Forest Carbon Coalition – Science Synthesis

Why is forest thinning an ineffective/inefficient method for limiting wildfires?

Wildfire severity is influenced by a number of natural and human-caused factors. These include temperature, aridity, wind, topography, the species and density of vegetative fuels (trees, brush, grass) in the forest, and the types and density of human-built fuels (houses, etc.). The presence of fire barriers (freeways, rivers/wetlands, fire lines built by firefighters, etc.) also can influence wildfire behavior.

Proponents generally advocate for thinning, arguing that removing trees reduces the amount and density of fuel so that, if a subsequent wildfire encounters the thinned area there will be too little fuel to support the fire and it will go out. This reasoning has validity, though, only in circumstances where the reduction in fuel can exert a controlling influence over a wildfire. In reality, these conditions often don’t exist and the thinning has little, if any influence on a fire’s behavior:

  • There is no certainty that a fire will ignite in subsequent years and encounter the thinned area before regrowth of natural tree seedlings, brush and grass restores fuels to, or above, pre-thinning levels. In many cases, the probability that a fire will actually encounter an area soon after it is thinned is near zero.
  • As it reduces fuel levels, thinning can intensify other factors that increase fire risks. For example, the reduction in shade from a thinned forest might raise ground temperatures and reduce moisture in and near the soil, so that, if a fire occurs, it can move more surely and quickly through the thinned area.
  • Other factors might be more important. The behavior of any given fire will depend on many variables, not just the amount and density of fuel. If temperature, aridity, and wind are all at high levels, they likely will dominate a fire’s behavior, so that it will move through a forest, thinned or not. Such fires are becoming more frequent, in response to past changes in climate, and are likely to become more frequent in the future.

In other words, when factors other than fuel influence fire behavior, thinning is ineffective. This is especially true when temperature, aridity, and winds are high enough so that thinning has zero influence on fire behavior. When conditions are such that a reduction in forest fuels might influence fire behavior, thinning is inefficient whenever the probability that a fire will encounter the thinned area is near zero. Thinning can be both ineffective and inefficient if, in  addition to reducing fuel, it increases ground-level fire risks by raising temperatures and reducing moisture.

Key research about the effects of thinning on wildfire:

Bradley, C. M., C. T. Hanson, and D. A. DellaSala. 2016. Does increased forest protection correspond to higher fire severity in frequent-fire forests of the western United States?

Link: https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecs2.1492.

  • “The relationship between forest density/fuel, mechanical fuel treatment, and fire severity is complex.”
  • For instance, thinning without subsequent prescribed fire has little effect on fire severity (see Kalies and yocum Kent 2016) and, in some cases, can increase fire severity (Raymond and Peterson 2005, Ager et al. 2007, Wimberly et al. 2009) and tree mortality (see, e.g., Stephens and Moghaddas 2005, Stephens 2009: Figure 6)…”.
  • “…the effects depend on the improbable co-occurrence of reduced fuels (generally a short time line, within a decade or so) and wildfire activity (Rhodes and Baker 2008) and can be over-ridden by extreme fire weather (Bessie and Johnson 1995, Hély et al. 2001, Schoennagel et al. 2004, Lydersen et al. 2014).”

Moomaw, W.R., and others (signed by more than 200 climate scientists and ecologists). 2021. Open Letter to President Biden and Members of Congress from Scientists: It is essential to Remove Climate-Harming Logging and Fossil Fuel Provisions from Reconciliation and Infrastructure Bills.

Link: https://johnmuirproject.org/wp-content/uploads/2021/11/ScientistLetterOpposingLoggingProvisionsInBBB_BIF4Nov21.pdf.

Key excerpts:

  • “We have watched as one large wildfire after another has swept through tens of thousands of acres where commercial thinning had previously occurred due to extreme fire weather driven by climate change. Removing trees can alter a forest’s microclimate, and can often increase fire intensity.”
  • “ In contrast, forests protected from logging, and those with high carbon biomass and carbon storage, more often burn at equal or lower intensities when fires do occur.”
  • “ We urge Congress to move in the opposite direction by shifting from more logging toward natural climate solutions that store carbon in mature and older forests and allow naturally regenerating forests to continue growing for greater carbon accumulation.”

DellaSala, D.A.2022. Testimony for Hearing on “Fighting Fire with Fire:Evaluating the Role of Forest Management in Reducing Catastrophic Wildfires” in the House Committee on Oversight and Reform, Subcommittee on Environment.

Link: https://oversight.house.gov/sites/democrats.oversight.house.gov/files/DellaSala%20Testimony.pdf.

Key excerpts:

  • “The bottom line – Large wildfires burning under extreme fire weather cannot be stopped by suppression, nor controlled by thinning and other forms of logging over large areas. …”
  • Attempting to moderate fire behavior at the local or landscape level, when large fires are mainly tracking global temperature increases and drought, is becoming increasingly ineffective.”
  • “ We can never spend enough to solve fire problems by thinning and other forms of logging and suppression, as the global climate spins off more “megadroughts” and “heat domes.” Just like we cannot stop hurricanes, we cannot stop large fires under extreme conditions known to create their own weather fronts.”