Sometimes, conservationists want trees to die. That’s true even for majestic icons like Douglas firs, evergreens that can grow to a towering 100 meters in the coastal mountains of the Pacific Northwest.
Which begs the question: What’s the best way to kill a tree?
Jim Rivers, a wildlife ecologist at Oregon State University, wanted to answer this morbid mystery. Not because of a macabre fascination, but because sometimes forest managers want to more dead trees on the landscape.
This isn’t about the kind of mortality that decimates entire stands of trees, from disease, pests or fires. It’s something more subtle. In the forest, large, dead standing trees are a bonanza for wildlife. Insects dine on the rotting wood. Birds such as pileated woodpeckers feed on the insects in turn, and chisel nests from the softening fibers. That’s just one example of the menagerie of furred and feathered creatures that build their homes inside dead and dying trees.
These standing dead trees are often called snags. “Snags support multiple functions within forest ecosystems,” Rivers said. “They provide vertical structure and contribute to nutrient flows and carbon cycling in addition to providing habitat for a diversity of organisms. But many have been lost from managed forests, especially large-diameter snags.”
To make up for that loss, people have been studying how to create more snags in the forest. In 2001 and 2003, the federal Bureau of Land Management sent crews into the woods in western Oregon armed with chainsaws and a stash of Phellinus pini, a fungus that can rot the center of tree trunks. At more than 1100 Douglas firs, they either lopped off the top and most branches, slashed a huge chunk of bark and wood from near the tree’s base, injected the tree with the pathogen, or some combination of the three measures.
Two decades later, a team returned to more than 800 of these trees to see what had happened. They found that the trees met very different fates, depending on the tools used. While nearly all the trees were still standing, the ones with their tops and branches cut off were much more likely to have died and become snags. Nearly all of the with the most extreme butchering – more than 90% – had died. By comparison, just 6% of the infected trees and around 4% of the trees with a chunk missing had transformed into snags, Rivers reported recently in Forest Ecology and Management.
“Mechanical wounding and fungal inoculation showed limited ability to create snags and promote structural diversity in the forest,” said Rivers. “And adding fungal inoculation to trees that have been topped with chain saws does not appear to be worth the additional expense and time.”
That doesn’t mean the only option is to cut the tops off trees, cautioned Rivers. If you want snags quickly, the new research shows that’s the best choice. But if you want to have snags cropping up decades later, it might make sense to turn to fungi and tree “wounding.”
The cheapest strategy could be mixing all three approaches at once, said Rivers, rather than paying to send crews into the woods multiple times over years. By diversifying during a single visit, snags will emerge on different time scales, some quickly and others in the distant future.
Think of it like a murder mystery. Sometimes the killer chooses swift death with a knife. Other times, a slow-acting poison. Only, in the case of creating snags, no one (except perhaps the tree) will consider it a crime.
James W. Rivers. “Fungal inoculations and mechanical wounding of trees have limited efficacy for snag creation two decades after treatment.” Forest Ecology and Management. Dec. 23, 2023.
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