Weber State University
Weber State University
Department of Botany
Antelope Island Field Trip
Fire Ecology
Fire Ecology is the study of how fire affects ecosystems. Impacts of fire differ depending on the type of ecosystem, the intensity of the fire and the time from the last fire. Some ecosystems, such as grasslands and chaparrals (mixture of grasses and shrubs) are prone to frequent fires. Fire also plays a role in other ecosystems, such as coniferous forests and aspen forests, although these are usually less frequent than in grasslands. Native plants growing in areas where natural fires occur have adapted and indeed depend upon fire for survival.
In order for a fire to occur, an ignition source and fuel are required. Naturally occurring fires are started by lightening strikes. Human sources include discarded cigarettes, unattended campfires and deliberately set, controlled burns. If there is a great deal of fuel present (wood and litter), fires tend to burn more intensely and for prolonged periods of time in a given spot. Factors such as fuel moisture, wind, and humidity all play a role in determining if a fire will spread, how quickly, and in which direction.
There are three major types of fires; surface, canopy and ground fires. Surface fires are common in grasslands, where there are no trees and the litter and vegetation is spread fairly evenly over the surface. Grasses provide little fuel and hence, fires move quickly over the surface of the ground. Surface fire are the least intense of the three types, so underground structures (roots, bulbs, etc.) are not affected. The soil surface may become quite hot, but nutrients and microorganisms below the surface are left intact. Canopy fires are more intense and spread through the canopies of adjacent trees. These typically occur in coniferous forests. Due to the fact that there is more fuel involved, the heat generated is greater than that found in surface fires. In the event that there is a great deal of dead wood on the forest floor and the vegetation is very dry, a ground fire can occur. These are the most intense fires, fueled by a great deal of wood. These also occur in coniferous forests. In ground fires, the heat is so intense that underground structures are lost, many soil microorganisms die, and organic matter and soil nutrients such as nitrogen can actually become volatilized and lost to the air. Intense fires alter the soil structure, pH, wettability, chemical composition, and soil microflora. It is not surprising, therefore, that legumes and nitrogen-fixing soil microorganisms are among the first types of organisms to re-colonize a severely burned area. Less intense fires have the advantage of breaking standing litter into smaller pieces, thereby making it more available for decomposition. Pathogenic fungi may also be killed, allowing more seedlings to emerge.
Many plant species native to areas that naturally burn at regular intervals have specialized seeds or seed structures (e.g. many cones) that will not open until there is a fire of appropriate intensity. In this manner, survival of the species can be better assured. Some plants, such as aspen trees, depend on frequent fires to prevent the establishment of competitors. Aspen forests are found adjacent to coniferous forests. Aspens are clonal, sending up new shoots from the roots. Entire aspen groves are usually made up of clones from one individual. Under normal conditions, young conifer seedlings will be killed in frequent fires. The aspens recolonize the area with offshoots from protected underground roots. Throughout the western United States, aspen forests are being lost to the encroachment of coniferous forests. This is a direct result of the long-held practice of fire suppression on public lands. When fires that would normally maintain aspen forests are put out, adult coniferous trees become established, thereby crowding out the aspens.
Fire suppression is used differently by land managers than it was prior to the huge fire in Yellowstone National Park in 1988. Before that time, forest fires were put out as soon as possible to preserve trees, habitat and ultimately, tourism. However, when the forests of Yellowstone caught fire, it became impossible to control. The forest had been allowed to grow so dense with mature trees, with so much litter, that the fires spread quickly, uncontrollably, and burned with incredible intensity. As a result, a great expanse of forest was lost. However, the area also grew back much more quickly and diversely than previously expected. The need for fire in terms of decreasing fuel for future fires, creating an age stratification among trees and allowing new seedlings to emerge and grow in clearings is vital in maintaining a healthy forest. This provides natural breaks for future fires due to the low fuel content of immature trees, adds to the diversity of habitats and therefore, increases the overall biodiversity within a forest. Hence, fire managers often now rely more on fire suppression plans focused on fire containment, rather than fire exclusion.
In many areas where fires would have naturally occurred if not for human intervention, land managers often deliberately set fires (prescribed burns) to mimic what should have occurred under normal circumstances. Prescribed burns are well planned and undertaken so as to allow the plants to go through their natural cycle while controlling the spread and scope of the fire. Unfortunately, these too can get out of control, although rarely, as evidenced in Los Alamos, New Mexico in May, 2000. There, a prescribed burn got out of control, burning 80 square miles including part of the Santa Fe National Forest, private ranch land, 405 housing units and causing $300 million worth of damage to the Los Alamos Nuclear Weapons Laboratory.
While a century of fire exclusion is likely to blame for the increased potential for large-scale, catastrophic fires in the western coniferous forests of the United States, this is probably not the case in the California chaparral regions. A USGS study in southern California found that even when prescribed burns are conducted to remove old stands of shrubs, the succeeding young shrubs fail to act as breaks against fires driven by the intense Santa Ana winds. Unfortunately, in California, human activities have dramatically increased the frequency of fires beyond the point where native shrubs can rebound. As a result, invasive species are taking over, changing native shrublands into nonnative grasslands. To make matters worse, these chaparral regions are home to many rare and endangered plants.
Antelope Island is prone to frequent fires, often started by lightning strikes. Land managers so far have mapped the major burns that have occurred there since 1994. Two of the last nine years have seen very large fires which burned between about 15 -20 % of the island each time. The key elements of the island’s fire suppression/control plan include development and implementation of a prescribed burning plan, green-stripping in appropriate areas to create a vegetation buffer zone, addressing fire equipment/training needs, reviewing and addressing the effectiveness of current lightning rod structures, and identifying and prioritizing critical areas to be protected.
Antelope Island used to be much more of a shrub community than it appears today. When people settled on the island, it was used to graze domestic livestock. This lead to overgrazing, decreasing the reproductive output of the native plants. Invasive, non-native plants, such as cheatgrass (Bromus tectorum) were allowed to become established. Cheatgrass is now a common problem in much of the Great Basin region. This grass produces seeds and dries out fairly early in the season, therefore providing fuel for fires. Shrubs that were once separated by bare ground are now connected by continuous cover of dry cheatgrass. When fire starts, it quickly spreads from shrub to shrub, burning far greater areas than would have occurred before. Cheatgrass and other invasive species can quickly take over a burned site. Overgrazing, coupled with more frequent fires have caused the shrubland to be converted primarily to a grassland over much of the island. This has caused significant damage to the island’s habitat and therefore its wildlife.
Re-vegetation efforts are underway on Antelope Island. The plan includes identifying and mapping vegetation and soil types, critical wildlife areas, and appropriate seed source areas, obtaining funding, prioritizing areas for re-seeding, and identifying actions to enhance the island tree canopy to increase both plant and animal diversity.
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