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by Kay Wiseman and Liz Clift

Fires in the west have become larger, more frequent, and more severe in recent decades. This upward trend is likely to continue, making it vital to understand the implications these massive fires have on our landscapes.

For nearly 100 years, fire management policy has established a best practice of  suppression to control potential fires and sources of fire. However, suppression can lead to the disruption of natural fire regimes essential for fuel load reduction, seed germination, and soil nutrient cycling. Despite best efforts for prevention, fires continue to ignite and spread and with nearly a century worth of fuel accumulation, a wildfire can quickly turn into a devastating threat.

While there is room for debate as to what defines a “severe wildfire” in differing ecosystems, it is well documented that for the United States’ dry western landscapes fuel accumulation has been a major component for increased fire intensity. Heavy fuel loads can have multiple negative impacts:

  • Prevent the fire from sweeping quickly across the landscape and concentrating the heat in localized areas. Intense, slow moving fires can cause combustion beneath the soil surface and spread underground. Underground fires can cross fire breaks and pop up several hundred feet in any direction putting firefighters and protected structures at risk.
  • Cause a fire to burn especially hot, which can lead to the death of soil microorganisms, effectively sterilizing the soil and can also create a hydrophobic layer in the soil—and making it difficult for water to penetrate This can cause recovery to take a significantly longer time.
  • Impact water quality, through increased erosion and downstream sedimentation.
  • Allow fires to reach into tall vegetation and trees creating large, fast-moving crown fires. Crown fires move quickly through trees releasing hot embers that travel farther than would be possible for low intensity ground fires. These travelling embers have the potential to ignite spot fires outside of the projected burn path—because crown fires can move independently of a ground fire. Some crown fires do not even touch the ground, which can leave a forest permanently damaged—and filled with more fuel for a future fire.
  • Present treacherous obstacles for firefighters traversing the landscape.

There’s a balance though between removing fuel and allowing some of the vegetative fuel to stay because of the role it plays in a biotic community. Vegetative material that could easily become (excess) fuel provides beneficial soil stabilization and habitat areas. Fuels, such as senesced grasses, may allow soil to hold moisture more effectively or provide bedding or nesting materials for a variety of animals, as well as providing cover to small prey animals. Larger fuel, such as tree snags or “standing dead” trees provide nesting habitat for a variety of birds as well as a rich environment for a diversity of beneficial insects and fungi that provide decomposition—thus enriching the overall nutrients within the soil (and which may also be forage for a variety of animals, including some songbirds). Trees, even dead ones, may also provide soil stabilization for many years.

Additionally, if these fuels are fully cleared soil is exposed to sunlight and wind creating dry conditions more conducive to increased fire intensity or increased erosion. The presence of fuels is also essential for topsoil production and nutrient cycling—nutrients from vegetative fuels cycle back into the soil as they decompose—which is another reason that it should be considered undesirable to completely remove fuel sources from the landscape. Therefore, fuel management can be a delicate balance between leaving too little or accumulating too much. Other than the human safety aspect, why should we concern ourselves with fire intensity? Let’s dive more deeply into the points made in the bullets above.

Fires that burn underground—especially those burning peat—can release inordinate amounts of stored carbon. That’s because peat, which makes up only 3% of the world’s surface contains 25% of the world’s carbon soil. What does that mean? It means that there’s currently about as much carbon in the air as there is in all the peatlands in the world.

That’s not all. According to Guillermo Rein, an expert on smoldering fires, “Once ignited, these fires are particularly difficult to extinguish despite extensive rains, weather changes or firefighting attempts, and can persist for long periods of time (months, years), spreading deep (5 meters) and over extensive areas of forest subsurface” (as summarized by Andrew C. Revkin, for the New York Times). These fires occur in tropical, temperate, and boreal forests around the world—because peat exists on all seven continents, including places you might not expect if you don’t know much about the landscape.

Although it’s often easy for anyone who isn’t a soil scientist to see the soil as dirt, it is, in fact, alive with microorganisms and invertebrates which are integral to overall soil health. Additionally, a complex mycorrhizal (fungi!) system exists beneath the soil, which is integral to soil health (in particular helping different plants receive nutrients). Hot fires can kill this mycorrhizal system, which can take years or decades to restore. These intense fires can leave soils not only hydrophobic (which has implications for loss of soil function and may increase risk of erosion or downstream flooding and sedimentation), but effectively sterilizes the soil, leaving it a “moonscape.” This isn’t a reference to the moon that controls the tides—it refers to a post-fire devoid of all vegetation, potentially for years after the initial devastating fire. Moonscapes burned so hot below ground that seeds, soil microbes, and even the compounding properties of inorganic materials in the soil have been expunged.

Without protective vegetation and woody debris, the soil is left exposed to the elements. Exposure and water repellency increase the erosive potential of soil and the subsequent runoff after fires cause stream sedimentation and low water quality, because plants that may have normally acted as “filters” to rainwater or other precipitation were removed from the ecosystem as a result of the fire. In addition to increased repellency, the absence of plants in the landscape means that there is little to slow down precipitation—which can further decrease water percolation into the water table.

Crown fires—especially in the absence of intense ground fires—can provide new opportunities for younger trees and understory growth to flourish. However, the loss of the tree canopy can result in greater vulnerability for some canopy dwelling organisms, can increase damage to lower forest strata during precipitation events (canopies can effectively slow down rain, hail, for instance), and increase surface soil temperatures, due to the absence of the shading properties a full canopy provides during the hottest months of the year.

Restoring areas hit by severe wildfire is resource intensive and expensive but is also necessary to immediately stabilize soil and prevent future erosion, along with reviving ecosystem function. There are a variety of options for short term soil stabilization, and remediation, which can help limit some of the impacts of a fire on an ecosystem (and the human community that depends on that ecosystem as well). Long-term solutions must take into account the current soil quality, options for soil remediation (if appropriate), appropriate (native) plant communities, planting plans, and land management to support revegetation. Other abiotic factors, as well as the pre-fire history of the landscape, may also provide important clues about what restoration techniques will work best for the landscape.

But, these are reactions. We should also be having proactive conversations about how fire management plans can be effectively and appropriately incorporated into a variety of landscapes. These conversations—and the implementation of plans—will likely help fire managers effectively and safely reduce the fuel load that’s resulted from decades of fire suppression.