FIRE ECOLOGY
SPECIES: Populus tremuloides | Quaking Aspen 



FIRE ECOLOGY OR ADAPTATIONS : 
Fire adaptations:  Quaking aspen is highly competitive on burned sites
[46].  Even where quaking aspen was a barely detectable component of the
prefire vegetation, it often dominates a site after fire.  Quaking aspen
has adapted to fire in the following ways [41].

1.  The thin bark has little heat resistance, and quaking aspen is
easily top-killed by fire.

2.  Root systems of top-killed stems send up a profusion of sprouts for
several years after fire.  

3.  Sprouts grow rapidly by extracting water, nutrients, and
photosynthate from an extant root system, and may outcompete other woody
vegetation.

4.  Following a fire, a new, even-aged quaking aspen stand can develop
within a decade.

5.  In contrast to most trees, quaking aspen is self-thinning.  Without
intervention, a mature forest of healthy trees can develop from dense
sprouts. 

Fire releases sprout primorida on roots from hormonally controlled
growth inhibition; removes canopy shade; and blackens the soil surface,
increasing heat absorption.  Increased soil temperatures aid sprout
production [22,83].  On cold sites, quaking aspen may be unable to
sprout until soil temperatures rise after fire [83].

Quaking aspen is able to naturally regenerate without fire or cutting on
some sites [123], but fire may be required for regeneration on others.
There are areas in Jackson Hole, Wyoming, where ungulate browsing has
been light, both historically and recently, yet stems have not attained
tree size since extensive fires in the 1800's [69].

Fuels and fire behavior:  Fuels are usually more moist in quaking aspen
stands than in surrounding forest.  Crown fires in coniferous forests
often drop to the surface in quaking aspen, or may extinguish after
burning into quaking aspen only a few meters [19,55,138].  Quaking aspen
stands often act as natural fuelbreaks during wildfires [55], and fires
sometimes bypass quaking aspen stands surrounded by conifers [138].  In
an analysis of fires in quaking aspen in National Forests of the
Intermountain West (USFS Regions 2, 3, and 4) from 1970 through 1982,
Bevins [19] reported that wildfires that burned thousands of acres
during extreme weather conditions usually penetrated less than 65 feet
(20 m) into quaking aspen.  Managers he interviewed used the terms
"abestos type" and "firebreak" to describe quaking aspen stands.  Bevins
reported that mixed quaking aspen-conifer types such as those on the
northern Kaibab and Dixie National Forests did sustain fires, however,
and burned substantial amounts of quaking aspen.  Throughout all three
Regions, a relatively few, large fires (>100 acres burned) accounted for
93.2 percent (or 1.12 million acres) of all quaking aspen burned.

Fire history:  Before and during the mid-nineteenth century, fires were
apparently more frequent, and larger acreages of quaking aspen and
quaking aspen-conifer mixes burned, than any time since.  A large
majority of the quaking aspen stands in Jackson Hole, Wyoming, date from
fires between 1850 and 1890 [69].  In central Utah, Baker [5] and
Meinecke [106] found few quaking aspen fire-scarred later than 1885.
Earlier fire scars were common and showed a 7- to 10-year fire
frequency.  Since quaking aspen is fire-sensitive, the fires were
probably of low severity.  Extensive sampling of quaking aspen in
Colorado found few fire scars dating later than about 1880 [37].

These data indicate that there has been a great reduction of fire
rejuvenation of quaking aspen in the West since about 1900.  Extensive
young stands of quaking aspen are uncommon in the West [65,151,46].
Conifers now dominate many seral quaking aspen stands.  Probable
contributing facters are:

1.  highly effective direct control of wildfires in the last 50 years,
especially in the quaking aspen type [46],

2.  reduction of fine fuels in quaking aspen/grass and quaking
aspen/forb types due to grazing [28,46], and

3.  cessation of deliberate burning by Native Americans [9,68,80].

Ungulates, fire, and quaking aspen:  In most areas, ungulate browsing is
probably not a major factor restricting postfire quaking aspen
regeneration.  Quaking aspen has increased in importance in the East
depsite browsing pressure from large white-tailed deer populations.  In
many areas of the United States, elk populations impact quaking aspen
very little.  Browsing elk had no significant impact on quaking aspen
sprout density after wildfire in New Mexico [115].  In some areas,
however, fire suppression coupled with heavy ungulate browsing has
reduced quaking aspen regeneration.  Failure of some stands in the Great
Lakes States to regenerate has been attributed to overbrowsing of
sprouts by white-tailed deer [145].  Overbrowsing has particularly been
noted in northwestern Wyoming, in Yellowstone and Grand Teton National
Parks and the Bridger-Teton National Forest.  Elk are the primary
browsers of quaking aspen in this area, although where moose populations
are high, moose have also removed considerable quaking aspen
regeneration.  Historic narratives and photographic evidence suggest
that ungulates were a major biotic influence on quaking aspen in this
region during the the exploration and settlement periods.  However,
fires were extensive during this period, so postfire sprouting of
quaking aspen and growth of palatable grasses, shrubs, and herbs,
probably produced a forage supply that dispersed browsing ungulates
sufficiently for quaking aspen to regenerate [69].

Coring of old quaking aspen stems in Yellowstone National Park showed
that most live, large quaking aspen established in a brief period
between the 1870's and 1880's:  a period of severe fires followed by
above-normal precipitation.  Elk, moose, and beaver populations were at
a historic low, and some wolves were present.  Neither this combination
of conditions nor significant quaking aspen regeneration has occurred
since then.  Elk populations were low in the 1950's and 1960's, but
fires were suppressed and the climate was dry.  In the 1910's, there
were numerous elk and beaver and few fires.  After the 1988 fires, elk
numbers were high and climatic conditions were dry.  In this region,
even large-scale burning does not seem sufficient for quaking aspen
regeneration [69,90,137]. 

Prairie:  Frequent fires on prairies and plains grasslands historically
helped control quaking aspen invasion [30].  Fire may have been only one
of several factors controlling quaking aspen, however.  Drought [76] and
ungulate browsing may have worked in conjunction with fire to curtail
woody plant invasion.  Fire alone may not control quaking aspen spread
[32].  Anderson and Bailey [2] reported that 24 years of annual spring
burning checked quaking aspen invasion onto tallgrass prairie, but
actually increased the number and cover of quaking aspen sprouts in the
area.  Elk Island National Park, Alberta, was described by early
settlers as a grassland with scattered quaking aspen groves.  By 1895,
extirpation of bison and severe reduction of other ungulates was
followed by expansion of quaking aspen.  Bison were reintroduced with
Park establishment, but fire was not.  Ungulate populations rose rapidly
and were culled in the 1930's and 1950's.  Grassland expanded with the
ungulates, while quaking aspen expanded when culling occurred [21].


POSTFIRE REGENERATION STRATEGY : 
   Tree with adventitious-bud root crown/soboliferous species root sucker
   Initial-offsite colonizer (off-site, initial community)

Related categories for Species: Populus tremuloides | Quaking Aspen