Park Scientists Summarize the Fire's Ecological Effects into 12 Points:
Overall: Yosemite is dominated by fire-maintained ecosystems. Nearly every inch of the park has burned many, many times in the past (granite excluded). Decades ago, however, management of the park was to protect the forests from fire, and that changed the way fires burn today. One hundred years of suppressing fires changed the forest structure and composition, leading to more severe fires today. Fire managers in Yosemite use prescribed fires to begin to restore forest structure, function and composition.
1. Prescribed fire is used as a tool to reduce hazardous fuels around communities and to restore forest structure and composition. Prescribed fires open the forest canopy, reduce tree density, alter tree spacing and species composition, and remove litter and duff. In meadows, prescribed fire is used to reduce conifer encroachment, maintain native plant communities, and reduce hazardous fuels around communities.
2. Yosemite’s fire managers strive to get fires back into the ecosystem to match the pre-Euro American fire-return interval. A fire-return interval is the average number of years between fires, varying between 3 and 75 years, for a particular vegetation type. Meadows were largely maintained by American Indians for purposes like wildlife habitat management and enhancements for particularly important plants for food or fiber. Yosemite’s fire management recognizes the impact that American Indians had on the landscape and tries to continue their legacy; many of the early views that we associate with Yosemite were a direct result of fire management by the American Indians. Our knowledge today of fire-return intervals is based on scientific studies of fire scars left in trees and of sediments in lakes containing ash and charcoal from previous fires and pollen from trees. Because trees put on annual growth rings, we can count how old trees are; we also count the number of annuli between each fire scar left in the tree to determine the fire-return interval. Some trees, like sequoias, contain a rich history of fire due to their long-life spans. Research tells us that sequoia groves burned frequently, approximately every 3 to 15 years.
3. Re-introducing fire into the ecosystem consumes surface fuels and kills some trees; thereby, reducing the expected severity of future fires. One of the goals of the Fire Management Program in Yosemite is to treat forests with fire to reduce fuels so those forests become more resilient to future fires. Mechanical thinning and logging removes part of the fuel load but leaves the duff and litter on the ground. In the past, lower-intensity fires at regular intervals reduced fuel loading and thinned regenerative growth, allowing the more mature trees to survive fires. Yosemite’s fire management seeks opportunities to conduct prescribed burns in order to create natural vegetation barriers and a more natural mosaic of fire severity and size.
4. Why would the park set a prescribed fire in August? The Big Meadow prescribed fire was initiated based upon a written and pre-determined “window” of very specific conditions (temperature, humidity, fuel type, wind speed, smoke dispersal, etc.). At the time of ignition all conditions were within this window. Prescribed fires may be done any month of the year depending upon the objectives of the burn.
5. Why couldn’t the meadow be burned in much cooler conditions? The park uses prescribed fire to accomplish specific objectives. In the case of the Big Meadow fire, those objectives were meadow restoration and the maintenance of fire resistant vegetation for the Foresta Community. For vegetation to burn it must be sufficiently dry and flammable to maintain a fire. The prescription for the Big Meadow burn was based upon these objectives.
6. Why did the Big Meadow Prescribed Fire escape? There is not a simple answer to this question, and the indirect answer requires us to go back in time to the creation of the National Park Service. When Yosemite National Park was created, the conventional wisdom was to protect parks from environmental threats, including fire. Today we know this was misguided management, but 100 years of fire suppression changed our forests in many ways. The forests of the Sierra were created and maintained by fire. If you take fire out of the equation, the forests change. When forests finally do burn, they burn differently--typically hotter and larger. We’ve kept fire out of the park for 100 years, and 100 years of forest growth created very densely packed trees and lots of litter and duff on the ground. Then, in 1990, on a dry summer day, there was a lightning strike near the Arch Rock entrance station on the west side of the park. The resulting A-Rock Fire burned a forest overstocked with trees and removed most of the canopy, leaving behind thick brush, tightly-spaced young trees and many heavy dead and downed logs. Finally on Aug. 26, 2009, the Big Meadow Prescribed Fire escaped into a combination of brush, snags and logs. Snags caught on fire and launched embers in many directions, making it difficult to catch, and dry shrubs and logs on the ground made it too hot to fight directly. The logs and brush wouldn't have been there without the A-Rock Fire, and the A-Rock Fire wouldn't have burned the way it did if we allowed five fires per century in the lower mixed conifer forests. The escape was unfortunate; however, one of the most important lessons from the Big Meadow Fire is that history has a lot to do with the way our forests burn today.
7. Scientists’ understanding of fire and of how to manage it has evolved. More than 30 years of ecological research and several large catastrophic fires in the park have taught fire managers that suppressing all fires only delays the inevitable, making the resulting fires (as well as their smoke) much more intense than they otherwise would have been. Unfortunately, 100 years of complete fire suppression throughout the United States has left much of our wild lands, including Yosemite’s landscape, prone to damaging fires.
8. Plants and animals are adapted to our fire-driven ecosystem. If plants and animals were not adapted, they wouldn’t be here. Animals will either walk or fly away or take shelter in burrows in the ground. Many of our plants depend on fire, and those that don’t live in places that rarely burn. Plants depend on fire to remove litter that allows seeds to germinate on bare soil. They depend on fires to recycle nutrients to growing seedlings, and they depend on fire to remove competitors, less adapted to fire. Forests that evolved with fire cannot survive without fire; neither can all the other organisms that live in the forests.
9. Other prescribed burns had been set in 2009. Yosemite Fire Management burned around 800 piles from tree-thinning projects along road corridors and around communities. They also burned 30-plus acres in El Portal to reduce fuels around that community. In term of acreage, Yosemite should be burning thousands of acres each year to keep up with forest succession, but because of a host of reasons, the park is limited to burning a fraction of that. External pressures include air quality, visitors’ experience, prescription windows (temperature, relative humidity, wind, fuel moisture, and smoke dispersion). Yosemite Fire Management tries to burn whenever these windows open.
10. The burn area looks different in different areas due to the pattern of intensities. You probably remember learning about the fire triangle in grade school; fuel, oxygen, heat. Fuel and oxygen and heat vary across the landscape as the fire is burning, thereby, creating a mosaic pattern of the burn. Where there is little fuel, fire burns cooler than where there is a lot of fuel. When the amount of oxygen increases in the form of wind, fire intensity increases. When fuels are preheated like they are on slopes (heat rises up the slope ahead of the flames and preheats the fuels), fires burn with more intensity. So the pattern of intensities across the burn produces a mosaic of severity. Fire managers like to see the mosaic pattern rather than a uniform pattern. A high-severity burned area might show "ghost-logs", or white lines on the ground, while a less-intense burned area, such as in the shade of a rock outcrop, might leave dead leaves still on tree branches.
11. When the burned area around Foresta re-grows, it is predicted to re-grow as chaparral initially. Before the 1990 A-Rock Fire, this area had been a mixed conifer forest. After that fire, it became a chaparral community with scattered patches of conifers. Now, after the Big Meadow Fire, this area will re-grow initially as a chaparral-dominated community. Because the patches of chaparral will be so large, the time it takes for the regeneration of conifers will be longer. Chaparral will dominate the canopy and will shade the growing conifer seedlings. Given time, this area will probably be a forest. There are enough trees left in the burn perimeter to seed the area and, eventually, ponderosa pines, cedars, and knobcone pines will come back. Yosemite’s fire managers plan to continue to use prescribed fire to restructure the forest gradually, thereby, preventing future A-Rock-type fires that are difficult to manage due to earlier fire suppression management policies.
12. A Burned Area Emergency Response Team has investigated all post-fire conditions. Strategies have allowed for mitigating any negative fire effects, including water quality and erosion. Yosemite’s long-term management plans for the area lookat damage assessments, emergency stabilization and then long-term rehabilitation. Management of the area will reflect the fact that fire is a natural process, and, in the park, we try to allow natural forces to prevail.