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Wildlife, Animals, and Plants
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FIRE EFFECTS
SPECIES: Populus tremuloides | Quaking Aspen
IMMEDIATE FIRE EFFECT ON PLANT :
Small-diameter quaking aspen is usually top-killed by low-severity
surface fire [88]. Brown and DeByle [26] found that as dbh increases
beyond 6 inches (15 cm), quaking aspen becomes increasingly resistant to
fire mortality. Large quaking aspen may survive low-severity surface
fire, but usually shows fire damage [26,94]. Moderate-severity surface
fire top-kills most quaking aspen, although large-stemmed trees may
survive. Some charred stems that survived low- or moderate-severity
fire initially have been observed to die within 3 or 4 postfire years.
Severe fire top-kills quaking aspen of all size classes.
Moderate-severity fire does not damage quaking aspen roots insulated by
soil. Severe fire may kill roots near the soil surface or damage
meristematic tissue on shallow roots so that they cannot sprout. Deeper
roots are not damaged by severe fire and retain the ability to sucker
[69,160,143,146].
Mortality does not always occur immediatedly after fire. Sometimes buds
in the crown will survive and leaf out prior to the death of the tree
[26]. Brown and DeByle [26] reported that quaking aspen trees died over
a 4-year period following fires in Wyoming and Idaho, although most
individuals succumbed by the second postfire year. Even when quaking
aspen is not killed outright by fire, the bole may be sufficiently
damaged to permit the entrance of wood-rotting fungi [94]. According to
Jones and DeByle [88], basal scars which lead to destructive heart rot
can be made on even good-sized aspen by "the lightest of fires." Basal
fire scars may also permit entry of borers and other insects which can
further weaken the tree [23].
DISCUSSION AND QUALIFICATION OF FIRE EFFECT :
Fire may kill (as opposed to top-kill) a deteriorating stand of quaking
aspen. A deteriorating stand on the Sweetwater drainage of the Wind
River Mountains, Wyoming, failed to sprout following a 1963 wildfire.
However, another 1963 wildfire in the Wind River Mountains, near
Pinedale, had the opposite effect on a deteriorating stand of quaking
aspen. Although the site was considered poor for quaking aspen due to
dry, sandy soil, fire only top-killed the stand. Browsing pressure on
sprouts was light, and postfire stocking was "more than adequate" for
regeneration [69].
The position of an individual tree on a slope, or within a stand, can
influence the degree of damage caused by fire. Even when damaged, trees
located near the boundaries of a fire can often maintain a live crown.
These peripheral trees may receive food supplies from the roots of
unburned neighbors. Quaking aspen on slopes generally show greater
damage than do trees on flatter areas. Flames moving uphill often curl
up the lee side of trees when fanned by upslope wind, charring the stem
further up its bole. The effect of slope is particularly pronounced (up
to 31-44% higher char heights) after fires of higher severity. This
relationship is presented in the following table [26]:
Probability of mortality
________________________
0.90 0.95
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dbh (cm) Average char height -
10 5 12
15 14 21
20 23 30
25 32 39
Uphill char height -
10 6 16
15 19 29
20 31 42
25 44 55
PLANT RESPONSE TO FIRE :
Quaking aspen sprouts from the roots and establishes from off-site,
wind-blown seed after fire [27,123,157]. It is the classic soboliferous
species described by Stickney [157]: a plant that sprouts from
carbohydrate-storing lateral roots (sobols).
Sprouting: Quaking aspen generally sprouts vigorously after fire.
Long-term growth and survival of quaking aspen sprouts depend on a
variety of factors including prefire carbohydrate levels in roots,
sprouting ability of the clone(s), fire severity, and season of fire.
Moderate-severity fire generally results in dense sprouting. Fewer
sprouts may be produced after severe fire. Since quaking aspen is
self-thinning, however, sprouting densities are generally similar
several years after moderate and severe fire. A low-severity surface
fire may leave standing live trees that locally supress sprouting,
resulting in an uneven-aged stand [12,13,28,123].
Quaking aspen burned in spring generally sprouts later in the growing
season and again the following year. Fires in mid-growing season
generally result in late-season sprouting. Quaking aspen burned in late
summer or fall usually sprouts the next spring [28].
Predicting postfire sprouting: Appyling prescribed fire in exclosures
in Yellowstone National Park, Renkin and Despain [133] found that root
biomass can be estimated from basal area, and both can be used to
predict local response of quaking aspen to burning. Sprout biomass
produced in postfire year 1 was positively correlated (r2=0.90, p=0.013)
with both prefire basal area and root biomass. On average, 11.5 metric
tons per hectare of root mass were required to produce 0.1 metric ton
per hectare of sprouts. Average sprout height was positively correlated
with basal area and root biomass (r2=0.85, p=0.004). On average, 25
square meters per hectare of basal area and/or 19 metric tons per
hectare of root biomass were required to produce 0.5 meter of sprout
growth.
Examples of sprouting: After the 1988 fires in Yellowstone National
Park, percentage of sprouts produced in spring, 1989, was significantly
higher (p=0.030) in burned stands (mean 82%) than on unburned stands
(mean 60%). The percentage of sprouts in fall, 1989, was also higher
(p=0.103) on burned stands (mean 82%) than in unburned stands (mean
65%). In spring 1990, sprout density averaged 80,000 stems per hectare
in burned stands and 27,000 stems per hectare in unburned stands. By
fall 1991, density was 38,000 stems per hectare in burned and 25,000
stem per heactare in unburned stands, respectively. Mean heights were
9.6 inches (24 cm) in spring 1990 and 10.8 inches (27 cm) in spring
1991. Browsing intensity was much higher in winter and spring (45-55%
of sprouts browsed) than summer and fall (5-10%). There were no
significant differences in browsing among burned stands, unburned stands
adjacent to burned stands, and remote unburned stands: Sprouts were
heavily browsed in all stand types [137].
Birch-aspen: Following a 1944 summer wildfire in Maine, quaking aspen
and paper birch sprouted vigorously, forming a dense stand. In 1951,
there were 40,000 to 45,000 stems (both spp.) per acre. Quaking aspen
dominated the stand; it averaged 20 feet (6 m) in height while paper
birch averaged only 6 feet (1.8 m) [96].
For further examples of quaking aspen sprouting response after fire,
refer to the FIRE CASE STUDIES section. Cases from Arizona, New Mexico,
Colorado, Wyoming, Minnesota, and Alberta are presented.
Seedling Establishment: Fire exposes mineral soil, which is an
excellent seedbed for quaking aspen [61]. Quaking aspen seedlings have
been noted following severe fire in Canada. Six years after fire in
northeastern Wisconsin, quaking aspen seedlings composed 20 to 35
percent of seedlings of all species present on the burn [79]. Kay [90]
reported good seedling establishment following 1986 fires in Grand Teton
National Park and 1988 fires in Yellowstone National Park. Height
growth was negligible, however, due to ungulate browsing. Density,
height, and ungulate use of quaking aspen seedlings on the Yancy's Hole
Burn, Yellowstone National Park, were [90]:
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Transect # Year Number/ha % browsed Mean height (cm)
1 1989 177,202 -- 62
1991 32,154 100 50
2 1989 141,362 -- 60
1991 46,148 100 57
3 1989 109,522 -- 53
1991 16,660 100 75
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Mean 1989 142,695 -- 58
1991 31,654 100 47
Renkin and others [134] are conducting a similar seedling study on
forested and nonforested sites in Yellowstone National Park; only
preliminary data are available at this time. They found that quaking
aspen seedlings were concentrated on wet microsites but widely scattered
on other site types. In 1989, quaking aspen seedling density on 14
plots ranged from 0.6 to 1,014 per square meter; average height ranged
from 2.3 to 11.1 inches (mean=5.1 inches) (5.7-27.8 cm, mean= 12.8 cm).
Quaking aspen seedlings were two to four times taller than lodgepole
pine seedlings on forested plots. In 1990, all plots had persistent
quaking aspen seedlings; in some cases the stem had died back but the
1-year-old roots had produced suckers. Density of surviving seedlings
ranged from 0.05 to 332 per square meter. Average heights had
increased, ranging from 3.6 to 15.6 inches (mean=7.8 in) (9-39 cm,
mean=19.4 cm). Quaking aspen seedlings on fenced plots averaged 12
inches (30 cm) in height; seedlings on unfenced plots averaged 5.36
inches (13.4 cm). Seedling survival was significantly greater (p=0.004)
on forested than nonforested plots. Survival was also influenced by
presence of ungulates, spring flooding, disease, and intraspecific
competition. Ungulate presence negatively influenced seedling survival
on unfenced plots (r=0.97, p=0.004). Plots submerged in spring showed
high seedling mortality. A fungus (Venturia tremulae) also contributed
to seedling death or dieback [134].
DISCUSSION AND QUALIFICATION OF PLANT RESPONSE :
NO-ENTRY
FIRE MANAGEMENT CONSIDERATIONS :
Prescribed fire is recommended for quaking aspen [2,25,123,143].
Currently, an estimated 600 acres (240 ha) of quaking aspen burns per
year in the Intermountain Region. At that rate, it will require 12,000
years to burn the entire quaking aspen type in that Region. It is
likely that seral quaking aspen will be replaced by conifers; stable
quaking aspen stands may become less productive [46]. In many areas of
the West, quaking aspen stands have lived longer than they did prior to
fire exclusion, and many stands are in a state of decline due to
advanced age [62]. Gruell and Loope [69] found that in Jackson Hole,
Wyoming, quaking aspen stands begin to deteriorate after about 80 years.
Houston [80] stated in 1973 that quaking aspen in Yellowstone National
Park were primarily large trees ranging from 75 to 120 years of age.
Applying fire: Prescribed fire is often difficult to apply in quaking
aspen stands because of the prominence of live fuels and often sparse
distribution of fine dead fuels [25]. Even if fuels are plentiful, they
are usually too moist to burn easily. Prescribed fire may be possible,
however, when live vegetation cures enough to contribute to fire spread
rather than hinder it. The combination of dry weather and cured fuels
occurs most often in early spring, late summer, and fall [131,138]. The
forest floor of a quaking aspen stand immediately after snowmelt is
covered by matted, cured surface vegetation and deciduous leaf litter.
Before leaf-out this mat is directly exposed to drying by wind and sun,
which increase fuel temperature and decrease fuel moisture. Without
rain, the withered leaves in the litter begin to curl, resulting in a
more favorable fuelbed for combustion and heat transfer. In Alberta,
these moderately severe, early season burning conditions can persist
from snowmelt until the first week in June [131].
In most years, leaf fall and autumn precipitation coincide, making fall
burning difficult. If September and October are dry, however, burning
may be possible. Surface fuels are dead and sometimes frozen, with a
continuous layer of loosely packed leaves, making quaking aspen more
flammable than at any other time of year [138].
Live fuel moisture varies greatly between understory species throughout
the growing season, but can be estimated well enough to determine when
to light prescribed fires. Brown and others [25] estimated that when
herbaceous vegetation is the primary fine fuel, at least 50 percent
curing is needed to sustain fire spread. Less than 50 percent curing
may be sufficient in stands with substantial conifers. Brown and
Simmerman [28] provide a method for appraising fuels and flammability in
quaking aspen to assist managers in choosing when to apply prescribed
fire and help determine proper conditions for burning. Five fuel types
in 19 community types common in the Intermountain West are presented,
accompanied by color photographs.
Prescriptions: Aspen parkland and northern forest - Bailey [174,175]
found that in Alberta, prescribed burning in quaking aspen forests and
parklands in spring was usually not successful above relative humidity
of 35 to 40 percent. He recommended that prescribed burning be
conducted 8 to 10 drying days after snowmelt, when air temperature is at
least 64 degrees Fahrenheit (18 deg C), relative humidity is less than
30 percent, and 3.3-foot (10-m) open winds are 5.4 to 21 miles per hour
(9-35 km/hr).
Bailey and Anderson [173] reported that in central Alberta, quaking
aspen forest in a grassland-shrub-quaking aspen forest mosaic was the
most difficult of the three vegetation types to prescribe burn. With
spring burning, backfires consistently gave poor results, frequently
going out within a few feet of ignition and yielding a maximum
temperature of only 550 degrees Fahrenfeit (288 deg C). Headfires were
hotter but gave variable results. Most headfire temperatures ranged
from 700 to 900 degrees Fahrenheit (371-482 deg C), but 14 percent were
in excess of 1,112 degrees Fahrenheit (600 deg C). Fire and fuel data
from the quaking aspen sites follow.
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| fire temperature 393 +/- 28* (deg C) | | total fuel 13,436 +/- 354 (kg/ha) | | ground fuel 11,704 +/- 337 (kg/ha) | | standing woody fuel 1,732 +/- 181 (kg/ha) |
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*standard error of the mean (SEM)
Perala [119] recommended this prescription for burning quaking aspen
slash in the Great Lake States:
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Months for burning dormant season
(all but June, July, & August)
Fuel model* D
Air temperature > 65 degrees Fahrenheit (18 deg C)
Relative humidity < 35%
Ignition component* 40-50
Energy release component* 14-17
Spread component* 4-7
Burning index* 13-21
Wind** 2.5-5 m/s
Number of days with less
than 2.5 mm rain > 5
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*from the National Fire-danger Rating System [176]
**measured 20 ft. above ground, or at average height of vegetation
cover, averaged over at least a 10-minute period
Canadian Forest Fire Behavior Prediction (FBP) System : Alexander and
Maffey [1] provide examples for predicting fire spread rate, fuel
consumption, and frontal intensity in quaking aspen types using the FBP
System.
Forage quality and fire: Three burned quaking aspen/shrub/tall forb
communities on the Caribou National Forest, Wyoming, showed increased
forage quality (better Ca:P ratios, higher elk digestibilty, and higher
crude protein and P levels) than adjacent unburned sites during the
first postfire year. By the second postfire year, there were no
significant differences between forage quality on burned and unburned
sites. Shrubs on the unburned sites were above browse level throughout
the study period, however, while shrubs on the burned site were still
accessible to elk in the second postfire year [47].
Related categories for Species: Populus tremuloides
| Quaking Aspen
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