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Climate Change and the Origin of Old-Growth

Douglas-Fir Forests in the Puget Sound Lowland

Linda B. Brubaker

Author

LINDA B. BRUBAKER is a professor of forest ecology and

Quaternary paleoecology, College of Forest Resources, University of Washington, Seattle, Washington 98195.

Abstract

The vegetation and climate history of the Pacific Northwest

is best understood from fossil pollen records and climatic

simulations by general circulation models of the Earth’s

atmosphere. Paleoclimatic simulations by these models provide a physical explanation of the climate associated with

past vegetation changes. During the maximum of the North

American continental glaciation (30,000 to 18,000 years

Before Present (years BP)), the Pacific Northwest experienced

dry-cold easterly airflows from a strong high-pressure system

centered above the massive continental ice sheet. Vegetation

west of the Cascade Range consisted of open forest-tundra

parklands with lodgepole pine, Engelmann spruce, and mountain hemlock interspersed with tundra communities. By about

10,000 years BP, the continental ice sheet had collapsed, and

summer insolation increased because of changes in the

Earth’s orbital features. These changes brought warm-dry

summers to the Pacific Northwest, and pollen records in the

southern Puget Lowland indicate a predominance of Douglasfir, oak, alder, and prairie herbs. Fires became more frequent,

as lake sediments show a marked increase in charcoal concentrations. The closest modern analog to this vegetation

may be the oak savannas and Douglas-fir forests of the

Willamette Valley. By 6,000 years BP, modern climatic

conditions were established, and pollen records in this region

show decreases in Douglas-fir, alder, oak, and grass pollen

and increases in western redcedar and western hemlock

pollen. Charcoal concentrations in sediments decrease

rapidly at this time.

Pollen evidence indicates, therefore, that the modern forest

composition of the Puget Lowland was established about

6,000 years BP. Although structural properties of these forests

cannot be inferred from the pollen record, both the structure

and composition of old-growth Douglas-fir forests can reasonably be assumed to have first established at this time. No

compelling evidence in pollen data and paleoclimatic simulations suggests that this forest type existed elsewhere before

6,000 years BP. Old-growth Douglas-fir forests thus developed relatively recently on an evolutionary time scale and

probably do not represent a coevolved complex of species

bound together by tightly linked and balanced interactions.

Introduction

Pacific Northwestern forests have changed profoundly over

the past 30,000 years in response to climate variations driven

by changes in large-scale controls of the Earth’s climate

system (such as solar radiation, sea surface temperature,

extent of land and sea ice, and atmospheric trace gases and

particulates) (Barnosky and others 1987). Both fossil pollen

records of past vegetation and physical models of global

17
atmospheric circulation clearly show that climate variations

follow unique paths as new combinations of boundary conditions establish and disappear with varying periodicities

(COHMAP members 1988, Huntley and Webb 1989). As

a result, the climate history of the Pacific Northwest, as in

other regions of the world, should not be viewed as a simple

sequence of latitudinal or elevational displacements of

modem climatic zones.

Plants have, therefore, not been able to escape climatic

change by migrating within zones of climate that are simply

displaced in space. Species continually face new conditions

and must adjust to them by evolutionary, ecological, or phenotypic mechanisms (Brubaker 1986). Genetic change in

long-lived species such as Northwestern conifers is probably

too slow to keep pace with rates of climatic change (Bartlein

and Prentice 1989). Thus, these species are probably not

perfectly adapted to current climates and should retain some

“memory” of past conditions in their current genetic make-up.

The short-term responses of Pacific Northwest conifers to

climatic change primarily involve ecological mechanisms

(for example, altered reproduction and establishment rates)

and physiological adjustments (for example, drought avoidance or tolerance mechanisms) that affect tree establishment

and survival.

Pollen preserved in lake and bog sediments have provided

most of the empirical evidence of past vegetation and climate

in the Pacific Northwest (see Baker 1983, Bamosky and

others 1987, Heusser 1986). Pollen data accurately reveal the

general composition of regional forests but are less precise

for reconstructing fine-scale forest features such as the composition and structure of individual stands. Stand conditions

are generally inferred from knowledge of stand characteristics in modem forest zones thought to be similar to ancient

forests. Plant macrofossils (such as leaves, seeds, and cones),

which are often present in such sediments, have been invaluable for confirming and refining interpretations of pollen

records (for example, Cwynar 1987, Dunwiddie 1986).

Fossil pollen records in the Puget Sound region indicate that

Western Hemlock Zone forests (Franklin and Dyrness 1973)

first established about 6,000 years ago. By inference, oldgrowth Douglas-fir forests date from this time. Because no

evidence has been found that Douglas-fir forests existed as a

zonal forest type elsewhere in the Pacific Northwest before

this time, this forest association is young on an evolutionary

time-scale and may not represent highly coevolved speciesinteractions.

In this paper, I will discuss forest and climate change in the

lowlands west of the Cascade Range over about the past

30,000 years, with emphasis on the Holocene (the present

interglacial, from 10,000 years ago to the present) because

pollen and plant macrofossil records are most abundant and

old-growth Douglas-fir forests first became dominant during

this period.

Sources of Paleoenvironmental Information

Pollen and Plant Macrofossils

Pollen and macroscopic plant parts are well preserved in

anaerobic lake and bog sediments. Pollen is much more

abundant than plant macrofossils because pollen (particularly

of trees) is often produced in large amounts and widely

dispersed by the wind. Owing to its ubiquitous occurrence,

pollen has been the most important source of information

about past vegetation in the Pacific Northwest (Baker 1983,

Bamosky and others 1987, Heusser 1986). Pollen data have

been supplemented in several studies by plant macrofossils,

however, to increase the taxonomic and spatial resolution of

vegetation reconstructions (Bamosky 1981, 1985, Cwynar

1987). Plant macrofossils provide finer taxonomic information than pollen because seeds and leaves typically can be

identified to species, but pollen is generally identified to the

genus for trees and shrubs and only to family for some important herbs (such as grasses and sedges) (Birks and Birks

1980). Macrofossils also reveal the local occurrence of plants

near the collecting site because leaves and seeds fall close to

the parent plant. Pollen, on the other hand, is often carried

several kilometers before falling to the ground and, therefore,

reveals the general composition of regional vegetation.

Fossil pollen data are interpreted primarily by making comparisons between modem and fossil pollen assemblages

(Birks and Birks 1980). Researchers look for pollen assemblages in modem sediments that match those found in fossil

sediments. If a close match is found, the modern vegetation

is considered to be a good analog for past vegetation. Fine

features of the vegetation, such as stand structures or local

variation in stand composition, are inferred to have been the

same as in the modem vegetation.

Radiocarbon dates or well-dated volcanic ash layers establish

the timing of past vegetation change. Ash from the eruption

of Mt. Mazama (about 6800 years BP, S. Porter, pers. comm.)

forms a well-defined layer in lakes and bogs throughout the

Pacific Northwest and provides an important time marker in

Holocene sediments of the region.

Because lakes are formed primarily by the action of glaciers

(such as glacial scouring and kettle formation), the spatial

coverage of lake pollen records in the Pacific Northwest is

related to the limits of continental and alpine glaciation (for

example, see Barnosky and others 1987). Although alpine

glaciers covered most of the Olympic Mountains and the

Cascade Range, only one study (Dunwiddie 1986) has been

done at high elevation, and the vegetation history in most of

these mountains remains unknown. Continental glaciers

18
Thousands of years before present

18 ,I s 12 9 6 3 0

I I I I I I I

Figure l-Major changes in external and internal boundary conditions of the

Earth’s climate system since 18.000 years BP (after Kutzbach and Guetter

1986). SJJA, SDJF: Northern Hemispheric solar radiation in June-August and

December-February, respectively, as Percentage difference from present.

Land ice as percentage of present. SST: global mean sea surface temperature

as departure from the present. CO2 as concentration in parts per million.

extended as far south as the central Puget Lowlands in Washington (Waitt and Thorson 1986). Consequently, lowland

pollen records are relatively abundant from central Washington to southern British Columbia but are scarce in southern

Washington and Oregon. Several excellent reviews of the

vegetation history of the Pacific Northwest have recently

been published (Baker 1983, Bamosky and others 1987,

Heusser 1986).

Paleoclimatic Simulations by General Circulation Models

Over the past two decades, physical models of atmospheric

circulation have been developed to examine the interactions

of the major external and internal controls of global climate

(for example, Hansen and others 1984, Kutzbach and Guetter

1986, Schlesinger and Zhao 1988, Whetherald and Manabe

1988). These models represent the current understanding of

Earth’s climate system and are being used to assess the characteristics of past and potential changes. The Community

Climate Model (CCM) of the National Center for Atmospheric essearch (NCAR) (Kutzbach and Guetter 1986) has

been used to examine the ability of one model to reconstruct

past climate change as recorded by fossil data (COHMAP

members 1988). The major boundary conditions of this

model are seasonal solar radiation, sea surface temperature, atmospheric aerosol and carbon dioxide concentrations,

and the volumes of land and sea ice (fig. 1). The degree of

agreement between paleoclimatic simulations and empirical

evidence of past climate measures the success of the model.

The model has been tested with paleovegetation and lakelevel records from North America, Europe, and tropics of

Africa and Asia. In general, the agreement has been good,

and the model is presently considered to be a reasonable

mechanistic explanation of the climatic variations that drove

past vegetation change. The comparison of these simulations

with vegetation history in the Pacific Northwest is described

in greater detail in Bamosky and others (1987) and

COHMAP members (1988).

Definition of Old-Growth Douglas-Fir Forests

and Their Identification in the Fossil Pollen

Record

Old-growth Douglas-fir forests of the Western Hemlock

Zone (Franklin and Dymess 1973) are defined by a combination of compositional and structural characteristics

(Old-Growth Definition Task Group 1986). Old-growth

forests generally may be identified by the presence of two or

more tree species with large diameters (typically Douglas-fir

plus one or more shade-tolerant associates, such as western

hemlock or western redcedar); a wide range of sizes and ages

and a deep, multilayered canopy; and substantial woody

debris in the form of logs and standing snags.

Because pollen records reveal the compositional aspects of

the vegetation, they indicate whether tree species that currently characterize old-growth stands were common in the

past. They cannot, however, provide definitive evidence that

those trees grew together in the same stands, or that past

stands had the structural characteristics of modem oldgrowth forests. In this paper, I assume that structural characteristics follow compositional characteristics, and thus that

old-growth Douglas-fir stands were first established in the

Pacific Northwest when fossil pollen assemblages first

matched pre-settlement pollen percentages of the modern

Western Hemlock Zone. Because of the poor resolution of

pollen data, the terms “old-growth Douglas-fir forests” and

“Western Hemlock Zone forests” are used interchangeably in

this paper.

Full- and Late-Glacial Vegetation and Climate

(30,000 to 10,000 years BP)

The last ice age in the Pacific Northwest culminated in the

Fraser Glaciation about 25,000 to 10,000 years BP (Waitt

and Thorson 1983). Mountain glaciers covered the Cascade

Range and Olympic Mountains from 29,000 to 22,000 years

BP (Porter and others 1983), but lowland areas remained ice

free until about 18,000 years BP, when ice advanced from

southern British Columbia into the coastal Olympic Peninsula and the Puget Lowland (Waitt and Thorson 1983).

These ice lobes coalesced and reached their maximum extent

in the southern Puget Lowland south of Olympia, Washington. Lowland ice began to retreat about 15,000 years BP.

19

7 iet stream@

. .

-Subtropical _L , ,..I__\ surface winds \

>

9 ka

Stronger onshore

flow (July)

Figure 3-Paleocliiatic conditions for the Pacific Northwest as simulated by the NCAR CCM model for 18,000 and 9,000 years BP (after Bamosky and

others 1987).

causing a strong high-pressure system, “the glacial anticyclone,” centered over the ice cap. Anticyclonic circulation

brought prevailing easterly airflows across the Pacific Northwest, particularly in winter. Because of cold-dry airflow from

the mid-continent ice cap, simulated temperatures, precipitation, and precipitation-minus-evaporation for 18,000 years

BP were substantially lower than at present. These simulations agree with fossil pollen and macrofossil records that

show tundra and xeric treeline species such as Engelmann

spruce in the Puget Lowland during the full-glacial period.

Interglacial Vegetation and Climate (10,000

Years BP to Present)

The present interglacial is called the Holocene. By convention, it is defined as the period from 10,000 years BP to the

present. By 10,000 years BP, lowland glaciers had receded

from the Puget Lowland into southern British Columbia,

and alpine glaciers were restricted to very high elevations

in the Cascade Range and Olympic Mountains (Burke and

Birkeland 1982, Thorson 1980).

Early Holocene (10,000 to 6,000 years BP)

Tree species indicative of cold climates had disappeared

from the Puget Lowland by the beginning of the Holocene.

Rapid changes in pollen and macrofossils suggest the onset

of substantially warmer and drier climate than at present.

Between about 10,000 and 6,000 years BP, pollen diagrams

from southern British Columbia to the central Puget Lowland

showed maximum percentages of Douglas-fir, red alder,

grass, and prairie herbs (Barnosky 1981, Leopold and others

1982, Mathewes and Rouse 1975, Tsukada 1982, Tsukada

and others 1981). Pollen diagrams from the southern Puget

Lowland show less Douglas-fir and alder but more grass and

oak pollen (Bamosky 1985), suggesting a north-south

vegetation gradient with a higher frequency of open, xeric

communities in southern areas. Pollen of mesic tree species,

such as western redcedar and western hemlock, was rare

throughout the region. In addition, charcoal concentrations

increased dramatically and spores of bracken fern became

abundant in sediments.

Old-growth Douglas-fir forests were not important at the

regional scale during the early Holocene because pollen

assemblages of this period differed substantially from presettlement pollen spectra in modem forests in the Western

Hemlock Zone (Barnosky 1981, Heusser 1977, 1986). The

general importance of oak and grass pollen and the scarcity

of mesic taxa in early Holocene pollen assemblagessuggest

that ancient vegetation may have been similar to modem

oak savanna and Douglas-fir woodlands in the southern

Willamette Valley of Oregon (Bamosky and others 1987).

This hypothesis must be tested with modem pollen data from

Oregon, however.

21
The most compelling climatic interpretation of the fossil

pollen data is that warm-dry climates dominated the Puget

Lowland during the early Holocene. Summer droughts were

probably more severe and fires more frequent than today.

Grassy areas, oak savannas, Douglas-fir woodlands, and riparian alder stands probably characterized the regional vegetation, with strong north-south gradients in the predominant

community types. Southern areas may have been characterized by extensive grasslands and oak savannas, and northern

areas by a shifting mosaic of dry Douglas-fir forests in early

successional stages. This vegetation gradient was probably

maintained by interactions of climate and fire.

Summer droughts were probably most severe in the southern

Puget Lowland, resulting in discontinuous tree cover and

sparse, dry fuels. These conditions would have favored

frequent low-intensity fires, which can maintain savannas

and grasslands effectively. Oregon white oak was probably

common because it readily resprouts after fire. The thick

bark of mature Douglas-fir probably allowed old trees to

survive fire at scattered locations. The establishment of

Douglas-fir would have been difficult, however, because of

the sensitivity of young trees to even low-intensity fires

(Agee, this volume). The distribution of parent materials

(coarse outwash, till, pro-glacial lake sediments) may have

controlled major spatial patterns of plant communities in the

southern Lowland. In particular, extensive areas of coarse

outwash were deposited south of the lowland ice sheet during

late-glacial times (Thorson 1980). The soils of these areas

were undoubtedly exceedingly droughty during the early

Holocene and probably supported only grassland vegetation.

Woodlands and savannas probably occupied finer textured

soils in the region. Small modem prairies south of Olympia,

Washington, may be remnants of extensive early Holocene

grasslands. Prairies also may have been maintained in part by

fires set by Native Americans who were present in the Northwest during the early Holocene.

The greater abundance of tree pollen in sediment records

from the central and northern Puget Lowland (fig. 2) suggests that these areas were more heavily forested. The

northern forests may have experienced fires of variable

intensity (Agee, this volume) that were less frequent than in

the savannas and grasslands to the south. Fire effects would

have varied depending on tire intensity. Low-intensity fires

would have removed understory plants but caused little tree

mortality. Moderate fires would have killed some trees and

opened growing space within stands, but intense fires would

have killed most of the trees in a stand, thereby reinitiating

succession. This type of variable fire regime characterizes

dry Douglas-fir forests of southwestern Oregon and results in

patchy mosaics of young and old stands of various tree densities. These forests may be reasonable modem analogs to those

growing in the northern Puget Lowland during the early

Holocene.

Pollen diagrams from coastal areas of the Olympic Peninsula

also show maximum values of Douglas-fir and alder pollen

in the early Holocene, but western hemlock pollen was more

common than in the Puget Lowland (Bamosky and others

1987, Heusser 1986). Oak and grass pollens were rare in

these diagrams. Apparently, maritime influences caused more

mesic densely forested vegetation along the Pacific coast

than farther inland. Coastal diagrams are nevertheless consistent with records from the Puget Lowlands, in that they show

forest types with drier affinities in the early than in the midto late-Holocene.

The only pollen records from Oregon were published nearly

50 years ago by Hansen (1947), who pioneered palynological

research in the Pacific Northwest. Hansen worked before

techniques such as radiocarbon dating and fine taxonomic

discrimination of pollen were developed. His diagrams, therefore, provide only very general descriptions of past vegetation. Nevertheless, they remain invaluable to current understanding of the forest history of the Pacific Northwest.

Hansen (1947), fortunately, identified the Mazama ash layer

at several sites. It is now dated and can be used as a basis for

comparing early- and late-Holocene pollen assemblages.

High pine-pollen percentages are a striking feature of earlyHolocene pollen diagrams from the central Willamette

Valley. Although the pine species comprising this peak are

not known, the conclusion seems reasonable that they were

species currently more abundant to the south (such as

ponderosa pine, Jeffrey pine, sugar pine, knobcone pine)

rather than lodgepole pine, which typically characterizes fulland late-glacial pollen assemblages in the Northwest. This

interpretation implies warmer-drier climates during the

early-Holocene than at present and is thus consistent with

interpretations of pollen records farther north. Regardless of

their climatic significance, early-Holocene assemblages in

western Oregon do not match mid- to late-Holocene spectra

from the same area or from sites in Washington and southern

British Columbia. One must conclude, therefore, that oldgrowth Douglas-fir forests were not common in Oregon

during the first half of the present interglacial period.

Climate simulations for the early-Holocene are represented

by the model for 9,000 years BP, when summer insolation

at 400

 N. latitude was 8 percent greater than present, the

Laurentide ice sheet was greatly reduced, and the influence

of the glacial anticyclone had essentially disappeared

(COHMAP members 1988, Kutzbach and Guetter 1986).

The simulated July temperature was higher, and precipitationminus-evaporation was Iower than at present,,because of increased insolation. At 9,000 years BP the Earth was closest

to the sun during the summer and the tilt of the Earth’s axis

was more extreme than at present, both of which increased

22
the intensity of solar radiation, Paleoclimatic simulations,

therefore, agree with pollen data for warmer and more xeric

forest associations during the early-Holocene.

Mid- to Late-Holocene (6,000 years BP to Present)

Holocene pollen records from the Pacific Northwest generally show the establishment of modem forest composition

about 6,000 years BP (Baker 1983, Bamosky and others

1987, Heusser 1986. See fig. 2). Pollen of western hemlock

and western redcedar increased, and pollen of Douglas-fir,

grass, and alder decreased about that time at sites from the

central Puget Lowland to southern British Columbia. Pollen

changes were similar in the southern Lowland, except that

Douglas-fir pollen increased during the mid-Holocene. The

changes in both areas suggest a shift to more mesic forest

communities and thus a trend toward a cooler and moister

climate. This interpretation is supported by decreases in

charcoal and bracken fern spores at about 6,000 years BP.

Fire frequencies certainly decreased, favoring the expansion

of fire-sensitive species such as western hemlock and western redcedar, and most parts of the region became densely

forested by communities that were similar to those of the

present.

Pollen changes in other parts of western Washington and

Oregon also suggest a more mesic condition in the mid- and

late-Holocene. For example, on the Olympic Peninsula,

Douglas-fir and alder pollen decreased, while western hemlock and western redcedar pollen (not identified in some

diagrams) increased (Bamosky and others 1987, Heusser

1977, 1986). In the Willamette valley, pine pollen decreased

and Douglas-fir pollen increased in the mid-Holocene

(Hansen 1947).

Although pollen data do not provide evidence of the structural features of the forests of about 6,000 years BP, the

simplest interpretation of these data is that old-growth stand

characteristics developed with the shift to modem forest

composition. Structural changes may have lagged somewhat

behind compositional changes, however, because features

such as large woody debris and standing snags require time

to develop. The length of time would have varied depending

on forest type. For example, only a few hundred years would

have been required for old-growth Douglas-fir stands to

develop, but possibly more than a thousand years were

needed for old-growth structures to develop in western redcedar stands. Northwest Indian cultures that depended on

wood from massive western redcedars developed 2.000 to

3,000 years after the expansion of western redcedar on the

Olympic Peninsula and Vancouver Island (Hebda and

Mathewes 1984), suggesting that the establishment of oldI

growth conditions lagged considerably behind the first

increase in western redcedar in these areas.

Pollen records from the Pacific Northwest provide convincing evidence for a climatic cooling and an increase in moisture during the second half of the Holocene. This interpretation is supported by geologic evidence showing the expansion

of mountain glaciers (Burke and Birkeland 1983) and by

paleoclimatic simulations. Models for 6,000 years BP are

based on climatic boundary conditions similar to the present

(COHMAP members 1988, Kutzbach and Guetter 1986).

Summer insolation was slightly greater than today, but other

boundary conditions were at modem values. By 6,000 years

BP, simulations for the Northwest show the summer precipitation increased and summer temperature decreased by comparison with 9,000 years BP and thus they corroborate the

climatic interpretation of the pollen dam.

Discussion

No pollen evidence indicates that old-growth Douglas-fir

stands were an important component of Pacific Northwestern

forests before 6,000 years BP. Additional data are needed to

confirm this statement, however, because the vegetation

history of western Oregon is very poorly documented. The

lack of data from Oregon is a particular problem for interpreting forest composition during full-glacial times. Douglas

fir forests did not exist south of the ice sheet in western

Washington, but no data exist to assess whether such forests

occurred farther south. Circumstantial evidence from climatic

simulations suggests that western Oregon was too cold and

dry for the large-scale survival of Douglas-fir forests, and

pollen records from northern California indicate xeric pine

and possibly juniper woodlands during this period (Adams

1986). Thus, for old-growth Douglas-fin forests to have

survived full-glacial conditions as an important forest type in

the Pacific Northwest is unlikely.

A sufficient number of well-dated pollen diagrams are available from western Washington and southern British Columbia to conclude that old-growth Douglas-fir forests were rare

or absent during the early-Holocene. Even the less-detailed

and undated diagrams from western Oregon provide convincing evidence that old-growth forests did not exist south of

the Puget Lowland in the early-Holocene. Even if Douglas-fir

forests occupied western Oregon during full-glacial times,

they must have disappeared as a zonal forest type during the

warm, dry climates of the early-Holocene and then reassembled about 6,000 years BP.

During the early-Holocene, mesic late-successional conifers

such as western hemlock and western redcedar may have

formed small populations in moist topographic depressions

or in stable riparian sites. These fire-sensitive species would

have been relatively protected from fire in such environments.

Both species presently occur in similar settings in warm,

dry summer climates east of the Cascade Range (Franklin

and Dymess 1973). At the onset of the cooler, wetter climate

23
of the mid-Holocene, such populations would have provided

seeds for the rapid colonization of upland areas. With this

expansion, modern zonal forests were established.

As in other parts of the world, the interpretation of forest

history in the Pacific Northwest is limited by the coarse

spatial resolution of pollen data. Extensive old-growth forests

typical of the modem Western Hemlock Zone were clearly

absent from western Washington before the mid-Holocene,

but pollen data cannot evaluate the possibility that local

old-growth stands occurred at restricted, scattered locations.

Future research should address this question by examining

pollen and macrofossil records from small collecting basins

because such sites could provide records of local stand

composition (Dunwiddie 1986).

Conclusion

Modem forests in the Western Hemlock Zone were established in western Washington about 6,000 years ago. These

forests did not migrate into the region as intact communities

from the south. Instead, dominant tree species of this forest

zone had differing late-glacial and early-Holocene histories

and responded individually to climate change.

Because climate is driven by a complex set of boundary

conditions that do not change in unison, new conditions

constantly arise and disappear on time scales of several

thousand years. Over these broad time scales, tree species

continuously reassemble in different combinations that

follow the rhythm of changing climate.

24
Fire History of-Douglas-Fir Forests in the

Pacific Northwest

James K. Agee

Author

JAMES K. AGEE is a professor of forest ecology and

chair, Division of Forest Resources Management, College

of Forest Resources, University of Washington, Seattle,

Washington 98195.

Abstract

The fire history of Pacific Northwest Douglas-fir forests is

varied and complex because Douglas-fir exists in a variety of

forest types over a wide range of environments. Douglas-fir

has been dominant over this region because of disturbance

by fire and the species’ adaptations to fire. Human-caused

fires have been locally important, but lightning appears to be

most significant in explaining fire history. A lightning fire

model based on climate suggests a strong north-south gradient in lightning ignitions. The western Olympic Mountains

have a very low probability of ignition by lightning; the

southern Washington Cascades have twice as much; the

western Oregon Cascades have another 60 percent more; and

the Siskiyou Mountains have twice, again as much. Our knowledge of fire return intervals based on forest age-class data

shows a parallel history, ranging from fire return intervals

over several centuries in the Olympics to several decades in

the Siskiyous. Most Olympic forests have developed as firstgeneration stands after historic fires; Siskiyou forests are

usually multi-aged stands that have experienced several fires.

Almost all of the old-growth Douglas-fir resource is a product of fire: it not only has created and maintained such

stands but has destroyed them as well. In the short term,

management strategies to perpetuate old growth can focus on

protection against fire. In the long term, we will be forced to

recognize a more dynamic management strategy, sensitive

not only to historic fire regimes, but also to those expected

with future climatic change.

Introduction

The fire history of Douglas-fir forests is varied and complex

because Douglas-fir exists in a variety of forest types with a

wide range of environmental conditions and associated species. Classical Pacific Northwest Douglas-fir forest is characterized by those low- to mid-elevation forestswest of the

crest of the Cascade Range extending into the coastal and

Klamath Mountain regions of California. This area is commonly known as the Douglas-fir region, and is the area to

which this paper is applicable.

Although the region has been named after Douglas-fir because of the dominance of this one tree species over a wide

area, Douglas-fir is a late-successional or climax species over

only a small portion of the area for which it is named. Its

dominance at the time of European settlement was largely

due to disturbance, primarily by fire, for many centuries

before such settlement. Pollen records establish that Douglasfir dominance in past millenia often coincided with charcoal

peaks in the pollen profile (Brubaker, this volume).

25
Such disturbance by fire was neither a unique event nor one

easily characterized. Fiie can occur at various frequencies,

intensities, and extents. Severity, defined as the effect on the

tree or the stand, varied because of these factors as well as

because of the age and composition of the stands. These variables, although initially introducing a great deal of confusion,

can be used to order our knowledge of the ecological effects

of fire in Douglas-fir forests. In particular, they can be used

to understand how old-growth Douglas-fir forests across this

wide region have been created, maintained, and destroyed by

this powerful agent of disturbance.

Disturbance as an Ecological Factor

The role of disturbance in ecosystems can be characterized

by describing the type of disturbance and its frequency, magnitude, extent, and variability (Pickett and White 1985, White

1979). Moving from this simplistic characterization to the

field application, however, is often quite difficult because of

the problems associated with separating these various factors

and the lack of independence between them. For Douglas-fir,

which can live 750 to 1000 years or more, a millenium of

history may have to be unravelled to understand the evolution of present species composition, structure, and.pattem.

Many disturbances in addition to fire may also be important

in stand history. Wind can cause catastrophic damage to

stands (Franklin and Forman 1987, Ruth and Yoder 1953).

Disease and insects can create locally patchy stands (DeBell

and Franklin 1987, Russell and others 1986). Landscapewide, however, fire appears to be the primary large-scale disturbance factor. Across the region, one estimate of average

fire return interval is 230 years (Fahnestock and Agee 1983),

about one-fifth the potential longevity of an individual tree

(Franklin and Dymess 1973). Fire, therefore, is likely to disturb this “average” stand long before Douglas-fir disappears

from the stand, even where Douglas-fir is an early seral

species.

Relative Adaptations to Fire

Douglas-fir has long been characterized as a species adapted

to fire (Flint 1925, Starker 1934). Its ability relative to other

species to adapt to the presence of fue is critical to understanding the ecological impacts of fire. The fire strategy (for

example, Rowe 1981, Wright and Bailey 1982) of major tree

species of the Douglas-fir region is summarized in table 1.

The relative success of Douglas-fir compared to its competitors depends on which other “players” are part of the stand.

Where western hemlock and western redcedar are the only

other species in a mature stand and a fire kills all the trees

but does not consume all the organic mat (which would encourage the invader red alder along with Douglas-fir),

Douglas-fir is likely to be a dominant in the new stand.

Douglas-fir is also likely to be a dominant after fire in a mature stand where Pacific madrone and tanoak are understory

Table l-Relative adaptations to fire of major species in the

Douglas-fir region

’ Invaders = highly dispersive pioneets. rapid early growth;

evaders = species with long-lived progagules stored in soil

or canopy; avoiders = fire-sensitive individuals, even to lowintensity surface fires; resisters = able to resist fires of low to

moderate severity; and endurers = resprouting species.

species. In a mature state in the presence of a moderate fire,

Douglas-fir generally exhibits a “resister” strategy; and

madrone and tanoak are “endurers” that are top-killed and

will have to resprout from the ground. If a young stand with

the same species mix is burned again in 20 years, however,

Douglas-fir as a small tree will not be fire tolerant, and as an

“avoider” will be killed, leaving the madrone and tanoak to

resprout again and dominate the post-fire stand.

This system (table 1) treats the species as a variable, but it

assumes that fire is something of a constant. As the examples

above illustrate, the frequency of fire can create different

ecological effects, as can different fire intensities. Over the

region, fire frequency, intensity, and extent vary considerably, which helps to explain the myriad ecological effects of

fire.

Regional Fire Patterns

Ignition Patterns

Ignitions by humans-Ignitions by humans are known to

have been important in certain forest and grassland types of

the Pacific Northwest. The dry Douglas-fir and ponderosa

pine forests of the eastern Cascades may have been burned

frequently by Native Americans (Barrett and Amo 1982).

West of the Cascades, however, the role of aboriginal burning

is much less clear for Douglas-fir forests. Burning,by Native

Americans was a common practice in the Willamette Valley

grasslands and oak woodlands (Boyd 1986), and they apparently burned the valley bottoms of at least some of the major

tributaries of the Willamette River (Teensma 1987). Norton

(1979) and White (1980) make a strong case for aboriginal

26
ignitions in prairies adjacent to the relatively dry Poget Lowland Douglas-fK forests. Botanical (Habeck 1961, Kertis 

1986, Thilenius 1968) and pedological (Ugolini and Schlicte 

1973) evidence suggest that prairie-forest ecotones frequently 

burned and that forest area has expanded over the last century 

at the expense of prairie. Outside of these dry lowland prairie 

and woodland areas, little convincing evidence exists that 

aboriginal ignitions were a significant ignition source, although future investigations may shed more light on the 

debate. 

Some of the earliest evidence for Native American fuesetting in the Douglas-fir region was collated by Morris 

(1934). who fast documented the acconntS of large historical 

fires in the Pacific Northwest Although Native Americans 

are implicated in anecdotal accounts as the source of several 

large fires, the evidence is not overwhelming that they ignited 

many fires in upland forests. The coastal Oregon Native 

Americans were the victims of some of these fires, having 

been driven to the waters of the Pacific Ocean to survive 

(Morris 1934). The source. of these fires is never clearly 

identified or placed in a cultural context. Legends tell of two 

other fxes, both in Washington, that occurred in prehistoric 

times. The Quinault tribe has a legend of a great fii that 

swept down from the Olympic Mountains perhaps 500 years 

ago, pushing the people into the sea, but the ignition source 

is not identified (Anon. 1983). In an area roughly bounded 

by Mount St. Helens, Mount Rainier, and Centralia, a 

500 000-ha fire is rumored to have occm~ed about 1800. 

This so-called “Big Fire” was supposedly set by the Cowlitz 

tribe against tie Nisqually uibe, OI was set by the Nisqually 

tribe as a means of generating rain during a drought 

(Clevinger 1951). The existence of two stories about the 

same fire make both suspect; alternatively, either or both 

could be true. At present, the case for widespread aboriginal 

fues throughout the Douglas-fir region is not convincing. 

Lightning ignition-The variability in regional lightning 

ignition patterns is illustrated by the application of a fmcycle model based on climate. The Olympic fire-cycle model 

(Agee and Flewelling 1983) generates both significant fii 

ignitions and sizes based on climatic parameters of the site. 

For this application, only the ignition portion of the model is 

presented because the size (and fue-cycle) portion of the 

model is based on historic relations between fue size and 

climate not available regionally. The ignition portion of the 

model presents the expected ignitions exceeding 1 ha within 

a 175 OOO-harea over a given period. 

The model requires four inputs, each used by the model in 

one of twelve IO-day periods during the fire season: probabilities of long-term drought, a rain exceeding 0.25 cm, 

occurrence of a thunderstorm, and an eaSt wind. The model 

was applied to four regional locations: the western Olympic 

Mountains, the Wind River area in southern Washington, the 

WASHINGTON 

OREGON 

McKenzie River area in the central Oregon Cascade Range, 

and the eastern area of the Siskiyou Mountains (JacksonvilleKerby) (fig. 1). The probability of long-term drought, defined 

as below-average annual precipitation, was set at 0.5, with 

additional cumulative years of drought at lower probabilities 

defined similarly to the Olympic model. Short-term drought 

was defined as 1 minus the probability of significant precipitation during IO-day summer periods and was gathered for 

each area based on records from nearby stations (Manger 

1925). Thunderstorm probability was determined from 

Pickford and others (1980) for the Olympics and from 

Morris (1932) for the other sites. 

As for any stochastic model, long-term expected averages are 

best calculated using many model repetitions; here, each simulation is based on averages from a lO,OOO-year run of the 

model. This represents not the LateQuatemary period, but 

only a long-term average (or expected value) of the effects of 

20th~century weather on ignition patterns. The results should 

be inteqreted as relative rarher than absolute. values, although 

the model predicted well the proportional seasonal distribution of Olympic fires (fig. 2). 

The probability of ignitions exceeding 1 ha increases as 

location shifts east and south from the western Olympics 

(fig. 3). The Wind River area has twice as many ignitions as 

the western Olympics, McKenzie River has about 60 percent 

more than Wind River. and the Siskiyou Mountain area has 

more than twice the number of ignitions as McKenzie River. 

27 

The notion of a “fire cycle,” or a rettan interval of regular 

frequency, is not as meaningful as in drier forest types, where 

with some measure of variability a roughly cyclic occurrence 

of fire can be assumed (compare mixed-conifer forest, as in 

McNeil and Zobel 1980). Rarely in a Douglas-fir stand is the 

fire record long enough or regular enough to infer a cyclic 

patter” of fire, particularly in the presence of climatic shifts 

that would alter any cycle in operation. Different patterns of 

historic fire have clearly affected the Douglas-fir forests of 

the region, however. 

Moist Douglas-fir forests--In the moist Douglas-fir forests 

of the Coast Range of Oregon, the Washington Cascade 

Range, and the Olympics, most forests are first-generation 

post-fire forests less than 750 years old. This patter” would 

suggest a fire return interval somewhat less than 750 years. 

The tie-cycle model of Agee andFlewelling (1983) could 

not reproduce a natural fiie rotation (essentially a tire. cycle) 

of less than 3500 years using 20th.century climate patterns, 

and eve” with significant alteration in climate input to the 

model, fire return intervals could only be brought down to 

about 900 years. They suggested that perhaps much larger 

than average events may have occurred in the past (also 

suggested by Henderson and Peter (1981) for the southeastern Olympics) as a result of short-term but very extreme 

changes in two OI more of the climate parameters that drive 

the model. 

Our knowledge of the dates of old-growth forest establishment is so weak as to preclude firm hypotheses about disturbance pulses of the past. The forests around Mount 

Rainier appear tc have had a major fre about 750 years 

ago (Hemstrom and Franklin 1982). and similar-aged stands 

have been identified in the southern and western Olympics 

(Agee, pers. obs.). A series of about 650-year-old or 450- to 

500year-old fires-or both-are apparent from the data of 

Henderson and Peter (1981) in the southern Olympics, 

Franklin and Hemstrom (1981) and Yamaguchi (1986) in the 

southern Washington Cascade Range, and Huff (1984) in the 

western Olympics. Although the forest age-class 

data are sparse. these are also times of sunspot minima identified by Stuiver and Quay (1980). using tree-ring analysis of 

carbon-14 activity. If large fires are associated with these 

periods of general global cooling, they may represent periods 

where altered synoptic weather patterns, particularly during 

the growing season, contained higher frequency of lightning 

and foehn (e+st) wind patterns. 

In moist Douglas-fir forests, long early se& tree recruitment 

(for example, 75-100 years for Douglas-fir) has been documented after disturbance by fiie (Franklin and Hemstrom 

1981). This pattern is not characteristic of all prehistoric 

fires. For example, Huff (1984) shows a @l-year recruitment 

interval for a fire in about 1465 in the western Olympics, and 

Yamaguchi (1986) shows that about 95 percent of Douglas-fir 

was recruited within 40 years after a fire in about 1300 “ear 

Mount St. Helens. Even on these sites, however, the regeneration period is decades long and probably represents some 

regeneration from rrees that initially colonized the burn and 

grew large enough to produce viable seed to help completely 

restock the stand. Lack of seed source, brush competition, 

and reburns-r a combination of these factors-have been 

identified as delaying regeneration on such sites (Franklin 

and Hemstmm 1981). Patterns of rebums on the Tillamook 

fxe of 1933 at 6-year intervals (1939, 1945, 1951: Pyne 

1982). at Mount Rainier in the late 19th century, and at the 

southern Washington Yacolt burn of 1902 (A. Gray, unpubl. 

data) are evidence these sites will rebum. High surface fire 

potential during early succession in Douglas-fir forest was 

identified by Isaac (1940) as a “vicious cycle” of positive 

feedback, encouraging rhizomatous bracken fern: this patter” 

was quantified by Agee and Huff (1987, fig. 4). Given 

sufficient sources for reignition (the original Yacolt and 

Tillamook burns and all rebwns are thought to have been 

human-ignited), the rebum hypothesis is likely to be true in 

certain areas. Whether reburns were a common event before 

European settlement in the moist portion of the Douglas-fir 

region is not clear, however. 

0 

* 

Id0 260 

Years after fire 

29 
After disturbance and over time, potential surface-fire behavior declines, particularly after crown closure, and then gradually increases in the old-growth seral stage (Agee and Huff 

1987). Rebums in roughly NO-year-old stands during the 

late 1400’s suggested by Henderson and Peter (1981) suggest 

that independent crown-fire behavior in these thick-canopied 

stands may be a” additional significant type of fire. Our 

present state of knowledge is insufficient to tell. 

Mesic-to-dry Douglas-fir forest-For many years, the patter” of stand-replacement fire summarized above was a paradigm for the Douglas-fir region. Recent work, particularly in 

the Oregon Cascades, suggests a higher fire frequency, and 

different ecological role, for fire in mesic-to-dry Douglas-fir 

forest, reinforcing the output of the fire-cycle model (fig. 3). 

A site in the western Oregon Cascades (Stewart 1986) near 

the H.J. Andrew Experimental Forest regenerated after a 

stand-replacement fire in about 1530, but it has had three 

partial-mortality fires since then, in about 1660, about 1860, 

and about 1890. Some of these fires were in the settlement 

period and probably reflect human-caused fxes, but the 

partial mortality associated with them is significant. Over a 

broader area several kilometers to the southeast, encompassing similar forest types, Morrison and Swanson (1990) 

suggest a natural fire rotation of 95 to 145 years over the last 

five centuries, well below that of the moist Douglas-fir 

forests of Washington. The patchiness of at least some of the 

fires is illustrated by a fireseverity map from Morrison and 

Swanson (1990; fig. 5A). A similar fire regime was noted by 

Means (1982) on dry sites in the western Oregon Cascades 

and by Agee and Dunwiddie (1984) for dry Douglas-fir 

forests in Washington’s San Juan Islands (fig. 5B). Another 

fne-frequency analysis was completed by Teensma (1986), 

“ear the area studied by Morrison and Swanson. Using 

conservative methods that did not recognize underburns with 

no resulting regeneration or substantial fire scarring of trees, 

Teensma estimated a natural fire rotation of 100 years over 

the last five centuries. If fires of moderate severity ax 

removed from the analysis, a stand-replacement mea” fire 

return interval is 130 to 150 years, suggesting that intense 

fnes are a significant part of the natural fire regime in this 

area, but that fires of lower severity also occur. Other stands 

500 years old or older exist without much evidence of 

recurrent fire. 

These studies strongly indicate that a variable fire regime 

with much higher frequency than found in the typical, moist, 

Washington Douglas-fir forest occtxs in the central Oregon 

Cascades, and in otber music-to-dry Douglas-fir forests. 

A Mortality 

B 

n High 

0 Medium 

El Low 

ee 8 

8 Douglas-fir 

0 Pacific madrone 

30 

REGENERATION FC+t 3 FIRE REGfMES 

Douglas-fir 

A#---- F’re -1 

Hemlock/Douglas-fir 

Hemlock 

C 

-__.._--- 

I 0 500 1000 

I Time since burn 

Huff (1984) has summarized the species response to disturbance regimes for a range of wet-to-dry Douglas-fir forests (fig. 7). If fne is absent for 700 to loo0 years on wet 

sites, Douglas-fir will drop out of the stand, and western 

hemlock or Pacific silver fu will be the primary seed source 

for postfiie regeneration (fig. 7C). On sites with fire return 

intervals in the 300- to 6CGyear rang-well within the longevity of individual Douglas-fir--mixed dominance of 

Douglas-fn and western hemlock or Pacific silver fir will 

result from a typically severe stand-replacement fire (fig. 7B). 

A stand-development sequence will ccctu as illustmted in 

figure 8 (modified from Huff 1984). At age ZOO+, the characteristics of old gmwth are usually present The Douglas-fir 

componew having developed after the previous centuriesold fire, provides the live-tree criterion, and those large 

Douglas-fz and western hemlock begin to supply the largelog component. 

When fire return intervals are reduced to 30 to 70 years in 

drier and warmer environments, western hemlock may not 

even be present on the site (fig. 7A), and a stand-development 

sequence similar to that shown in figure 9 may occur. Beginning after a stand-replacement fire, the Douglas-fu regenerating on the site may survive several moderate-severity fnes 

that thin the Douglas-fr (“resisters”), remove the understory 

white or grand fir (“avoiders”), and topkill the associated 

hardwoods, such as madmne and tanoak (“endure&‘). Several reoccurrences of such fires will create a stand with 

several age-classes of Douglas-fir (some of which are large), 

and an age-class of Douglas-fir and hardwoods representing 

regeneration after the last disturbance. Understory-tolerant 

conifers of other species will also be represented in this mostrecent regenemtion. Large logs are provided by residual 

Douglas-fir or sugar pine that have died from insect, disease, 

or the last fire, or have blown over. At age 250+, the stmcture of this stand may meet the old-growth criteria, having 

developed in a very different way than tlx. wet-site Douglasfr stand. Such stands will usually be intermixed with others 

that have experienced a stand-replacement event during one 

of the intermediate fires, so that the landscape is more patchy 

than in the wetter Douglas-fir forests. 

32