Fire Factors

Fire Factors

Plants are at a distinct disadvantage when it comes to escaping fire, they cannot get up and run. So, many plants have adapted specific traits for fire survival, using it to their advantage as an aid in reproduction or regeneration. These adaptations have made wildfires essential to some ecosystems. Fire ecology is an arm of the science which studies the origins, cycles and future stages and implications of wildfire, or lack thereof. There are four basic tenets researched through fire ecology which aim to define the relationship between fire and living organisms and their environment:

Fire Dependence- This applies to species of vegetation which are dependent on fire to complete their life cycle. Effects from wildfire eases their ability to regenerate and grow.

Fire History- This concept tracks how often fires occur in a region. A biological record of historical fires can be obtained from “fire scars”; the layer of charcoal that remain on living trees even as they annually add cells.

Fire Regime- This is the general integration of characteristics associated with a fire event such as intensity, severity, frequency and type of vegetative community.

Fire Adaptation- This refers to plant species which have specially adapted to fire survival throughout various life stages. Examples of fire adaptations evolved by trees include; serotinous cones (A cone or other seed case that requires heat from a fire to open and release the seed), fire resistant bark, fire resistant foliage, seed retention and the ability for rapid growth.

Fire changes the soil composition and temperature and may be responsible for maintaining productivity in soils where low temperatures or nutrient scarcity limits plant growth. Wildfires help to release nitrogen and other vital nutrients from woody vegetation and  move it back into soil, making it readily available for uptake and useful for the immediate regeneration that will follow.

Regeneration following a fire event will occur according to the traditional successional stages, each stage’s composition and length being determined by the ecosystem; soil characteristics, climate, natural disturbances, etc.. In general; fast-growing weeds and herbaceous plants may dominate at first, followed by saplings and small shrubs, consisting of pioneer species such as members of the willow and aspen families which can dominate a disturbed site for 10 to 30 years. Eventually the pioneers grow into mature trees and shade out the understory, which may be replaced with a more shade tolerant conifer species, this successional stage can last up to 100 years. However, depending on the ecosystem, pioneer trees will be almost completely replaced by well adapted, long-lived broad-leaved deciduous hardwoods (oaks, maples, hickories, ashes) in temperate forests or in boreal forests, a softwood conifer forest for the final stage. The climax successional stage leaves the forest floor littered with humus, detritus, small shrubs, leaf litter and deadwood, in other words, fuel for the fire which will begin the process again.

In the United States we have 6 distinct ecosystems adapted to regeneration and reproduction by wildfire:

Tallgrass Prairie: These ecosystems are characterized by grasses, shrubs, forbs and trees living in moist soils. Tallgrass prairies make up parts of Nebraska, Illinois, Kansas, Iowa and more eastern Midwest states. Fire in these types of communities aids in the elimination of invasive species, additionally low fuel loads in these prairies makes fires controllable and easily suppressed, if needs be.

Chaparral: Chaparral refers to various brushland ecosystems found in southern California and across the southwest. These communities tend to contain the most flammable plant species found in the country, many of which require or even promote wildfire. The leaves of many chaparral plants contain flammable oils and they remain small even at maturity. Many species have serotinous seed packets requiring fire for dispersal. Fire benefits this ecosystem by freeing up nitrogen and nutrients bound in plants returning it deficient soil.

Ponderosa pine: These trees can exist in pure stands or cohabit with Douglas fir trees, in the interior western states. They are usually found in communities also containing grasses, forbs and shrubs in regions receiving less than 25 inches of rain per year. The fire prone grasses within the community force the ponderosa pine to adapt with thick, resistant bark, deep roots and the drop of lower limbs, which decreases the chances of fire to reach their crowns. Fires in ponderosa pine stands helps to eliminate older, weaker or sickly individuals paving the way for regeneration and success of new trees, usually of the same species.

Douglas fir: The natural range of the Douglas fir is much of the pacific northwest including Oregon, Washington and British Columbia. These trees favor mixed forests in a region which receives 50 or more inches of rain per year. Doug firs readily regenerate on sites which have been disturbed by wildfire; nearly all natural stands in the U.S. sprouted following fire events. The most beneficial aspect of fire to these communities is the removal of the fuel load which vastly reduces the chances of uncontrollable crown fires.

Loblolly and shortleaf pine: These southeastern communities are not immediately resistant or responsive to fires, but after several years of growth their resilience improves. Fire benefits these communities by removing invasive species which creates a more favorable habitat for seedlings to grow and thrive.

Jack pine: Jack pines live amongst a variety of trees, shrubs, forbs and grasses within the Great Lakes states. The seeds of the Jack pine do not drop upon ripening but instead persist on tree branches for many years protected by a thick, heat-resistant cone. The intense heat of a forest fire will be able to open the bracts of the cone and release seeds onto a forest floor cleared of competing vegetation. The fire regime in these communities is tied to the life cycle of this tree, occurring most often in the fall.