14 May 2012
The US Forest Service is using sticks of high explosive to fell trees in forests that have succumbed to mountain pine beetle infestations. The practice began in Colorado and is now being trialled throughout North America. Blasting trees makes sense in certain situations, Gordon Ash of the USFS Montana told The Missoulian, the newspaper serving Missoula, Montana. Insect-killed trees in particular can pose hazards that healthy forests don’t. They often rot from the inside, making them prone to shattering or falling in unpredictable directions. Put that rotten tree on a cliffy hillside over a road, and there’s no safe way for a lumberjack or mechanical cutter to cut it down.Explosives also help when a cut tree has hung up on its way to the ground. Those trees can get so twisted or bent that their tension makes it almost suicidal to touch them with a chain saw. Better to blast from a distance than trigger a natural booby trap, Ash explained.
“You’d calculate the proper amount of explosive, and then fix that on the tree with shrink wrap,” Ash said. “You’d put it right where a face-cut would be, and sever it off right at the point where you put the explosive – almost like a directional fall. The idea is to link as many of those trees as possible to be efficient.”
While felling tress with explosive makes sense in theory, it takes a lot of on-the-ground practice before foresters feel safe to have it in the toolbox. Ash’s crew measured what their shock waves did to surrounding areas, how closely spaced the blasting pods could be, and other factors beyond what it took to detonate a tree trunk.
A blaster can exercise a remarkable amount of control in how the tree comes down. Drilling a hole in the trunk and putting explosives inside does very little damage to the wood, leaving a fuzzy stump. Shrink-wrapping explosives to the outside snaps the tree as if it were toppled by a wind shear. Proper placement can bring down 50 trees at once, laying them “like hair on a dog’s back” for easy decking or removal.
British Columbian loggers use explosives extensively. In addition to clearing hazard trees, they like to place ammonium nitrate-based explosives at the base of decrepit cedar trees to tip them over without shattering them. This type of explosive detonates more slowly than the emulsion-style sticks used in the United States. So they push more than they break, tipping the tree from the roots instead of snapping it at the trunk.
The USFS is experimenting to determine what kind of placements bring a tree down in the safest way, with the least explosive and the most efficient use of time and personnel.
Source: The Missoulian (c). To read more click here.