Quick Facts...
- The ability of a tree to withstand fire damage is based on the thickness
of the bark, rooting depth, needle length, bud size, and degree of scorch.
- Conifers are cone-bearing trees that reestablish only from available
seed reserves. Wind and wildlife aid in seed dispersal into the burned
area.
- Fire often prepares a suitable seedbed by exposing mineral soil necessary
for good germination.
- Aspen is unique among Colorado forest trees in its ability to sprout
new stems after a fire. Many shrubs and grasses share this ability to
resprout from under-ground roots undamaged by fire.
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| In 2000, the Hi Meadow Fire, southwest of Denver,
caused extensive damage to the area's natural resources. |
It's difficult to see the rebirth of a forest after a devastating fire. The
speed and extent of recovery depends on the severity of the fire, when it occurred,
and the plant community.
Nature has equipped many plants to recover quickly from fire. The root
systems of many plants run deep and are often protected from lethal temperatures.
Once the above ground stem is removed, roots are stimulated to send up
new shoots. The soil profile may contain many dormant seeds waiting for
the right conditions to germinate and grow. Some trees are so adapted
to fire that they rely upon it to regenerate new stands. Pine cones of
some species will not open and release the seed until exposed to the heat
of fire. Given these adaptations to fire, recovery of many plants will
occur, often rapidly.
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| Figure 1: Outer bark, inner bark, cambium, sapwood and heartwood of a tree.
Cambium must survive a fire for the tree to survive. |
Trees
In Colorado the major forest types are pinon pine-juniper; ponderosa pine-Douglas-fir;
lodgepole pine; spruce-fir; and aspen. Fire is more frequent in pinon-juniper,
ponderosa and lodgepole pine trees. These types of trees are typically lower
in elevation and dryer than the spruce-fir forest types of the subalpine zone.
Aspen is Colorado's major deciduous forest type. Response to fire varies considerably
among coniferous and deciduous trees. Colorado conifers are limited in their
response to fire. Unlike some deciduous trees and shrubs, the root systems of
conifers do not regenerate new stems or "suckers."
Conifer Trunks
In order for a conifer (a cone-bearing tree) to survive, some of the
roots, the cambium of the main trunk, and buds must survive. The cambium
is the sensitive layer of growing cells that produces the vascular system
which conducts water and nutrients through the tree (Figure 1). The bark
insulates the cambium from the damaging intensity of a fire; the thicker
the bark, the better the protection. Bark thickness varies with age and
differs by species. (Table 1.)
Buds
Buds are located at the terminal end of branches in trees. Buds begin
forming at the end of spring to provide tissues for next year's growth.
The foliage (needles) of a conifer provides some protection to the buds. Longer
needles provide more protection than short ones. When the fire occurs can impact
the development and survival of the buds. New buds may not have formed prior
to an early summer fire, which reduces its survival.
The amount of scorched foliage in the tree crown can predict conifer
survival to a certain degree. Even with severe scorch damage, the buds
may survive and grow the following spring. Basing survival estimates on
scorch alone can be misleading; foliage color after a fire can be deceptive.
Buds should be carefully examined – they should be firm and the terminal
stem flexible. The bud or stem should not break off easily.
Roots
Damage to roots depends, in part, on the nature and overall depth of the
root system in the soil profile. The amount and depth of the duff layer (needles,
leaves, and other recognizable litter on the forest floor) can impact a fire’s
effect and damage to the root system. Fast moving fires may not destroy the duff
layer and may cause little root damage. (Figure 2.)
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| The use of an axe can reveal the cambium layer sufficiently to evaluate
fire damage.
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Ponderosa Pine and
Douglas-fir
The degree of damage to roots, stems, and the crown determines whether
trees will survive a fire. Bark thickness plays an important role in the
survival of these trees. As a ponderosa pine matures, it develops a very
thick bark that insulates the cambium from damaging heat. Even if the
bark is considerably scorched, the cambium can remain undamaged.
Ponderosa pine roots run deep thus providing further protection. Hot slow
moving fires will often destroy the duff layer and cause root damage to
shallow rooted species such as Colorado blue spruce.
Trees beyond the pole stage (about the size good for fence and corral
posts) are very resistant to fire damage if they are not too crowded.
The crowns of larger trees are more elevated, thus protecting the buds
and foliage from heat scorch.
Cambium damage can be evaluated by chipping away a small section of bark
with an axe. A healthy cambium is a light tan or cream color. Dead cambium
is dry, brown or gray, and has a sour fermented smell. A large amount
of pitch exuding from deeply charred bark can also indicate cambium damage.
Crown scorch and bud kill is considered the principle cause of death.
In healthy, well-spaced stands mortality is usually low. Ponderosa’s
lengthy needles can provide sufficient protection to the buds, which are
large and well protected by heavy scales.
Douglas-fir shares similar bark characteristics with ponderosa pine.
Both are more fire resistant than spruce and true fir. Douglas-fir needles
are very short in comparison with ponderosa pine. These offer little protection
to the small buds. Douglas-fir saplings are more prone to loss than ponderosa
pine.
Trunks 9 inches in diameter or larger can survive low to moderate intensity
fires. If 25 percent of the cambium is damaged, a Douglas-fir will most likely
die. In addition, Douglas-fir has shallow lateral roots that are susceptible
to damage.
Currently, many ponderosa pine/Douglas-fir forests are over-crowded.
This leads to higher mortality rates due to fire than in well-spaced stands.
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| Cones of Ponderosa pine.
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Regeneration From Seed
Natural reestablishment of ponderosa pine and Douglas-fir can occur from
seed depending on the presence of cones on the tree. Most pines do not
develop cones every year. Cones of pine require two seasons to mature.
Cones typically mature and release their seeds to the wind in late summer
and early fall. In some cases cones may continue to mature on a top-killed
tree and release a viable seed crop. While most pines have variability
in cone crop production, the Douglas-fir is more regular.
In a severe fire only live trees around the perimeter of a burned area
may produce viable seed. Wind dispersal of the seed is often limited to
a few hundred feet from the seed-bearing tree; birds and rodents also
help distribute seed.
Even with a good seed crop, moisture conditions must be optimal for seed
germination and seedling survival. Fire effects on the forest floor will
impact the success of seedling establishment. Most conifers require bare
mineral soil for successful germination. The litter layer is often consumed
in slow moving fires, which exposes the necessary mineral soil.
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| Regeneration of aspen after a fire can occur from extensive
root damage. |
Aspen
Aspen can form extensive pure stands in Colorado, but are also present
to a greater or lesser degree in many other forest types. As a result,
the aspen component in a conifer stand can greatly increase after a fire.
In addition, while conifers successfully out-compete aspen in a non-fire
situation, after fires occur, aspen may regenerate in a pure stand. This
is due to the extensive suckering from roots when the main trunk of the
aspen is destroyed.
Thin aspen bark makes it susceptible to fire damage. Pure stands are
often missed or jumped in some fires due to the low flammability of aspen.
Again, the diameter of the trunk influences the trees resistance to fire.
Diameters of six inches or more are often quite resistant
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| Pinon pine destroyed by fire. |
Pinon-Juniper
Both piñon and juniper are very susceptible to fire damage and
are easily top-killed. Both have thin, highly flammable bark that provides
little insulation to the cambium.
Reestablishment of these trees is from seed; rodents and birds often
store large amounts of seed. However, this can be a very slow process.
Typically, a severely damaged stand will convert to a shrub community
with gradual reintroduction of trees at 60 to 100 years.
 | Lodgepole pine regeneration 13 years
after a fire. |
 | Some cones of lodgepole pine are serotinous, remaining closed
until intense heat from a fire forces them open.
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Lodgepole Pine
Lodgepole pine forests are very adapted to a natural fire regime. Regeneration
of new lodgepole seedlings can be rapid (as little as two to three years).
Lodgepole bark is thin in comparison to that of ponderosa pine. Temperatures
lethal to the cambium are common.
A unique characteristic of lodgepole pine is its serotinous cones. The
cone scales can remain closed for several years because of a resin coating.
During an intense fire this resin melts away allowing the cone scales
to open, thus releasing the seed. After a fire, a massive number of seeds
are released. An intense fire also exposes mineral soil to provide a good
seed bed.
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| Bare mineral soil exposed after a fire. |
Spruce-Fir
Stands of spruce and true fir occupy the highest elevations of Colorado’s
mountains; moisture and temperature conditions here are often less favorable
for development of an intense fire. Catastrophic fires are less frequent in
this zone; however, when fires do occur, they can be intense.
Colorado blue spruce can be a component in lower forest types. Its thin
bark and shallow roots make it very susceptible to fire damage. Branches
often grow low to the ground, which allows the fire to climb into the
crown and destroy the tree.
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| Wildflower regeneration after fire. |
Engelmann spruce and subalpine fir forests are common in higher elevations
of the state. Both trees share characteristics making them highly susceptible
to fire mortality. The bark is thin and ignites easily, the roots are
shallow and the branches grow near the ground.
Recovery of these trees after a fire can be difficult and slow. Both
are adapted to a cool and shady environment. Seedlings may become established
in small burns of 1/10 acre or less. Larger areas may not reestablish
because seedlings are intolerant of the intense sunlight at this elevation.
New seedlings may establish at the perimeter of a larger fire. The seedlings
require the shade the larger trees provide.
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| This stand of gambel oak resprouted quickly after a fire. |
Other Conifers
Limber pine and bristlecone pine are present in scattered stands throughout
the state. Fire is relatively uncommon in the zone where these pines occur.
Limber pine bark is thin but mature trees are more fire resistant. The
large seed of limber pine is attractive to Clark’s nutcracker, which
can be instrumental in caching and dispersing limber pine seed.
White fir is common in southern Colorado at mid-elevations. Like subalpine
fir it has a thin bark and is susceptible to top-kill in the sapling and
pole stages. White firs have shallow root systems, low growing branches
and foliage making it susceptible to burning.
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| This stand of gambel oak resprouted quickly after a fire. |
Shrubs, Grasses and Forbs
Unlike conifers, many shrubs, forbs and grasses readily sprout from underground
root structures after a fire. These root structures vary in size, shape
and depth in the soil profile. Fire severity directly impacts these structures
and influences which species regenerate. Slow moving fires destroy the
duff layer and heat the soil to lethal temperatures. Sometimes shallow
root structures are destroyed favoring those species with deeper roots.
However, when a forest canopy is so dense that there is little or no understory,
it may take considerably longer for grasses and shrubs to come in after
a wildfire.
Dormant buds can be located in the roots. These become the new growing
points for reestablishment after a fire.
Common Colorado shrubs, such as antelope bitterbrush, rabbitbrush, and mountain
mahogany, can sprout from the root collar (the point where the stem and root
meet). The root collar is rather shallow and may be destroyed in a slow moving
fire that consumes the duff layer.
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| Natural recovery of forb and grass community. |
Rhizomes (horizontal underground stems – not roots) occur deeper
in the soil profile. This depth protects the rhizome from lethal temperatures.
Gambel oak and chokecherry are common species with rhizomes in this state.
Gambel oak often resprouts readily after fire. Oak rhizomes are often
four to 20 inches deep in the soil profile. The density of Gambel oak
often increases after fire. Many types of willow also resprout quickly.
Forbs, which include many common wildflowers, have similar root structures
that resprout after fire depending on fire severity and depth of the root
structure. Wildflowers, such as Indian paintbrush, lupine, and columbine
resprout from an underground structure called the caudex. Some wildflowers
form underground bulbs or corms develop and new sprouts.
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| Effects of subsurface heating on postfire sprouting on rhizomatous
shrub (adapted from Wildland Fire in Ecosystems: Effects of Fire on Flora, GTR RMRS-
GTR-42-volume 2.) |
The seeds of many shrubs and forbs can persist in the soil for years
after dispersal. A fire can open an area to greater sunlight and warmth
stimulating germination of some seeds. Hard seeds with thick seed coats
are ruptured by fire allowing the seed to absorb moisture and germinate.
Raspberry, gooseberry, currant, plum, and chokecherry seed are stimulated
in this way.
Many grass species are rhizomatous and regenerate readily after fire.
Buds and grass meristems (growing points) are protected by being deeper
in the soil profile.
References
USDA Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory
(2001, May). Fire Effects Information System, [Online]. Available: http://www.fs.fed.us/database/feis/
[June 22, 2001].
Brown, J.K.; Smith, J.K. eds. 2000. Wildland fire in ecosystems: effects
of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol.2 Odgen, UT: U.S. Department
of Agriculture, Forest Service, Rocky Mountain Research Station, 257 p.
From Colorado State Forest Service, Colorado State University, Fort Collins,
CO 80523-5060; (970) 491-6303; Fax (970) 491-7736; www.colostate.edu/Depts/CSFS:
• 6.302, Creating Wildfire Defensible Zone
• 6.303, Fire-Resistant Landscaping
• 6.304, Forest Home Fire Safety
• 6.305, FireWise Plant Materials
• 6.306, Grass Seed Mixes to Reduce Wildfire Hazard
• 6.308, Soil Erosion Control after Wildfire
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