11 June 2012

Fires and forest density

Observations from an article in The Economist:
The wildfire raging in New Mexico’s historic Gila National Forest has already scorched some 230,000 acres (93,000 hectares) of woodland, making it the biggest in the state’s history... In many ways, the New Mexico blaze is a test case. It represents the first large-scale opportunity that firefighters have had to test a new approach to forest management. This relies on pruning the undergrowth and thinning out the stands, and then letting nature take its course when lightning strikes and triggers a blaze...

Actually, there is nothing new about the latest approach to forest management. Native Americans were practising similar forms of forestry long before settlers arrived from Europe. Where lightning had not done the job for them, they set fires to thin the forests—so grasslands and edible plants could flourish between the trees and attract grazing animals for hunting. Records indicate the land in pre-settlement times had no more than a dozen trees per acre. Today’s forests, whether natural or man-made, tend to be packed with well over 100 trees per acre...
There's more at the link, especially re the influence of forest density on water management.

Yesterday, I hiked three miles along a bicycle path in the Madison area; while doing so, I passed through some acreage that was being restored to "oak savannah" conditions by grubbing out the underbrush.  The contrast was striking.  For the previous half-mile the woods on either side of the bike path were so dense that one couldn't see any further than the line of first trees.  Where the restoration was underway, one could see for hundreds of yards.  This was being done in a suburban setting where wildfire would be impractical as a clearance mechanism; the community was relying on "elbow grease" and some funding from the sale of turkey hunting licenses. 


  1. In my neck of the woods, a particular family or families within the local First Nations were responsible for annual burns. Each year a different area was burned off. Recently a study was done on a nearby forest and tree "cookies" were cut. The researchers were able to determine that that forest had burns on a 15-year rotation. The family and the local and provincial governments now work together to schedule and start the burns.

  2. I do not know where that information came from, I personally talked to the head ranger shortly after the fire blew up and she stated that crews had been dispatched to fight this particular fire. In a very short time, it became clear the firefighters were in danger, then they were withdrawn. The Gila has had a record of trying to allow fires to burn naturally. I live about ten miles from the fire, it is spectacular in its destruction.

  3. Can you post pictures of the oak savannah if you go back there?

    1. Yes, actually I'm hoping to do that. The first time I was just looking for butterflies and hadn't read the Economist article yet. I expect to be back later in June or July.

  4. I work on forest issues in the west, and for about the last 20 years.

    It's a bit of an over-simplification to say that historic forests had "no more than a dozen trees per acre." In reality nature is highly variable. Fire history varied. Fires burned in a mosaic of burned and unburned patches. Some forest sites are natural "fire refugia" meaning that fire rarely affects those sites. Fires "skipped" some areas once, twice, or three times, resulting in various tree densities.

    Also, fires tends to stimulate a flush of tree germination and establishment. See, Baker, W.L., T.T. Veblen and R.L. Sherriff (2006). Fire, fuels, and restoration of ponderosa pine-Douglas-fir forests in the Rocky Mountains, USA. Journal of Biogeography. 2006. (“Low-severity fires were common, but high-severity fires also burned thousands of hectares. Tree regeneration increased after these high-severity fires, and often attained densities much greater than those reconstructed for Southwestern ponderosa pine forests. … Exclusion of fire has not clearly and uniformly increased fuels or shifted the fire type from low- to high-severity fires. … Both tree-ring reconstructions and forest reserve reports document that tree density was highly variable in Rocky Mountain ponderosa pine–Douglas fir forests near or before AD 1900, suggesting that the low-severity model is inappropriate in most cases.”)

    It's a great idea to reintroduce natural processes like fire, but many people want to replace fire with logging. Unfortunately, logging is not a very good surrogate for fire. Logging removes too many of the trees and virtually all of the trees that are killed by the logging disturbance. Fire on the other hand, kills trees and consumes the smallest, most hazardous fuels, but the large trees and large wood often remained in the forest and provided habitat for a wide variety of species, including woodpeckers, that excavate cavities,and a wide variety of secondary cavity users.

    The best idea is to use fire alone as much as possible, and when necessary pre-treat forests with careful removal of a portion of the small fuels before reintroducing fire.

    Brown, Rick. 2000. Thinning Fire and Forest Restoration, Defenders of Wildlife, December 2000. http://cbfinfo.com/cbf/wapro/Fire/Brown.pdf

    Peters, Robert L., Evan Frost, and Felice Pace. Managing for Forest Ecosystem Health: A Reassessment of the "Forest Health Crisis" Defenders of Wildlife. http://www.defenders.org/bio-fh00.html

    1. Thank you, Doug, for an excellent comment.

  5. According to '1491' and other texts that I have read, fire was actively employed by earlier Americans as a means of enhancing their food supply. The Great Plains were maintained by deliberate burning so that Bison would have a plentiful food supply. A Bison was a self-storing food supply; in addition to providing shelter, clothing, and fire fuel. So much easier than harnessing and plowing. The Amazon supported large agricultural cultures for many centuries through burning and maintenance burning, charcoal making. Charcoal maintains soil fertility for decades, perhaps eons.

    After witnessing the burning of the Okefenokee Swamp for 3 years, I can sympathize with people who are losing their precious trees, but fire is an active part of the ecology that maintains a dynamic balance in nature. Too much harvesting and burning and you have the scenario of the middle east, and too little and you have the bonfire waiting to ignite such as is in the west. Neither are very efficient at supporting human life as part of its ecology.


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