Smokey Bear: Fire in Nature

Some ecosystems depend on periodic fires.

In these fire-adapted areas, fire promotes plant and wildlife diversity and burns away accumulations of live and dead plant material (leaves, branches, and trees).

Ponderosa Pine in the Northwest and Intermountain West

  • Common in the southwestern mountains as far north as Washington and Oregon, and east to the Dakotas.
  • Natural fires in this ecosystem usually occur every five to 25 years.
  • These fires tend to be low-intensity ground fires that remove woody shrubs and favor grasses, creating open, park-like ponderosa stands.
  • The life history of ponderosa pine is well adapted to high-frequency, low-intensity fires. These fires burn leaf litter and other ground vegetation, and release soil nutrients, providing a good seedbed for ponderosa pine seeds.
  • Ponderosa needles on the ground facilitate the spread of low-intensity ground fires, and minimize the danger of crown fires (fires which spread from treetop to treetop), which can kill ponderosa.
  • In ponderosa pine stands, fire is generally prescribed on five- to 10-year intervals to reduce fuel loads. Shorter burn intervals have insufficient ground litter built up to fuel the fire, and longer periods may run the risk of causing tree-killing crown fires. Prescribed fires usually result in healthy maintenance of the forest.

Chaparral

  • Common in Southern California, Arizona, New Mexico and parts of the Rocky Mountains.
  • Natural fires in this ecosystem usually occur every five to 25 years.
  • Variations in species in these areas are attributed partly to the frequency of wildfires.
  • Fire adaptations include vigorous stump sprouting after fires by many shrubs, including the manzanitas, ceanothus and scrub oak. Chamise produces dormant seeds that require fire for germination; these seeds create a large seed bank during non-fire years.
  • Many of the shrubs, especially chamise, promote fire by producing highly flammable dead branches after about 20 years.
  • After a year, the plant community is dominated by annual grasses. Five years after a fire, chaparral shrubs once again dominate the ecosystem.

Lodgepole Pine Communities of the Rocky Mountains

  • Common throughout the Rocky Mountains of the western United States (Idaho, Montana, Wyoming), generally found in unmixed stands at higher elevations.
  • Natural fires in this ecosystem usually occur every 80 to 300 years.
  • Each subsequent stage of a lodgepole pine community displays different reactions to fire. Lodgepole pines need natural fire to regenerate and maintain healthy populations.
  • Lodgepole pines often produce serotinous cones, which are coated in a resin and need a natural trigger (in this case, fire) to open and release their seeds.
  • Suppressing fire can create a fuel buildup that is difficult to manage, creating more intense fires.

Alaska’s Boreal Forest and Tundra

  • Boreal forest is common in southern Alaska, extending as far north as Fairbanks. Tundra is found in the higher elevation of this zone, extending from the Brooks Range north to the Arctic Ocean.
  • Natural fire frequency in Alaska is various, diverse and depends on region, site characteristics and vegetation. Alaska’s boreal forests burn naturally every 24 to 300 years, while the Alaskan tundra natural fire frequency is much more infrequent and ranges from 175 to 1,023 years. Because of Alaska’s cool year-round temperatures, vegetation decays at a very slow rate, releasing nutrients slowly. Large amounts of nutrients are released after a fire in the boreal forest or tundra.
  • Fires in the boreal forest and tundra typically burn in a patchwork pattern, leaving a mosaic across the landscape.
  • After a fire, plant succession takes place, meaning smaller plants like grass begin growing first before giving way to shrubs and then, eventually, trees again. To maintain ecological diversity, Alaska’s plant and animal communities are highly dependent on fire regimes.

Midwest Prairie

  • Common in Nebraska, Illinois, Iowa, Kansas and the Ohio Valley.
  • Natural fires in this ecosystem usually occur in five- to 10-year cycles.
  • Primarily made up of grasses and forbs (flowering plants), with some shrubs and trees.
  • Growth of native species such as big bluestem, little bluestem and Indian grass all increase significantly following a fire.
  • Fire in tallgrass prairies burns aboveground biomass, killing woody plants, allowing sunlight to reach the soil, and changing the soil pH and nutrient availability.
  • Because grass is a light fuel, grassland fires are not usually intense.
  • When fire is removed from a prairie ecosystem, woody shrubs and trees eventually replace grasses and forbs.

Pitch Pine “Barrens” of the Northeast

  • Common in the northeastern states, especially on sandy soils.
  • Natural fires occur in pitch pine barrens every six to 25 years.
  • Many of these fires are severe crown fires, pushed by the wind from one treetop to the next.
  • Pitch pine has thick bark that protects it from heat. It can sprout back after fire, and it reproduces with serotinous cones (cones that are opened by fire).
  • Pitch pine barrens are home to many rare plants,insects, turtles and frogs. Some of these organisms require fire to shape their habitat or to survive competition from other species.
  • If many years go by without fire, pitch pine barrens accumulate large amounts of dead wood and pine needles, which can make the next fire too severe for the pines and other species to survive.

Jack Pine Communities of the Great Lakes Region

  • Common in the forests of the Great Lakes states.
  • Natural Fires in this ecosystem usually occur approximately every 125 to 180 years.
  • Jack pine is well adapted to fire.
  • Jack pine seeds have been known to still be viable after exposure to heat as high as 1,000 degrees Fahrenheit. That heat, however, opens the scales of the cone and releases the seed onto the ground, where the fire has removed much of the existing vegetation and leaf litter.
  • Jack pine seeds require contact with mineral soil to germinate, so fire serves to prepare the seedbed, reduce competition from other plants, and release the jack pine seed.

Oak-Hickory Forests

  • Oak-hickory forests are common in Pennsylvania, West Virginia, Kentucky, Tennessee and many other states.
  • Natural fires occur in oak-hickory forests every 25 years or less.
  • Native Americans burned some of these forests very often, possibly every year or two.
  • Most oaks have thick bark, so they can survive surface fires. Both oaks and hickories sprout from the base of the trunk after fire.
  • Fires remove shade and deep litter on the forest floor, creating perfect conditions for oaks and hickories to reproduce.
  • Oaks and hickories don’t reproduce well in shade, so other tree species take over if the forest doesn’t burn for a long time.

Southern Pine Communities

  • Common in areas from Texas east to Florida, and north to Maryland.
  • Natural fire frequency in this ecosystem ranges from one to 10 years, with some of these forests historically burning every year. Most of the southern pine forests were longleaf pines that burned every one to four years prior to European settlement. Natural fire in these systems included both lightning and burning done by Native Americans.
  • Lightning-ignited fires in southern pine communities are common.
  • Longleaf pine requires mineral soil for seed germination. Ground fires prepare the seedbed by removing leaf litter and other ground vegetation, and releasing soil nutrients.
  • Different pines react differently to the frequency of fire. Longleaf pines favor frequent fires; shortleaf and loblolly pines favor less frequent.
  • In cases where fire does not occur for 25 years or more, such as when fire is removed from the system or on wet sites where fire seldom occurs, hardwoods such as oaks and hickories gradually replace the native pines.

Science Topics
Wildfires
K-6, Middle School, High School
2nd Grade, 3rd Grade, 4th Grade, 5th Grade, 6th Grade, 7th Grade, 8th Grade, 9th Grade, 10th Grade, 11th Grade, 12th Grade, Adults

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