A tree’s gifts to a forest continue after its death. Standing dead trees, also known as snags, are so important that forest managers often purposefully damage trees so they’ll later die and form snags. Two decades after a series of snag creation efforts by forest managers, new research reveals which methods were most effective.
“In a natural setting—in a forested setting—those trees are incredibly valuable.”
Even when trees are dead, they provide vertical forest structure, habitat for dozens of species, and carbon storage. A dead tree also shuttles nutrients around the ecosystem as it decays and eventually provides wood that can shelter animals on the forest floor. “A lot of people say, ‘It’s a dead tree. It’s in my backyard—I want it out,’” said Jim Rivers, a wildlife ecologist at Oregon State University in Corvalis who authored the new study. “But in a natural setting—in a forested setting—those trees are incredibly valuable for many reasons.”
Natural snags often occur when there are so many trees that some die because of a lack of nutrients. But humans sometimes remove snags to sell them as lumber or to protect workers during timber harvesting. “When you’re actively managing a stand for timber and you’re constantly taking trees out, you don’t have that competition anymore,” said Cheryl Friesen, the science liaison for the U.S. Forest Service in Springfield, Ore. “You don’t get things dying in a way that’s really valuable to a lot of wildlife.”
So, particularly in the Pacific Northwest, forest managers started taking measures to create snags some 20 or 30 years ago in places they may not occur naturally, Friesen said. But it’s been difficult to monitor the progression of these snags. Even when purposely damaged, trees die and decay slowly, over a period that exceeds a typical Ph.D. program. That’s where the new study comes in. Rivers has pulled together longer-term monitoring projects, which are rare, according to Friesen.
In 2001 and 2003, in two areas in southwestern Oregon managed by the U.S. Bureau of Land Management, forest managers used several techniques to produce future snags, then marked the trees with metal tags and logged their GPS coordinates. Some had their upper branches lopped off, a method known as chainsaw topping. “That’s the [method] that we typically see on the ground,” Rivers said. Other trees were wounded at the base with large chainsaw cuts or dosed with a wood-munching fungus. Last, another group of trees received both the fungal inoculation and chainsaw topping.
Nest Holes and Peeling Bark
In 2021, a field crew visited those sites and found 809 of the marked trees, about 98% of which were still standing. That wasn’t surprising, as dead trees can stay upright for 100 years or more, Rivers said. But he was surprised that few trees that had been wounded at the base died during the monitoring period. Chainsaw topping was much more effective, killing up to 99% of trees when a large number of branches were removed. Fungal inoculation didn’t kill many trees on its own or seem to aid chainsaw topping in bringing a tree’s demise. Researchers estimate the slow-growing fungus lives up to 100 years, Rivers said, and it may take 4 or 5 decades to cause significant decay.
Such information can help guide forest managers, Rivers said. For instance, chainsaw topping appears to be a good bet for turning trees into snags within a couple of decades, but it’s probably not worth the cost and effort to add a fungal inoculation. Mechanical wounding may be a better option when forest managers seek a slower decay. For instance, a forest crew could use both methods in one outing to create many snags that will die at different times, spreading benefits across a longer period. The study didn’t examine girdling, another common technique that cuts a ring through the bark around a tree’s circumference.
“Every tree that is left as a snag is one that isn’t brought to the mill for harvest.”
The field crew also examined other ways in which the trees were serving the ecosystem. For example, they estimated the percentage of each tree that was covered with holes, an index of how much the tree had been used by woodpeckers, one of the few animals that can get into a dead tree and carve out holes for nesting, Rivers said. Once they leave those cavities, other organisms use them. A decrease in bark cover hints at a tree’s usefulness to bats, as some roost beneath peeling bark. And a broken trunk suggests that much of the dead tree’s wood has landed on the forest floor, where it can provide a haven for ground dwellers, such as some small mammals and salamanders. Chainsaw-topped trees with the most branches cut off proved most beneficial in these areas as well, with the highest percentages of cavity cover, peeling bark, and cracks along the trunks.
Ultimately, if forest managers want to create snags that provide habitat over long periods, they should probably use multiple techniques, Rivers said. Future monitoring studies could reveal how long it takes for the method of wounding a tree’s base to reliably produce a snag. In another 2 decades, the trees treated in 2001 and 2003 may be snags, whereas trees that were topped may be on the ground. Rivers also hopes to study how the economic costs of creating snags compare with their ecological benefits. “Because every tree that is left as a snag is one that isn’t brought to the mill for harvest.”
—Carolyn Wilke (@CarolynMWilke), Science Writer
