Forests and Fire Notes

Forests and Fire

Ecological Role of Fire

  • Fires create landscape patches, ranging from totally burned, partially burned, and unburned areas.
  • The burn mosaic results in varied vegetation regrowth rates, creating patches and increasing heterogeneity.

Heterogeneity

  • Landscape-level heterogeneity is increased.
  • Stand-level heterogeneity is also increased, leading to:
    • Different aged patches
    • Different vegetation types/sizes/foliar nutrition
    • Edge effect
    • Increased soft/hard mast production
    • Snags/Downed woody debris
  • Examples of wildlife responses:
    • Quail and turkey benefit from more open conditions.
    • Red-cockaded woodpeckers (RCW) benefit from regular, low-intensity fires that kill hardwoods.
    • Scrub jays benefit from periodic fires that recreate scrub habitat.
    • Moose benefit from shrubby growth on the forest edge.
    • Deer benefit from resprouting trees.

Fuel Reduction

  • Fires reduce fuels such as litter, woody debris, and ladder fuels.
  • Regular, low-intensity fires reduce the risk of extremely hot, uncontrollable fires.
  • The distance between surface fuel and tree crowns makes it more difficult for crown fires to start.
  • Periodic fires spread through surface fuel.
  • The surface fire may not reach the tree crowns.
  • The fire consumes small plants, while taller trees escape with scorched bark.

Seed Germination

  • Fires reduce the depth of the organic material layer, improving seed germination.
  • Blackened litter layer warms soils and promotes faster nutrient cycling and early seedling establishment.

Seed Release

  • Many species are adapted to fire through cone serotiny.
    • Examples: lodgepole pine, jack pine, black spruce.
  • Seeds are released when the seedbed is prepared.

Fire-Dependent Species

  • Many tree species are fire-dependent and fire-adapted.
  • These species are outcompeted by shade-tolerant species in the absence of fire.
  • Examples: Upland Oak Resprout and New Lodgepole Pine Recruits
  • Without fire, low-value, poor-quality, shade-tolerant hardwoods often occupy land best suited to growing pine and upland oaks.
  • Woody plants encroach into grasslands without fire.

Pest and Pathogen Control

  • High temperatures and smoke directly kill insect adults, larvae, and eggs, fungal spores (e.g., root rot), and parasites (e.g., mistletoe).
  • Reduced tree density after fire decreases susceptibility to insect attack.

Other Benefits

  • Aesthetics.
  • Removal of logging or other woody debris.

Simplified Ecological Role of Fire

  • No Fire:
    • Dense tree stands rob lower plants of sunlight. Many die and accumulate as deadwood and litter. Fires here burn hotter and are more dangerous.
  • Fire Cycle:
    • Fire starts again by lightning or humans.
    • As the forest gets older, food for wildlife is reduced.
    • As the forest grows, it provides a home and food for various kinds of wildlife.
    • New growth produces a variety of food.

Fire as a Natural Disturbance

  • Fire is a natural disturbance in terrestrial ecosystems.
  • Fire regime = size, severity, intensity, timing.
  • Ecological role of fire includes:
    • Creating heterogeneity.
    • Increasing wildlife habitat diversity.
    • Reducing fuels/preventing catastrophic fires.
    • Preparing seedbed/releasing seeds.
    • Perpetuating fire-dependent species.
    • Managing competing/undesired species.
    • Controlling pests/pathogens.

Controls on Fire

  • General Fire Triangle: Heat Source, Oxygen, Fuel.
  • Fire Behavior Triangle: Weather, Topography, Fuel.

Fuels

  • Aerial Fuels: All green and dead materials located in the upper forest canopy including tree branches and crowns, snags, moss, and high shrubs.
  • Surface Fuels: All materials lying on or immediately above the ground including needles or leaves, duff, grass, small dead wood, downed logs, stumps, large limbs, and low shrubs.
  • Ground Fuels: All combustible materials lying beneath the surface including deep duff, roots, rotten buried logs, and other organic material.

Fuel Characteristics

  • Surface fuels are grouped into 6 classes to aid identification:
    • Grass fuels: support very fast spread, increasing with wind and slope.
    • Grass/Shrub fuels: spread slower than grass but can be more difficult to control.
    • Shrub/Brush fuels: often require more wind for fire spread, like crown fires.
    • Timber Understory: has live and dead ladder fuels that can cause crown fire.
    • Timber Litter fuels: are not influenced by living plants. Burn intensity varies by load.
    • Slash fuels: have heavy dead loads. They burn intensely but spread more slowly.

Dead Fuel Moisture

  • Moisture and flammability of dead fuels are governed by the weather conditions around them.
  • Fine dead fuels, like grass and litter, gain and lose moisture from hour to hour. Peak burning conditions typically occur late in the afternoon.
  • Larger dead woody fuels dry more slowly and usually burn most readily during the peak season.

Fire Types

  • Ground Fires: Burn on or below the forest floor in the humus layer down to the mineral soil. Consumes organic material beneath the surface litter, usually exhibiting smoldering.
  • Surface Fires: Most common; burn along the floor of a forest. Burn loose debris on the surface, which includes dead branches, leaves, and low vegetation.
  • Crown Fires: Burns in the elevated canopy fuels. Spread rapidly by wind and move quickly by jumping along the tops of trees.

Fire Type Comparison

  • Surface Fires:
    • When fire is on the ground
    • Smoldering, Creeping and Spreading
    • Little fire spread
    • Minimal flaming, less than 1'
    • White smoke, Combustion of ground fuels
    • Intermittent surface fire, Slow spread
    • Visible open flames 1' - 4'
    • Little torching, Generally white smoke
    • Vigorous surface fire, Flames 4'-8'
    • Flammable canopy can ignite, Moderate to fast (grasses) spread
  • Crown Fires:
    • When trees and shrubs burn
    • Running, Torching and Spotting, Crowning, Erratic and Extreme
    • Single tree to group tree torching
    • Surface flames 8' - 12'
    • Moderate to fast spread, Gray to black smoke
    • Crown fire front at head, Fast spread rates
    • Black to copper smoke, Long-range spotting
    • Extreme fire environment, Extreme intensity
    • Turbulent fire, Chaotic spread, Interface fuel involvement

Fire Impacts and Fire Regime

  • Fire impacts on ecosystems depend on the size, severity, intensity, and frequency.
  • These factors together constitute a fire regime.

Fire Regime Controls

  • Controls of fire activity across temporal and spatial scales:
    • Microsite (Seconds): oxygen, heat
    • Fire Behavior (Days): weather, fuel, topography
    • Wildfire (Decades to Centuries): climate, ignitions, vegetation, fuel

Fire Size

  • Influences lateral flow of materials and organisms.
  • Can shift vegetation density and composition of the ecosystem.

Fire Severity

  • Magnitude of change in resource supply or environment.
  • Typically, is the amount of organic matter (plants/soils) removed by the disturbance.
  • Major factor determining rate and trajectory of vegetation development after disturbance.
  • Measures of Fire Severity:
    • Unburned/Low Severity: Less than 25% tree mortality, limited effects on soils.
    • Moderate Severity: 25-75% tree mortality, moderate effects on soils.
    • High Severity: Greater than 75% tree mortality, extensive mineral soil exposure.

Pyrodiverse Landscapes

  • Mixed severity effects produce pyrodiverse landscapes.
  • LOW INTENSITY, MODERATE INTENSITY and HIGH INTENSITY are all part of a pyrodiverse landscape.

Fire Intensity

  • Technically, energy release per unit area and time.
  • E.g., Fire intensity = rate of heat production (depends on mass, consumption rate, and energy content of fuel).
  • Fires can be intense but not severe (Keeley 2009).
  • Measurement tools: Pyrometers, Thermal Infrared Cameras, Thermocouples, Digital Thermometers.

Fire Frequency

  • How often the fire occurs.
  • Ecosystems are most resilient to disturbances they frequently experience.
  • Affects the ability and amount of time available for recovery.

Historical Fire Frequency

  • Fire scars on tree rings are used to determine historical fire frequency.
  • Sedimentary charcoal records are also used.

Longleaf Pine Forests

  • Most well-known fire-adapted forest of Alabama.
  • Once covered between 10-13 million acres of the state; 90 million acres across region.
  • Found in a variety of landforms from deep sandhills to mountain ridges up to 1,500 feet in elevation.
  • Dominated by longleaf pine, but also contain slash and shortleaf pine and sometimes fire-tolerant oaks like blackjack, turkey, and post oak.
  • Original stands have been impacted heavily by logging, grazing, tapping for naval stores, fire suppression, conversion to loblolly, slash, agric. Of the original 90 million acres of longleaf, less than 5% remains; considerable effort in recent years to restore longleaf systems which have numerous T&E species.
  • 29 federally protected animals and plants, including the red-cockaded woodpecker ( Picoides borealis), Eastern indigo snake (Drymarchon corais couperi), gopher tortoise (Gopherus polyphemus) and Cooley’s Meadowrue (Thalictrum cooleyi).
  • Of the 290 species of amphibians and reptiles that occur in the Southeast, 170 are found in the remnants of the longleaf pine forest.

Fire Regime in Longleaf Pine Forests

  • Burned about every three years (i.e., low intensity but high occurrence).
  • Fire kept shrubs and other fire-intolerant hardwoods low in stature, giving the forest a park-like appearance.

Longleaf Pine Adaptations

  • Seedlings go through a “grass stage” for several years until released.
  • Early growth is slow but can maintain growth for a long time due to grass stage.
  • Fire in the grass stage is important to control brown needle spot disease. Spores live in the soil.
  • When it rains, spores can be “splashed” onto needles, causing brown spots, which reduce its growth and can ultimately kill it.
  • Fire removes any infected needles off the longleaf pine and reduces the number of viable spores in the soil. Grass stage seedlings are resistant to fire.
  • Once height growth is initiated, trees are susceptible to fire for a few years.

Historical Perspective

  • As noted by Mobile, Alabama botanist Charles Mohr in 1901, “the pine forests are open. The crowns of trees scarcely touching one another … the floor of these open pine forests is covered with a carpet of grasses and other herbs, mostly perennials, which, under the mild climate of this zone, retains its verdue for the better part of the year.”

Understory Diversity

  • One of the most diverse understory plant communities in the entire country.

Fire Ecology in Longleaf Pine Ecosystems

  • Fire consumes litter on the forest floor, creating optimal conditions for germination.
  • Historically, low-intensity fires frequently moved through the landscape. Today, these fires are mimicked using prescribed fire.
  • Fire reduces competition from hardwoods and other pines, maintaining open conditions necessary for longleaf pines and native plants, while providing food and habitat for wildlife.

Longleaf Pine Growth Stages and Fire

  • Grass Stage: Young longleaf resemble a clump of grass. The dense needle cluster protects the bud from fire and will quickly regrow post-burn. Most grass-stage growth occurs underground as seedlings develop extensive root systems.
  • Bottlebrush: Longleaf bolts in height with no branching. Fire recycles nutrients back into the soil.
  • Sapling: Lateral branches emerge at 6-10 ft in height.
  • Mature: Lower pine limbs are pruned by fire, keeping the canopy above most flame heights. Thick plates of bark protect the inner trunk from fire. Native understory plants and longleaf needle litter provide fine fuels to carry fire across the forest floor.

Longleaf Pine Vulnerability to Fire

  • Longleaf is resilient to frequent fires but is vulnerable to fire at certain stages:
    • Prior to the grass-stage as new germinants.
    • During height growth before bark thickens.
    • While