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The Importance of Fire - Fire and Ecosystems

Fires are a natural part of most ecosystems, and the drier the environment, the more important fire is as a force driving the structure of an ecosystem. Wildfires need a source of ignition, usually lightning, and fuel. The fuel for the fire is the accumulation of organic material such as fallen leaves, twigs, and branches on the ground. The dryness of the fuel determines the probability that a fire will start, but once the fire generates enough heat, even damp material will ignite. It is the amount of fuel per hectare that determines the intensity of the fire. If there is little fuel, the fire just creeps across the land; it never develops the intensity to leap up into the crowns of trees. However, if there is plenty of fuel, the fire builds in intensity and can move quickly across a landscape, engulfing everything in its path. The intensity of the fire makes a huge difference to wildlife. A low-intensity of the fire that moves slowly may do no more than scorch plants, and it gives mobile animals the chance to flee. Because the heat from this low-intensity fire does not penetrate more than a few centimeters into the ground, some animals can survive by burrowing into the soil. Because many animal species can neither flee nor burrow, their numbers are seriously deduced during a fire. This relatively cool fire may not damage the root mats of trees, shrubs, long-lived herbs and grasses. Trees may have their bark blackened and may lose all their leaves, but if the fire has not penetrated the bark, the trees can usually resprout from fresh buds lying beneath the charred surface. After a year or two, the regrowth is so complete that it is hard to tell that the area ever burned. By turning leaves and wood to ash, the fire releases mineral nutrients and actually speeds the cycling of nutrients. Consequently, ash is a fertile growth medium rich in potassium, phosphate, and many micronutrients. One nutrient that is lost as a result of fire is nitrogen. As much as 70% of the nitrogen in burned leaves and wood goes off in smoke. It is common to find that the first plants to flourish after a fire are adapted to living in nitrogen-poor soils and can fix their own nitrogen (legumes).

Ecosystems that burn every two to three years, such as the long-leaf pine barrens of the southeastern United States, are fire-adapted systems. Here, the seeds of many plants only germinate following fire. (Indeed, in California where fire suppression has been the rule many fire adapted species are only seen after a region has burned. Because these species, such as the Giant Sequoia, cannot germinate until the seed coats have been scarified (broken) by fire the National Park big trees have literally been protected out of existence in many places.) These fire type seedlings have an excellent chance of lying in bare ground (because the previous occupant has been burned), and nutrients are plentiful due to the ashing of organic matter. The fires crawling through the understory of the pine forest also prevent invasion by hardwood species, preventing them from growing up into the canopy. Without regular fire the pines would be outcompeted by the oaks, and the entire ecosystems would change. Some endangered species, such as the red-cockaded woodpecker, are absolutely reliant on mature pine woodlands. These systems require a delicate balance of fire in which the burns are sufficiently frequent to eliminate competing trees, but never so intense that they kill the pines.

Less benign fires are the large, intense wildfires that incinerate whole forests. These fires may have 10 to 100 times the energy per square meter of the low-intensity fires, and the heat can penetrate deep into the earth, killing roots and burrowing animals. The above ground portion of trees are killed, and the organic component of the topsoil may catch fire, leaving fertile ash but a poor soil structure that is very vulnerable to erosion. The ecosystem will recover from such fires, but the process takes longer, and it is more likely that isolated populations of rare animals will have been eliminated. A forest burned this completely may take more than 300 years to regain its full measure of habitat diversity.

The dry Mediterranean climate of California creates forests and scrublands that burn readily. Wildfire is a natural part of that ecosystem, but as urban development invades the forests, fires are suppressed to protect the real estate. Satellite imagery to compare the number and size of fires in southern California (with fire suppression) and Baja California (no fire suppression) has demonstrated that without fire suppression there are more fires that cover less than 800o hectares. With fire suppression, there are fewer fires in total, but there are more large fires that burn 100 to 10,000 hectares. These data indicate that, although fire suppression reduces the number of fires, the fires that do occur burn htter and longer. The explanation lies in the process of fire suppression, which relies on the ability to extinguish fires quickly. An unwanted result of this control is that there is an accumulation of fuel on the forest floor. When a fire does start, it has a huge supply of fuel and generates a blaze that cannot be held in check. An uncontrollable inferno results; witness the fires of Yellowstone National Park in 1992, California in 1993, and Florida in 1998. The recent southern California fires, 2003, devastated many more acres of developed and wilderness lands than any other West Coast fire in recorded history. Tree ring analysis strongly suggest that because fires were natural before the advent of western Europeans, that the California lands have never been subjected to such great destruction. It is well known from California studies that Native American used fire as a means to maintain the Oak woodlands such that chaparrel (greasewood which is highly flamable) did not grow up around the acorn producing crop trees and endanger the Native American food supplies. The grasslands of the midwest (where the buffalo roam) were constantly burned from both natural sources and Native Americans. Since the Buffalo likes to roam where there was grass and not brush, the indigneous people recognized that they could attract the buffalo herds if they had good clear grassland. If the fire in grasslands is suppressed the shrubs and thorn bushes soon take over and the grass no longer grows. This has happened in the southwestern United States grasslands and desert lands.

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