United States 

Department of 

Agriculture

Forest Service

Pacific Northwest 

Research Station

Research Paper

PNW-RP-592

May 2013

Natural Tree Regeneration 

and Coarse Woody Debris 

Dynamics After a Forest Fire 

in the Western Cascade Range

Martin J. Brown, Jane Kertis, and Mark H. Huff
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Authors

Martin J. Brown is an independent research consultant, Portland, OR 97215; 

Jane Kertis is an ecologist, Siuslaw National Forest, 3200 SW Jefferson Way, 

Corvallis, OR 97331; and Mark H. Huff is an ecologist, U.S. Department of the 

Interior, National Park Service, Mount Rainier National Park, 55210 238th Ave E, 

Ashford, WA 98304-9751.

Cover photos

Plot number 16 at 7 and 14 years after fire. At 7 years, there is abundant regeneration of Pseudotsuga menziesii under a canopy of snags. At 14 years, the snag 

canopy is thinning out owing to snag fall and fragmentation; tree, shrub, and herb 

regeneration mix but trees are the tallest elements.

This work was performed under USDA Forest Service PO AG-04T0-P-10-0057.
Abstract

Brown, Martin J.; Kertis, Jane; Huff, Mark H. 2013. Natural tree regeneration 

and coarse woody debris dynamics after a forest fire in the western Cascade 

Range. Res. Pap. PNW-RP-592. Portland, OR: U.S. Department of Agriculture, 

Forest Service, Pacific Northwest Research Station. 50 p.

We monitored coarse woody debris dynamics and natural tree regeneration over 

a 14-year period after the 1991 Warner Creek Fire, a 3631-ha (8,972-ac) mixedseverity fire in the western Cascade Range of Oregon. Rates for tree mortality in 

the fire, postfire mortality, snag fall, and snag fragmentation all showed distinct 

patterns by tree diameter and species, with Douglas-fir (Pseudotsuga menziesii 

(Mirb.) Franco) more likely to survive a fire, and to remain standing as a snag, than 

other common tree species. Natural seedling regeneration was abundant, rapid, and 

highly variable in space. Densities of seedlings >10 cm height at 14 years postfire 

ranged from 1,530 to 392,000 per ha. Seedling establishment was not concentrated 

in a single year, and did not appear to be limited by the abundant growth of shrubs. 

The simultaneous processes of mortality, snag fall, and tree regeneration increased 

the variety of many measures of forest structure. The singular event of the fire has 

increased the structural diversity of the landscape.

Keywords: Fire severity, monitoring, coarse woody debris, reforestation, snag 

recruitment, regeneration.
Summary

We monitored natural tree regeneration and coarse woody debris dynamics over 14 

years following the 1991 Warner Creek Fire, a 3,631-ha (8,972-ac) mixed-severity 

fire in the western Cascade Range of Oregon. The results provide a detailed picture 

of the variety of natural postfire conditions in westside Cascade forests, and demonstrate how a single mixed-severity fire can increase variety in forest structures.

We studied 24 permanent plots representative of the burned area and its mix of 

fire severities, which ranged from 0.1 to 100 percent of prefire canopy killed. Plots 

spanned higher elevation forest dominated by Abies amabilis (Douglas ex Louden) 

Douglas ex Forbes and Tsuga mertensiana (Bong.) Carrière, and lower elevation 

forest dominated by Pseudotsuga menziesii (Mirb.) Franco and Tsuga heterophylla 

(Raf.) Sarg. Tree mortality resulting from the fire was strongly related to diameter 

and species, with larger diameter trees and P. menziesii more likely to survive. Tree 

mortality continued in the years after the fire. Forty-four percent of trees alive at 1 

year postfire were dead at 6 to 7 years postfire, and 55 percent at 14 years postfire. 

This postfire mortality rate was higher in plots with higher initial fire severity. 

Much of the new snag volume quickly transferred to the ground via snag fall and 

fragmentation. Small snags, and A. amabilis snags, were more likely to fall. Larger 

snags were more likely to fragment. 

Natural seedling regeneration was abundant, and highly variable in space. Densities of seedlings >10 cm height at 14 years postfire ranged from 1,670 to 392,000 

per ha in the higher elevation zone, and from 1,530 to 47,500 per ha in the lower 

zone. New seedlings were observed over all study visits, in patterns demonstrating 

that establishment was not concentrated in a single year. The growth of shrubs was 

abundant but did not appear to be limiting tree regeneration.

The mixed-severity fire, followed by variable mortality, snag fall, and tree 

regeneration, led to an increase in variability, among plots, of several metrics of 

forest structure. The prominence of P. menziesii among both survivors and new 

seedlings gave some stands a more distinctly two-cohort structure, which may 

have the potential to develop old-growth features relatively quickly. The fire has 

increased structural diversity on the landscape, a legacy that should persist as these 

stands develop.
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Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

Introduction

Fire is a defining element in the ecology of moist, productive forests of the Pacific 

Northwest dominated by Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). In 

these forests, which are common in Oregon and Washington west of the Cascade 

Crest, and which stretch into portions of British Columbia and California, highseverity fires are a major initiator of new forest stands, while less severe fires 

contribute to the development of multicohort stands (Agee 1996, Oliver and Larson 

1996). Tree mortality caused by fire creates snags, which are crucial as wildlife 

habitat (Hutto 2006), but are not a permanent fixture of the landscape, because they 

decay and fall.

One current theme in forest management is emulating the range of historical 

conditions fostered by natural fire and regeneration (e.g., Halpern et al. 1999). In 

particular, concepts about the effects of disturbance have played a role in plans to 

promote the development of “mature” or “old-growth” forest structures (Zenner 

2005), for example, with thinning (Comfort et al. 2010). But these efforts, as well as 

general scientific understanding, have been stymied by a scarcity of clearly quantified, direct field observations of natural forest establishment and of snag creation 

and decay after fire.

When the Warner Creek Fire burned in the western Cascade Range of Oregon 

in October 1991, the area became a focal point for debate over forest management 

in the zone of the northern spotted owl (Strix occidentalis caurina). A 1700-ha 

portion of the burn was set aside as a “natural succession area,” to be allowed to 

develop with little human interference (USDA FS 1993). This report is one outcome 

of that decision. It documents natural changes at the fire site from 1991 to 2005. 

The results to date serve as a detailed test case to evaluate several existing notions 

about forest regeneration and coarse woody debris (CWD) dynamics. They should 

also provide land managers with an example of the range of forest structures and 

regeneration trajectories that might be expected in the wake of a large, mixedseverity fire.

In this region, natural regeneration of forests after disturbance has been a subject of debate among foresters and ecologists. Most of the discussion has concerned 

regeneration after harvest rather than fire. Although it is not clear whether these 

two processes are identical, discussion of them has occasionally been conflated, so 

we consider both as background to our study. In the first half of the 20th century, 

natural regeneration by seed was the standard method of regeneration after harvest 

(Isaac 1938), but early surveys of harvested land gave the technique a reputation 

as being unreliable for the purposes of silviculture. Isaac (1938) reported that 20 

percent of harvested areas in the Northwest had completely failed to regenerate, 
2

Research paper PNW-rp-592

while another 50 percent had regenerated inadequately. Planting nursery-grown 

seedlings became standard practice in harvested areas and is still strongly encouraged by Oregon law, which requires the state forester to approve any plan for 

natural reforestation (Oregon Revised Statutes 527.745). Since the 1960s, regeneration research in this region has focused on optimizing the survival and growth of 

planted seedlings, and few quantified descriptions of natural regeneration after 

harvest have been published. One study showed densities of naturally regenerating 

P. menziesii at >800 trees/ha 40 years after harvest (Miller et al. 1993). In general, 

natural regeneration after harvest was considered unreliable owing to unpredictable 

seed source and the presence of competing vegetation (Minore and Laacke 1992, 

Tappeiner et al. 1997b).

For some, these expectations extended beyond harvested sites to burned areas. 

After the Biscuit Fire in southwestern Oregon, Sessions et al. (2004) warned that 

“without planting and subsequent shrub control... it could take more than 100 years 

to even establish conifer forests.” Some “retrospective studies” have echoed this 

idea of slow or sparse regeneration after fire. These works examine current-day 

stands and project their structure backward through time, suggesting that at some 

sites, establishment of P. menziesii can span a century or more (Franklin and 

Hemstrom 1981, Huff 1995, Tappeiner et al. 1997a), with trees growing in the open 

at low densities (e.g. about 100 trees/ha, Tappeiner et al. 1997a). 

In contrast, direct field observations of natural regeneration after fire show 

more abundant regeneration. Such studies have been accumulating from all over the 

region since the 1970s, but to our knowledge have not been synthesized, as we do 

in figure 1. These studies show P. menziesii establishing in considerable densities 

(often 1,000 to 5,000 trees/ha) in a few decades after disturbance, over a wide range 

of Pacific Northwest forest types and climate zones. Similar numbers can be found 

in at least one retrospective study (Winter et al. 2002). It is unclear if the slow, 

sparse patterns from some retrospective studies are an ecologically distinct mode of 

regeneration, or an artifact of method, as Tepley (2010) has suggested.

Questions also persist about the mechanics of seedling establishment, in particular its continuity and the importance of fortuitous seed years. In a study at the 

Warner Creek Fire site unrelated to ours, Larson and Franklin (2005) found abundant regeneration 11 years after the fire, except in low-fire-severity areas, and attribute it to a bountiful seed year and a legacy of viable seeds that survived the fire. 

This suggests generally that the regeneration occurred in a single pulse. A different 

postfire study, from northern California, shows steady ongoing establishment over 

more than a decade (Shatford et al. 2007). Another issue is the role of potentially 

competing vegetation. Field experiments in harvested areas have suggested that the 

In this region, 

preconceptions about 

the speed and density 

of natural regeneration 

after fire have not been 

tested against direct 

observations.
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Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

simultaneous regeneration of shrubs alongside seedlings can hinder survival and 

growth of planted seedlings (e.g., Stein 1999). However, Shatford et al. (2007) found 

no interference. It is unclear if these divergent results arise from chance or something substantially different about the postfire and postharvest environments.

Less controversial than natural regeneration, but equally important to forest 

function, is the fate of coarse woody debris created in the fire. Federal land management agency biologists try to create or maintain natural snag densities using 

historical guidance provided by information systems such as DecAid (Marcot et 

al. 2010), and predictive models like the Coarse Wood Dynamics Model (CWDM) 

(Mellen and Ager 2002). Snag abundance and longevity after fire are influenced by 

several key factors, including the types of trees most likely to perish and become 

snags, and the length of time those snags remain standing, without breaking or 

falling down. Other studies have suggested strong effects of diameter and species; 

in particular, P. menziesii appears to be considerably more resistant to death by fire 

(Ryan and Reinhardt 1988), and, once converted to a snag, more resistant to falling (Parish et al. 2010). The expectations of managers, and realism of models like 

CWDM, could be improved with detailed data about tree mortality, snag fall, and 

snag fragmentation from a postfire landscape. 

Figure 1—Literature review of direct observations of natural regeneration after fire. Open 

symbols represent seedlings of any species, while filled symbols represent Pseudotsuga

menziesii only. The reference line represents a token silvicultural standard for regeneration, 

494 seedlings per hectare (Oregon Revised Statutes 527.745).
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Research paper PNW-rp-592

The Warner Creek area was a promising place to study CWD dynamics and 

tree regeneration because the fire spanned two major forest elevation zones and 

involved a range of fire severities. The central region of the western Cascades is 

characterized as a mixed-severity fire regime, in which a single fire can create a 

landscape mixture of higher and lower severity patches or stands (Tepley 2010). 

The Warner Creek Fire clearly illustrated this pattern. Added to this spatial variety 

in severity was some variety in the prefire age, stature, and composition of the forest stands, and some potential variety in postfire phenomena such as regeneration. 

These multiplying contingencies might add to the variation in forest structure on 

the landscape, or alternatively, the main structural effect of the fire might be one 

of simplification.

Our study asked:

1. What were the rates and patterns of fire-related tree mortality, snag fall, 

and snag fragmentation?

2. What were the rates and patterns of postfire tree establishment and growth? 

Was establishment momentary or ongoing? Was tree regeneration negatively affected by competing vegetation?

3. How did the structure of Warner Creek forests change from prefire conditions to 14 years postfire? Are postfire stands more or less variable in structure than prefire ones?

Although basic questions of forest ecology provide the structure of this work, 

the pragmatic concerns of land managers inform the way the results are reported. 

Many graphs and tables present results in relatively detailed, unreduced form—

more detailed than is strictly necessary for the testing of hypotheses—so that 

readers can see the true range of natural variation and extract the information most 

relevant to them. 

Methods

Study Area and the Fire Event

The Warner Creek Fire burned 3631 ha of land on October 10–23, 1991, in the 

Cascade Range near the town of Oakridge, Oregon, approximately 43° 42' N., 122° 

12' W. (USDA FS 1993). Elevations in the burned area range from roughly 700 to 

1800 m. Physiography is diverse, with broad valley floors; steep, dissected and 

straight slopes; bowl-shaped headwalls; and broad ridgelines. The climate displays 

transitions from maritime to Mediterranean and produces wet, mild winters and 

warm, dry summers. Approximately 90 percent of precipitation (120 to 200 cm/yr) 
5

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

occurs October though March, mostly as rain, with snow accumulating at higher 

elevations. East winds can occur during the dry months of summer and early fall 

(Gedalof et al. 2005, Kushla and Ripple 1997).

Forest stands in the burned area can be divided into two zones based on elevation and major tree species. The “western hemlock zone” (<1500 m) is composed 

mostly of the “western hemlock series” (McCain and Diaz 2002), where Tsuga heterophylla (Raf.) Sarg., Pseudotsuga menziesii, and Thuja plicata Donn ex D. Don 

dominate the tree layer. Some dryer sites in this zone are of the “Douglas-fir series” 

(McCain and Diaz 2002), where the tree layer is dominated by P. menziesii in association with Arbutus menziesii Pursh and Castanopsis chrysophylla (Douglas ex 

Hook) A. DC. The “Pacific silver fir zone” (>1500 m) is composed of forests of the 

“Pacific silver fir series” and “mountain hemlock series” (McCain and Diaz 2002), 

where the canopy is dominated by Abies amabilis (Douglas ex Loudon) Douglas ex 

Forbes and Tsuga mertensiana (Bong.) Carrière, respectively, in association with P. 

menziesii, Abies procera Rehder, and Chamaecyparis nootkatensis (D. Don) Spach.

The fire burned in a patchwork of severities ranging from near 0 percent 

canopy killed to 100 percent crown consumption, according to a map of live cover 

created from aerial photos in the weeks after the fire (Bailey 1991). About 1700 

ha were set aside as a “natural succession area” where postfire processes were to 

be largely undisturbed by human activity (USDA FS 1993). Our study plots were 

located in this area. However, several treatments applied from aircraft may have 

affected our sites. About 750 ha out of the total burn area of 3631 ha were fertilized, and about 1200 ha out of 3631 ha were seeded with the annual grasses Lolium

annuum (Bernh.) and Hordeum vulgare (L.) (USDA FS 1992). L. annuum was 

noted in 5 of our 24 plots at 1 year postfire, but had largely disappeared by 6–7 

years postfire.

Fire Severities and Study Plots

Our sampling objective was to survey ecological responses across a set of plots 

representative of the mix of fire severities in the burned area. Fire severity is an 

inconsistently applied but unavoidable concept in fire ecology (Keeley 2009). There 

is a need to provide an index of significance of a fire, but because actual measurements of energy flux, or “fire intensity,” are usually impractical, fire severity 

describes an observed change in matter—for example, the percentage of canopy 

scorched or removed, observed via satellite or aerial photo (Odion et al. 2004), the 

percentage of stems or basal area killed (Larson and Franklin 2005), or the visible 

effect on soil (Smucker et al. 2005). We chose to use a measure of canopy damage 

because this could be evaluated both from the air and the ground.

Our sampling objective 

was to survey 

ecological responses 

across a set of plots 

representative of the 

mix of fire severities in 

the burned area.
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Research paper PNW-rp-592

Our initial estimate of fire severity came from a November 1991 map of 

percentage live tree canopy cover created from post-fire aerial photos (Bailey 1991). 

In these dense forests, it was reasonable to assume that prefire live canopy cover 

was near 100 percent, so we subtracted live cover after the fire from 100 percent to 

calculate a new map of percentage cover killed by the fire. We selected prospective 

areas for plots based on this map and located plots randomly within those areas. 

Once plots had been established, we evaluated fire severity within each one by 

examining the canopies of all trees individually, from the ground, as described in 

more detail under “Stem Survey in 1992,” below.

The distribution of fire severities across the Warner Creek burn and across 

our study plots are compared in figure 2. There is a general agreement in the “U” 

shape of the lines. The biggest divergence is among the lowest fire severities, where 

we surveyed more low-severity (10 to 30 percent canopy killed) plots, and fewer 

Figure 2—Distribution of fire severity in the burned area and in our samples. “Acres burned” comes 

from an aerial photo survey completed shortly after the fire (Bailey 1991). “Complete plots” have all 

survey data for all study years. “All plots” are all plots in table 1, complete or not.
7

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

very low severity (<10 percent killed) plots than the aerial photo map displayed. 

However, estimating very low fire severities using aerial photos is difficult. We 

conclude our survey is reasonably representative of the entire burned area, and 

acceptable given the nature of field research and the difficulty of the terrain.

We installed 24 plots: 13 in the Pacific silver fir zone, and 11 in the western 

hemlock zone (table 1). Each study plot was a rectangle, 30 by 24 m, oriented with 

the 30-m sides roughly parallel to the slope. This 720-m2 area was divided into a 

grid of 80 fixed 3 by 3 m cells. Once established, we visited each study plot three 

times, first from June through September 1992 (1 year postfire), next from June 

through September 1997 or 1998 (6 or 7 years postfire), and finally from June 

Table 1—Plots studied

Plot Fire Prefire Years trees Years seedlings

no. severitya Forest series ageb Elevation Aspect Slope surveyed surveyed

Percent Years Meters Degrees Percent

Pacific silver fir zone:

2 0.1 Pacific silver fir >200 1436 218 55 1992, 1997–1998 1992, 1997–1998

5 35.9 Mountain hemlock >200 1672 342 26 1992, 1997–1998, 2005 1992, 1997–1998, 2005

6 100 Mountain hemlock >200 1652 316 33 1992, 1997–1998, 2005 1992, 1997–1998, 2005

7 100 Pacific silver fir >200 1573 170 66 1992, 1997–1998, 2005 1992, 1997–1998, 2005

9 85.9 Pacific silver fir 100–200 1539 130 35 1992, 1997–1998, 2005 1992, 1997–1998, 2005

10 13.7 Pacific silver fir >200 1506 24 30 1992, 1997–1998, 2005 1992, 1997–1998, 2005

11 100 Pacific silver fir 100–200 1603 20 55 1992, 1997–1998, 2005 1992, 1997–1998, 2005

12 58.3 Pacific silver fir >200 1561 76 55 1992, 1997–1998, 2005 1992, 1997–1998, 2005

13 100 Pacific silver fir 100–200 1487 222 70 1992, 1997–1998, 2005 1992, 1997–1998, 2005

14 5.7 Pacific silver fir 100–200 1567 126 65 1992, 1997–1998 1992, 1997–1998

23 77.7 Pacific silver fir 100–200 1686 50 80 1992, 1997–1998, 2005 1997–1998, 2005

24 96.2 Mountain hemlock 50–99 1713 310 42 1992, 1997–1998, 2005 1992, 1997–1998, 2005

25 85.2 Pacific silver fir >200 1524 340 5 1992, 1997–1998, 2005 1997–1998, 2005

Western hemlock zone:

1 100 Western hemlock >200 1329 182 57 1992, 1997–1998, 2005 1992, 1997–1998, 2005

3 10.9 Western hemlock >200 945 58 78 1992, 1997–1998, 2005 1992, 1997–1998, 2005

4 1.5 Western hemlock >200 1032 3 65 1992, 1997–1998, 2005 1992, 1997–1998, 2005

15 18.8 Douglas-fir >200 1219 235 49 1992, 1997–1998, 2005 1992, 1997–1998, 2005

16 100 Western hemlock >200 1170 240 60 1992, 1997–1998, 2005 1992, 1997–1998, 2005

17 28.6 Douglas-fir 50–99 981 200 63 1992, 1997–1998, 2005 1992, 1997–1998, 2005

18 65.5 Western hemlock >200 896 210 19 1992, 1997–1998, 2005 1992, 1997–1998, 2005

19 27.9 Western hemlock >200 829 320 50 1992 1992

20 6.4 Western hemlock 50–99 853 240 53 1992, 1997–1998, 2005 1992, 1997–1998, 2005

21 100 Douglas-fir 100–200 1237 164 53 1992, 1997–1998, 2005 1992, 1997–1998, 2005

22 100 Douglas-fir 100–200 1198 158 30 1992, 1997–1998, 2005 1992, 1997–1998, 2005

a

 Fire severity = the percentage of canopy killed, evaluated from ground observations at 1 year postfire.

b

 Prefire age = the prefire age of dominant trees.
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Research paper PNW-rp-592

through September 2005 (14 years postfire). A few plots were not resampled in 

1997/1998 and 2005 owing to difficult field conditions (see table 1).

Stem Survey in 1992

A survey of all stems (standing trees, live or dead) provided information about tree 

and snag structure, tree mortality, snag fall, and snag fragmentation. In 1992, we 

recorded the following information for every stem >2.5 cm diameter at breast height 

(DBH) in each grid cell in each plot: species, live or dead status, diameter, and by 

extension, diameter classes frequently used within the Forest Service: saplings and 

poles combined (diameter 7.7 to 22.8 cm), small (22.9 to 53.2 cm), medium (53.3 

to 81.2), large (81.3 to 121.8 cm), and giant (>121.9 cm). We assigned each snag a 

decay class from 1 to 5, with class 1 being recently dead and class 5 being almost 

completely decayed (Cline et al. 1980). We assumed that class 1 snags found in 

1992 were casualties of the Warner Creek Fire, which allowed us to reconstruct the 

prefire tree and snag structure of each plot.

Not all trees and snags were evaluated for height during the 1992 visit. We 

measured the height of the first three trees and snags encountered of each species 

in each diameter class per plot using a clinometer and distance tape. We estimated 

the volume of snags by assuming they were shaped as the frustum of a cone, with 

taper constants for each species taken from a sample of recently fallen snags. We 

estimated the volumes of snags that were not measured for height using average 

observed snag heights from each diameter and decay class.

During this visit, we also evaluated canopy condition for every stem that had 

a canopy (live, or snag decay class 1), by recording the proportions of canopy that 

were live, scorched (dead, but needles still visible), or consumed (dead, with needles 

consumed). It was not unusual for a single tree canopy to display more than one 

condition. We used this survey of canopy condition for individual trees to calculate 

a measurement of fire severity for each plot, averaging the percentage of dead 

canopy across all the trees in each plot, and weighting by stem basal area.

Stem Surveys in 1997/1998 and 2005

We repeated this stem survey during a second set of field visits (spread over 

1997/1998) and a third (in 2005). On these visits we relocated specific individual 

stems with the help of the 80-cell grid, diameter measurements, and 1992 field 

notes. On these second and third visits, we noted any change in the status (live/

dead, decay class, height, standing or fallen) of each stem. About 8 percent of 

1992’s stems (mostly small-diameter snags) could not be relocated in 1997/1998; 
9

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

in all likelihood they had fallen and become part of small-diameter debris on the 

forest floor. During the 1997/1998 and 2005 field visits, we measured heights of 

nearly all standing trees and snags with a laser rangefinder. We ignored height 

decreases of <10 percent from 1992 to 1997/1998, because they may have resulted 

from the change in height measurement method. We did not record canopy condition in 1997/1998 or 2005.

Definition of fallen and fragmented snags— 

When an entire snag, including its root wad, toppled to the ground between two 

visits, we characterized the snag as “fallen.” When some part of the snag remained 

standing between two study visits, but decreased in height by ≥10 percent, we characterized the snag as “fragmented.” 

Coarse woody debris on the ground— 

To measure log volume on the ground, we inventoried all logs >7.6 cm diameter 

in each 3 by 3 m cell in each plot in 1992, 1997/1998, and 2005. This inventory recorded the lengths of logs in five Forest Service diameter classes mentioned earlier. 

We classified each log by decay class (Sollins 1982), with class 1 being recently 

dead and class 5 almost completely decayed. Within each plot, cell, diameter class, 

and decay class group, we calculated the volume of logs by assuming that logs were 

cylinders and applying the observed length and the quadratic midpoint diameter for 

the diameter class. Taper was assumed to be negligible as log length was only being 

measured within the relatively small 3 by 3 m cell. We estimated the prefire (1991) 

volume of logs as the 1992 log volume, minus the 1992 volume of logs with decay 

class 1, which we assumed had been killed and felled by the fire.

Regeneration cover survey— 

To make an assessment of the abundance of regenerating vegetation, we evaluated 

cover in eight 3 by 3 m cells in each plot. In 1992 and 1997/1998 we recorded the 

total cover of shrubs, tree seedlings, and herbs in each cell without reference to species. We use the word “herbs” in a general sense to also include ferns and mosses. 

In 2005, we recorded those totals, but in addition recorded a cover value for each 

species in each cell, and a maximum height for the most abundant species of each 

life form (seedling, herb, or shrub). Later, we summarized these cover values and 

heights in several ways. We averaged cover values for each life form among cells 

to create a cover value for each life form in each plot. We calculated the maximum 

height for each life form in each cell, and then averaged those heights to create a 

representative height for each life form in each plot.
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Research paper PNW-rp-592

“Subplot” and “cell” surveys of regeneration density— 

To reduce the risk of trampling tiny tree seedlings, we did not initially study tree 

establishment in the 720-m2 plot area. Instead, we established eight circular “seedling subplots,” each with a radius of 1 m (area 3.14 m2

), in a standard geometrical 

pattern just outside the 720-m2 plot. In 1992, 1997/1998, and 2005, we visited each 

seedling subplot and counted all seedlings. We tallied seedlings by species in height 

classes: <1 cm, 1 to 10 cm, 11 to 30 cm, 31 to 100 cm, and 101 to 300 cm. Seedling 

species were identified using keys in Franklin (1961). Abies species are difficult to 

distinguish when they first germinate, and some error in identifying Abies probably 

occurred.

By 2005, the vigorous growth of individual trees and shrubs was, in places, 

overwhelming the small 3.14 m2 seedling subplots. Any future monitoring would 

require a larger subplot. Also, the vigor of the regrowth suggested that our concerns about trampling had been overblown. Accordingly, in 2005, we also counted 

seedlings in eight of the “cells” composing the larger tree plot. These were the same 

9-m2 cells where we evaluated shrub and herb cover. This “cell survey” of seedlings 

uses a total area almost three times as large as the “subplot survey.” However, to 

keep labor to a manageable level, in the cell survey we only counted seedlings >10 

cm in height.

Sample sizes and calculations— 

Our primary unit of study is the “plot:” a defined area that is sampled in some detail, mostly for the purpose of creating summary statistics (such as tree density per 

area, snag volume per area, etc.) that can be compared among plots. Our sample 

has 24 plots. Because difficult field conditions made some surveys impossible in 

1997/1998 and 2005, some surveys were missed (table 1), so effective sample sizes 

range from 19 to 24 when the vegetation zones are examined together, 9 to 13 for 

the Pacific silver fir zone, and 10 to 11 for the western hemlock zone.

For some results, our data offers another sampling perspective with larger 

sample sizes. Because our set of plots is arguably representative of the burned area 

(fig. 2), the complete collection of live and dead trees that make up those plots 

should also be representative of the trees in the burned area. We used this sample 

of >1,500 stems, without reference to plot, to calculate tree mortality proportions, 

snag fall proportions, and snag fragmentation proportions for species and diameter 

classes. Confidence limits for these proportions were calculated using the normal 

approximation of the binomial parameter. We then converted these proportions 

to annualized mortality, snag fall, and fragmentation rates for easier comparison 

with the literature. Because the second sample period (1997/1998) spanned 2 years, 

relevant annualized rate calculations used a midpoint year value of 1997.5. 
11

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

Results

Overall Trends

Several overarching results apparent in the survey data are illustrated in figure 3. 

The fire is the immediate cause of widespread, but not complete, mortality among 

trees > 2.5 cm DBH, visible as a sharp declining line between 0 and 1 years 

postfire (top row of the figure). This mortality is also reflected in a corresponding 

increase in snag density (top row) and volume (second row). Between 1 and 6–7 

years postfire, there is some additional mortality, visible as a decrease in live tree 

density, and an increase in snag volume. Despite the increase in snag volume, by 

6–7 years postfire, snag density is actually declining, and log volume increasing, as 

snags created by the fire fall to the ground. This increase in log volume continues 

through 14 years postfire. Meanwhile, the ground layer is becoming occupied with 

herbs, shrubs, and thousands of new tree seedlings. Seedlings (bottom row of fig. 3) 

established and grew in such numbers that by 14 years postfire many have DBH’s 

> 2.5 cm and are showing up as increases in live tree density (in the top row). The 

abundance of regeneration is also visible in appendix 1, which photographically 

compares three plots between 6–7 and 14 years postfire. 

The clarity of these trends in averages obscures an intriguing variety in the 

unsummarized plot values for those same variables, notable in figure 4. When the 

standard deviations of key structural measurements are plotted across time, as in 

figure 5, it suggests that the Warner landscape at 14 years postfire is not merely different than its prefire state, it is notably more variable. Appendix 2 photographically 

illustrates an assortment of regeneration and canopy conditions at 14 years postfire.

In the following sections, we explore each of these trends in more detail before 

returning to investigate the increase in structural variability suggested by figures 4 

and 5.

Live Tree Populations and Mortality in the Fire

Live tree densities are listed by time period in table 2. Before the fire, the majority 

of living stems in Pacific silver fir zone plots were smaller diameter (mean 20.5 

cm DBH) Abies amabilis or Tsuga mertensiana with an occasional P. menziesii or

Abies procera of larger stature (42 to 141 cm DBH). In the western hemlock zone, 

the majority of stems were larger P. menziesii (mean DBH 60.5 cm, range 36 to 157 

cm), with some smaller diameter Thuja plicata and Tsuga heterophylla mixed in.

Only 28 percent of trees >2.5 cm DBH alive before the autumn 1991 fire were 

still living in autumn 1992. Survival was strongly associated with tree diameter, as 

figure 6 (top left panel) shows. There was a significant correlation between diameter class midpoint and survival rate (Pearson correlation = 0.83, p = 0.01), with 
12

Research paper PNW-rp-592

Table 2—Live trees of sapling size (2.5 cm DBH) or above in the Warner Creek fire area

Years Density Diameter at breast height

Species after fire Mean Minimum Maximum Mean Minimum Maximum Frequencya

- - - - - - - - Trees/ha - - - - - - - - - - - - - - - Centimeters - - - - - - -

Pacific silver fir zone (n = 11 plots):

Abies amabilis 0 474 14 1,514 20.5 8.7 39.1 11

1 76 0 222 26.0 14.4 55.6 6

6–7 22 0 125 9.7 4.2 13.7 3

14 37 0 194 12.8 5.5 19.8 3

Abies procera 0 19 0 56 78.1 42.5 111.8 6

1 11 0 42 80.1 55.9 111.8 5

6–7 5 0 28 73.0 69.9 76.2 2

14 15 0 125 33.4 5.1 72.4 3

Pseudotsuga menziesii 0 35 0 194 99.4 49.2 141.4 4

1 5 0 42 140.5 139.7 141.4 2

6–7 5 0 42 140.5 139.7 141.4 2

14 254 0 1,542 28.3 5.1 141.4 6

Tsuga mertensiana 0 107 0 403 37.7 21.6 68.6 8

1 37 0 292 44.6 12.1 104.1 5

6–7 16 0 153 22.2 7.6 36.7 2

14 15 0 139 23.7 10.2 37.3 2

All speciesb 0 665 208 1,778 30.4 13.7 54.9 11

1 133 0 431 43.1 26.2 66.0 7

6–7 49 0 222 62.6 32.3 139.7 4

14 323 0 1,542 14.9 5.1 42.6 8

Western hemlock zone (n = 10 plots):

Pseudotsuga menziesii 0 260 28 486 60.5 35.9 157.5 10

1 124 0 319 71.5 46.8 157.5 7

6–7 106 0 347 75.1 47.4 157.5 6

14 813 28 4,458 45.7 5.1 157.5 10

Thuja plicata 0 49 0 222 40.5 19.1 85.9 5

1 11 0 69 62.5 38.1 85.9 3

6–7 7 0 42 65.2 38.1 96.5 3

14 7 0 42 64.9 38.1 95.7 3

Tsuga heterophylla 0 69 0 222 27.4 14.2 53.3 7

1 6 0 28 44.0 20.3 76.2 3

6–7 3 0 14 45.7 15.2 76.2 2

14 4 0 14 31.3 5.1 73.7 3

All speciesc 0 422 125 583 44.1 28.1 94.5 10

1 151 0 319 59.7 45.0 102.6 7

6–7 121 0 347 63.7 47.4 113.5 6

14 829 83 4,472 39.1 5.1 112.6 10

a

 Frequency = the number of plots on which the species was observed. 

b

 “All species” for the Pacific silver fir zone include less common species Abies grandis, Calocedrus decurrens, Chamaecyparis 

nootkatensis, and Pinus monticola. c

 “All species” for the western hemlock zone include less common species Abies grandis, Acer macrophyllum, Calocedrus decurrens, 

Cornus nuttallii, and Taxus brevifolia.
13

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

Figure 3—Mean values for key stand metrics compared to time since the fire. Each point 

represents the mean of plots for that vegetation zone for that study period. Top row: mean live 

and dead stem densities per plot. Second row: mean snag, log, and total coarse woody debris 

volume per plot. Third row: mean cover of regenerating herbs, shrubs, and tree seedlings per 

plot. Fourth row: Mean densities of regenerating tree seedlings per plot, in three height classes: 

all sizes, >10 cm high, and >30 high. Seedling densities are in thousands per hectare. N = 11 

plots for Pacific silver fir zone, 10 plots for western hemlock zone; for seedling density panel, 

N = 9 plots for Pacific silver fir zone, 10 for western hemlock zone.
14

Research paper PNW-rp-592

Figure 4—Raw data for four key stand metrics: live tree density (trees >2.5 cm diameter at breast 

height), snag volume, shrub cover, and seedling density. Each circle represents the observed value for 

one plot in one study period. All available plots used.
15

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

Figure 5—Standard deviation of key stand metrics compared to time since the fire. Identical to 

figure 3 except that each point represents a standard deviation instead of a mean.
16

Research paper PNW-rp-592

Figure 6—Tree survival rates calculated for several intervals since the fire, with 90 percent confidence limits. All rates expressed in 

annual (per-year) terms. DBH = diameter at breast height. Top row: survival rates for 15-cm DBH classes centered on 7.5 cm, 22.5 

cm, etc., with all species combined. Bottom row: survival rates for species (all DBHs combined), ABAM = Abies amabilis; ABGR 

= Abies grandis, ABPR = Abies procera, CADE3 = Calocedrus decurrens, PSME = Pseudotsuga menziesii, THPL = Thuja plicata, 

TSHE = Tsuga heterophylla, TSME = Tsuga mertensiana. Rates are not reported for species or DBH groups with ≤5 live trees to 

start, or for which calculated confidence limits ranged to <0 or >1.
17

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

~82.5 cm-DBH trees approximately seven times as likely to survive as 7.5-cm-DBH 

trees. The very largest diameter trees (>100 cm DBH) appeared to have a lower 

survival rate (fig. 6, top left panel) during this timespan, though the confidence 

limits for this result overlap substantially with the next lower size class.

Species showed apparent differences in susceptibility to fire, as shown in figure 

6 (bottom left panel). Pseudotsuga menziesii and Abies procera were significantly 

more likely to survive than were Abies amabilis and Tsuga heterophylla (Fisher’s 

exact tests, one-tailed, p < 0.02). By 1 year postfire, about 50 percent of the P. 

menziesii had survived, with survival in all diameter classes; while A. amabilis and 

T. heterophylla survival rates were below 16 percent. Although part of this distinction between species comes from the typically larger DBH of P. menziesii, some 

differences between species persist when diameter class is held constant. Table 3 

has detailed survival rates by species and diameter class.

Delayed Tree Mortality

There was notable mortality of trees during the postfire period (i.e., after 1 year 

postfire). Of the 215 trees alive at 1 year postfire that could be tracked to the end of 

the study, only 56 percent were alive at 6 to 7 years postfire, and 45 percent alive at 

14 years postfire. Such survival rates are easiest to compare with other work when 

converted to annualized (per-year) terms. The annualized survival rate is defined 

as the proportion of trees alive at the beginning of the timeframe that are alive at 

the end, expressed as a per-year probability of surviving. This rate, for all available 

trees and plots, was 0.915 for the early postfire period (1 to 6–7 years postfire), and 

0.960 for the later postfire period (6–7 to 14 years postfire). 

Most of the postfire mortality occurred during the early postfire period, 1 to 

6–7 years postfire. During this period, survival rates followed a trend with diameter 

class that was similar to, but less dramatic than, the trend for 0 to 1 years postfire 

(fig. 6 top row). There was a significant positive correlation between diameter class 

midpoint and survival rate in this period (Pearson correlation = 0.91, p= 0.01), 

but only when the largest diameter trees, >90 cm DBH, were excluded from the 

analysis. Some species differed in survival (fig. 6, bottom row, middle panel). Abies 

amabilis survived at a significantly lower annual rate (0.80) than P. menziesii (0.97) 

(Fisher’s exact test, p < 0.001). Although some of this difference is likely due to the 

larger diameters of P. menziesii, table 3 suggests that this difference persists when 
18

Research paper PNW-rp-592

diameter class is held constant. In the later postfire period (6–7 to 14 years postfire), 

there is no significant correlation based on size, and there were too few surviving 

trees for statistical comparisons among species.

Postfire mortality also has a relationship to initial fire damage to the stand. 

Figure 7 considers plots where fire severity (the percentage of canopy dead by 

1 year postfire) was less than 100 percent. It shows the proportion of trees dying 

between 1 and 6–7 years postfire rises significantly with fire severity.

Table 3—Annualized survival rates, by species, diameter class, and postfire timeframe

Annualized survival ratesa

DBH (cm) midpoint for 15 cm groups

b

Time

postfire Species 7.5 22.5 37.5 52.5 67.5 82.5 97.5 112.5 All sizes

0 to 1 yr Abies amabilis 0.084 0.286 0.300 0.278 0.300 0.158

Abies grandis 0 0.400

Abies procera 0.647

Calocedrus decurrens 0.625

Pseudotsuga menziesii .174 .483 .585 .611 0.684 0.714 0.500 0.502

Thuja plicata 0 .286 0.222

Tsuga heterophylla 0 .273 0.101

Tsuga mertensiana .333 .438 .278 .308 0.341

All species .093 .237 .372 .467 .507 .667 .667 .455 0.280

1 to 6–7 yrs Abies amabilis .903 .764 0.801

Abies grandis 0.882

Abies procera 0.896

Pseudotsuga menziesii .941 .951 .962 0.967

Thuja plicata 0.918

Tsuga mertensiana .819 .903 0.864

All species .886 .855 .908 .921 .952 .964 .857 0.915

6–7 to 14 yrs Pseudotsuga menziesii .934 .950 .976 .939 0.949

All species .965 .942 .952 .981 .956 0.960

a

 Survival rate = the proportion of trees alive at the beginning of the timeframe that are still alive at the end of the timeframe. Expressed as annualized 

rates. Rates are not reported for species/DBH groups with ≤5 live trees to start with, or for which calculated 90 percent confidence limits (not shown) 

spanned to <0 or >1.

b

 DBH = diameter at breast height.
19

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

Snag Fall and Fragmentation

As with fire-related mortality, snag fall in the wake of the fire followed strong 

patterns by diameter class (fig. 8, top row). Smaller diameter snags were more likely 

to fall. Results are easiest to compare to other work when expressed as annualized snag fall rate—i.e., the proportion of snags standing at the beginning of the 

timeframe that have fallen by the end of the timeframe, converted to a yearly rate. 

The annualized snag fall rate for new snags (i.e., snags created by the Warner Creek 

Fire) of 2.5 to 15 cm DBH was 0.159 between 1 and 14 years postfire, whereas for 

45 to 60 cm DBH snags it was 0.007, and for 60+ cm DBH snags, no falling was 

detected. The great majority of snags that fell between 1 and 6–7 years postfire 

were <20 cm DBH.

Snag fall may be related to species as well. Abies amabilis snags were especially likely to fall (fig. 8, bottom row), with an annual rate of 0.137 from 1 to 6–7 

years postfire, compared to 0.005 for P. menziesii (Fisher’s exact test, p < 0.001). 

Though much of this difference is due to the predominantly small diameters of 

A. amabilis snags, some differences between species seem to persist even when 

diameter class is constant; see the detailed rates in table 4.

If a snag did not fall, it was likely to lose some of its height through fragmentation, especially if it stood for more than 7 years. Annualized fragmentation rate is 

the proportion of snags standing at both the beginning and end of the timeframe 

Figure 7—The relationship of mortality in the postfire period (i.e., the percentage of trees 

dying per plot from 1 to 6–7 years postfire) to fire severity (the percentage of canopy dead 

in the plot by 1 year postfire). Pacific silver fir zone plots are circles, western hemlock zone 

plots are squares.

Mortality in the 

postfire period had a 

relationship to initial 

fire severity.
20

Research paper PNW-rp-592

Figure 8—Snag fall rates calculated for several intervals since the fire, with 90 percent confidence limits. Rates are expressed in 

annual (per-year) terms, and were calculated using only “new” snags, i.e., those killed by the 1991 fire. DBH = diameter at breast 

height. Top row: survival rates for 15-cm wide DBH classes centered on 7.5 cm, 22.5 cm, etc., with all species combined. Bottom 

row: survival rates for species, all DBHs combined: ABAM = Abies amabilis; ABGR = Abies grandis, ABPR = Abies procera, 

CHNO = Chamaecyparis nootkatensis, PSME = Pseudotsuga menziesii, TABR = Taxus brevifolia, THPL = Thuja plicata, TSHE 

= Tsuga heterophylla, TSME = Tsuga mertensiana. Rates are not reported for species or DBH groups with ≤5 snags or where 

confidence limits ranged to <0 or >1. Confidence limits not calculated for snag fall values of 0.
21

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

Table 4—Annualized snag fall rates, by species, diameter class, and postfire timeframe

Annualized snag fall ratesa

DBH (cm) binned into 15 cm groupsb

Time

postfire Species 7.5 22.5 37.5 52.5 67.5 82.5 97.5 112.5 All sizes

1 to 14 yrs Abies amabilis 0.228 0.062 .017 0 0.109

Abies grandis .042

Abies procera .031

Chamaecyparis 

nootkatensis .014

Pseudotsuga menziesii .037 .016 0 0 .018

Thuja plicata .041 .019

Tsuga heterophylla .048 .073 .069

Tsuga mertensiana 1.000 0 .035

All species .159 .043 .018 0.007 0 0 0 .059

1 to 6–7 yrs Abies amabilis .236 .019 0 0 0 .137

Abies grandis .059 

Abies procera 0

Chamaecyparis 

nootkatensis 0

Pseudotsuga menziesii 0 0 0 0 .005

Taxus brevifolia 0 0

Thuja plicata .065 .033

Tsuga heterophylla .118 .040

Tsuga mertensiana .171 0 0 .048

All species .179 .015 .007 0 0 0 0 .071

6–7 to 14 yrs Abies amabilis .210 .104 .033 0 .082

Chamaecyparis 

nootkatensis .023

Pseudotsuga menziesii .060 .023 0 0 .026

Tsuga heterophylla .075 .123 .092

Tsuga mertensiana 1.000 .046 0 0 .032

All species .133 .073 .027 0.011 0 0 0 0 .047

a

 Snag fall rate = the proportion of snags standing at the beginning of the timeframe that have fallen by the end of the timeframe. Converted to per-year 

rates as described in the methods. Rates are not reported for species/DBH groups with ≤5 live trees to start with, or for which calculated 90 percent 

confidence limits (not shown) spanned to <0 or >1.

b

 DBH = diameter at breast height.
22

Research paper PNW-rp-592

that have lost at least 10 percent of their height, converted to a yearly rate. Between 

1 and 6–7 years post fire, 17 percent of the 230 snags that could be followed for 

breakage lost height, for an overall annualized fragmentation rate of 0.034. But 

between 6–7 and 14 years, 52 percent of the 223 snags that could be followed lost 

height, for an annualized fragmentation rate of 0.092. Of the snags that remained 

standing through all observation periods (1 year, 6–7 years, and 14 years postfire), 

49 percent broke once, and 12 percent broke twice.

Snags with larger DBHs were more likely to fragment (fig. 9, top row). The 

snags in the 7.5-cm DBH group had an annualized fragmentation rate of 0.022 

between 1 and 14 years postfire, while those in the 67.5-cm DBH group had a rate 

of 0.116 (fig. 9 top row). All of the snags in the 82.5-cm DBH group that remained 

standing from 1 to 14 years postfire, and from 6–7 to 14 years postfire, fragmented. 

This particular result caused us to adjust the calculation of annual snag fragmentation rate for the 82.5-cm group. A raw proportion of 1.0 over any span of years 

converts to a yearly probability of 1.0, which is deceptive, and also makes the confidence limits we have calculated nonsensical. For the 82.5-cm group we provided 

no confidence limits, and estimated the raw proportion of 1.0 as 0.99, an estimation 

that is reflected in the annualized rates.

There may be differences in fragmentation rate between species (fig. 9, bottom 

row). Between 1 and 6–7 years postfire, Tsuga mertensiana was most likely to 

break, with an annual fragmentation rate of 0.112, higher than Tsuga heterophylla, 

Abies amabilis, and Pseudotsuga menziesii (Fisher’s exact tests, all p < 0.04).

Between 6–7 and 14 years postfire, the pattern was different: P. menziesii was most 

likely to break, with an annual fragmentation rate of 0.162, higher than T. mertensiana, T. heterophylla, and A. amabilis (Fisher’s exact tests, p <= 0.05). Though some 

of these differences are undoubtedly attributable to the effect of diameter, some 

differences persist when diameter class is held constant (table 5).

The increase in fragmentation rate from the earlier postfire interval (1 to 6–7 

years) to the later one (6–7 to 14 years) was common among three major tree species (fig. 9 bottom row). Fragmentation rates of Abies amabilis rose from 0.009 to 

0.085. P. menziesii and Tsuga heterophylla also rose dramatically. Tsuga mertensiana did not rise, but had relatively high rates in both sampling periods, 0.112 and 

0.091.

The broken portions of snags falling to the ground were diverse in size, but 

included some very large pieces of wood. Between 1 and 6–7 years postfire, the 

mean absolute height loss (± SD) per broken snag was 13.5 (±8.8) m, with 10 percent of height losses >28 m. Between 6–7 and 14 years postfire, the mean absolute 

height loss was 12.8 (±10.2) m, with 10 percent of height losses again >28 m. When 

Snags with smaller 

DBH’s were likely to 

fall, whereas snags 

with larger DBH’s were 

likely to fragment.
23

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

Figure 9—Snag fragmentation rates calculated for several intervals since the fire, with 90 percent confidence limits. Rates are 

expressed in annual (per-year) terms, and were calculated using only “new” snags, i.e., those killed by the 1991 fire. DBH = 

diameter at breast height. Top row: fragmentation rates for 15-cm wide DBH classes centered on 7.5 cm, 22.5 cm, etc., with all 

species combined. Bottom row: fragmentation rates for species, all DBHs combined: ABAM = Abies amabilis; ABPR = Abies 

procera, CHNO = Chamaecyparis nootkatensis, PSME = Pseudotsuga menziesii, TABR = Taxus brevifolia, THPL = Thuja 

plicata, TSHE = Tsuga heterophylla, TSME = Tsuga mertensiana. Rates are not reported for species or DBH groups with <=5 

snags or where confidence limits ranged to <0 or >1. The calculation of annual rates for the 82.5 cm diameter class involved an 

approximation; see “Results.”
24

Research paper PNW-rp-592

Table 5—Annualized snag fragmentation rates by diameter class, species, and postfire 

timeframe

 Annualized snag fragmentation ratea

 DBH (cm) binned into 15 cm groupsb

Time

postfire Species 7.5 22.5 37.5 52.5 67.5 82.5 All sizes

1 to 14 yrs Abies amabilis 0.020 0.065 0.052 0.050

Chamaecyparis 

nootkatensis .027

Pseudotsuga menziesii .052 .298 0.298 .108

Thuja plicata .037

Tsuga heterophylla .081

All species 0.022 .042 .090 .092 .116 0.298 .070

1 to 6–7 yrs Abies amabilis 0 .009

Abies procera .071

Chamaecyparis 

nootkatensis 0

Pseudotsuga menziesii .071 .118 .097 .049

Taxus brevifolia 0 0

Thuja plicata .040

Tsuga heterophylla .033

Tsuga mertensiana .112

All species .018 .011 .050 .091 .089 .034

6–7 to 14 yrs Abies amabilis .033 .095 .088 .085

Chamaecyparis 

nootkatensis .042

Pseudotsuga menziesii .079 .186 .459 .162

Tsuga heterophylla .061

Tsuga mertensiana .088 .098 .042 .154 .091

All species .029 .054 .105 .094 .129 .459 .092

a

 Snag fragmentation rate = the proportion of snags that remain standing from the beginning to the end of the timeframe, but lose >10 

percent of their height. Converted to per-year rates as described in the methods. Rates are not reported for species/DBH groups with 

≤5 snags to examine, or for which calculated confidence limits (not shown) spanned to <0 or >1. Rates for the 82.5-cm DBH class 

reflect an approximation; see “Results” text.

b

 DBH = diameter at breast height.
25

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

expressed as proportions of prior height lost, the proportions were similarly variable: 0.46 ± 0.19 between 1 and 6–7 years postfire, and 0.46 ± 0.25 between 6–7 

and 14 years postfire. 

Regeneration Species, Cover, and Relative Height

While trees from the prefire stand were dying, falling, and fragmenting, new 

vegetation was emerging. At 14 years postfire, our regeneration cover survey and 

cell seedling survey recorded 15 tree species, 35 shrub species, and more than 70 

herb species regenerating in plots which, in the immediate aftermath of the fire, had 

little or no apparent living understory. The complete list of species and cover values 

is provided in appendix 3. At 14 years postfire, the most abundant shrubs in the 

Pacific silver fir zone were Ceanothus velutinus Douglas ex Hook. (average cover 

per plot 12 percent), Rubus lasiococcus A. Gray (10 percent), and Rubus parviflorus

Nutt. (5 percent); in the western hemlock zone, they were Ceanothus velutinus (15 

percent), Whipplea modesta Torr. (9 percent), and Rubus ursinus Cham. & Schldtl. 

(8 percent). The most abundant herbs (including ferns and mosses) in the silver fir 

zone were Tiarella trifoliata L. (3 percent), Anaphalis margaritacea (L.) Benth. (3 

percent), and Fragaria vesca L. (1.8 percent); in the western hemlock zone, they 

were Lactuca muralis (L.) Fresen. (7 percent), unidentified mosses (6 percent), and 

Polystichum munitum (Kaulf.) C. Presl (2 percent).

The 15 tree species recorded regenerating at 14 years postfire (app. 3) were 

nearly identical to the list of tree species alive before the fire (table 2, including 

footnotes). All species recorded as live trees before the fire, including relatively 

uncommon species such as Pinus monticola Douglas ex D. Don and Taxus brevifolia Nutt., were reflected in the regeneration. Two species, Arbutus menziesii and 

Crataegus douglasii Lindl., were detected in regeneration but not in prefire stands. 

Seedling cover increased in the years after the fire. In the Pacific silver fir 

zone, mean seedling cover rose from around 2 percent at 1 year postfire to 30 percent at 14 years postfire; in the western hemlock zone, it rose from <1 to 15 percent 

(fig. 3, third row). Shrubs dominated the understory, reaching mean values of 35 

and 46 percent at 14 years postfire (fig. 3, third row). Herb cover (including ferns 

and mosses) seemed to peak earlier, and at 14 years postfire was equal to or lower 

than tree and shrub cover in both zones.

Although shrubs were often more abundant than tree seedlings in terms of 

percentage of cover, tree seedlings were frequently taller. Figure 10 compares the 

mean recorded heights of shrubs and tree seedlings in each plot during the regeneration cover survey. When the cover and height for shrubs and trees are compared 

with Wilcoxon paired samples tests with both vegetation zones combined, shrubs 

Although shrubs were 

often more abundant 

than tree seedlings in 

terms of percentage of 

cover, tree seedlings 

were frequently taller.
26

Research paper PNW-rp-592

had significantly higher cover than tree seedlings (p < 0.05), while seedlings had 

significantly greater height (p < 0.03), with a mean of 1.4 m compared to 1.1 m for 

shrubs. This height difference is visible in the concentration of points in figure 10, 

which are above the reference line representing equal heights. If the analysis is split 

into zones, shrubs did not differ in cover from seedlings in the Pacific silver fir 

zone (p > 0.1), while trees were consistently taller there (p < 0.01). In the western 

hemlock zone, shrubs had greater cover than tree seedlings (p < 0.001), but their 

heights did not differ (p > 0.5).

Regeneration Establishment, Density, and Growth

Our seedling subplot survey evaluated the count of seedlings rather than their cover 

values. In the subplot survey, seedlings were detected on all burned plots in one or 

more study visits. The consistently small size of seedlings at 1 year postfire (when 

100 percent of seedlings in the western hemlock zone, and 99.1 percent of seedlings in the Pacific silver fir zone, were <10 cm tall), in addition to the presence 

Figure 10—Comparison of shrub and seedling heights observed in study “cells” at 14 years postfire. 

Eight 3 by 3 m cells were studied per plot. In each cell, the maximum height of each life form 

(seedlings and shrubs) was recorded. Those eight values were averaged to create a mean representative height per plot for each life form, which is graphed here. The reference line represents equal 

heights. Circles are plots in the Pacific silver fir zone; squares are plots in the western hemlock zone. 

In nonparametric paired-samples tests, seedlings are consistently higher than shrubs for both zones 

combined, and for the silver fir zone individually; for the western hemlock zone there is no significant difference see “Results.”
27

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

of cotyledons and only one growth whorl, suggested that all of the seedlings we 

observed established after the fire. Although the subplots contained 16 different 

species of seedlings, listed in table 6, the great majority of them came from just 

three species in each elevation zone. In the Pacific silver fir zone, 88 percent of all 

seedling records over all sampling years were Tsuga mertensiana, Abies amabilis, 

and Pseudotsuga menziesii; and in the western hemlock zone, 95 percent were P. 

menziesii, Thuja plicata, and Tsuga heterophylla. Some of the less common species 

in the seedling subplots, such as Crataegus douglasii (see notes to table 6) had not 

been recorded in the prefire stands.

The total density of seedlings in the subplots was high. In the Pacific silver 

fir zone, mean density of all seedlings per plot was 67,500/ha at 1 year postfire, 

60,200/ha at 6–7 years postfire, and 85,500/ha at 14 years postfire (fig. 3 bottom 

row; table 6). In the western hemlock zone, these figures were 14,300, 22,600, and 

19,500/ha, respectively. Results from the cell survey, available for only 14 years 

postfire, were slightly lower, perhaps because this survey did not count seedlings ≤ 

10 cm height: a mean of 61,400/ha for the Pacific silver fir zone and 11,200 for the 

western hemlock zone.

A substantial number of the earliest seedlings survived and grew. Figure 11 

shows seedling density by height class and time postfire. In general, mean seedling 

density in the tallest height classes (31 to 100 cm and 1 to 3 m) increased from 1 to 

14 years postfire, while densities of the smallest height class (0 to 10 cm) declined 

from extreme levels. The smallest height classes did not diminish to zero, however. 

Mean density of seedlings 0 to 10 cm at 14 years postfire was about 6,800/ha in the 

silver fir zone and about 3,800/ha in the western hemlock zone (fig. 11). 

This last detail suggests that new seedling establishment was not limited to the 

single large group established by 1 year postfire. The ongoing nature of seedling 

establishment is confirmed by an examination of seedling counts in the individual 

subplots. In the Pacific silver fir zone between 1 and 6–7 years postfire, seedling 

counts increased in 36 percent of subplots. Although new seedling establishment 

could have occurred in any subplot, we could not track seedling death—so it is 

accurate to say that new establishment occurred in a minimum of 36 percent of 

subplots. Between 6–7 and 14 years postfire, new seedling establishment occurred 

in a minimum of 43 percent of Pacific silver fir zone subplots. In the western hemlock zone between 1 and 6–7 years postfire, new seedling establishment occurred 

in a minimum of 56 percent of subplots; between 6–7 and 14 years, it occurred in a 

minimum of 35 percent of subplots.

Examining data at the subplot level also demonstrates the extreme nature 

of spatial variability in natural seedling establishment at this fire site. Whereas 

New seedling 

establishment was not 

limited to the single 

large group established 

by 1 year postfire.
28

Research paper PNW-rp-592

Table 6—Seedling densities by species, from the seedling subplots

Pacific silver fir zone (n = 9 plots) Western hemlock zone (n = 10 plots)

Years postfire Years postfire

1 6–7 14 1 6–7 14

Thousands per hectare

Abies amabilis Mean 9.8 10.9 11.2 0 0 0

SD 14.7 18.1 21.5 0 0 0

Minimum 0 0 0 0 0 0

Median 2.4 3.2 2.4 0 0 0

Maximum 45.0 56.9 67.2 0 0 0

Abies grandis Mean 4.5 0.9 1.7 0 0.1 0

SD 7.1 1.9 3.3 0 .3 0.1

Minimum 0 0 0 0 0 0

Median 2.0 0 0.4 0 0 0

Maximum 21.9 5.6 9.9 0 .8 0.4

Abies procera Mean 4.0 1.9 1.8 0 0 0

SD 9.3 3.6 4.4 0 0 0

Minimum 0 0 0 0 0 0

Median 0 0 0 0 0 0

Maximum 28.3 10.7 13.1 0 0 0

Pseudotsuga menziesii Mean 12.7 6.3 6.1 11.0 13.3 13.9

SD 14.7 7.2 7.4 19.3 12.8 13.2

Minimum 0.4 0 0 0 .8 0.8

Median 4.0 4.8 2.4 0.8 9.9 11.1

Maximum 41.0 21.1 18.3 58.1 37.8 40.6

Thuja plicata Mean 0 0 0 3.0 2.9 3.3

SD 0 0 0 6.0 7.3 6.2

Minimum 0 0 0 0 0 0

Median 0 0 0 0 0 .2

Maximum 0 0 0 18.7 23.5 19.1

Tsuga heterophylla Mean .1 .1 0 .2 3.3 1.9

SD .3 .4 .1 .4 5.0 3.2

Minimum 0 0 0 0 0 0

Median 0 0 0 0 0.6 0

Maximum .8 1.2 .4 1.2 14.7 9.2

Tsuga mertensiana Mean 35.7 39.7 64.4 0 0 0

SD 84.2 100.2 172.9 .1 0 0

Minimum 0 0 0 0 0 0

Median 3.2 .8 6.0 0 0 0

Maximum 258.2 305.6 524.8 .4 0 0

All species combineda Mean 67.5 60.2 85.5 14.3 22.6 19.5

SD 100.3 117.0 191.4 21.8 23.7 19.7

Minimum 9.9 3.2 .4 0 .8 .8

Median 37.4 21.1 26.7 1.4 15.7 16.9

Maximum 329.1 370.0 594.5 58.1 74.8 66.1

a

 “All species combined” includes minor species that were observed in the seedling subplots: Acer macrophyllum, Arbutus menziesii, Calocedrus 

decurrens, Chamaecyparis nootkatensis, Crataegus douglasii, Pinus monticola, Prunus sp., Salix sp., and Taxus brevifolia.

SD = standard deviation.
29

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

Figure 11—Mean seedling density per plot and time postfire, by seedling height classes. Sample size for each bar is 9 plots for the 

Pacific silver fir zone and 10 plots for the western hemlock zone. Seedling densities are in thousands per hectare. At 14 years postfire, 

two data sets are available, “subplots” and “cells” (see “Methods”). Cells are larger but were not surveyed for seedlings 0 to 10 cm in 

height.

seedling density varied widely among plots, from 400 to 595,000/ha, density in the 

individual subplots, when expressed in the same units, ranged to more than 1 million. Some 3.14-m2

 subplots had dozens or hundreds of seedlings while neighbors 

had few or none. Coverage was patchy. Figure 12 compares seedling density per 
30

Research paper PNW-rp-592

Figure 12—Relation of seedling density to stocking for three combinations of seedling size class and 

time postfire. Stocking is the percent of subplots or cells occupied by at least one seedling. Seedling 

density is in thousands per hectare. Each symbol represents one plot: circles for the Pacific silver fir 

zone, Xs for the western hemlock zone. Reference lines represent token published silvicultural standards for regeneration. On the Y axis: 494 seedlings per hectare (Oregon Revised Statutes 527.745). 

On the X axis: 50 percent of milacre plots (Stein 1992), comparable in size to our seedling subplots, 

and 70 percent of 4-milacre plots (Stein 1992), comparable in size to our seedling cells.

plot to a rough estimate of “stocking” (the percentage of subplots or cells occupied 

by at least one seedling). Stocking of ≤ 50 percent can be associated with mean 

densities as high as ~25,000/ha.

Seedling density in plots was not strongly correlated with site environment 

and history variables. Comparing seedling regeneration to aspect was problematic, 

because Pacific silver fir zone plots frequently had northerly aspects, and western 

hemlock zone plots southerly ones (table 1). Given that caveat, there was no significant difference in mean density of seedlings at 14 years postfire between northerly 

and southerly sites (Mann Whitney U-test, p > 0.2). There was a weak positive 

relationship between seedling density and fire severity. The lowest densities were 

associated with the lowest fire severities. Using the cell survey data, at 14 years 

postfire seedlings of 30 cm height or greater could be predicted with Y = 0.468 + 

0.0084 * X (r2

 = 0.29, p = 0.01, n = 21), where X is fire severity, in terms of percentage of canopy killed by 1 year postfire, and Y is log(seedling density in thousands 

per hectare, plus 1). In practice this is not a particularly useful predictive relationship because the 90 percent confidence limits are very large, for example at fire 

severity 100 percent the range of predicted values is 1,000 to 100,000 per hectare. 
31

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

Structural and Compositional Change and Variation

The fire and its aftereffects changed forest structure in the burned area substantially. Table 7 compares structural metrics between prefire conditions and 14 years 

postfire. At 14 years postfire, the mean basal area of trees is only 19 percent of 

its prefire value in the Pacific silver fir zone, 50 percent of its prefire value in the 

western hemlock zone, and 35 percent of its prefire value overall (Wilcoxon signed 

ranks tests, p < 0.05). The mean live tree density does not change significantly 

in this interval, in either elevation zone or both zones combined (p > 0.05), as 

regenerating stems replaced killed trees. However, the size distribution of live trees 

changed (fig. 13, top row). In both vegetation zones, the mean density of the largest 

trees (≥ 122 cm DBH) declined about 33 percent, to about 5 per hectare. Sapling 

and pole sized trees (2.5 to 22.9 cm DBH) either suffered a small net reduction in 

density (as in the Pacific silver fir zone) or increased tremendously (as in the western hemlock zone), owing to regeneration. Meanwhile, trees in intermediate diameter classes (“small,” “medium,” and “large” trees, between 23 and 121 cm DBH) 

were decimated, with mean values declining 91, 92, and 94 percent in the Pacific 

silver fir zone and 83, 43, and 50 percent in the western hemlock zone. Older and 

younger cohorts became more distinguishable.

Table 7—Summary of structural changes between prefire conditions and 14 years postfirea

Whole burned area Western hemlock zone Silver fir zone

Magnitude Variation Magnitude Variation Magnitude Variation

Live tree basal area 35b 113 50b 185 19b 65

No. live tree species 51b 72 56b 53 47b 99

Mean live tree DBH 77 170c 89 181c 49 105

Live tree density 101 289 195 947 48 114

Mean snag diameter 132b 64 140 70 128 51

Pseudotsuga menziesii as a 

percentage of basal area 140b 79 114b 69c 178 104

Snag height 194 136 185 151 211 154

Pseudotsuga menziesii as a 

percentage of density 200b 91 144b 67c 352 140

Log volume 275b 138 372b 187 230b 108

Snag density 347b 309c 373b 269c 327b 360

Total CWD volume 456b 383c 569b 445c 402b 386

Snag volume 771b 646c 895b 841c 708b 582

a

 This table compares the magnitude and variation of forest structural metrics at 0 and 14 years postfire. Magnitude is the mean at 14 years postfire, 

expressed as a percentage of the prefire mean. Variation is the standard deviation at 14 years postfire, expressed as a percentage of the prefire standard 

deviation. 

b

 Indicates magnitude differs between the two time periods, according to a Wilcoxon signed ranks test (p < 0.05).

c

 Indicates variation differs between the two time periods, according to a Levene test for equality of variances (p < 0.05).

CWD = coarse woody debris. DBH = diameter at breast height. No trees <2.5 cm DBH are included.
32

Research paper PNW-rp-592

Figure 13—Mean live tree and snag densities per plot and time postfire, by standard Forest Service DBH (diameter at breast 

height) classes: saplings and poles combined (diameter 2.5 to 22.8 cm), small (22.9 to 53.2 cm), medium (53.3 to 81.2), large (81.3 

to 121.8 cm), and giant (>121.9 cm). Sample size for each bar is 11 plots for the Pacific silver fir zone and 10 plots for the western 

hemlock zone.
33

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

Tree species diversity, among live stems >2.5 cm DBH, changed as well, with 

a reduction in the mean number of tree species recorded per plot, from 3.1 to 1.5 

in the Pacific silver fir zone and 3.4 to 1.9 in the western hemlock zone (Wilcoxon 

signed ranks tests, p < 0.05). Because P. menziesii was prominent both among 

survivors of the fire and in new regeneration, its importance in stands increased. 

Table 7 shows that the proportion of stand basal area represented by P. menziesii

increased, on average, 78 percent in Pacific silver fir zone plots, 14 percent in 

western hemlock zone plots, and 40 percent over the whole burned area (increase 

significant for whole burned area and western hemlock zone, p < 0.05, Wilcoxon 

signed ranks test). The proportion of live tree density >2.5 cm DBH represented by 

P. menziesii rose as well, 251 percent in the Pacific silver fir zone, 44 percent in the 

western hemlock zone, and 100 percent overall (all p < 0.05). 

Tree mortality contributed to related changes in the diameter distribution of 

snags (fig. 13, bottom row). At 14 years postfire, mean snag diameter over both 

vegetation zones combined is 41.8 cm, 33 percent larger than in prefire conditions 

(Wilcoxon signed ranks test, p < 0.05). Metrics of CWD abundance, including snag 

volume, snag density, log volume, and total CWD volume increased significantly 

in both zones (Wilcoxon tests, p < 0.05). For example, average log volume in the 

western hemlock zone at 14 years postfire was 364 m3/ha, nearly four times the 

estimated prefire value. The length of logs per hectare increased by similar proportions, rising from 1418 m/ha to 4665 m/ha at 14 years postfire in the western 

hemlock zone, and from 2399 m/ha to 5044 m/ha in the Pacific silver fir zone. Log 

volume increases occurred across a range of diameter classes, suggesting that the 

ground was receiving a steady influx of falling fire-killed wood of various sizes, 

not just a few immense boles.

While the mean values of these structural metrics were changing, the variation 

around those means was changing as well. The raw distributions in figure 4, and 

the standard deviations in figure 5 suggest an increase in variability for many structural variables. Table 7 reports on changes in variability for numerous structural 

metrics between prefire conditions and 14 years postfire. It compares variability 

with the Levene test for equality of variances (NIST 2003), because that test is less 

sensitive than the usual F-test to departures from normality (Zar 1999), which were 

frequent in our data. 

Table 7 suggests that structural changes might be collected into several categories based on relative changes in magnitude and variability. Between 0 and 14 

years postfire, several metrics increased in both magnitude and variability: snag 

density, snag volume, and total CWD volume. Snag height may also fit into this 

category, although test results for it are not significant. In general, we would expect 
34

Research paper PNW-rp-592

variability to rise with magnitude. However, one key metric increased in variability, 

even though its magnitude did not change significantly: mean DBH of live trees 

(p < 0.05 for the western hemlock zone, and the whole burned area). Mean live tree 

density may also fit into this category, though test results for it are not significant. 

Meanwhile, in the western hemlock zone, two metrics decreased in variability, even 

though their mean values increased: the proportions of P. menziesii stems in total 

plot density and total plot basal area (p ≤ 0.05). 

Discussion

Our study of the Warner Creek burn shows that the first 14 years after a forest fire 

can be a very dynamic time—one that belies the image of forests of the Pacific 

Northwest as timeless or slowly developing. We observed rapid and dramatic 

changes: dominant organisms died from the stress of a disturbance, biomass in the 

form of CWD moved quickly from the canopy to the ground, and new individuals 

of both shade-intolerant and tolerant species established and grew in the tens of 

thousands per hectare These processes were not uniform across the burned landscape, and only a portion of the natural range of variation we observed resembles 

the “stand replacement” paradigm of succession.

The Warner Creek Fire and subsequent forest development occurred in a 

limited area in a specific timeframe, so our work is a detailed case study rather 

than a systematic review of postfire development. To what extent can our results be 

generalized? Although specific numbers might be most relevant to mature forests of 

the central Cascade Range of Oregon, we take some confidence from the way our 

results correspond with other field reports from the wider region of Pacific Northwest forests dominated by P. menziesii. Meanwhile, the ecological mechanisms 

observed or implied should have even wider relevance. 

Tree Mortality and Fire Severity

We found that initial mortality from the fire (i.e., between 0 and 1 years postfire) 

followed strong patterns by tree diameter and species (figure 6; table 3). Larger 

diameter trees were less likely to be killed by the fire, perhaps because of the effect 

of increased bark thickness (Ryan and Reinhardt 1988). Pseudotsuga menziesii

lived up to its reputation for resistance to fire (Minore 1979; Ryan and Reinhardt 

1988). Although Calocedrus decurrens (Torr.) Florin and Abies procera had higher 

calculated survival rates than P. menziesii (fig. 6), these species were represented 

in our sample by a low number of high-diameter individuals, so those results are 

tentative.

The first 14 years after 

a forest fire can be a 

very dynamic time—

one that belies the 

image of forests of 

the Pacific Northwest 

as timeless or slowly 

developing.
35

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

These mortality trends continued in the next half decade after the fire (fig. 6). 

Between 1 and 6–7 years postfire, the annualized survival rate of 0.915 (table 3) is 

consistent with a population of damaged trees, such as those experimentally topped 

by Huff and Bailey (2009), whose survival rates after 18 years were equivalent to 

an annualized rate of 0.917. Moreover, on a plot basis, we found that the magnitude 

of this “postfire mortality” rose with fire severity (fig. 7). Thus, many trees that 

survived the fire were injured or weakened by it. Trees injured by fire may be 

more susceptible to stresses such as drought, fine-root mortality, insect and disease 

colonization (Agee 2003), and loss of fungal symbionts (Borchers and Perry 1990). 

Notably, postfire evaluations of the Warner landscape found little evidence of tree 

mortality related to outbreaks of insects and diseases (USDA FS 1993). Between 

6–7 and 14 years postfire, annualized survival at Warner increased to 0.960 (table 

3), approaching the >0.99 that might be expected for undisturbed mature forest 

(Franklin et al. 1987).

While the phenomenon of delayed mortality in fire-damaged trees is common 

knowledge among forest scientists, it has largely undiscussed ramifications for the 

widely used, but inconsistently executed, measurement of fire severity (Keeley 

2009). When fire severity is measured by tree mortality or visible damage to the 

canopy, the time of measurement could strongly affect the result. Delayed mortality 

on the level we report (almost 9 percent per year) could substantially alter estimates 

of fire severity if the estimates come multiple years postfire. Estimates of fire 

severity should include an explicit reference to the time of measurement.

Snag Fall and Fragmentation

While tree mortality added a huge density and volume of snags to the burned area, 

that wood quickly began moving to the forest floor. Snag fall and fragmentation 

added so much wood to the ground—thousands of meters of log length per hectare—that it probably constitutes a significant ecological disturbance in itself, a kind 

of rain of logs.

This huge transfer of biomass from the canopy to the ground was not the result 

of extraordinarily high snag fall probabilities, but rather the immense population 

of snags available to fall. Although several authors (Bagne et al. 2008, Ohmann 

2002, Russell et al. 2006) have reported higher snag fall rates in disturbed forests, 

the probabilities we recorded do not seem unusually high. When species and size 

classes are matched as closely as feasible, our postfire snag fall rates (table 4) are 

only slightly higher than those reported by Wilhere (2003) and Mellen and Ager 

(2002), and similar to those reported by Ohmann (2002), for undisturbed forest. 

Like Wilhere (2003) and Ohmann (2002), we found an effect of DBH on snag fall 

Snag fall and 

fragmentation added 

so much wood to the 

ground that it probably 

constitutes a significant 

ecological disturbance 

in itself, a kind of rain 

of logs.
36

Research paper PNW-rp-592

rate (fig. 8; table 4). Like Parish et al. (2010) and Mellen and Ager (2002), we found 

a tendency for low snag fall rates on the part of P. menziesii (fig. 8; table 4).

Snag fragmentation has been less discussed in the literature, but our results 

suggest it plays a major role in delivering wood to the ground. Comparing figures 

8 and 9 (or tables 4 and 5) shows that for 6–7 to 14 years postfire, snags were often 

more likely to remain standing and fragment than fall down. This seems especially 

true for P. menziesii. Like us, Everett et al. (1999) found that P. menziesii snags 

are less likely to fall than other species, but very likely to lose height. Moreover, 

because larger snags are much more likely to fragment (fig. 9), the volume of wood 

falling to the ground per snag fragmentation event will likely be larger than the 

typical snag fall event. Given the massive size of some of these snag fragments (a 

mean height loss per event 12.8 ±10.2 m, with 10 percent of height losses >28 m), it 

is clear that a lot of kinetic energy, as well as wood, is being delivered to the forest 

floor, perhaps destroying some existing regeneration and reexposing mineral soil, 

as well as increasing fuel loads.

Because of snag fall and fragmentation, the large population of snags at the 

Warner Creek Fire site is only a temporary phenomenon. If the snag fall rates we 

observed between 6–7 and 14 years continue, snag density will be back near prefire 

levels in two more decades, with considerable additional losses in snag volume 

owing to snag fragmentation.

Natural Regeneration of Trees

We found that natural regeneration and growth of trees on the Warner Creek Fire 

site was generally abundant and rapid, although extremely variable in space, both 

within and among stands. This abundance occurred while other vegetation, notably 

the shrub Ceanothus, was regenerating profusely as well. Regeneration did not 

seem to be focused in a single pulse of establishment, as numerous new seedlings 

were observed on all study visits, and individual subplots showed increases between 

visits. Seedling numbers in the Pacific silver fir zone were particularly large, with 

mean seedling densities above 60,000 per ha. This is considerably higher than 

the several thousand per hectare reported by Agee and Smith (1984, p. 813) for a 

subalpine forest in the Olympic mountains, 3 years after a fire. Beyond this reference there is little in the literature about natural regeneration after fire in the Pacific 

silver fir zone. Most of our discussion will focus on the western hemlock zone.

The speed of reforestation in our seedling results contrasts with some retrospective studies, which portrayed natural reforestation of P. menziesii stands as slow or 

sparse (Franklin and Hemstrom 1981, Huff 1995, Tappeiner et al. 1997a). However, 

none of our results are unanticipated when compared to the handful of quantified 

Natural regeneration 

and growth of 

trees was generally 

abundant and rapid, 

although extremely 

variable in space.
37

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

direct observations of postfire regeneration in the literature graphed in figure 1. 

For example, our mean abundance of 13,900 P. menziesii per ha at 14 years postfire 

(table 6) fits into the range established by Hofmann’s (1917) map, 0 to >37,000 per 

hectare. Similarly, Shatford et al. (2007) found 83 to 8188 conifers per hectare in 

regenerating P. menziesii stands 18 years postfire. Like us, both of these sources 

report a wide range in the age of seedlings, from 1 to 11 years in Hofmann (1917), 

and 1 to 18 years in Shatford et al. (2007). This last finding may be the most interesting one, because it suggests that the natural postfire environment can provide 

multiple, ongoing opportunities for seedling establishment and growth. Natural 

reforestation may not be dependent on a single fortuitous season.

Minore and Laacke (1992) list four factors that likely affect the speed and 

abundance of natural regeneration: seed source, availability of suitable substrate, 

protection from heat and moisture stress, and the presence of competitive vegetation. It seems unlikely that seed source was limited on the Warner site, given the 

huge numbers of seedlings. A local silviculturalist characterized seed in the year 

following the fire as generally sufficient or abundant (Bailey 1992). Larson and 

Franklin (2005) have suggested that many seeds survived the Warner fire in the 

canopy. P. menziesii that survived the fire were typically larger, older individuals, 

which tend to produce more seed than younger ones (Hermann and Lavender 1990); 

such trees may have been littering the ground with seeds even as they took a decade 

or more to die from the stress of the fire. Even in stands that suffered 100 percent 

mortality and near-total crown consumption, the patchy nature of the fire may have 

left seed trees nearby. We did not assess the availability of substrates, but P. menziesii is reputed to prefer exposed mineral soil (Minore and Laacke 1992). At the 

Warner site, high-severity fires exposed some mineral soil, and the steady rain of 

falling logs (from snag fall and fragmentation) may have exposed more even after 

revegetation had commenced. 

Heat and moisture stress have long been acknowledged as major constraints 

on tree regeneration in this region (e.g., Isaac 1938). Locations that are dry for 

physiographic and microclimatic reasons, such as south-facing slopes, are the most 

likely sites for “regeneration failure” following logging; it can be desirable to leave 

some shade as protection for the youngest seedlings (Minore and Laacke 1992). At 

Warner, we found no relation between aspect and regeneration; one south-facing 

plot had 3,500 seedlings >30 cm tall per ha in the cell survey. The Warner study 

site, unlike a logged stand, contained an effective source of partial shade: a canopy 

of dead or dying trees. Though we know of no measurements of the transmittance 

of light through a snag canopy, it might be approximated with measurements from 

a deciduous canopy in winter, where average transmittance for visible radiation was 
38

Research paper PNW-rp-592

about 40 percent of incident values (Brown et al. 1994). In contrast, closed-canopy 

P. menziesii canopies transmit on average only 1 to 2 percent of visible radiation to 

ground level (Parker et al. 2002). As snag fall and fragmentation bring that canopy 

to the ground, light and heat to seedlings will be increased, but the newly fallen 

logs—which at Warner, were in the thousands of meters per ha—may also increase 

the availability of shaded microsites. Little et al. (1984) proposed that such microsites were important resources in the regeneration of subalpine forests featuring 

Abies amabilis and Tsuga mertensiana.

A protective shading effect may explain why we, like Shatford et al. (2007), 

found no relationship between seedling abundance and shrub cover, despite fears 

that shrubs may be a major restraint on reforestation in burned areas (Sessions et 

al. 2004). Whereas experimental work in harvested areas has demonstrated that 

mechanical removal of shrubs can increase the growth of planted P. menziesii

seedlings (Stein 1999), in a more natural setting encroaching and even overtopping 

vegetation may provide a measure of heat and moisture protection for seedlings and 

saplings (Irvine et al. 2009), allowing them to survive until they overtop the shrub 

canopy.

Future Development and Management Implications

A decade ago the efficacy of natural regeneration for replacing forest cover in the 

wake of fire was in doubt (e.g., Sessions 2004). We found that natural regeneration 

at the Warner fire site often far surpassed some silvicultural standards relevant to 

planting operations, such as 494 seedlings per hectare (Oregon Revised Statutes 

527.745), or >50 percent of milacre plots (Stein 1992). (The Oregon standard also 

has a more subjective condition, namely, that the seedlings must be “free to grow,” 

that we did not evaluate.)

When considered in the light of other direct observations of postfire regeneration collected in figure 1, it seems clear that Douglas-fir forests of the Pacific 

Northwest are capable of regenerating themselves in a wide range of circumstances. 

The results in figure 1 span a number of climate zones and topographical conditions, from moist sites in the Olympic Mountains (Huff 1995) to dryer and hotter 

areas in the Klamath and Siskyou Ranges (Donato et al. 2009, Shatford et al. 2007). 

Although it is not clear if the resulting forests are optimal for any particular purpose 

(wood production, habitat for a particular species), there is little doubt that tree 

cover is returning within one or two decades rather than the centuries that had been 

feared. The next time a mixed-severity fire burns in this region, it could logically 

be argued there is no need to plant nursery-grown seedlings.

There is little doubt 

that tree cover is 

returning within one 

or two decades rather 

than the centuries that 

had been feared.
39

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

The regeneration we observed differed from planting schemes in its spatial 

variability within and among stands. At 14 years postfire, density of large (1 to 3 m 

tall) seedlings in both elevation zones ranged from near 0 to more than 20,000 per 

ha (fig. 12, app. 2), with considerable variation within plots. In contrast, planting 

prescriptions in this area rarely exceed a few thousand per hectare (e.g., 3,000, Scott 

et al. 1998), and are usually executed in a regular geometric pattern. A prescription written for portions of the Warner burn which were not part of the designated 

natural succession area ranged from 500 to 1,000 per hectare (Dewitz 1992). If the 

goal of planting prescriptions in large treated areas is to emulate natural response to 

fire, their variation should be increased, both within and among stands.

In fact, practically every aspect of the fire event and ecological aftermath 

showed high variability, from the mixed severity of the fire to the processes of mortality, snag fall and fragmentation, and new vegetation growth. These compounding 

factors increased the variety of structural conditions on the landscape (table 7), and 

future conditions should be considerably more variable than they would have been 

without the fire. 

Two kinds of pathways expounded by Zenner (2005) seem apropos. Stands 

with high mortality and high regeneration fit into the “catastrophic” mode of stand 

development, where a single cohort of P. menziesii dominates the stand; future 

structural diversity comes from varying growth rates of different tree species and 

gap formation, a slow process that delays old-growth characteristics for centuries. 

Meanwhile, stands with moderate severity fit into the partial disturbance model 

where large remnants and successful regeneration immediately take on a twocohort structure that may develop old-growth characteristics more quickly. 

Areas of high severity and low regeneration might be demonstrating the slow, 

sparse pattern suggested by retrospective studies like Tappeiner et al. (1997a), 

where trees establish and grow in low densities, but get big quickly. These low-density stands may look like regeneration failures but appear to be a completely natural 

feature of response to a mixed-severity fire. They may have interesting attributes 

for forest health and conservation. Stands with lower initial densities may develop 

old-growth characteristics more quickly than stands with higher initial densities 

(Acker et al. 1998, Bailey et al. 1998). And a lower density of trees might allow a 

lengthy shrub phase of succession, where nitrogen-fixing shrubs like Ceanothus

enhance soil nutrients. Management plans for large burned or treated areas might 

consider incorporating such areas in a mix of treatments rather than aiming for the 

universal return to complete tree cover.
40

Research paper PNW-rp-592

It is common in Oregon and Washington forest management to refer to the 

landscape as a “mosaic” of mixed forest types and structures. With a variety of fire 

severities and variety of postfire responses, the Warner Creek Fire area demonstrates how such mosaics are created.

Acknowledgments

The authors extend their thanks to field crews that collected the data in this report. 

These crews included Danielle Robbins, Kate Richards, Jesse Fitzpatrick, John 

Foster, Luther Skeels, Bob Haberman, Barb Perrone, Miranda Miller, Ginger Craig 

and Lupin Loel. Thanks also to Tim Bailey for providing a map of estimated fire 

severity over the Warner burn, and to Bruce Kern for help with field reconnaissance 

in 2003.

English Equivalents

When you have: Multiply To get:

Centimeters (cm) 0.394 Inches

Meters (m) 3.28 Feet

Hectares (ha) 2.47 Acres

Square meters (m2

) 10.76 Square feet

Cubic meters (m3) 35.3 Cubic feet

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Appendix 1—Temporal Changes in Three Plots Between 6–7 

and 14 Years Postfire
47

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

Appendix 2—Photos of the Variety of Site Conditions at 14 Years 

Postfire
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Research paper PNW-rp-592

Appendix 3

Percent cover of regenerating species at 14 years postfire, from the 

regeneration cover surveya

Silver fir zone Western hemlock zone

(n = 11 plots) (n = 10 plots)

Standard Standard

Mean deviation Mean deviation

Trees:

Abies amabilis 9.2 10.0

Abies grandis 1.9 2.5 t t

Abies procera 2.2 4.9

Acer macrophyllum 0.3 0.8

Arbutus menziesii t 0.1

Calocedrus decurrens t t

Chamaecyparis nootkatensis 2.5 8.3

Cornus nuttallii t t

Crataegus douglasii t t

Pinus monticola t t

Pseudotsuga menziesii 8.5 12.5 17.1 22.3

Taxus brevifolia t t

Thuja plicata 0.3 0.5

Tsuga heterophylla 0.2 0.5 1.1 1.7

Tsuga mertensiana 7.6 10.7

All trees 30.3 20.0 15.4 18.9

Shrubs: 

Acer circinatum 0.2 0.8 0.5 1.1

Acer glabrum 0.1 0.1

Amelanchier alnifolia 0.1 0.2

Arctostaphylos t 0.1

Arctostaphylos columbiana 0.5 1.6 0.2 0.4

Berberis nervosa 0.4 1.2 6.6 5.5

Ceanothus integerrimus 2.2 6.6

Ceanothus sanguineus 5.3 12.7

Ceanothus velutinus 11.9 26.4 15.5 21.7

Chimaphila umbellata t t t 0.1

Garrya fremontii 0.3 0.8

Gaultheria shallon 0.9 1.8

Holodiscus discolor 0.1 0.2 0.3 0.6

Pachistima myrsinites 0.3 1.1 t 0.2

Prunus emarginata 0.6 1.1

Rhododendron macrophyllum 0.4 0.9 0.7 1.5

Ribes cereum 0.1 0.2

Ribes lacustre 0.9 1.2

Ribes sanguineum 0.2 0.5 t t

Ribes viscosissimum 2.5 3.0

Rosa gymnocarpa 0.3 0.5 0.6 0.9

Rubus lasiococcus 10.4 14.9

Rubus nivalis 0.1 0.3

Rubus parviflorus 4.9 12.8 0.5 0.9

Rubus spectabilis 0.1 0.2
49

Natural Tree Regeneration and Coarse Woody Debris Dynamics After a Forest Fire in the Western Cascade Range

Silver fir zone Western hemlock zone

(n = 11 plots) (n = 10 plots)

Standard Standard

Mean deviation Mean deviation

Rubus ursinus 0.7 1.3 7.6 12.2

Salix sp. 0.1 0.2 0.1 0.1

Sambucus racemosat 0.1

Sorbus scopulinat 0.1

Sorbus sitchensistt

Symphoricarpos mollis 2.1 6.2 0.5 0.9

Vaccinium membranaceum 4.8 7.1 0.2 0.7

Vaccinium parvifoliumt 0.1

Whipplea modestat 0.1 9.411.6

All shrubs 34.9 30.3 29.5 46.1

Herbs, ferns, and mosses: 

Achlys triphylla 0.7 1.1 1.9 3.8

Adenocaulon bicolor 0.2 0.3t 0.1

Anaphalis margaritacea 3.0 2.8 0.5 0.7

Anemone deltoidea 0.1 0.2 0.2 0.3

Angelica genuflexatt

Aquilegia formosa 0.1 0.5

Asarum caudatum 0.7 1.9

Aster foliaceustt

Aster occidentalist 0.1

Bromus vulgaris 0.9 1.2

Calamagrostis canadensis 0.3 0.8

Campanula scouleri 0.8 1.7 0.2 0.4

Carex sp.t 0.2

Carex inops 0.1 0.2

Carex kelloggiit 0.1

Carex luzulina 0.1 0.2

Carex mertensii 0.1 0.2

Carex rossii 0.3 1.0

Cirsium callilepist 0.1

Cirsium vulgare

t 0.1

Claytonia sibiricat 0.1

Clintonia uniflora 1.2 1.7

Cystopteris fragilis 0.1 0.2

Dactylis glomeratatt 0.8 1.8

Dicentra formosat 0.2

Disporum hookeri 0.2 0.6 0.1 0.1

Elymus glaucus 1.1 2.2

Epilobium angustifolium 1.4 1.7 1.1 1.6

Epilobium minutum 0.1 0.2

Festuca rubrat 0.1

Fragaria vesca 1.8 3.9 0.3 0.4

Galium oreganum 0.4 1.0

Galium triflorum 0.3 0.3 0.5 0.6

Glyceria elata 0.7 2.2

Heracleum lanatum 0.2 0.4
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Research paper PNW-rp-592

Silver fir zone Western hemlock zone

(n = 11 plots) (n = 10 plots)

Standard Standard

Mean deviation Mean deviation

Hieracium albiflorum 1.0 0.7 0.4 0.3

Hypericum perforatum 0.1 0.2 t 0.1

Lactuca muralis 0.8 1.5 6.9 16.7

Lactuca pulchella t t

Lathyrus polyphyllus t t 0.6 1.8

Linnaea borealis t 0.1

Listera borealis 1.0 3.0

Lupinus polyphyllus 0.8 1.1

Madia sativa t t

Mertensia paniculata 0.1 0.2

Mitella sp. t 0.1

Mitella breweri 0.2 0.6

Osmorhiza chilensis t 0.1 0.1 0.1

Pedicularis racemosa t 0.1

Penstemon sp. 0.1 0.4

Penstemon cardwellii 0.1 0.2

Phacelia hastata 0.1 0.4

Poa canbyi t 0.1

Polystichum munitum 0.1 0.2 2.2 3.9

Pteridium aquilinum 1.1 2.7 1.1 2.2

Pyrola secunda 0.5 0.7

Rudbeckia occidentalis 0.1 0.3

Senecio triangularis 0.1 0.2

Smilacina racemosa t 0.1

Smilacina stellata 1.5 4.0 0.1 0.2

Stellaria calycantha t t

Tiarella trifoliata 3.2 6.9 0.3 0.9

Trientalis latifolia 0.1 0.4 0.5 0.7

Trifolium latifolium t 0.1

Trillium ovatum t 0.1

Trisetum canescens 0.2 0.8

Trisetum cernuum 0.7 1.4

Valeriana sitchensis 0.4 1.2

Vancouveria hexandra 0.1 0.2

Vicia americana 0.1 0.4

Vicia oregana 0.1 0.2

Vicia sativa t 0.1 0.5 1.4

Viola sp. 0.2 0.8 0.4 1.3

Viola canadensis t 0.1

Viola sempervirens 0.9 1.9 0.3 0.9

Xerophyllum tenax 1.0 1.4

All herbs, ferns, and mosses 25.6 16.5 15.4 18.4

a

 t = trace (<0.1 percent).
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