It’s difficult to see the rebirth of a forest after a wildfire. The speed and extent of recovery depend 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.
Pinecones of some species will not open and release the seed until exposed to the heat of the fire. Given these adaptations to fire, recovery of many plants will occur, often rapidly.
How does fire affect different forest and tree anatomy?
Forests Types
In Colorado, the major forest types are:
- Pinon pine-juniper
- Ponderosa pine
- Douglas-fir
- Lodgepole pine
- Spruce-fir
- 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 these parts of the tree must survive:
- Roots
- The cambium of the main trunk
- Buds
The cambium is the sensitive layer of growing cells that produce the vascular system that conducts water and nutrients through the tree. 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.
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 it can impact the development and survival of the buds. New buds may not have formed before 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 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 the root system. Fast-moving fires may not destroy the duff layer and may cause little root damage.
How does fire affect different tree species?
Ponderosa Pine and Douglas Fir
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 of a fence post) 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 ax. 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 are considered the principal 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 lose than ponderosa pine. Trunks nine 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. The Douglas-fir also has shallow lateral roots that are susceptible to damage.
Currently, many Ponderosas pine/Douglas-fir forests are overcrowded. This leads to higher mortality rates due to fire when compared to well-spaced stands
Regeneration From Seeds for Ponderosa Pine and Douglas Fir
Regeneration
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.
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 seeds. 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.
Moisture conditions are critical for seed germination and seedling survival. Fire effects on the forest floor will impact the success of the 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.
Aspens
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.
While conifers successfully outcompete 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. The diameter of the trunk influences the tree’s resistance to fire. Diameters of six inches or more are often quite resistant.
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.
The reestablishment of these trees is from seed; rodents and birds often store large amounts of seed. A severely damaged stand will convert into a shrub community with the gradual reintroduction of trees at 60 to 100 years.
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 seedbed.
Spruce Fir
Spruce-Fir Stands of spruce and true fir occupy the highest elevations of Colorado’s mountains. The moisture and temperature conditions here are often less favorable for the 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. 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.
In larger burned areas the seeds may not re-establish 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 that larger trees provide.
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 them susceptible to burning.
Shrubs and Forbes
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 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.
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 the fire. Oak rhizomes are often four to 20 inches deep in the soil profile. The density of Gambel oak often increases after a 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. 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 the 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 a fire. Buds and grass meristems (growing points) are protected by being deeper in the soil profile.
Author, Disclaimer, and References
Author
By R. Moench (08/20)
Moench, nursery manager, Colorado State Forest Service
Published 11/02. Revised 8/20.
Disclaimer
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Milan A. Rewerts, Director of Cooperative Extension, Colorado State University, Fort Collins, Colorado. Cooperative Extension programs are available to all without discrimination. No endorsement of products mentioned is intended nor is criticism implied of products not mentioned.
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) 4916303; Fax (970) 491-7736; www.colostate.edu/Depts/CSFS:
- 6.302 Creating Wildfire Defensible Zones
- 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
