Where are Snags are Likely to Fall?

From the latest USFS R&D newsletter — subscribe here.

“Most dead trees fall within 10 years after a wildfire. To keep firefighters and those involved in restoration efforts safe, Forest Service researchers and partners developed a new tool that maps where dead trees are likely to fall.”

A related study, “Spatial and temporal assessment of responder exposure to snag hazards in post-fire environments,” notes that:

“Snag hazard increased significantly immediately post-fire, with severe or extreme hazard conditions accounting for 47%, 83%, and 91% of areas burned at low, moderate and high-severity fire, respectively. Patch-size of severe or extreme hazard positively correlated with fire size, exceeding > 20,000 ha (60% of our largest fire) 10-years post-fire when reburn becomes more likely. After 10 years, snag hazard declined rapidly as snags fell or fragmented, but severe or extreme hazard persisted for 20, 30 and 35 years in portions of the low, moderate and high-severity fire areas.”

FWIW, I recently talked with loggers on a fire salvage timber sale, less that one year post-fire, on private land. They said it’s common to see or hear trees fall spontaneously, even with light winds.

5 thoughts on “Where are Snags are Likely to Fall?”

  1. Snags can persist for decades. It depends on a lot of factors, especially size.
    I have seen snags still standing from a fire that occurred in 1873.

    Of course more trees fall after fire. This does not offer a way to weigh and balance the competing interests in snag habitat, natural regen, carbon storage and all the other ecosystem services provided by unlogged post-fire ecosystems.

    • Yes, snags can persist for decades. There are very large ones (well, portions of them) still standing in my area from a fire about 120 years ago. However, 100-year-old ponderosa pines, for example, usually fall within 10 years, in my experience. Lodgepole pine can last longer.

      • From my understanding and observation, low intensity fires, which were much more common in the past, causes pitch buildup at the PP tree base that lengthens the lifespan of PP snags way past 10 years.

        • Greg: “. . . low intensity fires were much more common in the past” in many places, but certainly not everywhere. In instances where low-intensity fires were more common, people were directly involved. In many such areas, until recently, firewood was systematically gathered for generations. When broadcast burning was done in order to harvest seeds, hunt, or rejuvenate huckleberries or pasturage, it was typically performed in areas with far less ground fuels, very few (if any) ladder fuels, and many fewer trees per acre. A lot, lot fewer snags in the past than now, and in large part due to those reasons.


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