Features
fires:
Yellowstone
A
decade
later
Ecological lessons learnedin the wake of the conflagration
A
top a ridge in Yellowstone has already yielded up a sparse forest
National Park in 1984, a freak
summer wind-perhaps a tornado or a downburst from a thunderstorm-leveled an ancient lodgepole pine forest, piling up a head-high
maze of logs. In the notorious summer of 1988, when wildfires burned
one-third of the park, a fire front
swept across the same ridge, incinerating everything under four inches in
diameter and charring the rest. The
result was a scene so desolate that a
network television crew chose it as
the backdrop to declare Yellowstone's "legacy in ashes."
A few years later, the park service
built a wooden boardwalk across the
blackened ground at that site atop the
Solfatara Plateau above the Grand
Canyon of the Yellowstone River. At
the end of the walk, an interpretive
sign tells of the blowdown and the
subsequent wildfires and proclaims:
"After two consecutive deforestations, this site can still reseed with
grasses and shrubs, but it may remain a meadow for decades."
That sign went up against the advice of Don Despain, then a park
service biologist and one of the architects of Yellowstone's "natural
fire" policy, which mandates fighting fires only when they threaten
lives or property. According to
Despain, the sign reflects the stillpervasive myth that forest fires often
"sterilize" soils. Today, however,
Despain, who is now with the US
Geological Survey (USGS), can wear
an I-told-you-so smile as he points
out the sign to visitors. That's because only a decade after the fires,
the charred ground beyond the sign
by Yvonne Baskin
February 1999
of knee- to shoulder-height pines.
"The soil was not sterilized by
this hottest of fires, and it hasn't
become a meadow," Despain told a
group of journalists and researchers
who visited the site last May. "It's
just a temporarily less dense lodgepole pine forest."
In fact, Yellowstone's forests are
regenerating even more rapidly than
Despain and other fire ecologists had
expected. And this rapid rebound
from the ashes exemplifies the recovery that biologists are seeing throughout the park as they take advantage
of a once-in-a-lifetime opportunityno comparable wildfires have swept
the region since the early 1700s-to
study the impact of this immense force
of nature on a near-pristine Western
landscape. (For early reviews of the
ecological consequences of the fire,
see BioScience 39: 678-722.) As they
began their tenth summer of postfire field work, many of these researchers gathered to review their
findings at a symposium at Montana
State University in Bozeman commemorating the 125th anniversary
of Yellowstone National Park.
"The recovery is certainly proceeding much faster than I would have
predicted," concluded Norman Christensen, of Duke University, after the
symposium. The lodgepole and Douglas-fir forests are largely replacing
themselves, as are the grasslands and
sagebrush on the park's northern
range. The elk and bison herds were
only briefly and minimally affected.
In the streams, the cast of invertebrate characters and the physical features have changed, but that hasn't
bothered the fish. Furthermore, invading weeds have gained little
ground in the burned areas.
Effects on landscape diversity
Not only have the park's major ecosystems retained their identity-forest as forest, grassland as grasslandbut the fire did not turn the once-patchy
landscape into monotonous stretches
of forest and range. Christensen noted
that, before the fire, it might have
been plausible to assume that such a
massive disturbance would have a
homogenizing effect, "resetting the
clock" by knocking vast areas back
to a uniform stage in plant succession. But because both the burn intensities and the responses of the
vegetation varied, the fire actually
reset different patches to different
stages, and the post-fire landscape is
as patchy and highly variable as the
pre-fire landscape.
Monica Turner, of the University
of Wisconsin-Madison,
and colleagues have been documenting the
dramatic mosaic of burn severities
across the Yellowstone landscape,
from unburned patches to areas
swept by all-consuming crown fires,
and tracking the complex patterns of
succession the fires set in motion.
One of the most enduring legacies of
these differences in fire severity is
the extreme variation in the initial
density of lodgepole pine regrowth.
Lodgepole pine forests, which cover
nearly 60 percent of the park, represent the postcard image of the
Yellowstone landscape. Despite their
dominance, the pines do not create
uniform and monotonous stands.
After the fire, in fact, ecologists Jay
Anderson, of Idaho State University,
and William Romme, of Fort Lewis
College, in Durango, Colorado,
found that the density of new pine
seedlings varied by as much as four
orders of magnitude at different sites.
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Blackenedtrunks, or snags, dominate a lodgepole pine forest near Swan Lake Flat
in Yellowstone National Park in August 1989 (left), approximately 1 year after
wildfires burnedone-third of the park. On the tenth anniversaryof the fires, in May
1998 (right), the burned snags were still standing but a dense crop of lodgepole
seedlings was replacing the burned trees. Photos: Jay Anderson.
In September 1988 (left), shortly after fire swept through this Douglas-fir stand
north of Tower Junction in Yellowstone National Park,the ground looked like "the
bottom of a barbeque pit," according to ecologist Jay Anderson, of Idaho State
University.Butby the following July (right),a lush carpetof forbs had sproutedfrom
the roots and seeds that had survived in the soil. Photos: Jay Anderson.
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These variations reflect not only how
hot the fire burned but also the soil
type, elevation, patch size, and, most
important, levels of a trait called
is, production of
serotiny-that
closed cones-at different sites. And
high levels of serotiny, in turn, appear to be a legacy of past fires.
A lodgepole pine bears only one
of two types of cones: cones that
open and drop their seeds at maturity or serotinous cones, which remain closed and hanging in the
canopy, sometimes for decades, until a forest fire cracks the resin bonds
on their scales and allows them to
open, releasing seeds into what
Romme calls a "perfect post-fire
nursery bed." Surveys by Anderson,
Romme, and colleagues found that
the proportion of serotinous trees in
pine stands in their Yellowstone study
sites ranged from 0 to 48 percent.
And frequent fires select for serotiny:
Lower-elevation stands that burn
more frequently tend to produce a
higher proportion of serotinous trees,
Anderson pointed out.
After the 1988 fires, these highserotiny sites also produced the highest densities of seedling regrowth because levels of serotiny, along with fire
severity, determine the seed supply
available after a burn, Anderson said.
In fact, he and his colleagues found
that seedling densities increased exponentially with the proportion of
serotinous trees in the pre-fire stands.
Anderson's group counted anywhere
from 80 seedlings per hectare at highelevation sites where no serotinous
trees had endured severe crown fires
to "doghair" thickets of 1.9 million
seedlings per hectare in a low-elevation stand where half the trees were
serotinous. Seedling densities also
ranged from 4 to 24 times higher in
moderately burned sites than in severely burned crown-fire sites.
Despite these findings, Anderson
noted that only a "modest proportion" of trees need be serotinous to
ensure that an existing forest is replaced, even after a severe fire. In
fact, Anderson's group found that
even on the most severely burned
BioScienceVol. 49 No. 2
high-elevation lodgepole pine sites,
within 5-6 years enough seedlings had
sprouted from seeds produced by the
earliest recruits to replace the stand
that had burned. (Density in these
stands will still not equal that in lowerelevation stands because of differences
in soil types and other conditions.) If
differences such as tree density persist in Yellowstone's regenerating
plant community, then they are likely
to produce lasting variations in ecosystem processes such as nitrogen cycling and plant productivity, Turner
said, providing a long-term signature
of fire-created landscape diversity.
In another Yellowstone ecosystem-the
Douglas-fir forests that
dominate only a small percentage of
the landscape-recovery from the fire
has been slower than in the pine
stands. One such site is a muchphotographed hot burn that occurred
near a road pullout a mile northeast
of Tower Junction. Symposium organizer Linda Wallace, of the University of Oklahoma, arrived there
shortly after the fires to find a soda
bottle that had been left sitting atop
a metal trash can. The fire had blazed
so hot that the bottle had melted to
the can. Despain recalled that walking through the area left him black to
his knees, "like walking through the
bottom of a barbecue pit." If any site
in the park looked sterilized, it was
that one, Anderson agreed.
Yet that and other Douglas-fir
stands, like the pine forests, have
defied 1980s notions about hot fires
sterilizing the soil. Indeed, by the
following July, that charred fir forest was a flower garden, so dense and
photogenic with a carpet of deep
pink fireweed and yellow heartleaf
arnica that pictures of it appeared in
National Geographic and other
magazines. Today a thick stand of
knee-high firs seems to be restoring
that forest to its original density,
Anderson said. Even the hottest fire
seldom affects more than the top
inch of soil, he said, leaving seeds,
bulbs, roots, and rhizomes below
that depth to resprout.
Contrary to pre-fire expectations,
February 1999
dispersal of seeds from surrounding
unburned forest has not played an
important role in recovery to date,
Turner noted, although half of the
crown fire patches lie within 50
meters of a "green edge" in the burn
mosaic, meaning that there are potential seed sources nearby. Like the
pines and firs, most herbaceous plants
in the burned forests came from
resprouting survivors and the seeds
they produced, she said.
Legacy in the soil
Whereas tree seedlings are the most
visible legacy of the fire, the thousands of fire-blackened trunks, or
snags, that tower above the new
young pines indicate to researchers
that the fire has also set in motion
long-term changes in nutrient cycling and soil processes. Some snags
may stand for a century or more, but
many thousands of others have already toppled to the ground, creating what foresters refer to as coarse
woody debris. Botanist Daniel
Tinker, of the University of Wyoming-Laramie, said that colleagues
often refer to him in jest as a
"morticulture" specialist because of
his focus on dead wood. Yet coarse
woody debris is "so full of bugs,
fungi, and nesting birds that it could
be livelier than living trees," he said.
Rotting logs, which can take a
century to fully decay, are also important for soil structure and nutrient cycling, Tinker pointed out, raising the question of the long-term
consequences of logging practices
that virtually eliminate most of this
debris. The millions of rotting snags
in Yellowstone-criticized
by some
politicians and timber industry officials as a waste that should be salvaged-are actually providing a crucial ecological benchmark by which
to judge and compare the soil in sites
of repeated clear-cuts, Tinker said.
The impact on elk
Sagebrush and grasslands on Yellowstone's northern range comprise only
a small percentage of the park, yet
they supply critical winter forage for
elk and bison herds. Indeed, the
northern range hosts a wildlife spectacle unique to North America that
has earned it the designation
"America's Serengeti." Wallace and
others noted that the grasslands have
already recovered to pre-fire conditions, although it may take 20-30
years more to recover mature sagebrush. Wallace was surprised to find
that although the fires burned onefourth of the winter range, they took
only a temporary and insignificant
toll on large mammals, especially elk
and bison.
Extensive surveys carried out on
foot and horseback and by helicopter right after the fires found that the
blazes had killed less than 1 percent
of the park's large mammals, but a
significant elk die-off occurred during the following winter. However,
two different models-an ecosystem
model used by Wallace and colleagues to study animal survivorship
and a model of landscape carrying
capacity developed by Colorado State
University ecologist Michael Coughenour-put most of the blame not on
the loss of forage grasses to fire but
on the severity of the 1988-1989
winter.
Coughenour found that the ecological carrying capacity for elkthe number of animals the food supply could support-dropped
by 80
percent in the winter following the
fire. However, his model attributed
most of that decline to the weatherboth summer drought and heavy winter snowpack on the northern range.
The fire had a multiplier effect, reducing carrying capacity by an extra 12
percent over the weather effects
alone, Coughenour said. His model
also predicted slight declines in elk
carrying capacity in the two succeeding winters, but after that, increased
grass in the burned areas was expected to support slightly higher elk
numbers for several decades.
Documenting the actual impacts
of the fire on the elk herds is difficult
because of confounding factors, such
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as the higher numbers of elk killed
by hunters when the harsh winter
and forage loss caused more elk to
venture outside the park, Coughenour said. Still, he concluded, "the
impact of the fire pales beside the
impact of the drought and the heavy
winter snowpack. The system and
the animals have endured similar
things [fires and severe weather] in
the past and they seem quite resilient
in the face of these periodic largescale disturbances."
Aspens struggle to rebound
Elk themselves are getting much of
the blame for heading off what many
had expected would be a post-fire
renaissance of aspen in the park. For
60 years, biologists and park officials have been concerned about a
very visible decline in both aspen
stands and riparian willows. Elk
browse on both species during the
winter, and the decline of these trees
and shrubs has helped to fuel an
impassioned debate among ecologists, wildlife biologists, range scientists, and various park critics over
whether the park has too many elk.
Other researchers have suggested that
the decline of aspen is a legacy of
past fire-suppression policies rather
than elk, because aspen grow as
clones that may cover many acres,
and fire stimulates the sprouting of
new shoots from the roots.
Romme reported that aspen have
been resprouting vigorously from
roots in the park's burned stands.
Also, in higher-elevation burned forests, where aspen clones were not
present before, aspen have sprouted
from seed-quite an unusual event,
Romme said. The source of the seeds
is unknown because aspen seeds can
remain viable in the soil for only a
few weeks. Despite these encouraging events, researchers are not seeing
the abundant regeneration of fullheight trees that Romme and others
had predicted, perhaps because elk
nibble down the shoots each winter.
However, Romme is not ready to
pin the blame for the loss of aspen
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entirely on elk. Even in earlier decades, when the park actively culled
elk, those that remained still browsed
the aspen. Most of the mature aspen
now found on the northern range
grew to full size in the late 1800s
under a unique convergence of ecological conditions that has not occurred since, Romme noted. At that
time, poaching and market-hunting
of elk and trapping of beaver had
probably knocked down the numbers of those aspen eaters, and wolves
were plentiful to help in ungulate
control. Moreover, the climate was
wetter and fire frequency was low,
meaning that the aspen shoots that
did sprout after a fire had a better
chance to "escape" to full height
before the next fire.
The park is now taking a waitand-see approach to elk numbers to
find out whether recently reintroduced wolves will thin the herds,
Romme said. Meanwhile, the park's
aspen clones are still sending up
shoots, and Romme and others will
be watching to see if any of them, or
the new seedlings growing at higher
elevations, survive to maturity.
The scene in streams
When watersheds burn, changes in
streams are often dramatic, thanks
to treefall, debris and sediment flows,
bank erosion, loss of plant cover,
changes in water temperature, and
shifts in the aquatic food web. The
streams in Yellowstone were no exception. The fires have led to continuing, often dramatic physical
changes in the park's streams, sending new gravel and piles of tree trunks
into some sections, for instance, or
deepening others, according to studies by G. Wayne Minshall and colleagues at Idaho State University's
Stream Ecology Center. And the remodeling is still in process.
The composition of the aquatic
insect community that feeds the
also
park's trout populations
changed dramatically as water conditions and food resources in streams
in the burned areas changed. Ten
years later, the invertebrate community still hasn't returned to its prefire composition. Aquatic ecologist
Timothy Mihuc, of the Illinois Natural History Survey, has followed the
changes in Cache Creek, a stream in
the northeast corner of the park,
since the drainage burned. As expected, he said, after the riparian
vegetation burned, the amount of
litter falling into the stream declined
dramatically. Once-shaded reaches
emerged into full sunlight, increasing algae production in some places.
This shift in food sources provided
an advantage to insects that eat food
colproduced in-stream-so-called
lectors and scrapers, such as mayflies and riffle beetles-over the normally dominant shredders, which
feed on fallen litter and detritus.
But Mihuc found that the feeding
habits of stream invertebrates are
"very plastic," and half of the invertebrates in Cache Creek now seem to
be functioning as generalists, feeding on both in-stream food and litter. The real survival test was not a
question of food preferences, but of
whether an invertebrate could survive the harsher physical environment of the post-fire stream. "One
mayfly [Paraleptophlebia heteronea]
we tested in a feeding experiment
grew like gangbusters eating burned
detritus, but it didn't increase in the
post-fire stream because it requires
stable flow and substrate conditions," Mihuc reported. He is still
watching to see whether the stream
invertebrates will revert to their previous food preferences and whether
detritus-feeding shredders will return
to dominance.
Despite the physical and food web
changes in streams, Yellowstone
National Park Service biologist
Daniel Mahony and USGS biologist
Robert Gresswell have detected no
negative impacts on fish populations.
"Fish populations can recover rapidly when connectivity [between
streams] is maintained" so that fish
can recolonize, Gresswell said. The
best way to minimize the impacts of
fire on fish is to "stop spending
BioScience Vol. 49 No. 2
money trying to stop fires and spend years during the past 14,000 years.
it preventing fragmentation of Thus, there is no evidence of a predictable long-term fire cycle, the two
aquatic systems," he added.
Can we predict the big fires?
How long will it be until Yellowstone
faces another conflagration like the
one in 1988? The commonly cited
presumptionis that Yellowstone has
a 200- to 300-year fire cycle tied to
forest succession-the time it takes
for lodgepole pine forests to mature
and age, creatingan excess of woody
debrisand setting the stage for 198 8scale fires. But in fact, several lines
of evidence have now convinced geographer Cathy Whitlock, of the
Universityof Oregon, and other scientists that such a predictable vegetation-driven fire cycle does not
exist. Instead, Whitlock said, "climate is the driverof fire in this park."
The key is whether the year is wet or
dry, she said, althoughif fuel is abundant, the resulting fires will be exacerbated. The summer of 1988, for
instance,was the driestin morethan a
century,with high temperaturesand
extremelyhigh winds producedby a
seriesof dry high-pressuresystems.
One line of evidence for a climate-fire link comes from studies of
charcoal particles in lake-bottom
sediments. Whitlock and graduate
student SarahMillspaugh have been
radiocarbondating charcoal records
from several Yellowstone lakes to
reconstruct the fire history of the
regionin postglacialtimes.Theyhave
found that fire frequency tracks the
intensity of summer droughts and
that mean intervals between major
fires have ranged from 50 to 500
February 1999
concluded. Whitlock said that tree
ring records of fire history mapped
by Romme and Despain mesh well
with the fire frequencies indicated in
the lake-bottom charcoal records.
Further support for the wide variation in fire frequency comes from the
work of geologist Grant Meyer, of
Middlebury College, in Vermont,
who has been examining post-glacial fire history in Yellowstone by
searching alluvial fan sediments for
ancient debris flows-fast-moving
slurries of water, mud, rock, and charcoal that can sweep down bare, newly
burned hillsides after a thunderstorm.
By extracting charcoal samples from
50 of those ancient flows and dating
them using radiocarbon techniques,
Meyer has found that the stand-replacing fires that produce such flows
were more frequent in warmer,
drought-prone periods, such as the
Medieval Warm Period (900-1300)
than in cooler, wetter periods, such
as the Little Ice Age (1550-1850).
"Big fires that create sediment and
debris flows are being driven by climate," Meyer concluded.
What about the possibility that
human activities promoted such
fires? Romme and Despain believe
that the human influence on the timing and intensity of wildfires has
been overstated. The 1988 fires
sparked intense public debate over
the park's fire policy. Since 1972,
fires have been fought only when
they threaten lives or property, but
many park critics blamed the intensity of the 1988 fires on 85 years of
suppression efforts before 1972,
which allowed fuel to accumulate.
Romme contends, however, that suppression efforts were probably ineffective in the high country until the
late 1940s, with the advent of slurry
bombers and other fire-fighting technology, and so played little role in
setting the stage for a major blaze in
the forests.
The human role in igniting the
fires was also probably negligible,
Despain believes. In the summer of
1988, there were 249 fire starts, of
which 7 were responsible for 95 percent of the area burned, and 3 of
those 7 were human-caused. Nevertheless, Despain pointed out, lightning might have done the job if humans had not. On one day that
summer, monitoring systems recorded more than 2000 lightning
strikes in the park. There is never a
dearth of ignition sources, Despain
noted, so if conditions are right, fires
will start.
The right conditions apparently
mean drought, which raises a question for the future: Was the record
drought of the 1988 summer an
anomaly? Whitlock's answer is, "we
live in anomalous times." If predictions of global warming prove accurate, both the distribution and character of ecosystems in the park and
the frequency and intensity of fires
will shift, just as they have since
glacial times. The only thing that we
can predict with certainty, ChristenLI
sen noted, is change.
Yvonne Baskin is a freelance science
writer based in Bozeman, Montana, and
San Diego, California.
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