
1997
Nelson
Forest
Region
Fitted age classes;
negative exponential
wk
350
Hawkes et al.
1997
121 000 ha,
near
McGregor
Model Forest
(wk2)
Forest inventory,
proportion in
Age Classes 3–7
wc
350
Pollack et al.
1997
Fitted age classes;
Nelson
negative exponential
Forest
Region (wc4)
wc
350
Dorner et al.
2003
Arrow Forest Rollback method using
FIP data
District
(wc1)
1880s–2000 2-yr-cycle
budworm
1800–1970
Stand-replacing 0.04–
events
0.06/10 yr
(80% CI)
1911–1950
Stand-replacing 0.19–
events
0.48
219
0.34
294
Stand-replacing
events
1850–1950
Stand-replacing 0.071–
events
0.188
Stand-replacing
events
1700–1860
61% of
landscape in
patches
> 1000 ha
Used Age Classes 3 & 4 to
avoid effects of fire suppression, harvesting,
reburns.
23% of
landscape in
patches
> 1000 ha
Includes ESSFwk2 and
SBSvk.
153
0.14
532–1429
794
170
Small, frequent
disturbances
28–3598**
90/237*
Episodic, large
events
39–3218
112/247
52–6471
174/535
Episodic, large,
age-dependent
events
Outbreaks synchronized
between forest districts, but
size and intensity varied.
Length of reduced-growth
periods was 7–11 yr.
*Left value is the mean
interval, assuming equal
hazard of harvesting; right
value assumes harvesting
targeted old stands.
**Min./max. numbers
reflect 95% confidence
interval in time-sincedisturbance estimates,
derived from 100 runs of
stochastic model. The 3
scenarios reflect different
assumptions about
disturbance dynamics.
33
TABLE 2D (Concluded)
Guidebook
mean
Zone & return
subzone interval
Author
(yr)
Location
Method
Disturbance rate
Fire cycle/
mean return interval
Period
of record
Disturbance
agent
1700–1860
Small, frequent
disturbances
23–2459**
92/173*
Episodic, large
events
35–2385
104/184
47–6765
183/326
24–7132**
92/173*
35–5919
103/184
34– >10 000
138/326
Range
(%/yr)
Mean
(%/yr)
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Engelmann Spruce-Subalpine Fir (ESSF), continued
wc
350
Dorner et al.
2003
Arrow Forest Rollback method using
FIP data
District
(wc4)
Episodic, large,
age-dependent
events
wc
350
Dorner et al.
2003
Arrow Forest Rollback method using
District
FIP data
(wcp4)
1700–1860
Small, frequent
disturbances
Episodic, large
events
*See comment above.
**See comment above.
*See comment above.
**See comment above.
Episodic, large,
age-dependent
events
Stand-replacing 0.01–0.03/
events
10 yr
(80% CI)
566
Fitted age classes;
negative exponential
Stand-replacing
events
222
Field sampling and
landscape modelling of
historic fires in different
valley orientations
Stand-replacing
fire
wv
350
Steventon
2001
Lakes/Morice Forest inventory, negative 1800–1970
TSA, Prince exponential, truncated,
Rupert
Region
vc
350
Pollack et al.
1997
Nelson
Forest
Region
vc
350
Rogeau 2000
Greater area
around Mt.
Revelstoke
National
Park (93 754
ha)
w
350
None known
34
110–180
145±15
34–59% < 10 ha
1–2.5%
> 1000 ha
Avg. = 100 ha
(depending on
season)
Since calibration and
verification of model were
not provided, interpret
results with care.
MFRI from field data in
park alone ranged from
18 to 48 yr.
Size classes of fires not
separated by BEC subzone
and include ICHwk.
TABLE 2E Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones
in British Columbia: Interior Cedar–Hemlock (ICH)
Guidebook
mean
Zone & return
Author
subzone interval
(yr)
Location
Method
West Arm
Demonstration
Forest,
Kootenay
Lake
Dated fire scars (n = 7
trees, not crossdated),
age structure in 4–5 plots
in 3 stands
Period
of record
Disturbance
agent
1762–1890
Low-severity
fires
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Interior Cedar-Hemlock (ICH)
dk
150
None known
dw
150
Quesnel &
Pinnell 2000
Stand 1
(midslope):
11.1 (n = 3
scarred trees)
Suggest that age structure
indicates higher-severity
fires too.
Stand 2
(upperslope):
24.4 (n = 4
trees)
dw
150
Pollack et al.
1997
Nelson Forest Fitted age classes;
Region
negative exponential
dw
150
Dorner et al.
2003
Arrow Forest
District (dw)
Rollback method using
FIP data
Measured canopy gap
along transects in 16
different site and age
units
Stand-replacing
events
1700–1860
101
Small, frequent
disturbances
13–>
10 000**
Episodic, large
events
25–9652
Episodic, large
age-dependent
events
42–>
10 000
45/215*
*Left value is mean
interval assuming equal
hazard of harvesting; right
value assumes harvesting
targeted old stands.
64/227
125/932
Gap processes
**Min./max. numbers
reflect 95% confidence
interval in time-sincedisturbance estimates,
derived from 100 runs of
stochastic model. The 3
scenarios reflect different
assumptions about disturbance dynamics.
Most are
< 150 m2,
max. = 600 m2
mc
200
Coates &
Burton 1997
Date Ck.,
north of
Hazelton
mk
150–200
Pollack et al.
1997
Nelson Forest Fitted age classes;
negative exponential
Region
(mk1)
Stand-replacing
events
101
mm
200
None known
mw
150–200
Pollack et al.
1997
Nelson Forest Fitted age classes;
Region (mw2) negative exponential
Stand-replacing
events
129
6.6% of area in gaps
35
TABLE 2E (Continued)
Zone &
subzone
Guidebook
mean
return
interval
Author
(yr)
Location
Method
Period
of record
Disturbance
agent
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Interior Cedar–Hemlock (ICH), continued
mw
150–200
Gavin prelim.
results (mw2)
mw
150–200
Dorner et al.
2003
un
Paleoecological record
of charcoal in lake
sediment
Arrow Forest Rollback method using
FIP data
District
(mw2)
1700–1860
Small, frequent
disturbances
33–
3598**
81/237*
Episodic, large
events
44–3218
94/289
Episodic, large,
age-dependent
events
66–
>10 000
161/788
Small, frequent
disturbances
19–
>10 000**
133/334*
Episodic, large,
events
32–
>10 000
151/401
Episodic, large,
age-dependent
events
34–
>10 000
207/472
*See comment above.
**See comment above.
None known
vc
250
None known
vk
250
Dorner et al.
2003
Arrow Forest Rollback method using
District
FIP data
wc
200
None known
wk
250
Pollack et al.
1997
wk
250
Johnson et al. Glacier
1990 (wk1)
National
Park
36
Includes ESSFwc4.
Intervals
of 200 yr
in a row,
also 500–
800 yr
intervals
Nelson
Forest
Region
(wk1)
1700–1860
Fitted age classes;
negative exponential
Time-since-fire
distribution from standorigin map developed
from field sampling
1519–1988
*See comment above.
**See comment above.
Stand-replacing
events
226
See also Hoggett (Hw
looper) 2000.
Stand-replacing
fire
80 yr
(1519–1760)
110 yr
(> 1760)
Includes the ESSF.
Periods of change were
determined a posteriori
from changes in slope in
the time-since-fire
distributions. See Huggard
& Arsenault (1999, 2001)
and Reed & Johnson (1999)
for a critique and debate of
this method.
TABLE 2E (Concluded)
Guidebook
mean
Zone & return
subzone interval
Author
(yr)
Location
Method
Period
of record
Disturbance
agent
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Stand-replacing
fire
150–240
181±19
Small, frequent
disturbances
Episodic, large
events
Episodic, large,
age-dependent
events
24–
>10 000**
133/225*
36–
>10 000
149/256
40–
>10 000
209/418
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Interior Cedar–Hemlock (ICH), continued
Field sampling and
landscape modelling of
historic fires in different
valley orientations
wk
250
Rogeau 2000
Greater area
around Mt.
Revelstoke
National
Park
(93 754 ha)
wk
250
Dorner et al.
2003
Arrow Forest Rollback method using
FIP data
District
(vk1)
xw
250
1700–1860
34–59% < 10 ha
1–2.5%
> 1000 ha
Avg. = 100 ha
(depending on
season)
Since calibration and
verification of model were
not provided, interpret
results with care.
MFRI from field data in
park alone ranged from
18–48 yr.
Size classes of fires not
separated by BEC subzone
and include ESSFvc.
*See comment above.
**See comment above.
None known
37
TABLE 2F Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones
in British Columbia: Interior Douglas-fir (IDF)
Guidebook
Zone & mean return
subzone interval
(yr)
Author
Location
Method
Period
of record
Disturbance
agent
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Interior Douglas-fir (IDF)
dk
Standmaintaining
4–50,
standreplacing 250
Gray &
Riccius 1999
Pothole Ck.
research site,
near Merritt
(dk1)
Dendrochronological
analysis of fire scars on
23 trees and snags
(crossdated)
1693–1967
Low-severity
fires
dk
Standmaintaining
4–50,
standreplacing 250
Heyerdahl
unpubl.
Mid-Stein
Valley (dk2
and un)
Dendrochronological
analysis of fire scars on
107 trees (crossdated)
1700–1996
Low-severity
fires
dk
Standmaintaining
4–50,
standreplacing 250
Douglas 2001
Cariboo
Forest
Region
(Williams
Lake to
Clinton;
dk3, 4)
n = 23 trees, 11 stands
(not crossdated)
1839–1989
Low-severity
fires
dk
Standmaintaining
4–50,
standreplacing 250
See Gray et
al. 2002 and
MSxk
dk1
dk
Standmaintaining
4–50,
standreplacing 250
Iverson et al.
2002
Cariboo
Forest
Region
(Williams
Lake to
Clinton;
dk3)
Dendrochronological
dating of fire scars (265
samples), and increment
cores (crossdated) of 44
stands
1750–2000
Low-severity
fires
38
13
1967
AD
(low
severity)
Maximum fire interval
46 yr, between 1788 and
1834.
15–47
for 38
sampled
stands
21
1968
AD
(low
severity)
Return intervals for lowto mid-severity fires only.
5–44
15
Median MFI
at each plot
ranged from
5 to 49;
across all
plots was 22.
No significant difference in
frequency across pure lowelevation Fd, mixed Fd–Pl,
and pure Pl high-elevation
strata.
Mixed severity in mixed
Fd–Pl and pure Pl.
Another study of 14 stands
in Cariboo to be completed
March 2003. (L. Daniels
pers. comm.)
TABLE 2F (Continued)
Guidebook
mean
Zone & return
subzone interval
(yr)
Author
Location
Method
2 sites,
Rocky
Mountain
Trench
(dm2)
Dendrochronological
analysis of 7–9 trees/site
(crossdated), increment
cores from all trees > 20
dbh in 50 × 50-m plots
Disturbance rate
Fire cycle/
mean return interval
Period
of record
Disturbance
agent
1694–1883
1683–1894
Low-severity
fires
19 and 14
Stand-replacing
events
121 (dm1)
111 (dm2)
Range
(%/yr)
Mean
(%/yr)
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Interior Douglas-fir (IDF), continued
dm
Gray et al.
Standmaintaining in press
4–50,
standreplacing 250
dm
Standmaintaining
4–50,
standreplacing 250
dm
Pollack et al.
Standmaintaining 1997
4–50,
standreplacing 250
dw
None known
Standmaintaining
4–50,
standreplacing 250
mw
None known
Standmaintaining
4–50,
standreplacing 250
un
See Gray et
al. 2002,
MSdk, and
MSdm
1883,
1894
AD
dm2
Nelson
Forest
Region (dm1
and dm2)
Mean stand age of
inventory age classes,
fitted against negative
exponentional model
Study used mean stand
age, not age of oldest trees.
None known
ww
Gray et al.
Standmaintaining 2002
4–50,
standreplacing 250
Haylmore
Ck.,
Squamish
Forest
District
Dendrochronological
analysis of 8–11 firescarred trees
(crossdated), and
increment cores in 2
elevational transect
1555–2001
Low-severity
fires
1–94
ww
Gray &
Standmaintaining Riccius 2000
4–50,
standreplacing 250
5 watersheds
near
Pemberton
Dendrochronological
analysis of 3–9 trees per
5–9 ha in each of 5
watersheds (crossdated),
increment cores for ageclass distributions
1674–1969
Low-severity
fires
6–23
6–7
South aspect.
Forest cover maps mark
IDFww area as CWHds;
one transect includes
ESSFmw.
1906–1969
AD
Did not use a period of
reliability.
39
TABLE 2F (Concluded)
Zone &
subzone
Guidebook
mean
return
interval
(yr)
Author
Location
Method
Period
of record
Disturbance
agent
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Interior Douglas-fir (IDF), continued
ww
(?)
Agee et al.
Standmaintaining 1990
4–50,
standreplacing 250
Desolation
Peak, Skagit
Valley,
Wash.
Dendrochronological
analysis of fire scars
and increment cores for
97 plots in 7 vegetation
types
1573–1985
Low-severity
fires
xh
Riccius 1998
Standmaintaining
4–50,
standreplacing 250
Lower Stein
Valley (xh2)
Dendrochronological
analysis of 356 fire scars
(crossdated)
1683–1972
Low-severity
fires
9–28 for
6 sampled
river
terraces
17
xh
Gray et al.
Standmaintaining 2002
4–50,
standreplacing 250
Glossy
Mountain,
near
Ashcroft
(xh2)
Dendrochronological
analysis of 13 fire-scarred
trees (crossdated), and
increment cores in
elevational transect
1621–2001
Low-severity
fires
1–54
11
South aspect.
Calculated a point
frequency and assumed fires
burned entire 40-ha
transect.
xh
Gray et al.
Standmaintaining 2002
4–50,
standreplacing 250
Spences
Bridge (xh2)
Dendrochronological
analysis of 9–11 firescarred trees
(crossdated), and
increment cores in 2
elevational transects,
includes IDFdk1 and
PPxh2
1786–2001
Low-severity
fires
North
aspect:
1–54
South
aspect:
1–54
North aspect:
12
South aspect:
10
Calculated a point
frequency and assumed fires
burned entire 40-ha
transect.
xm
Blackwell
Standmaintaining et al. 2001
4–50,
standreplacing 250
Churn Creek
Protected
Area
Dendrochronological
dating of fire scars
(n = one 10-ha site, 8
trees)
1680–2000
Low-severity
fires
3–36
xw
Standmaintaining
4–50,
standreplacing 250
40
None known
52
14
Near meadow
encroachment study of
Lepofsky et al. 2000.
Juxtaposition of forest types
commonly not found together believed to contribute to relatively long MFI
for IDF plots.
Did not use period of
reliability.
Most fires
1972
burned only a
AD
few ha
(low
severity)
Very small study area
(28 ha).
TABLE 2G Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones
in British Columbia: Mountain Hemlock (MH)
Guidebook
mean
Zone & return
subzone interval
Author
(yr)
Location
Method
Period
of record
Disturbance
agent
11 000 yr
Fire
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Time-since-fire
Mean
(yr)
Range
Mean
Median
1200
50–
9220
BP
Median
1550 BP
Patch size
Remnant
size
Comments
Mountain Hemlock (MH)
mm
350
Hallet 2001;
Hallet et al.
2003;
Lertzman et
al. 2002
Fraser
Valley,
3 sites
mm2/mmp
Dating charcoal in lake
sediment and soil
mm
350
Lertzman &
Krebs 1991
Cypress,
lower
mainland
1250 m of
transects in
4 stands
Area in canopy gap for
turnover rates
un
wh
Gap processes
556–111*
Note that comparing MFI
between sites from paleo
data sites is appropriate
when obtained from point
sources (small catchments
or different points for soil
record) or when study area
is known.
Fire interval calculated from
dated charcoal samples
which were > 300 yr apart.
0.0025–0.11 ha
*Depending on time to fill
gap; see text.
Lertzman et al. 1996 also
looked at gaps in MHmm1,
but had only 3 plots.
None known
350
None known
41
TABLE 2H Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones
in British Columbia: Montane Spruce (MS)
Guidebook
mean
Zone & return
subzone interval
Author
(yr)
Location
Method
Period
of record
Disturbance
agent
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Montane Spruce (MS)
?
150
Reed 1994
Kootenay
TSA
Maximum likelihood
Stand-replacing
events
51
(95% CI=
35 to 101)
Does not incorporate agedependent susceptibility,
which is very likely
significant in the MS.
dm1
150
Pollack et al.
1997
Nelson
Forest
Region
(dm1)
Fitted age classes;
negative exponential
Stand-replacing
events
107
Does not incorporate agedependent susceptibility,
which is very likely
significant in the MS.
dm1
150
Gray et al.
2002
Columbia
Lake,
Invermere
Forest
District
Dendrochronological
analysis of 16 fire-scarred
trees (crossdated), and
increment cores in
elevational transect
including IDFdm2
18
South aspect only, standreplacing event evidence on
north aspect.
Calculated a point
frequency and assumed fires
burned entire 40-ha
transect.
dk
150
Pollack et al.
1997
Stand-replacing
events
108
Does not incorporate agedependent susceptibility,
which is very likely
significant in the MS.
dk
150
Masters 1990
Kootenay
National
Park
Time-since-fire
distribution based on
stand-origin map
1508–1928
Stand-replacing
fire
60
(1508–1788)
130
(1788–1928)
Study area combines MSdk
and ESSFdk.
dk
150
Gray et al.
2002
Lone Peak,
Cranbrook
Forest
District
Dendrochronological
analysis of 17 fire-scarred
trees (crossdated), and
increment cores in
elevational transect
including IDFdm2
1756–2002
Low- to mixedseverity fires
1–43
14
South aspect only, standreplacing event evidence on
north aspect in ESSFdk.
Calculated a point
frequency and assumed fires
burned entire 40-ha
transect.
dc
150
None known
dv
150
None known
Glossy
Mountain,
near
Ashcroft
Dendrochronological
analysis of 16 fire-scarred
trees (crossdated), and
increment cores in
elevational transect
including IDFxh2, dk1
1735–2002
Low- to mixedseverity fires
21–71
39
South aspect only.
Consider a point frequency,
assume fires burn entire
40-ha transect.
un
Fitted age classes;
negative exponential
Low- to mixedseverity fires
2–44
None know
xk
150
Gray et al.
2002
xv
150
None known
42
1763–2002
TABLE 2I Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones
in British Columbia: Ponderosa Pine (PP)
Guidebook
mean
Zone & return
subzone interval
(yr)
Author
Location
Method
Invermere
Forest
District
1:20 000 air photo
interpret. of remnants
in 2 fires
Period
of record
Disturbance
agent
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Ponderosa Pine (PP)
dh
Stuart-Smith
Standmaintaining & Hendry
1998
4–50,
standreplacing
250
Mixed-severity
fires
Single trees:
20–33 trees
per ha,
avg.
±1 SD
Clumps:
1.8 ± 1.3 ha
Islands:
6.8 ± 5.8 ha
xh
Standmaintaining
4–50, standreplacing 250
Riccius 1998
6 terraces in
Lower Stein
Valley (xh2)
Dendrochronological
analysis of 356 fire scars
(crossdated)
1683–1972
Low-severity
fires
9–28
17
xh
Standmaintaining
4–50, standreplacing 250
Gray et al.
2002
Murray
Creek, near
Spences
Bridge (xh2)
Dendrochronological
analysis of 11 fire-scarred
trees (85 scars
crossdated), and
increment cores
1596–2002
Low-severity
fires
1–26
7
Most fires
1972
burned
AD
2–4 ha*
(low
severity)
Fires subjectively selected
and sample size very small
(n = 2).
Very small study area
(28 ha), topographically
dissected.
High risk of Pl, Lw, Fd,
dwarf mistletoe.
Calculated a point
frequency and assumed
fires burned entire 40-ha
transect.
43
TABLE 2J Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones
in British Columbia: Sub-Boreal Pine–Spruce (SBPS)
Guidebook
mean
Zone & return
subzone interval
Author
(yr)
Location
Method
Period
of record
Disturbance
agent
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Sub-Boreal Pine–Spruce (SBPS)
1922–
1961: 34.4
± 118.6 ha
(mean ±
SD);
7.1 ha
(median);
1575 ha
(range
from
median)
Field sampling was lowintensity (e.g., only 1 firescarred disk and 2 cores
collected/fire margin), but
spatially extensive.
91% of area originated
during or since 1869 fire.
Assumptions behind firecycle method violated as
2 large fire events affected
at least 50% of study area.
Also quantifies fire
severity across landscape.
Provided patch size
distribution.
dc
100
Francis et al.
2002
Chilcotin
Plateau
(80 000 ha of
307 000-ha
study area)
Field and
dendrochronological
verification of fire
margins and stand-origin
dates; and airphoto
interpretation to
extrapolate field data
1665–2001 Low-,
moderate-,
(POR:
1831–2001) and highseverity fires
47 (1869–
1961)
68 yr (1869–
2001)
(includes
SBPSdc)
61 yr (1869–
1961)
SBPSdc only
64 ± 40
(from fire
scars)
1922–1961:
1369 ± 5867 ha
(mean ± SD);
32.5 ha
(median);
35 042 ha (range
from median)
mc
100
Steventon
2001
Lakes,
Morice, &
Bulkley
TSAs; Prince
Rupert
Region
Negative exponential
distribution rolled back
to account for harvesting
& overlapping
disturbances
1800–1970
91
40% of
landscape
in patches
> 500 ha
Used Forest Inventory
Classes 3 to 7 and did not
account for reburning of
older patches.
Used age data from audit
plots to examine
inaccuracies in forest
inventory data.
mk
100
Francis et al.
2002
Cariboo
Region
(98 200 of
327 300 ha)
1700s–1961
Field and
dendrochronological
verification of fire
margins, stand-origin
dates and 1:20 000
airphoto interpretation to
extrapolate
60 (1869–
1961),
adjusted for
overlap,
includes
SBSdw1 &
dw2
1700s–2001:
325 ± 1490 ha
(mean ± SD)
Not adjusted for
overlap
Field sampling was lowintensity (e.g., only 1 firescarred disk and 2 cores
collected/fire margin), but
spatially extensive.
Used time-since-fire map
and attempted to account
for overlapping fires with
“ 1/3 overlap rule.”
Assumptions behind firecycle method violated
because 2 large fire events
affected study area.
Provided patch size
distribution.
Stand-replacing 0.06–0.09/10 yr
events
(80% CI)
Low-,
moderate-, and
high-severity
fires
1.67 (1869–
1961),
includes
SBSdw1 &
dw2
170 (1922–
1961),
assumes no
overlap
44
TABLE 2J (Continued)
Zone &
subzone
Guidebook
mean
return
interval
Author
(yr)
Location
Method
Period
of record
Disturbance
agent
Low-severity
fires
Disturbance rate
Range
(%/yr)
Fire cycle/
mean return interval
Mean
(%/yr)
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Sub-Boreal Pine–Spruce (SBPS), continued
xc
100
Douglas 2001
Chilcotin
Plateau
n = 15 trees (6 stands,
not crossdated)
1834–1989
xc
100
Francis et al.
2002
Chilcotin
Plateau
(219 000 of
307 000 ha)
Field and
dendrochronological
verification of fire
margins and stand-origin
dates; and airphoto
interpretation to
extrapolate field data
1665–2001 Low-,
moderate-,
(POR:
1831–2001) and highseverity fires
5–44
14
47 (1869–
1961)
68 (1869–
2001),
includes
SBPSdc
45 (1869–
1961),
SBPSxc only
45 ± 26,
from fire
scars
1922–1961:
1557 ± 4610 ha
(mean ± SD);
140.8 ha
(median);
36 639 ha (range
from median);
56% < 50 ha
1922–1961:
19.4 ±
53.1 ha
(mean
± SD);
6.8 ha
(median);
623 ha
(range
from
median)
Field sampling was lowintensity (e.g., only 1 firescarred disk and 2 cores
collected/fire margin), but
spatially extensive.
91% of area originated
during or since 1869 fire.
Compare time-since-fire
mapping and
reconstructed individual
fires.
Assumptions behind firecycle method violated because 2 large fire events
affected at least 50% of
study area.
Also quantifies fire severity
across landscape.
Provided patch size
distribution.
Other studies: Vera 2001;
B. Hawkes et al.
unpublished data.
45
TABLE 2K Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones
in British Columbia: Sub-Boreal Spruce (SBS)
Zone &
subzone
Guidebook
mean
return
interval
Author
(yr)
Location
Method
Near
McGregor
Model
Forest, 10
sites within
165 000 ha
Dendrochronological
dating of ring patterns in
host and nonhost species
Period
of record
Disturbance
agent
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Sub-Boreal Spruce (SBS)
1700–1980s Spruce bark
beetle
40–100
?
125
Zhang et al.
1999
dh
125
None known
dk
125
Steventon
2001; Cliff
Manning
Forestry
Services 2001
Lakes,
Morice, &
Bulkley
TSAs, Prince
Rupert Forest
Region
Forest inventory; field and 1800–1970
dendrochronological
verification of fire
margins and stand-origin
dates; airphoto
interpretation to
extrapolate field data
Stand-replacing 0.09–0.13/
events
10 y
(80% CI)
dw
125
DeLong 1998
637 662 ha,
near Prince
George
Forest inventory, used
only Age Classes 3 & 4
Stand-replacing 0.36–
events
0.75
0.55
dw
125
Francis et al.
2002
Cariboo
Forest
Region
(dw1 & dw2,
327 300 ha
including
SBPSmk)
1700s–1961
Field and
dendrochronological
verification of fire
margins, stand-origin
dates, and 1:20 000
airphoto interpretation to
extrapolate
Low- ,
moderate-,
and highseverity fires
1.67
(1869–1961),
includes
SBSdw1 and
dw2
1911–1950
mc
125
Steventon
2001; Cliff
Manning
Forestry
Services 2001
Lakes,
Morice, &
Bulkley
TSAs, Prince
Rupert Forest
Region
Forest inventory; field and
dendrochronological
verification of fire margins
and stand-origin dates;
airphoto interpretation to
extrapolate field data
1800–1970
Stand-replacing 0.06–0.09/
events
10 y
(80% CI)
mc
125
DeLong 1998
280 416 ha
near Prince
George
Forest inventory,
proportion in
Age Classes 3 & 4
1911–1950
Stand-replacing 0.67–
events
1.25
mh
125
None known
46
90% of
spruce
killed in
1960s
93
133–181
60
(1869–1961),
adjusted for
overlap,
includes
SBSdw1 &
dw2
170 (1922–
1961),
assume no
overlap
Used Forest Inventory
Classes 3–7 and does not
account for reburning of
older patches.
56.7% of landscape in patches
> 1000 ha, 76 ha
(mean), 418 ha
(SD), max. 7693 ha
Used Age Classes 3 & 4 to
avoid effects of fire
suppression, harvesting,
reburns.
1700s–2001:
SBSdw1, 297 ±
1183 ha (mean
± SD);
SBSdw2, 387 ±
1666 ha (mean
± SD).
Field sampling was of lowintensity (e.g., only 1 firescarred disk and 2 cores
collected per fire margin),
but spatially extensive.
Used time-since-fire map
and attempted to account
for overlapping fires with
“ 1/3 overlap rule.”
Assumptions behind firecycle method violated
because 2 large fire events
affected study area.
Provided patch size
distribution.
Not adjusted for
overlap.
133
0.96
104
Same pattern found over
165 000 ha, suggesting
widespread high-severity
outbreaks in past.
Used Forest Inventory
Classes 3–7 and does not
account for reburning of
older patches.
72% of landscape in
patches > 1000 ha,
296 ha (mean),
1602 ha (SD),
max. 19 030 ha
Used Age Classes 3 & 4 to
avoid effects of fire
suppression, harvesting,
reburns.
TABLE 2K (Continued)
Guidebook
mean
Zone & return
Author
subzone interval
(yr)
Location
Method
Period
of record
Disturbance
agent
Disturbance rate
Range
(%/yr)
Mean
(%/yr)
Fire cycle/
mean return interval
Range
(yr)
Mean
(yr)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Sub-Boreal Spruce (SBS), continued
mk
125
Andison
1996; DeLong
& Tanner
1996
Entire
subzone
790 000 ha
north of
Prince George
Forest inventory; field and 1700–1954
dendrochronological
verification of fire
margins and stand-origin
dates; airphoto
interpretation to
extrapolate field data, and
field work to divide Age
Classes 8 & 9.
Stand-replacing 0.07–0.4/20 yr
events
(rollback based
on 80-yr MFI), 0.07
–0.19/20 yr
if no rollback
years
mk
125
DeLong 1998
1 125 396
ha,
near Prince
George
Forest inventory, used
only Age Classes 3 & 4
1911–1950
Stand-replacing Plateau:
events
0.42–
1.16
Montane:
0.28–0.54
Plateau:
0.79
Montane:
0.41
mm
125
None known
mw
125
None known
un
125
None known
vk
200
Hawkes et al.
1997
121 000 ha,
near
McGregor
Model Forest
Forest inventory, used
only Age Classes 3–7
1850–1950
Stand-replacing 0.016–
events
0.083
Mean
0.047
vk
200
DeLong 1998
395 603 ha,
near Prince
George
Forest inventory, used
only Age Classes 3 & 4
1911–1950
Stand-replacing 0.02–
events
0.1
0.06
wk
200
DeLong 1998
583 700 ha,
near Prince
George
Forest inventory, used
only Age Classes 3 & 4
1911–1950
Stand-replacing Plateau: Plateau: 0.37
events
0.29–0.44
Montane: Montane:
0.15–0.24 0.2
80–100
31% of
landscape
in patches
> 10 000 ha
Plateau: 127
Montane:
244
1205–
6250
1966
Plateau: 270
Montane:
500
1–73 ha
(3–15%
of burns)
Plateau: 78% of
landscape in patches
> 1000 ha, 179 ha
(mean), 1858 ha (SD),
max. 41 787 ha
Montane: 74% in
patches > 1000 ha,
213 ha (mean,)
906 ha (SD),
max. 10 458 ha
Used Age Classes 3 & 4 to
avoid effects of fire
suppression, harvesting,
reburns.
23% of
landscape in
patches
> 1000 ha, 0.08–
13549 ha
(range), 39 ha
(mean)
Includes ESSFwk2/wc3.
Age Class 8 was avoided
because of inaccurate ages.
However, because 90% of
landscape fell in Age Class
8, it follows that return
intervals are relatively long.
13% of landscape in
patches > 1000 ha,
62 ha (mean),
146 ha (SD),
max. 1082 ha
Used Age Classes 3 & 4 to
avoid effects of fire
suppression, harvesting,
reburns.
Plateau: 38% in
patches > 1000 ha,
96 ha (mean),
270 ha (SD), max.
2514 ha
Montane: 39% in
patches > 1000 ha,
74 ha (mean),
224 ha (SD), max.
1931 ha
Used Age Classes 3 & 4 to
avoid effects of fire
suppression, harvesting,
reburns.
47
TABLE 2L Database of research on disturbance frequency and patch characteristics of various disturbance agents in biogeoclimatic zones/subzones
in British Columbia: Spruce–Willow–Birch (SWB)
Zone &
subzone
Guidebook
mean
return
interval
Author
(yr)
Location
Method
MacKenzie
TSA,
1 327 000–
1 549 306 ha
(4 different
terrain units)
Spatially explicit
modelling of historic fires
in different terrain units
(see comment).
Also interpretation of
1950s airphotos, and
analyses of MOF fire
database.
Period
of record
Disturbance
agent
Disturbance rate
Fire cycle/
mean return interval
Range
(%/yr)
Mean
(%/yr)
Range
(yr)
Mean
(yr)
0.3–0.5
0.32–0.45
60–330
220–303
(± 18,
1 SD)
Time-since-fire
Range
Mean
Patch size
Remnant
size
Comments
Spruce–Willow–Birch (SWB)
dk
200
None known
dks
200
None known
mk
200
Rogeau 2001
mks
200
None known
un
200
None known
vk
200
None known
vks
200
None known
48
< 1860
Stand-replacing
events
MOF fire
database
1942–1991:
2272 ± 3526 ha
(avg. ± 1 SD)
Details on verification and
calibration of the
STANDOR model are not
provided; interpret results
with care.
4 units had some
proportion of BWBS, SWB,
and SBS.
Historic regimes simulated
using probability of
lightning ignitions, based on
MOF database > 1950.
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