[Terrapreta] Life cycle of forest fire char?

[David Yarrow]

not all forest fires burn the same.  most burn hot and fast into ash.  some burn slow and smolder, leaving behind some char amid the ash.  differences are due to variations in weather conditions and species mixes.  dry pine burns hot and fast.  moist hardwoods tend to burn slower, smolder and char.  generally, these last few years, we see more hot, fast fires.  and lots more megafires that rapidly burn tens and hundreds of thousands of acres.  generally in any fire, most undergrowth and smaller trees are reduced to ash; char is the lesser, less frequent outcome.

a few years ago we did an ancient forest survey in a deep, steep, wide ravine west of schenectady, NY.  we discovered there were few elder trees except a few fire resistant oaks.  as near as we could determine by direct site study, about 80-120 years ago, a fire swept the ravine of most young trees and many species.  in studying trunk cores extracted with an increment borer from larger, older trees, tree rings revealed a layer of char when the tree was partially burned, scorched and charred on the outside, but not killed.  eventually burn scars were encapsulated and overgrown, becoming embedded in the trunk.

at another site a few miles nearer schenectady, we found 250+ year old oaks, white pine and hemlocks, with a few large pitch pine, which seldom gets over 100 years.  one 2-foot diameter pitch pine was down lying across the trail, so 4-inch thick cookies were chainsawed from the trunk.  we counted 120+ rings in the cookies; very old for a pitch pine.  to our surprise, the inside of the trunk was cracked open in a cavity lined with char.  our best guess is the tree was struck by lightning and ignited, but survived, recovered, and in 50 years, encapsulated and sealed off the charred crack split open by the lightning.

in any event, char isn't easily eroded by weather, corroded by chemistry, or digested by biology, and thus does not break down and vanish from soil.  reduced carbon in char isn't degraded by normal natural processes.  with no hydrogens, oxygens or other atoms (electron-enriched) attached, normal degradation processes can't eat away at the carbon.  thus, unlike oxidized ash and other combustion products, char won't disappear from soil, but instead stays residents for decades, centuries and millennia.  this long-term retention of charred carbon in soil is why archaeology is able to use radiocarbon analysis to date ancient objects and sites.

in forest fire conditions, entire woody objects are seldom combusted into char.  more usually, char only forms on woody surfaces.  in controlled pyrolysis, temperature, oxygen and pressure are controlled to maximize char production.  but it very rare to create such conditions in a forest fire.
  ----- Original Message ----- 
  From: Green Waste Recycle Yard 
  To: terrapreta at bioenergylists.org 
  Sent: Tuesday, January 29, 2008 7:57 PM
  Subject: [Terrapreta] Life cycle of forest fire char?

  As their counterargument, they said that all the char from a forest fire was essentially gone a few years later, therefore (in their view) it must have been biodegraded as a nutrient. If true (that it is gone), I suggested that most of it was removed by water run-off, but I actually don't know. So I'm asking here. What happens to char produced by a forest fire?

  1) Is it mostly not pure charcoal, and thus does get utilized as a nutrient?
  2) Does it wash/blow away because it is on the surface?
  3) Does it enter into the soil?
  4) Does it actually "go away" as they claimed? Or is this just simplistic observation on their part?
  5) Some combination of the above, or other explanation?

  No need to go into a detailed response if you can simply point me to appropriate articles, etc instead.

  There is going to be a lot of resistance from old school anti-burners, so it is important to know how to answer their questions and help them reframe their thinking.

  TIA,
  Bernie




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