Below the Blaze: How Wildfires Affect Soil
Soil is the most basic building block of any ecosystem. Healthy, nutrient-rich soil provides the basis for plant life, which in turn supports wildlife. However, it is not often we stop to consider soil, especially when distracted by smoke overhead and fires along the ridgeline. To understand post-fire environments, you must first look beneath your feet, to the dirt and ash left behind.
If you were to walk through a burn zone a week after a fire, you would see ash and charred logs. Your feet would slip through loose soil as you trudged over the scarred earth, fully exposed to the sun overhead. To the untrained eye, it would look like a wasteland, incapable of supporting further life. To an extent, this perception would be correct.
In the Sawtooths, high-intensity, stand-replacing fires are common. While a natural feature of the area, high-intensity wildfires pose some degree of immediate risk to the forest. The intense heat of the recent fire can actually make the soil repel water. Coupled with the felling of trees and upturning of root systems, which typically hold soil in place, the potential for soil erosion skyrockets. Some studies suggest that erosion into streams is 35-50 times higher in places where fires have recently burned. The effects of post-fire soil erosion can be especially harmful in riparian environments. When loose sediment filters into the waterways, it tends to settle in pools, ultimately smoothing out the streambed. This leads to a decrease in habitat and less diversity in the aquatic ecosystem.
High-intensity fires also pose a risk to the chemical makeup of soil. Nitrogen fixing microorganisms are crucial to plant growth as they capture nitrogen from the atmosphere and transform it into a form that is usable by plants. Unfortunately, these microorganisms are susceptible to heat and often are lost in higher intensity burns.
However, it is important to note that fire is an issue of many degrees. While high-intensity fires can pose considerable risks to soil health, low-intensity fires can actually improve soil quality. Low-intensity fires often leave root systems intact, reducing the risk of soil erosion and many fires can increase nitrogen availability through chemical transformation. While vegetation is constantly recycling nutrients back into the soil through leaf litter and downed logs, fire accelerates this process. Post-fire soil tends to be heavy in many of the nutrients plants rely on.
Even with high-intensity fires that decrease nitrogen-fixing bacteria and fracture root systems, a natural solution arises in the form of pioneer species. Lupine often follows hot on the heels of wildfires. The deep purple flowers grow well in the open sunlight of burn areas and provide additional perks for the rest of the ecosystem. As part of the legume family, lupine’s roots are ideal habitat for nitrogen fixing microorganisms and also latch on to the soil, holding it back from further erosion.
Lupine’s contributions are exhibitive of a common truth: the environment has natural ways of achieving balance. While high-intensity, stand replacing fires can initially have negative effects on soil, succession will gradually rectify these consequences and re-create an environment capable of supporting a healthy forest.
However, alterations in the pattern of wildfires, due to human-caused fires and climate change, have placed considerable caveats on this promise. In general, higher severity fires are becoming more common. As a result, there is a greater chance of losing nitrogen fixing microorganisms, meaning soil takes longer to fully replenish. In higher intensity fires, more vegetation is also destroyed, in turn increasing the risk of soil erosion as there are less root systems to hold the soil in place post-fire. More area is also being burned, equating to an overall greater amount of soil eroded. From 1970-1989, there were 13 recorded fires in the US that burned 100,000 acres or more, otherwise known as mega-fires. From 2010-2017, there were 45 mega-fires. Many fire ecologists project that mean wildfire size will continue to grow over the coming years.
Our methods of fighting and managing fires are changing. Many public lands today employ low-intensity prescribed burns as a safeguard against higher-intensity fires. Where high-intensity fires do occur, practices such as the application of mats to prevent soil erosion, re-vegetation, and fertilizer application facilitate the accelerated recovery of the soil and general ecosystem.
With or without such preventative measures, the relationship between fire and soil provides an apt metaphor for our greater relationship with fire in the West. At times our fire story is morose as high-intensity wildfires damage valuable infrastructure and threaten lives. However, fire is also a necessary part of the ecosystem. The plants and animals of this region are adapted to cycles of fire and can even draw benefit from low-intensity fires. It’s a system of give and take that humans are still growing accustomed to, but, with time and understanding, I believe that we too can harmoniously interweave ourselves into the natural cycle of Western wildfires.
Hailey is a naturalist at SIHA. While she grew up in Texas, she has spent her adult life traveling around the country and working at various nature centers, schools, and camps. Her favorite subject to teach about is succession. When not teaching, Hailey enjoys backpacking through the wilderness and climbing high peaks.