Forest fires can be devastating events. We hear in the news about many forest fires, the homes destroyed, and the lives lost1. Fires also have effects on the forest itself, and soil plays an important role in bringing the forest back to life.
Forests are important ecosystems. Of course, we think of trees when we think of forests, but they are also filled with animal life, big and small2. Soils provide many ecosystem services in forests. They store carbon in soil organic matter, and help to filter and hold water. Lumber that builds our buildings comes from forests, and they are also valued for recreation and spiritual and aesthetic purposes. Forest soils are integral to providing these many products and services.
Fire is a natural part of many forest ecosystems, and can serve to renew and invigorate forests. “Prescribed fires” are planned by land managers to manage forests. Natural fires and prescribed fires are usually low intensity and less damaging than some of the wildfires the U.S. has seen recently. Ashes left behind after low intensity fires can increase the availability of nutrients and organic matter to the soil and to plants. Some seeds also need the heat of fire to germinate. Finally, just like pruning bushes in your home garden helps your plants, the removal of dead or competing vegetation by fire can help invigorate forests.
Forest fires can also damage forest ecosystems, including soils. The extent of damage is related to the temperature of the fire and the speed at which it moves through the forest. Dry forest floors and soils can make the forest burn hotter. Extremely hot fires can sterilize the upper layer of the soils by killing the soil microbial life. They burn off large amounts of carbon stored in the soil, and change the ability of soil to absorb and retain water. Very hot fires that burn off the forest floor leave the soil bare, and can create water-repellant soils. Increased erosion has been documented after extreme fires, with mudslides and landslides adding to the damage from the fire.
The area damaged by wildfires has increased with recent droughts and an accumulation of dead wood in the forests, and the damage in terms of cost and lives has increased. Climate change is likely contributing to this increase in drought conditions3. While many of the largest forest fires have occurred in the west, this year, extensive and damaging wildfires became news in the Eastern U.S., most notably around the Great Smoky Mountains4.
Forests can recover quickly from fires depending on the severity of fire. The low-intensity fires prescribed by land managers help add nutrients to the soil and rejuvenate plant life. Burning can add charcoal to the soil, and may result in a short pulse of nutrients in the ash. Burning off the dead plant life, or weedy plants, increases sunlight to the forest floor. This often causes a flush of vegetation in response to the increase in light reaching the soil and available to plants.
Forests recover from fires through germination of seed stored in the forest floor. Some trees even rebound by sprouting branches from basal buds of trees that have been killed. Birds and other animals may also bring in seeds. Some tree species require fire for their seeds to germinate. For example, Jack pine seeds are sealed close with a resinous bond that requires high temperatures to open and liberate the seeds.
For larger, more destructive wildfires, active efforts to assist recovery may be needed. Such activities include use of silt fences, slash and mulches to prevent erosion and sedimentation, and re-seeding or replanting the area in order to get vegetation established.
Forest fires have been part of nature before the days of prescribed burns, careless humans or arsonists. They can improve the soils in the forest, which in turn helps plant life. Some types of plants need the heat of fire to germinate. Recovery from large wildfires may require some help. The forests will, in time, heal themselves.
Answered by Mary Beth Adams, research soil scientist, US Forest Service
- The Soil Science Society of America is sensitive to the destruction to humans – and wildlife – that forest fires present. Our many sympathies go out to the families affected by fires in the United States and beyond. Our goal is to provide science-based information about the hope that soils bring to forest life.
- To learn more about creatures that live in the soil, visit these Soils Matter blogs:
- The initial fire at Chimney Tops was allegedly set by arsonists; the secondary fires in the Gatlinburg area were caused by high winds and made worse by drought conditions. (http://wate.com/2016/11/23/4-smokies-trails-again-closed-due-to-another-wildfire/