Woodman, Spare That Snag

Study offers surprising finding that dead trees benefit wildlife, and don’t increase the severity of wildfire



Tom Brossart/ Roundup

A new study suggests that dead trees, like these off the Mogollon Rim Forest Road 300, may actually be good for the forest.

The Dude Fire swept through the forest, leaving a bristling landscape of charred matchsticks.

Cut them all down?

Bark Beetles moved through a stand of towering Ponderosa Pines, leaving naked, dead snags behind — seemingly begging for a lightning strike.

Cut it down before a bolt of lightning sets it on fire?

The U.S. Forest Service and other land managers have seen danger in the dry, dead snags of trees killed by fire, drought and beetles. For many decades, the Forest Service paid contractors to go in and cut down dead snags.

More recently, the Forest Service has often given “salvage” contracts immediately after a fire, to clear out dead trees and downed timber.

On the face of it, that policy would seem a perfectly reasonable way to prevent renewed fires since the dried out snags would seem logically much easier to set on fire than a live tree, full of moisture.

Not necessarily, concluded University of California Davis researchers studying a huge tract of land in the San Bernardino Mountains of California.

The findings, published in Open Forest Science Journal, concluded areas with dead trees were no more likely to catch fire than unburned forest. The size of the trees — whether dead or alive — proved the main factor in determining fire severity.

That’s bad news for Rim Country, since tree densities in the past century have increased from about 50 per acre to perhaps 1,000 per acre, according to various estimates — with millions of acres dominated by thickets of small, crowded trees.

The researchers based their conclusion on a study of satellite images taken before the 2003 Old and Grand Prix fires to determine which areas had dead trees, mostly as a result of the drought compounded by a massive bark beetle infestation.

Researchers then compared the subsequent fire history in areas with lots of dead trees and areas that hadn’t been heavily affected by the beetles or the tree die-off.

The number of dead trees had no connection to the severity of the fires, although the study didn’t address the question of whether fires were more likely to start in areas with lots of dead trees.

The researchers concluded that the Forest Service should not try to remove dead trees, except perhaps close by developed areas.

That’s because previous research has concluded that those dead snags also host more wildlife than any other sort of tree in the forest. For one thing, the dead and dying tree attracts a variety of insects, which provide food for a whole food chain of other animals.

Moreover, the dead snags provide homes for many larger animals — especially cavity-nesting birds like woodpeckers. Many of the birds that remain in the forest throughout the winter depend on finding or excavating hollows in dead trees. Those over-wintering birds play a disproportionate role in protecting the forest from insect infestations, since they will glean even hidden insect larva in brood chambers inside trees.

Many tree squirrels, which serve as an ecological cornerstone, also rely heavily on dead snags.

Studies by researchers from Northern Arizona University concluded that dead snags in the ponderosa pine forest may actually harbor up to 80 percent of the overall wildlife activity.

The study could prove controversial, since it undercuts one of the prime arguments in favor of salvage logging after a fire or beetle infestation. Such salvage sales represent one of the few types of sales still widely available to timber companies, whose harvest has declined dramatically in recent years.

The U.S. Forest Service spends hundreds of millions of dollars every year cutting down dead trees, trying to reduce the threat of severe fires. Generally when the Forest Service awards a contract for a salvage sale, it costs far more to prepare the sale than the timber company pays for the wood.

However, forest managers have hoped that the salvage sales would reduce the severity of fires. Most of the forests of the Southwest are adapted to frequent, low-intensity ground fires that consume saplings, downed wood and duff on the forest floor, but don’t generally kill mature trees. However, a severe fire in thick and overgrown forests will climb the ladder of small trees into the branches of the large trees and so consume everything in its path. If the fire burns through an area with a lot of dead wood on the forest floor, it can burn so hot that it effectively sterilizes the top layers of soil — sometimes fusing the soil so that it has trouble absorbing water during rains.

That logic provided a powerful argument in favor of even money-losing timber salvage sales.

But the UC Davis study has now cast some doubt on the basis of that argument, with results suggesting fires don’t necessarily burn hotter in areas with many dead trees.


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