What ever you do, stay out of the burned zone, is the message experienced foresters, researchers, and firefighters have for residents returning to their homes after the Fourmile Canyon and Reservoir Road fires. The blazes that left hundreds homeless leave another hazard in their wake: falling trees.
“Really hot fires burn the tree, its roots, and all,” says Greg Sundstrom, Assistant Staff Forester with the Colorado State Forest Service, “those trees are likely to fall anytime between now and ten years from now.” This makes the burn zone an area that curious homeowners and curiosity seekers should avoid.
Left unstable without anchoring roots, charred trees begin falling immediately. “When I was on the firefighting crews in California, they’d have us eat our lunch squatting on our feet. This was so we could move quickly when we heard a tree coming over, to get out of its way,” he said, describing the thud-thud-thud of falling trees.
But not all trees killed by fire die right away. “We’ve looked at survivorship of ponderosa pines following fire in the Black Hills,” says Dr. Skip Smith, Department Head of Forestry and Rangeland Stewardship at Colorado State University. “Immediate kill from the fire, where the foliage is consumed, is obvious. But what we saw is that trees will die for four or five years after a wildfire. It may appear green but eventually dies.”
If the cambium – that area of living cells under the bark – and the phloem experienced a lot of heat, those damaged spots girdle the tree. It may be alive right now, but next year or the year after, it dies, he says.
Assessing trees for survivorship can be tricky, cautions Smith, but here are a few things to look for. Check the crown, looking for percentage of scorched foliage. If more than half of the crown remains green the tree has a good chance for survival; less than half and the tree’s odds diminish. Total consumption, where the crown has burned off, means the tree is killed.
Look for deep charring around the base of the trunk. Ground fires burning duff, leaf litter, or other fuels against the trunk can kill the tree at ground level. Like a pencil standing on its point, these trees are hazardous and will fall. But if sixty-percent or more of the base is unharmed, it might survive, says Smith.
The good news is that ponderosa pines become resistant to fire, developing a thick, corky bark as they grow. This insulates the cambium from heat. Other trees aren’t armored from the heat, such as Douglas fir or lodgepole pines; they often fall victim to the flames. Unable to regenerate from roots, pines succumb completely. But other trees, such as Aspen, are remarkably resilient and will rise from the ashes to shade the land.
“Aspen will sprout up after fire if the heat into the soil isn’t great,” said Dr. William Jacobi, Professor of Tree Pathology and Extension Specialist at Colorado State University. “In fact, often they’re not even noticed until after wildfires remove other trees.” Native bushes are supposed to burn, he says, and come back quickly.
The big question for these areas is whether invasive plants will crowd into areas where smaller vegetation has burned off. “The problem, now that invasives have moved into the forest, is that the invasives often do better after fire. Seeds and roots survive, and they grow back before the natives,” he said.
At this time of year, experts recommend people whose property burned should stabilize the soil before snow flies, remove hazard trees, and plan revegetation for spring. Contact the Colorado State Forest Service for a list of tree contractors.
- Fourmile Fire: Mitigation Efforts In Areas West Of Boulder Under Review (huffingtonpost.com)
- Fourmile Canyon Fire: A Sign of Things to Come in Pine Beetle-Ravaged Colorado? Maybe Not. (ecopolitology.org)